Little high speed stumbles

[amc49]

Look here......................any time the engines got to where they would not rev all the way out on me and popping once getting up there they had bad valve seal when I yanked the head. The exhaust seats will be dead. Another way to tell if you are sensitive enough to engine note is that they will seem to act like carbs not synced, there will be a burbling note at cruise that sounds like one carb is off sync but is actually at least one cylinder not putting out as much as the others. Other than that the engine will run on all 4 cylinders and seem pretty good, it will just not rev all the way out. It really IS making less power but they make enough you think it's fine at first as the loss is not much, later though it gets steadily worse. One other indicator is that everything you can think of is right, there is nothing else that can be wrong. In that I don't think the advancer is your issue, I've had no trouble with them at all. You should be able to tell it anyway by the amount of spark scatter you get while setting timing with a light.

The bikes were built in a era of low lead fuel which actually was not as bad as zero lead, even a little helped tremendously. The valves are heat treated only and a very thin layer, it's not remarkable to have to change 2-3 valves every time you pop a head on these; they are shortlived in modern zero lead gas and ethanol only made it worse. I call around 25K miles as about the time to need valvework if you playbike it enough. Why the compression numbers matter more than usual. I consider the engine type to not be as robust over the longterm as the SOHC, the bigger single valves have half the number to leak and bigger ones stabilize better in the valve guides anyway.

How most of them die, they begin to not pull out max rpm and then it slowly drops lower and lower.

 

[Chuck Hahn]

Well whats a day to yank a head off gonna hurt me. Guess what Im doing this weekend. AMC...so the valve SEATS were burnt on ones you took apart or the edges of the valves themselves?? Kinda prepping for what I should be looking at once i take the head off.

 

[amc49]

They will be somewhat burned but brinelled as well, the carbon bits beat the crap out of the seats to make them all dented and scarred up. You need to do both surfaces and that means new valves as they cannot be cut, even lapping them too much can remove enough hardcoat to then let the bare metal show and then the valve dies like lightning as the base material underneath is super soft. The smaller valves use about the same clearance as the SOHC but the smaller length and size lets that same clearance act like it is larger. The relative wallow then lets valve wobble not hit correct to then tear seats up worse.

The intakes usually do better as the heat is not nearly so great.

The engine has to remove to yank a head.............

 

[Chuck Hahn]

Got engine out and head off..only took 3 1/2 hours. So there is a lot of carbon on the valves but none look that ashen white that ive seen before on smoked valves. BUT now that its open Im gonna remove all the valves, marking each one as i remove them with a Sharpie, and take a good look at things. Got the head off which was my goal for the night and so it is. Shower and chill time now.

Valve removal will commence tomorrow after work. And yes i will try to remember the camera...we all know how much we like pictures.

 

[Chuck Hahn]

So its been a while but i figured it out. Took the head off and checked every valve and lapped them. Only one had intake valve with what I didnt like and that was visible with my magnifying glass. Looked like a small dimple from maybe a chunk of carbon getting smashed. Replaced that valve. New NOS advancer went in also. Got it all back together and it till did the same thing. Had taken off the carbs and verified i hadnt mixed up jets etc etc etc. and nothing out of place. triple checked float hts and fuel levels...all spot on. Now im getting mad.

SOOOOO...i thought they had put these aftwrmarket so maybe the jetting is a bit fat. Took out the air filter and it got bad at around 50 MPH instead of 70ish like before. Put the air filter and spring thing back in BUT thi time left the airbox cover off....ran like a scladed dog without so much a a blip!!!

So now we know it wasnt getting enough air and was sort of like it was slightly being choked. So this leads me to ask you guy this. So instead of reyanking carbs and going through all the "lets try this size main jet" drama, do you think a K&N air filter would be more breathable over these stock paper filter and thus resolve the poor intake air issue???