The Stovebolt Site Forums The Shop Area Engine Shop ’56 235 running poorly

So I’ve been troubleshooting this for over a month - ‘56 235 in my ‘41 p/u:
Idles fine at approximately 21” vacuum until around 1200-1300RPM lowers slightly but past 1300 or so RPM the vacuum decreases significantly until nearly zero at WOT and engine begins to run poorly.I tried pouring a very small amount of gas down the carb at wot to detect any change in engine sound or rpm and there was none. Replaced spark plugs and cleaned them when they began to look black, rebuilt carb, timing seems ok, new points and condenser, new fuel pump and filter.
As of now, I’m focusing on either fuel starvation from carb or restricted exhaust. Also, on the street it runs fine as long as the RPMs stay low. It does seem to lose slightly more power at the higher throttle when at full operating temperature. Haven’t taken truck too far from the house due to the same exact symptoms every outing.
I know this topic has been addressed in older posts but nothing I’ve read has solved the problem or the remedy was never specified in the posts. TIA…

how are you measuring vaccum and are you 100% certain your vacuum advance is working? What is your timing set at? Did you confirm this with a timing light?

Vacuum gauge, albeit from HF, it seems to be correct. Vac advance turns with mild acceleration then goes back to original position when vacuum (on gauge) begins to decrease as throttle increases.
Timing was set to 10°BTDC, if I remember correctly. Yes, with timing light. If timing were off wouldn’t I get backfiring through carb? Everything seems perfect until just before 1500rpm. Above that, it’s bad.

If you have access to a Dwell Meter it might give you some answers.

This is VERY unlikely, but I long-distance trouble shot a similar symptom 6 or 7 years ago that drove me nuts (usually a short trip). I had an old dwell/volt/ohm meter that I was no longer using that I shipped to Kansas City for the guy to use and we had 3 or 4 phone sessions to figure out the problem.

His truck worked well at low engine speeds, but by about 900 RPM the points started floating a little (and dwell was going down. By 1500 RPM the dwell dropped to near zero (points open all the time) and the engine refused to go faster.

Turns out the brand new set of points was missing the stainless steel spring and was depending on just the brass spring to close the points. The brass spring (really there to conduct electricity, not be a real spring) didn't have enough tension to keep the points from floating. The owner had never closely examined a set of points, so he didn't know anything was wrong. Replacing that set of points with another new set solved the problem.

I had the same problem as Bill described above drove me crazy the new points I had came with the stainless spring loose in the box ,thought it was for a different application ,turned out the spring needed to be fitted in as it came loose .same type of symptoms idled nice but on acceleration would run rough unless you feathered the gas .

Lucky me to have ordered an old analog Sears tach/dwell meter. I didn’t use the spring provided, either. Should’ve known… Hopefully, a set of new points WITH spring will solve this problem.

Find out what's restricting the exhaust system. You can confirm an exhaust blockage by loosening or disconnecting the exhaust pipe at the manifold. Low vacuum at higher RPM than idle is almost always caused by a plugged exhaust system. I've seen it frequently, so I'm not just guessing.
Jerry

Use a set of Blue Streak DR-2227P points.

Eureka!! Got a new set of Blue Streak points (spring on this time) and running like a clock. Never occurred to me that it was floating at such a low rpm. I’m sure the mufflers need to be thrown overboard as well. I know they are at least 30 years old. Regardless, I’m as happy as can be tonight. Thanks for good direction!