Peter Jenkins Posted November 15, 2020 Author Share Posted November 15, 2020 E.D. Having taken a careful look at the engine and the stub prop shaft and what you have said, I've come to the conclusion that the problem lies at my end! Anyway, before we get there, here are the photos you asked to see. The first one shows the front bearing and also a short extension shaft that sits above the drive shaft. This looks like a stop for the collet to which you referred. The next photo shows the collet fitted inside the prop driver. This extends aft beyond the prop driver itself and it looks like when tightening up the prop nut this will push the prop driver aft compressing the split collet onto the drive shaft. So far so good! This photo shows the stub prop shaft. The LHS is shorter and the RHS. If I screw the RHS into the crankshaft it bottoms whereas the LHS threaded shaft is 5 mm shorter than the drilled shaft. So, at least it was round the right way. The problem I think was that I could not get the shaft tight enough with just engine compression to provide the tightening torque. The next photo shows the stub shaft part screwed in. The land on the stub shaft does contact the front edge of the crankshaft. With the prop driver in place, it was possible to see that the collet contacts the rear shaft part I referred to earlier and keeps the prop driver from contacting either the crankcase or the front of the bearing. You can just see the gap (the black line) between the two in the photo below. So, as I had some 2 stroke racing oil (for my petrol mix) to hand, I turned the airframe over so the engine exhaust port faced upwards. I then used some blue threadlock on the stub shaft that would screw into the crankshaft and screwed that in by hand. I then rotated the crankshaft so that the transfer ports were partially blocked by the piston and poured in some oil. Hand turning the engine provided resistance to rotation although I accept that there would still be some air above the piston. I screwed the nut you see in the photo above until it stopped on the land. Then holding the engine with one hand I tightened the nut as much as I thought reasonable. Rapidly turning the crankshaft to hit the hydraulic lock the other way, I unscrewed the prop nut. Pleased to say that the stub shaft remained tight! I'll leave that to cure overnight and, weather permitting, give the engine a run tomorrow. If all is well, then I'll just put the red thread lock on the shelf for another opportunity! If the engine kicks off the stub shaft, then I shall repeat the above actions using red thread lock this time. Does that sound a reasonable course of action? Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted November 16, 2020 Author Share Posted November 16, 2020 Tried to run the engine today. Used 5% nitro fuel and checked the plug was a dull red when connected to the power panel glow output. Every time it started it spat the prop off but not the prop shaft that stayed fimly attached. So, one problem solved. Thanks for the ideas chaps. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted November 21, 2020 Share Posted November 21, 2020 Hi sorry for the late reply .The engines crank looks ok . I think the problem lies with the brass collet or the prop driver . When the prop nut the prop driver should be pushed back onto the tapered collet and compress onto the crankshaft and locking all three parts together. This should , once tightened need a puller to release the prop driver if needed This grip on the crank shaft is what prevents the stud or nut coming undone . Its nothing to do with the stud that screws into the shaft .The pic of the collet in the prop driver doesn't look correct. Is that a hairline crack at about 1 o'clock ? or just an oil mark . The gap in the collet is too big and has been opened with a saw blade . When the collet is fully inserted into the prop driver it should sit proud . This then sits onto the crank up against the bearings inner race. I would put money on the Prop driver being stretched / cracked or the collet being faulty or both . Re insert the stud and look for a replacement prop driver and collet. If the rest of the engine is good then its worth doing as they run really well and last .Just Engines used to distribute them and might have the parts The prop driver is easily damaged , stretched or cracked in a crash or has been overtightened . I will look but don't think I have any MVVS parts in my spares box anymore E.D. Edited By Engine Doctor on 21/11/2020 15:52:44 Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted November 22, 2020 Share Posted November 22, 2020 A thought just occurred to me. I wondered if MVVS were available with opposite rotation crankshafts for pushers and twins. A quick google revealed this statement, "All MVVS glow engines can be ordered with counterrotating shafts." If the engine is backfiring due to the timing being for reversed rotation, that could be what the problem is. How about trying to start it in reverse? I was reminded of a work colleague who had had a rather peaky enduro bike modified for road use. He was sitting at the traffic lights, with the thing popping and banging away as it usually did at idle. As the lights went to amber he revved up, dropped the clutch and shot off the line... ...into the car behind him. Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted November 22, 2020 Author Share Posted November 22, 2020 Hi Robin I think that's unlikely as it was flown by the guy I bought it from and it has a standard prop. Thanks for the thought though. Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted November 23, 2020 Share Posted November 23, 2020 Hi Peter, As you are content that the crankshaft is the correct rotation, my thoughts are inclined towards pre-detonation. Could there be a build up of carbon on the piston, causing a hotspot? Is it possible you are running a higher nitromethane content than the previous owner? In which case, a drop in compression ratio or a change of fuel is required. Have you tried adding a second plug washer to the glowplug? That can sometimes help. A cooler plug may also work. If the information on this site applies to your particular MVVS engine, you should be running either straight (non-nitro) fuel or a maximum of 5% nitro. MVVS recommended fuels Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted November 23, 2020 Author Share Posted November 23, 2020 Hi Robin, if you look above I have tried 5% nitro fuel but with no joy. I may give the additional washer under the plug but there is a stack of stuff I have to do first so might be some time before I can try that. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted November 25, 2020 Share Posted November 25, 2020 Posted by Robin Colbourne on 22/11/2020 18:52:29: I was reminded of a work colleague who had had a rather peaky enduro bike modified for road use. He was sitting at the traffic lights, with the thing popping and banging away as it usually did at idle. As the lights went to amber he revved up, dropped the clutch and shot off the line... ...into the car behind him. Hi Robin . A lot of petrol engines used port induction and would happily run in either direction if the ignition timing was adjusted to suit. In fact some bubble cars in the 1950s used this feature for a reverse function ( Messerschmitt and Sachs) and had a separate set of contact breakers for the different engine rotation. You simply turned the ignition key in the oppersite direction. So your friends motorcycle probably had badly set ignition timming and bounced back in the wrong direction , a bit like bounce starting a four-stroke glow. Front induction engines like the MVVS won't run in reverse unless a correctly timed crank is installed. Pre ignition would only throw a prop if severe . The problem will I'm sure be a mechanical fault with the prop driver unless the engine internals have been modified that we are unaware of. Its also worth checking that the head shims have not been removed . MVVS came with a set of different thickness shimms for different applications. This in itself wouldn't cause the prop to keep coming loose but would make the engine more fussy to start and tune. Quote Link to comment Share on other sites More sharing options...
Geoff S Posted November 25, 2020 Share Posted November 25, 2020 My 1932 TT Replica 600cc Scott (twin cylinder deflector piston 2 stroke) was prone to run backwards. The clue was a slight cough when ticking over. It could be embarassing but I nevr shot backwards into a car A friend had a Messerschmitt in the late 50s and I had a little go in it (probably uninsured?) but only forwards. They actually had 4 gears in reverse - so 60+mph! I bet it was exciting. Geoff Quote Link to comment Share on other sites More sharing options...
Keith Berriman Posted November 25, 2020 Share Posted November 25, 2020 Having had a closer look at the collet and propdriver set up picture it looks as if they could be worn or something When I had this set up on my Iearly Irvine engines they looked to be a tighter fit in the prop driver Just my observation Quote Link to comment Share on other sites More sharing options...
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