The Stovebolt Site Forums The Shop Area Engine Shop 261 - no power above 1500 RPM

This is a long story about my (1958-1962) 235 engine that no longer runs; so take a seat.

the engine is set up as follows:

Fenton intake and headers

Two Rochester BC carbs, balanced with a Uni-Sync tool

Original series coil, distributor, wires; plugs are three posts which center over the tip of the spark plug.

Dual, 2 inch exhaust pipes, with LOUD mufflers

the lifters are solid and valve lash is adjusted by the book

I set the timing by using a vacuum gauge; I advanced it until the improvement in the gauge stopped and then returned to the high point.

the engine is hard to start and, initially at 1500 - 2000 rpms, does not run smoothly. Once the engine warms up (3-4 minutes) the engine runs smoothly at 1,000 rpms and is immediately responsive to the throttle
I can not get the engine to run under load; when not under load the engine responds immediately . When not under load it idles from 1,000 rpm to 4,000 + rpm immediately and snaps back immediately every time. suction is identical in both carbs.

When i pulled my 235 engine powered 1951 Chevy truck into the garage it would not idle below 1,800 rpm but otherwise accelerated great up through 4,000 rpm. I fixed the high idle problem and installed a used 261 cam and later a new 261 cam.

With both camshafts I could not get the engine to run over 2,000 rpm, every time I shift from 1st to 2nd it's like the fuel is turned off; the engine coughs, sputters and has zero power/acceleration.

What I don't understand is the truck easily [u]ran[/u] above 1,500 rpm even with then broken the rocker shaft and now it doesn't.

the vacuum gauge shows 14 pounds (?) of suction, close to the end of the red stating "advance distributor timing" and just before the green area stating the engine is running correctly. When running, the needle on the vacuum gauge is rock solid, NO movement.

I absolutely assure you the timing mark is in the right place, before installing the fan onto the water pulley I made an adapter to bolt onto the vibration damper so i could rotate the engine slowing and carefully while setting the valve lash. The settings were done with the pistons exactly on TDC.

Part of me wants to remove the carbs and re-set them up again. i hesitate because they run great now and did before i changed camshafts.

The other approach seems to be looking closer at timing and that's probably where i'll start on Friday.

Any and all help appreciated. Thanks, Ted

Things you might check...

Are you getting enough fuel?

Whats the fuel line pressure?

Have you swapped in a new set of points and a condenser?

Loose vacuum line?

Mike B

One too many carburetors!
Jerry

Let me get this straight,

235 engine, 261 camshaft,
dual carbs, dual exhaust,
engine ran before cam change with same set up as now
14 inches of vacuum at idle,
Is that about right?

If the intake and exhaust system was working with 235 camshaft but not with the 261 camshaft, you have to suspect something is wrong with the cam profile or grind, or that the lift and duration is just to much for the 235 engine with a marginal intake system. A bigger camshaft moves the power band up the rpm scale, it also lowers the vacuum, you should have 18-19 inches of vacuum, not 14.

Are you 100% sure the camshaft is timed to the crank correctly? Dots are lined up?

Don't worry to much about the timings actual number, turn it till it starts and runs and keep messing with it to see how it acts. If you find it's way off its mark, yet runs better, suspect the camshaft again.

As a test, block off one carburetor, doesn't matter which one, then get the truck where it will run the full rpm range without any trouble. Swap the block off plate to the other carb and get it running correctly. I have used a simple aluminum gasket cut from a Coke can to block carbs with, then I don't have to mess with the fuel systems. Once happy with the way it runs, go back to both carbs and see how it acts.

You stated, " the truck easily accelerated up to 4000 rpm, but wouldn't idle under 1800 rpm " How did you fix this? Was this acceleration full throttle or normal driving? Sure sounds like a lean condition, not enough air flow through the carburetor to pull the required fuel out.

Joe

What kind of carb heat do you have?

Have you tried it on one carb?

No matter what other things you need to work out with your set-up, read Jerry's post carefully.

My only un-professional slight change/clarification to Jerry's statement: "1/2 or more too much/many carburetors!

I have never set ignition timing like you described, sounds to me like it would be to far advanced. Not saying it's wrong just never done it that way.
2)what cam gear did you use? are you sure the relationship between the mark and the keyway are identical to both cam gears? They may be the same gear for both motors, I don't know, if you have doubts degree the cam.
3)Pull a plug wire and look at the spark, make sure it is strong.

Hello,

It runs great! I took it out yesterday and from stop to over 60 mph it just keeps pulling! I've still got some minor tuning to do but nothing serious. The solution came from two comments, one to check for a strong spark and one to check the points and condenser.

The spark getting to the sparkplug was weak; the points were 40% pitted and the point setting/dwell was out of specifications. I had spark and the truck idled okay and when not under a load the change to higher RPM wasn't a problem. Under load the spark at the plugs simply was not strong enough to make up for the necessary change in timing.

I also went back to the engine initial timing setup. I had set it up at about 8-10 degrees advanced and needed to set it up at TDC, zero advance; it fired right up and runs strong. My dwell reading is just inside the required range but I will adjust the timing to see if I can maximize performance.

Thanks to all of you for thoughts and help, I really appreciate you!

Have a great day,

Ted