Engine problem, dying

#1
Ok so here is my problem. Engine was running good with timing at 30* but I adv it 2* in search of the extra bit of power (arc adjustable flywheel) and the bike did not run good. I set timing back to 30 and now the engine blows flames and dies when you hit the throttle hard. I have gone thru the carb completely, float checked all good, reset timing from scratch (tdc, pointer, etc), no change so I did a leak down and found a big leak out of the exh valve. Is it a coincidence that it crapped out after the timing change or could the change somehow cause a leak or a problem? I pulled the head and lapped the valve. Now waiting on a gasket. How do you guys check for a good seal after lapping the valves prior to installation?
 
#2
I vaguely remember something about using a dye of some sort, and twisting the valve in the seat looking for a solid mark.
But alas, I've slept since then.
 

trinik7597

Active Member
#3
Sounds like valve overlap what is your lash set at you might want to loosen up the exhaust a little bit ... depending on your fuel and cam you should run fine at 32° that is stock advance on all of arc's fixed wheels
 
#4
I have a bright light above the bench, and peer in to the port with the chamber side of head to light.
Some pour gas, rubbing alcohol, piss, in the port and see if it leaks.
 
#5
No luck yet. I replaced the exh valve and now I don't have the big loss thru the exh but still not running. Replaced the carb, no luck same symptoms. Valve lash now loose no luck. WTF? Big flames out of the exh and won't run only idles and barely. I must be forgetting something.
 

trinik7597

Active Member
#6
Need a little more info ... what motor ?? Is this your animal or something else ?? What head ?? What cam ?? What is your coil gap set at sounds to me if you are blowing flames out of the exhaust that your exhaust valve is opening on the compression stroke or too soon
 
#9
Need a little more info ... what motor ?? Is this your animal or something else ?? What head ?? What cam ?? What is your coil gap set at sounds to me if you are blowing flames out of the exhaust that your exhaust valve is opening on the compression stroke or too soon
Briggs 305, shaved head about 12:1 CR, 308 dyno cam. I took the head off and set timing to 31* with a degree wheel and dial indicator. Lash .003, also checked keys on crank and gear, all good. I rechecked the carb again and all seems good, needle stops the flow and the floats are within range. This bike ran great a week ago. I do believe that it is timing related but I must be doing something wrong now.
 
#12
It really seems and sounds like its out of time....but you've done checked it .. I'm sure you've changed the spark plug...but I have to ask ?
 
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#13
It really seems and sounds like its out of time....but you've done checked it .. I'm sure you've changed the spark plug...but I have to ask ?
Plug has been changed. Anything else comes to mind? Tomorrow I will go back to the original carb (cleaned twice), reset timing (forth time), adjust valves (forth time) and see what happens. I even changed the fuel. I watched the ARC youtube video about the flywheel and I have done it exactly like that every time.
 
#15
its a piece of acrlyc with a piece of rubber glued on. I originally made a metal one but when a head has a leak i could not tell where or what seat was leaking. I made this plastic one and if It does not hold a vacuum I fill chamber with alcohol and then pull a vacuum and see where the bubbles are coming from.
 

joshua. c.

Well-Known Member
#16
the only other thing I can think of is checking if the breather is working properly and that compression is good. it sounds like you've checked everything else. could it be bad gas ? or a clogged fuel line?
 
#17
Ok guys I found the gremlin. I had recently changed the flywheel nut for one to be used with a starter. This nut apparently does not have the right threads and it gets loose even after tightening to proper specs. The flywheel was slipping out of position but since I was setting it right I never thought about it. Now I'm using a key and it stays put. Thanks to all.
 
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