Here is a picture of two plugs (1&4) after 15 minute ride that I done after the sync and on this ride there were backfire on deceleration. these plugs were new. I put them just before the sync procedure.
what can one learn from this plug read? Is it carbon fouled? Is it to cold ? It doesn't look like hot or lean ?
The plug says that one or more throttle positions are too rich. And that quite likely you are getting the same build up in the exhaust, providing fuel for the pops.
It doesn't say anything about fuel mixtures when throttle is closed (deceleration position). The plugs are on their way to being carbon fouled. The black coating on the center electrode insulator will shunt away spark energy, leading to misfires. At minimum, they need a full power full load hard run, to see if temps will get high enough to burn off those deposits. You know, like what the police call drag racing.
Another thing to look at is ignition timing. Early timing on the wasted spark cylinders can light the mixture in the pipes. Also check for stuck advancer.
The compression test I did showed low compression (105 for all 4 cylinders). can this explain the backfire?
If all 4 were the same, no worries. I expect you use a compression tester for large auto engines. These add volume to the cylinder and force lower readings than book value, where they used a small engine tester to establish printed numbers.
A dyno report should tell you what needs changing in your carbs, by sniffing the exhaust. The Dyno tech, may even tell you how much change is needed.
