A forum for owners of Yamaha TTR250 trail and enduro bikes!

Alf c h · Member

Misfire cruising at 1/2 throttle and higher

TTRfan · Super Guru

Unlikely but maybe the cam chain has jumped a tooth on the camshaft sprocket? It would give the same symptoms.

Easy enough to check. Tank off, make sure the TTR is clean underneath (so no dirt will fall into the engine), remove the cam cover, put engine at TDC and check the lines on the cam gears line up with the head as they should. There are some photos in the "removing cylinder head" sticky biggrin

Brian

andypwc · Veteran Member

This sounds familiar

Jak · Veteran Member

If you haven't changed the plug cap try that, check the connections and the earth to the coil. Change the coil (coils tend to cause a misfire after they have got hot if they are faulty). I had similar problem on mine but it was not always there, which I would assume it would be if the cam timing was out. I changed the coil and that seems to have cured it. I had a spare coil of something else in the garage but I think they are pretty cheap from Brian.
Cheers Jak

TTRfan · Super Guru

andypwc wrote:
This sounds familiar

Thanks for the reminder Andy!

Andy had the same problem and a thorough clean of the carb didn't sort it out. Swapping coils had no effect. Cam timing checked and all perfect. In order to rule the carb in or out as the cause, we swapped carbs and, success, everything worked fine. There was nothing obviously wrong with Andy's carb so we swapped the emulsion tube and main jet from my "good" carb into Andy's "bad" carb and, hey ho, misfire gone!

Still not sure why but the test will be if my carb will work with Andy's emulsion tube and main jet hmm

Brian

PlumasDude · Senior Member

Maybe replace the jets rather than clean em.

Alf c h · Member

Since the comments from Brian about the emulsifier tube I contacted the dealer and in discusion it came to light that the
ultrasonic cleaning was never done due to the pristine condition of all the parts.

I have since examined the emulsifier tube, main jet, and pilot jet. All in excellent condition.

I'll wait here a little longer for more suggestions before I make the decision to take the bike to the dealer.

I guess they can do electrical checks with instruments rather than trial and error with changing parts?
Ref comment from Jak regarding the spark plug cap. The plug lead is moulded to the coil and cap. i.e. all in one piece.

Thanks to all who have shown interest so far.

It is still a mystery that this misfire was sudden. No hint of gradual deteriation in performance.
As stated in my original post. No running concerns since new in 2002.

Alf

ttboof · Super Guru

Time for the might be a cause suggestion

if the kill switch is full of dirt and moisture it can at times track across and cause problems.

If your runnung a side stand switch and the springs letting the stand flop around it may intermitenty cause strife (very common on dr400s )

A heavy set of keys can wear the ign switch but tend to cause probs from first start

as discussed a carby can look clean but unless you get right in theres no way of tellng for sure

these are just things ive come across on varoius bikes over the years

PlumasDude · Senior Member

When you finally do get this figured out let us know what fixed it. Curious that it gets rough right about where your main jet would be primary, and that the jets haven't actually been touched in the carb clean, so what might be the most likely cause may not have been eliminated. Any chance the needle clip moved?

Alf c h · Member

Problem Resolved.

I really wanted to isolate cause between Fuel and Electrical.

I managed to get the sense of the misfire whilst stationary at home in the garage.

Took a chance that the high revs was not an annoyance to neighbours.

As mentioned before the bike starts ok and warmed up.I then increased the revs by the adjusting the throttle stop to a point where the hesitation began.

I then pulled the choke out a small amount [about 1.5mm]. The engine ran smoothly. This points to fuel?

Being disappointed about the dealer not carrying out an Ultrasonic bath procedure I found that Ron, a colleague in the motorcycle club had a bath and volunteered to have a go. Between us we removed the jets [all visually clear] and a small diaphragm/needle assembly. Put the carb body and the separate jets in the bath and set it running. Ron advised that 5 mins. would be sufficient. For those who are not familiar, the bath fluid consists of water and some additive [like strong washing up liquid].

Ron restores bikes and cars so has good experience. After the bath, the carb body was blown through with compressed air to clear the passageways etc...

At home, let carb body dry out on radiator overnight. Reassemble all parts and fitted to bike. Tested and misfire no more!!.

Ron advised to let carb run dry after each future run.

Of course it’s not known the exact location where the fuel restriction occurred. The tip to run the carb dry was to protect from Ethanol solidifying.

My theory is that we in the club had trail rides about every 2/3 weeks and I had never let the carb run dry.

About 9 months before the problem the trail rides stopped and long periods between starting the bike may have been enough to let Ethanol solidify and cause the problem???