Engine HATES idling in gear. How should i keep it from loading up?

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1972 Fastback (Factory sprint, but now a bright red Mach 1)
351c 2v(big ol cam, holley dominator intake, roller set, edelbrock 4bbl)

fmx
WHAT DO YOU GUYS THINK? IM ALL EARS.

HERES THE SITUATION

When I turn the key, it starts right up. I keep it at around 1700rpm till it warms up to 160ish. There, it idles fairly well at 850-900. The problem comes in when I put it in gear (reverse especially) The rpm drops down to approximately 350 and I really have to sweet talk it into not loading up and stalling. This includes keeping rpm up and controlling speed with brakes. Typically ive gotta restart it a few times. This is when backing out of places, stop lights, (etc).

Basic rundown of drive train:

351c 2v

mid/low range comp cam (not an RV but not full race) flat tappet

Stock FMX I am fairly confident that it is a stock torque converter too.

Edelbrock 4 barrel carb

Holley street dominator (yes I know its a single plane and I know thats really not good for this scenario. I'm putting on a dual plane this weekend):)

P.S. This engine is not fully broken in, only about 30 miles. I'm trying to get it reliable enough to drive it on the freeway and get the piston rings seated and the compression up.

Perhaps I need a lower stall converter? ALL COMMENTS APPRECIATED THANKS GUYS!!!

 
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You most likely have a vacuum leak. Your symptoms sounds very classic of a vacuum leak. PCV or open port on a carb. As Chris indicated, toss a vacuum gauge on it.

 
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Only 30 miles on it? You mite have a classic manifold leak ...+2 above...Vacume gauge will show if that is happening.

A lower stall will only it worse....Even a low stall torque converter does not lock till 1600-1800 rpm's thou...Dout it is the issue.

 
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1971 Mustang Convertible, factory 302, disc brakes and A/C. Started to rebuild it all original but elected to go full tilt and rebuilt everything in the drivetrain, a/c, cooling, etc. etc. etc.
I had a very similar experience after having my motor built, 302 bored .30, Edelbrock 650 Thunder series carb, Dart Pro 1 aluminum heads, JBA mid length s/s ceramic coated headers, Edelbrock Endurashine air gap RPM dual plane intake manifold, duals with 40 series flowmasters and H pipe, roller rockers and most importantly Comp Cam model 297TH Thumpr camshaft.

Being that my car is an automatic with a/c, pwr brakes and pwr steering it was not pulling enough vacuum to run correctly. It only had 8" of vaccum max, I had to change to a milder cam and went with the XE262H and had no more problems.

BTW I tried a newer larger vaccum cannister, electric idle advance for the a/c and every possible solution in vain. The cam would have been fine for a 4 speed, not so much for my C-4 with the other options and a 2400 stall converter.

:s

 
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across the leading and trailing edge of the intake manifold and the top edge where it sits against the gaskets and heads on the sides. A can of carb cleaner in these areas on an idling engine will make it speed up noticably. Check around any vacuum tree and inspect the vacuum hoses for splits and any tees for cracks also look for uncapped vacuum ports on the carb. The choke will have a slight designed leak in it, at least on a Holley, don't do anything with that as it cools the choke coil and it will burn up without it.

I would specifically look at the vacuum port for "Ported vacuum" as most people now run unported vacuum to the vacuum advance (it is a long and complex conversation, but I agree with using manifold vacuum) one should have a hose to the vacuum advance canister on your distributor and the other should be capped.

Check for a vacuum modulator line to the transmission that may have split, burned dried up fallen off etc. Basivcally if there is a vacuum line, you need to check it.

If your car is or was air conditioned from the factory there is a coffee can looking canister for vacuum to the AC controls a hole in it will cause problems.

The PVC valve's hose and connections can be a huge source of leaks due to the large line size.

You can buy vacuum caps and unhook and cap off any and all suspected leaks.

and if the intake manifold and the heads aren't a proper match it can leak horribly internally and be a bear to find I've heard.

You need to get this straight or that engine will never seat the rings properly.

 

Finmach

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Vacuum leak would be my number one reason too, but check the spark plugs too. Mine 351c had no spark plug cap at one cylinder at all. It reacted similarly, when on N position, engine stayed awake but when on D it barely kept alive. Just something to check.

Vacuum modulator from the transmission might do the same symptons if it's going broke or is broken allready.

 

Bill73Ragtop

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former owner (25 yrs) of 1973 Convertible, 351c, A/C, RA
+1 on the intake manifold leak. Had the same problem a number of years back. Almost impossible to find, but once I removed and reinstalled the intake manifold as a last ditch effort to solve the problem, that is what it turned out to be.

 
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just a Q- since you have a 351c 2V with FMX, same as mine...do you still have a throttle position solenoid installed? I ask because they dont always function properly when they get old. they can stick and cause idle adjustment to be irregular when the idle is set when the thing is open when it should have been retracted. It is electric, sits right on the hot intake, the wires get stiff/crack. all sorts of problems.

 
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351c 2v(big ol cam, holley dominator intake, roller set, edelbrock 4bbl)

fmx
just a Q- since you have a 351c 2V with FMX, same as mine...do you still have a throttle position solenoid installed? I ask because they dont always function properly when they get old. they can stick and cause idle adjustment to be irregular when the idle is set when the thing is open when it should have been retracted. It is electric, sits right on the hot intake, the wires get stiff/crack. all sorts of problems.
I dont believe so, i have electronic choke if thats what your asking. however i have noticed that on the throttle linkage on the carb can stick in a spot that is reasonable and basically closed but when you push down on it, or press on it, the idle goes down about 150-200rpm. Its weird but not crippling.

 
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i am not talking about an electric choke. im talking about the small electric piece mounted to the intake manifold on the drivers side just ahead of the throttle linkage. it adjusts the throttle closed position after you cut the ignition, when functioning properly.

 
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1972 Fastback (Factory sprint, but now a bright red Mach 1)
351c 2v(big ol cam, holley dominator intake, roller set, edelbrock 4bbl)

fmx
Do you know the cam specs?
Yes,

Comp Cam **ADV DURATION.=262 INTAKE, 275 EXH.** DUR. @ .050=218/224**110 SEPARATION**LIFT=.513/.519***

I'm pretty sure that its the cam thats too big.

 
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1972 Fastback (Factory sprint, but now a bright red Mach 1)
351c 2v(big ol cam, holley dominator intake, roller set, edelbrock 4bbl)

fmx
I put on an Edelbrock performer dual plane on yesterday, it has been running much better than the single plane but still shows similar symptoms unless i set the idle to 1200 in park. Only there will it reverse decently.

 
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What carb? I agree with Jeff you should have no problem using a stock torque converter with that cam.

If you are absolutely certain there are no vacuum leaks then I think it is a matter of ignition and carb tuning.

 
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