back firing now and then.

[toymechanic.]

7785 miles on bike in signature. I soaked the carb and rebuilt them with new parts. The bike will only run with choke fully on. It ploops thru the carbs and back fires thru the exhaust. Before I remove the head to check the valve for rust I wanted to confirm, that valve seating is my problem. Compression on #1,2,&4 125-135, compression # 3 80-108. This is telling me that a valve is either stuck open or is rusty and leaking. Is my assumption right or should I be looking at the carb or somewhere else.

Thanks for the help.

Gary

[focus frenzy]

do you have the air box on?

[flyingace]

Low compression can be valves or rings or even a hole in the piston crown. Put a tablespoon of oil down the plug hole and check compression again. If it's higher, it's rings. If not... do you have adequate valve clearance on #3 valves? If not, adjust and check again. I would think a stuck open valve would create even less compression than stated. Maybe the valve is burned/corroded and has a small hole in the seat area? You just have to open it up and look.

[mark]

I don't think that even with a cylinder that low you'd need to run with the choke fully on. Did you need full choke before?

When you do a compression test the throttle needs to be wide open or carbs off the bike. If you didn't do that your measurements won't be right. Did you adjust your floats excatly right when you put the carbs back together? Did you adjust your air screws correctly?

I assume you synchronized the carbs also? That needs to be done after a rebuild too. Valve adjustment correct?

[LW_Icarus]

Im with mark, with only one cylinder below 100 psi it should run ok, and maybe have a soft idle/off idle. it takes 100 psi to make the combustion process happen the way its supposed to, in any gas engine.
popping back could be related to that but more likely youve got a lean situation, either a vacuum leak (check the boots) or a restriction in the fuels supply. that could start at the petcock, unlikely, or be in the carbs idle circuit, which is highly likely due to the fact its the smallest passages in the carbs.
mine had been run with busted boots, and fried the rings in 2 and 3, they were both <80 psi, but even after I fixxed that I needed to clear the pilots before the bike ran right.

[toymechanic.]

BACK FIRING

I have the air cleaner on witha K&n filter installed in the stock box. Tank is off, I'm running it on a gravity feed hanging fuel aux tank. It has not ran enough to connect the synchronizer up. Wide open throttle when checking compression. I bought it not running, and don't know how long it has set and when the last time it was running. I'm thinking that possibly the number three valves were open and may have slightly rusted. I do agree that some of the symptoms point toward running lean since the choke has to be on. I did not check or really look at the intake boots when I had the carb's off. I have not had time to mess with it since Wednesday. The next time I mess with it, I'm going to check out the boots. and run a compression check again. I'll post afterwards.

Thanks for the help!

Gary

[toymechanic.]

I had two wires crossed. on spark plugs 3 and 4. I blow in the breather hoses. I thought it would never start up after that. I held the throttle wide open and it finally started and idle smooth and everthing(I had no idea that blowing in them hoses would fix everything). I hooked up the synchronizer, balanced it out and it great. I can't wait to ride it. If I wasn't so busy working on other people atvs and m/c, I would time. WELL it is those side jobs that allow me to my my GS.

Thanks for the help guys,

Gary the Toymechanic