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The TTR250 FAQ web pages -> TTR250 -> Carb stuff -> Engine stops on hard accelleration
Post Info TOPIC: Engine stops on hard accelleration
mmwain


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Engine stops on hard accelleration
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Can't find this one anywhere in the 5 pages of carb posts so here goes...

Bike has a very intermittent fault, will run perfectly for a long time then suddenly stops completely or misfires on mid/high throttle.  The problem started after stripping the bike for frame paint and valve guide seals, but didn't do anything to the carb, just took it off and put it back on a couple of weeks later.  Sometimes happens when coming off the throttle downhill to a junction, sometimes when accelerating hard from a junction, in which case it will run perfectly till you get to the last couple of gears - then just stops pretty instantly and coasting in gear gets nothing, just like it's run out of petrol.  Put the choke on and it runs again (badly, but will maintain around 50mph).  Choke off, instantly dies, choke on it runs again.  Sometimes when it's  not so bad and starts to misfire as if it's running out of petrol, giving lots of throttle gets a good response initially, indicating that a shot of fuel is coming out of the accelerator pump, but that doesn't last, it may continue running really badly with choke improving it and reducing the throttle will sometimes make it run smoother and after a few moments there you can gradually increase throttle and it might go back to misfire or might continue accelerating back to normal running!  If you stop and wait a couple of seconds the bike will then fire up and idle perfectly but dies on throttle, a few more seconds and it will drive off as if nothing happened!  It's not connected to tank fuel level or the breather (as undoing the filler cap doesn't fix it).  Reserve makes no difference.  I flushed the tank and tap, got a small amount of detritis out but made no difference and there is no restriction in flow down to the carb.  Stripped the carb, all the jets and floats out, all looked super clean, no apparent wear, float bowl looked like new, accel pump confirmed working.  It ran good for 200 miles after that, then started again.  Often goes 50 miles without an issue, then suddenly does it for no apparent reason and tends to do it a few more times once it's happened before clearing up and running nicely for another 50 miles or so.  A new air filter has made no difference, nor does fuel from other sources.  The inlet rubber is fine and no problem with the sealing on the inlet/outlet of the carb.  There's no water in the fuel/float bowl and it's not connected to riding through water, I often ford with the carb under water with no problems.

The only thing that I can think of that could cause this is the float valve intermittently sticking SHUT in a way that it is somehow freed and allowed to fall open by stopping the engine - but I just can't see how this can happen - there's quite a head of fuel pushing it open! It looked in super condition when I stripped it and did not appear to be in any way sticky, same for the floats.

Has anyone else had this problem and knows what the cause is?

Thanks in advance!

Mike



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mossproof


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That's a puzzler! Agreed it sounds like fuel starvation from the float valve. Did you get as far in as the float valve gauze filter on your stripdown? (Really stupid place to put it Mr Teikei!)
Is the interior of the fuel pipe all intact? If there's some delamination inside it could be closing off the flow sometimes.
When it happens, if you undo the floatbowl drain screw does fuel flow out well? If not, that would indicate the supply to the bowl is being interrupted, rather than the supply to the piston via the jets and pumper.
Good luck,
Simon.

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mmwain


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That's a puzzler! Agreed it sounds like fuel starvation from the float valve. Did you get as far in as the float valve gauze filter on your stripdown? (Really stupid place to put it Mr Teikei!)
**** I don't believe I did as I had no idea it existed!  It was on a Welsh campsite between getting back from a rideout on choke for last mile or so and going for a curry while being eaten alive by mossies!  I was fully expecting to find crap in the main jet and bowl (something being occasionally sucked over the main jet would give this symptom I think) and totally surprised to find it all as clean as new.  By strange coincidence the guy in the next tent was also having problems and also stripped his KTMs carb which was absolutely full of crap.  He cleaned it all out and next day is made no difference at all whereas I'd found nothing at all and my bike ran faultlessly all day!  Anyway, hopefully that's the vital piece of missing info so I'll concentrate on back flushing that this w/e and see what comes out.  It may be that messing around with it there, including turning it upside down etc, was enough to move something away that eventually got washed back down to where it had previously been. ***

Is the interior of the fuel pipe all intact? If there's some delamination inside it could be closing off the flow sometimes.
*** I don't think that's the problem, not that sort of tube and blown through it a few times from both ends.  Pretty sure I replaced it as well during the course of the problem as it was on and off so many times it wouldn't seal any more. ***

When it happens, if you undo the floatbowl drain screw does fuel flow out well? If not, that would indicate the supply to the bowl is being interrupted, rather than the supply to the piston via the jets and pumper.
*** I've not done that as it seems quite disrespectful to the rest of the ride to start pfaffing around getting tools out and doing diagnosis when you know that stopping the engine for about 5s will cure it!  Which may well mean that after 5s the fuel would come out normally anyway!  I'm still struggling to see how that bit fits with the inlet filter being partially blocked though.  How would stopping the engine make that unblock?  Unless it somehow results in a little bit of blow-back which somehow gets just enough positive pressure into the carb to blow back up the fuel line just enough to move the rubbish off the filter?   Maybe a long road ride prepped ready to try that if it happens, but I'll try plan A above first, watch this space, results should be in by Monday evening!  ****


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mossproof


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Think your bike caught something nasty from it's orange neighbour. I've heard they're contagious!
Pics of float valve seat/filter here:
ttr250.com/Carb/TTR250_carburettor_strip_and_rebuild_guide.htm

If there's something moving about in there it might be restricting the bowl refilling, then you get a bowlfull of riding before she conks again, or the crud moves and the bowl refills ok. Clutching at straws a bit I know. Good luck!

PS Sorry if I'm teaching granny to suck eggs, but take care with the float pin! Removal with Halfords automatic centre punch (wound down to low force/impact) is a good idea - the float support posts are really weak.

Breakdowns are a good opportunity to take the p*ss. Your mates should be glad of the opportunity! When my brother had some problems with his XL600, one of the guys said "Sh*t in the carb". Brother's reply was "A kind offer mate, thanks, but I don't think it will help!" Well, we were giggling about it for hours after anyway...

(Turned out to be a valve seat dropped. No way crapping down the air box snorkle was going to help that!!)



-- Edited by mossproof on Thursday 23rd of August 2018 10:52:02 PM

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mmwain


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Read everything I could find on the carb, got the pin out no problems by twisting the out end with vice grips (like moles but better) to free it slightly, then compressing the in end against the in post with pliers.  Once it moved a little it pulled out from the out end quite easily with the vice grips doing a twisty/pulling motion.  Investigated float and it was in perfect condition, spring load working fine, no ring and couldn't make it jam in any way.  Despite warnings it would the difficult the housing came out quite easily with a little twisting before pulling and there was quite a bit of rubbish above the filter, mix of rubbery bits that don't match the current feed tube so must have been there for ages before the engine rebuild, general dirt and a small stone about 2mm.  I gave it a good flushing with petrol down from the tank into filter material but that was all I got.  My gut feeling is that it wasn't enough to cause a full blockage but only time will tell, only done about 16 miles so far in little trips to work.  Hopefully doing a full day up the Usk Valley on Sunday so will see what happens then! Also took the opportunity to make sure all the drains were properly clear and fix the breather outlet that was weeping after being disturbed on the engine strip, just needed the hardened end cutting off.



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mossproof


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Any luck Mike?

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ttboof


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Fingers crossed its fixed 

Interesting selection of crud in the way 



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mmwain


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Now done 3 full ride outs including a 150 miler up the Fosse Way, some fairly severe rock climbing stuff up the Usk Valley and the Bristol to Bath run (5.5hrs so not the most direct route!!!) plus regular commuting and had no further problems so think it's cracked!  

Thanks Mossproof!

Here's a picture of it running perfectly!  Someone commented on how perfectly it runs at idle last week.  No tacho but it's clearly ticking over very slowly and still not missing a beat.   Shame it uses so much oil though!



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mossproof


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Excellent news!

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ttboof


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Sounds like a good result 



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