351C IDLE SPEED PROBLEM, FMX CLUNK?

[andy72]

Hi everyone,
This question is about idle speed...and tranny clunk.
I rebuilt the Holley 4 barrel 8 years ago. It has always acted like this, so not a new issue.
There is a major difference, 300-400 rpm, between idle in park and idle in gear.
When the engine is cool, idling at 600, putting it into gear will drop the idle to 200 and stall the engine.
So, i've always had the idle up to about 1,000 so when in gear, its about 600. This way it doesn't stall at stoplights.
When hot, it's racing along at 1,100.

Now i've always noticed that at this high RPM, when you shift out of Park, there is a very pronounced, almost violent, CLUNK in the Tranny as it engages Reverse, or again as you go to Drive.
It jerks the car so bad going into gear that the hood has closed itself when it was up.
When researching this "idle" issue today I ran across a thread from this website. A guy said his car did this and that the problem was the transmission.
He said the "clutch plates stuck or bind together" causing the idle speed difference.

This has me thinking in a different direction than a carburetor issue.
This car sat outside for over 10 years before I acquired it in 2015. Been driving it limited last 3 years.

Any thoughts? Do I have a transmission problem possibly?

If have a carbureted 302 with a C4 and a carbureted 460 with a C6. Neither act like this.

Thanks for any thoughts...

 

[Sheriff41]

It sounds like you may have your idle speed set too low. Try setting your idle speed at 625-650 rpm with the transmission in DRIVE and the AC turned off. Also, check you ignition timing and set it at 14°-16° BTDC with the vacuum hose(s) removed from the vacuum advance and plugged.

The clunk could be caused by broken motor mounts or transmission mount.

 

[vintageman]

I second Sheriff41's comment but would also check your fast idle setting on the opposite side of the carb. If you have a choke, it is kind of right behind the choke and is hidden. You have to hold the throttle in the open position and you will see the screw for it drop down into site under the choke housing. I had a similar issue with my earlier Holley where the fast idle was keeping my idle at around 1100-1200 RPMs. It took a little playing with it but I finally got it to idle at around 800 in Park and about 600 in gear. Good luck.

Tom

 

[mrgmhale]

There are a few other things that will allow a powertrain to have a louder than usual "clunk" when going into gear. The high idle speed in Neutral/Park is going to contribute to the problem. There should be a drop in RPM when placing an idling engine into a forward or reverse gear. A 400 RPM drop sounds a little high to me. I am thinuing you may be running at too lean an idle air/fuel mixture. That may be caused by an intake manifold vacuum leak. There are a few different ways to test for a vacuum leak. One of those ways is to use a smoke test, which will help show exactly where any intake leaks are occurring. The one video I like is at:

https://www.youtube.com/watch?v=TmV_HjeiuCc

If you find any intake manifold leaks, correct them before trying to adjust the idle speed and idle air fuel mixture settings. I have some videos I posted showing how to adjust the carburetor idle speed, and the fast idle speed when the choke is (partially) on. It does not sound like your idle speed has an issue beyond it being set to a fairly high speed in Neutral to help prevent stalling the engine when it is put into gear. But, after checking for and correcting any intake manifold problems getting the carb set up correctly is important. The carb I am showing in my videos listed below is a stock Autolite 2100 2v unit. Your carb will be set up in a different manner, but the concepts are the same. Set the high speed idle first, then the running idle (with the TPS if so equipped), then the base idle (with the TPS off if so equipped), and idle mixture screws. I also suggest you set the base ignition timing before dealing with the carb idle settings. I cover that in one or more of the videos in the links below.

https://youtu.be/mwLwwUtI1cE


The link below includes a rebuild of the carb before the adjustment. kip forward in the video to get to the adjustment section.

https://youtu.be/U-ETjhRqUwQ


After the carb is adjusted,, if you are still running at high RPM in Neutral the clunking will likely remain an issue. Here are some other areas you can check...

It was suggested above to check the engine mounts. That is not unreasonable, but I do not see engine mounts cause a problem util there is some power applied to the engine and the engine ends of rocking out of its settled position, at which point it will make a significant sound. But, if the rubber insulation of an engine mounts has been eroded away, it is possible any engine noise will be transmitted with a metal to metal contact. In that eventuality I would concur an engine mount that is failed could make a clunking sound more pronounced.

There is also a transmission tailshaft mount that could possibly contribute to a slunking sound. But, like the engine mounts, usually I see those causing problems on a heavy load, not at idle speeds. But, as with the engine mounts that is no reason to overlook it as a (partial) cause of a pronounced klunk.

If either U-Joint is seriously worn putting the tranny into to gear csn certainly cause a loud clunking ound. It is easy to check a U-Joint for excess wear to the point where a loud clunking sound is heard. If you do not know what to look for there are a lot of videos in YouTube on the subject. Just search in YouTube on, "check for bad u-joint" and a lot of videos pop up. I have not read or watched all of the videos, but I have come across many that are reasonable. For me, rather than tearing things apart to inspect the U-Joint, I would prefer to forst trying to rotate the driveshaft in different rotational directions when the wheels are on the ground to see if you get any excess movement with either U-Joint. If you see excess movement, or hear any noise from the U-Joint(s) it is time to replace the offending part. If you have a new U-Joint being installed, be sure to pack it with fresh grease so it is properly lubricated. If you are going to use the existing U-Joints I suggest lubricating them with fresh grease also to both provide proper lubrication, and to help soften any sound they may be contributing to the clunk you re hearing.

While trying to rotate the driveshaft back nd forth you my feel or hear domr clunking coming from the driveshaft front you where is slides onto the transmission output shaft. It is very unlinkly the toke and output shaft splines have worn to the point where you hear a clunk from there or see/feel/hear the impact of highly worn splines. But, you can help reduce or minimize any clunking when going into gear by unbolting the rear U-Joint from the rear axle pinion shaft, then sliding the front yoke out of the transmission, Then pack some wheel bearing great into the yoke and output shaft so when you reinstall the yoke the grease is spread out between the roke and output shaft, You may need to indsert and remove the yoke a few times to mke certin the grease is applied along the length of the yoke, and between all sliding spline points. When reassembling the driveshaft make certain the rear U-Joint retaining U-Bolts are properly tightened. There is no need to over-tighten the U-Bolt nuts.

Yet another area a clunk could be coming from is the rear axle pinion gear when it is slammed against the ring gear when going into gear. If the rear differential has enough wear to allow a clunking sound, you will likely have other issues with the rear axle gear set. Those rear axle gear sets are very durable, and do not wear much as long as there is adequate lubrication (using the correct rear axle lubricant) in the rear differential system. So, I really dount you will have excessive play when manually rotating the pinion input in both directions back and forth. If you are getting excessive movement and/or sounds from the rear axle gears you will need to rebuild or replace the differential gear set. I sincerely doubt that will be the case, but it is just another stome to be overturned and examined.

I know this was a long read. But I am hopeful it is helpful. If the clunk continues, or is corrected, please post the information so all of us find out how you ended up dealing with it. I am going to be doing some acoustical waveform testing on a series of driveshafts and U-Joints over the next few weeks. If I come scross any other ways to test U-Joints for excess noise/wear I will be posting it in this forum. That may happen a while after the testing processes are being worked out, which may extend until Car Season 2024 if the testing and recordings of various U-Joints is not done before Winter Hibernation 2023 - 2024 up here in Snow Country (outside Rochester, NY.

 

[cjstaci]

Do what mrgmhale said. He is right on the money.

 

[Rozz]

Awesome job as always and great job explaining how everything works together.

 

[KSpence]

Hi everyone,
This question is about idle speed...and tranny clunk.
I rebuilt the Holley 4 barrel 8 years ago. It has always acted like this, so not a new issue.
There is a major difference, 300-400 rpm, between idle in park and idle in gear.
When the engine is cool, idling at 600, putting it into gear will drop the idle to 200 and stall the engine.
So, i've always had the idle up to about 1,000 so when in gear, its about 600. This way it doesn't stall at stoplights.
When hot, it's racing along at 1,100.

Now i've always noticed that at this high RPM, when you shift out of Park, there is a very pronounced, almost violent, CLUNK in the Tranny as it engages Reverse, or again as you go to Drive.
It jerks the car so bad going into gear that the hood has closed itself when it was up.
When researching this "idle" issue today I ran across a thread from this website. A guy said his car did this and that the problem was the transmission.
He said the "clutch plates stuck or bind together" causing the idle speed difference.

This has me thinking in a different direction than a carburetor issue.
This car sat outside for over 10 years before I acquired it in 2015. Been driving it limited last 3 years.

Any thoughts? Do I have a transmission problem possibly?

If have a carbureted 302 with a C4 and a carbureted 460 with a C6. Neither act like this.

Thanks for any thoughts...

My first reaction is that youre idle is too high, then I saw you say it's at 1100 in park, which is indeed too high. Take it to the extreme. Imagine if the RPM is at 2000 and you drop it into Drive. You're essentially dropping the clutch on an automatic. Too much shock. Get that idle to 800 or so in park.

When it goes into drive, you shouldn't drop too low...600-ish. Look for a vacuum issue. I'm wondering if you have full/some vac to the distributor, or if you have an outright leak - something to create a big RPM difference like that. Start disconnecting vac lines and plugging them just to see if anything affects your RPM at idle. Also worth seeing if there's a vac leak under the carb. Also check your initial timing.

For the clunk, if your tranny is OK, the next thing on the driveline is the driveshaft, so check those u-joints. Wouldn't be the first time I'd seen that. It'd happen with your foot on the brake and shfting from D to N to R too ... clunk forward and in reverse... classic u-joint issue.

Hope some of this is helpful

 

[andy72]

OK Thank you everybody... so far!
It's always hard on the initial post to include all the info. To much writing...
Initial timing is at 6BTDC
new u joints, ect. blah blah
As a few said, I will look into fast idle issue. Not something I ever had to deal with.
Idle is way not right!
I've had a Holley on a 302 of mine for 20 years.
I've had an Edelbrock on my 460 for 25 years.
They slip from neutral into drive like butter. Slight RPM drop, that's it.
This is something completely different.
Thanks MRGMHALE, lots of info. Motor does have new intake gaskets... I'll go thru all..
As KSpence said, when my fast idle is rippin at 1,100 in neutral or park, that drop into drive or reverse is shocking.
When the engine is up to full temp., the idle is way to high. Give it a rev, nothing. just settles back down to over 1,000.
I'll let everbody know how things go..

A few pics of before and after... It's why I want it to be right!
Thanks again

 

[midlife]

First thing I would do is get the engine warmed up and check to see if the choke is still set. The butterfly plate should be vertical at this point. If not, the problem is in the choke, a rather easy fix to your high idle.

 

[Precision Transmission]

Quote: He said the "clutch plates stuck or bind together" causing the idle speed difference.

You're kidding us, right?

 

[andy72]

Precision Transmission...cracked me up!
Seriously read that from a post from 7173mustangs from about 2013.
Precision, assuming you are a transmission"guy", I have a question.
Here's where I'm at..did 5 minutes ago. Warmed the car up. Lowered the idle to 600rpm.
When I shift into Reverse, the whole car has a violent jerk. It REALLY snaps into gear. When I go down to Drive,
also a pretty good jolt to the whole car. I wish I could post a video. Something is not right. This is idling at 600. My other two early
Fords slip in and out of gear smooth. Any idea what could be going on with my FMX?
I had the hood open, shifted in to Reverse, the whole car snapped backwards so hard, it shut my hood. This is with new hood springs from NPD. Supposedly the correct "flat wound".

I have two separate issues .
I think the carburetor idle speed is one issue. Whether cold or warmed up. The transmission seems to be another.
but anyway..Midlife, the choke is not set completely properly, but when I open the blade completely, no change in RPM.

 

[mrgmhale]

As hard as it may be to believe, I have some other things to consider - not my fault. In a post a little above this one Midlife mentioned something about the choke plate positioning, and it triggered me to add to this thread. Blame Midlife! heh heh... Actually, I feel badly that I did not think to add the following earlier, as it is indeed pertinent. What Midlife mentioned is something I should have added. All hail Midlife! What? Crickets? Nobody is honoring him for having brought forth a perfectly valid possibility for the high idle condition? Okay, if that be the case I honor Midlife for his thought (this and may others)...

Okay, back to being serious. When you replaced the prior carb (oem I assume) with a 4v carb, I am betting the new carb (also) has an electric choke. The reason this is important is because aftermarket electric chokes require 12 volts DC switched current. The oem electric choke Circuit #4 is powered off the alternator stator terminal, which output 1/2 the voltage of the alternator positive terminal that leads to the battery (through the Starter Relay large positive terminal), and the stator current is AC, nor DC. The AC vs DC is not significant, as either will send power through the heater and warm it up. BUT... the stator voltage being about 6 to 6.8 or o volts, vs battery voltage, is significant as the lower than needed voltage will result in the choke coming off too late, which can result in the fast idle speed staying engaged too long and/or the choke plate being partially closed which will result in an excessively rich air/fuel ratio being sent into the combustion chamber.

I have had two times in the past few years, where inadequate voltage caused a problem. The first time was on the 73 Mustang Convertible with its original 2v carb. The electric heating element failed and the engine was dumping a lot of black smoke out the exhaust. I figured one of two things happened, wither the electric choke failed or the 2v Power valve ruptured. I popped the air clear top off and say the choke was partially closed despite the engine being well warmed up. I simply replaced the choke bimetallic spring housing, which included a new electric heater element, adjusted the choke housing to the proper index pointer location, and all was well. I later on produced a video on how to properly adjust the high speed idle setting in the following video (and other settings also):

https://youtu.be/mwLwwUtI1cE

The other possibility involving the electric choke is, again, using the oem connector which is powered by the Circuit #4 stator terminal on the alternator. What you need to do with an aftermarket carb that requires 12 volts of switched power is to change the power leading to the choke housing heater terminal. I have see where some folks use the "I" terminal off the Starter Relay. That is switched power, kind of, but it is also running at a reduced voltage (110 volts or a little lower) as it is powered from two different sources. In the Ignition Key CrankStart position it is getting full battery voltage via an internal copper disk that bypassed the ignition resistor wire circuit. BE AWARE, THIS DOES NOT MEAN IT IS GETTING 12 VOLTS WHILE CRANKING... hen the engine is being cranked over the voltage drops to 9.6 - 10.5 or so volts - so all the Ignition Bypass Circuit can send to the Ignition Coil is whatever the battery can deliver (9.6 - 10.5 or so).

The other power source for the Starter Relay "I" terminal is through the ignition resistor wire that also feeds the Ignition Coil's positive primary terminal.

As for how to get switched battery voltage to the Electric Choke, there are a few ways. One way is to use a splice off the Windsheld Wiper Motor (Circuit #63, Red wire). The other is to splice into Circuit #640 used to power the carburetor Throttle Position Solenoid. Another more esoteric way, is to use a Power Relay where the trigger voltge can come from the #63 or #640 circuit, and the load power sent through the Relay is then sent to the Electric chike. There really is no good reason I can think of to use a relay in this situation. It will work, but it is like using a bazooka to kill a cockroach - needlessly messy and more complex than necessary.

The second time I had an electric choke problem was with the 73 Mach 1 with its Holley carb. The prior owner had the electric choke on the Holley hooked up to the Starter Relay "I" terminal, which as described above was too low. But, somehow or another the choke would open fully soon enough to not cause a problem for me. It turns out it is because the choke cap was set to a very lean position, so the bimetallic spring inside the choke cap housing provided very little pressure to close the choke, thus with just a little warming it would open the choke fully pretty quickly.

Well, the other month I decided to rewire the choke heater on the Mach 1, and in my case I decided to use the Windshield Wiper Motor Circuit #63, as the prior owner had for some reason cut off or hid the Circuit #640 that was once present for the 302 2v carb's Throttle Position Solenoid. No matter, not a big deal. Circuit #63 is plenty adequate, and is a load protected circuit (Circuit Breaker inside Windshield Wiper Switch protects Circuit #297 (> #63) from Ignition Switch, and Fusible Link Protected circuit (#37) that feeds the Ignition Switch.

And, for those just waiting to see if I am going to say I have a YouTube video showing how to do an electric choke battery voltage bypass, your moment has come. Yes, I did one of those videos also. It is at:

https://youtu.be/WDcskIPfLcE

The rewired electric choke circuitry worked out nicely. The choke comes off more quickly than before, and if it were not for the tendnency of our Holley to run a little rich anyway I might have had to reset the choke cap index pointer position. So far it has been fine, so I am leaving well enough alone.

 

[mrgmhale]

OK Thank you everybody... so far!
It's always hard on the initial post to include all the info. To much writing...
Initial timing is at 6BTDC
new u joints, ect. blah blah
As a few said, I will look into fast idle issue. Not something I ever had to deal with.
Idle is way not right!
I've had a Holley on a 302 of mine for 20 years.
I've had an Edelbrock on my 460 for 25 years.
They slip from neutral into drive like butter. Slight RPM drop, that's it.
This is something completely different.
Thanks MRGMHALE, lots of info. Motor does have new intake gaskets... I'll go thru all..
As KSpence said, when my fast idle is rippin at 1,100 in neutral or park, that drop into drive or reverse is shocking.
When the engine is up to full temp., the idle is way to high. Give it a rev, nothing. just settles back down to over 1,000.
I'll let everbody know how things go..

A few pics of before and after... It's why I want it to be right!
Thanks again

After I posted a suggestion to check for intake manifold leaks it occurred to me I have said this before, but I never really gave a good way to test an intake manifold for leaks. In the past I would hit suspected leakage areas with a small squirt of carb cleaner. If the engine increased in RPM for a moment it was a good indication of a leak being present. But, I had to be careful the carb spray mist was not drifting into the carburetor intake air horn, otherwise I would be likely getting a "false positive.


But, despite the disopportunity for a false positive I found that techniques worked pretty well. But over the past several years I have seen others post how they use a smoke machine to locate gas leaks (intake and exhaust). I thought that was interesting, but I never pursued the technique because I was not getting any indication of intake or exhaust leaks on our pony cars. But, after pondering more on the way to diagnose an intake manifold gasket leak I decided it was high time to get a smoke machine. I purchased one via Amazon, and it arrived yesterday. I can hardly wait to begin playing with it. For anyone interested in getting one, this is the one I chose:

Solary Automotive Smoke Machine Leak Detector - 12V DC EVAP Vacuum Diagnostic Tester for Pipe System​

https://www.amazon.com/gp/product/B0C7MKZPK3/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1

 

[hotrodjunkyard]

Hi everyone,
This question is about idle speed...and tranny clunk.
I rebuilt the Holley 4 barrel 8 years ago. It has always acted like this, so not a new issue.
There is a major difference, 300-400 rpm, between idle in park and idle in gear.
When the engine is cool, idling at 600, putting it into gear will drop the idle to 200 and stall the engine.
So, i've always had the idle up to about 1,000 so when in gear, its about 600. This way it doesn't stall at stoplights.
When hot, it's racing along at 1,100.

Now i've always noticed that at this high RPM, when you shift out of Park, there is a very pronounced, almost violent, CLUNK in the Tranny as it engages Reverse, or again as you go to Drive.
It jerks the car so bad going into gear that the hood has closed itself when it was up.
When researching this "idle" issue today I ran across a thread from this website. A guy said his car did this and that the problem was the transmission.
He said the "clutch plates stuck or bind together" causing the idle speed difference.

This has me thinking in a different direction than a carburetor issue.
This car sat outside for over 10 years before I acquired it in 2015. Been driving it limited last 3 years.

Any thoughts? Do I have a transmission problem possibly?

If have a carbureted 302 with a C4 and a carbureted 460 with a C6. Neither act like this.

Thanks for any thoughts...

What duration is your camshaft??

 

[andy72]

Camshaft unknown. Mild, from sale info. To answer earlier posts, (mrgmhale), carb is 4160 series Holley. 7154 stamped. No electric choke.
Hard to find info on 7154 number.
ANYWAY,
As I said yesterday on my 3:19 p.m. post, I believe I have a a transmission problem.
I just realized I could post video..
Watch and listen as I engage Reverse ... and also drive... something not right.
Very Hard engagement of the gears... and that's with my foot on the brake of course..
What think?

 

[Don C]

Make sure your kickdown is properly adjusted. Make sure your vacuum modulator is connected and operational.

After you get the engine warmed up and the choke is operational and open, shift the transmission into gear and adjust the idle speed. Make sure the timing is set correctly, make sure you don't have any vacuum leaks.

Here's some additional information:
https://itstillruns.com/diagnose-ford-fmx-transmission-problem-4810525.html

 

[cjstaci]

I assumed your engine was stock. You say it has a mild cam, that could be the issue. It may need a higher stall converter. It’s very difficult to diagnose the problem without all of the information, and being able to rule out problems,

 

[andy72]

Regarding the harsh clunk when shifting, at idle, into Reverse.. (and into Drive, but not as bad as reverse)

Fluid level is good
kick down is adjusted properly
Vacuum modulator tests good
U joints are new
there is some slop in the differential, but my other Stang has more slop, and it doesn't bang like this problem with my 72's FMX
idle speed is 600

This is from a post on a website called Mustang Forums describing the exact complaint I have:
posted question:
"I have a 70 Mach 1, FMX auto transmission and there is a violent jolt when I go from Park into drive or reverse, it's worse when the car is warm, ie it doesn't happen when i'm reversing the car out of my garage, apart from that the gears shift great when driving, there is no knocking or delays when going through the gears, i've currently got the car booked in with a Mustang specialist as I had to have the carb rebuilt, but any ideas what the problem could be, do you think the transmission could need rebuilding? I sure hope not, we've got a new modulator valve on order as that is the cheapest option at the moment, thanks for any help"
Reply:
"FMX's have a problem with the pressure valve sticking in the pump. It's due from lack of fluid changes. If all else is ok, you can drive it that way but it will need a rebuild soon."

Anybody heard of the "pressure valve sticking in the pump"?
I've kind of run out of this to check to determine why my FMX has such violent engagement of Reverse and Drive..

Any other thoughts?
Time to take it somewhere to confirm I have an issue

 

[KSpence]

OK Thank you everybody... so far!
It's always hard on the initial post to include all the info. To much writing...
Initial timing is at 6BTDC
new u joints, ect. blah blah
As a few said, I will look into fast idle issue. Not something I ever had to deal with.
Idle is way not right!
I've had a Holley on a 302 of mine for 20 years.
I've had an Edelbrock on my 460 for 25 years.
They slip from neutral into drive like butter. Slight RPM drop, that's it.
This is something completely different.
Thanks MRGMHALE, lots of info. Motor does have new intake gaskets... I'll go thru all..
As KSpence said, when my fast idle is rippin at 1,100 in neutral or park, that drop into drive or reverse is shocking.
When the engine is up to full temp., the idle is way to high. Give it a rev, nothing. just settles back down to over 1,000.
I'll let everbody know how things go..

A few pics of before and after... It's why I want it to be right!
Thanks again

if you suspect a vac leak, there is the old school way of using a spray, like carb cleaner, and lightly spraying the edges of the intake, the carb-to-intake seat, etc. If the engine RPM comes up, you have a vac leak. Like others have said, check hoses and ports too (PCV valve, heater vac line, vac advance line, etc).

Beautiful work on the car ... I'm closer to the 'before' but working on it
Your video startled me at first - "RH Drive car?!?" ... oh yeah, Apple does that on selfie mode (why oh why...I dont know).
Best of luck...

 

[71coop]

I realize that there is a lot left to figure out here - but my guess is the torque converter is too tight for the engine combination.