The Stovebolt Site Forums The Shop Area Engine Shop My 235's got some idling issues.

Good evening fellow bolters. I've got kind of a strange situation with the engine in my 6400. I didn't figure it'd matter where this post went since it was an engine problem. Anyway, here's the reader's digest of the scenario: Spark plugs and wires were ancient, and the fuel pump was barely spitting fuel into the fuel filter before the carburetor. However, the truck ran like a top and idled great. Being pro-active, I changed the fuel pump, hoping to get a little more pressure since I thought the old one was worn out. I also put new spark plugs and wires on and gapped the spark plugs at .035. I installed all these new goodies and now, for some reason, the truck idles so rough that it'll barely run without the choke. It seems to smooth out when the RPM's are up, but it'll barely idle. I'm stumped. The fuel pressure doesn't seem to have changed any. I re-checked the plug gap and re-traced the spark plug wires twice to make sure they were in the correct location. We set the timing before we started fiddling around with any of it, and before I started changing plugs and the fuel pump it ran fine. Sorry the post is so long, but I wanted to squeeze in all the gory details. Any ideas where I might start troubleshooting? Any thoughts would be dearly appreciated, because my bolt is starting to frustrate me. Thank you!
P.S. I'm 99% sure my motor's a 235, because it's in a 6400 series truck and the vin plate says it's rated at 103 horsepower. However, I cross referenced the spark plugs for both the 216 and 235 and they call for the same plug. The old plugs I pulled out were also gapped at .035

You might need to replace your points, condenser, rotor and distributer cap.

I thought about adjusting the points but I've never done it before. I'd like to replace the rotor and cap but I've gotta order them. The condenser did cross my mind though. My father's got a 57 Pontiac Star Chief that started running erratically and it had a completely rebuilt engine, new fuel pump, etc. After we screwed around with it for about 6 hours we figured out it had a bad condenser. Thanks for the input, Doug.

take the line off of the fuel pump coming from the tank and blow air thru it into the tank, then put a new filter on and clean the bowl in the carb. the new pump has awoken the dirt devils in your fuel tank.
same thing happened to mine, had to clean out the tank and replace the rubber hoses under the cab.

Jake Schaeffer,

I agree with Doug...when you timed the engine, did you use a timing light, remove and plug the vacuum line, and what mark did you use for timing. This could have a bearing on the idle.

Were the new wires solid copper, or solid wires?

Stuart

We pulled the vacuum line off the vacuum advance diaphragm mounted on the distributor shaft and plugged it. After some searching, we found the mark on the flywheel that was supposed to line up with the arrow on the flywheel housing and timed it off of that. The shop manual we had said to time it at 0 degrees, which seemed to help before this fiasco. I'm not entirely sure what kind of wires the new ones are, I didn't notice. I can go dig the box out of the trash though, I've still got it. It wouldn't suprise me if there's gunk in the carburetor. I put an in line filter in before and after the fuel pump, but even after we pulled the fuel tank out and cleaned it, there was still crud in the fuel filters. I'll have to try and blow the lines out. I may have gotten dirt in them when I replaced the pump.

I had a rough idle problem with my 235 some years back after I had done a complete tune up and rebuilt the carburetor.
After much examination and elimination we found a split in the flare of the steel vaccum line. I replaced the line and solved the problem.
The idle was not excessively rough and this might not be the source of your problem, but do check that line.
Bill

if the above fails i would check your float level to be sure the NEW fuel pump isnt pushin just a bit TOO MUCH fuel past the needle an seat in the carb ?
Just a thought ,take care
Tim

Thanks again for all the feedback, everyone. I kinda ran out of ideas. I'll have to tear into it and see what I can find.

If it runs correctly at road speed, the problem is likely a partial blockage of the idle jet in the carburetor. There is a quick fix you can try which might prevent haviing to do a rebuild on the carburetor. When the failure to idle showed up on my dads old 1936 truck, he would crosss the two middle sparkplug wires. Then he'd start the engine, rev it up a time or two to make it backfire through the carburetor. The backfiring would clean the idle jet. Saw him do it a number of times.
meansam

Sam,
Thats a good way to ruin a rod bearing or wrist pin.

It seems to me that the only difference between this planned backfire and a regular firing would be when it happens...at the wrong time with the valves open! This would actually create less cylinder pressure as it's blowing out the carburetor.

I have done this on numerous occasions with no horrible consequences.

Stuart

Interesting development: Checked my fuel pressure last night, new fuel pump is pushing about one and a half psi. Fuel lines are all clean, and I installed new fuel filters. Put a different new rochester carburetor on there I had sitting in a box in my garage. Set the timing as far advanced as it would go, it still seems like it wants more advance. It idles now but not very smooth. The engine seems to smooth out on acceleration. The plugs might be a little cold for the motor. I thought about putting one or two stages hotter in it just to see what happens. Stuart, if I'm not mistaken the new wires are solid wire, not copper.

The octane selector is not the only way to advance the timing; it is only for fine tuning. The timing can be avanced further by loosening the distributor clamp and advancing the entire distributor. I typically set the octane selector to null and set the distributor to the book, then adjust the octane selector to optimum running.

Be careful not to lift the distributor while turning it; you don't want to pull the distributor shaft tang out of the oil pump and have no oil pressure. Learned that over fifty years ago.

Well, I learned something new. Thank you Baldy. I suppose I should have done a better job cleaning up the area around the distributor because I assumed that the distributor clamp was permanently mounted to the distributor. I didn't know it had a locking collar on it. As soon as it's daylight again, I'll have to go out and play with it a little. Any thoughts on what timing should be set at? I've heard that you have to advance the engines like crazy to run off of today's gasoline. I know when I timed my Chevelle I set it at 10 degrees initial advance. Is that a good ballpark for a 235? P.S. Thanks for the advice about the oil pressure, the 235's worn out enough. I'd hate to make it worse...lol