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TartanMac10/01/2018 19:12:05
225 forum posts
142 photos

This is a little out of the ordinary O have tested my Mvvs 90 and the power has been good but I always got a bad stutter and then it would quit it I throttled from idle slowly. I was online and found a thread on modifying the engine from what I think was the guy in the Netherlands who used to sell mvvs engines and he said the transfer ports were too small and need to be opened up and that is the weakest link in this engine as they cover the port a little.




But if you look at the second pic of the transfer ports the black band on the right is the only area that is free to flow gas the metal band is actually the metal casting right up against the cylinder liner and the dark band on the left is the angle on the liner. It only has 1/4 of the port free to flow gas. Has anyone out there modified one of these before as it looks heavily restricted. But still turns a 16x6 @ 8,500 rpm from memory and a bolly 14.5x8 @ 10,000 also from memory. You can slam the throttle quick and it work but not slow.

Oh check the heading on the news paper lol.

Edited By TartanMac on 10/01/2018 19:13:25

Denis Watkins10/01/2018 19:42:07
2803 forum posts
139 photos

I would spend a lot of time TM, checking the carb, the fuel and the plug, before grinding commences.

Make sure the motor is not overpropped

OS No8 - OS No10 cold plug


Edited By Denis Watkins on 10/01/2018 19:45:46

Percy Verance10/01/2018 19:45:19
6637 forum posts
111 photos

Hi TartanMac

I had an MVVS .91 way back when, and the problems you describe seem strange. I certainly never had any issues with the throttling or pick-up on mine. Mine went like a train, and pulled a Flair Giles around on an APC 15x7 with great authority. Easily a match for an OS. I specced mine from new with the *Prettner* carb though, so I imagine that gave it an edge. The issue you highlight regarding the transfer ports leaves me a bit baffled, as mine ran so well. Bob Cotsford on this forum bought it off me, and as far as I'm aware it's still giving him good service.....

The chap in the Netherlands was P.E. Reivers, whom sadly is no longer with us. His site was an excellent resourse for MVVS owners, it's a great shame both Mr Reivers and his site have gone.

TartanMac10/01/2018 20:41:20
225 forum posts
142 photos

Yeah it is weird that the engine does that I ran it with the novarossi plug recommended for the engine by mvvs and at 10k it is making around 2.1-2.2up so 2.4@11K is pretty much online with the right power. The carb is the mvvs butterfly carb that I put new o rings on. It also has new bearings . I then tried an 8.5mm irvine carb it ran the exact same. It has bags of compression too starts very easy on bouncing it backwards off compression. I have tried with both carbs Lean Settings where I get lag on quick throttling up and also A tad rich I find the same thing happen it is weird. If I follow the directions on removing that material it will be quite a lot of metal.

Bob Cotsford10/01/2018 20:52:29
7269 forum posts
415 photos

I've still got PV's old engine along with a second newer one. The old one runs on a pipe, the new was running on a JE Pitts silencer and both behave perfectly, the piped engine on a 14*8, the Pitts a 15*6. Until I buried it PVs engine had a butterfly carb, now it has the standard barrel carb like the newer one. I don't remember it hesitating at all when it had the butterfly carb but that was a few years ago now.

Chris Walby10/01/2018 20:55:54
487 forum posts
71 photos

The height of the ports are effectively the timing of the engine, if it were a 4 stroke you could change cam timing to change the performance and here is the twist, if you retained the old cam you could always put it back to original.

Your problem is once you have removed the material it will be difficult or impossible to put it back!

I could be wrong (most likely am!) but the higher the port the more top end you will get - at the cost of bottom end power. That's why they put reed valves, power valves etc to get the best of both worlds (but at the cost of complexity).

Just my 2p, but I would still say its carburation.

TartanMac10/01/2018 21:12:38
225 forum posts
142 photos

Chris the actual liner is not what I will change it is the casting behind it. The timing is on the liner I won't touch that it is the actual transfer port from the bottom of the case to the port hole in the liner. It is like taking the liner in your hand then covering up part of the intake ports. That is what the casting is doing. I will try the engine at the weekend again just in case I have made some mistake. Bob if yours had that carb it must be the same internally. It doesn't knock or anything. I really like the engine though it is very powerful torque wise. I don't want to mess it up.

Steve Hargreaves - Moderator13/01/2018 16:56:42
6605 forum posts
182 photos

If you don't want to mess it up TM then my advise would be don't touch it.....2 strokes are very sensitive to port shape & height & IMHO if you mess with the ports you risk ruining the engine. All engines are a compromise to a certain extent & the designer will look to get the best compromise between torque, power, noise, fuel consumption & will not have sent the engine out into the world unless it is as good as he can get it.....

Yes you might be able to get more power but it will be at the expense of low down torque....or more noise....or higher fuel consumption....possibly all three...

MVVS engines usually throttle pretty 10cc unit ticks over like a Swiss whatever is causing your problem I doubt you will find the solution by messing with the ports....most definitely don't touch the liner....if you damage the chome plating in one area it will almost certainly start to peel off the rest of the liner.....

Just my 2p....wink 2

TartanMac13/01/2018 19:24:32
225 forum posts
142 photos

You are probably all right on this. I will run it tomorrow on Laser 5% and see how it runs. I have the butterfly carb Irvine 8.5mm and the MVVS 9mm Q40 carb that some used as an upgrade (so I've read) I read that 8.5mm is the original size and the MVVS carb is similar to the Irvine at least in that it has a spray bar right across and is 8.5mm. I looked at the port right across from the exhaust with the piston @bdc and it is very strange there is not enough material to grind or cut it further up the port.

There was some crash damage on this engine when I got it but I don't see anything that can cause such an erratic stutter coming off idle. I think the bottom case might have a very very very slight bend in it. I remember replacing the bearings and when cold the crank would bind slightly but when heated up it would not. So I took out the bearings and put the crank in and turned it it sheared a very very small piece of metal off and then when put back together there was no binding at all nice an free and huge compression.

However when I ran it there was jet black oil out the exhaust. Now I have taken it apart again I can see the skirt of the piston has hit the back plate and very very slightly bulged it out and in doing so the black residue that was all round the piston has been coming off and I think that is what was coming out the exhaust. But it is completely free to turn over. I don't think it is enough to cause the weird running of the engine. It acts like it is going to throw the prop off.

Denis Watkins13/01/2018 19:54:37
2803 forum posts
139 photos

Just for reference, jet black oil is aluminium being rubbed down into the oil, so something is misaligned and wearing away.

On inspection, the offending part will be brightly polished when found and cleaned up.

TartanMac13/01/2018 20:16:00
225 forum posts
142 photos




As you can see the transfer port is completely covered. The port on the casing angled in beside the piston so the only area to transfer gas is the width of the port on the liner and the depth from piston to casting being the thickness of the liner that is a very small area it is difficult to get a picture of and visualise the pic. I will have to grind a little off the back plate before running it I think. But really I would like to check how round the piston is and cut of the distorted part of possible hope you can understand by the pics they are not great.

Denis Watkins13/01/2018 20:56:00
2803 forum posts
139 photos

Talking of backplate

The big end hitting the backplate would easily make the jet black oil

And it is there that you might find a polished surface

If so, the big end and con rod has moved towards the backplate

Is the crankshaft fully home on the bearings ?

You will sort it

TartanMac13/01/2018 21:00:50
225 forum posts
142 photos




Thanks Denis yep the piston is starting wear. It still has the horizontal scuff marks but way cleaner than it used to be.

John Emms 113/01/2018 22:50:17
229 forum posts

I was going to say that there was a slight mod to the MVVS glow carbs, maybe a little short of 20 years ago to improve throttling. But then I saw the carb - that is of a type that I don't recognise.

The answer, as always, is a short e-mail to MVVS.



TartanMac13/01/2018 23:29:03
225 forum posts
142 photos
Posted by John Emms 1 on 13/01/2018 22:50:17:

I was going to say that there was a slight mod to the MVVS glow carbs, maybe a little short of 20 years ago to improve throttling. But then I saw the carb - that is of a type that I don't recognise.

The answer, as always, is a short e-mail to MVVS.



Yes I can remember reading about that on the forums when searching around. They had an 8mm carb on there at one time but I think it was for making all there engines with the same carbs 61-77-90 but then I think they went to 8.5mm and settled on that. They had this carb early on but I think it was costly to make it is very nice just like an O.S type 86 carb without the pump and fuel shutoff return line. The needle on the carbs changed also much like the difference between the webra ultra mix and the pro mix the low speed had been rounded instead of straight cut at the tip. I think they changed the shape of the mains aswell. Yes I think I will send an email.

TartanMac13/01/2018 23:44:37
225 forum posts
142 photos

For anyone wondering about the carb




Percy Verance14/01/2018 07:57:12
6637 forum posts
111 photos

Isn't that the "Prettner" carb? When Just Engines were MVVS agents Paul Landels offered the option of this carb over the (then) standard i tem. When I purchased my .91 I chose to go with this carb, and was very impressed with the throttling and power it provided.

I'm not sure I'd tinker with the port openings. MVVS have been producing engines almost as long as OS, and certainly seem to know what they're about.

Brian Cooper14/01/2018 08:37:54
352 forum posts
17 photos

If the carb needs replacing, an ED carb is worth considering. These exquisite carbs give a boost to the top end RPM and give ultra reliable idling with a crisp pick-up.

They can be bought from Weston UK.


Robert Welford14/01/2018 09:55:33
84 forum posts
1 photos

I have new a unrun MVVS 77 and a 91. After reading this thread yesterday I took my engines out of their boxes and had a look.The carburettor on the 91 is the same as yours. However, the exhaust port is not restricted as yours is by the liner. The opening in the liner is the full width of the crankcase exhaust port, but slightly restricted at the top by less than 1.0mm.

I was wondering given the head on these engines can be rotated through 90 degrees, so that they can be run as either a side or rear exhaust engine, has the cylinder ever been rotated and the liner set in the incorrect orientation?

Percy Verance14/01/2018 10:42:56
6637 forum posts
111 photos


Re: ED carb.....

I used one of these to pep up my Merco .61 a couple of decades ago. It certainly did the trick, and improved the throttling greatly. The downside was the horrendous increase in fuel consumption. Even Oliver Reed would have struggled to match it........


Being able to rotate the cylinder 90 degrees to match (say) a tuned pipe manifold was/is a useful feature, but it had to be done from new. It wasn't advisable to move it once the engine had been run and had bedded in.

Edited By Percy Verance on 14/01/2018 10:49:45

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