My 235 runs bad at higher RPMs. Need help.

[NeverDone50]

Backstory: Everything electrical has been replace and updated to 12v. Point gap is .022, spark plugs brand new and gapped .035. Timing is set to 5 BTDC. I have two rebuilt Rochester 1 barrel carbs and the motor ran the same on both. Mechanical advance and vacuum advance work. Distributor is original, but cap, rotor, condenser, and points have been replace.

Here's what it's doing. The motor idles well. As you start driving it, through every gear at about the same RPM, it peaks and doesn't gain any more power. The motor sounds like its not firing right, either electrical or fuel related. Low end it seems to run great. I suspected bad spark plugs, so I got brand new ACDELCO plugs. Then I suspected the carburetor, so I put on another one. No difference.

[OrrieG]

Sounds like:

1) fuel starvation, fuel pump may be going bad. Pressure regulator with gauge should show 4-8 pounds thru RPM range. Might also be a jetting issue with the carbs.
2) timing not advanced or advancing enough.
3) possibly coil is crapping out at higher load.

Note that the 235 is not a high RPM engine. The attached dyno info show max torque at 2000 rpm, max hp at 3600 rpm. These truck were designed for in town utility use and farm road driving when 45 was considered pretty darn fast (wife grandfathers quote).

[1-ton of fun]

valves floating? really i wouldn't rev a straight 6 past 4k rpm's unless it has been built to do so

[chevyguyase]

Take a fuel sample and set it a side for a few minutes and check for contaminated gas. Hook a vacuum gauge to manifold vacuum and check to see if your vacuum drops at cruise speeds to check for a restricted exhaust. Check your fuel pump for pressure and volume of fuel, you can have good pressure and still have low volume. Check your filter even it's new, these old truck tanks are full of rust.

[YukonXL04]

Originally Posted by OrrieG

Sounds like:

1) fuel starvation, fuel pump may be going bad. Pressure regulator with gauge should show 4-8 pounds thru RPM range. Might also be a jetting issue with the carbs.
2) timing not advanced or advancing enough.
3) possibly coil is crapping out at higher load.

Note that the 235 is not a high RPM engine. The attached dyno info show max torque at 2000 rpm, max hp at 3600 rpm. These truck were designed for in town utility use and farm road driving when 45 was considered pretty darn fast (wife grandfathers quote).

This is great info for me. Thanks alot!

[dwcsr]

Also check to see if the distributor shaft is wobbling. Make sure you don't have a clogged exhaust

[Speedbumpauto]

In particular, make sure the exhaust flapper valve shaft is not rusted in the almost closed position. It's used to block exhaust for intake manifold heat on cold starts. If it is free turning from closed to open, make sure the spring allows it to open at operating temperature. Those things could be a real pain and, as others have said, it sounds like a restricted exhaust.

[NeverDone50]

What's the best way to tell the exhaust is blocked? The thermostatic valve spring on the manifold is broke so unhooked the spring so it stays open. I have the feeling of its not the exhaust, it may be the timing. I advanced the timing a little and it made a slight difference. When I say high rpm, I meant high for this motor. The truck barely goes over 30 mph in fourth gear.

[chevyguyase]

As I stated in my post above, hook up a vacuum gauge to a manifold vacuum source. Rev the engine to cruise speed you should have 17 -18 inches of vacuum at cruise speed rpm. If the vacuum starts to fall lower as you hold the rpm steady your exhaust is restricted. Or you can do it the hard rust in your eye way by lowering the exhaust pipe at the exhaust manifold remembering to inspect the heat riser which is part of your exhaust manifold. You use the same gauge to check your fuel pressure, 4-8 psi, and volume, you should be able to get a pint or better with a few seconds of cranking. I would put the spring back on the riser, it warms the underside of your intake manifold and aids cold drivability.

[joedoh]

check the firing order. when I was 20 I bought a three year old firebird with a "bad motor" for $800 once with this same problem, wouldnt rev. had two plug wires reversed, close enough to idle well, until the advance kicked in and 2000 rpm was plllbbbt. after the "new motor" it was a burnout champ.

[mr57]

Originally Posted by dwcsr

Also check to see if the distributor shaft is wobbling. Make sure you don't have a clogged exhaust

What he said....

[Coupeguy2001]

check to see that a previous owner did not add a fuel filter under the truck. If it
is partially plugged, it will not pass enough gas to get that old fart going