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Allan G, Falcon_52, mondtster, Oil Changer, splash
Total Likes: 7
Original Post (Thread Starter)
#851875 06/18/2021 9:56 PM
by Richrd
Richrd
I've been working on a well worn '71 t100r that should have been put out of it's misery. But now after new bores, pistons, valves and guides, it starts one kick and feels pretty good.

except for backfiring on decel. Ive checked for air leaks and made sure exhaust fittings are tight, but it still does it.

what am I'm missing?
Liked Replies
#851891 Jun 19th a 12:59 AM
by Hillbilly bike
Hillbilly bike
Mikuni technical info say the popping is from a sudden lean mixture when the throttle is closed at higher rpm. They state some popping is normal is normal for a highly tuned engine. Many Japanese carb bikes have a decel valve that as far as I understand adds a little fuel to limit the noise....
3 members like this
#853011 Jul 4th a 02:46 PM
by triton thrasher
triton thrasher
There is always unburnt fuel in the exhaust. There is also heat.

Combustion (banging) does not usually happen in the exhaust of a well-tuned engine because there is not enough oxygen present.

An air leak into the exhaust near the valve allows enough oxygen in, to support combustion. The effect is loud banging on over-run.

A weak pilot mixture can have a similar, but usually not so loud, effect. The oxygen than was drawn in on the inlet stroke has not all been used.

Needle position shouldn’t have much, if any, effect with the throttle closed.
2 members like this
#851945 Jun 19th a 11:47 PM
by tiger_cub
tiger_cub
A sign of an air leak into the exhaust, I would double check all your clamps and gaps especially at the head.
1 member likes this
#853186 Jul 7th a 02:19 AM
by desco
desco
Splash,
Call these guys. They did my 72. Beautiful work, very reasonable and great customer service;
https://www.shopevengineering.com/services/12315/top-end-engine-rebuild
Call them. It was awhile ago.
1 member likes this
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