Backfiring worse after MAC 4-1 install

[ccs]

Hello Everybody;

Well... I got my MAC 4-1 header installed (did not connect the muffler/baffle as I like the deep throaty sound).

My bike (an 82' GS850G) had started to backfire a little with the stock exhaust on it -- didn't worry about it cause I planned on putting the MAC 4-1 on it.

So after I installed the MAC 4-1 the backfiring is way worse and very loud (sounds like a gunshot quite a bit).

There's not much backfiring at idle, but some on take off, and more during acceleration, and pretty bad while just running down the road.

I know I need to adjust the idle mixture screws and re-synch the carbs, but do I need to re-jet?

If so, the specs I saw say main jet is a #115. I want to order the jets before I pull the carbs so I'm thinking about getting 120 or 125's.

Also, do any other jets need to be replaced?

Any advice would be appreciated.

and... lastly can I ride the bike? (Sure don't want to blow the engine, but it's my only transportation and I need to ride 25 miles and back several times durnig the next week or two -- don't know when I'll get the time to do all this stuff.)

TIA.

Charles...

[Guest]

i think you're not likely to have to re-jet if it's only the exhaust you changed and the rest is stock, so don't pull the carbs appart just yet
try getting the mixture and sync right first

[Tom MLC]

Sounds like you're running lean. You should not need to re-jet just for the header. Are you running pods? Open air box? Those will contribute to more airflow & will probably require a re-jet.

Definately try to adjust each mixture screw to achieve the "best rpm". Then a sync. If these things don't help, you may have to re-build the carbs. You likely have a few blocked pasages restricting the fuel flow. Also causing a lean condition.

It is not healthy to run your bike this way. You should try to resolve this asap. Your bike will be happier, and so will your neighbors!

[earlfor]

Generally, Mac exhausts do not need rejetting. However, that is if they are installed as intended. Taking out the baffles voids that. If the bike was already popping with the stock exhaust, certainly installing a Mac without baffles will make it worse. If you cant rejet immediately, the best thing to do until you can is at least put the baffles in it. Also, adjusting the idle mixture literally adjusts the IDLE mixture. It will do NOTHING for a lean main jet condition. I would not be inclined to run the bike if its "shotgun" lean.

E.

[nabrams]

Earl is right on the money. I've got the MAC 4-into-1 on my K - came with the bike. The seal between the pipes and the cylidner head was not perfect (slight leaks) AND my baffle was not sealing properly. Lean and backfiring in mid-range was the result. Properly sealing that stuff up made a huge difference.
No re-jet required in the final analysis (although I did play with larger main jets at first to try to compensate - big mistake).

[Guest]

Don't run the bike without the baffle because the lean condition could harm your bike. If you had popping with the stock exhaust the exhaust wasn't at fault but the carbs or motor. I would check valve clearance and clean the carbs. Make sure the airbox is sealed along with the intake boots.