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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Sat Jan 03, 2015 9:03 pm

Joined: Wed May 15, 2013 5:24 pm
Posts: 34
Location: USA
model: 750 Paso
year: 1988
With stock coils you should not increase the plug gap. The atmosphere in the combustion chamber is such that there is less resistance enabling a wide gap at say cranking/starting speed. But with increased compression and fuel at higher RPM the resistance is subtaintially higher. I would make the gap even less than stock to .020. Fine wire plugs are better for weak ignition spark because the smaller electrode uses less voltage to spark the same gap as the original plugs. The precious metals have better conductivity and longevity for corrosion and carbon build up.
Also how much compression does that cylinder have compared to the good one?

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Sat Jan 03, 2015 9:32 pm

Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Hi,
Truth is that i have not checked the compression because the evidence is in the electrical system. A lack of compression would not effect the electrical system. I use an inductive timing light and can see when the spark drops away. At idle the front cylinder is firing and idles normal. When the RPM is raised the timing light stops flashing indicating no spark to the plug. I verified this visually with a spark plug. At idle there is spark, above idle its gone.
Thank you!

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Sat Jan 03, 2015 10:50 pm
 paso grand pooh-bah

Joined: Tue Jul 29, 2008 9:50 pm
Posts: 3174
Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
Quote:
With stock coils you should not increase the plug gap. The atmosphere in the combustion chamber is such that there is less resistance enabling a wide gap at say cranking/starting speed. But with increased compression and fuel at higher RPM the resistance is subtaintially higher. I would make the gap even less than stock to .020. Fine wire plugs are better for weak ignition spark because the smaller electrode uses less voltage to spark the same gap as the original plugs. The precious metals have better conductivity and longevity for corrosion and carbon build up

wrong wrong wrong where is the increased compression in a stock engine ? less voltage is just that less voltage

The precious metals have better conductivity= less resistance even with a stock coil you must widen the gap

the only thing that changes when you change to a precious metal plug is the resistance of the plug, so to get the same energy out of the coil YOU MUST INCREASE THE GAP. IT IS BASIC IMMUTABLE PHYSICS !!!!

E_{total} = \mathrm{KE}_{system} + \mathrm{PE}_{system} + U_{system}

BAD COMPRESSION WILL NOT KEEP THE PLUG FROM FIRING, GO TO THE BACK OF THE CLASS

_________________
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

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Sun Jan 04, 2015 3:42 pm

Joined: Wed May 15, 2013 5:24 pm
Posts: 34
Location: USA
model: 750 Paso
year: 1988
Higgy, you missed my point. The cylinder compression does increase from start up cranking speed verses high RPM operation. Thus a wide plug gap may work at idle but not with added fuel and higher compression at high RPM. I have no disagreement about precious metal plugs. It seems to me a lot of people see them as some kind of spark amplifier. Which they are not. They just lose less of the voltage output from the ignition coil than standard plugs do because of better conductivity and fine wire electrode.
And my point of a small plug gap is that because of the output of the stock coil which for most bikes up to the late 90s, is less than individual plug coil stacks of today's bikes which still have plug gaps less than .040. Cars have wider plug gaps because the coils are bigger having larger wire spools. And the average car doesn't have a compression ratio like a current sport bike that are now near 13:1, but the bikes still use smaller plug gaps.
Can I move from the back now?

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Sun Jan 04, 2015 4:28 pm

Joined: Wed May 15, 2013 5:24 pm
Posts: 34
Location: USA
model: 750 Paso
year: 1988
Since you restored a spark at idle with the original rear trigger coil. And bypassed the the bikes wiring to get primary ignition coil voltage. Similar to what Derek did how about the wire from the trigger coil back to the CDI. At the least try new wire once it is out of the engine case. Or really be sure and would be no fun but replace from the trigger coil out. Or another less invasive way than soldering at the coil, first try getting the wire away from the rest of the harness and pull and manipulate it while running the bike.

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Sun Jan 04, 2015 5:00 pm
 paso grand pooh-bah

Joined: Tue Jul 29, 2008 9:50 pm
Posts: 3174
Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
the biggest plus of precious metal plugs is they last longer as they are less susceptible to corrosion than most alloys.
lower resistance does contribute to their longevity for sure but is just a contributing factor.

Our bikes do not have 13:1 compression

750(Paso & Sport) is 10:1
906 is 8.2:1
907 is 9.2:1
sport is 10:1

The stock coils are perfectly fine once the voltage supply issue is repaired

Almost every Automoble or Motorcycle these days has moved to direct ignition which changes everything and is irrelevant here, apples and oranges

one of the first things recommended for all of our bikes is the wiring update to fix the problem these bikes have with voltage supply to the coils and ignition switch problems associated with too many amps.

being 23+ years old if you have not replaced the spark plugs wires at the very least you would be lucky if the bike runs at all

so if you have in fact done the minimum required to solve basic issues all these bikes have you should in fact be able to increase the plug gap from the standard 0.25 to at least 0.35. If you have also replaced the coils with more modern units 0.35 I think you will find 0.35 to be the minimum to get reliable spark with today's fuel

Reducing the gap on these bikes to 0.20 is going to give you alot of misfire issues and would indicate to me you have problems with the ignition circuit
jm2c

Ccwpaso, don't take my ribbing too seriously Just been quiet around here, this has been the liveliest discussion around here in a long time and the only point is to help moto319 to get his Paso going again

_________________
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

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Sun Jan 04, 2015 5:15 pm
 paso grand pooh-bah

Joined: Sun Jul 18, 2004 12:00 am
Posts: 5182
Location: southern Germany
model: 750 Paso
year: 1987
spark plugs, HT leads, coils, CDI units were swapped, wiring and voltage checked and all ok.
If one of these things was not ok there would be no spark on the cylinder, weak spark at idle and improvement at higher rpm or intermittent spark at higher engine temperatures but not the current symptoms.

If the ignition timing was wrong (belt skipped a tooth, pick up mounting bracket not positioned correctly), if the compression wasnt ok or if the pick-ups were connected to the wrong CDI unit or the CDI units to the wrong coils the engine would not start or idle right or cut out when throttle is opened but youd always have spark.

Never heard of a defective flywheel and if it was there wouldnt be any spark on both cylinders.
Air gap is set correctly which imho only leaves the option of the pick-ups.

Did you measure the resistance? It should be 220 ohms +-5. (strange that the Haynes manual for SS/Monster says 95-105 ohms +-5 as if you check part numbers they are the same #036946260)
What makes me wonder is that you wrote that you had swapped them as well. (is there a chance that when you combined the new and the old one you took the old one of the front cylinder?!)

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Mon Jan 05, 2015 9:39 am
 paso grand pooh-bah

Joined: Thu Nov 03, 2011 10:13 am
Posts: 585
Location: Scotland
model: 907 I.E.
year: 1994
I don't know what coils the 750 Paso runs but on the bikes with the Kokusan ignition (600/750/900SS & M600/750/900 and 750 Sport) they benefit from an increased gap to 0.9mm (36 thou). The coils handle it quite happily and this reduces the chances of plug fouling. I don't think though that spark gap is a factor in the OP's problem.

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Mon Jan 05, 2015 5:03 pm

Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Hi,
I greatly appreciate all of the responses! I have a thick skin so no worries on the ribbing!
I tightened the spark plug gap to .020" and seem to have a better spark while i have it but the problem persists. I did a compression test and have 100psi in both cylinders.Where this seems low its good enough to run. Does any one happen to know what the resistance value of the primary winding of the coils is supposed to be?
Thank You All!

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Mon Jan 05, 2015 5:16 pm
 paso grand pooh-bah

Joined: Sun Jul 18, 2004 12:00 am
Posts: 5182
Location: southern Germany
model: 750 Paso
year: 1987
according to the service manual page M14: coil H.V. circuit resistance 8.800 ohms +-20%. coil, L.V. circuit resistance 40 ohms (can that be right?)

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Mon Jan 05, 2015 5:38 pm

Joined: Wed Dec 17, 2014 12:29 am
Posts: 11
Location: US
model: 750 Paso
year: 1988
Hi,
My experiance with ignitions tells me the primary windings are typically 5ohm and as low as .5 ohm,secondary windings are around the 11K range.
Thank You!

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Mon Jan 05, 2015 6:28 pm
 paso grand pooh-bah

Joined: Tue Jul 29, 2008 9:50 pm
Posts: 3174
Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
From my experience with the 906 3 ohms is what is required. If in fact your coils are 5 ohms it may just be the root of your issue
Anyone here have any experience with 5ohm coils on the 750 ? I know there are a few companies offering them but if I remember they are for racing use only, or is the other way round ?

anybody ?

The 907 is different as I found out the hard way not too long ago

_________________
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

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Mon Jan 05, 2015 6:54 pm
 paso grand pooh-bah

Joined: Sun Jul 18, 2004 12:00 am
Posts: 5182
Location: southern Germany
model: 750 Paso
year: 1987
it might be that its a typo in the manual and it should be 4 ohms.
Quote:
I know there are a few companies offering them but if I remember they are for racing use only, or is the other way round ?

yes, its actually the other way around. I.e. many shops that sell the black 5 ohms Dyna coils for Ducatis note that these are for normal use while the green 3 ohms coils are for race use but simply because they get hotter.
Other shops only sell the green ones. 3 ohms is the minimum resistance that should be used, lower resistance could damage the CDI units.
Using 5 ohms coils is just fine.
Quote:
The stock coils are perfectly fine once the voltage supply issue is repaired

Lets say the work but you definitely notice the difference if you replace them with stronger ones.

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Mon Jan 05, 2015 10:03 pm
 paso grand pooh-bah

Joined: Tue Jul 29, 2008 9:50 pm
Posts: 3174
Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
Yes but to take advantage you need a wider gap

_________________
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

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 Post subject: Re: 1988 750 Paso Loosing SparkPosted: Tue Jan 06, 2015 3:11 pm

Joined: Wed May 15, 2013 5:24 pm
Posts: 34
Location: USA
model: 750 Paso
year: 1988
Getting back to your 1/3/15 post you seemed confident about eliminating the coils. Using the good cylinder find a second coil that works, seems like you have spares. And get it on the other cylinder. It seems like you are second guessing your self, just don't want to see you going in circles on your troubleshooting.

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