re- jetting

[Rdltmc3]

So i have a slight backfire on the exhaust side, i also have some pretty free flowing mufflers (basically straight pipes)

should I be re-jetting the carbs to help out here?


also where is a good place to buy jets for our carbs and what would be a good size to increase too?


exhaust is the only mod.

[CYBORG]

As much fun as re-jetting is, I don't think it will help your problem. You might start by adjusting the air jets a little. If you feel its running to lean, decrease the air in the air/fuel ratio

[CYBORG]

Popular thinking is that changes to the exhaust will have little effect on running issues. However a little back pressure is is required

[robin1731]

Jetting is different from bike to bike. You can start with one size bigger. But there is more than one jet in these carbs. Then air jets too not just fuel. Takes time and patience.

[Rdltmc3]

Air/fuel screw on carb #2 seems to do nothing, but the bike runs perfect so I haven't put much time into it.

[CYBORG]

That might be the source of the backfire

[Rdltmc3]

slow idle jet correct? clogged usually or needs replacing?


If I was going to go in there and fix that what benefits or downfalls would i have by going up one size in the jets?

[CYBORG]

Clogged more then likely. Or the passage is clogged

[gregforesi]

These bikes are a bit touchy on the intake side, but not very touchy at all on the exhaust side. I'm going to vote for a clogged idle jet.
The idle jets are .35mm or .013". Any little piece of crap that breaks loose somewhere can clog them. Since your #2 idle adjustment screw isn't doing anything, I'd pull the rack, drain it, flip it over, pull out the #2 idle jet, clean it out with a piece of wire the appropriate diameter (i.e. anything that will fit it without hurting it) and then throw it back together.

You can get drills from places like Production Tool Supply that will allow you to open up a jet to the size you like.
If you think a jet is too large, you can solder it closed and then use the correct size drill to open it up to a smaller size.

Generally speaking - a lean condition can cause backfiring through the intake. A rich condition can cause backfiring through the exhaust.

Benefits or downfalls from jetting...
It's about the mix for the particular motor. Too big and you waste fuel and maybe foul plugs. Too lean can be a very bad thing. Hard to start is a minor problem. Running lean can also cause high heat in the combustion chamber and that can burn a hole in a piston.

The bottom line is that no matter how you jet it, or how you tune it, mine will always be faster.

[Rdltmc3]

The bottom line is that no matter how you jet it, or how you tune it, mine will always be faster.

Thanks, very inspiring.

[gregforesi]

Thank You. Always happy to provide inspiration so long as it doesn't interfere with that quote.

[Whiskerfish]

If your idle mix screw is not doing anything that needs fixed. I went up a size on the main fuel on my 75 and probably could go another yet. I am running a oil type air filter and a 4 into 1 headers with no back pressure. The engine felt like it was running out of gas at Wide Open when approaching the red line and the plugs matched that feeling. Got my jets from Sirusconic I believe. Short of a before and after dyno run there is no real way to quantify the value of the effort.

[robin1731]

Air/fuel screw on carb #2 seems to do nothing, but the bike runs perfect so I haven't put much time into it.

Do the other 3 make a difference if you move them? If so then the number 2 carb is where you need to direct your attention. If nothing changes when you turn a mix screw in or out 2 or 3 turns that idle circuit is most likely plugged. When set properly 1/2 turn will/should make a difference. But that can be hard for some to notice. So try 2 or 3 turns. That should be big. If you turn a mix screw all the way in and the engine doesn't want to die that circuit is plugged.

[Rdltmc3]

its only on #2

[gltriker]

Air/fuel screw on carb #2 seems to do nothing, but the bike runs perfect so I haven't put much time into it.

If it runs perfect..... leave it alone.