MarVil info sought

OK, found it now!
Also found this site that I'd not seen before
Regards,
Kim Siddorn

the 98cc one) up & running. I'm surprised to discover jpg's, but cannot find what I need Hi Kim, a good few years ago I had a couple of Marvils & found out the unusual speed control device where the shaft cam for the magneto points are controlled by a centrifugal mechanical blocking device that at the running speed fly's out at operating speed to prevent the points breaking & restricts the speed! see for a couple of booklets.

Thanks Dave. Yes, I'd discovered the brass segment - or the lack of it in one case - a while ago. My first Marvil didn't have one & gathered speed to two strokey fashion, reaching its maximum around 2,000 RPM, perhaps a little more. The carb restricted its breathing and it hunted until it warmed up & then eight - or possibly 16! - stroked cheerfully at maximum revs.
Although no harm befell it, it took me some time to figure it out & it was not a cheerful companion. I loaded it with the dynamo it came with (never seen another of these bed plates BTW (I wonder what it was for) & that naturally made it run more smoothly. A primitive form of speed control was possible by shading the choke, but I wasn't happy exhibiting it in that state. I have suffered over noisy neighbours myself & would not wish it upon others!
My gravity lift carbed Homelites all stay at home for the same reason, although the APU from a Boeing B17 is hilarious to watch. It sits on a base supported by (effectively) valve springs & leaps about in a most demented fashion. You'd swear it was dancing to some inner melody of its own, but humans cannot hear it for the howl of the exhaust and fan!
I'd love to haul the twin out & show it, but the 110volt dynamo may need a field coil rewind & - being unloaded- it makes even more fuss than its single cylinder brother!
Regards,
Kim Siddorn

I stripped down the carburetor today. I was rather surprised to find that it is complete as they have a number of easily-dropped bits that can skitter off unseen. I've had one apart before, so was especially careful.
I'd assumed it was Mazac or similar, but no, it's aluminium & at some time a cack-handed fiddler has broken off one side of the flange & another more sensitive hand has gas welded it back on - pretty fair job too considering the thin sections involved.
For those unused to these curious direct lift carbs, let me take you through it. From the base of the carb proper, a long pipe descends into the cast aluminium fuel tank to which the engine is bolted. Both fuel pipe and engine base must make a good seal to their faces or the fuel will not rise as it should. At the foot of the pipe is a ball valve that stops the fuel being pushed back down the pipe by back pressure. It also makes this type of engine more easily started by hand.
The depression caused by the air flow raises the brass ball off its seat & allows the petroil to rise up the centre tube, across the bore and about another 1.5" to the top of the carb. At its top there is a needle with a fine thread that controls the mixture. (I screwed it right in then out 2.25 turns.) The needle allows the fuel to spill out of the tube which is itself a close fit in the carb body. The metered fuel now pours down the outside of the tube until it meets the bore where the rushing air pulls it into the engine. There is no means of regulating the air supply except for the choke lever. & there is no provision for an air filter.
Trapped between flange & crankcase, is a curious device that I can best describe as a toothless five spoked clockwheel with a centre spindle pointing away from the engine. It sits in the recess in the carb that is provided for it and appears in the parts list, so it is obviously supposed to be there. The parts list is unco-operative & just describes it as a "Spider" - very useful. Anyone like to speculate with me as to its function?
The engine is a constant-speed device & engine speed can be adjusted between 1,250 and 2,000 RPM. It is controlled by a brass segment in the points cam rising out of its slot under centrifugal force and holding the points open. It works very well actually and can be finely adjusted by changing the weight of the spring against which it acts. There is a coil spring under compression in a little tube attached to the body of the points box & it operates on the back of the moving point. It is adjustable, bringing more or less pressure to bear on the points arm. I can't quite figure out why, but I assume it is there to steady the arm, giving fine control to the points.
Once you have decided upon an engine speed, the engine is run under load until it has reached a decent working temperature. Final adjustments are made by taking off the chrome plated cone on top of the carb and finely adjusting the mixture until the engine two strokes properly and complete combustion is achieved.
All this will take some time & effort, but all two stroke owners expect that their enigmatic beasts will be cranky & need more maintenance than a desmo Ducati!
Regards,
Kim Siddorn

Kim,
That was an interesting read, I had a Marvil many moons ago driving a crop Duster and could never get on with it so sold it. I sometimes wish I had your tenacity for digging deep into the engine, I spent yesterday from 16:00 to 22:00 on the village playing fields enjoying some of the best local bands performing free for a charity event organised by the village, a great afternoon / evening. There must have been at least 500 people there and not a spot of bother to be seen even though the bar was open for the whole period. I'm going to spend today cleaning in and out of the caravan ready for Dorset and making sure it all still works but not so in depth as you.
Martin P

Thanks Martin. I've used it as the basis for my "Featured Engine" in our monthly club magazine!
Regia did the Heckington Village show & the Saturday evening was a brass band concert ending with the 1812 & lots of fireworks. It was hard to see how many were there, but the organiser told me afterwards that their advance sales showed nearly 2,000 - as you say, a great evening & not a cross word.
Regards,
Kim Siddorn

Keep forgetting to send you shekels for book.
Martin P

kimsiddorn wrote (snip):
Should be a light (usually broken) spring and fiber disc on the spindle which together form a non-return valve as MarVil has no piston controlled inlet port.
NHH

That makes sense - thanks Nick, There was neither disc nor spring ...
Regards,
Kim Siddorn

Looked in another Marvil carb & there was both disk & spring! I've replicated the disk from the cap of a Prit Stick but the only 1" lightweight springs I can find are from retractable ballpoints & they are too strong. I've got it back together now & it will fire occasionally, but I think the spring pressure is too great & at starting speeds it doesn't stay open far or long enough to breath in a lungful of mixture.
The original spring? Alas, it found another more interesting place to live and ATM I cannot find it ;o))
Regards,
Kim Siddorn

kimsiddorn wrote (snip):
I wondered what the cursing coming from the west was all about ;-)
They are skinny little springs and must have a pretty hard life in a revvy two-stroke - I'm not suprised it made a bid for freedom!
NHH