Any Ideas ???

[pirie593]

Took the Mk 7 Pursang out for my inaugural VMX racing on the weekend, only to be disappointed by the machine.

Serviced on Saturday; checked spark etc.  Clean airfilter.  Started second kick and ran well.  Took a bit to clear out old oil but then motor was sharp.

Sunday, two laps of practice.  Started second kick and ran ok for first lap, then seemed to bog down a bit and I had to rev it more than previously, but quickly came on song.  When I slowed at end of practice, engine revs stayed high for several seconds, then died away.  That happened each time I opened up throttle to more than half.

Engine died as I pulled up in pits.  Then would not restart.

Changed plugs.  No go.

Plenty of compression.  Plugs very wet.  Tried all sorts of coercions.  Kicked over with plug out.  Plenty of spark.  It even fired with one plug out, but would not run.

Pushed for miles, and it would fire and run on idle but die when open throttle.

Gave up after an hour of that.

Any ideas on what might be ailing the old bugga???       

PS  fuel 3 weeks old.

[mx250]

One of the carby jets vibrated out/loose.

[EML]

Yep, I'm with Gray-ham, a jet has dropped out-happens alot on the Wasp/EML Yamaha with all the vibration.
I do recall however the story of a particular XL175 that had a similar problem some years back-it would run fine with the tank off but would stall when it was refitted. Checked the coil-thought it was squashing the wiring etc... or pulling the harness apart et al. Wasn't until the mechanic turned the tank over that he found a frog up in there and when the tank went back on he sat on the coil and shorted it out!!
So have a look for frogs-o.k.

[mx250]

Yep, I'm with Gray-ham,

It's 'Grey em' - I ain't ham that's gone off

[Bullymad]

Plugs wet means too much fuel, not enough spark or dodgy fuel

Plug spark must be blue in colour.

Pull carby and strip so you can inspect the internals.

Drain fuel and replace regardless

[EML]

Sorry Grey-em, what no frog? will just have to look further. Crank seals is another option but not familiar with Spanish seals- or any Spanish wild life really-though there was one time in Villa Real on the Portugese border- we were heading to Moron-de-la Frontera for a GP, side cars of course, but hey that's another story!!

[mx250]

Moron-de-la Frontera for a GP,

Portuguese for, 'moron of the fronteer' or 'on the fringe of society' or 'on the fringe of insanity' not sure which. Any, or 'all of the above', would seem appropriate for sidecar racing of any form .

[EML]

Like, I said it's another story. Moron is actually in southern Spain and the track there was cut out in a desert area with a bull-dozer, 2 or 3 blades wide, and looked more like a BMX track. It hadn't rained in years and suggested to Hansi that as it a round of the world championships, just maybe these guys should water the track. Their answer was that there was no water so NO they couldn't water the track!.
Hansi put together a meeting of the top teams and it was agreed to boycott the event. Suddenly, after a quick call to the local politician water began arriving-however it came in truck loads all through the night, keeping the whole camp awake, and when we climbed out of the sack next morning, the place was awash-big puddles everywhere held in place by the levy banks formed by the bull-dozer blade. Shit, what a mess-big puddles followed by dusty dry spots so the goggles got a good coating of each in turn.
"Idiots of the bush" indeed.   

[felix7]

Pirie ... what are you doing?? That's exactly my scenario on Sunday when I had a putt around Narrogin (VMXWA) on my Mk 7 250.
It was a complete bitch to start so kindly Ray Wann put my bike on the electric starter. After a minute it fired but soon after (at the start-line, natch) it died and wouldn't kick start.
The plug (5) was wet. Replace plug with a hotter one (9) and add more fuel (lessen the oil a fract) and back on the electric crank. It went well, with plenty of top end but felt always like it was going to die down low. I stalled but it kicked to life almost immediately. The second race ended with a stall. My fault - possibly picked a higher gear for the corner (it was one of the five gears in between the 35 neutrals).
No chance to start it by foot though lost about 5kg in sweat.
Pulled the plug and it was sooty.
Clearly, too much fuel.
I have a 34mm Mikuni. Changes I've done the day before include a thorough oiling (Castrol spray-on stuff) of the air filter and fresh fuel.
I have pulled the carby off and I'll now pull that down. i suspect a jet is wrong; the oil filter may have been too hard to breathe through, hence it'll run rich; or I have embraced Buddism when I should have ticked the Catholic box.
Please let me know how you go. I'll keep you up to date.
Thank the gods it's not just me
- Felix

Perth
   

[pirie593]

Felix,

I will let you know what I find with the carby (Mikuni of course).  I will tackle that first, since my thoughts that it could be a crank-case seal keep getting pushed to the back of the brain box in the hope that something simpler is discovered.

I replaced a very viscous filter oil (it ran fine with that) with a less viscous variety, so sucking through shouldn't be the problem.

PS  this is one of Graham Taylor's bikes.  Cheers

[mike1948]

Was there any sign of excess smoke from the exhaust?  White in colour, blown primary side seal.  No excess smoke, good compression, but no go, maybe magneto side seal gone.  Either suggests main bearings loose/worn.  Easy fix.
Good luck.
Mike

[pirie593]

Mike,

No excess smoke when I went out for practice; in fact exhaust looked good, and when back in the pits, a nice grey tinge.  But thereafter, it wouldn't start, so couldn't check gas colour.

Thanks for the tip.

Cheers

[felix7]

My Mikuni is in for a look-see now, though I had a tamper last night and noted the jet needle's circlip was on the leanest setting (at the top of the needle) and that the choke function was fine.
I've seen no evidence of seal issues with the engine.
I don't know the sizes of each of the jets so when I find that I'll post them up. Practically, for the engine to turn into such a swine after running reasonably for a few races (sorry, putts) shouldn't show up as a mis-matched jet.
I'm sure I can get it all right with the proper spark plug and perhaps a clean out of the carby which may have crap in it for some reason (the main one being taking the bike into a situation that contains dirt, dust and water).
Keep the comments coming.
- Felix

PS: Pirie .. mine's ex-Graham Taylor, too!

[Bullymad]

Guy's are you running the original points system

If you are what about the condensor???

A dodgy condensor will allow you to start the engine but will fail when under load.

Just a thought

[pirie593]

Bullymad,

After owning a couple of CZ's in the early 70's, I went into therapy.  It took around 20 years to finally erase the word "condensor" from my conscious and subconscious.....oh my god, its BAAAAAAAAAAAAAAAACK....