Post by Phantom Phyxer on Dec 24, 2013 17:50:14 GMT -5
Ok, here goes. I have a spare engine which I rebuilt. After installing and running it for a couple of hundred miles I found the right plug to be black and sooty. The Carbs are adjusted properly because they work fine on the original engine. I made sure they are syncronized. It doesn't appear to be blowing any smoke and seems to run fine except for the black plug. Could a problem in the valve train cause this? The valves are adjusted properly but could a weak or broken valve spring cause this? When I rebuilt it, I found a bent valve so I replaced it. I just lapped the valves and then poured gasoline in the combustion chamber with the head on the bench with the plugs in to make sure there were no leaks. Before lapping them they leaked pretty bad. My other engine runs fine, I got this engine to use as a spare just in case. I rebuilt the top end on my original engine and had no issues. I have the engine on an engine stand but before I take it apart, is there anything I should check? Thanks.
Welcome aboard. Hope what follows is helpful, but if it isn't, keep coming back.
A few things come to mind. First off, 90% of "carb problems" originate elsewhere, usually in the ignition. If you have breaker points, inspect carefully. Next, your carbs may have worked well on your previous motor, but don't assume that they've continued to work well; you can't tell when something is going to fail. Inspect them on the bench and make sure the float valve (needle and seat) is shutting off fuel. If not, look for a leaking float or dirty/worn float valve. Re. the head, have you done a compression check? In pre-1980 carbs the needle jet is sealed with an O-ring, and a rich condition can result, especially in the cruising range, if it fails. If compression is good, a leak down test can tell you more. I once spent a lot of hours looking for the source of an idle stumble on a high compression big bore motor. Compression was dead even at 180 lbs on both sides, but a leak down test showed a 20% pressure loss on the right cylinder--you could feel the breeze coming from the right side intake bell. Tear down revealed a cupped valve. A bent valve will often damage its guide; clearance specs and inspection procedures are given in the factory manual, available as a free download at www.biker.net .
Good luck and Merry Christmas to you!
Last Edit: Dec 24, 2013 22:11:56 GMT -5 by grizld1
Post by Phantom Phyxer on Mar 20, 2016 12:51:08 GMT -5
Hi, well I took the head back off and looked it over. It turns out the intake valve I had replaced previously was incorrect, at least it didn't look like the rest. It was a "new replacement". Well I found a NOS item on ebay, still in the wrappers so I went for that. I also found that due to the previously bent valve, I had a bad valve guide which was probably causing the rich running condition. Found a NOS replacement as well and took the head to a machine and had them both installed professionally. Well got it all back together and was out riding for a couple of hours when suddenly got a backfiring from the same cylinder and the bike did not want to idle. Once I got home I inspected the plugs which both were burning clean. I found the only way to make the bike run right was to adjust the offending cylinder mixture screw out 4 turns. The left side is at 2. I changed the right cylinder carb and got the same result. I checked the valve clearances and they are spot on and the valve are opening and closing properly. I guess now its time to do that compression and leak down test. What should compression read on a warmed up engine?
r80rt: :)This is a splendid write up, THANK YOU!!!!
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