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

Just to add a little more insight into re-jetting.

I seem to find that people in countries that use an Ethanol blend of fuel, do not need to enlarge the main jets as much as countries that do not use Ethanol blend. It seems the Ethanol blend is harmful to performance and a decrease in jetting sizes is the end result. Also, different countries have different climates, so this will help to confuse things.

If the jetting is not right to begin with then re-jetting can be hard to know what setting is best for your particular climate, elevation, fuel quality and modifications made to your bike. In this situation you would be ''stabbing in the dark'' so to speak. It would be best to start with the standard jets sizes that YAMAHA brought the bike out with. This may not be the best for your particular climate etc., but should at least run nonetheless.

If you find that the standard YAMAHA sizes do not work, I would suggest that either the pilot or main circuit is blocked, diaphragm pump is faulty, or the float level is not correct. Although, this may not be the case in some circumstances.

If all of the above has been checked and it still has problems, see below.

Symptoms from idle to 1/3 throttle- Pilot Jet

First make sure that the fuel metering screw O-ring is not perished or damaged and the pilot circuit is not blocked.

Rich

Does it start without choke when cold?- if so a smaller pilot jet may be needed.

Turn your fuel metering screw all the way in- does it still idle?- if so a smaller pilot jet may be needed.

Does it run better with the fuel metering screw turned to 1.5 - 1 turns out?- if so a smaller pilot jet may be needed.

Does it splutter just off idle up until 1/3 throttle?- if so a smaller pilot jet may be needed.

Is all of the above symptoms accompanied by popping in the carb when the throttle is let off from 1/3 throttle to idle?- if so a smaller pilot jet may be needed.

Lean

Does it run better with the fuel metering screw at 3 - 3 1/2 turns out?

Is it hard to start without choke or does not idle without choke- if so a larger pilot jet may be needed.

Does it backfire in the carb when the throttle is applied from idle to 1/3 throttle- if so a larger pilot jet may be needed.

Does it get red at the header pipe when idling or 1/3 throttle- if so a larger pilot jet may be needed.

Symptoms from 3/4 throttle through to full throttle- Main Jet

Please note that some of the following symptoms can be from the wrong pilot jet installed, so if you have any trouble at idle to 1/3 throttle, see Symptoms from idle to 1/3 throttle- Pilot Jet. Also, not all sypmtoms will be present at once, so unless way out of specification,  you will not have all sypmtoms described in the topics below.

Lean symptoms

Choke helps at wide open throttle.

Header pipe gets red at the header pipe (even at night).

Backfires/pops in carb when throttle is applied quickly or from low revs to high.

Feels like it's running out of fuel (not to be confused with splutter-rich symptom).

Runs good but lacks power-slightly lean

A hot metallic smell coming from exhaust (extremely hard to tell).

Spark plug is clean but has a whitish deposit- extremely lean

Spark plug is very clean- lean

Rich symptoms

Starts up without choke (it should take quite a few turns to start when cold without choke usually).

Splutters until warm & then has flat spots.

Splutters at 3/4, unless the throttle is slowly rolled on.

Bike stalls easy.

Flat spots in carb.

Engine does not idle down fast enough when at roughly 4000 RPM & the throttle is let off.

Smells like un-burnt fuel.

Black smoke that has an unburnt fuel smell.

Hits the rev limiter too easily.

Sounds a bit like a two-stroke.

Carb pops when throttle is let off.

Black, soft,sooty deposit on plug.

Perfect

Bike runs great through all gears & idles smoothly.

Spark plug is a chocolate brown color.

Yeah I saw that someone is using a Mikuni #52.5 but they have done a lot more than just an air filter change. I would like to know what the best size conversion is myself, so if you find a better size conversion, let me know.

Try using a screwdriver bit that fits into a socket to loosen the screw. If you can't get it off, you can just take the slide out without removing the diaphragm. Just be careful not to force the slider too far.

If the screw is not stripped, you can use an impact driver to remove it. If you don't have one, I'm sure that your local dealer does, so it may be the best method of removal. It would not cost much to have it removed for you.

If you can explain your problem better, maybe I will have better advice on the problem.

Are there any other symptoms other than ''a bog''?

What sort of ''bog'' is it- Does it splutter, pop, backfire etc.?

Does it idle right?

It sounds lean from how you described your problem, but jumping to conclusions will not help.

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I know you said that you checked your accelerator pump is squirting properly when throttle is applied (and carb is full), but if unsure it doesn't hurt to check again.

If your float level is not right, it will cause the same problem as described also, so this should be checked also.

Try taking off your airbox side cover to see if it gets better or worst. This may help to diagnose the problem.

First thing to check in this situation is that your pilot curcuit or pilot air curcuit is not blocked.

There is little holes (air jets) at the airbox side of carb that runs through the inside of carb. The upper hole is for the pilot curcuit and goes through the side cover. The main jet air intake goes through the carb & to the main jet. These must be clean!

Also, make sure that O-ring on your fuel metering screw is not perished.

So I ordered a Mikuni n102/221 135 small round main jet as well as a vm28/486 42.5 pilot jet. They have not come in yet, however I just lowered the clip on my needle one groove. My bike not only had more power on top, but it smoothed out my idle a little bit as well. It still starves for fuel at idle, but it ran better on bottom. I find this weird since the needle should have no effect on idle...?