VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc 1956 235 Chevrolet Inconsistent miss

I have a 1956 235 hydraulic lifter engine in my 37 Chevrolet. 2000 miles on rebuild with 2 webber carburetors, fendon headers with dual exhaust, mini hei distributor, new plugs and plug wires. Engine has an inconsistent miss whether at idle or holding rpm's above idle. I've heard a lot of 6 cylinders that have this "flubber" type sound at the tail pipe. Engine runs great under a load and has great power. Trying to figure out if I can eliminate this or if it's just part of a 6 cylinder. Attached is a short Youtube vid with the sound. Anyone with this experience please chime in
[yt]https://www.youtube.com/shorts/krAyntcQBNo[/yt]

Timing may be advanced too far or dual carbuetors not sycronizes,

Not really sure if you even have a problem but there's nothing wrong with seeking perfection. I know on my dual carb setup, I was really able to tune things in better with O2 sensors at each exhaust bank. And with the flow meter bubble thingy that you put on top of (or beside) each carb, that too exposed some tune deficiencies.

I also got a better valve adjustment after really heating up the engine i.e., drive it around a bit before adjusting everything. Get the entire engine (top and bottom) up to temp... then adjust.

You've got a few miles on it now. Check your plugs for matching color. That might tell you if you're rich or lean or if something else is off.

Maybe take this time to go through and re-torque the top end. It'll need to be done eventually anyway.

Bottom line, if it runs good and everything is doing its job... now you're just piddling.

Good point from C-nut about timing. Try setting your timing with a vacuum gauge if you haven't already. I've had good luck with that technique.

Skidplate,
I am not familiar with the technique of setting your timing using a vacuum gage. Please explain, I would like to try it.

I haven't use a timing light or any kind of gauge in years. I adjust to the fastest/smoothest idle then use the octane selector to fine tune if necessary. I'm assuming setting it to vacuum would be set to the highest vacuum.

There are a few good YouTube videos on how to do it. Really simple. Connect your gauge to your intake manifold vacuum (I use the vacuum that feeds the distributor). The logic is, the higher the vacuum, the more air is being sucked through the engine and therefore the closer to peak performance you will attain. I can't remember the exact explanation but that's the gist of it. For example, I'm at 5000' elevation so standard timing marks may or may not be the best timing for my LOW oxygen/fuel ratio environment. It would seem that VACUUM doesn't care. High is good. Low is bad.

That along with the Flow Gauge. Both kinda work together. (This is only necessary when you have multiple carbs. Ya don't need the Flow Gauge when there's only one) Adjust for vacuum, then match up the carbs with the Flow Gauge. Back and forth until everything peaks and matches. Kinda cool when you get it all "Right".

NOTE: And since I'm at said "high elevation" I installed Variable Jets and O2 sensors at the exhaust. With those, I've been able to tweak my fuel mixture to better match to modern air/fuel readings 12-14 to 1 ratio. That too has helped a lot with dual carb tuning. The O2 gauge will help with your AT SPEED tweaking also. It's not for everyone but it DOES help.

Timing may be slightly off but I think more likely carburettor balance, I have problem getting dual carbs even by ear.
Tony

FWI the OP hasn't been back on site since he started the thread.