351C intake swap carb issues

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Tataocb

Well-known member
Joined
Jul 11, 2016
Messages
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Location
Houston, TX
My Car
1973 Mustang Convertible 351C 2v, auto.
I recently swapped my stock 2 bbl intake and carb for a Weiand X-cellerator I bought used and a new Edelbrock 650 cfm Thunder series carb. My car is a stock 73 351C 2v with a duraspark ignition (bought it like that from the PO) and an FMX. I am having a couple of issues after completing the swap.

1. Explosions through the carb on start up at random. Sometimes when I go to start the car there will be a single explosion through the carb. It seems like it blows a whitish smoke (could it be mixture?) up through the carb, but no fire. After that, I turn the key again, and it starts up just fine. It does not always do it.

Took it for a test drive and found 2 issues:

1. It seems the car is having trouble getting to move from a stop or very low speed. Kind of like the transmission takes a couple seconds to engage. The transmission suffers from "morning sickness" where I have to wait for the car to warm up before it engages gear properly. But I was not having these issues at slow speeds or coming off a stop once it had warmed up before the swap.

2. The car bogs down like its going to stall and even heard a popping sound (like an explosion) one of the times I accelerated to WOT.

The initial timing is currently set to 12 degrees and I am not running any vacuum advance as the total timing (mechanical advance) goes up to about 35 degrees (harmonic balancer marks only go up to 30). I set the idle mixture screws and it idles at 900 rpms. I checked for vacuum leaks with carb cleaner spraying around the base of the carb, the intake manifold, vacuum fittings, and vacuum lines and did not find any leaks. The total manifold vacuum is at 15 at idle. I checked the firing order and it was fine. My next step will be to check the spark plugs.

What is the best way to check the s park plug wires?

Could these 3 things be related?

What else do you suggest I check?

Thanks in advance for any help.

 
Not all that great at carbs and all but I have a question and a thought. In your explanation you said it didn't do the lag in the trans engagement when warmed up before, did it do it cold though? I do know that an oversaturated mix on WOT can feel like its going to stall and is likely to backfire, topfire or a whole range of bad things. You may have 2 issues that are now working to present problems together. A laggy trans in an auto could be the trans itself or the stall converter going bad. After putting on a new carb and intake if you're running too rich then the hesitant response of the engine to get to rpm could be slowing down the rpm so the stall converter doesn't engage. I'll leave it up to other members to help you lean it out as I'm not that great with a carb but the principles I get. I'd try to go lean and see what the car runs like then, and remember if you get the idle/ acceleration rpm playing nice you still might need to look to the trans or the stall.

 
A single plane intake with an otherwise stock drive train is a very poor match. The fuel atomization is poor and low speed torque is weakened and coupled with a numerically low rear gear, causes off idle stumbles and poor performance. Chuck

 
I removed the wires and cap from the distributor. I was going to try to remove it, but could not remove the bolt with the tools I had so I left it in there; and completed the intake swap with the distributor in place.

I installed the plate to "convert" the intake to a dual plane intake. Other than that everything is stock including the 2.75 rear gears. I eventually plan to change them to something close to 3.89.

I set the timing to 10 degrees and took it for a drive after it warmed up. I noticed that while in Drive, when I came to a stop sign, the car felt like it was in neutral. I accelerated and the car did not move until 2-3 seconds after the RPMs started increasing; that's when the transmission seemed to engage and the car started moving. I dont have the tach installed in my car so I could not tell what the RPMs were. But it did that every time I came to a complete or almost complete stop during the test drive. I also had detonation twice, once while cruising and accelerating to try to downshift, and the other when the car was parked in the driveway and I was hitting the gas to see if it would detonate.

What causes detonation, primarily under heavy acceleration?

I do remember that I heard detonation when I hit the gas hard the last time I took it for a drive before starting the swap. That caught my attention, but did not think much of it. However, I never had this issue where the car felt as if it was in Neutral while it was in Drive, other than during the "morning sickness" warm up period. Could this mean the transmission is starting to go?

 
Have you checked the trans fluid level? Check while hot in neutral. Your fluid could be low causing the shifting issues. If I understand your notes correctly you now have your initial timing at 10 BTDC so according to your first post you are all in at 33 using only the mechanical advance. At what RPM is this? You adjusted the timing from 12 to 10, how did you do this if you could not get the bolt out for the distributor? You need an OHM meter to check the plug wires. Also, 15 inches of vacuum at idle on a stock 2v seems low.

 
I have to ask if any of these issues were present before the Carb/intake swap? A single plane manifold is not a great choice for your application an Edelbrock performer would be much better with a 600 cfm Holley 4160 or 4150. Others have mentioned that as far as your transmission issue (which is an issue) goes you need to check your fluid level. This is done with the car parked on level ground, engine running and in park. Use only type F fluid to top off. FMX transmissions are pretty good. The FMX in my 72 has 155 K on it without any internal repairs needed since new. Repairs that were required were a fluid change,pan gasket+filter,vacuum modulator,tail shaft bushing and seal. Admittedly the selector shaft seals leaks as they all do and will be addressed soon.

 
Before we go any further, "detonation" is not back-firing through the carb. Detonation is usually caused by too much spark advance for the octane rating of the fuel or a weak mixture or overheating. It sounds 'pinkily pinkily' when you put your foot down.

Back-firing thru the carb could well be a sign that the accelerator pump shot is not big enough (pump jet too small) or coming in too slow (pump cam too gradual). I recently experienced both these problems tuning a 'too big' carb for my engine. At first if I tried to 'wing' the throttle from idle in neutral there was an ear-splitting bang (no air cleaner) and the engine died immediately. This situation is easy to test so try it first. I did get my carb tuned, once I could drive it I used an AFR meter & it took 0.15 secs off my 1/4 mile time.

 
Before we go any further, "detonation" is not back-firing through the carb. Detonation is usually caused by too much spark advance for the octane rating of the fuel or a weak mixture or overheating. It sounds 'pinkily pinkily' when you put your foot down.

Back-firing thru the carb could well be a sign that the accelerator pump shot is not big enough (pump jet too small) or coming in too slow (pump cam too gradual). I recently experienced both these problems tuning a 'too big' carb for my engine. At first if I tried to 'wing' the throttle from idle in neutral there was an ear-splitting bang (no air cleaner) and the engine died immediately. This situation is easy to test so try it first. I did get my carb tuned, once I could drive it I used an AFR meter & it took 0.15 secs off my 1/4 mile time.
Backfiring can be caused by the timing being too far retarded, inlet valve still open. I'm no mechanic, but I think I'm right.

Also Duncan, would that be Santa Pod you're at? Been a loooong time since I was living in the UK and for all I know it's long gone but??

Geoff.

 
Before we go any further, "detonation" is not back-firing through the carb. Detonation is usually caused by too much spark advance for the octane rating of the fuel or a weak mixture or overheating. It sounds 'pinkily pinkily' when you put your foot down.

Back-firing thru the carb could well be a sign that the accelerator pump shot is not big enough (pump jet too small) or coming in too slow (pump cam too gradual). I recently experienced both these problems tuning a 'too big' carb for my engine. At first if I tried to 'wing' the throttle from idle in neutral there was an ear-splitting bang (no air cleaner) and the engine died immediately. This situation is easy to test so try it first. I did get my carb tuned, once I could drive it I used an AFR meter & it took 0.15 secs off my 1/4 mile time.
Backfiring can be caused by the timing being too far retarded, inlet valve still open. I'm no mechanic, but I think I'm right.

Also Duncan, would that be Santa Pod you're at? Been a loooong time since I was living in the UK and for all I know it's long gone but??

Geoff.
Hi Geoff

I used to live 10 miles from "the Pod" & spectated there for years. However I have recently "emigrated" to Yorkshire & my nearest track is York Raceway where I have been running in American Super Stock. I did get down to the Pod for the 'Dragstalgia' meet where a lot of us northerners joined in the southern Nostalgia Super Stock. It was quite funny, all the guys from the north got thru the first round! I also run a BBC comp altered team & we finished 2nd out of 34 entrants in our class - not bad in our first season!

The Pod is going stronger & better than ever - easily the best strip in Europe. Competition there is intense. Right now they have torn up the asphalt to lay a brand new concrete track. We've been trying to raise money to save the York track & get it resurfaced - have just made the minimum target of £100,000.

Dunc (apologies to OP for hijacking thread)

 
Does the FMX require a kickdown rod connecting the carburetor throttle shaft to the transmission? I can't remember...

If not, free bump for you...

 
Well I feel stupid! I went back and checked the firing order and I had mixed up the wires on one side of the motor. I fixed that now and have not gotten the backfire (thanks for the correction) at startup, but I still get it when I floor it while driving. The car seems like its going to die when I floor it and then I get the backfire. Could this be because I do not have the vacuum advance hooked up to any port? It has too much mechanical advance, so I just did not hook it up to any port and capped the ports in the carb.

 
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