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Bike wont idle, will start with revs held, runs rough, tried everything.

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Hey everyone!

So! i've been lurking here for ages, reading just about everything about my bike for the past year or so, but alas, i am stuck. (it was about time i signed up and said hi anyways!)

I feel like i've tried just about everything so i am at your mercy! it's a long one so i apologize, and it was my first experience really working on the bike!


So to start at the beginning...

Around November (the last time i rode it) the bike was starting to sound a little rough. Sounded a bit like knocking on the engine, but otherwise started first try and had no visible exhaust smoke etc...

Put the bike away for winter storage. Got the bike out of winter storage, charged the battery, cleaned and lubed everything, fresh fuel, fresh engine oil. Bike wont start.

Replaced the spark plug, no joy.

As the bike would start with revs but have no idle, i thought maybe it could be the idle jet in the carb, so stripped it down, cleaned everything, blew compressed air through everything, reset everything to factory settings (including float height) as per mr haynes. put it back on the bike. still the same problem. (all jets are completely clear, can see through them)

Then i thought it could be an air flow problem / leakage so i sprayed some wd40 around all the rubbers, hoping to find some bubbling or to hear the engine rev higher. absolutely nothing, all rubbers look and seem fine. (replaced the air filter with a nice new one too, let it soak in oil then air dried for 4 hours)

By now in my mind i've eliminated carb, and air flow, so i check the fuel pipe / flow, all seems fine. checked the oil filter, which was a bit of a state so decided i would replace this after the next step...


Checked the spark plug again, and its black. looked like a mixture of carbon and oil. (dried black powder with some wet black still there) tried a bunch of different settings with the carb, cleaning the spark plug after each failed attempt.

My logic (or lack of) then decided oil must be getting into the combustion chamber for it to foul the plugs, need to get it open to eliminate rings, piston, head gasket and valves.

tear it all down, found the oil ring had worn to below specs, and the piston was slightly under spec too... but the cylinder measured a perfect 57mm everywhere with the digital calipers 

SO! ended up buying a new piston, rings (both STD size) honed the cylinder,  refitted and torqued everything (new gaskets and seals of course) tensioned the camchain at TDC on compression stroke, adjusted the valve clearances (exhaust side was double what it should be) put everything back together with new fuel, oil and oil filter.


Bike wont start, even with choke, but will start if you give it revs and hold it. as soon as you back off the revs the bike dies. and it still sounds a bit rough with some white smoke coming from the exhaust


I'm genuinely confused as to what to try next!

If anyone has any ideas then i'd gladly welcome them, only thing i havn't checked is the valve seals, which could be tomorrows job...


Cheers in advance!



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Ok,  am thinking, the choke system isnt working, ,  the flaot bowl has a small well,  and a bras pipe dips in there, thers a small pinhole at the bottom , that feeds the well,  see it in this pic, 


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  • Moderator

that white smoke is no doubt just water vapour like all engines produce before they fully warm up, I'd investigate what blackhat said...further proof would be if it makes absolutely no difference switching the choke on or off when you do have it running?

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just a thought .. how come the exhaust valve clearance was double what it should be ? my guess a broken valve guide/bent valve .. which in turn would give the bike low compression which would follow through which the way you can only get the bike to run e.g Revs ..

I would compression test the engine, both wet & dry and work out from the (possible differences) what is broke.

@Airhead The white smoke may be oil burning that is running through the possible break in the valve guide .. at this point the valve guide oil seal is ineffective and allows oil to run into the combustion chamber and therefore burns as wispy white smoke  rather than water vapour

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I know some bikes need the filter soaked but don't think yours is one of them! Start with the simple stuff first.

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Wow! i wish i'd checked back sooner, weren't expecting such a quick response!

Blackhat - I've just taken the carb back off and i'm looking at the float bowl... found the well so i'm spraying carb cleaner down it just incase i missed it when i cleaned it last time. (using it again on all jets etc... just to make sure) so will report back once i've stuck it back on the bike.

Airhead - yep, choke had no effect at all but then again i had no idle to properly test it with. I'm blasting it with carb cleaner and if that doesnt work i might stick it in an ultrasonic bath.

Jim R - I'll test the carb issue first as its easier to check, but will certainly check the valves / guides after. My compression tester should arrive tomorrow though so hopefully should help diagnose it!

Tommy xs - Mr Haynes told me my bike needs an oil soaked one, but i did try starting it without the air filter and it didnt make a difference :(

I'll be sticking the carb back on in half hour or so, so hopefully will have some happy news!


Thanks again guys, will keep you posted!


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So! I put the carb back on the bike, but it still had the same problem.

I thought maybe i hadnt waited long enough with the carb cleaner so took it off and blasted through every possible orifice to make sure its all clean.

Upon putting it back together i somehow snapped the main jet (really didn't turn it that hard!) so i've ordered the new one.

While i wait for that to arrive i'm going to inspect those valves... any good references i should follow? i find the haynes manual quite limiting...


Cheers again!


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I don't claim to know anything about your bike but is the cam chain stretched? This could explain it sounding rough and the clearance being wrong. High revs might be compensating for this.

Is there a poor electrical connection. Only other things I can think of are coil pack/cdi unit but if it runs at high revs and cuts out at idle the timing could be all tits up due to a worn chain, this will also give poor compression.

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R1Junky - I'm pretty sure the camchain is fine... it has very good tension and everything seems to match up nicely at TDC on compression stroke. I thought maybe the valve just hadnt been done up tightly. not sure!

One thing that tripped me up though was my bike has an automatic camchain tensioner which isnt mentioned in the haynes manual, but after some research they were apparently added in the later models. (mine is a 2003 which is the last year of manufacture)

is there an easy way to check the CDI?


Airhead - Yep! I sprayed carb cleaner in through point A and it came straight out of point B, then tested it with a small amount of petrol which disappeared down through the pinhole.

I also checked points C and D as well all is clear. hmmm!

Jays float bowl.jpg

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Just an update:

Compression tester finally arrived, and lo and behold, its only reading 70psi. (mr haynes tells me is should be a minimum of 114psi)

Retried again with a squirt of oil through the plug hole, still reading 70psi.

So! logically... it's not piston or cylinder (its new, with rings, and fresh hone) as the squirt of oil made no difference.

I've already tried a brand new head gasket. All bolts etc... torqued to haynes manual specs.

So logically it's gotta be the valves.

Just pulled the head off the bike again and waiting for the remainder of oil to drain out so hopefully will know more tomorrow.

Any tips on what to keep an eye out for?


Cheers again guys!


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tip the head upside down and put petrol/turps in the "bowl" if it drains you need have the valves ground (try those kits first) and retest. if it still does it, the seats will need recutting by an engineer

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Spot on Drewpy, has to be the head if the piston and barrel are ok. I would suggest that you also fit some new valve seals as well just to be sure, that can lower compression and let oil into the barrel.

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Hey everyone...

So, i think i've now officially given up...

Finally had the the parts arrive and went to work...

lapped both valves with rough, then fine paste which removed all carbon deposits. tipped the head upside down and put fuel in the bowl, no leak. yay!

fitted new stem seals just to be safe, rebuilt it back up, set valve clearances back to the manual specs... fresh head gasket (again!) etc...

Decided to do a compression test once all was assembled... STILL EXACTLY THE SAME! reading around 70 PSI!...


I'm now genuinely lost. it's got a new piston, with new rings, ring gaps are in correct locations, and the cylinder has a fresh hone. head gasket has been swapped twice now, and the valves when measured were all within tolerance of the manual, and have fresh seals and seats.

Cant see any cracks anywhere either...



Thanks for your help anyways guys, think im just gonna list it for parts on ebay which is a shame as half the bike is brand new parts! :(

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NO DON'T DO THAT..! It's always worth another go, where are you in the world, someone on here might be nearby and able to help. If all else fails THEN think about binning it but give it one more go.

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Haha yeah sorry i'm just frustrated, been trying to get this bike running almost every weekend since February!

I'm based down in southeast London (SE9 area)


Maybe i should push it to the nearest motorbike garage which is about 1 mile away and see if they can suss whats happening.

I refuse to be defeated by a damn machine! :)

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there's a thread somewhere on this forum about helping other peeps and from which area?


have a search and see if you can find it



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  • 4 weeks later...

Forgot to update this!

Just for anyone else having similar issues... mine was a very stupid one.

I hadnt noticed that the ridge that the camshaft pully sits on had actually worn down (maybe the camchain had been overtentioned?) and as a result was pivoting on the woodruff key, essentially causing variable valve timing!

I'll get round to swapping it out one day but for now it shall sit in the back garden. In the meantime i've upgraded to a Yamaha XVS 125 dragstar which has a fraction of the miles and is so much more bike for the money!.


Thank you again everyone!


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