1988 Paso Ignition Issues

discussions specific to the 750 Paso

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murphus
Posts: 51
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

1988 Paso Ignition Issues

Post by murphus »

Thought I was finally going to get my new-to-me 1988 Paso running and road-ready, but it's giving me some fits, running only on the vertical cylinder.

Engine has 7,026 miles on it, was last on the road approx. 2003. In addition to changing all fluids and belts I just rebuilt the original Weber carb employing jetting updates suggested here and 4.5 auxiliary venturis. Prep included a thorough bath in an ultrasonic cleaner. The basic issue is it's only running on the rear vertical cylinder. My attempts at diagnosis so far have returned the following results:

Compression check shows an almost dead even 150psi in each cylinder
Valve lash as been inspected and found to be spot on.
Battery voltage: 13.2 volts
Coil input voltage: 10.2 volts (obviously there's an issue here, but not convinced it's THE issue).
Coil resistance: HV horizontal cyl. coil = 12.23k ohms, LV horizontal coil = 3.9 ohms
HV vertical cyl. coil = 12.37k ohm, LV vertical coil = 4.1 ohms
Factory specs are 8.8k ohms HV and 4 ohms LV - Is my HV resistance too high?
Swapped coil triggers: No change.
Swapped coil leads so front coil fired vertical cylinder and vise-versa: No change, rear cylinder fires on what was formerly front cylinder coil, which would appear to negate coils as the issue.
Checked ignition pulse to coils with a test light across the +/- contacts on the coils: Front horizontal cylinder returns regular lighted pulse regardless of which coil is used. Rear vertical cylinder (the running cylinder) lights the test light for a few pulses and then the engine dies. I presume this might be because of the afore-mentioned low voltage combined with the test light drawing enough voltage it keeps the coil from properly firing.

When I first got the bike running some months ago on the flat-slide Mikunis it was then wearing I had several occasions where the horizontal cylinder did not seem to be firing. I sprayed starter fluid in the carb throat and it caught and then appeared to run. I didn't run it long as at the time all I was looking for was proof of concept, e.g., the engine would run. For whatever reason, I didn't think much about it at the time.

Following that memory, I tried starting it on the horizontal cylinder only. I gave the horizontal cylinder carb throat a light spray of starter fluid and it would haltingly fire/burble, but only with the throttle held fully open. It would not clear its throat and actually run.

Somehow I don't feel like it's a carburation issue, but then there's the old saw that says 99% of electrical issues are electrical, and vise-versa. Anyone have any thoughts? I'm stumped.
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
koko64
Posts: 91
Joined: Sun Feb 24, 2013 11:14 pm
model: other
year: 1999
Location: Melbourne Australia

Re: 1988 Paso Ignition Issues

Post by koko64 »

Curious what your plugs have to say. Also interested to see how it runs with the Mikuni flatsldes back on.
If it is indeed an ignition problem and its not the Ignition modules (Kokusan boxes?), plug leads, wires to ignition modules or a bad coil, then that only leaves the pick up coils/flywheel triggers in the case.
Have you ground a plug to the head with ftont fylinder plug cable attached to check for spark? It must be ground to the head to avoid ignition damage. Use a spare plug and turn the motor over to check for a good spark.

Some have powered the Ignitor modules via a relay for a solid 12 volts if the stock wiring is weak. I was able to buy the sub loom for the Ignition when mice chewed the wiring on a Monster. If your bike uses the Kokusan Igniton modules then the monster sub loom may well retrofit (with some adjustment).

Just some thoughts and I'm curious how it works out.
2013 M1100Evo Street
1999 M750 "Scrambler"
2004 GSXR750 Track
murphus
Posts: 51
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

Re: 1988 Paso Ignition Issues

Post by murphus »

Front/horizontal plug becomes badly fuel fouled. I have of course have replaced it multiple times during this process to ensure clean spark. I do get spark to the front cylinder. I have since somewhat isolated the forward cylinder and gotten it to fire, albeit intermittently and using spray. I am turning toward it being a carburetion issue. I'm wondering if for whatever reason I'm not getting a vacuum signal in the right venturi. The Weber does have damaged fuel/air ports from a previous owner tightening the fuel/air screws down so far they started to blow out through the venturi side. Yesterday I pulled the Weber and am readying to reinstall the flatslides. Out of town for the next week so won't be able to report back for a bit.
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
koko64
Posts: 91
Joined: Sun Feb 24, 2013 11:14 pm
model: other
year: 1999
Location: Melbourne Australia

Re: 1988 Paso Ignition Issues

Post by koko64 »

:thumbup:
Will be watching with interest.
2013 M1100Evo Street
1999 M750 "Scrambler"
2004 GSXR750 Track
murphus
Posts: 51
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

Re: 1988 Paso Ignition Issues

Post by murphus »

Well, it turned out to be an electrical issue after all. Reinstalled the Mikuni flat slides, fresh o-rings on the manifolds and both slides set exactly the same using the drill bit method. Fired right up - on the rear cylinder only. Rechecked compression just in case I'd gotten a faulty reading previously, still a solid 150-152 each cylinder. As previously noted, I only had 10.2 volts to the coils, which on recheck was now 9.3 volts. So, I installed a relay for the coils and trigger units (they get their power from the same circuit) to ensure a solid 12+ volts to both. Whilst doing same I looked at the connectors at the triggers. I had already replaced the small spade connectors on the 2 wires from the pickup at the plastic connecting block to each trigger as the wires were frayed. Looking at them, it occurred to me the spades weren't necessarily pushed all the way home as they were a somewhat loose fit in the connector. On my last check, I did have a signal from each trigger to the coils. Regardless, I pushed them home from the backside using a very small screwdriver. I then tried firing the engine, and it ran perfectly. Wow.

I only wish I'd followed best practice of only changing one thing at a time so I would know which change actually made the difference. I'm left believing it was likely a two-pronged issue: Loose or incomplete connection at the trigger providing a somewhat haphazard signal to the coil and low voltage exacerbating the poor signal. Regardless, it sounds glorious and pulls like the proverbial train, at least on the stand. Steady idle and immediate throttle response. Glutton for punishment that I am, I think I'll go ahead and reinstall the Weber and see what it does. I figure the worst it can do is run shitty. And if it does, I'll switch back; but at least I'll know it should run. I'll report back in case anyone's interested in how this turns out.
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
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randtcastell
Posts: 318
Joined: Fri Nov 13, 2009 4:33 pm
model: 750 Paso
year: 1987
Location: San Francisco Bay, California USA
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Re: 1988 Paso Ignition Issues

Post by randtcastell »

Congratulations! We are all very pleased and excited for you. Thank you for keeping us in the loop. We value your time and effort in writing post to share with us. Cheers!
1987 Paso 750
1973 Honda CB450
2017 Kawasaki Ninja300 track bike
koko64
Posts: 91
Joined: Sun Feb 24, 2013 11:14 pm
model: other
year: 1999
Location: Melbourne Australia

Re: 1988 Paso Ignition Issues

Post by koko64 »

Glad you found the issue.

Looking forward to your carb comparison!
2013 M1100Evo Street
1999 M750 "Scrambler"
2004 GSXR750 Track
murphus
Posts: 51
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

Re: 1988 Paso Ignition Issues

Post by murphus »

So I installed the Weber. Actually, I installed 2 Webers; the original that came with the bike and a spare I stumbled onto for silly cheap. I expected the original to perform poorly, as a previous owner or someone had screwed the fuel/air mixture screws in so tight they were actually blowing out the port into the venturi. Never seen anything like it. I've seen bent needles from doing the same, but never the carb body damaged. It did run, but I could not get a clean or stable idle.

Per Romus & Higgy, I set it up with 150 mains, F27 emulsion tubes, 60 idle jets and 4.5 auxiliary venturis. Float set to 43mm and running the stock fuel pump. l also greased the throttle plate bearings to stave off any possible vacuum leaks there. It pulled somewhat cleanly at anything above 1/3 throttle, but it clearly wasn't a useable proposition.

I then swapped jets, etc., to the spare, which I'd already stripped, cleaned and given an ultrasonic bath, as I did with the original. I removed the throttle plates so I could replace the throttle shaft bearings with sealed units. Bit of a PITA, but not too bad. Installed, it required full choke to start and wanted to spit if I gave it any throttle. It took a bit to get it to want to run, and then it became apparent I was only running on the rear cylinder. I checked to see if I'd lost the signal to the front coil (per my previous issue which started this post), but everything was good. I was pretty sure this time that I didn't have an ignition issue so I swapped the Mikunis back in and it fired right up, running perfectly on both cylinders. I haven't done a post-mortem on the Weber, but I have to wonder what I missed that's keeping it from working even somewhat properly. I expected it to at least work reasonably well. I'm disappointed, but on the plus side it's running excellently, and from everything I've read here I'll be glad to be running the Mikunis instead of the Weber.

A couple of observations that may or may not be of interest for the Weber Files:
Emulsion tubes in original carb were F36.
Emulsion tubes in spare carb were F44. The spare carb is of unknown origin. It's identical to the original except that it has capped air screws so you can't adjust them. Perhaps that dates the carb? I'm assuming it must have come off a 906, perhaps California spec?

Following Romus & Higgy's writeup I purchased F27 emulsion tubes. A comparison of F27 and F44 shows they're almost identical, but the F44 has the second set of holes that Romus & Higgy suggest adding to the lower-middle part of the F27. For whatever reason, F44 tubes appear unavailable, but they also appear to have the characteristics Romus & Higgy were looking for.

For now, I think that closes my interest in the Weber. I really wanted to return my Paso to stock, but on the other hand I really want to actually ride it, and clearly that means sticking with the Mikunis. Just need to find filters I actually like then fit the bodywork and hit the road, as I think I've tended to most of the bike's issues. Fun stuff, these odd, old Italians!

Richard
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
User avatar
Mc tool
paso grand pooh-bah
Posts: 1794
Joined: Wed Dec 05, 2007 12:35 am
model: 906 Paso
year: 1990
Location: Newzealand

Re: 1988 Paso Ignition Issues

Post by Mc tool »

Hi Richard , one thing I found with the weber is that I likes the air box cover (with snorkles ) to be fitted . I found it did not want to run well at or around idle , spitting as you describe . :-)
I wish I was young again............Id be heaps smarter than last time
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