Saving Old Seagulls

hi
my forty plus starts well,runs well then after about 15-20 mins it starts to "hunt" (rev speed fluctuates up and down).
if i stop the engine then re-start it is fine initially and then starts to hunt again after the same period of time.
ive replaced the fuel line,stripped and cleaned the carb,cleaned out the tank
but the problem wont go away.
any thoughts anyone....

I'd possibly be thinking blowing head gasket or leaking crankcase halves. I'd try very hard not to think about cracked block.

There's a 40+ in the workshop waiting in the queue that does much the same. The crank case leak is just above the tiller mounting stub. Pressing a finger over the edge nearly cures it!

Peter

oh dear
that doesnt sound to promising.
i will investigate all of the possibles you mentioned.
thanks peter.
ken

I had a Forty Plus with a crankcase leak that was giving some trouble and I cut a gasket from thin gasket paper and reassembled with that between the halves. The tolerances are loose enough that the added thickness of the gasket paper isn't a problem. The edge of the gasket that meets the base of the cylinder block *must* be trimmed accurately and square with a rasor blade.

On reassembly, assemble loosely and bolt up to the cylinder block finger tight to ensure proper alignment. Tighten the crankcase bolts finger tight and check the engine turns over freely before final tightening.

I always dress paper gaskets with Hylomar blue and have had absolutely no trouble since from crankcase leaks.

Its an easy enough job if all goes well but if the engine has been used extensively in salt water, the nightmare will be getting the bolts holding the cylinder on undone without damage. Do your self a favour and liberally coat the threads of all bolts into the block you've undone with Hermetite Red before reassembly to prevent them rusting solid. DONT remove the cylinder head unless its gasket has failed or you *have* to replace it.

thanks for the replies guys,you were absolutely spot on.
the problem was a tiny leak on the crankcase joint.
i didnt bother replacing gasket (lazy sod).
i just smeared a little "instant gasket " on to the leaky bit which was about 4-5 mm across.
left it for a day or two and it now works fine.
i will do the job properly eventually,but it will do for now.
thanks again
ken

As a matter of interest how did you find it? A mechanic once told me that if you spray WD40 around a joint you can find a leak ie it'll start to run well. He was talking about an LPG system (the vacuum bit) on a car but I assume it would work on a 2 stroke.

Hugo.

snatch wrote: i will do the job properly eventually,but it will do for now.

Worry not......... You've done a proper job if you only put silicone instant gasket between the two crankcase halves. That is all the factory did!

There is not meant to be a gasket between the two halves although there should be a paper gasket, part # SJ/1300, between the block and the assembled crankcase.

Not an expensive item; a pound or so from John. Alternatively a piece of Kellogs cornflake packet is perfectly acceptable at 2:00 am ...... Many would also submit that a dressing of Hermetite or Hylomar on paper gaskets is worthwhile.

Peter

Perhaps I wasn't entirely clear, cutting a *thin* gasket to fit between the case halves is certainly *NOT* 'factory'.

On Hylomar, less is better. You can buy it as a spray, but its a lot cheaper just to take a glob of it and thin it till its brushable with celulose thinners. Apply a thin even coat and let dry till its firmed up before assembly. DONT use it on a modern thin narrow rubberised fibre gasket or on any sort of rubber seal, you *will* have problems. Smearing it on thick is also guarenteed grief.

Hermetite red is good for reusing copper cylinder head gaskets (after annealing them). Its also good on screw threads you dont want seawater getting down. I also use it on pipe joints if I'm having difficulty (or from past experiance) getting a good seal. NOT my first choice for most uses.

Silicone RTV is evil stuff. Unless sold as neutral cure or oxygen sensor safe, it gives off Ethanoic acid (vinegar if you've forgotten your chemistry) which can lead to all sorts of corrosion problems, It has a bad rep for blocking oil ways (not a problem on seagulls) and requires a perfectly clean, absolutely oil free surface to stick properly. If you do get good adhesion, its absolutely great until you have to get the joint apart again as if its over a large surface area you may well have to part it with a rasor blade or dental floss! Its often be ****ing difficult to get the last bits off when cleaning up joint faces, especially if its got somewhere it shouldn't have. It also sets in the tube after opening in anything from a couple of weeks to a couple of months so its a right pain to keep any handy.

OTOH there is a long well documented history of using it successfully on Seagull crankcase halves, so if I had a tube on the go and didn't mind having to be fussy about degreasing the parts, I would'nt hesitate to use it, Its NOT a bodge for this application.

For many applications the correct dressing is a light smear of oil or grease, or even nothing at all, especially for modern preformed composite gaskets.

hi
thanks again for all your replies.
i found the leak the same way i find any leak in anything,simply by brushing on a concentrated soapy water mix.
i used to be a pump engineer in the army and even in these high tec days there is no better way of detecting a "blowing" type of leak .
if you go to a calor gas factory there is actually a bloke employed to do nothing but paint on a soapy water mix onto every filled gas bottle (regulator mainly) which comes down the production line.