The Stovebolt Site Forums The Shop Area Engine Shop Engine misses while cold.

Hey all,

Just started having a problem 2 months ago where my 235 engine misses and sometimes back-fires when I step on the gas. When I'm cruising, the engine doesn't miss. It's only when the engine is cold, and when I'm stepping on the gas that it happens. I replaced the fuel filters and even rebuilt the carburetor to no avail. I checked the spark plugs, coil, distributor, and verified the valve lash is correct. I was a bit concerned that the problem might be a failed head gasket (blow-by between cylinders). I torqued down the head bolts and that didn't help. But I'm thinking it's not the head gasket because the problem goes away when the engine is warm. Next I was thinking about retarding the timing to see if it might be a timing issue. Does anyone have any suggestions? Has anyone had a similar problem? It seems to have gotten worse in recent weeks.

Hy Chenzo, have you checked the torque on the intake/exhaust manifold bolts, and the carb mounting bolts?

Take the air cleaner off and look down the carb while you open the throttle. The accelerator pump should squirt a stream of gas like a water pistol. If your shooter is bad, it will stumble badly when it is cold. I'm not questioning your ability to rebuild the carb, but things can go wrong.

The accelerator pump squirts out gas just fine. The problem doesn't happen when I stomp on the gas pedal. It happens when I'm holding the gas pedal down to accelerate (i.e. after the accelerator pump is done doing it's job). Thanks for the suggestion though.

By the way, this problem started before I rebuilt the carburetor and continues after the rebuild. The carb needed the rebuild regardless (was leaking gasoline slowly at the seams), which is why I ventured down that path in the first place. Unfortunately it didn't resolve the problem.

I'll check the torque on the intake/exhaust manifold bolts. I'd be surprised if that were it though. If there is an intake manifold leak, the problem would be worse at idle (when manifold vacuum is highest), not when the throttle is open. The engine idles very nice. It's when I hold the throttle open that the stalls and backfires occur.

To add to the problem description and some of the stuff I've tried:

The misses, stalls and backfires tend to happen more when under load (while accelerating with the pedal down, even part way down, climbing a hill, etc.) I don't have the problem when driving a constant speed on a flat road (regardless of how fast, and how far down I have the pedal pushed).

When the misses/stalls do occur, it can be pretty dramatic. It feels like the engine dies for just a moment (more than one cylinder misses in a row) and it feels like the truck is going to stall completely. But after another moment, things kick back in (may get a backfire or two at this point) and the engine runs fine again. This can happen many times while accelerating. Once I start cruising, even if it's a high rpm cruise with the throttle opened up, I no longer have the problem.

The problem slowly diminishes after the engine is warmed up (~20 to 30 minutes of driving). Once warm, I have zero symptoms and can drive the truck all day at any speed and at any acceleration without any hesitations or misses.

Originally I thought the carb was getting starved of fuel, but after swapping out the fuel filters and making sure the fuel lines are clear, the problem persists. I checked the carb float level when I rebuilt it. When the problem happens, there doesn't appear to be a shortage of fuel. The problem is just as bad (and just as frequent) with the choke closed completely (running rich) as it is with the choke open. I think I've exhausted all the possible problems with the carb.

I think I need to snag me a vacuum gauge and test cylinder compression. I hope it's not a blown head gasket, but I'm running out of options. I also want to check exhaust back pressure. The muffler on the truck is pretty nastily rusted and I wonder if it's clogged with rust. Still, neither explain why the problem only happens when it's cold...

It just doesn't sound like a head gasket. I doubt that a compression check would tell you much. I would strongly suspect ignition if you have eliminated the shooter in the carb. Check for burned points, a bad condenser or you could have a broken spring in your centrifugal advance. Coils usually get worse with heat, but one can't take anything for granted.

Hy Chenzo, could it be that ice is forming around the carb base restricting flow until the engine warms up? Is your heat riser/spring working correctly?

Is your vacuum advance working?

Kurt

I did check the top end if the distributor looks good (points, button, etc.) but I do need to verify the centrifugal advance is working. It would be hard to narrow the problem down to a coil problem without just flat out replacing it. The resistance on the coil is fine, so if there is another coil problem, it's not obvious.

Thanks for the suggestions.

I doubt it's an ice issue. It's already in the 70's here in Tucson and it's been very dry (less than 10% humidity). My 1955 1/2 doesn't have a heat riser, so there is nothing to check in that department.

If it wasn't I should expect to have overly retarded timing, right? Overly retarded timing wouldn't give me a problem with backfires, it would rob my engine of power, so I doubt it's a vacuum advance issue. I'll check it though.

There is a good chance that you are advanced too much to perform cold. The centrifugal advance advances your timing as rpm increases. The vacuum chamber allows it to advance under light load load conditions and retards it when the vacuum drops. It may not be dong this. One way or another, a backfiring engine when it is cold is usually an indication of too much advance. The only way to really check a coil is to try a different one. But I really don't suspect the coil.

So the problem appears to have fixed itself. I was playing with timing, advancing and retarding it to the point of being extreme, and and the truck let out a nasty little backfire and blew a rather large hole in the muffler (there were little peices of muffler laying on the garage floor). As it turns out, the problem completely went away with the giant gash in the muffler. I re-tweaked the timing and took a peek inside the muffler to find layers of rust flakes inside it. I'm sure it was badly clogged. Looks like I was having a back-pressure problem all this time. At least I can drive it now without the stuttering, even if it is a bit on the loud side.

Now it's a "hot-rod".