MVVS 45 headscratcher

[Martin Harris - Moderator]

A clubmate has an MVVS 45 (2 stroke petrol) which was last used a few years ago. He is in the closing stages of an extended build and is using this engine.

He attempted to run it at the club this weekend and it exhibited some strange responses to attempts to tune it - no adjustments made since it was last used. It started relatively easily but refused to transition to high rpms, sounding rich. Leaning the low needle didn't help at all. After some fiddling, it started and ran pretty much perfectly - until I observed that it seemed to me to be running backwards! Various hands were cautiously positioned behind and in front of the prop and my diagnosis confirmed...

I have to admit very little experience with reed valve 2 strokes so perhaps someone on here might be able to help?

After this, the original conditions returned until (I had started assisting someone else by this time) it refused any attempt to start. Spark was checked and confirmed as present, fuel was being delivered and compression felt fine.

Later reference to the MVVS instructions revealed a couple of warnings not to open the throttle if it started backwards as it would "cause damage" to the engine. I've looked at the only parts I can think of that could conceivably be affected - the reed valves - without dismantling further than removing the carb and they appear intact - what sort of damage could running a 2 stroke backwards cause?

Why would the engine run so well backwards but not in the correct direction? It does appear that the Hall sensor magnet passes the sensor at TDC so that would account for the ignition timing working correctly - ignition advance being calculated by the ignition module. The pump diaphragm is energised from a tapping on the crankcase so no carb orientation problems...

Next move is planned to be to reset the carb to factory start settings and run the engine on test. At one point, I believe the plug cap was found to be loose so this might have damaged the ignition module but the spark was present after this was noticed and I'd assume it would be high tension components at risk? We may have a second module available after chatting to another member today...

Have I missed anything obvious?

[Bob Cotsford]

Have you tried it with a timing disc and test light to check when it's actually sparking? Test sets are only a couple of quid from HK, in fact I think I've got one spare if you get stuck. Any chance that the key locating the prop driver/magnet could have jumped ship? I had an older MVVS and the timing setup on that was odd with the pickup being offset from TDC. I know later models were more conventional though. Other than that, does the reed look ok? Does it seem to seal, no chips or splits? I guess the biggest risk running backwards would be overstressing the crank and conrod as the timing would should be so far out.

[Martin Harris - Moderator]

Thanks Bob - another clubmate I was talking to yesterday mentioned that he had a timing kit so we should be able to do a check on this point. However, on the crankshaft loading issue, my assumption is that with the sensor position at TDC, direction will not affect timing? Unless the key wasn't ever fitted and the prop driver has co-incidentally slipped to TDC from an advanced position, I don't think there's much chance it's moved, especially as the owner says it hasn't been dismantled.

I've only checked the reed valves visually through the inlet tract and they appeared OK. Blowing into the crankcase tapping for the pump with the piston sealing the transfer ports revealed slight leakage out of the inlet port but I assumed that as the valves are simply thin carbon fibre sheets resting against the ports, a perfect seal at rest was not to be expected. Have you any experience of checking these at rest or is it a case of running pressures creating an effective seal?

Edited By Martin Harris on 08/05/2018 10:16:32

[Bob Cotsford]

The pickup may be at TDC but the auto advance function will retard the timing by about 28-32 degrees at low speed and advance at high rpm. Reverse that and you have a very odd timing setup! I think they calculate when to spark based on the timing pulse interval once the motor is over a certain RPM or after so many pulses (to allow for hand starting) so it's predictive in nature. It doesn't know when it's running backwards!

[Martin Harris - Moderator]

That's why I'd assumed that the spark would have the required amount of advance, whichever way the engine was running...if it's time based with the trigger being at TDC wouldn't it would fire at the calculated point before TDC whichever way it was running? Even more head scratching while I try to work this out!

Edited By Martin Harris on 08/05/2018 18:40:21

[Bob Cotsford]

ah but how would it know which way it was running?

[Martin Harris - Moderator]

That's exactly why I'm having trouble working out why it wouldn't run the same in either direction! (I do appreciate your interest by the way and trying to understand your theory - I'm not just arguing for the sake of it!)

The way I see it, the ignition unit doesn't know which way the engine is running so it only knows when and how often the sensor (at TDC) is triggered. It then decides, e.g. I'm going to fire the ignition when I calculate it's at 30 degrees before TDC - in 5.678 mS...

I may very well be wrong but that's the only way I can see that it could work...

[Bob Cotsford]

All of this is hypothetical BS based on vague memories of an apparently knowledgable discussion of MVVS ignition units carried out on RCGroups.  I understood it to be a problem with where the index point is taken and some MVVS units didn't use TDC as the index, rather they used something like the retarded firing point which may have been in advance of TDC for a standard CCW running direction.

My MVVS (branded Evolution as it happened) only had a single magnet and iirc it was a about f in that it didn't use the same logic as other ignition units so if the EI died you had a scrap engine. Vallach rings a bell for the EI manufacturer.

On a different tack, I think the piston and liner ports may be optimised for CCW running though I wouldn't have thought that could do much damage when run backwards.

Edited By Bob Cotsford on 08/05/2018 20:20:06

[Martin Harris - Moderator]

Thanks again for your interest, Bob, I'd wondered about that last point but although it may be rotating the other way, isn't the piston simply going up and down - the gases still flowing through the ports at the same time due to compression of the crankcase and the ports being uncovered? Unlike a conventional glow 2 stroke, I don't think there's any element of inlet timing like there might be with a crankshaft port?

I still can't see why with the pick-up at TDC the ignition system would work any differently whichever way it rotates. Obviously, if the pick-up was at an advanced position then things would be very different. But as I'm a 4 stroke man at heart, I may be missing something significant! My only petrol 2 stroke uses a magneto with fixed timing which will only run it in the correct direction.

The "running backwards damage" mentioned in the instructions really intrigues me as I still can't see what could be damaged...

[Chris Walby]

I could be completely wrong, but when we worked with a company that was making electronic units for the grass bike engines the theory explained to me was:

if you time the ignition for TDC you can only have TDC (as you most advanced point) and can retard if you want. electronics cannot predict the future! So you can't say fire 10 degrees before you detect the firing point with the magnet/switch and waiting for nearly another revolution is too inaccurate.

We would set the ignition for 42 BTDC and the electronic unit would add more delay the slower the engine RPM down to a max value. The only issue was that it needed to see at least 1 revolution before it knew the engine was rotating which for hand starting a single cylinder 500cc engine on 16.5:1 compression proved a tad exciting.

Manufactures can choose any point to put the magnet/electronics, but for ease TDC is easiest point to find in relation to the piston position, but in reality it is say 30 degrees BTDC (its fully advanced state), but only the manufacturer really knows what their settings are.

The Japanese motorcycle manufactures used three or four capacitor circuits set to various resonant frequencies that adjusts the delay time (not a curve, but cheap). Now electronics don't like being left off for long periods of time so its possible the electronic unit as partially failed (you can only really tell by replacing it with a know good one).

Lastly if an engine (2 stroke) is timed for say TDC it will run in either direction, but the more you rev it the less power it will produce, what you really need to do is advance the ignition point and this it what I don't understand with your previous comment. It should have well advance ignition (in normal rotation direction) and if it was going backwards (crankshaft) the more you rev it the more retarded the ignition hence it won't rev out if it has a fair prop on it). This is where the manufacturer advises it not running it backwards and very retarded ignition normally results in very hot exhaust.

If I think of anything else I'll post again.

PS we grew to "dislike" 2 strokes with a passion due to melting multiple pistons, hence my preference for a nice 4 stroke on methanol!

[Martin Harris - Moderator]

Thanks Chris. As I can watch the piston through the exhaust port, I can say with certainty that the sensor is mounted so that the magnet is dead central with the sensor at TDC. As this is supposed to be a "performance" engine rather than something powering a leaf blower, it only really makes sense that the spark timing will be somewhat advanced from TDC, so the electronics must be doing the job. I think Bob described a viable scenario a few posts back where sensor info can be processed in order to advance the firing point as RPMs increase. Start timing can be nicely retarded for an easy start - not like the magneto compromise position of more basic conversions.

So, there remains the possibility that the driver has slipped if TDC is not the correct point - unlikely as it's keyed (according to the manual) although it's impossible to check without stripping the engine, as the driver arrangement hides the keyway - but what are the chances of it slipping to exactly TDC? While on the subject of the prop driver, it seems to be retained by the propeller mounting shaft so perhaps there's a danger of it loosening if the engine is run backwards - maybe the explanation for the repeated warnings in the manual?

I'll know more if I get the chance to bench run it - the owner has the ignition model buried in his model but another one should be available for test. My suspicions are that its later refusal to run at all may relate to the ignition module possibly being stressed by running with a loose plug cap - although it was still producing a spark. It could just be that the adjustments to the mixture may have caused it to flood so we'll start at the standard settings...

Edited By Martin Harris on 09/05/2018 00:03:48

[onetenor]

All side ported and reed valve motors have the ability to run backwards. On sparkies this is governed by the spark timing. I used to have people calling me out to their bike or scooter with 3 or 4 reverse gears,and no ideas as to why. My first question was always Have you had the points out? The answer was always yes. What had happened is that as they came to put the points back in the lead was very tight when in the right set of holes so they assumed this was wrong to be so tight so put them in the holes that left slack in the lead. Hey presto engine runs backwards An easy few quid for me for putting it right.. Five minute job.. Glows and diesels of course run without sparks so are only timed by their porting of course, Side ported or reed valved. An article in IIRC Aeromodeller mag showed how to convert an E.D. Bee from disc valve to reed valved. This delighted my mate Peter who was having trouble finding pusher props for the Bee powered pusher he built. He did the conversion and the engine ran better than it had before. He had to clean the back plate up a bit where the old valve had marked it so the reed made a good seal. I never saw a better running Bee in either direction. I can only think that the damage warning relates to the bearings. When a set of bearings have been running in one direction for some time they develop a wear pattern. Reversing the direction of rotation can accelerate  the wear in the races and balls. This may be the reason for the warnings,

Edited By onetenor on 09/05/2018 02:21:51

[Chris Walby]

Onetenor, Bearing rotation makes no difference, its things that have threads are important as they have a habit of undoing if going backwards !

I think Martin is on the right track with:

• Get it on the bench so everything else is isolated (tank and plumbing etc)
• Strip and double check the carb diaphragms and for any air leaks etc
• Set carb to standard manufactures spec and try to start
• Remove and check reed block, if the petals don't seat it will give all kinds of odd running issues
• If that fails change for known "good" ignition unit

As it ran okay a few years ago it should be sound, just IMHO a lot of standing around something has perished/failed.

I am sure with your usual methodical approach you'll get to the bottom of the issue.

PS are the reeds metal of fibre?

[Martin Harris - Moderator]

They are carbon fibre Chris.

Unfortunately, I don't think we're any the wiser as to why an engine which was exhibiting symptoms of faulty carburation suddenly ran perfectly in the reverse direction...

[Bob Cotsford]

That's the joy of life Martin, it's infinite mysteries

[Martin Harris - Moderator]

So true!

[bert baker]

For my two pennies,,,

A recent strip down of a DA 100 that had had two years off work, found two stroke oil had tuned more or less turned to grease,,,sticking the flappy pump valve shut ad also the fine round mesh filter was blocked with same stuff..

I had a MVVS 28, that would fire once then quite,,

I have both test tools for spark unit testing and a buzzer tool for the,,

The tools showed the pick up was ok as well as the spark unit.

I changed the spark unit out of desperation and all came back to life,,test tools hmmm more like a general guide,

Has it definitely got the original spark unit fitted ,,, not got mixed up with another

[Engine Doctor]

Posted by Martin Harris on 07/05/2018 22:15:42

I have to admit very little experience with reed valve 2 strokes so perhaps someone on here might be able to help?

Later reference to the MVVS instructions revealed a couple of warnings not to open the throttle if it started backwards as it would "cause damage" to the engine. I've looked at the only parts I can think of that could conceivably be affected - the reed valves - without dismantling further than removing the carb and they appear intact - what sort of damage could running a 2 stroke backwards cause?

Have I missed anything obvious?

Running a read engine backwards is fine on some simple engines. Larger petrol engines have pistons that have a thrust side ,designed to run in one direction . Also the cylinder porting has probably been designed to support the piston correctly while running in the intended direction. Old side port diesels like comp specials and Mills and similar glows were happy running in either direction .

I'd start by changing the ignition unit . They will only advance the spark in one direction as speed increases. A retarded spark is usually for ease of starting without kicking back. 45cc kicking back is not nice ! Make sure that A, correct voltage is used for ignition unit and B ,Don't under any circumstances turn engine over if ignition is switched on unless lead is connected to the plug as this can easily damage ignition unit.

[Engine Doctor]

Sorry posted twice. Doh!

 

Edited By Engine Doctor on 09/05/2018 15:58:25

[Martin Harris - Moderator]

That's basically the plan ED. I suspect that the ignition unit may have been damaged while the engine was run with a loose plug cap.

I can't see any basis for the timing other than RPM information and the unit triggering the spark at a calculated time before (or more likely after as Bob pointed out, it can't make a prediction!) it has sensed the crankshaft position. This more or less removes the ignition unit as the cause of the phenomena of the engine refusing to tune with conventional adjustments - it could, with a lot of throttle juggling, be coaxed through transition but was still not running right and didn't respond to the high end needle once there - but when it started in reverse, it idled well, transitioned cleanly and peaked out nicely - that's the enigma!

[Engine Doctor]

Is it possible the prop driver has moved on the crank or the hall sensor has moved or is faulty? For it to spark at exactly TDC seems like timing is retarded . Has the magnet in the prop driver fallen out or has it been replaced ? the polarity of the magnet can effect the ignition . Retarded timing will cause running  that appears like a rich mixture and very hot running and a loud flat exhaust note instead of a crisp pop.

 

Edited By Engine Doctor on 11/05/2018 09:26:13

[Martin Harris - Moderator]

Although the "rich running" when retarded fits the symptoms, I think it's unlikely unless the key has sheared (or was never fitted) and the driver has slipped so that the magnet is exactly at TDC. I think we can discount casual "owner interference" as the driver appears to be retained by the propeller mounting shaft (see the diagram I posted earlier) which I suspect may be Loctited and would need a determined effort to disassemble. The sensor mounts on a fixed location so wouldn't be a factor.

My assumption is that the sensing is done at TDC but the ignition unit processes the information to decide when to fire the spark based on calculated RPM and crankshaft position information from the sensor pulses.

We do have a member who has an identical engine so it will be easy to check when he brings it along

An interesting point is that the driver has two magnets 180 degrees apart with opposite polarities. I suspect this may be so that it can be used on a twin cylinder version? I think this is a "red herring" in regards to this engine's problems though - if there was a 90 degree difference, for example, it would be different as it could then be used to check direction of rotation.

My money is tentatively on an ignition control module fault but it doesn't explain the better reversed running quite as well as your theory! It will be very interesting to check the other engine's magnet position...

[onetenor]

iBang Good have been popping up an ad all week for new ignition module complete kit with wiring etc etc. I tried to find it on the site today and 169 items were turned up. I looked at 2 pages but they were for bikes and cars. The one they advertised was for about 28 quid or so if you fancy ploughing through the site to find it It Iooked very comprehensive.

[onetenor]

iBang Good have been popping up an ad all week for new ignition module complete kit with wiring etc etc. I tried to find it on the site today and 169 items were turned up. I looked at 2 pages but they were for bikes and cars. The one they advertised was for about 28 quid or so if you fancy ploughing through the site to find it It Iooked very comprehensive.

[onetenor]

iBang Good have been popping up an ad all week for new ignition module complete kit with wiring etc etc. I tried to find it on the site today and 169 items were turned up. I looked at 2 pages but they were for bikes and cars. The one they advertised was for about 28 quid or so if you fancy ploughing through the site to find it It Iooked very comprehensive.