Yamaha TRX850 Enthusiasts Forum

My bike is backfiring pretty badly on the right cylinder when it's warm, missing some performance down low & at about 6K RPM (bit of a hesitation), & there is carbon in r/h side inlet.

The back (forward?) firing is pretty constant, every second or so when the bike is warm & idling for a little bit. When you come off the throttle to idle, it takes maybe ten or 20 seconds to start backfiring.

Cleaned & balanced the carbs to no avail, I haven't changed plugs yet, but I can't see the plugs being an issue... Coils should be fine, one is a few months old & the other was tested at the same time...

My engine is new from the wreckers, I think I've done about 3000 ks since I put it in, & I did the valves before I put the engine in, (all were fine except for one) so I wasn't expecting to have to do the valves for a long time! I guess sitting for god know how many years at the wreckers wasn't great for it...

I'm assuming I need to do the valves again, any thoughts?

someone recently had a similar problem finding maybe the pilot jet was blocked.(may have been Get fuk't Bob racing) There is a pilot air jet maybe this could be the one not giving you the mix you want and I think it's accessable from the air intake side of the carbs too from memory

M.V. wrote:My bike is backfiring pretty badly on the right cylinder when it's warm, missing some performance down low & at about 6K RPM (bit of a hesitation), & there is carbon in r/h side inlet.

The back (forward?) firing is pretty constant, every second or so when the bike is warm & idling for a little bit. When you come off the throttle to idle, it takes maybe ten or 20 seconds to start backfiring.

Cleaned & balanced the carbs to no avail, I haven't changed plugs yet, but I can't see the plugs being an issue... Coils should be fine, one is a few months old & the other was tested at the same time...

My engine is new from the wreckers, I think I've done about 3000 ks since I put it in, & I did the valves before I put the engine in, (all were fine except for one) so I wasn't expecting to have to do the valves for a long time! I guess sitting for god know how many years at the wreckers wasn't great for it...

I'm assuming I need to do the valves again, any thoughts?

Sounds to me like a forward fire due to really fat mixture (as it´s when it´s get hot) that fires when meating air and hot downpipe.

cobbadiggabuddyblooo wrote: someone recently had a similar problem finding maybe the pilot jet was blocked.(may have been Get fuk't Bob racing) There is a pilot air jet maybe this could be the one not giving you the mix you want and I think it's accessable from the air intake side of the carbs too from memory

I just stripped & cleaned the carbs on sunday, that was my first thought too.

Killerwhale wrote:Sounds to me like a forward fire due to really fat mixture (as it´s when it´s get hot) that fires when meating air and hot downpipe.

How fat? Running a K&N with standard jetting, 2.5 turns out on the pilot, float heights 15mm...

I guess I could try cooling down the down pipe & see if it goes away...

Edit: I'd like to avoid doing the valves if I can help it, it was easy enough when the engine was out, but in the bike it's a different story!

Carbon deposits in the inlet tract you say, surely must indicate charge getting back past the inlet valve seats?
Do a compression test to be sure.

Mincehead wrote:Carbon deposits in the inlet tract you say, surely must indicate charge getting back past the inlet valve seats?
Do a compression test to be sure.

One should always read everything twice if you missed something important!!
...and they enlongate when hot wich should explain it comes after a while..