Engine revving its T#ts off

[Bitten]

Hi All

I just want to throw this out there as I am not sure what was going on.

I took my RM465 out for a bit of a ride a week or so ago at Louee (NSW) only did about an hour or so on and off throughout the day and all went super but towards the end of the day on 2 occasions the bike when first started began revving its tits off till the point I had to stall it (the kill switch works but on this occasion wouldn't kill the engine). I let it sit for a minute or 2 and all was well, did a few laps and than a bit later it did it again.

It has a brand new carby, new cables, new spark plug. The throttle wasn't sticking as far as I could tell.

Any ideas?? - I would like to understand what it was as I am coming to CD9 and would like to get on top of it prior if I can.

Cheers

Bitten

[Simo63]

I know it's a simple thing but a mate had the same problem at the Conondale Classic on the weekend and I spotted his problem ... the carby top was loose ... well in his case it had actually come off 

Yeah yeah I know .. didn't have to be einstein to see but anyway if you've recently replaced the cables maybe the carby top has worked it's way a bit loose??

Always start with the simple things but if not then it might be something more complicated like sucking air through seals or gaskets 

Good luck

[John Orchard]

Yeh I'd check the main things first, like loose carb top, cracked/loose inlet manifold sucking in air, air leak at base gasket or left crank seal.

Does the bike run a flat-slide?  I had a problem back in the day with a 250Z, the Castroll TT oil that I was using would turn to a gummy substance when mixed with water (either from washing the bike or riding in the rain) and cause the slide to 'hang-up' at part throttle.  At part-throttle the intake vacuum is so strong it would pull the slide hard against the carb body, over powering the return spring, preventing it from fully closing.

[Bitten]

Thanks guys

It runs a 38mm flat slide

I will look at this but it has just gone through a full rebuild and not saying that my mechanical skills are unfallable but it has had all new seals, gaskets etc and has had no more than about 2 hours riding since all of which have been faultless

The intake manifold is what was on it and it has what looks like little surface cracks on the outside but inside in seems intact and structurally sound

I will give it a good going over and give it a kick over on the weekend and see if anything presents itself

Cheers

[John Orchard]

My call is the slide not closing off properly, maybe stretch the throttle return spring a touch; it could have compressed over time, stretching it will increase the closing pressure but not increase the resistance at the twist grip by much.

What 2-stroke oil are you using?  and at what ratio?

[philthy]

I have exactly the same problem with my yz250g.Had a complete rebuild including main bearings and seals.Runs perfectly for a lap or two and then just revs out like yours.The local bike shop has just split the motor again and found that the left hand side crank is worn where the main bearing fits,goes on tight but then has a loose spot.Theory is at this stage is that the seal is ok until it gets hot then goes to pieces with the excess movement.Hope this isn't your problem!

[Bitten]

The oil I am using is Motul 800 2T (Ester) at 30:1, I hope it is not that either Phil or I will be doing a quick lap and lists of resting in between at CD9

[TM BILL]

Start and run the bike at idle (turn up idle screw if you have to ) then spray some ether product (start yer bastard or such like ) all around the intake manifold base gasket etc , even take off mag cover and spray around main seal . If there is an air leak revs will rise as it sucks in the dope . Wiggle carb around to flex manifold incase cracks are opening under load or vibration.

If it is a sticking slide try this (this is hard to do in the heat of the moment when its reving its guts out ) but feel for excessive free play in throttle cable , roll throttle to closed stop and slowly back until tension is felt if it differs from normal slide is probably sticking , if the slide is stuck up in the bore a bit in theory the cable should be slack at throttle end .

Good luck .

[smed]

Could also be running out of fuel,lean burn,check the breather on the tank cap & also fuel flow through the tap & fuel line

[Bitten]

Thanks Bill

I will give it your suggestions a try, will have to be this weekend as I am a bit busy for the next few days

Smed, It may be that simple, I can't remember how much fuel was it it at the time and I have already drained it and didn't take any notice of how much came out although I would feel pretty stupid if it that was the issue

I have put out the feelers for a new intake manifold as this is about the only part that is not new on it so I might as well replace that as well!! - Alpha in the states have them but if I can get locally I may save a few$$

Just a side question is there a local supplier for new reeds for older bikes ('82 RM465) as I was using the ones that were on it and there is no telling how old these were as well

Cheers

[John Orchard]

I think you can still get Boyesons on ebay

[Lozza]

Try Mick Hone Motorcycles for old Suzuki bits. You could also have a crack in your cylinder

[TM BILL]

Most bike shops still have lots ot packets of reeds on the shelf that have been sitting there since the 4 joke revival 
they will usually cut you a sharp deal on a set or two as they want rid of them . You can trim them to fit with a sharp scalpel and redrill the mounting holes if you have to 

[Bamford#69]

Hi,
from your discription this sounds like it is "Dieseling" , the spark plug is acting like a glow plug, it is glowing red hot ,(I mean a glow plug as in a model aeroplane engine) that why the kill switch wont work and it is not responding to throttle opening  , after hard use  it over revs  and when the motor and plug cool down it  runs as normal .

It may be running lean , maybe advanced a little too much, the spark plug is probably the wrong heat range  , that is not conducting heat through the head, and  is running hot , it also could be your plug is projecting too far into the combustion chamber, check how far the plug is in when  the head is off,

cheers 

[Bamford#69]

Hi,
I should have explained how easy it is to check if this is cause of the problem; ride the bike until this happens again , stall the the bike to stop the motor , quickly remove spark plug, it will be really hot, and replace with new cold spark plug, and restart motor.
cheers