My MK3 does not rev beyond 4500rpm

[javifoto70]

I completely restored my MK3 with new pistons and rings, restored head with new head gasket getting good compression, new triple primary chain, new clutch, new Boyer dual output coils, new Boyer MKIV Ignition kit, new AMAL PREMIER 932 pack 113 (932/33 - 932/34) carbs, new negative ground electric wiring with Motogadget M-unit control box.

The fact is that the engine does not rev over 4500rpm on not any gear, it vibrates and does not want to rev.

The bike starts at first kick, idles and revs perfectly up to 4000rpm, the valve clearance are perfect, the sparks have a good color. The bike runs smooth on road and has good torque up to 4000rpm.

Using a stroboscope light the marks are spot on on right cylinder but strangely the light emitted when checking the left cylinder is weak, it only produce an erratic light around idle but does not produce light at all beyond idle.

I suspect Boyer ignition problem:

I checked the rotor/stator captors cables and they are ok, I connected a direct ground cable from negative battery to the Boyer Control Box, I connected a ground cable from engine to frame.

I made Boyer dual output coils test: the resistance between the primary terminals with the wires removed. Should read from 2.5 to 4.5 Ohms and I read 4.6 Ohms. The resistance between the primary terminals and the center core of the coil is infinite resistance. Between one of the primary terminals and one of the plug wire terminals is also infinite.

But the resistance one plug terminal to the other should be 15,000 to 20,000 Ohms and I read 27,000 Ohms.

Is it normal for the Boyer to produce stroboscopic light output only on one cylinder?

Could this be the origin of the problem?

 

[MexicoMike]

The strobe light should be equally strong on both cylinders. Offhand it sounds to me like one cylinder is not firing under load. Could be a problem with the Boyer, the ignition coil for that cylinder, the ignition wire to that cylinder, or the spark plug itself.

 

[L.A.B.]

new Boyer dual output coils,

Has it been twin plugged or do you mean one dual output coil?

Is it normal for the Boyer to produce stroboscopic light output only on one cylinder?

No, and as it's a wasted spark system then the Boyer is providing one output to both cylinders at the same time so more likely to be the coil, HT lead, plug cap or plug I would have thought. Do you still have the original 6V coils you could try?

 

[kommando]

The problem that you can get with a bad earth between the head and the battery terminal giving a weak spark on one side only applies to two coil setups, a dual coil creates its own loop as each internal coil has opposite polarity. So to have an issue with one spark and not the other is as @LAB suggests with the HT side, so coil, HT leads etc.

I would as a test however extend the engine to frame earth directly up to the battery terminal. Commando frames are not good conductors of electricity.

 

[Matchless]

Does the engine misfire when it gets to 4500 rpm or just not want to rev any further? If not then it sounds like the cam timing is wrong.

 

[johnm]

Not a solution to your problem but try reversing the strobe pickup on the plug lead on the weak side. That may sort out the strobe

 

[MichaelB]

Are the chokes fully open?

 

[DennisMo]

As already said by our wise Moderator, the Boyer is working fine as it triggers both coils at the same time and one cylinder is working fine.

I suggest you check individually the resistance of the bad cylinder's spark plug (from centre firing electrode to where plug cap clips onto the plug),
then the plug cable resistance, then the coil resistance (both the primary and secondary circuits). A good idea is to then check same on the good cylinder and then compare these side by side.
Dennis

 

[BrianK]

As already said by our wise Moderator, the Boyer is working fine as it triggers both coils at the same time and one cylinder is working fine.

I suggest you check individually the resistance of the bad cylinder's spark plug (from centre firing electrode to where plug cap clips onto the plug),
then the plug cable resistance, then the coil resistance (both the primary and secondary circuits). A good idea is to then check same on the good cylinder and then compare these side by side.
Dennis

Ditto that. And try switching the plugs, and then the wires, from side to side. Does the problem migrate also?

Apropos of plugs, as I and others have noted elsewhere on the forum, there are a lot of counterfeits out there. E.g., I have a slew of ostensible "NKGs" that ohm out at ridiculous levels. The problem does seem to manifest itself more as revs rise.

Good luck and please report back so we can learn from your experience.

 

[javifoto70]

Has it been twin plugged or do you mean one dual output coil?


No, and as it's a wasted spark system then the Boyer is providing one output to both cylinders at the same time so more likely to be the coil, HT lead, plug cap or plug I would have thought. Do you still have the original 6V coils you could try?

The Boyer is a one dual output coil. I tried to interchange the HT leads but I cannot detach the leads from the oil, it looks like sealed together. It is a test I would like to do to be sure the problem is not in the lead.

I have not the original 6V voilà. I have instead 6v coils from a 1979 Moto Guzzi, would they be compatible?

 

[javifoto70]

The problem that you can get with a bad earth between the head and the battery terminal giving a weak spark on one side only applies to two coil setups, a dual coil creates its own loop as each internal coil has opposite polarity. So to have an issue with one spark and not the other is as @LAB suggests with the HT side, so coil, HT leads etc.

I would as a test however extend the engine to frame earth directly up to the battery terminal. Commando frames are not good conductors of electricity.

I already extended the earth to the battery negative terminal but the problem was still there.

 

[javifoto70]

Does the engine misfire when it gets to 4500 rpm or just not want to rev any further? If not then it sounds like the cam timing is wrong.

No misfire, it just does not rev any further.

 

[javifoto70]

Not a solution to your problem but try reversing the strobe pickup on the plug lead on the weak side. That may sort out the strobe

I already reversed the strobe pick up. On one side it just did not produced any light, on the other side produces a weak light only at idle and no light when revving.

 

[DennisMo]

Do measure the resistance from the spark plug boot inside metal contact, to each terminal on the coil.
For both coils it will be megohms.
Also each sparkplug from centre electrode that sparks to the contact outside where the spark plug cable boot clips on

 

[johnm]

Are the chokes fully open?

I always forget to ask that. After 50 years of pulling Amal Concentrics to pieces you tend to assume everyone knows how the choke works.

Cable tight. Choke off. Opposite to many bikes and cars.

 

[javifoto70]

Since I cannot exchange the HT leads I am considering buying a new double output coil and eventually also new spark plugs. Do you suggest any particular reliable coils and sparks plugs?

 

[johnm]

Since I cannot exchange the HT leads I am considering buying a new double output coil and eventually also new spark plugs. Do you suggest any particular reliable coils and sparks plugs?

When all else fails follow the instructions of the ignition manufacturer.

I have a Pazon on my bike. I use the 6 volt coils sold by Andover Norton and NGK BP7ES plugs. I use a NGK resister cap and solid copper HT leads. If you buy cheap plugs off the internet they may be fakes. You tube videos and the NGK site will tell you how to spot fakes. You need to follow the ignition manufacturers instructions regarding the requirements for any resistor plugs or caps. Different systems have different requirements.

Andover Norton - IGNITION COIL (6V) PVL 45222A, 55.1153, 06.2092 1.5/8" (40mm) diameter - 99.0576 https://share.google/6l1F8shBScEoeabny

 

[javifoto70]

I always forget to ask that. After 50 years of pulling Amal Concentrics to pieces you tend to assume everyone knows how the choke works.

Cable tight. Choke off. Opposite to many bikes and cars.

Yes I know, certainly I set choke off on both carbs once the engine starts. Strangely the engine starts up, idles, push and revs perfectly up to 4000rpm.

 

[javifoto70]

The HT leads are the ones coming off the dual coils, the spark plugs are new NGK ones.
Anyway I will double check the Boyer ignition instructions and troubleshooting guide.

 

[johnm]

A year or two ago a friend had real problems with running on one cylinder on his bike. We swapped everything carbs, plugs, ht leads, coils, wiring, had the ignition unit checked.

In the end it was the trigger unit down on the end of the camshaft. You have to be careful to get the clearances of the rotating and static parts correct. Sometimes they need spacing out.