CIS Problem - HC too high

1bar · 05-09-2005, 07:36 PM

I have a '77 930 that runs really well. Doesn't burn oil, has good even compression all around, less than 10k miles on a complete rebuild, and pulls like stink. Unfortunately, I can't get it through smog. It passed with flying colors last year.

The idle CO measured 8.9% and HC was 1244 at the DEQ station (limits in Oregon are 2.5% and 300, respectively, at idle). Took it back to the shop and adjusted CO. HC reaches a minima of ~1050 at about 3% CO. Lower CO the HC starts to climb, as it does with higher CO.

If you come off idle - to 1800RPM or higher - the HC drops to less than 100, and CO stays at what it was set to. Drop back to idle, and the HCs climb again.

This is really frustrating - I can't drive it if I can't smog it, and I'm stuck.

I've had a couple of suggestions, primarily centered around the fuel distributor. Apparently its possible to have a scratch or something stuck in one of the ports that causes uneven fuel flow to some cylinder. The car WAS stored last winter with a nearly empty gas tank (I know, dumb...). Maybe some crap got in the fuel system past all the filters?

Anyone here have any ideas? Can you confirm the possibility of a faulty fuel distributor, or perhaps an injector? Could anything else cause this kind of problem?

Any help would be welcome...

SC-targa · 05-10-2005, 04:28 AM

Does it idle smoothly and pull smoothly at low RPM? Maybe the injectors are dirty and have a poor spray pattern? Just a wild guess though.

Jerry Kroeger

Paulporsche · 05-10-2005, 04:51 AM

1bar,
Have you tried a search? It seems there was a lot of discussion on this a few months ago, including getting the car properly warmed up, disconnecting one injector, running lots of Techron, swapping tires to a different size, using a different gear etc.

ruf-porsche · ruf-porsche's Garage

Brought my Targa in last summer for it annual smog test. Had a stuck injector so it was only running on 5 cylinders. It pass with flying color, best smog report the car ever got. Got the injector fix and now it running on all 6 again.

MobileWrks · 05-10-2005, 06:53 AM

Heres a few things that may work. If you have a kragen, theres some stuff called LUCAS fuel treatment. I had a 77 that had poor spray pattern, add some of this stuff, next tank refilled with supreme gas... went away..(drive with high RMP's) Lucas is a lubricant, and works...
Also they have some stuff called. garnteed to pass smog.. (shhh, did that on my 94 ford aerostarw/226K..work van) drop HC's, how, i have no idea,saw it on craigslists, and it worked...
MAKE sure you leave your car running while you wait..keep it HOT
Good luck, let us know the results
Jimmy.....

MotoSook · 09:15 AM

You can't just lean it out and expect the numbers to be right. You have to change air, fuel and ignition to balance the reaction and attain an acceptable idle. I started a tech article on getting CIS tuned for emissions, but never finished it. I'll dig it up tonight if I don't get mobbed at the house tonight. I'll post some tips to help...

john walker's workshop · 10:34 AM

HC is wasted fuel for a number of reasons, not all CIS related. does it have good leakdown, new plug wires and connectors, cams other than stock, air pump gone, vacuum leaks, timing advanced, tight valve?...etc, etc.
try killing one cylinder at a time, by loosening the fuel line bolt on the fuel distributor, while hooked up to an exhaust machine, and see if any one cylinder, when deprived of fuel, causes the HC to drop down more than the rest. that may help narrow it down.

ruf-porsche · ruf-porsche's Garage

What causes high HC (hydrocarbon)? High HC is caused by incomplete combustion; in other words, the air and fuel that went in to the combustion chamber was not completely burned, now the unburned fuel has ended up in the tailpipe. Problems that can cause incomplete combustion include:

An ignition misfire. If a spark plug does not spark, the air/fuel mixture in the combustion chamber does not burn and then goes out the tailpipe.
Incorrect air fuel mixture. If there is not enough fuel in the combustion chamber to ignite, the fuel that is there goes out the tailpipe.
Low compression. If the compression is too low the air/fuel mixture will not ignite and the unburned fuel will go out the tailpipe.
A bad catalytic converter. The converter lives in the exhaust pipe and burns unburned gas as it passes through.

Did you car have any pollution equipment removed from it, air pump, cat converter, etc?

1bar · 05-15-2005, 03:41 PM

Hey, guys. A lot of great ideas, and lots to address.

First, the car has SC cams, has had some mild port cleanup (grind and flow), and has had the air injection system removed. The thermal reactors are removed as well, and the exhaust is replaced with a Euro system as suggested by Bruce Anderson's book. This car never had a cat - it's most like a 78SC fuel system, no O2 sensor and no Catalytic converter.

All these changes made a significant improvement in performance, and the car passed smog last year with all these changes already in place.

The idle is rough, like the car wants to be a top fuel dragster. "Lumpy" is how someone described it. That's at 900 - 1100RPM. Once it's up past 1800RPM or so, this goes away. I assumed it was the cams, but maybe not. I've sprayed all over looking for vacuum leaks, and can't find anything. I think I mentioned that the car has good compression (remember - this is a really low compression engine!). It pulls very smoothly, but again below 2500 RPM this thing is roughly equivalent to a Volkswagen bus - no torque at all, and no fun at all until you cross 3500 and the boost cuts in.

I've completely ignored the other part of the equation - spark - as several of you reminded me. I'll check for any ignition issues (weak spark at idle somewhere?), and try disabling a cylinder at a time to see if I can find one that seems to be the culprit.

What the heck - I wasn't already busy anyway...

stlrj · 05-15-2005, 06:02 PM

Sounds like you need to get it to run on all cylinders at idle. I would start with your ignition.

Joe