Miss in idle /running rough

mrrolleyes · 05:48 AM

Welp I had some grounding issues after going crazy with cleaning. Starts right up now. So that’s solved. Now this is my headache

76 911s 2.7 cis with permatune cdi

I know the idle light on the permatune will have a flash when you hear the miss

https://youtu.be/T_1pqRbirSo

Replaced the following

Plugs
Wires
Cap
Rotor
reset points
Fuel system is all new up front from restore

Idle is about 900

After it warms up it will idle a bit more solid but under heavy throttle It breaks up

Also tried a new exhaust ( always swapping these ) and when I let off at a light it drops very low. I don’t think that’s exhaust related but part of my bigger issue.

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T77911S · 10-03-2018, 03:11 AM

you need to say year and what you have, CIS, carbs, other?

"idle light on permatune"?

I would consider trashing the permadoom and putting an MSD and coil.
the MSD has a light that shows it is getting a trigger from the dist, is that what you are talking about?

if the permadoom has a light that flashes ONLY when it misses then I would look at the dist. and the wire from the dist. since you have points I would also remove the dist and take it apart and clean all the ground contact points starting with where the points contact the dist. also clean the body of the dist where it contacts the engine case.

even with that miss the engine sounds really good.

to me the miss made me think of an injector that could be leaking just a little. I have seen carbs do it too where they can drip and make it miss.

toddetch · toddetch's Garage

I haven't had a chance to watch the video, but your description (idle at 900, warm up, then miss and running rough) is the exact description of a gremlin I chased for a couple of months. I worked with my mechanic back and forth on things. In the end, he was taking a look at mine (80 Targa) and he happened to have an 81 or 82 sitting there. He swapped the CDI boxes and it solved the issue. The mechanic said he had never seen that failure mode - the CDI not working properly only after warmup. Anyway, I put in the Classic Retrofit CDI+ (I had already replaced the coil with the same one Classic Retrofit sells) and the problem was completely solved. For whatever that's worth... I guess I would suggest maybe seeing if you can swap with a known good CDI and see if that fixes it.

mrrolleyes · 10-03-2018, 05:50 AM

Quote:

Originally Posted by T77911S

you need to say year and what you have, CIS, carbs, other?

"idle light on permatune"?

I would consider trashing the permadoom and putting an MSD and coil.
the MSD has a light that shows it is getting a trigger from the dist, is that what you are talking about?

if the permadoom has a light that flashes ONLY when it misses then I would look at the dist. and the wire from the dist. since you have points I would also remove the dist and take it apart and clean all the ground contact points starting with where the points contact the dist. also clean the body of the dist where it contacts the engine case.

even with that miss the engine sounds really good.

to me the miss made me think of an injector that could be leaking just a little. I have seen carbs do it too where they can drip and make it miss.

fixed..

everything has been replaced or cleaned or reset. its so hit or miss.

Quote:

Originally Posted by toddetch

I haven't had a chance to watch the video, but your description (idle at 900, warm up, then miss and running rough) is the exact description of a gremlin I chased for a couple of months. I worked with my mechanic back and forth on things. In the end, he was taking a look at mine (80 Targa) and he happened to have an 81 or 82 sitting there. He swapped the CDI boxes and it solved the issue. The mechanic said he had never seen that failure mode - the CDI not working properly only after warmup. Anyway, I put in the Classic Retrofit CDI+ (I had already replaced the coil with the same one Classic Retrofit sells) and the problem was completely solved. For whatever that's worth... I guess I would suggest maybe seeing if you can swap with a known good CDI and see if that fixes it.

i just had one for testing and i sent it back...

dfhtrhjn · dfhtrhjn's Garage

I had my injectors clog. It would start an run well cold because it was getting extra fuel cold, then warm it would miss; new injectors fixed the issue.

Warm it would mostly miss at idle, but then started missing with throttle as well.

Also, I love the sound of your exhaust.

mrrolleyes · 10-03-2018, 07:14 PM

fixed the idle drop... just adjusted it and its fine now.

still a similar idle. gets better when its warm but not perfect.

rolling the throttle over 4k it starts breaking up bad.

time to do some injectors and hope its the issue.

T77911S · 10-04-2018, 02:41 AM

do you have the vacuum controlled WUR?

if so, I would check it out.
above 4k it should start to make the mixture go richer

mrrolleyes · 10-05-2018, 05:28 PM

injectors came in today but...

a few buddies said check the points. so i regapped to 12 and the idle was a bit smoother but broke up bad at 2500+. regapped to 15 and idle is very rough.

mrrolleyes · 10-05-2018, 06:40 PM

a few of the injectors have some play in them so the seals are bad.

off to swap them out.

Jonny H · 10-05-2018, 11:35 PM

^. The plastic injector inserts can become loose in the intake runners too. Spray some brake cleaner on them and see if the engine note changes.

mrrolleyes · 10-06-2018, 06:59 AM

Quote:

Originally Posted by Jonny H

^. The plastic injector inserts can become loose in the intake runners too. Spray some brake cleaner on them and see if the engine note changes.

i started the injectors and needed boots too

... so another few days until they come in. a couple have major play in them