Re: Mototrans 250 Scrambler restoration: she's a runner!

[themoudie]

Aye Al,

Woah!

This is increasingly reading the same as a problem that I have with my Moto Morini 125H engine. This uses a Ducati Electronicca alternator, 6v 4 coil charging system, along with a single power coil and a trigger coil, mounted on the alternator stator plate and within the alternator rotor, powering and sending an ignition signal to a CDI/coil unit. The trigger for the trigger coil is inside the rotor.

Your MotoTrans has a similar alternator stator, but you might not be able to view the coils as they may be encapsulated in resin. However, the Saches ignition trigger will be mounted in the upper part of the front offside compartment of the engine timing chest. And the timing is adjusted by moving the trigger in relation to the rotation of the disc on the shaft.

My Morini 125H will readily start and idle all day at 1,100 - 1,200rpm. When the throttle is gradually opened the engine revs will increase to approximately 2,000rpm, at which point the engine starts to hesitate and if the throttle isn't opened any further, this irregular running will continue, but the exhaust pipe will start to glow red hot! If the throttle is gradually opened further, the revs may increase to 2,500rpm, by which time the back firing and spluttering is very pronounced and then the engine will stop dead.

From having spent many hours trying to solve this conundrum, along with other Morini 125H owners suffering exactly the same symptoms, we have proved that the engine timing is not advancing!

Having re-read your post of Thu Sep 28, 2023 7:37 am on this thread, I suspect that you need to be contacting Saches to find answers as to how to establish whether their CDI unit is faulty, or the trigger unit that sends electrical signals to the CDI is faulty.

In my opinion your problem lies with the ignition and not the carburation. By all means check the ignition timing, but in your previous posts you have said

My scrambler has been technically ready to go for the last 12 months or so, and it was starting, running and revving well in the garage with no load on the engine. Not road-tested though.

and

So I got her started, warmed-up for a few minutes, then took her out on the road for the first time since 1981. I have some carb tuning to do, as the engine wouldn't run without the choke, was unwilling to rev, and generally seemed gutless.

and

The gearing is higher than I remembered, so that resulted in a stall, but restarted without problems and I had a sedate circuit of the local roads in first & second gear. It felt like the engine wouldn't tolerate third gear.

On reflection these observations read as though there is a gradual breakdown of an electrical component and very similar to my experience with the 125H. At present all my 125H testing has to be carried out on the stand, as putting any load on the engine causes it to die. At one time the engine wouldn't rev sufficiently to achieve more than 60Km per hour, but this gradually declined, with time and deterioration of the trigger coil internal insulation.

So, I do not think that the engine timing, or the ignition timing are at fault. Something within the ignition components has cried "Enough!", sufficient to stop the ignition advance.

Electrickery is not my forte and with the 125H, the whole of the Morini Riders Club expertise has been brought to bear to try to solve the problem. I believe the problem has been located and it is due to the polarity of the signal coil not sending the correct signal to the CDI circuitry to advance the ignition when the desired engine "RPM" are reached. Hence my advice to go back to Saches. Just because it is a new system, doesn't mean that it doesn't have a component failure!

I hope this helps, good health, Bill

[A.I.M]

Thanks Bill, that's an interesting bit of lateral thinking on your part... and it does seem to make a lot of sense given the symptoms I'm finding. I will indeed ask Sachse for their views on this and order any parts necessary. If that is the problem it would be incredibly frustrating as I had hoped that the Sachse ignition would be a "fit & forget" reliability enhancement.

I think I will also check the static ignition timing and valve timing next week: if nothing else it will prove to myself that I can do it.

Cheers,
Al

[themoudie]

Morning Al,

I think I will also check the static ignition timing and valve timing next week: if nothing else it will prove to myself that I can do it.

I would agree, take your time, it can all be a bit Zen!

Also, whilst a digital multi-meter is very useful, the readout can "dance about" and an analogue multi-meter needle can provide a more steady reading, when trying to obtain critical readings, such as voltages from ignition coils, whilst the engine is running.

Not having ever dealt with Saches, I don't know what their warranty period is, or their after-sales service. I just hope that they are more helpful than HPI in Belgium were, when I had problems. A chocolate tea strainer comes to mind! I have gone down the Electrex route and to date, after over 11,000 miles (18,000Km) I am very pleased.

I wish you success, with the sleuthing.

Good health, Bill

[Duccout]

Hi Al,

Sorry for the late reply, I've been away. I will try to post a picture of the timing marks, but there are some pictures of them on this site so try a search for 'timing marks'. The problem is that Mototrans camshafts used a keyway in a different position to the Italian ones, which throws the valve timing out if parts are mixed up, so it is a good idea to check the valve timing with a degree disc.

I hope that you have made some progress.


Colin

[A.I.M]

I'm currectly waiting for a reply from Sachse about whether there are any diagnostics that I could to to work out whether there is a problem with the ignition advance, and / or which components to replace. I also tried switching to a different advance curve, but no change. I also re-checked the electrical connections of the ignition system.

The engine starts incredibly easily and idles beautifully but hesitates and ultimately quits if I rev the engine or try to pull away in first gear. Since the old carb (refurbished) has given slightly different symptoms than the brand-new one, I will try changing over again and see what happens.

I'm working towards checking valve & ignition timing (no degree wheel or piston stop yet) and I've been searching old posts on the site to work out how to deal with the "two dots on the bottom bevel gear" issue, and I think I have found the right picture from Eldert ....

16979130747387308636152122094775.jpg

...and the advice to use the left dot, not the one that aligns with the keyway from Jon.

So not there yet, unfortunately but I have a better idea of what to do next.

[Duccout]

Hi Al,

That's the picture. One for is correct for Mototrans and cams and one for Ducati Italy cams, and the only real way to tell if the engine is timed right is with a degree disc on the crankshaft. Having said that, the engine would still run a tooth out (although a bit flat) so I would still be looking at ignition or carb problems.

Good luck,

Colin

[Jon Pegler]

Using the left dot is not a hard and fast rule.
Most Mototrans bikes used the left dot as standard with the Mototrans camshaft, but as these bikes are now almost 50 years old it's best not to assume the correct parts are fitted.
Do as Colin suggests and use a degree disc to check the camshaft opening and closing events.

Jon

[A.I.M]

Thanks Bill, Colin & Jon, your sage counsel is hugely appreciated.
I am fairly confident that the camshaft is the original, I have been the (2nd) owner of this bike since it was 3 years old / around 8,000 km (now 49 years and 15,000 km), and I knew the original owner and the bike's history when I bought it. However I am aware that original is not necessarily the same as consistently manufactured with these bikes.
Nevertheless I do plan to check valve & ignition timing, as I was not present the last time the head was removed and reinstated, nor did I check the timing marks. I'm certainly not discounting anything at this point, and I fully appreciate that this problem is likely to, frustratingly, have more than one source.
Just to muddy the waters a bit more, I changed back to the brand-new Amal 627 carb and managed to get a 10 km test run today, on single-track roads only suitable for 2nd - 3rd gear. Starting and idling is not quite so good with this carb, but on the road the engine was much more willing this time - but still wouldn't tolerate running without the choke, and the road I chose wasn't suitable for 4th or 5th gears.
Again, two carburettors each set-up with exactly the same jets, needles etc, but producing completely different running characteristics. More work to do....
Cheers,
Al

[Duccout]

Hi Al,

I wish that we could be more help.... Obviously the fact that the bike runs on choke points to fuel problems - which could be ignition problems......

I would stick with the new carb, as at least you know that wear is not clouding the issue. You can make up a degree disc for the timing quite easily, you just need an 8mm bolt welded to a short length of 12mm studding and two 12mm nuts and washers; the discs are available on line. You can then at least check the ignition timing and verify that the valve timing is correct.

It is strange that the engine will only run on choke. Although ignition problems and carb problems can seem the same, I would have betted that your problem is fuel starvation or an air leak.

Cheers,

Colin

[mrkprsn]

I am going to go out on a limb and suggest that something in the head might have changed since you originally ran the engine. It feels fuel related but you've already tried 2 different carbs. If it were me, I might inspect the head and its components. Turn the engine over and see if the valves are opening and closing correctly and at the right time. I might also pull the head and do a gas "drip down" test. Turn the head over and fill with gas and see if any leaks out. Good luck...Mark