Wildgoose Chase Moto Guzzi

General Category => General Discussion => Topic started by: Tom H on July 28, 2021, 05:04:51 PM

Title: Well That's Interesting. Distributor Rotor. Found The Issue!
Post by: Tom H on July 28, 2021, 05:04:51 PM
I was dynamic timing one of my bikes. It seemed like my timing light was not working right. I was getting like a ghost spark and the timing marks were all over the place on one side. Even tried another light, same issue.

I decided it might be my old cap or the rotor.

I had a spare rotor and cap (no name set, not Marrelli), slightly used almost new pulled from a bike with running issues. I installed both and tried to start the bike. It would not start. I pulled a plug cap and grabbed a spare plug. It fired, but still the bike wouldn't start.

Hmmm......Well let's try the old rotor since it's the easiest thing to change back to. Old rotor in and it fired right up. New rotor back in, no start. Hmmm???

I looked at my old and the new rotor. Everything looked exactly the same, even measured it. Use my DVM and checked to see if the copper strip had continuity, all good. Checked for a short to ground from the strip, all good.

Damn thing should work!!!! I guess even when one looks good and is basically new, it could be bad. Maybe a lesson here somewhere??

BTW: the cap did seem to solve my timing light issues. Marks are now solid. Maybe the old cap was tracking or whatever it's called from dust or something.

Tom

Title: Re: Well That's Interesting. Distributor Rotor.
Post by: guzzisteve on July 28, 2021, 05:15:54 PM
I have a box of old caps & rotors, just keep using them unless I have an issue. I clean up the strip on rotor and cut posts in cap. They last forever. I only put new stuff on someone else's bike.
Title: Re: Well That's Interesting. Distributor Rotor.
Post by: elrealistico on July 28, 2021, 05:24:14 PM
Clearance/resistance between the rotor end and the contatcs for the wires to the coils or plugs? Not familiar with the setup for that, but maybe too much gap on those or the center rotor to cap contact if it has one
Title: Re: Well That's Interesting. Distributor Rotor.
Post by: Don G on July 29, 2021, 01:16:43 PM
I had something like that happen to my 73 Eldo, lost spark ,so I grabbed the cap and rotor off of another running loop and that was the cure. DonG
Title: Re: Well That's Interesting. Distributor Rotor.
Post by: Tom on July 30, 2021, 04:09:02 PM
Only thing out all of that is that maybe the length is not the same to make contact to the rotor cap.
Title: Re: Well That's Interesting. Distributor Rotor.
Post by: CalVin2007 on August 01, 2021, 01:23:47 PM
 To check to see if the spark is jumping downward through the rotor to the distributor shaft (it happens!) remove the cap,leaving the rotor in place. Pull coil wire end out of cap and hold 1/8" above rotor.Key on,crank engine. If the spark jumps to the rotor it's shorting through it to ground. If it doesn't jump it may try to jump to YOU so use something non conductive to hold the wire.
  Found this several times when I worked on cars for a living back in the dark ages... :wink:

  Terry
Title: Re: Well That's Interesting. Distributor Rotor.
Post by: moto-uno on August 01, 2021, 05:22:58 PM
  And I'd agree with a previous remark that carbon tracking could easily prevent plug sparking and not be
too noticeable when taking a glancing look inside , good to get it running though . Decades ago I had
similar issues when trying to time a Le Mans 2 , to eventually find the drive gear on the end was badly
worn and introduced considerable wobble in the points cam . Replaced it and the timing was rock solid
from idle on up .  Peter
Title: Re: Well That's Interesting. Distributor Rotor.
Post by: Tom H on August 01, 2021, 09:25:59 PM
Well I measured everything again. Seems the same dimensions as the working one.

Calvin, I think I might just try that with the bad rotor. Maybe that is why it will not run the bike.

FYI: To be clear. The bike seemed to be running fine, no issue I could notice, I was just trying to check the timing with a light. I was having an issue that I thought was my timing light. Bought a new one. Same issue. On my shelf was a newish , but used cap and rotor, new style no name.
Installed them and the bike would not start. Removed new rotor, installed old rotor. Started right up. ALSO, timing light issue was gone.
Installed the new rotor again to see what happens. AGAIN, no run.

I did clean the old cap with a toothbrush and Qtip. I will install it shortly and see if the timing light issue returns. If it does, bad cap. If gone, the cap needed the residue/dust removed from inside it.

Tom
Title: Re: Well That's Interesting. Distributor Rotor. Found The Issue!
Post by: Tom H on August 02, 2021, 07:42:06 PM
I put my old cleaned up cap back on with the old rotor. Fired right up. With my old timing light I checked to see if I still had issues with the lights advance adjustment. Worked like it should.

For S&G I put the new rotor in. No start. Tried CalVin's  idea and tested his way with the cap off. I had spark to the new rotor. Pulled new rotor and put my old one in. NO sparks.

It appears that the new rotor is being grounded somehow??? I tried my multi meter on the ohms setting to try to find the short. I'm not an electromagician, so maybe I missed something. I did try all the ranges on the meter.

(https://i.ibb.co/XLfn867/dist-rotor.jpg) (https://ibb.co/XLfn867)


So...For whatever reason, the rotor is shorting out the spark!

Edit: The new rotor would allow a spark at the plugs, but it was very weak looking. Probably because most of the energy was going to ground?

Tom
Title: Re: Well That's Interesting. Distributor Rotor. Found The Issue!
Post by: nc43bsa on August 03, 2021, 12:38:55 AM
As mentioned previously, the spark is probably grounding through the plastic of the rotor to the rotor shaft.  A multimeter will not detect this.