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I am a new user and find this site extremely useful. I got a 906 Paso and I recently changed the belts and checked the valve clearances and everything looks a-ok until I went to start the bike.
had a hard time getting it started and after it warmed up a bit i started hearing poping sounds in the exhaust. I made checks and both cylinders have spark. It appears that fuel are getting pushed out of the carb instead of getting pull into the intake runners. I unpluged the rear plug boot and the bike runs "smooth" with just the horizontal cylinder even with some revs. If i do the same for the vertical cylinder the bike wouldn't even start at all. This leads me to believe my vertical cylinder is not firing or firing at the right "timing". Is it possible that my marelli box is bad?

Does anyone have an idea? Any advice would be appreciated

Apparently it is said that 99% of electrical problems are carby related. But first replace plug(s) . Aways start at the top, Tank has and gives fuel, check filter, Fuel line to carb, good flow, restrictions at carb fuel intake, check filter, Take off carb and clean with carb cleaner, blow out all orifices and check clear, replace and check for manifold air leaks, this relates a lot to popping noises. Check compression with gauge, ensure all electrical conections are in good order, check battery voltage and ensure good earth connections at earth points. Just the basics obviously. but hope you have some joy . Regards Marty .
Just an add on , best not to run with plug lead off expesive damage to electrcal components can result, Cheers

So you changed the belts and it immediately didn't run right? I would say that you have the timing of the one belt wrong. You should verify this before looking at anything else.

+1 on j slocum.
When all 3 timing marks on belts line up, you`re right on.

thanks a lot for the quick replies!

I thought so at first so i went back to check clearances and marks on pulley/belts. All match up though. check and seal carb as well as i can as well. wonder if it's possible that my valves are not sealing completely as i did rotate them during the clearance adjustment process. Thinking maybe some crud is keeping the valves from giving a complete seal? I am not getting any vacuum in the intake. The horizontal cyclinder/carb is working perfect though. I plan to rotate the engine until both valves are open at the same time then give the spark plug hole a burst or two of compressed air. think that might help or hurt? I really don't want to pull the head!!!

Duclover888,
Still sounds like the belts are the problem. The dots could all be lined up, but be 360 deg out. Particularly if it seems that fuel is getting pushed out the carb rather than drawn in.

Fraser.

interesting. this thing about 360 degree out of phase. can you explain futher?

duclover888 wrote: I unpluged the rear plug boot and the bike runs "smooth" with just the horizontal cylinder even with some revs. If i do the same for the vertical cylinder the bike wouldn't even start at all.

It's very dangerous to make engine run with pipes unplugged!
Hope you'll put a park plug connected to the ground next times, to save your marelli box!
Anyway, cam timing control and a compression test (trottle fully open!) will tell you anything about valve sealing, as carbon slags are hard to clean from valve seats

thanks for all the help. it turns out to be a poorly sealing intake valve.

Set too tight????

Glad to hear you go it taken care of.

duclover888 wrote:interesting. this thing about 360 degree out of phase. can you explain futher?

If I remember correctly......It happens every now-and-again when a person changes belts and accidentally rotates the crank between changing each belt. Often, the timing marks are only valid for the horizontal cylinder, so the vertical can get confusing. This is because when going to TDC on the compression stroke for the vertical cylinder (after having it on TDC for compression on the horizontal), the piston will come up on TDC for the exhaust stroke before it comes back around for TDC for compression.

If you set your timing at TDC on the exhaust stroke, you will be 360-degrees off on the timing of that cylinder. I've been told the bike will run, just that it will be similar to skipping and the bike will have very little power..

Desmo, that's correct. Many years ago I had a bevel drive 750 Sport, when the motor was out and the vertical cylinder head off my brother came along and pushed the piston down. As there were no spark plugs in the piston came back up. Not knowing this I fitted the head back on and of course it would not start. It was 360 deg out. Fortunately no valve damage, just a lot of head scratching (on my part) to figure out what was going on.
Fraser.