Tohatsu M70B misfiring

[panula]

Hello,
experiencing top end RPM low and poor acceleration on my tohatsu m70b. Furthermore, starting difficulty with misfirngs occurring already at low rpm till the motor shuts off.
Cheked out ignition coil primary resistance with good ohmeter: 0.21ohm
Cheked out ignition coil secondary resistance with good ohmeter: 4500ohm
Cheked out alternator coilresistance with good ohmeter: 0.24ohm
Cheked out Exciter coil output at cranking speed with DVA: all three values widely above the specification listed in service manual (one 40V instead of 9V, other two 155V instead of 35V)
According to the service manual, i hence checked the pulser coils with DVA at cranking speed: one pulser coil outputs 4.55V, the other two coils output 3.4V.
Is the procedure above correct? If it is, does it mean i should replace the pulser coil?
Are pulser coils sold separately or should i buy the pulser coil assembly?

Thanks for your attention


Luca

 

[pvanv]

Re: Tohatsu M70B misfiring

There are 2 exciters, 2 alternators, 3 pulsers. Pulsers come as as unit.

When did this problem start? Are the carbs clean and synced OK?

 

[panula]

Re: Tohatsu M70B misfiring

There are 2 exciters, 2 alternators, 3 pulsers. Pulsers come as as unit.

When did this problem start? Are the carbs clean and synced OK?

Thanks for your reply.

Problems started some days after a complete engine refit due to engine siezing.

• Cylinders have been grounded by professional workers.
• New cylinder-head
• 3 New pistons (+0.50) with rings
• New thermostat
• New gaskets
• Cylinders comprssion tested. New values are around 900kPA (130psi) in each cylinder.
• Carbs have been unmounted and checked. they have been blowed and cleaned, then mounted and synced.
• Timing and carbs arms have been set according to service manual specs.
• New sparks

After refit, the engine started without any problem. Perfect idle as well as acceleration.
Problems at HIgh RPM came the day after, but still instantaneous engine start up
Ignition Coil testing revealed problem. No continuity or sputtering behaviour in Coil Resitence testing. More accurate checking revealed no continuity in spurkplugs. All 3 repleced with 3 brand new NGK plugs.

Then misfiring problems came up together with starting problems. Explosions in combustion chambers occurre at wrong times. When it happens, water in the flushing barrel litterally blows up.

Magneto checking with multimeter and DVA adapter, revealed the wrong values listed above.

 

[TOHATSU GURU]

Re: Tohatsu M70B misfiring

What service manual are you using? Unless you left out test results you didn't check the coils correctly...And what are the results of your CD module tests?

 

[panula]

Re: Tohatsu M70B misfiring

What service manual are you using?

I'm using an aftermarket service manual, but I have to say I found it quite messy...The manual deals with 2 or more different Tohatsu models at the same time and illustations are poor.

Unless you left out test results you didn't check the coils correctly...

the manual's guided procedure is as follows:

IGNITION SYSTEM CHECK	INSTRUMENT	LEADS COLORS	READING	REFERENCE RANGE
Ignition coil resistence PRIMARY Resistence SECONDARY Resistence​	Ohmmeter	B/W to B spark cable to B​	0.21 Ohm 4800 Ohm	0.16 - 0.24 Ohm 3000 - 5500 Ohm​
Alternator resistence	Ohmmeter	W to Y	0.24 Ohm	0.21 - 0.31 Ohm
Pulser coil peak Volts output (cranking speed)	Mutlimeter+DVA	B to W/R-W/B-W/L	3.4-3.4-4.5 V	4.75-5.0 Peak V.
Exciter coil peak Volts output (cranking speed)	Mutlimeter+DVA	W/G to Br/W W/G to W/Y Br/W to W/Y	40 V 155 V 155 V	8.1- 9.9 peak V. 34.2-41.8 peak V. 35.1-42.9 peak V.

Isn't correct?

And what are the results of your CD module tests?

I tested CDI as follows:

IGNITION SYSTEM CHECK	INSTRUMENT	LEADS COLORS	READING	REFERENCE RANGE
CDI peak Volts output (cranking speed)	Mutlimeter+DVA	B to B/W-B/W-B/W	6.5 V	198/220 peak V.

Did i miss something?

One more thing:
Despite such a poor reading value in CDI output, the spark plugs fire normally..... strange, isn'it??

Thank you very much for your attention.

 

[TOHATSU GURU]

Re: Tohatsu M70B misfiring

There are a bunch of different measurements that you need to take. I'm at a boat show and do not have access to a manual until Monday, but Paul can probably look it up before then. The CD tests are extensive and you may have a problem there. Generally one electronic part fails...Sometimes its a combo.

 

[pvanv]

Re: Tohatsu M70B misfiring

The Factory service manual uses resistance tests for the CD, as well as the ignition coil, and the pulsers and exciters. You MUST use a good analog meter, not a digital unit, and the test battery of the meter cannot exceed 3v.

Pulsers should each be 80-120 ohms.

Exciter w/g-br/w should be 216-324 ohms. Exciter br/w-w/y should be 16-24 ohms.

The CD tests are an extensive table; it's on page 49 of the old style service manual (dedicated to the M60B and M70B), and takes up the entire page. I could help you with the readings, but I can't reasonably translate the entire table into this post. You could email me: paul@obersheimersails.com, and I can give you some info.

 

[panula]

Re: Tohatsu M70B misfiring

Paul, Elvin, thank you very much for your consideration and attention.

Paul, I will obtain a good analog meter (I've used a digital one, so far) and I will follow your specs to check pulser/exciter resistences. I will text in this topic as soon as I got the readings.
It's very kind of you to help me in this matter. I will email you for further details.
Thanks.
Regards.

Luca

 

[levi_tsk]

Re: Tohatsu M70B misfiring

panula, let me know if you need any parts my m60b just bought the farm and the engines use the same electrical stuff

 

[panula]

Re: Tohatsu M70B misfiring

The Factory service manual uses resistance tests for the CD, as well as the ignition coil, and the pulsers and exciters. You MUST use a good analog meter, not a digital unit, and the test battery of the meter cannot exceed 3v.

I just bought a good analog multimeter and here are my readings:

Pulsers should each be 80-120 ohms.

My reading: 170 ohms each

Exciter w/g-br/w should be 216-324 ohms.

My reading: 250 ohms

Exciter br/w-w/y should be 16-24 ohms.

My reading: 20 ohms

Does it means that exciters are good, while the pulsers need to be replaced?

The CD tests are an extensive table; it's on page 49 of the old style service manual (dedicated to the M60B and M70B), and takes up the entire page. I could help you with the readings, but I can't reasonably translate the entire table into this post. You could email me: paul@obersheimersails.com, and I can give you some info.

Paul, I sent You an email. I hope I'm not bothering you too much.

Thanks.

Luca

P.S.:

panula, let me know if you need any parts my m60b just bought the farm and the engines use the same electrical stuff

thanks for your offer, levi_tsk. I'll take you into account.

 

[pvanv]

Re: Tohatsu M70B misfiring

Any inductive unit (coil) can drift a lot and still be OK, so pulsers may still be OK. I would proceed with the CD tests. I'll return your email when I'm in the office tomorrow...

 

[panula]

Re: Tohatsu M70B misfiring

I tested CDI strictly following manual's table (thanks Paul!) .

I wrote impedence readings in red:
I wrote OK when reading is equal the reference value.
Mismatches may vary a lot: from 10 to 50% of reference values.
Mismatched values are always lower than reference values.
I noticed that impedences with smaller reference values, returns greater variances when tested.
On the other hand, all the readings above 180KΩ are equal to references values.

Are these variances still acceptable or the CDI needs to be replaced?
Thanks.
Luca

 

[pvanv]

Re: Tohatsu M70B misfiring

The readings are a bit off, but I wouldn't condemn the CD based on those numbers alone. Are you still sure that the CD outputs to the ignition coils are below 10 volts (read with a DVA or oscilloscope? Should be well above 50; ideally around 100. Lower voltages may fire the plugs at slow speeds, but will break up as the RPM increases.

 

[panula]

Re: Tohatsu M70B misfiring

HI everyone,
Sorry for not have been updating for one month.
I was double, triple checking all measurements and settings and I found out an unforgivable mistake I made when changing spurkplugs, after complete engine?s refitting.

After refit, the engine started without any problem. Perfect idle as well as acceleration.
Problems at HIgh RPM came the day after, but still instantaneous engine start up
Ignition Coil testing revealed problem. No continuity or sputtering behavior in Coil Resistance testing. More accurate checking revealed no continuity in spurkplugs. All 3 replaced with 3 brand new NGK plugs.

Then misfiring problems came up together with starting problems. Explosions in combustion chambers occurre at wrong times. When it happens, water in the flushing barrel literally blows up.

In order to change the spurkplugs, I removed the 3 coils ?..When reassembling them, I have unintentionally exchanged CDI output leads to the coils: I run B/W cables through the lower side of cylinder head instead of passing them on the upper side. Consequently, I plugged the lower coil to the shorter lead, while the upper coil was connected to the longest one. To put it in an easy way, the only coil which was giving correct timing to the cylinder was the middle one.

? I'm slightly ashamed. I made a boo-boo :facepalm:

This is my Tohatsu video, filmed 5 months ago, after bloody coil exchange:

Now I have put the leads in the correct order, but the problems are even worst:
? Engine cranks but won?t start.
? At cranking speed, mistimed ignition causes explosions throughout the exhaust.
? Off value in CDI readings persists.

Did my mistake cause the CDI to be damaged?
What kind of damages might have occured due to my mistake?

Thank you very much for your attention.

Luca

 

[pvanv]

Re: Tohatsu M70B misfiring

I would take it from the top:
- Make sure you are firing in time from the flywheel. Check that you don't have a sheared magneto key that has allowed the flywheel to fire everything off time.
- Verify that #1 coil is firing #1 cylinder; #2 doing #2, and #3 firing #3.

 

[panula]

Re: Tohatsu M70B misfiring

I would take it from the top:
- Make sure you are firing in time from the flywheel. Check that you don't have a sheared magneto key that has allowed the flywheel to fire everything off time.

YUO ARE A GENIUS, Paul!!!!!

Before removing the fywheel, I just removed the plastic cover and the nut on the top of the crankshaft....I looked into the hole and.... NO MAGNETO KEY inside the seat

• Flywheel removed
• magneto key spotted just below the flywheel:facepalm:
• KEY placed in its seat
• parts reassambled

everything working!!!!!
THANKS.

I'll upload a short video as soon as possible.

 

[pvanv]

Re: Tohatsu M70B misfiring

YUO ARE A GENIUS, Paul!!!!!

Before removing the fywheel, I just removed the plastic cover and the nut on the top of the crankshaft....I looked into the hole and.... NO MAGNETO KEY inside the seat

• Flywheel removed
• magneto key spotted just below the flywheel:facepalm:
• KEY placed in its seat
• parts reassambled

everything working!!!!!
THANKS.

I'll upload a short video as soon as possible.

Glad it worked out for you. The clue was that once you verified firing order, you were firing into exhaust on all cylinders... leading me to think the magneto itself might be out of time. I didn't recall whether you'd had the flywheel off or not, but now it's clear that someone did, and accidentally dropped the key. BTW, the taper of the flywheel to the crankshaft must be assembled DRY (if necessary, clean both sides with thinner -- acetone works well), and make sure the nut is properly tightened.