hi guys put new main bearings in yesterday ,seemed ok,ran engine up felt it was overheating but as you do entered trial today ,ok first lap then stopped . and will not start ,has electronic ignition ,199 model ,plenty power, and spark ,but did NOT replace crank seals ,question would this cause the over heating due to air leak kind regards murray

Yes it can - air leak will make the engine run weak and overheat...I'd always change the seals if the mains need doing - no point not doing them as the cost is so low.

I'd guess the plug will look lean if this is the case - should be a chocolate brown.

Also check the ignition timing - should be 2.4mm BTDC for a 199 I think - if youve rebuilt it maybe the timing slipped or you set it wrong?

Other causes could be fuel stavation or blocked exhaust - although both would cause drop in power which you didnt see.

hope this helps

yea thats the line i was thinking stupid not to have done those seals ,the plug did look chocolate in colour i thought that was an improvement from the usual black ha ha!! will order and fit see how it goes thanks very much for your reply murray

How do you know it was overheating? Symptoms?

If the timing side seal was sucking air it would affect the way the bike ran, weak throttle response and over-run on shut off. The clutch side would draw and burn oil.

If it was overheating significantly it would start to knock badly, something you'd wouldn't fail to notice

hi it was a bit smoky i thought this would clear itself ,the whole engin felt hot right through to the club foot being very hot,lucky it was a scottish windy and cold day otherwise i might have done some damage.it had great response to throttle(well normal) and did not over run thanks again will maybe replace clutch side seal regards murray

Another Issue might be air leaks between carb and engine,

I would check the gasket between manifold and cylinder and

the rubber connection nozzle between manifold and carb for

- cracks in the rubber nozzle,

- hose clips, do they sit thight?

To check for air leaks before disassemble the side seals, ... you can

demount the ignition cover, (be sure having neutral engaged), then

start the engine let it idle then spray WD40 / brake cleaner to the

places where you think that you might have an air leak, if the engine

rev will change when you have an air leak as the engine will suck in

the WD40/brake cleaner areosol then you then know where to look at,

(only a little amount of areosol is needed!).

ok will spray with wd40 if she will start after new seal thanks again

oh one thing i noticed no small o ring in front of bush on main shaft. and no o ring on gear shaft in front of spacer. could this play a part? murray

yes and remember the gasket under the seal carriers The o ring is very thin and can be easily missed out or broken

hello again put in new crank seals and o rings ,still hot,strip and clean carb, dump fuel , no smoke but still hot,then remembered had not annealed copper gasket on head, plus tightened head on bench not in frame.-----------------ran for 20min normal temp still to trial but looks good.------- thought i would post the result as i once got a great tip on how to fit a kickstart return spring from the land of Aussie,

hello again did a wee trial bike still overheating ---took head off to try new base gasket and saw looks like some gear oil on crank --stripdown again ---cases split ,cleaned,ready to reassemble tomorrow now the big question ----i would like to put a thin film of sealant on doth sides of the paper gasket i have been given green hermetite does any one have any advice on other sealers say at halfords or any other tips to take up any slight differances around the crank kind regards murray

When I fit gaskets and seals I usually just use grease Make sure both mating surfaces are flat and clean and the cases are pulled up evenly Remember an air cooled engine will run fairly hot when trialing.

Have you checked your timing ?

hi fitted electronic ignition from bultaco uk it has felt good and ridden well .will assemble tonight with a wee bit of sealant on gasket-- if this dosent work will inquire how to alter timing --thanks for all your help guys cheers

Hi , make sure the crank is running true before the assembly and also check that you have the correct clearance main bearings, as both these things can cause overheating.

Cheers Greg