The Speed Demon is on and running okay for the most part. I do have a weird problem with the idle surging. It jumps around from 600 to 800 rpms and wants to stall at times.

It happens after the engine is warmed up. Seems to run fine when cold. I haven't plugged off vacuum lines yet to check for a leak, I'll do that tomorrow. But could a vacuum leak cause a surge like that?

Manifold vacuum is holding steady at 17hg at idle, so I don't think its a power valve issue. (I think the Demon has an 8.5 power valve). It also happens with the vacuum advance line disconnected & plugged. So its not a vac advance issue either.

I did change the stop bushing on the MSD distributor. I'm running 18 initial and 18 mechanical now. Too much advance? No detonation detected.

Only other change was I had to tee the PCV line into the smaller inlet on the manifold (Demons aren't PCV friendly).

Maybe a lean condition? But why the surge?

Any tips or areas to check would be greatly appreciated. I found some hits in the archive, but its one of those topics that is hard to find without sifting through hundreds of posts.

--Jack

 

Hey Dude,

I think power valve is 6.5, but I doubt that is the issue. I just wanted you to know.
What springs do you have in the distributor. What source is the vacuum advance setup to? manifold or ported?

Thanks,
Joe

 

Hi Joe,

Bet you loved to hear the words "Demon" again after what you went through!!

No vac advance setup right now. Dist is set at 18 initial and 18 mechanical all in at 3000. Confirmed with a light.

 

I love demons, yeah right!
Is the distributor mechanic advance only? or is the vacuum advane capped?
which advance springs are in the distributor? In ohter words, what rpm are you fully advanced at?
Thanks,
Jo

 

Vacuum adv is capped.
Fully advanced at 3000 rpms.

 

I had to put a small spacer under mine as the accelerator pump linkage was hitting the manifold.

 

Thank goodness I don't have that problem. I have 1/2 inch hood clearance now. That's with a 1 1/4 drop base and 2 1/2 inch element.

 

Hey,

I think you should setup the ignition fully before you start messing with the carb. It will make a big difference at idle. Otherwise you will be chasing your tail!

Try this:
1. Start by removing the vacuum advance and setting the total timing to 36 total.
From what you have mentioned, you have the black bushing in which gives you the least amount of advance, 18 degrees. That is good, leave that in!

2. Makes sure it is coming in around 2500 or lower. (with msd, try one light silver and one blue spring.) I have the two lightest springs and mine comes in at about 950. Try to get it in as quick as possible with any detonation.

3. After that is complete, check the vacuum reading and make sure it is over 13 hg.
The reason for this is that most of the MSD distributors have the B26 (Napa part number VC1808??) unit. This unit is fully advanced around 12 hg. You wat it fully advanced at idle. The unit provides 16 crankshaft degrees (8 distributor degrees) of advance.

4. Plug the vacuum advance into a manifold port. This is the one that has full vacuum at idle.

This is what you will end up with:

Total timing without vacuum advance: 36
Initial timing without vacuum advance: 18
Initial timing with vacuum advance: 34 (18 initial + 16 degrees vacuum advance)
Total timing with vacuum advance: 52 (36 total + 16 degrees vacuum advance)
Fully advanced by: 2500 RPM (at the max, shoot for something lower)
Vacuum without vacuum advance: greater than 13 hg.

After you plug in the vacuum advance, the idle and vacuum will go up. That is when you can start tuning the carb.

Let me know how it goes.
Thanks,
Joe


If you want me to help, just ask and I will come over and tune it with you.