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

IdahoGuy · Member

Weird Sputtering Problem

Lin · Senior Member

Sounds like not enough fuel is reaching the carb. Maybe strip the fuel shut-off tap on the tank, check for anything trapped there. I understand some TTRs have a fuel strainer gauze behind the needle valve, maybe blocked.

One other thing - later TTRs have a restrictor in the primary exhaust pipe where it joins the muffler. Not sure yours will though, however we have experience of exactly the same symptom as you describe. It was instantly cured by removing the baffle.

IdahoGuy · Member

Lin, I replaced the screen behind the needle valve as part of the set previously. Today, after reading your reply, I removed the shutoff valve and found that the screen on the Reserve side was broken off. The local Yamaha dealer had a replacement valve in stock, so I replaced it and still have the same problem.

One of the guys at the parts counter at the dealer suggested that the problem may be caused by the accelerator pump in the carb. Has anyone dealt with changing one of those out?

TTRfan · Super Guru

IdahoGuy wrote:
Lin, I replaced the screen behind the needle valve as part of the set previously. Today, after reading your reply, I removed the shutoff valve and found that the screen on the Reserve side was broken off. The local Yamaha dealer had a replacement valve in stock, so I replaced it and still have the same problem.
One of the guys at the parts counter at the dealer suggested that the problem may be caused by the accelerator pump in the carb. Has anyone dealt with changing one of those out?

The part that can fail is no 38 on the pic below. The diaghragm can wear through and leak. Easy to change - just held in by two screws. Worth changing the two O rings at the same time and cleaning it all up.

Brian

carb fiche.jpg

IdahoGuy · Member

I pulled the accelerator pump off and sure enough, the diaphragm looked like it was about to start flaking into chunks and was also very dirty. Guess I shoudl have expected that from buying a bike that had been left sitting for over a year. I ordered a new part and new o-rings from the dealer for about $30, and once they come in I'll let you know how it turns out.

IdahoGuy · Member

I installed the new accelerator pump and after some cleaning on the nozzle that supplies the fuel I was able to get that working. However, even after tuning the fuel mixture to the "best running" point, it still coughs and sputters from about 1/4 to a little past 1/2 throttle. Low end and high end seem to run fairly smooth, but there is very little power through that range as well. Anyone have any more ideas? I've worked a lot on larger engines for cars and trucks and most of my small engine experience to this point had more to do with lawn mowers and things of that sort, so this sort of problem has me stumped.

Lin · Senior Member

Remove the carb. Remove the main jet. Unscrew the emulsion tube (brass tube the mainjet screws into) It has very tiny holes drilled in it's wall. You said the bike had been sitting for a year. There is a chance they are covered in corrosion.

-- Edited by Lin on Thursday 25th of August 2011 06:55:56 AM

Dullthud · Veteran Member

Bent needle might cause that too.

Andy

IdahoGuy · Member

I'm pulling my hair out trying to figure this thing out. I've adjusted the clip on the needle, and still can't get things to run right. The holes in the main jet nozzle looked clear, but I cleaned them all out just to make sure there wasn't something that I couldn't see very well. Still the same result, although adjusting the needle clip seems to change the range of throttle that results in the sputtering.

I'm thinking I should check the float level, but the steps in the manual require a special tool and more removing and reinstalling of the carb if adjustments are needed. Can anyone give me some details on checking the float height with the carb removed and no special fuel level guage?

Lin · Senior Member

Fuel Level. You can make that "special tool" by just using a length of clear plastic tube.

Float Height. The manual says 1.04~1.08 inch (26.5~27.5mm) I found the bike responded better with the float height at 25.0mm (1.00 inch)

For what it's worth, checking the fuel level with a plastic tube is a whole lot easier than pulling the damn carb off to check the float height. If the fuel level is within spec (or slightly higher) you know float height is not the problem.

IdahoGuy · Member

It runs!! I checked the float, cleaned out all of the vent hoses and finally completely disassembled the carb and soaked it in solvent to clean everything out really well. I'm not sure what was going on in the carb, but the solvent soak seems to have fixed the problem.

I would like to say thanks to everyone for throwing out suggestions. It actually helped a lot and the new suggestions kept me from giving up.

Camoracer34 · Member

Sounds like the same problem I'm having. Tried nearly everything. Jets are clear, clened carb like 7 times. I'm at a loss. Maybe a good soak will wor for me.