The Stovebolt Site Forums The Shop Area Engine Shop Dead spot at 1/3 throttle

Hey all, been driving my 56 for the last couple days and noticed something. The engine runs great at light throttle or heavy throttle, but if I am accelerating somewhat quickly it kind of bogs for a second and then takes off. Seems to be the same spot in the RPM range no matter what gear. It feels like a flat spot in the torque curve.

Engine(261) has had a fresh valve job, a rebuilt B series carb, new plugs with proper gaps, points have been checked. I have adjusted the valves a couple times, but not since I started driving her.

Any ideas?

When the carb was rebuilt was the float level set correctly? Check the carb for a good squirt of gas when you move the linkage, look down the throat of the carb. check the vacum advance to see if it moves when you rev the engine, or disconnect the vacum line and suck on it to see if the vacum unit moves.

Sounds like the accelerator pump is weak or not working.

Joe

Seems to me that if the float level were wrong it would quickly run short of fuel at full throttle if you held it open for a little bit i.e., a hard run. Or are you referring to a short burst of WOT?

I have triple checked the float levels.

Accelerator pump is brand new. Vacuum advance seems to move correctly.

So as long as the throttle is not moving, it runs OK? Light throttle, full throttle but not moving throttle? Is it the right carb for the engine, size wise?

I know you said the pump was new, but have you checked it lately? Does it dump fuel down the carb when you open the throttle?

If you carefully drive through the rpm range, does the engine run OK?

You need to determine if it's a true rpm related problem or it just happens to be you notice at the same rpm.

It would be pretty strange to have it act up at a certain rpm and still run ok on either side of it. Just for kicks, check the looseness of the distributor shaft, if the upper bushing is worn, it could cause some troubles.

You should also check the motor mounts, make sure the engine is not torquing over and pulling wires loose or pulling the choke closed. Stranger things have happened.

Joe

how about the gasketn the bottom of the carb, is the vacum port open

Thanks Joe ill check on those things. I drove her about 40 miles today, I also noticed going up a hill that where it tends to bog it backfired. I applied a little more throttle and she took off!

If I'm gentle on the throttle it runs fine from idle to a certain rpm (no tach, so I don't know exactly where in the rpm range) then for about 500 rpm it feels flat and once it moves past that it gets power back.

Tommy I'm 90% certain the hole in the carb spacer is open, but I'll double check that.

I would check timing. Sounds like your engine is it of time a little.

Mine ran like crap for a couple of years till I replaced my modified head with a stock one! I used dad's flow bench and ported the exhaust side and added bigger valves. On the intake side, 250 ci, I added epoxy to raise the floor and get a better short turn radius into the port. I gained a decent amount of air flow doing this with out increasing the port size, I actually decreased the port size and figured the air speed would increase which helps low to mid lift power. On paper it looked really good, about 15% air flow increase.

Driving was a different story, it ran noticeably better at full throttle and off idle. It ran terrible at pulling hills, and like yours ran good up to a point then start to cut out, it even broke a motor mount it was running so bad. All I had to do was stay in the throttle (keep pushing down) or let up. I tried everything I could think, changed cams, distributors ( points to HEI and back several times), intakes (one carb to three and back) harmonic balancers, torque converter, even a rear axle I thought was bent.

Turned out the head was causing it. The exhaust would flow close to 90% of what the intake was, normally you would want 75 to 80%. This high exhaust flow was sucking the fuel mix right out the intake during valve overlap causing a extreme lean condition. At full throttle the air speed was enough to over come it. Those long slight hills were the worst, sometimes I was down from 65 to 40 by the time I made it to the top.

That head is now on the rack collecting dust.

My point of all this is to keep working at it. Lean conditions do strange things. Yours sounds a lot like mine did.

Go back into the carb and make sure the fuel enrichment circuit of working right. You might swap in a different spring if it uses one or a different metering rod. It should get worse if you go the wrong way.

Joe

Finally got a timing light and checked it. Someone had the octane selector maxed out and the timing way off. Zeroed it out and set the timing to then BB and she runs like a hot rod now. No hesitations at all!

I did notice that the arm on the distributor hold down that connects the vAC advance has a bit of play in it that changes the timing a couple degrees is that normal?