CIS and rough running 79SC
 

Here is the situation. My elder 79SC came out of the shop running a lot better but not just right. Under lying items have been addressed: ie valves recently adjusted, compression checked (but not differental check), new plugs, new plug wires, vacuum leaks traced and repaired, new fuel tank, filter, accumulator, leak at tank outlet fixed. Engine has a recent top o/h and was re-ringed. It was sitting for several years but I've put about 1000 miles in the last few weeks.

Adjustments got the idle perferct and it runs cold and hot very smoothly up to 3000 rpm then it picks up a miss under load. No load OK but I don't have anyplace cheap to run it steady at 4000 rpm in 5th. 3rd and 4th at 4K is OK.

System pressure is 5.0 warm and 3.5 cold. Pressure at 20c was reset from 2.2 to 2.8 bar. Timing set to 0 degrees with vac adv disconnected. Co was set at 2.58 hc and 105PFM. Idle set for 950 at 175F oil temp.

I had been using injector cleaner for three tanks of gas, but not in the last tank. Was advised that the cat was contaminated. There are some exhaust leaks. Also, I had a 1948 studebaker that has more pep at 2500 - 3500 rpm. It pops lightly when cold.
I feel the miss is fuel related. Electrics all check out.

Now the question. Is it possible that the Cat clogged with injector cleaner or the exhaust leaks can cause a miss around 3000 rpm under load?

Any ideas or suggestions welcome.

old tee,
i wouldn't rule out ignition. could very well be timing or an advance distributor issue...the fact that it's at high rpm suggests it. the fact that it runs fine and idles fine otherwise suggests that it isn't fuel related to me.
ryan

I would guess bad wires arcing except you say they are new. I'd still take a look in the dark for arcing at higher RPMs.

Agree - smells like ignition related

Will check.

SC with a Miss...
 

Pull the cap on the distributor and give the rotor a slight twist in each direction. If the advance weights and springs are working correctly, the rotor will turn slightly and then spring back to its static position. If the advance weights are frozen, the rotor will not move or will move and then stay in the advanced position. My guess is that you are not getting the proper advance as the engine tries to rev up. Another problem could be the Bosch CDI box is not healthy. They can develop problems where the car will run but not run well due to a poor spark.

Good Luck,

Fred Cook
'80 911SC coupe

One reason why I feel it is fuel related is because before the adjustments it was lean and had pep and performance. Will check the distributor tomorrow anyway. Thanks.

Mr. Cook. Pulled the dist cap today. Snap back was there. Buffed up the rotor button and will run it after dark tonight. Last night was just too darn cold in MD. MN, SD, MI, upstate NY go ahead and give your snide remarks. :D

Real porschephiles work on their cars even in much worse conditions! Try 5' of snow in 4 days in upstate New York! Only problem is finding the car.
sounds a lot like your warm or cold control pressure is wrong or something got clogged up in the distributor with all the work they did.

Ran it tonight. Not a spark seen anywhere. My question remains. Can a CAT clogged with injector cleaner or exhaust leaks cause a miss at 3000 rpm? Has anybody seen this kind of problem and what was the solution? SmileWavy

i dont think injector cleaner will clog a cat. and a clog cat just makes a car feel anemic. i dont remember a miss.

do not think it is the cat. clogged cats lack power, sometimes car starts but will hardly run. Try breaking the inlet flange to the cat to rule it out. If it is just a miss, sounds like it may be electrical.

Two things off the top of my head:

- have the injectors checked and cleaned if necessary, techron can't cure everything

- test for electric power loss as revs go up, we covered this in another thread in the last few weeks: higher revs = more current draw by the coil to make more frequent sparks. A known good coil can't compensate for low volts*current.


After the last few winters here, I'm beginning to think the snow is broken....

Thanks thebaer,

Injectors have been removed, clean and tested.

The voltage check is a good idea. I have a Fluke electronic volt meter that should do the job.
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