454 stumbles at 2500 rpm

[wjvineyard865]

New carb and float level is set halfway up sight glass but I’ve tried it at a few different places and didn’t make a difference

 

[Snoots]

Pick-up coil? Carbon tracking inside the dist cap?
At night park somewhere dark, raise the hood and have someone rev it to 2500 rpm's. Check for arc flashes around the spark plug wires.

 

[WebMonkey]

my vote is an ignition problem. when the time between sparks diminishes, the problem arises.

so i'm GUESSING there is trouble in spark delivery.

had this trouble in a motorcycle. low rpm smooth running, higher rpm stumble like a 'rev limiter'.
turned out the harness from the stator was rubbed by the flywheel and dampened the 'juice' at higher rpm.

good luck!
'monkey

 

[Matt69olds]

Rig up a cheap vacuum/fuel pressure gauge to the fuel line ahead of the carb. Tape the gauge to the windshield so you can see it while driving. If it maintains pressure, it’s time to start looking at ignition components. If it doesn’t, inspect the fuel line. Check for kinks, rusty lines, split rubber hose, etc. if the pump is sucking more air than fuel, the engine is t going to run very well. Next, disconnect the fuel line from both the fuel pump and fuel tank, try blowing some air thru the line. Make sure it’s not clogged.

 

[Rusty Nail]

Running a 37-38 year old distributor by chance?
I'm at spark output voltage.
Somebody else said "pick up coil" and I bet he's ALSO right.

 

[wjvineyard865]

Two new distributors on it so far just to make sure. I’ll rig up the vacuum guage so I can watch it while I’m driving. In gear the truck is about 15. I know it eventually gets to 0 or close at WOT but what am I watching for while just cruising?

 

[Rusty Nail]

Battery cables then.

Ya know one time I put a new alternator in a Honda and it ran much better.

Didja check the halogen fluid?

 

[85K304SPD]

Have you changed carburetor jets? The holleys are usually jetted pretty rich out of the box. What jets are in it? Did you blow the power valve with a back fire?

 

[Matt69olds]

Two new distributors on it so far just to make sure. I’ll rig up the vacuum guage so I can watch it while I’m driving. In gear the truck is about 15. I know it eventually gets to 0 or close at WOT but what am I watching for while just cruising?

Most vacuum gauges also have a scale for fuel pressure. Tee the vacuum gauge into the fuel line so you can monitor fuel pressure.


Engine vacuum should stay fairly constant at steady cruise. If the vacuum drops off rapidly while the engine stumbles (at steady throttle) that’s a symptom of a restricted exhaust.

 

[wjvineyard865]

Have you changed carburetor jets? The holleys are usually jetted pretty rich out of the box. What jets are in it? Did you blow the power valve with a back fire?

I haven’t changed the jets on the new one yet. I did replace power valve just to make sure. Didn’t fix it

 

[wjvineyard865]

Most vacuum gauges also have a scale for fuel pressure. Tee the vacuum gauge into the fuel line so you can monitor fuel pressure.


Engine vacuum should stay fairly constant at steady cruise. If the vacuum drops off rapidly while the engine stumbles (at steady throttle) that’s a symptom of a restricted exhaust.

Ok I taped a guage to the windshield. Got down to about 3 wot and while cruising around 50mph it was around 10. When it stumbled it dropped off for a second but jumped right back up to around 15. But the power hasn’t come back yet

 

[wjvineyard865]

Battery cables then.

Ya know one time I put a new alternator in a Honda and it ran much better.

I ordered a new alternator. I’m gonna give it a try. It’s just weird that it’s at exactly 2500 rpm that it happens every time

 

[Bextreme04]

Ok I taped a guage to the windshield. Got down to about 3 wot and while cruising around 50mph it was around 10. When it stumbled it dropped off for a second but jumped right back up to around 15. But the power hasn’t come back yet

Are these vacuum numbers or fuel pressure? Either way, its not right.

 

[wjvineyard865]

Are these vacuum numbers or fuel pressure? Either way, its not right.

Vacuum. What should it be in gear?

 

[Rusty Nail]

An '84 @ 2500 rpm... 84.... Big block... Th400?

84.
2500 is TCC speed is the thought.
Does that activate fuel return too? Thinkin so.

One gas tank or two?

What transmission please?
need positive ID / dont guess