The Stovebolt Site Forums The Shop Area Engine Shop Engine won't idle

My truck has suddenly developed a problem where it won't idle. It runs as long as I give it gas but the second I let up on the accelerator, it dies. A quick look under the hood shows that the heat riser spring has fallen off. The valve was in the "off" or warmed-up position. Could this possibly cause the problem I'm having? Float bowl in the carb is full, gas seems to be getting where it's supposed to go, all fuel line connections are tight. I know this could be caused by a number of things but I'd just like to know if the missing heat riser spring could be one of them. FWIW, the stalling happened after the truck had been sitting for a while, so I was running a cold engine, with the riser valve in the warmed-up position.

First thing that comes to mind is a vacuum leak, how about the choke and fast idle/hand throttle-did you make any changes to them? If the engine is still warming up and you have some choke assist, the heat riser should still be closed to some degree. As a comparison, my '57 283 V8 2 bbl. in the 2 ton truck did not have a heat riser out of the factory, choke and hand throttle needed until it's warmed up enough to idle and drive.

What happens if you nurse the RPM long enough to get up to normal coolant temp? It sounds like you've got piece of debris somewhere in the idle circuit. I've cured a problem like that by revving the engine up and putting my palm over the air horn to draw liquid gas through all the carb passages- - - -sort of a power flush with liquid gas instead of vapor. Let the engine catch up to avoid a flooded stall by clearing the airway and opening the throttle a little more. Repeat as required.
Jerry

Have not run it long enough to answer this question. It was kind of an emergency to get it home since it was cutting out at every stop sign and red light. I will try the power flush thing as soon as I get a chance. Thanks.
Did not make any changes to choke. Throttle is non-operative on my truck, ie, not connected.

"Throttle is non-operative on my truck, ie, not connected."

Better think that one over again.

The knob on the dashboard with the "T" on it. That's what I thought he meant.

Throttle- - - - -carburetor linkage- - - - -gas pedal- - - -"foot feed"- - - -etc. Whatever you choose to use to make the engine run fast enough not to stall.
Jerry

Pull a spark plug to look for black soot.
Fix your throttle cable. Mine has got me home more than once when it started acting up from a hole which showed up in my accelerator pump
on my YF2100 (ethanol related).
What carburetor are you running? What engine?

joeflanigan Oldest chevy 6 trick there is: cross your 2 middle plg wires start and run engine uncross wires if it fixes it clean the float bowl of chunks that can be sucked-up in the main jet .

What carburetor are you running? What engine?

Rochester model B, 235 engine

Please explain the science behind this procedure.

There is none. The engine will run on the other 4 cylinders, but firing the spark plug at the top of the exhaust stroke on two cylinders accomplishes exactly nothing.
Jerry

I was trying to do the math in my head on this and keep coming up with the same answer as Jerry.

Both intake and exhaust valves would be open, so any fuel/air mix remaining in the cylinder may light off and cause a pop out both valves.

There is virtually no overlap on a stock stovebolt cam. Watch the rocker arms sometime on a valve adjustment. The exhaust valve closes before the intake rocker even begins to move. That's the main advantage of the 261/Corvette cam- - - - -much longer exhaust duration for better cylinder scavenging.
Jerry

Sorry, I just assumed the Chevy would have a cam similar to my GMC 228.

Topic seems to be getting off track. To the OP...

Might be time for a carb clean/rebuild.

Our truck is not used much - only about 500 miles per year. The bowl is often empty due to evaporation. So, I expect to have to clean the carb every couple of years. Getting ready for Spring, I just disassembled the carb this afternoon and the parts are now sitting in solvent. Engine was running lean last time I ran it.

Rough idle and idle mixture screw not doing anything when you turn it is a clue that carb needs cleaning. (Plugged up passages as Jerry suggested)

I tried Hotrod Lincoln's suggestion of clamping my hand down over the carb while the engine was running at 1000 rpms. Did this several times with no effect. Getting the engine temp up doesn't have any effect either. It just will die once I let my foot up off the gas. I currently have the carb apart. I studied the idle circuit in the manual and shot both compressed air and carb cleaner through the passages and they are clear. I did notice that the last time I rebuilt the carb, I had forgotten to put the spring into the discharge pump (the thing with the ball and the T shaped thing that guides a tiny spring). Everything else looks OK. I checked the manifold bolts and found most of them to be a bit loose. Not finger loose by any stretch but nowhere near tight, either. I also discovered that the terminal in the ignition coil had a good amount of corrosion. Another issue I have had for quite some time is an occasional cough out the carburetor on acceleration, which I created a thread on a while back. I never did solve this. Valves have been adjusted and operate freely. I know that a lean condition can also cause this. Anyway, I am soaking all the carb parts and will be rebuilding it with a new kit. I'm aware of the ethanol issue. I do use Stabil in my gasoline, if that makes any difference. Thanks for the responses. I will report back with how things work out.

Have you checked your ignition timing?

Another easy thing to check….see if your vacuum advance is moving as it should when you blip the throttle. Make sure it is turning the distributor and allowing it to return to position.

Another possibility is point gap if you're running a conventional ignition system. As the rubbing block wears, the point gap closes up, which retards the timing. At some point, the timing gets late enough that the engine won't idle. Raising the RPM gets the centrifugal advance into play and helps the timing advance enough to run.

So I decided to get a rebuilt carburetor. I got it from National Carburetors in Jacksonville, FL, which does only carburetors. I also got to watch them test it on an engine and it ran well. I didn't want to do this. My original, which I rebuilt 11 years ago, never gave me any trouble until recently and there were so many unknowns with it that I thought it would be wise to start from scratch. I installed the new carb but there's still a problem. It stalls at idle, runs rough, accelerates rough, etc. Its running really rich and the spark plugs are fouled black, which was the case with the old carburetor, too. It runs best when the idle mixture screw is turned all the way in and seated, which obviously it should not and which tells me it's getting WAY too much fuel. i looked down the throat of the carb with the engine running to see if there was gas dribbling down the throat and it is not. So both old and new carburetors produce the same effect which tells me I need to look . . . where? The fuel pump? Could it be delivering too much pressure?

Did you try any of the ignition system suggestions? Can’t completely rule out a fuel issue, but I’d sure start with ignition and then come back to fuel if need be.

Getting fixated on a single issue usually gets frustrating and expensive. At least try checking out the ignition system.
Jerry

Because I had a new ignition coil handy and it was easy to try, i put it in but there was no change. I did the same thing with a new ballast resistor I had handy. Again, no change. I checked the points and they were at .015. I reset them to the recommended .019 I have not checked the condenser.
I just opened up the new carb and checked the idle circuit. The needle and seat were clean, but the seat itself was fairly loose in its threads. I tightened that up but there is no change. Blew air through the carb's passages to clear any obstructions. No change in the way it runs. I have black soot at the tailpipe and also on the plugs.

Switched out the condenser with a spare I had and the result is the same. I cleaned the plugs and ran it again. They are completely black after a very short time. I suppose the options at this point are it either is getting too much fuel, or it can't efficiently burn the fuel it is getting.

Did you check the timing or at least play with it? My 235 seems to be quite responsive to small changes in timing.

Also check the function of your vac advance.

Have not done those things yet. Will do them next.

So basically you installed a rebuilt carburetor which you saw being tested on another engine, and it ran well.

You installed it on your engine, and it runs poorly.

This would seem to indicate that there is an issue with your engine other than the carburetor.

Given that you have tested a different condenser, and a different coil, and readjusted the points (lots of guessing, but some suggested here); I would suggest some testing, but not with new parts.

(1) compression test
(2) a fuel pressure test
(3) at least checking (and probably readjusting) the timing.
(4) a firing voltage test would also be interesting

Dr. Werner Von Braun allegedly stated that one test is worth 1000 expert opinions. I happen to believe he knew what he was talking about.

Jon

Soot in the tailpipe doesn't mean what it used to. Modern fuel always leaves soot in the tailpipe of any vehicle - old and new. It isn't an indication of rich running anymore.

There must be something wrong with my daily drivers, then- - - - -the tailpipes on both of them are clean, and almost white.

Joe: I'm sorry to hear that you wasted money on a rebuilt carb.

Actually, my old one had issues. There was a lot of play in the body around the throttle shaft and the hole where the fuel inlet was so loose it was getting harder to get anything to screw in there without it leaking. Plus it used to have an automatic choke (the mounting is visible on the side of the carb) but someone converted it to a manual choke. I got to thinking I know nothing about this carb. This truck was so jerry rigged when I got it 11 years ago. Hell, someone used a lamp cord to wire the tail lights. I know the things I mentioned can be fixed but my thinking was to start off with something that is known to be good. I know it's frowned on here but that was my thinking. The choke on the old one stopped working completely and this one does work, so there's that. I was getting a new one regardless.
Anyway, since my last post, I checked the timing and adjusted it so that it is a few degrees BTC, which is where I've been told it should be and where I've always kept it. It runs better, but it's rough, seems to speed up and slow down on its own, and spews out a lot of rich smoke. There's a huge black spot on the pavement under my tail pipe. Adjusted the idle mixture and speed, but there's not a whole lot of improvement. I didn't know where to go next on the ignition front without further advice, so I tested the fuel pump. I did Hotrod Lincoln's suggested test. It pumps just shy of a pint of gasoline in 30 seconds of running. On the pressure test, it registered about 4.5. But when I shut the engine off, the needle never went back to zero as it is apparently supposed to according to HRL's description of the test. I waited several minutes. The needle didn't go back to zero until I disconnected the hose. So i thought I'd mention that, anyway. I had to call it quits for the day so did not get to the vacuum part of the test. Compression test and firing voltage test will be next.

I had a similar situation that turned out to be dirty ignition switch contacts. Try bypassing it and install a jumper wire from hot terminal on the starter to the ballast resistor

Thank you

It's beginning to sound like you've got internal engine problems. Next on the agenda needs to be a compression test, or a cylinder leakdown check. There's also a very good test that Sun used to recommend called a "power balance test". The engine is run at a fast idle on only two cylinders, with a steady throttle set by the curb idle screw. The RPM and manifold vacuum is recorded when running on each pair of companion cylinders. Low powered cylinders are much easier to spot using that procedure, compared to a compression test.

Also, have you done a valve adjustment, and checked for flat camshaft lobes?
Jerry

OK, a compression test will be next. I can also do a leakdown test if you think one is better than the other. Have never done either but have researched it and think I can probably figure it out. Do you think that should be done before a voltage firing test? I have not touched the valves during this episode. Checking my records just now, I see that they are due for adjustment now. I have not checked for flat cam lobes and would need advice on how that's done. The engine has fewer than 25,000 miles on it. Could the camshaft be damaged? Also, what did you make of the fuel pump reading 4.5 pressure but not going back to zero when the engine is shut off? Does that indicate a problem?

I think that just signifies a good needle and seat in the carb and a good check valve in the fuel pump.

Listen to Jerry and follow his advice. But if you want something quick and easy to do while you are sourcing the compression tester or leak down tester….can try pulling one plug wire at a time. Be careful….easy to get zapped.

Each wire you pull SHOULD make the engine run worse by roughly the same percentage. If you pull a wire and it produces no change at all…that cylinder either isn’t firing or has a bad misfire. Might help you narrow down and eliminate simple things like fouled plugs, bad spark wires, or valve issues (bad adjustment, bent pushrod, etc).

My well used and somewhat smoky 235 pulls down about 50 rpm on every cylinder once a wire is pulled. It’s probably half worn out, but it’s at least worn out evenly.

Did you ever check the vac advance?

Edit: On an engine that’s running badly already, it might be tough to interpret changes. But it’s a quick and easy test nonetheless. I’ve also seen a safer variation where you probe the spark plug boot with a grounded test light, rather than pulling the wire.

The vacuum advance will not hold a vacuum. It won't even build up a small amount temporarily. I can hear the diaphragm moving but it holds zero pressure. I tested it with one of those gun-type vacuum testers with a gauge on top.
Getting ready to do a compression test now. The instructions say to warm the engine to normal operating temp before doing the test but I doubt I can keep the thing running that long, so not sure what to do there.

Hot or cold, specific compression pressures don't matter. You're looking for a percentage difference from one cylinder to another. A valve adjustment needs to happen first, as a tight valve will give a false compression reading for a specific cylinder. A flat cam lobe should be pretty obvious if it's severe- - - -watch the rocker arms and/or valves for a difference in the distance they move up and down. It doesn't take much cam lobe wear to cause a visible difference in valve lift, especially since a stovebolt rocker arm moves the valve something like 1.75 times the cam lift distance. A worn exhaust lobe can affect several cylinders since exhaust gas that doesn't make it out of the tailpipe backs up into the intake the next time the intake valve on that cylinder opens. That messes with manifold vacuum and the mixture other cylinders receive. A burned or too-tight intake valve can take out more than one cylinder also, for the same reason- - - -exhaust gas backup into the intake manifold.

Vacuum advance will only affect light throttle cruise and gas mileage- - - - -not low to medium speed and performance. At any throttle opening other than "feather-footing" the gas at highway speed, there will be little or no vacuum advance.
Jerry