Return of the bog monster

About a month and a half ago my VFR started running very poorly. Barely started, smelled of fuel, bogged down under throttle. I couldn't figure out what it was. I took that opportunity to check the valves, drain the coolant, replace the air filter and plugs. Put her back together and fired her up. Found that one of the plug caps was arcing so I replaced the two front cylinder plug caps and all was well. Rode it another 1000 miles and this morning she barely started, smelled of gas when it did, and bogged down with throttle worse and worse as it warmed up. I know it's not carb related, it is at all rpms and sounds like it's cutting out. And suggestions?


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I'm pretty confident this is going to be an ignition issue. Smell of fuel and bogging down tells me you have lost spark, probably in 2 cylinders. The fact it's intermittent makes me think it could be the pulse generator. Basically there are 2 magnetic pickups (each one does 2 cylinders) that tell the CDI when to poop out a spark. I had a bike with aftermarket electronic ignition. Bogged down intermittently, hard to start. Turned out to be one of the pickups going bad.

Could also be the cdi, or coils. I would go through the whole ignition circuit.

 

Wow excellent help! I was curious to see whether or not those cdi's went bad or not. I really does feel like 2 cylinders are down. It is worse when hot. Cold start up is choppy but MUCH smoother


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So far it appears a pulse generator (if that is indeed what it is) are discontinued and unavailable elsewhere (used or new). 94-97 doesn't fit. Anyone know what about them goes bad? Maybe I could replace wiring inside or what not and salvage it that way?

 

Internal wiring starts to break down over time due to heat. Electrex world sells generic pulse pickup coils. They have dimensions on their website. You maybe able to find one that fits. I believe rick's motosports sells them to.

If you want to identify easily what cylinders aren't getting spark, auto stores and harbor freight sell inline spark testers. No need to pull the plugs and ground them on the side of the crankcase. Tester sits between lead wire from coil and the spark plug has a light that will light up if spark is present. They're only a few bucks.

Pulse coils can be tested with a resistance check.

 

You're incredibly helpful! I will test those things as soon as I can and hopefully nip this in the bud. I love the bike and hope to enjoy it for years to come. Thanks again for the quick and knowledgable replies


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A good tester should simulate the conditions under compression. Spark in free air does not take as much juice.

 

Could be something simple like a loose or dirty connector. Electrical issues can be the hardest thing to trace down on a bike. Specs for the pulse coil ohm wise should be in the service manual

 

Update. So far I've only had a few minutes to check anything out. Last night I pulled the fairings and started the bike. It was running choppy so I thought I'd check to see if a cylinder wasn't getting spark. The first coil I pulled was 4 (right front cyl). I disconnected the leads and the idle remained the same. I plugged them back in, got a little spark as the connectors made contact with the coil blades. I unplugged the left front cyl coil and the bike obviously went down to 2 cylinders and nearly died. I took cyl 4 coil and installed it on the left side and it ran on 3 cylinders again. At this point there were only 3 coils hooked up. I took the left side coil and hooked it up to cyl 4. No change. So the coil is good. I can't be my pulse generator because that controls 2 cylinders at a time. But when it bogged so bad I couldn't ride it, it felt like only 2 cylinders... So I'm not sure. I'm going to get it hot because that seems to be when it runs worse and see which cylinders are losing spark


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Pull the spark plug and see if it is fouled. You said new plugs 1000 mi ago, maybe it got fouled again. You sid you replaced caps, did that include high voltage wires too? Rubber in wires maybe breaking down and cause HV leakage. Again, what may produce a spark in free air may not under compression. If plug is fouled, could be one carb running rich or burning a little oil?

 

Well 1000 miles ago the carbs were professionally gone through and sync'd. Right now it's only running on 3 cylinders but warmed up runs more on 2. I did NOT replace the plug wires allyance, they're really not that expensive I probably should anyway. The viffer is in the shop I work at for the weekend so I'll have to pull the plugs Tuesday. My coworker suggest those inline spark plug lights to look at what the spark is or isn't doing.


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Looks like you found it.

Obviously from this data the number 4 cylinder is not sparking-- probably the plug is fouled. How hard is it to pull the plugs and inspect?

How did it foul? Likely a fuel system issue, floats set too high, mixture screw setting too rich, etc.

 

That's possibly Kenny. I have the fairings off so removing the front plugs is a matter of removing the lower radiator mount and pushing it forward.


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So the bike running on 3 cylinders all the time was a matter of a blocked carb. The running on two cylinders when warm was my left front spark plug cap failing. I ordered and installed Taylor 8mm Pro Comp spark plug wires and caps. Bike runs like a champ! Thanks for all the great advice and direction


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