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Yosh-Mikuni TMR carb backfire on decel

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DRZ400S year 2003

Ex Japan

Yosh-Mikuni TMR flatslide carb with pumper

27.5 pilot jet

Fuel screw 2.5 turns out

FMF powercore exhaust system

Air Injection system (PAIR) disconnected and ports plugged

Location -Cambodia


Bike backfires and sometimes stalls on decel when ridden in rain (stalling not as bad now that PAIR is disconnected)

Being a tropical climate, only slight temp drop when its raining.

When its not raining, bike now runs perfect through all circuits, ie pilot, needle jet, and MJ

The confusing thing is, when its raining thats more humidity/water, thus less O2 , bike should run rich ?

However the slight temp drop means denser air,more O2, bike should run lean?

So which is the overiding factor, humidity of temp drop?

I'm guessing I need to go richer on the pilot

As pilot jets must be ordered from overseas, is it worth a try to raise the needle by one notch to give a richer mixture in the PJ/NJ circuit overlap

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Is there any science to calculating "how much" bigger I should go on the pilot jet.

I now have 27.5

Next two sizes up are 30 and 32.5 according to Sudco catalogue.

The bike idles best at 2 turns out on fuel mixture screw which I believe is within spec.

Will going bigger on the pilot jet to cure the backfire on decel create a problem with setting the idle mixture.

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a slight pop on decel is perfectly normal and a sign of proper mix.heavy explosions are not normal.

if the bike idles correctly at 2 turns the pilot jet is fine.

make sure your pipe is properly sealed.

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No exhaust leaks. Had the system apart, repacked the muffler, applied high temp sealant to joints, Head manifold gasket OK.

However, the FMF powercore with 1 1/4" spark arrestor is fairly loud and I assume provides very little back pressure.

The weather is fine now, around 32 C, and backfire (loud bangs) is reduced to occaisional with popping on decel all the time.

Peak idle on a hot day is achieved with the fuel mixture screw between 1 1/4 and 1 3/4 turns out.

Bike still idles adequately at 2.5 turns out with backfire slightly reduced from the 1 3/4 turns out setting

The loud backfiring on decel problem is still there on cool days/rain and there is probably not much I can do it about it .

I am starting to think that the combination Yosh-Mikuni TMR flat slide carb and FMF powercore exhaust is a tempermental setup when it comes to weather changes.

I suspect a standard exhaust system would cure the problem but I'm not prepared to give away the horsepower.

You may remember a few weeks ago I had a hanging idle problem

FYI this turned out to be excessive play/wear in the linkage that controls the throttle valve.

Shutting of the throttle did not move the throttle valve all the way back to closed, relying on gravity or blipping the throttle to move the valve down that last 1mm

Got a machinist to repair the throttle linkage and hanging idle disappeared.

However, with the throttle valve now closing off properly, the backfiring on decel got worse.

It seems that under the condition of rapid decel I could use a bigger pilot jet /richer pilot circuit but for all other situations the current setting is correct.

Would be interested to hear any other suggestions

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