no compression/runs bad when started :((

[mrbrnz]

i'm having a bit of a problem , I got a new head and put it on and off about 10 times with no compression so I lapped the valves, put it on this morning and had compression, kicked it a few times, it started for a bit and then no compression again, kept kicking and bang, full compression, started it up but it doesn't run well and the head ticks allot. I can't figure it out :\ Is it something to do with the anti kickback on the cam or what? What could cause it to just have on and off compression like a switch. It's got compression now but the head ticks like mad

 

[thump*140]

done the valve clearances since you reassembled it and had it running?

 

[mrbrnz]

i just redid them, ticking went away, I did them when the head was off, seems it changed when I chucked it back on, the bad running was due to the spark plug tip getting whacked or something when it came off, put a new one on and it purs like a kitten, caught me off guard when I nearly looped it in 2nd, didn't realise milling the head, lapped valves, HD valve springs and porting made so much difference. HEAPS of compression now too.

 

[thump*140]

lol whodathunkit eh? ha ha. yeah shaving the head etc etc usually will bump up the ponies a tad.. lol
glad to hear it's ticking over properly mate.

 

[mrbrnz]

haha i'm one big epic fail when it comes to engines. I've learnt a lot from rebuilding it though. It's probably cheaper for me to bring it to a shop but experience is invaluable.

still wonder what caused the on/off compression though?

 

[thump*140]

the valves would have.. had the same problem when i changed cams in the 140... one kick, too much compression... next kick, no compression... You should find the compression will remain constant now. if not, there may be a smurf in there somewhere blocking the ports... ???

 

[mrbrnz]

yeah maybe something got lodged between the valve and the port. yeah compression seems to be even now. I'm stoked. Took me 2 days, bloody stressful. Hope my compressions not too high when I take the methanol out of the tank.

 

[thump*140]

if you can find E85 or ethanol petrol, run that... it'll help to some degree... better still, man up, go syphon some avgas out of your nearest light aircraft. :

 

[headsmess]

i know this one!

its the anti kickback alright....you swap the cams with the springs as well? and did it already have a kickback assembly in it? the two plates and a spring loaded pawl that mount on the rocker shafts.

sounds like its bent/deformed if it has one, but! (and if it doesnt have one fitted anyway)
on the end of the camshaft,opposite the sprocket boss, theres a little ring with a lump on it. the pawl stops this from turning, until the cam runs backwards...like when it has kickback... then the ring holds the exhaust valve open to decompress the cylinder

press the bearing off, remove this little doodad, and replace bearing...happened to me and had me scratching my head for days...but i didnt have an AKB fitted...

watch your leg without the AKB

 

[mrbrnz]

yep I swapped cams too, both had anti kickback mechs, I had a feeling it had something to do with it but I couldn't see how it worked, to me it just looked like it stopped the roller moving one way and let it move the other way but it didn't look to do anything. It's all working now miraculously but i'd love to figure out how it works. the stupid thing has made me scratch my head for 2 days and i've taken the head on and off about 10 times and same with valve springs/keepers, what a pain.

 

[headsmess]

if you look at the roller, youll see it has a face higher than the cam base circle,with flat spot... the pawl holds it still so the flat spot is in line with the rocker...as soon as it turns, the face interferes with the rocker, stopping it from fully returning, and holding the valve slightly open... only by a few mm...

the real sign this is the problem is when it starts ok, but starts blowing flames out the exhaust when you attempt to rev it....

mine also lasted a week without an issue at one time, then started doing it again until i removed that part.... check the pawl is working properly, or try swapping them over, etc... then just removing the whole shebang...

 

[mrbrnz]

makes sense now, I've had a proper look, I think maybe with the heavy duty valve springs the rocker may have had enough force to hold it in decomp position. Hopefully thats the last we'll see of that. I noticed rocking the bike backwards was enough to get it to unlock the last time it happened.