still backfiring

[Guest]

Ok, after the carb. rebuild-new o-rings. valve adjust., carb sync.,mixture screws at 2 1/2-3, sealed airbox, new airfilter, one step up on main jets (Mac exhust, stock airbox) bike has more power than ever and is running well, no bogging or anything like that. The only problem is although it is backfiring less (alot), it still backfires when letting off throttle in 4th gear and some in 3rd. My health hasn't allowed me to ride much and do a plug chop. I did pull the plugs and they all seemed somewhat light black, and not sooty. I was wondering if I should shim the needle jets? Any opinions much welcome! terrylee

[niclpnut]

make sure there are no gaps in the connection between the muffler/midpipe, and the header collector, and that the exhaust bolts to the head are nice and snug.

Possibly another 1/4 to 1/2 turn out on the mixture screws.

Black on the insulator or on the base ring of the plug?

Nic

[OldVet66]

I know the VM's require the clip on the needles to be dropped one notch to raise the needles on the 750's for four into one pipes, stock air box, not sure about CV's on the 850 but it makes sense that it would want to be a little richer on the needles.

[terrylee]

Did check the exhaust and tightened headers, also it is a four into 2 system. Will turn screws alittle. thanks guys! terrylee

[OldVet66]

Four into two should be basically stock settings I would think unless the Macs are more free flowing. Have you replaced boots and O-rings on the engine side or checked for proper seal on the air box boots?

[terrylee]

Got three bikes so can't remember if I did replace o-rings in carb holders, think I will just order some and replace anyway to be sure. Mac is pretty free flowing it seems to me. Did turn screws up about a 1/4 turns and seemed to help alittle. Rode wife on bike this last weekend and she said she didn't really notice it very much, but I quess I just listen for it and worry. terrylee