'88 Paso 750 starts on one cylinder only. Spark good.

[sgusinde1263]

I have a 1988 Paso 750 that was a non-runner. I am attempting to resurrect it from the desert where is sat outside with a tarp cover. After a few months work and recovery I got the bike to run. However, I found it only starts on one cylinder (vert). The horizontal does not fire.

The spark is the first thing I checked - good there. The compression is the same in both cylinders. The fuel delivery is good.

I now have a small tank set up on the bars and fuel runs directly into the Weber carb. The Weber is a 44 DCNF 107 that has been rebuilt by me. I thought I did everything right, and am now checking things again. On the first rebuild I ran air through all the vents and made sure the jets were clean.

If I squirt some starting fluid down the intake, the bike fires like a demon. It will not draw fuel from the regular feed into the one intake. No air leaks that I can find, and like I said - it does fire on fluid.

Looking at the carb the second time, the only odd thing is that the float level was very high - doesn't look like the way I re-assembled it.
Also the four hole gasket on the carb base was wet in the rear where is covers the two slanting grooves.

Before I get flamed by all the experts out there - I HAVE looked all through this forum and found some good articles on jetting and total carb operation (from the "show pictures of your Weber" posting). But I still am lost on why one intake does not work. The jetting I can look into after the bike starts and runs.

I am probably missing something very simple and just need a nudge in the right direction.

Thanks to all. I've enjoyed this forum for many years, and received some good advice on my first 750 Paso long ago; it was my first Ducati and got me started on ownership of various Ducatis over a long period.

[sgusinde1263]

Addendum to first post:

I live in the Portland, OR area. If anyone has some ideas and would like to stop by and offer an opinion on my amazing 375cc Paso, drop me a PM. I keep a supply of good wine on hand for visiting Ducati mechanics.

[paso750]

Often what seems to be a carb issue turns out to be an ignition issue. You say you have spark and that it runs properly with starter fluid. As that ignites very easily the spark may not be strong enough to properly ignite the fuel. Therefore measure the voltage at the coils (orange wire) with ignition on and when turning over the engine. It's common that you'll read 10 volts or less. If so check the download topic for the wiring/relay modification.

G.

[higgy]

As G has relayed you most likely have an electrical issue as in not enough voltage to the coil on the offending cylinder.

[sgusinde1263]

OK after the reply from G. I tested voltage. This is what I got.

With the ign off, I read no voltage to either coil on the double orange wire connection.
With the ign on, I read 9.6 to both coils
With the ign on and starter going, I read approx 7.5 to both coils.

The resistance across the poles reads:

Vert 5.2 ohms
Horz 4.8 ohms

The spark from both plugs is "blue-ish" However, when the wire and plug are next to the frame on the horz I can see some spark "leaking" from behind the cap to the frame (this is visible only in the dark).

So, what do you think G. and Higgy?

[paso750]

As expected the voltage is way too low. Probably at a higher rpm there would be enough voltage for the front cylinder to get spark and run as well.
Ideally the voltage at the coils should be the same as measured at the battery. The culprit causing a high resistance is often the ignition switch.
Do the relay mod and I bet it'll run on both cylinders. You'll find it here. P750 wiring modifications
https://app.box.com/s/waxvk1l9g47phfybd6n9
Don't let it confuse you. There are different levels for this electrical mod. The most simple is to add a relay and take the voltage from the battery. This will not fix the voltage drop but the mod bypasses the ignition switch to get full battery voltage to the coils.The ignition switch will only trigger the relay.

The coils' resistance is ok. As they and the ignition wires age it may make sense to replace them sooner or later. However, they are not your current problem.

G.

[sgusinde1263]

Thanks G. From what i see you are recommending the coil relay mod over the power relay mod. You are right, it seems much simpler.

However I have a single orange wire going to the front coil and then another orange wire going from that same post connection to the rear coil. The rear coil already has 2 orange wires and the added cross wire from the front makes 3, all going to the same post.

The mod diagrams only show 1 orange wire to each coil. Hmmmmm?

[sgusinde1263]

OK G. I got it. On the coil mod it doesn't make an difference where the sparker wires go, just take the one from the kill switch and connect to the relay. In my case 2 sparker wires go to one terminal on the back coil (the or/blk goes from a sparker to the rear terminal on the back coil) then a wire crosses to the front coil and another wire connects there from the kill switch.

I know which that wire is, so I'm good. The wiring diagram was a better help that the photo - that only showed one wire to each coil.

Thanks for the help. Sept is a busy month, so results may take a while. I DO have other bikes, and I'm waiting on a carb kit for my 1980 Vespa, and that goes back together first.

[paso750]

I'm waiting on a carb kit for my 1980 Vespa, and that goes back together first.

I'm currently finishing a Malossi 221cc engine for my '85 Vespa T5.