Yamaha TRX850 Enthusiasts Forum

Hi all,

Since I'm having backfires in my exhaust I would like to check my carburator for any flaws which may cause these backfires. Therefore I'm looking for a manual of the Mikuni carburator BDST 38/2. I've surfed the internet and I found several Mikuni manuals for all kind of carburators but none for mine.

So I'm wondering is there a manual available for the BDST 38/2 and if so where can I get a copy?

punica78 wrote:Hi all,

Since I'm having backfires in my exhaust I would like to check my carburator for any flaws which may cause these backfires. Therefore I'm looking for a manual of the Mikuni carburator BDST 38/2. I've surfed the internet and I found several Mikuni manuals for all kind of carburators but none for mine.

So I'm wondering is there a manual available for the BDST 38/2 and if so where can I get a copy?

When fires in the exhaust it´s actually called front fires...back goes up the carbs...ANYWAY...
Isn´t the carbs pretty well covered in the servicemanual in the downloads??

Often it´s unburnt fuel wich ignite when going into warm headers wich having fresh air....you´ve checked for any leaks in the headers?? On the downside when trying to check what it is also a lean mixture in pilot will bang when closing throttle...

@Killerwhale: your remark on bangs when closing throttle is spot on. Any tips on how I best can check the lean mixture in the pilot? I've already checked most logical parts for leaks where false air could enter the engine like holes in the downpipes.

On the side: so frontfires is when I have bangs in my exhaust and when I would have had backfires I should have had bangs in my carburator?

punica78 wrote:@Killerwhale:
On the side: so frontfires is when I have bangs in my exhaust and when I would have had backfires I should have had bangs in my carburator?

Most people call all bangs backfires so don´t hang up on that....

I would have tried bigger pilot jets...but as i have a dude with reasonable prices on dyno runs i would actually have it up on the dyno measuring the air/fuel ratio.

But....my bike was fat as hell on the fuelscrews so i went down a size in pilotjets and it started popping as hell on close throttle so i adjusted that with the fuelscrews and then it was spot on with no bangs....mind you i have FCR´s...

If its poppimg and banging on the over run then it's probably too rich

Has your fuel comsumption increased ? Worn emulsion tubes?

try lifting the needle a notch and see if that helps

wont lifting the needle make it richer ??

@Phuk72: Fuel consumption has not increased. Currently I can do 100km with 6,5 liters of fuel. I was also thinking that problem may lay in the emulsion tubes or the gasneedles but I don't have any reference point on how a healthy emulsion or gasneedle should look like. Hence my search for the manual.

punica78 wrote:@Phuk72: Fuel consumption has not increased. Currently I can 100km with 6,5 liters of fuel. I was also thinking that problem may lay in the emulsion tubes or the gasneedles but I don't have any reference point on how a healthy emulsion or gasneedle should look like. Hence my search for the manual.

Pop at throttle off would tell me that it´s lean anyway...

Killerwhale wrote:

punica78 wrote:@Phuk72: Fuel consumption has not increased. Currently I can 100km with 6,5 liters of fuel. I was also thinking that problem may lay in the emulsion tubes or the gasneedles but I don't have any reference point on how a healthy emulsion or gasneedle should look like. Hence my search for the manual.

Pop at throttle off would tell me that it´s lean anyway...

So the first thing that I should do is correct the mixture by lifting up the needle a notch?

punica78 wrote:

Killerwhale wrote:

punica78 wrote:@Phuk72: Fuel consumption has not increased. Currently I can 100km with 6,5 liters of fuel. I was also thinking that problem may lay in the emulsion tubes or the gasneedles but I don't have any reference point on how a healthy emulsion or gasneedle should look like. Hence my search for the manual.

Pop at throttle off would tell me that it´s lean anyway...

So the first thing that I should do is correct the mixture by lifting up the needle a notch?

I would try the fuelscrews at first....no i lie....i would do the dyno at first...
But screw them in recording the turns....if at 1,5 turn out....try 2 turns....if it fix problem good.....if not and still pops go leaner pilot and go back on the fuel screw. If under 1 turn and over 2 turns you should check the pilot...

steve speed wrote:wont lifting the needle make it richer ??

good point well made

Killerwhale wrote: I would try the fuelscrews at first....no i lie....i would do the dyno at first...
But screw them in recording the turns....if at 1,5 turn out....try 2 turns....if it fix problem good.....if not and still pops go leaner pilot and go back on the fuel screw. If under 1 turn and over 2 turns you should check the pilot...

Might help, but remember the screws are for small openings, needles are for middle and mains for full. Check all vacuum hoses also!