calling all CIS gurues... I have a question

cnielsen · 03-16-2005, 06:38 AM

So my 73.5 911T has developed a new problem and I was wondering if anybody had some input...

The car starts fine when cold and actually runs fine for the first few miles till it warms up, then.... It happens....

The car will not take throttle. It idles great and runs great over 3,000 RPM but will not run smooth at all in between. It coughs, spits, and backfires (glad I have a pop-off valve) in the 1K - 3K range. I have to rev it to over 3K and then slip the clutch in order to drive the car otherwise it will start bucking uncontrolably while you try to give it gas. Even revving it up at a stop light produces the same effect...you can't rev it smoothly, you have to give it gas fast to get over the 3000 RPM mark.

I took the injectors out and bench tested them at a shop and they seemed to spray fine. The injector seals still seemed supple. I tried making the fuel mixtre richer because it kind of acts like it is lean but that did not make a difference.

Any thoughts...

ruf-porsche · ruf-porsche's Garage

Check your mixture with a meter instead of adjusting it without. It could be lean because when you first start up the engine you have the cold start injector adding fuel to increase the mixture.
Check to see if the plate is moving up and down in the airbox and that is centered.

mede8er · mede8er's Garage

I took the injectors out and bench tested them at a shop and they seemed to spray fine. The injector seals still seemed supple. I tried making the fuel mixtre richer because it kind of acts like it is lean but that did not make a difference.

My simple rule of thumb is: low speed missing points to ignition probs (points, plugs, etc.) and high speed missing points to fuel probs....this is not always an absolute however, but since you've already adjusted A/F without results I'd look at ignition.....

Paulporsche · 03-16-2005, 06:57 AM

Have you checked your warm control pressure?

Usually the WUR goes out of whack so that the cold CP is too high, giving a too lean condition until warmed up. In your case maybe the WUR is OK for start and warmup and out of spec for warm.

Was there any difference at all when you richened the mixture? Are you sure you actually richened it?

oneblueyedog · oneblueyedog's Garage

This might help-

http://members.rennlist.com/jimwms/CIS/CIShome.html

But it may be Ignition. On my SC I had a CD box act up under high heat conditions causing an unburned fuel condition and burning up my CAT Conv. Ran fine until it reached a certain tempurature.

Distributor could be a factor also.

Paulporsche · 03-16-2005, 07:06 AM

After reading the above comments from mede8er, I would like to add:

Have you checked the usual tuneup items, like points, cap, plugs, wires and timing?

Have you checked for vacuum leaks? Is your popoff valve closing correctly?

It is also possible that not only is your sensor plate sticking as ruf-porsche says, maybe your plunger is sticking in the fuek dist?
How about your valve adjustment?

cnielsen · 03-16-2005, 08:13 AM

Wow, thanks for all the good leads. You tend to get tunnel vision with a problem like this. I was certain that the injectors were the culprit but this gives me some new areas to look into. Parts for the 73.5 CIS motor are getting hard to find. Is there any retrofitting on later parts, like from a mid 70's or an SC?

I will look into the sticking sensor plate. That makes sense. Actually, one day I left the mixture setting tool in the fuel distributor while I drove it and it caused the plate to remain partially open. This created the same symptoms...only much, much worse.

I will check that tonight.... too many projects and broken things... Arggggghhhh

mikemdd · 03-16-2005, 08:31 AM

Just curious...I noted the comment about the popoff valve not closing which could cause an air leak. I've seen this noted by others also. Maybe an obvious question, but how does this cause an air leak when it is still in the intake side of the throttle body? The air may not go through the filter, but is still in the intake path. I'm sure I'm missing something.

Argeo · 03-16-2005, 08:36 AM

Was points on my CIS with same problem. Try adjusting or buy new set. I think they are like $15.00.

ed martin · 03-16-2005, 08:36 AM

I had a similar problem with my 73.5 CIS. Turned out that the air box had breaks in the seams. Fixed the problem with some deck caulking.

911AtLast · 03-16-2005, 08:40 AM

I've been following this thread with interest - my car is back on the road after I changed a blown airbox.

When it's warmed up it's almost impossible to drive in traffic - idling badly and backfiring.

I did remove the hot air blower to check the motor at the same time - I'm now wondering whether the excess heat on that side of the engine compartment is causing problems with the CD box?

Going to try putting the blower back on tomorrow....

Warren

cnielsen · 03-16-2005, 08:41 AM

More great ideas to check. I am going to have to print this thread and make a detailed list of things to check.

Thanks a ton, keep 'em coming

Argeo · 03-16-2005, 09:41 AM

HI Warren, not sure what you run for a CDI but I use PErmatune on my 77 C3 with zero blower with no problem. Maybe something wasn't properly reattached?

Don't know but good luck.

David · David's Garage

I've seen this problem caused by vacuum leaks, ignition timing, and a bad WUR. If you don't have a set of CIS pressure gauges, you need them, might as well buy some now.