engine and drivetrain High RPM stumble?

[jrhone]

This may be related to my breakdown last week. I now get a high RPM stumble around 5k RPM. Where should I look? Fuel pump? Pertronix? Carb? I had a leak in the accelerator pump on the weber 38 carb. Gonna replace the carb this weekend. Anyway to test the fuel pump? Or the Pertronix?

[PaulTWinterton]

My first thought is fuel starvation.  The pump is pulling fuel, but if the filters are clogged, it can't suck enough to feed the engine properly.  Hence stumbling at high rpms, is the first symptom of clogged filters.

[airborne85]

Just going along with the fuel angle..

 

Maybe after resolving the unknown problem from last week you've maybe enhanced the flow of fuel...Maybe turning down the fuel just a skoosh on each side of your 38/38 or one side at a time....Adjust just enough not to create another variable...Or just turn down the screw that controls both barrels, small small increments...Which I'm sure you already know..

 

Or it could be running lean which would mean do the above in reverse

 

You'll get it

 

ira

[jrhone]

Oh it's a new fuel filter. I replaced that just because. My first thought is starvation as well and that's why I'm going to replace the pump. I have heard when that fail they can pump not enough fuel or too much fuel. Also a leaky carb in the accelerator pump could be letting air in as well as starving fuel at high RPMs. Maybe? Gonna swap the carb tomorrow and see if the problem persists.

[TobyB]

Mine used to do that when the carb holddown nuts started to back off...

 

t

[jrhone]

Yeah I think it's the carb. Will be swapping it out in the morning. Here is a video of the stumble. Kinda hard to hear it's right before I shift. Also here is a video of the fuel leak from the carb.

Stumble on acceleration

trim.D7B0D532-EF66-4E75-BB2A-D567AB69EE9D.MOV

Leak from carb

trim.45ECF090-C868-4B84-9A09-A162E37D5C1A.MOV

[jrhone]

I think I fixed the problem. Let's mark this one solved. I swapped the weber 38 for another one this morning and all stumbles are gone. I would bet the carb had a lot to do with my breakdown last week as well. No leaks or stumbles now. The swap was a peace of cake. Thanks for the info!!!

[jimk]

Those damn Webers! 

[OriginalOwner]

So there's your project:  take apart the apparently bad Weber and look for the problem(s) ..... and thus become a Subject Matter Expert for yourself and others.

 

Cheers,

 

Carl

[jrhone]

Well I think its just tired….its in need of a rebuild if one thing is leaking, I would bet ALOT of the seals are bad.  

['76mintgrün'02]

I will give you ten dollars for it. 

[TobyB]

Oh, hey- take the top off the old one and push on the WOT (acceleration) diaphragm.  The one with the spring and the long rod attached.

 

I've had all sorts of odd problems when those get old and brittle- just as you're describing.

 

There's not much to wear out permanently in a Weber downdraft that a rubber kit can't fix.

 

t

[Nuthatch]

Even brand new 38s, however, are very sensitive to fuel pressure, and won't tolerate anything less than full flow from a healthy pump. 

[jrhone]

Thanks. Yes I'm gonna swap the fuel pump just because. I wanna rule that out and make sure I'm getting consistent fuel pressure.