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 Post subject: 1988 750 Paso Loosing Spark
PostPosted: Wed Dec 17, 2014 9:27 pm 
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Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Hi, New Here!
I'm working on an 1988 750 Paso that is loosing spark in the horizontal(front) cylinder just above idle. It has spark at idle,but as the rpm rises it looses spark. I have replaced the triggers, coils, swapped in a known good CDI Box, I've swapped CDI boxes Front to rear, I've added additional grounds, I've continuity tested the circuits looking for shorts and grounds. Ive jumped the positive voltage circuit, eliminating the rest of the bike. I've replaced spark plug caps. I've disconnected the stator to eliminate any weirdness as well as the regulator.......none of this has helped. I'm hoping someone may have encountered this problem and may be able to direct me in the right direction.


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Wed Dec 17, 2014 10:14 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5100
Location: southern Germany
model: 750 Paso
year: 1987
that`s interesting as often it`s the other way around cylinder drop out at idle and spark at higher rpm.
Sounds like you`ve already tried a lot. Was the bike stored for a longer time ?
You didn`t mention if you replaced the spark plugs. Is the spark plug of the front cylinder clean and dry or wet and black ? Is the ground wire from the CDI unit to the coil ok, is there still full 12V on the front coil when the cylinder drops ?
What is the condition of the wiring from the pick-ups to the CDI units?
In case you worked on the pick-ups is the clearance to the flywheel correct?
Is the compression ok? Do all pulley marks line up correctly?
A lot of questions ;)

G.


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Wed Dec 17, 2014 10:20 pm 
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paso grand pooh-bah
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Joined: Tue Jul 29, 2008 9:50 pm
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Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
Did you replace the High Tension wires ?Spark plug wires in english


Always start with the basic stuff first,plugs, wires, coils,voltage supply to the coils in that order once the basics are verified and known to be working then check the generation side of things pickups, coil wiring and spark module

doing it any other way waste time and money

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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Fri Dec 19, 2014 6:03 am 
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Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Thank You for the responses. I have replaced the spark plugs and wires. The wiring has been checked and additional ground ciruits have been added. As my customer has indicated to me, the bike was running fine and he stopped for a drink break and when he started the bike it was running on one cylinder. Using an inductive timing light and using a spark plug i visually verified the loss of spark at the front cylinder. the real stumper is the trigger output voltages are consistant and equal and the supply voltages are stable at the CDI boxes and at the coils. I suspected a loss of dwell in the coil due to an inaccurate air gap at the triggers or a bad coil, but i visually verified the air gap and used multiple coils to eliminate the coils. (The coils i have been using all pass resistance tests). Thank You all for your time and input.


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Fri Dec 19, 2014 8:45 am 
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Joined: Sun Jan 16, 2005 12:00 am
Posts: 257
Location: The Netherlands
model: 750 Paso
year: 1989
moto319 wrote:
Thank You for the responses. I have replaced the spark plugs and wires. The wiring has been checked and additional ground ciruits have been added. As my customer has indicated to me, the bike was running fine and he stopped for a drink break and when he started the bike it was running on one cylinder. Using an inductive timing light and using a spark plug i visually verified the loss of spark at the front cylinder. the real stumper is the trigger output voltages are consistant and equal and the supply voltages are stable at the CDI boxes and at the coils. I suspected a loss of dwell in the coil due to an inaccurate air gap at the triggers or a bad coil, but i visually verified the air gap and used multiple coils to eliminate the coils. (The coils i have been using all pass resistance tests). Thank You all for your time and input.


I was thinking of the pickups also. They are located on the left side of the engine, behind the altenator cover. This is also the side where the 15t sprocket is fitted. You should be able to measure resistance of the yellow wires, while turning the crank.
Léon

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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Fri Dec 19, 2014 10:53 am 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
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Location: southern Germany
model: 750 Paso
year: 1987
what additional ground circuits did you add, what spark plugs are you using and how did you "visually" verify the air gap ?
If you hadn`t said you verified there`s no spark I would have added a carb problem (synchronization, clogged jet) to the list.


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Mon Dec 22, 2014 5:41 pm 
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paso grand pooh-bah
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Joined: Sun Aug 01, 2004 12:00 am
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Location: siena, italy
model: 750 Paso
year: 1988
Just one more question: did you try to switch upper and front coils? Just switching wires should do.
Any time I happen to face such a problem, I switch parts up-front to find the fault one

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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Tue Dec 23, 2014 9:51 am 
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I had a interesting problem with my son's 916 Strada last year. It would start fine from cold but once it had run for 2 or 3 minutes would develop a misfire. Like the OP we started by replacing the plugs then swapping the coils around, then the plug leads but got nowhere. I then tried the coils from 907 with no change, then the 907 ignition amplifiers still with no change, then the timings sensors and finally the TPS. None of which made any difference at all. Only the ECU was left.
We had managed to establish that the misfire seemed to be on the vertical cylinder and on a whim my son took a wire directly from the ignition amplifier output to the vertical coil. The problem vanished! On close inspection we found that the loom had been pinched, such that the wire to that coil was almost severed, creating a high resistance. It was ok at a cold start but the cable must have got hotter at the damaged point, increasing the resistance and limiting the current through the coil until it started to misfire. We stripped back the loom and replaced the wire. End of story.
The point though, is that none of the individual components were faulty. The fault lay in a wire joining them. So it may well be worth checking the integrity of the wiring and all the connectors especially if all else has failed. A connection which makes at tickover may fail at increased revs, so no spark.

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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Tue Dec 23, 2014 10:30 am 
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paso grand pooh-bah
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Joined: Tue Jul 29, 2008 9:50 pm
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Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
Value of a voltage drop test = Priceless

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There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Sat Jan 03, 2015 3:51 am 
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Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Thanks for the input!
Ill start from the beginning.....this has turned into a 'Cant see the forrest for the trees" type thing so im hoping one of you might be able to point out what i may be missing even though im staring right at it!
I started out with a bike with no spark in the front cylinder.
I checked battery voltage and charging output, it was good.
I checked spark plug and cap. I replaced spark plugs and caps still no spark in front cylinder.
I checked resistance of front coil primary and secondary...it matched the rear cylinder coil
I checked voltage at coils from CDI boxes...they were the same front and rear
I checked the voltage to from the kill switch to the coils, then to the CDI boxes these measurements seemed low to me at just arouns 11VDC.... but since the rear cylinder is firing normally, i religated this to memory (I later voltage drop tested the system from the battery to the coils and found the drop at the ignition switch)
At this point i procured a pair of known good coils and tried them, no change(Coils eliminated)
I tested the voltage from the Crank triggers and found the front cylinder trigger to be unstable and I replaced the trigger assembly. Setting the air gap at the .027" or .7mm as stated in the service manual. At this point i have spark at the front cylinder but only at idle...if the RPM is raised the spark drops again! GRRRRR!
I have tested the trigger voltage again and even though the spark drops, the voltage output of the triggers are consistant and uniform front and rear. Something i didnt have previously.
I thought i may have an impeadance issue with the front cylinder trigger so today Swapped it with the rear cylinder trigger from the old assembly.(which was firing the rear cylinder)...now im back to square 1.
I added grounds from the battery to the ground point of the CDI boxes and from the battery to the coil mount brackets...the voltage to the coils is consistant and equal front to rear at near battery voltage.
Thank You Everyone!


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Sat Jan 03, 2015 3:22 pm 
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Joined: Wed May 15, 2013 5:24 pm
Posts: 33
Location: USA
model: 750 Paso
year: 1988
Sorry if I missed that you tried it, but have you swapped the ignition modules to see if the rear misfires instead?


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Sat Jan 03, 2015 3:24 pm 
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Joined: Wed May 15, 2013 5:24 pm
Posts: 33
Location: USA
model: 750 Paso
year: 1988
CDI boxes


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Sat Jan 03, 2015 3:35 pm 
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Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Hi,
I swapped CDI Boxes front to rear and procured a known good CDI box and tried it with no improvement.
Thank You!


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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Sat Jan 03, 2015 4:25 pm 
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paso grand pooh-bah
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Joined: Tue Jul 29, 2008 9:50 pm
Posts: 3159
Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
just a thought,
I don't see anywhere that you said the type of sparkplug you installed

If it is Iridium try widening the gap to .8mm or even more

the standard gap is not enough resistance to allow the coils to provide enough energy to resist the rigors of charge compression on precious metal plugs so you must widen the gap to compensate



Also what coils are you using, after market coils like Dynacoil also require a wider gap even with copperplus plugs to take advantage of their higher voltage capabilities

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Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie


Last edited by higgy on Sat Jan 03, 2015 5:29 pm, edited 1 time in total.

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 Post subject: Re: 1988 750 Paso Loosing Spark
PostPosted: Sat Jan 03, 2015 5:18 pm 
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Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Hi,
The coils are stock part number and the spark plugs are NGK's Per their application chart.I thought about the gap issue, and widened it to .040"
Thank You!


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