Breaking up through the RPM range

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05.27.25, 7:09 PM
The Big Bad Wolf

Breaking up through the RPM range

I've been chasing a problem for a bit now and want to see if anyone else has experienced it.
At 32 degrees of timing at around 3500 rpm the engine starts to spit and spudder but then clears out around 4-4200 rpm.
If I advance the timing to 36 degrees it breaks up around 3800 and if I push it to 40 degree timing it breaks up around 4 and then clears up around 4800. I quickly backed the timing back down to 36 degrees. All of this was on a drive under load.

The fuel pump is a year old, it has petronix instead of points and the fuel lines are all clean. carburetor was rebuilt after this all started.
I'm going to order a couple jets to see if that helps but for some reason I have a feeling it won't help.

Has anyone had this happened to them before?
05.27.25, 9:03 PM
BLS

Put a set of points in it and see if that clears it up. I've heard of several mysterious petronix problems. Points are pretty inexpensive...
05.27.25, 10:08 PM
The Big Bad Wolf

Quote:

Originally Posted by BLS

Put a set of points in it and see if that clears it up. I've heard of several mysterious petronix problems. Points are pretty inexpensive...

I almost went to the parts store at the last race and tried that. Before swapping around jets I'm going to try this thanks.
05.28.25, 1:46 AM
Dietmar

Breaking up

You said this was happening under load but did not say if it is on a straight or coming out of a corning.
If it is the latter, then the carb is flooding. Check the float and float level plus the needle valve under the carb top.

Dietmar
www.quixoteracing.com

Quote:

Originally Posted by The Big Bad Wolf

I've been chasing a problem for a bit now and want to see if anyone else has experienced it.
At 32 degrees of timing at around 3500 rpm the engine starts to spit and spudder but then clears out around 4-4200 rpm.
If I advance the timing to 36 degrees it breaks up around 3800 and if I push it to 40 degree timing it breaks up around 4 and then clears up around 4800. I quickly backed the timing back down to 36 degrees. All of this was on a drive under load.

The fuel pump is a year old, it has petronix instead of points and the fuel lines are all clean. carburetor was rebuilt after this all started.
I'm going to order a couple jets to see if that helps but for some reason I have a feeling it won't help.

Has anyone had this happened to them before?
05.28.25, 11:02 AM
Steve Davis

Just because the carb was 'rebuilt' doesn't mean it's "correct". Borrow a known 'good one' from someone if possible. Check your air correction jet (CAREFULLY). Couple weeks ago, I cranked my engine in the garage with the air filter off..and noticed that the a/c jet was MOVING. It had vibrated loose, but thankfully did NOT hop out and fall into the running engine. I reinstalled with a 1/8 'dab' of rtv to keep it from coming loose again. I'll also try to remember to check it occasionally. Also found carb top screws 'not as tight' as I wanted... and found a pinhole leak in the accel pump diaphram. Any or all of these could cause break up as you describe, but none did.
Manifold sucking air at the head junction? Nuts tight on the bottom of the carb? Air adj jet set properly? Emulsion tube of carb loose? One or more valves adjusted too tight? Leaking air at the head exhaust ports? crack in exhaust system somewhere? Sucking AIR somewhere between fuel cell and pump and carb?
The list could get pretty long but these are a few things to check... OH.. 2 spark plug wires crossed (1 & 2 or 3 &4)?
Steve, FV80
05.28.25, 4:17 PM
BLS

I thought the key was the fact that it starts and then clears up at a bit higher RPM. Not many things that will do that, at least that I'm aware of.
Dietmar and Steve know as much as anyone - maybe clear up the circumstances when it occurs a bit?
05.28.25, 4:29 PM
kea

Breaking up

Make sure your battery isn't dropping voltage with the vibrations increase at higher RPM's. Broken plates internally will short, dropping voltage and causing a miss.
05.28.25, 9:31 PM
stonebridge20

The absolute first thing I go to with any kind of misfire is the master kill switch.
Pull one cable off of one lug and attach it to the other lug, essentially using one lug as a connection of the two cables and see if your issue goes away.
05.29.25, 2:19 PM
billtebbutt

Quote:

Originally Posted by stonebridge20

The absolute first thing I go to with any kind of misfire is the master kill switch.
Pull one cable off of one lug and attach it to the other lug, essentially using one lug as a connection of the two cables and see if your issue goes away.

Oh my. Been there. Learned the hard way to look at this early in the diagnostic process......

I/we have a tendency to over complicate the process by being biased to complicated problems. I was beside myself a month ago as my fresh-off-the-dyno Kent would only start/run on 2 cylinders once I got it back in the car. Naturally, my mind goes "Pertronix!" immediately. When in fact, I eventually figured out that I had absentmindedly re-fastened the plug leads in typical British firing order 1-3-4-2. Which as you know, is NOT the Kent firing order......

Good luck
bt