CIS stumble when warm

jhelgesen · 12-19-2002, 04:48 AM

Hi guys, hoping someone could share some insite, don't have a lot of practice with CIS engines yet.

I'm working on an engine for a friend, 77 911s with sportmatic tranny, 2.7 cis. Stumbles accelerating from idle after it warms up, starts fine cold and warm.

So far I have:

Adjusted valves (got rid of a tick)
Installed new plugs
checked timing
installed ignitor
re-checked timing
run a can of techron through with a few gallons of gas

Other than the sportmatic being a PITA, it runs pretty strong.

Got any suggestions?

Thanks!

Targa77 · Targa77's Garage

I would check the sparkplug wires with an ohmmeter. Can't recall from memory what the resistance should be but do a search on this site and I'm sure that you'll find it.

makaio · 12-19-2002, 05:59 AM

Have you checked the CO?

bigchillcar · bigchillcar's Garage

Ditto on the last suggestion, check the CO by a gas analyzer. Also check that nothing is causing the air sensor plate to bind and that it's centered and seated properly...this will cause a stumble on acceleration. If memory serves me correctly, I think plug wire resistance, in general, needs to be between 1,000 - 10,000 ohms per foot. Otherwise, it sounds like the cold start valve is working properly and your check valve and accumulator are fine by virtue of a healthy warm start. I'm guessing the air filter is good, how about the fuel filter. You could always check the spray pattern of the injectors. Check for the presence of a vacuum leak by spraying some carb cleaner around the air box, hoses, etc. and listen for an increase in idle speed to confirm a leak. CIS cars hate vacuum leaks. Good luck!
Ryan

jhelgesen · 12-19-2002, 06:35 AM

I've seen mention of checking the CO and fuel pressure. Are these tools I'll need to borrow, or are they within the price reach of the average guy?

I'll try checking for vacuum leaks this afternoon, check the plate, and maybe change the fuel filter (think I have a spare). I flushed some crap out of the tank, so there may be some junk in there.

The car has sat for over a year without running, so its had a few things to tune up.

BTW, thanks to everyone on the board, did a search on the ignitor installation, it was right on the money. Instrucions in the box were crap, you guys got it fired right up.

Thanks

stlrj · 12-19-2002, 07:32 AM

Before you go any further, finish your ignition tune up with a new cap, rotor and ignition cables.

Joe

beamonk · 12-19-2002, 07:40 AM

Make sure the vaccum advance is working properly, fixed my stumble.

bigchillcar · bigchillcar's Garage

Best bet for setting CO is to go to a local garage with a gas analyzer that will let you stick the analyzer in the tailpipe yourself. You'll need a 3 mm allen wrench to find the hole between the fuel distributor and the air sensor plate. Little adjustments mean a lot. Don't give it any gas while adjusting or it can cause internal damage. Adjust, then give it some gas, let it stabilize and take a reading. I think your manual for your year car will say what percentage...1.5% maybe?

Fuel pressure gauge and hoses aren't that expensive and could be picked up at an Autozone type store. Gauge needs to read up to 100 psi and from the gauge it should go to a 'T' fitting with one side of the 'T' having a valve that can open and close.

New distr. cap, rotor and ignition wires are probably a good idea. Expect to pay ~ $50-60 for the cap and rotor and ~$125-175 for wires.
Ryan

47silver · 47silver's Garage

i lean towards spark plug wires and if the dwell if the electronic ignition does not calculate that.

gary