Hi, I have a 5.8L 351 engine with a 4 barrel carburetor. I have been updating everything on it. Recently replaced plugs, starter, voltage regulator, ignition module, coil pickup in distributor, distributor rotor and cover, It has two problems:

Problem 1:
Engine runs good on warm to hot weather, but when it gets cold, as in the 30's, it begins to idle very very rough and backfires a lot. If left alone it normally just dies. Starting is ok, but the idle is just the pits. Once the coolant warms to normal operating temperature, all returns to normal. It is not the fast idle cam because I actually have it set on high fast idle. On warm weather the engine will idle almost too high because of it. On colder weather it is enough to keep it idling until it warms up. On cold weather as in winter, it just doesn't do it. Even when I press the accelerator to keep the revs up, it still backfires.

Problem 2:
Carburetor throttle valve doesn't fully return to its idle position after acceleration, leaving the engine with a higher than normal idle. I have cleaned the carb, and tightened the throttle springs to see if this would take care of it. Neither has worked. The problem is in the last few fractions of an inch. Pushing the throttle lever with my hand returns it to its normal idle position.

Any assistance would be greatly appreciated.

 

You seem to have a couple of problems with the carb.
Usually when you get a backfire it's due to a lean fuel condition,but could also be due to the way you have the plug wires routed.
Off the top-- did you have this problem before you changed the parts ?
There is the possibility of a vacuum leak that seals itself after the engine gets warm.
Have you been able to check all the vacuum hoses at the carb and also the carb mtg gasket although they rarely gave any trouble.
What you can try is to disconnect the throttle linkage from the carb and see if the butterflys return to the almost closed position. There has to be some opening or the engine will stall. That should help isolate the return to idle problem to the carb or the linkage. If there is a vacuum leak from the PCV hose it will probably stall when returned to idle. After you check those items we can go from there.

 

Thanks for the response. What I know of car repair is self taught and I have to admit I haven't figure out yet the finesse of troubleshooting and actually identifying what is wrong.

Before I go out to the van, I would like to confirm something. You said that a LEAN mixture, or not enough fuel in the mixture, creates backfire. That took me by surprise. Because I thought too rich a mixture does it, I was actually considering leaning it more. Glad to know I was heading in the wrong direction because to adjust the idle screws I have to take the carb out. So to make sure, you are saying I should enrich the idle mixture by turning the idle screws out, right? At this point, they are turned at 2 3/4. Once I know for sure I will go out adjust and look for vacuum leaks.

As to the throttle:
- the funny thing about this is that the throttle will return to its idle position, or being flush with the curb idle screw, when the engine is off. With or without the linkage it works. The problem is when I turn the engine on. That's when the throttle gets stuck open just before it touches the curb idle screw. When I press down on the throttle, it sets in and the engine returns to a slower idle speed.

 

I have to think on this , but is there a solenoid on the side of the carburetor that could be holding the throttle open? If so, it is supposed to be adjusted to give the correct idle speed then drop off when the key is shut off to prevent dieseling. Some set ups used a solenoid to increase the idle speed when the A/C was turned on. You should be able to adjust the idle mixture without removing the carburetor. Since you are way past emission requirements , the better way to adjust the mixture is to turn the screws in slowly one at a time until the engine speed begins to drop then back it out a half turn and leave it there. If the mixture screws are difficult to get to, sometimes you could place a piece of hose (small hole) on the screw and rotate it to make the adjustment.

Getting back to the cold weather problem,,,,,,, is the automatic choke working properly?It sounds like it isn't because it is responsible for cold weather starting and running until the engine warms up and the choke is fully open.

 

Good news. The rough idle and backfire was a result of a manifold leak. I adjusted the bolts to spec yesterday afternoon. I immediately noticed and improvement in power. This morning I started the van early in the morning and it idled without backfiring. Slightly rough the first 30 seconds or so, but significant improvement.

By the way, I have been doing some reading on carbs and read that lean mixtures do cause misfiring. Thanks for the correction.

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Thanks for the tip on setting the idle mixture. I have been baffled about their setting. The Haynes manual on Holley carburetors says the initial setting is 1 1/2 to 2 turns but I talked to a professional who claims these carbs don't work with anything less than 3 turns. Because I can only reach one screw when the carb is installed, I have been unable to do any kind of tuning with a running engine.

The idle mixture screws in the carb are at the bottom under the gas well. I have lots of vacuum hoses right in front of them as well. I can't use a hose to turn them because they are recessed and the typical carb adjustment kit sold in auto part stores won't do it either.

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as to the sticky throttle issue...

Yes, there is a selenoid, but I have yet to see it work, so I have considered it a non functioning part to be replaced some day down the road. When I have checked this sticking throttle issue I did not notice the selenoid being the problem.

 

Technically speaking, if you were able to get the engine to smooth out some with the one mixture screw you were able to adjust, what you could do is; count the number of turns required to bottom out the screw you were able to adjust (turn the screw in gently to bottom it or you could score the taper and the adjustment is a little more difficult to refine) then adjust the other screw to approximately the same amount of turns open. This should get you close to the desired mixture adjustment. Once you do this you won't forget how. Sounds like you are making good progress, stay with it !!