Plug fouling and valve recession

Darryl.Richman · Post by **Darryl.Richman** » Mon Jun 30, 2008 11:42 pm

Two things come to my mind: if you replace the jets, you should replace the needles as well, as they both wear. Also, did you verify the float settings on the carbs?

I haven't read anything in your posting that makes me think valve seat recession. The classic symptom for this is having to constantly readjust the valve clearances because they close up in a few hundred miles.

Your change in compression could be because of technique, or it could be because the valve seats have some carbon on them.

Good luck!

tricyclerob · Post by **tricyclerob** » Tue Jul 01, 2008 1:41 am

Thanks, these are the same things I mentioned to him. With only 13,500 miles on the bike, I did not see how valve recession could have occured. I asked if he was sure the plugs were carbon fouled and not oil fouled, thinking a stuck ring from sitting. But he said no oil smoke. Any other suggestions out there?

schrader7032 · Post by **schrader7032** » Tue Jul 01, 2008 7:14 am

I would look at the choke mechanism. These parts are handed and only go on one side or the other. It is also easy to assemble a side upside down so that the choke is always on or partially on. Also check to be sure that when the lever at the handlebar (I think that's where it is on the '81) is off, the lever at the back of the carb has also gone to the full off position. The lever on the carb should be up against the bottom stop. Snowbum's website has a section on carb rebuilding and shows (poorly) the position of the choke mechanism:

http://bmwmotorcycletech.info/bingcv.htm

When the idle screw is mentioned, I assume he's talking about the idle mixture screw. There's an idle speed screw and the mixture screw. The mixture screw controls a gas circuit...turn it in to the stop should shut all gas off. If the bike still runs unchanged, then he's correct in that gas is still getting through. Could be the choke mechanism as suggested. Could be the butterfly plates in the bore of the carb...not sure if these were removed as part of rebuild. I'm not sure how much they affect the amount of gas, but they can sure create flat spots in acceleration. The butterfly has beveled edges and a mark on it which is supposed to indicate proper installation orientation. When installed properly, there should be a consistent amount of light all around its edge when it is seated.

If the diaphragms are torn, that could create a rich running situation and no acceleration. Try taking the air intake tubes off the carbs, start the engine, and look at how the slides are responding to throttle input. They should move up and down with throttle input. If they are sticking down, they could be creating a higher vacuum and thus pulling in more gas.

That's some more ideas...Kurt in S.A.

tricyclerob · Post by **tricyclerob** » Tue Jul 01, 2008 9:26 am

Kurt and Darryl, I fowarded both of your responses to my friend. I think the choke plate idea may have some merit. I've read of others getting them in wrong during a carb rebuild.
There is always a logical reason for these problems but sometimes when you've been working on something for a while, it's hard to see the forest for the trees.
It usuallly boils down to a short between the headphones. It's such a relief when the problem is solved, but half the time it's somthing simple and you're kicking youself for not seeing it sooner.
thanks for the replies, and if anyone thinks of anything else ,don't be shy, post it.
!00 [air] heads are better than one. thanks again.

tricyclerob · Post by **tricyclerob** » Tue Jul 01, 2008 2:22 pm

From the owner, sounds like he really covered the carbs?

In brief response, everything in the carbs is replaced. Needle jets, jet needles, idle mixture screw, floats, float valves with the extra expensive Bing alcohol free system. The carb butterflies seat perfectly with the idle screw backed out. It is open slightly when adjusted for correct idle. I have checked the choke assembly several times and have good documentation from the carb rebuild class so am confident it is ok. Diaphrams are also new.

Everything logical pinpoints a fuel delivery problem, I just can’t find it. My thought re the valves is if they were not seating correctly full combustion would not take place. I have attached photos of the offending spark plugs. Condition is identical side to side. New plugs look like this within 50 miles. I am impressed the bike actually runs.

schrader7032 · Post by **schrader7032** » Wed Jul 02, 2008 9:35 am

Hmmm...any chance he can go back to the simple float system? I've heard that getting these new Bing alcohol-free floats set up is a bit tricky. Not sure the reason for going this route. The claim that Bing makes regarding fuel efficiency doesn't pan out as far as I've read from users. I believe the idea is based upon keeping fuel delivery the same on both carbs when leaning the bike or side loads are encountered. The way we ride our bikes doesn't induce any side loads. A sidecar might, but I don't think that's the issue here. I believe these new floats were useful in aircraft where side loads are typical.

Suggest that a leakdown test be performed to evaluate the state of the engine. The leakdown test is best in that it will identify the source of loss of compression, especially if the valves are a problem.

What about the health of the electrical system? What type of plugs are being used? What's the resistance of the wiring system, through the coils, from one spark plug cap to the other spark plug cap? On the '81, there should be 5K ohms resistance in each wire (due to the built-in resistance on the cap) and about 5K ohms for each coil. From one cap to the other, it should be around 20K ohms. A weak spark might be contributing to the problem.

What's the timing like? Does the full advance mark appear in the timing window when the RPMs are about 3000-3200 RPM? Above this RPM, the advance mark should remain solid in the window.

Kurt in S.A.

VBMWMO · Post by **VBMWMO** » Mon Feb 15, 2010 2:52 pm

This was a message from a fellow "Airhead". Any ideas? Almost sounds like 2 diff.issues to me.

My new ride Is a 1981 R00RS which had 13500 miles. About 500 of these miles were over its last 10 years. I have put on 1400 miles since I got it and have been fiddling and tinkering nonstop. The problem I cannot cure is spark plug fouling. The bike runs ok, pulls strongly under acceleration but tends to stumble between 1200 to 2800 revs at constant throttle. Even with brand new plugs they are carbon fouled in 30 miles. I started with valve adjustment, ignition timing, then went on to rebuild the carbs. I have had the carbs apart multiple times and am reasonably convinced they are assembled correctly. I have tried different size jets and needle jet positions all too little avail. The standard idle screw position is 1.25 turns. There is no change if it is at .75, 1, or 1.25 turns. With the idle screw turned in (clockwise) all the way the engine revs just increase and does not stall as you would expect. There is too much fuel getting to the cylinder. I took the heads of for inspection but apart from carbon buildup they appeared ok. I don’t have a valve spring compressor tool, but no light appeared when I shone a flashlight in from the inlet and exhaust port. I am wondering if I don’t have the dreaded valve seat recession issue. The compression was just ok when I first got the bike. 135 and 128. I figured this would get better with use, but last time I checked it was 90 in both cylinders. You would wonder the bike would even run like this. It does and there is no blue smoke and oil usage is minimal.

The issue is driving me crazy, but I have been busy with work, (which I should be doing right now) and have going at it when time permits. I am to close to the problem at the moment and really need an accurate diagnosis. Any bright ideas (or dumb ones) gratefully accepted. Sorry I missed the Elk Neck event, but that was the weekend I pulled the heads to see if that was the problem