mobylette gasket repair

my 1970 mobylette keeps blowing a head gasket.is there a factory gasket or should i make my own?where can i find a aftermarket/factory gasket?

Re: mobylette gasket repair

first find out why the gasket blows..

Is the head warped? Is a head bolt stripped?

Did you do any modifications to the engine?

once you find out what's up, gaskets are easy to cut from thin aluminum sheet.. scissors and sharp razor blades and some care.. Clamp it between two pieces of wood or aluminum to drill nice round bolt holes in the thin metal.. or use a tapered reamer.. no biggee

Re: mobylette gasket repair

thanks joew, bolts aren't stripped and the head is as straight as an arrow.thanks for the gasket tips. also i've heard if the muffler is clogging that it may affect the gasket and piston performance.....b

Re: mobylette gasket repair

clogged exhaust will make it run poorly but blown head gaskets are from too much pressure / heat / whatever or some mechanical fault.. strange to be blowing gaskets on a little low powered engine like that..

what exactly do you mean by a "blown" gasket? Describe what happens or what you see.

Re: mobylette gasket repair

twice now only after a couple hours of riding on the new gasket and just upon restarting the engine, some darker smoke will exit the exaust then total decompression. i'll turn the flywheel easily with my hand and can actually feel where the air is escaping from. upon further inspection i am able to see the empty space where gasket used to be.

Re: mobylette gasket repair

check anything that affects a tight head-to-cylinder seal.. Flat, clean and undamaged head and cylinder surfaces.

Flatness can be checked with a flat piece of plate glass, and a sheet of very fine (maybe 320 grit) wet-dry sandpaper.. wet it and move the head around on the paper.. the dull scratch pattern should cover the entire gasket surface area. Any bright areas that don't make contact can mean a warped head.

All 4 head bolt/studs/nuts are in good shape.. no stripped threads.. not substitutes bolts of incorrect size or length..

head bolt/nuts are torqued properly. ("manual":http://www.mopedriders.org/article_view.php?faq=2&fldAuto=12 says "tighten without excess.. i dont see a torque chart)

Torque with a small torque wrench to about (only) 10 pound feet.. gradually.. in a criss-cross pattern.

10 pound feet aint much.. Without a small torque wrench (I have an inch-pound wrench for small stuff like this) it's easy to under or over torque these bolts.. stripping them can mean major problems. Uneven torque can result in a warped head.

Re: mobylette gasket repair

I have a suggestion.. Get a new gasket in there (home made or factory, whatever). Torque the bolts to specs, then run the motor at idle until it is hot.. do NOT put it under load.

Let the motor cool back down (mostly anyway, 15 minutes), and then re-torque it. It's pretty common for new gaskets to take a 'set' and need an immediate re-torque after one heat cycle.

ALSO - if the mating surfaces are not spotlessly clean (some carbon, oil, etc) this will comprimise sealing. clean the surfaces with some carb cleaner and a soft brass or aluminum brush prior to assembly.

Hope this helps.

Re: mobylette gasket repair

thanks for the info, i'm gonna try that today.it makes a lot of sense so i'll let you know how it goes ....thanks again...b

Re: mobylette gasket repair

super useful info thanks...wish me luck..b

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