Carb backfire cause?

[Bluechrome]

Hi

My 1987 Venture idles well. It revs up nicely in the garage. The pilot screws are out 2 1/2 turns and the 4 carbs are well synchronized. I have run a can of Seafoam through it too.

However when I am on the road one cylinder backfires and/or misses. When I rev the bike (in the garage} without the air-cleaner I occasionally see a flash in the #2 cylinder (left front cylinder). I have tried opening and closing the #2 pilot by 1/2 turn but this makes no difference.

Any ideas what I could try?

Many thanks.

[Chaharly]

Pull that pilot screw out completely and run an E or B guitar string through the orifice. Got mine running on a sweet 4 cylinders that way. If that doesn't do it start by checking for vacuum leaks.

[frankd]

They all pop out the carbs when the air filter is open. When you rev it up in the garage, you don't really get on the main jet very much. When you are underway, the main jet is used any time you open the throttle. Check the carb diaphrams first, but I suspect that your real problem is a clogged main jet.

 

Now that I think about it, there is another possibility.....High loads require a much hotter spark to fire the plugs. Make sure you have GOOD spark to all 4 cylinders.

[Bluechrome]

Thanks for the advice. I pulled the carbs - (wow what a job to get the cables disconnected). Three diaphragms had pinholes so I have ordered new ones. I replaced the 'o'-rings on the pilot screws ($8 each!) and cleaned all the jets. As soon as I get the diaphragms I'll try it out.

Thanks again.