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

pavlos · Member

Tank gas cap leakage

TTRfan · Super Guru

I did a search and found an old post from 2013 that may (or may not) help:

"I took some time out of rebuilding my 325 engine to have a look at some spare caps. Using a short length of clear pipe I was able to easily blow down through all 3. There was no feeling of restriction and I could get a lot more air through than even a big jet could pass petrol.

I honestly can't see why it would be necessary to drill the cap as the other two holes will still be smaller confuse But I am happy to be persuaded wink

Brian"

Blue petrol cap dismantled 001.jpg

Blue petrol cap dismantled 002.jpg

pavlos · Member

I'll give a try to disassemble the original yamaha cap with keylock barel.
Maybe it has similar design for rebuild.

TTRfan · Super Guru

Sorry - I didn't spot that yours was a metal tank model and therefore has a different cap.

I haven't taken apart one of those caps but maybe someone else has?

Brian

mossproof · Super Guru

Here in the stickys

ttr250.activeboard.com/t51137706/locking-fuel-cap-key-barrel-removal/

Not quite what you're after, but it shows how it all fits together, including where the breather hole is.

Have fun - it's one job you can do in the light, warm and dry indoors

-- Edited by mossproof on Tuesday 26th of January 2021 09:25:19 PM

PurpleWheels · Veteran Member

I have just completed a disassembly, cleaning and reassembly of a metal tank fuel cap with a key barrel, as described in the sticky above, except I was not able to remove the metal ring around the barrel for full access. I was able to spray brake cleaner and WD40 through the keyhole and from the opposite side until it came out clean and barrel turned round smoothly. I also thoroughly cleaned all the other parts including the small air passages. Unfortunately I didn't take any pictures this time, but the sticky above has good photos.

The reason for this was that my engine would occasionally start stuttering and finally stop as if it ran out of petrol while riding. It would only start and run again with choke fully out and revving the engine, and even then with difficulty. Typical symptoms of a clogged jet I thought, and due to this I have opened and cleaned the carburettor 3 times without ever finding any impurities in the fuel or in any of the jets or in the passages of the carburettor body. I started getting frustrated with the bike. This would happen with the engine hot, usually after riding at higher speeds or uphill.

A few days ago a mechanic friend suggested that this could well be due to clogged air holes in the fuel cap, and at disassembly I did find dirt and rust that could well have limited or blocked the air passage in the cap. I haven't had an opportunity to go for a longer ride yet, so this theory is yet to be confirmed. Should it happen again I'll also just unscrew the cap to let air in, in case the fuel cap would still be blocking the air. and creating vacuum in the tank.

/Simo

pavlos · Member

Well, in my situation gas fumes found wayout by the keylock barel preventing any engine stuttering. So following prompting from Brian and mossproof, i've disassembled the metal fuel tank cap and clean up the air passage paths and metal bead.

Total vent compartment has one path for gas evaporation and one path to filter out (probably ) the intake air into tank.

The path for gas evaporation has enclosed a metal bead and a copper washer cap. Also i found that the gasket, between the handcap & ventilation parts, was a dirty and rigid (probably by the long-term exposure in warm climate here..) so i soaked this in wd40 to come back its elasticity.

Finally, the last part that need to replace is the o-ring between the keylock barel and the washer (that is showing in mossproof's stickie post pictures).

Thank you all guys!!!

mossproof · Super Guru

Hope this solves your engine running problem! Thanks for the extra pictures.