Firing problems
 

I just did my 500 mile valve adjustment after rebuilding my '71 911T engine. It has webers which I also had to rebuild. I removed all the spark plugs and 5 of them looked ok but one looked like it was straight out of the box and was completly clean. This was in cylinder number 5. Compression was at 135 and I have spark. when the engine is running and I remove the lead going to number 5 there is no change which tells me it's not firing. when I look down into the carbs and pump the trottle I can see gas squirting out so I know it's getting that far. Where would be the best place in the carb to check for blockages??, could the timing be out that much that one cylinder would not fire?? If anybody has any ideas please let me know
thanks
Martin

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Check that #5 sparkplug lead with an Ohmmeter or multimeter for end-to-end continuity. Some wire sets have a resistor end at both the distributor and the sparkplug. Compare the total resistance with one of the good leads. In cold weather the CDI-ignition develops enough Voltage to jump accross an open resistor, but above 80 degrees F ambient there will be a miss.

Individual ends can be replaced by 'unscrewing' from the Neoprene wire, and while you are troubleshooting, run the engine at night and look in the engine compartment for 'corona' ... high Voltage leakage from the wires. The Neoprene wire lengths can be replaced, and I recommend AC-Delco SS-550 bulk wire, a high-quality 7 mm Silicone-jacketed wire with a stainless steel core.

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Warren Hall
1973 911S Targa

Thanks for the advice, but I'm fairly sure the lead is ok. last night I ran the engine and put in an old spark plug into number 5 lead and there seemed to be plenty of spark. If it were to change with temperature then there should still be fouling on the plug even if it was only running every now and again during the 500 miles, no?. I forgot to mention when I removed the plug again after running the engine for a minuite or two there was no sign of gas on the plug.
thanks for any other help

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[This message has been edited by seery (edited 06-22-2000).]

Well, then check the sparkplug to be SURE it is not shorted!

If it is not getting any gas to the cylinder, then you need to check for blockage in the port with a rag or paper towel, etc.

It would take a very odd or strange carb. problem for the cylinder to not get ANY gas from the accelerator pump, idle circuit, and main circuit!

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Warren Hall
1973 911S Targa

How does your engine sound? When you replaced the "clean" plug with an old one did the engine sound like it was running on all 6? I can't immagine that you ran this motor for 500 miles on 5 cylinders and didn't notice a rough idle. You may just have a bad plug. Run the engine for a longer mount of time with the old plug in place and see what happens.

Check out this thread www.spyderclub.com/ubb/Forum1/HTML/000189.html . There are a couple of lines here, one is about a carb problem that sounds like yours. Have you checked the balance of your carbs particularly on that bank? With the air cleaner off and the engine running, when you rev the engine is it sucking through barrel?

Well it sounds like a blocked jet or something. It does suck through #5 barrel as good as the rest. I thought the idle was a little rough but I didn't think it would run that good on 5 cylinders and thought it just needed a good tune up. I'll check it our tonight and see what happens. I had one other thought, if I got the plug leads mixed up, like 6 going to five and 5 going to six would there be any chance that number 6 cylinder would work and 5 would not?
thanks again for the help

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My spark plug leads would not allow the #5 wire to be on the #6 plug. Plugged jets would suggest that two jets are both plugged, wouldn't it?

If you felt that the car needed a tune-up, then clearly this is why. The cylinder is sucking, the plug is firing. If it weren't it would smell like gas and the car would backfire. I think you're getting spark but not gas.

I'm a bit of a neanderthal at times, and I might gently pour a little gasoline into the carb while it idles to see what happens. There'd be a fire extinguisher within three feet and I would not use much gas. I'm crazy though, maybe you're not.

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'83 SC

I think you need to remove that right side Weber and find out what went wrong with your rebuild ... the problem CAN'T be fixed with it sitting there on the engine!!! I hope you have an exploded diagram of the carb. ... because I think you are going to need it!!!

At this point, all of our guessing is POINTLESS ... the problem IS INSIDE the carburetor!

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Warren Hall
1973 911S Targa

Thanks for all the advice guys. Well I think I solved the problem last night, the main jet was blocked, the idle jet was clean but the tip of the mixture screw (the one with the spring around it) had a bit of crude on it so I guess this was blocking the idle circuit. I cleaned these out with carb cleaner and it worked. The idle is still a bit rough but I still have to get a tune up. What a difference that extra cylinder makes when driving though. I also filled up with swepco and the tranny shifts alot better, it's well work the money. What would be a fair price for getting the carbs balanced the mixture set right? I'm not sure if I'd ever get them right myself and I'm hoping to use this as a daily driver.
Thanks again
Martin.

It's been a long time but let's see if memory serves me well. My first was an everyday driver with webers and I miss them to this day. Once you get a feel for them, they are easily serviced and diagnosed.
The balancing is relatively simple. start with the air correction screws screwed in and backed off about two turns. Set the idle to the prescribed rpm (I think it's around 2000-2500..I forgot that part) Using a unisyn set so the indicator ball will rise to midrange on the first stack tested, go from one cylinder to the next and adjust the air jets to equal settings. you may do it a few times to get it balanced, but it will come to you. you may have to re-set the unisyn and repeat the process, but it will get easier.
one VERRRRRRRRYY important point:
Make doubly sure that the idle and main jet passages and the jets are free of any type of debris or dust. You've already found how critical they are. Even the tiniest obstruction of an idle jet causeds a lean misfire. Easily diagnosed, just loosen the holder with the stubby scredriver while it's running and you'll find the faulty jet as it won't influence the engine when moved.
Good luck. You'll find lots of tricks the webers can perform and they're a lot more interesting than injection systems.

I'd put my MFI with that belt comming off the cam driving the pump up against webers for interest any day.

My guess is that the 'fair' price would be whatever a Uni-Syn goes for these days!

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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 06-23-2000).]

I do agree with Fred. I sing the praises of CIS cuz I understand them somewhat and they work very well. But frankly carburetors are both more elegant to me, and more......tough. they look awesome, neater and smaller. The sucking sound they make is cool. And they are straightforward and reliable.

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'83 SC