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Posted by: cwpeden Dec 26 2012, 01:34 AM

I'm trying to help a friend solve an idle problem. Idle is low to the point of stalling and backfiring.

Here's what we have tryied so far:

Grounds are good
Timing is good
Dwell is a litlle high
Vacuum hoses are good
Cyl head temp sensor good
Ballast resistor good
Points good
Cap good
rotor good

Been through two trobleshooting guides with no results.

Need help figuring out a peculiar problem. When I take the vac lines off the dist I get the the usual high revs but they dont go away when the lines are plugged. Could this be a symptom of a bad vac can on the dist?

Any thoughts

Posted by: 914itis Dec 26 2012, 05:44 AM

Was timing set with the vac lines plugged? Try to cap the vac lines going to the dizzy and report back.

Posted by: cwpeden Dec 27 2012, 03:24 AM

Timing set with vac lines off the dist and plugged.

Thing is, everything seems to work fine except for idle. Smells like too much gas.

Posted by: Bleyseng Dec 27 2012, 10:48 AM

MPS is correct number (037) and tests good with a vac tester at 15hg for 5 min?

Posted by: cwpeden Dec 27 2012, 03:55 PM

Does MPS affect idle mixture?

If it was bad I would expect a vacuum leak and high idle. Not 700 down to stall....right

Posted by: euro911 Dec 27 2012, 04:34 PM

There's not much vacuum being drawn above idle, so a leaky MPS shouldn't affect the mixture too much at higher throttle positions.

Have you checked out PBander's site on troubleshooting?

Posted by: 914_teener Dec 27 2012, 04:57 PM

I think what Geoff is saying is that your post doesn't rule it out.

Mark....I think vacuum is highest at idle...with due respect... he mentioned at idle.

Could be both MPS and distributor.....ask me how I know. I would suggest you rule out the MPS first.

Posted by: euro911 Dec 27 2012, 05:36 PM

I guessed I missed the high idle part, Rob.

Not discounting a bad MPS, conducting a simple test with a Mighty-Vac will tell. However, a slow leaking MPS still works OK (not as good as a fully solvent one, of course).

The lower the idle, the higher the vacuum, so yes, a real bad MPS will affect the idle mixture.

Posted by: Cap'n Krusty Dec 27 2012, 06:06 PM

Ummm, Yeah. What "ballast resistor"? 914s don't have a ballast resistor.

The Cap'n

Posted by: euro911 Dec 27 2012, 06:17 PM

Valves adjusted correctly?
Dwell should be somewhere between 44 & 49 degrees (optimally)
Idle speed adjustment on ECU (pot wiper or trace corroded)?
Idle adjustment on throttle body?

Posted by: cwpeden Dec 28 2012, 02:10 AM

I meant the inline resistor on the CHT sensor. not sure why I called it a ballast resistor. Must have been thinking of something else.

Will check the MPS though and probably need to pull the dist. for a thorough cleaning. and vac can test. I'll bet the ground wire in the dist is loose too.

Just to clarify. With everything hooked up the idle is low to the point of stalling and backfiring. When I pull the hoses to set the timing the idle goes up as expected, but dosent return to normal when the hoses are plugged. Should it?

thanks for the suggestions.

Posted by: euro911 Dec 28 2012, 10:17 AM

Are you sure the two vacuum hoses to the dizzy are going to the correct vacuum ports? (not reversed)?

Posted by: 914_teener Dec 28 2012, 01:03 PM

Good point Mark.

I had the advance can go bad once and the mech advance plate inside the dizzy in dire need of lube when I first got the car. I would check those out as well. Sounds like the dizzy to me. Make sure everything is working and lubricated in there.....if it is the stock unit.

Posted by: cwpeden Dec 29 2012, 06:41 PM

I'm feeling the same myself, about the condition of the dist. Think I'll just pull the good dist out of my car with the pertronix and see what happens.

Posted by: cwpeden Jan 11 2013, 01:29 AM

Finally had the chance to go to my friends place tonight to work on his car. Brought my good MPS and Dist. with me.

Tested his MPS and it was good, vacuum wise. Swapped mine in anyways and no change.

Swapped in my dist. with pertronix and started right up smooth idle at 1100 rpm and settled down abit.

I have his dist on my bench and now I get to learn about distributor rebuilding. Cant be that hard.....right?

Posted by: euro911 Jan 11 2013, 01:44 AM

If you don't already have a 'MightyVac' get one. They're relatively inexpensive and an invaluable automotive diagnostic tool serving many applications.

Vacuum canisters, MPS', AARs, and hydraulic systems.