Been having some trouble with my 72 911T ( 2.8) misfiring and running rough,
so I got the carbs off and sure enough one was full of water. Cleaned and rebuilt them, put them back on the car and it fired up right away. Set the timing up to balance the carbs and found that the factory specs for the timing (5 ATDC at 900rpm) don't seem to work very well and make it nigh on impossible to set up the carbs. Found that the car runs best with the timing around 10 degrees BTDC. I'm thinking this may well be because the car has carbs and not MFI as it came from the factory with. Looking around the web I've read quite a few articles saying that with webers on the 911 you should run with the timing advanced. Anyway, though I got it running OK there is still a bit of spitting up through the velocity stacks. Not constant, but it happens. No amount of adjusting has been able to remedy this. I tried the test whereby you remove a plug lead at a time while the engine is running and monitor the drop in rpm and if it's a fairly even drop then one is pretty close with regards to the tuning and balancing of the carbs. Although what I did find when removing plug leads 4 and 6 (the 2 cylinders spitting up through the stacks) is that there was no change to the idle of the car when removing the leads. I've gone through the ignition, changed the plugs, tested the wires and have now come to the conclusion I have a couple of burnt valves. On inspecting the plugs, 4 and 6 are a lot darker and have some soot on them. Does this sound like burnt out valves to anyone or can anyone think of anything else I should try?

I guess my next step is to go buy a compression tester.

 

A bad cap could cause cross firing, as could leaky wires. A compression test would also be in order.
did you check your float levels ? The fuel bowls could be overflowing.

 

Also, check your idle enrichment screws. Loosen the locknut and count the turns until bottomed out. Generally speaking, 1.5 - 3 turns is the ballpark. Count the turns so you can return to original settings if prudent. I am a firm believer that 90 % of all carb problems are ignition related. Make sure you have good spark. A cracked cap could cause crossover spark, spitting etc. Unless your valves are totally out of adjustment, it is most unlikely that you would burn an intake valve.

 

yup

 

Checked and double checked float levels, as they were wrong first time round. As for the ignition circuits I swapped the cap and leads out of my other (known good) 911. As for the air adjustment screws the haynes weber manual says to screw them all the way home and only adjust them when balancing individual vernturis on each carb. I have played with them to see if it makes a difference, but as of yet it hasn't. It did occur to me that the idle circuits could be dirty (though they shouldn't be), as the car goes really well and doesn't feel like it's running on just 4 cylinders. Also, there isn't the vibration one would expect from 2 cylinders on one bank not firing. Would it be a good idea to hold the engine at about 3500rpm and then remove the troublesome plug leads to see if it has an effect? Would that prove dirty idle circuits?

 

BTW, you dont have those wires crossed do you ?

 

No, they're all labeled. And that's the weird thing, it idles very smoothly and drives really well. Just doesn't seem like a 6 cyl car thats running on 4 cylinders.

 

I think you are right. I think you need to blow out all the pasages in the carbs.

 

What kind of gas is in your tank ? Water in the fuel filter ?

 

No the gas is fine and I have a see through filter king filter on it so you can see the gas.

 

First off, your method of doing a power balance test by pulling a live spark plug wire while the engine is running is NOT safe, I would never do that, especially considering you were the guy that was complaining about too much fuel in the carbs (fuel vapors+secondary ignition sparks=BAD NEWS). The proper way to do a power balance test is to put a short length of rubber vacuum hose in between the ends of the individual spark plug wires and where they plug into on the distributor cap. Then connect a test light or something similar to a good ground and touch the other end to the vacuum hose that's connected to the cylinder you want to disable. This is much safer!

OK now that the safety lesson is out of the way, it did seem that you found something during your power balance test, that cyls 4 and 6 are dead. Before you do anything else, do get a compression tester. This will let you know if your problem is a mechanical engine problem that no amount of fiddling of the carbs will fix. The compression numbers should be within 20% of each other as a general rule of thumb. If one or more cylinders are low, then you will need to do a leakdown test to help narrow down your problem. You really want to be under 5% leakdown, but under 10% is passable. If you find major leakdown on a cylinder, listen for where the air is escaping from. Air coming from the carbs (intake) means a problem with the intake valve(s), air from the exhaust indicates an exhaust valve problem (this would be the case if your have a burnt exhaust valve like you are speculating) and air from the oil filler would indicate piston rings that are not sealing enough.

If your compression/leakdown numbers are OK, then of course your problem is either fuel or spark related and we'll go from there.

---Chris A.

 

Thanks for the reply. Good point anout the safety aspect. I think it's time I bought a compression tester. My only query with that is the location of the sparkplugs on the 911. As they're recessed I'm not sure I'll be able to get a good enough seal on the tester as it will be difficult to get any sort of wrench on it. Has anyone else had experience with this?

 

Use a leakdown tester, no tools required - except a compressor.
Pete

 

Bump. How's the compression Adam ?

 

I haven't had the time to get the car to a workshop yet. I was going to buy a regular compression tester, but couldn't see how I would get a good seal what with the plugs being recessed and all and it being difficult to get a wrench of any sort on it.