The Stovebolt Site Forums The Shop Area Engine Shop 55 235 runs terrible at high rpm

I wonder if anyone has an idea what's going on with my truck. It starts and runs fine below about 1500 rpm.(just a guess because I don't have a tach) As soon as I try to run it any faster it starts spitting, sputtering, and popping. I've been driving it for about 2 years, and the problem just started recently. I've changed the points, condenser, plugs, and fuel filter, but have seen no change. It doesn't seem to be affected by load or road speed, only rpm. It runs fine from idle all the way to wide open throttle, as long as it's at low rpm. At higher rpm it doesn't matter where the throttle is, it just sputters, pops, and runs like hammered crap. It's a '55 235 converted to 12V in a '49 3800. Any ideas?

Sounds like an ignition problem. I would suspect points first off, but it could also be timing, condenser, and vacuum advance. Does the distributor rotate when you goose it?

Kurt

I'm thinking it's got to be ignition as well. The vacuum advance does not work(no movement of distributor when I goose it), but it hasn't worked for as long as I've owned the truck. The points and condenser are new, although the spring on the points does seem a little soft. Hmmmm...maybe the points are floating at high rpm. By the way, I watched your video, groovy 'Burb man.

I would guess the carb needs a rebuild,it sounds like the high speed jet is clogged.

Pete

i agree with pete52

I rebuilt it 2 years ago, but I guess that doesn't guarantee anything with the corn gas we have now. It'd be easy enough to pop the top plate off and have a look. I'll do that the first chance I get.

More likely a weak or broken valve spring if it runs OK at lower RPM's.

if the vacuum advance doesn't work there's no use in trying to trouble shoot any further until that's fixed

Bill

I have a 55 235 in my 58 and mine was doing the same thing but worse on hills. I ordered a new vacuum advance and installed it . Runs like a brand new engine with lots of pep and could tell a world of difference in pickup.I ordered my advance unit from Obsolete Chevrolet. I would pull a vacuum test on the advance unit first before buying one.

There's also an extra spring for the points that reinforces and makes it stiffer. I just put in a new set where the spring seemed very soft. Then I found the extra, or reinforcment spring in the package. It goes right alongside the one that's already installed on the points. Maybe you left that out?

Sounds like it also may be leaning out when fuel is most needed under load.....I'm guessing contamination within the carb may be a culprit??!!!!

I agree on the fuel leaning out under a load.

Joe Flanagan, I think you might be on to something with the "extra spring" thing. I didn't know what it was, and I threw it away. As soon as I have the time I'll go get another set of points so I can install the extra spring. I'll just keep the new set of points for next time and reuse the spring then. I had never seen an extra spring with a set of points before...strange. The more I think about it, the more sure I am that the points floating at high rpm, due to the weak spring has got to be the culprit. I'll let y'all know how it turns out, when I get the time to try it.

I believe you will find that was not an "extra" spring but the only spring.
Look carefully at the one that doesn't come off and you will find that it is a copper strip with almost no spring ability at all.It carries the current instead of the spring which is usually installed beneath the conductor strip.
Also be aware that the spring can sometimes be installed improperly and not load the contact arm.I had to correct condition that for a man one time.
When the spring is properly installed it loads the contact arm against the breaker cam.
But am I not reading correctly when I think you said the condition existed before the points were changed?

Is it "popping" through the carb? Or backfiring through the exhaust? If it is then i would check out the cam and springs like Dave said, It will backfire through both of them if springs are broken or bad,weak. The engine will run ok low RPM but when it try's to climb higher on RPM the springs cant handle it.Now this is only if you have done all what the other guys have told you to do.

Well, I finally had time to work on my truck today. I went to NAPA and got a new set of points that were normal(without the stinkin seperate spring). I came back to the shack and put them in. That was all it was, the old truck's running like a song again. Thanks for the advice and moral support, everybody.

I like to thank joe flanagan for his comment on leaving out the extra spring on the points. Your comment was my eureka moment. I too left that spring out (not being too familiar with points) . It solved my high speed miss (1500+ rpm), & I've been chasing that problem for awhile. Thanks buddy