Hey all, new member here! Got a '78 F100 now with a 351M. I had a similar issue a while back with a '80 Crown Vic. Multiple trips to mechanics on both and could never seem to get this figured out. I am no mechanic by any means but will tinker around here and there where I can. This is our daily farm truck so figured I'd bounce this off you guys before I spend a day opening the carb and whatnot.

The issue is as title suggests with warm starts, but I mean WARM. Like after driving back roads at 55mph for at least 30 mins consistently. Park it and let sit for a bit, go into the store, etc. When you start it back up, it immediately fires, idle sounds slightly lower than normal, but no issues. Now when you put any load on it that requires a little extra power, brake+reverse+steering or brake+reverse to drive+steering, the idle struggles so low it constantly stalls. Especially in tight spaces where you can't really give it gas, requires several starts just to get it out of the parking lot. Then when you do get it going, same problem in drive with the brake pressed at a stop light. Doesn't seem to have enough power, oil pressure lightly flickers, have to feather the brake petal to keep it from stalling. No problems when it's cold or warm, only when it's HOT. The more times it stalls and you have to restart it, the worse it seems to make the problem. Now under the right circumstances sometimes when I fire it, if I very very lightly tap, and I mean tap, the gas when this happens in park, I can get the curb idle to jump and hold steady. But it's rare. It's not just annoying but putting a lot of stress on the starter. A completely uneducated guess makes me think it has something to do with a sticky choke. Has a mechanical choke with the stock 2 barrel carb. Any ideas on where to start?

 

Welcome to FTE

At first I was going to say choke but then you said it works by cable so its not coming on when shut down.
What if you give it a little choke will it run / idle better?

I would say you are having heat soak of the carb.
Motor is run at temp for a bit then shut down while in the store come out and most have issues of a lot of cranking before it starts.
Yours starts but has a run issues I would say both issues are heat soak and that is because of todays gas blend and carbs.
Todays gas is made for EFI not made for carbs.

If the carb has not been rebuilt in a long time it would not hurt to pull it apart for a good cleaning and make sure all is good with it.
I would look into a carb to intake spacer that will not conduct heat to the carb when it is shut off as a start.
It has also been posted to add up to 1 gallon of diesel to a tank (20 gallon) at fill up as this helps change the make up of the gas so it will not have this issue.
I got to tell you I have not done this as I don't have this issue with the 300 six (running EFI exh. manifolds) in my 81 F100 truck.
BTW this in not an issue with just your years trucks but any v8 with a carb, have not heard of many with the 240 / 300 with this.
Dave ----

 

Sounds like your idle is set too low. Could be timing, vacuum leaks ect but I'd start by setting the idle mixture and speed first.

Does it act like it's running rich ?

 

Try taking the cap off the gas tank when it does it. I have seen problems like this...Vac. builds up in gas tank.

 

Very good thought.

 

So I have a 77 351M, and I am experiencing the same thing. Choke is adjusted correctly, and works as should. Fast idle to curb idle operating normally. As the truck is warmed up it idles great, probably a little too high but that’s because when it gets good and hot it exhibits these same symptoms. Truck cranks fine, but doesn’t want to idle intermittently, when hot. Lots of time it will die as I push the clutch in and coast/brake to a stoplight. I can feel the engine die in the brake pedal, but it will do it as well without braking.

The fuel cap wasn’t venting, so that was fixed and it still does it, but not as bad.

I have rebuilt the carb and bushed the throttle shaft to fix that leak. New fuel tank. Only thing I haven’t replaced is the pump but it doesn’t seem to be a fuel getting to the carb issue. When I get onthe truck it responds very well and pulls until it runs out of gear.

I’m beginning to start chasing down carb heat as an issue. The times it’s really done it, the truck dieseled for 2-3 seconds after I got home and killed ignition. I run 91 non-ethanol, but I’m thinking it’s still to blame. I still have the EGR plate on but disconnected, and I believe I’m going to look to trying one of the phenolic spacers instead to try to cure it.

 

If you can get to the fuel line near the Carb. try wrapping a rag with ice cubes around it. It will cool the fuel and tell you if it is a fuel/heat problem.

 

Put a vacuum gauge on it and see what you have. If necessary leave it connected while you drive and check it when its acting up. Make double sure the engine is tuned properly. Since you have a manual choke try choking it a bit to see if it helps. If it does that means you probably aren't getting enough fuel. If it makes it worse doubt its fuel delivery problem. Could be accelerator pump in carb. Also remove the cap on the distributor and make sure the vacuum advance mechanism is moving freely and returning home.

 

This is becoming a problem with all carb'd engines. Motor fuels have changed drastically over the years, the refiners are using fewer and fewer heavy chains in gasoline to boost the volume of product refined. This is making for gasolines that have a much lower vapourization threshold. Throw ethanol in the mix and it becomes even worse. In fuel injected vehicles this is of no consequence, such is not the case for carbed engines.

So there a few ways to rectify the problem one way is to add some heavy chains back to the gasoline to raise it's vaporization threshold, the easiest way to achieve this is to add a gallon of diesel to a tankfull of fuel. This has shown to work nearly all the time with no negative effects, as long as you are not running cats.

Then there is reducing the actual amount of heat absorbed by the intake and inturn the carb. Now, fortunately, the 335 Series engines have dry intakes so this is way more effective than in many other engines.

Disabling the EGR if fitted and if you are able, will help keep the induction system and the carb cooler..

Blocking off the heat crossover in the intake will go a long way in helping to keep the carb and intake cooler With modern fuels, it not really needed unless you are operating at sub-zero (Fahrenheit) temps and even then it will just mean a bit longer warm up.
Finally, you can add a valley pan such as below (yes this one fits the 351M/400) to keep internal engine heat and hot oil form heating the intake.
With these changes/mods you will no longer have vapour lock/vapour flooding issues.

 

If you do a search this topic is a common thread. I am led to believe, as with many others, it a dynamic issue with no one right answer. I decided to start over and check common problems. Verified all my caps on the vacuum octopus were good, as well as tested the lines. All seemed fine. Tightened the carb gasket, wasn’t loose, but let it know I was thinking about it. Then I went out and drove the fire out of it to get it hot. Then let it heat soak 15-20 minutes, and cranked it up to make sure it was good and pissed, and it started idling better, but still rough, gave both sides about another 1/4 turn on the idle screw and it smoothed out. Ran the truck as I normally do, and it seemed to run cooler (on the factory gauge)with a little more fuel mixture at idle( could be placebo, as I know that’s expected) and never missed a lick hot. I think my version of the “truck is hot” and the truck being warm through drive cycles were two different things and the engine needed a little more fuel at operating temp. We will see tomorrow as I’ve been driving it daily in 80 degree Alabama spring. I run 9-10 miles one way in stop and go on the way to work, so the truck is pretty hot when I get there. Usually run to the bank 1/4 mile down the road about 40 minutes after and it will be a good test. That seems to be when it likes to act up.