[chriss69]

509ci, 8-71, dual Holley 750 blower carbs, 28 deg total timing.

The usual suspects have been tried, changed or tweaked:
Adjusted timing
Reduced idle
Removed carbon build-up
Changed plugs and wires
Replaced distributor cap and rotor
Futzed with throttle adjust screw & idle mixture screws

About to try:
Rocker panel kill switch for MSD 6AL box (cut all power to box)

Did not try:
Bore small holes in throttle plates
Throttle solenoid
In-line diode as suggested by MSD


The run-on is still happening. Is there something else I should look? Is blower motor run-on different in any way from N/A engine run-on? Thanks in advance for your input.

[onovakind67]

Does it run-on or does it just keep running like nothing has happened?

[chriss69]

It runs-on, may or may not diesel, then dies.

[onovakind67]

The MSD fix is usually prescribed for an engine that won't shut off at all.
What's the initial timing?

[chriss69]

Initial is 8.

[onovakind67]

I would try raising the initial advance substantially while keeping the total advance under control.

From BDS:

Blown motors should run as little as 16 degrees or as much as 26 degrees initial advance with the total advance of about 32-36 degrees at 2500 RPM to 3000 RPM. Specific timing requirements depend on compression, blower drive ratio, engine load, camshaft, and fuel octane....

Blowers love ignition advance. Especially initial advanced. For most blower applications, the initial timing should be set at 16-24 degrees initial advance with a total of 32-36 degrees advance, in all by 2500-2800 RPM. More advance can be run if there is not a detonation problem. Some vacuum advance systems will have as much as 45-50 degrees total advance under part throttle, cruise conditions.

[JimM]

agreed. Blower or not, a "standard" cause of deiseling is too little initial advance, which results in too much throttle opening to maintain an idle.

[chriss69]

Originally (few days ago) it was 17 initial, 36 total. Seemed way too high plus it ran-on, so we brought it down. Unfortunately, the run-on continues. By tomorrow morning the MSD will be on a seperate switch, may or may not be a good idea but it's just another thing we're trying. So now we've tried 8 initial and 17 initial, both with run-on. It's a little frustrating given the other things we've tried.

[JimM]

What about plugs?
blower motors like COLD plugs

[68Camaro396/375]

Could be an Octane issue.

[chriss69]

Plugs are one step cooler than those recommended by the engine manufacturer for the same motor w/out a blower. 509 was originally dyno tuned for 93 pump gas, I have not tried going to higher octane. I suppose I should have tried this simple solultion first.

[chriss69]

I feel a little frustrated regarding the timing. I've read cases here where 18 initial 34 total works great (for my exact motor, blower combo no less!), others recommend less than or around 30 total at most. I'm really beginning to feel like hard-wiring the MSD is a waste. I need to hone-in on what timing my combo really wants.

[camaroman7d]

Running your initial that low is going to cause you problems. The engine will run a lot hotter and you are giving up a lot of power. My initial is 18* with a total of 34, boost retard hooked up to drop out a few degrees under boost conditions. With your timing at 8* I bet your plugs are glowing red or at least very hot along with your headers, this alone can cause run on. I am not sure if you have carbon build up yet but, if so it could be causing your run on too. How high is your idle set? Many times that alone will be the problem. Are your carbs dialed in/well tuned? This will have to be gone over everytime you make a timing adjustment (unless it's very minor)

[chriss69]

Idle is set at around 900. When we set the timing to 8/28 the headers did glow, we thought it was because it was running lean, so we up'd the idle a bit. We're trying to dial in the carbs. What is your recommendation for initial/total timing based on this thread?

[chriss69]

After some discussion we've decided not to install the kill switch on the MSD. We're now shooting for around 16-18 initial 34-36 total timing (sort of where we began), idle around 800-1000rmp. Not certain how we will go about dialing-in the carbs. We're thinking that maybe a power valve is shot causing excess fuel to leak into the engine, so we're using the sight plugs to see if any of the fuel bowls drain with time after shutoff.