86 VF500F middle rpm issues under load.

My VF500F is running OK, but not great. It feels like it is missing a bit in the middle of the RPM range when under load. It sounds great at all RPM ranges when in neutral. The trouble is most noticeable under moderate acceleration when cold between 4k and 8K. Low RPM and high RPM seem to be fine, and while still present when warmed up, less noticeable. The valves had recently been adjusted, carburetors synchronized, new battery, spark plugs replaced last fall and still look good, none of which seemed to have an effect. A few months back, I was struggling with a fuel supply issue, but I think that issue is resolved and the bike runs well once the RPM's are up.

I was talking to a guy when having the tire on my other bike replaced and he thought it might be electrical or grounding issue, coil collapsing, or faulty pulse generator; indicating that the coils have to work the hardest in the middle rpm range. I made sure the connections from the battery were clean, and the resistance in the coils are similar and within spec.

Any advice on where to start would be greatly appreciated. I would like to start with the easiest to do an least expensive things first. I don't have much access to spare parts to swap out and try so I have not tried swapping out the coils or spark plug wires yet. Is there a definitive way to confirm they are good/bad besides replacing and seeing if things work better?

Thanks,
Chocula

 

It sounds like it may be the cdi unit starting to go out, would be worth checking resistances on that.

 

Any idea what the specifications are for the CDI? My manuals provide don't provide any information other than if timing is off, replace. Hopefully I will have some time tomorrow to go through the ignition system and retest everything.

Chocula

 

I think there is some info on that in the archives. I've got two 500's sitting in my garage waiting to be worked on. It always seems like you have either plenty of time or plenty of money but never both at the same time. In my pursuing of all the forums I came across this guy that rebuilds them
http://http://home.comcast.net/~tjhafner/V4-spark.htm

It's worth checking out he has links and other. Things on other vfr and v4 forums, hope this helps

 

Just try playing with the "choke" a bit while riding. If a small amount of choke improves the problem, you're running too lean in that midrange, and it's probably not an electrical issue. The fact that the problem is worse when cold also points to a lean a/f mixture. Maybe just re-clean carbs or start experimenting with washers to raise the slide needle.:distracted:

 

I was able to take the VF500F out for a bit this weekend and playing with the choke dramatically improved the issue. Thanks for the suggestions!

 

I had the carbs on my '86 cleaned and balanced last year but it has an annoying mid-range "flat" spot. The mechanic, a former VF500/700 racer, says the newer reformulated gas is the problem. He recommends a DynoJet stage 1 kit, but at the moment I don't have the cash for the kit, so I can't say if it will make any difference. Perhaps other owners here have tried it or something similar.

 

Just a quick update, using better quality fuel with the highest octane rating I can find seems to have helped quite a bit. I still notice the issue, probably because I am always looking for it, but is not nearly as much as before.

 

Are you running a stock exhaust ??

 

Yes, everything on the bike is stock except the radiator, fan, and horn were swapped with parts from an '84.

 

I was running the best gas I could find but it didn't help. At the moment I'm not riding because I'll a little too poor to afford insurance. Ain't that a stinker?

 

Sounds to me like your pilots are slightly clogged, might be just one or two of them. These are super easy to change, not so easy to clean because of the super tiny hole. You can get a new set for about 20 bucks on ebay, I bought a couple sets just to change them out and clean them before they're too clogged. I put 40's in mine and it they work great, nicer, stronger feel at take off.

 

Thinking about going to 40's in my '86 due to not being able to tune out a lean surge. I know the carbs are different in '84 but I am already at 3 turns out on my fuel screws. Did you have the same issue? Also, I sure am getting tired of pulling the carbs just to clean clogged pilots.

 

my issues were more of the clogged type where it needs choke just to idle and run well even after warming up, new/clean pilots do the trick, you can use anywhere between 35 and 40's, probably not much of a difference in most places. I just used dumb logic in that I figured it'll take a slightly larger hole slightly longer to clog than a smaller one. That's pretty much the only reason I went to 40's vs. 38's and they work just fine. I think that with ethanol fuel, carbs are just destined to be clogged without using some additive. I don't know of any additives that work though, I don't ride my bike often enough to be able to judge.

are you at sea level? maybe try a little richer mains too if the pilot thing doesn't work, those are cheap too....about 10 bucks for a 4 pack. I think you probably have 90's? I went with 107.5's and they work great for me. I tried all the way up to 115's but that was a bit too rich.

http://www.powersportparts.net/Main...ort=13&cat=1144&search=keihin&show=150&page=1

that's considering you're lean, if you're too rich at midrange with 90's, which I doubt, it could be worn needles or emulsion tubes/needle jet holder...look at your plugs to see if they're white (lean) or black (rich)... it's a can of worms but it could be fun if you're into that sort of thing... ooops, never mind you said choke helped so that spells lean... carry on

you're probably already familiar with something like this but you probably could also try shimming your needles and/or making sure they're clean... you could also have a one or more slightly torn vacuum diaphrams, did I mention the can of worms?? oh, we all know that 1/4 is less than 1/3 (shown wrong in the picture).

 

Thanks jrod, I was thinking the same thing about the 40s clogging less but I really should just ride it more. Pretty sure this time around it was the gas going bad because it's been awhile since I put gas in it. Spring, I think... This was after I flushed the tank and coated it. My needles are already shimmed but the mains are stock size. She pulls hard on the gas and the plugs look pretty good. The persistent lean surge is driving me crazy. I also have not adjusted the valves.
Go Chargers!

 

To eliminate or reduce the lean surging, try opening up the mixture screws about 1/2 turn. (Up to 3 1/2 turns out is OK.)

Also, just mentioning that a worn out chain and sprockets can cause a mechanical surging that can mimic a lean fuel condition.