Need some help troubleshooting an 84 VF500

Hi,
Looking for some help troubleshooting my 84 VF500 that I thought I had recovered after a fire last year. Had the bike running fairly well after replacing the fire-damaged parts and was using her as an around-town bike. Bike started to run a bit rough so I pulled the carbs (had rebuilt them already) to make sure I hadn't picked up any dirt in the jets. Got her back running, but she was even rougher than before. Took her out for a test run and she quit after about 2 miles of running poorly. Could not get the bike to restart, but did experience some serious backfires while trying! Left her for the winter and started again today. Pulled the tank and air cleaner and connected fuel to accessory tank. Pulled one of the rear cylinder leads and found weak spark, but spark. Surprisingly, she started right up but only on the two rear cylinders. Would only idle with full choke. Front pipes stayed cold. Swapped ignition units with no change. Then swapped ignition coils, and still only running on the rear cylinders. After restoring the coils (spark plug leads and wires to ignition), the bike refused to start at all. Pretty sure I have everything back as it was, but now I get no firing at all. Did check a front spark plug lead and found a weak spark. Checked the coils, 3 - 3.2 ohms primary and about 14k across the ignition leads on both, so seemed ok. Had real difficulty reading continuity across the ignition leads, but got intermittent values of 5-14k testing all 4. Think it was just my ohmmeter probe wasn't making good contact with the wires? Plan to check the pulse generators tomorrow, but any recommendations on anything else I can check. Since I swapped ignition coils and ignition boxes and both seemed to fire the rear cylinders, does that indicate that they are ok? Coil numbers seem ok. Plug leads, I am not so sure? Any thoughts?

 

It sounds like you're swapping parts instead of diagnosing. That can work with new parts going in; so are yours new or also used? Electronic components can fail (or test as bad) only when they are hot, so that is what you want to check them, once you get it running.

As always, it is air, fuel, spark...or some combination of them! Smarter VF500 people will surely reply.

 

By swapping, I mean that I swapped the two ignition boxes and two coils already on the bike from the rear to the front cylinders and vice versa, to see if I could identify one bad component. I haven't replaced anything yet, as I cannot figure out which component or components might be bad. I am thinking that my testing so far may indicate that the coils and ignition boxes are not my problem? But I would appreciate some input. So that would then leave pulse generators, ignition leads and spark plugs? Thoughts?

 

What's the state of the fuel tank look like with regards to rust? My first thought was that it sounds like blocked pilot circuits after having a bunch of shit get pulled into the carburetor. Yes, you can pull the bowls off and remove the pilot jets--but that doesn't necessarily mean the full pilot circuits are actually clean.

 

ASSUMING THE CARBS ARE CLEAN AND PROPERLY ASSEMBLED, pulse generators would be my next step. Unfortunately, it will be confirmed by part swapping. The resistance tests might help you decide you are on the right path, but they are not conclusive. The only thing that will absolutely confirm is installing a known good unit.

 

That is what I have concluded also. I checked the Pulse Generators this morning via resistance check, and found the unit supplying the front cylinders way low. Reassembled everything again and it started right up on the two back cylinders. Timing light to all four leads showed power to both rear plugs and none to the fronts. So some used pulse generators are now on order. Hope this resolves the issue and I can get her back on the road! Thank you all for your input.

 

Just a quick postscript, replaced the pulse generators and the bike started up but was still not running on the front right cylinder. Pulled apart again, and found I probably broke the ignition lead while troubleshooting (it was fire-damaged and/I meant to replace it anyway). Also re-engineered the fuel line for my gravity flow feed, and she now runs clean and strong. Thanks for the help.