Ignition break up at 4000+ rpm

[Ziegelsteinfaust]

I have a mid 90's Corvette LT1 engine in my C10 with a gmpp 4 barrel intake, Q-jet, and pertronix equipped distributor. The engine was rebuilt with 10-1 compression, and not the factory 10.5-1. The heads are gasket matched to the intake, slight porting, and back cut valves.

The truck pulls hard to 4000/4500 rpm where it just rolls over, and you can hear then engine kind of make a bum bum bum sound.

Right now I am convinced it's ignition break up because a bad module.

All plugs check out fine, and the timing light works on all cylinders.

The truck drives great under normal circumstances, and mpg is right where it roughly should be. I have no issues hauling heavy weight either.

Any ideas on something else to check?

Or at the next oil change. I am going to just put the distributorless ignition back in.

[Willshook]

Making sure the Pertronix has 12V would be my first check.

I wouldn't go back to the OptiSpark for anything...

[Dead Parrot]

Did it always do this or is it a recent development?

Possible the cam is RPM limited.

I would change the plugs first. Possible you got a bad set or they somehow went bad after installation.

[MySons68C20]

Go ahead and put a new module in and see if that works.If not it never hurts to have an extra.


"Yes, a bad ignition module can definitely limit the RPMs of an engine, as it controls the spark timing and intensity, which directly affects how well the engine can combust fuel at higher RPMs, potentially causing the engine to lose power and struggle to reach higher revolutions per minute when faulty.
Key points about ignition modules and RPM limitations:

Spark generation:
The ignition module is responsible for triggering the ignition coils to create the spark that ignites the air-fuel mixture in the engine cylinders.

Weak spark at high RPM:
When an ignition module is failing, it might not be able to produce a strong enough spark at higher RPMs, leading to incomplete combustion and reduced power.
Symptoms of a failing ignition module:
Besides limited RPM, other symptoms include rough idling, engine misfires, difficulty starting, and a check engine light."

[Ziegelsteinfaust]

Originally Posted by Dead Parrot

Did it always do this or is it a recent development?

Possible the cam is RPM limited.

I would change the plugs first. Possible you got a bad set or they somehow went bad after installation.

I belive it to be recent. I rarely take it above 3000 rpm to do anything.

The cam is rmp limited, but that should start to happen around 5500 rpm. Not 3500/4000.

[Ziegelsteinfaust]

Originally Posted by MySons68C20

Go ahead and put a new module in and see if that works.If not it never hurts to have an extra.


"Yes, a bad ignition module can definitely limit the RPMs of an engine, as it controls the spark timing and intensity, which directly affects how well the engine can combust fuel at higher RPMs, potentially causing the engine to lose power and struggle to reach higher revolutions per minute when faulty.
Key points about ignition modules and RPM limitations:

Spark generation:
The ignition module is responsible for triggering the ignition coils to create the spark that ignites the air-fuel mixture in the engine cylinders.

Weak spark at high RPM:
When an ignition module is failing, it might not be able to produce a strong enough spark at higher RPMs, leading to incomplete combustion and reduced power.
Symptoms of a failing ignition module:
Besides limited RPM, other symptoms include rough idling, engine misfires, difficulty starting, and a check engine light."

I don't have another module, and the HEI i have is not compatible with my engine. Wrong distributor gear material. Nor do I own wires for it.

[MySons68C20]

Originally Posted by Ziegelsteinfaust

I don't have another module, and the HEI i have is not compatible with my engine. Wrong distributor gear material. Nor do I own wires for it.

$50.00 buys a module for the Pertronix....

[Willshook]

It’s worth taking a look at the Pertronix reluctor and making sure the gap is right - basically go through the initial setup again as a check.

I would still be looking at voltage fall-off - worth a check.

You can replace the distributor gear; it’s just held on by a drive pin.

I’ve had outstanding luck with all the Pertronix stuff going back to a kit I put in a ‘70 MGB 40 years ago and a bunch since them. So I would invest in troubleshooting vs. swapping.

A local shop that works on classics might have an Olde Skool distributor machine and be able to run it up.

Agreed that a module swap is reasonably cheap and easy.

Cams aren’t “RPM limited” such that they break up like this - they simply stop making power…

[Dead Parrot]

Originally Posted by Willshook

.....

Cams aren’t “RPM limited” such that they break up like this - they simply stop making power…

My Ford Escape with variable cam timing reacted much like OPs description of his problem when some aspect of the variable part failed and it got stuck. Decent low and mid range but very much an upper limit + something that felt like missing.

I also agree that a high end miss is very much in the realm of an ignition issue. Also worth checking the little button at the top of the dizzy cap and the cap itself for cracks and burn trails.

[Willshook]

Originally Posted by Dead Parrot

My Ford Escape with variable cam timing

GEN II SBCs (LT1/LT4) do not have VVT.

...and regardless, VVT is not an "RPM limited cam"...

[Ziegelsteinfaust]

Played with it yesterday morning.

I took the old switch power I installed to turn on/off my msd distributorless ignition, and routed it to the coil to power the distributor.

The engine ran better, but acted like it was constantly lean. Not having jets bigger then 75's I already using for my Q-jet. I drilled out a set of 71's I had extra's of. Honestly it feels like I gained 30hp/tq. I can carry a burnout a good bit further then before. Plus it is generally more responsive as i drive. Jet size is roughly 77.5 now, and is a bit rich. I have thicker rods i can use to bring down the effective jet size.

Down side. Ignition break-up or what ever the issue maybe is now at 3500 rpm, but it's alot better to that point.

Unfortunately I am sick, and I won't be able to investigate anything further for a few days. All i have done was a couple of small adjustments to hot idle speed, and idle afr. It runs more then good enough to let it ride for a bit till I am in a position to pull the carb off to get to the distributor easier to see if the gap is to big.

[Ziegelsteinfaust]

Originally Posted by MySons68C20

$50.00 buys a module for the Pertronix....

Thier a $100 now

[MySons68C20]

This is the one I found for $45.00. Don't know if you have Advance Auto where you live.

https://shop.advanceautoparts.com/p/...B&gclsrc=aw.ds