Help Please! '84 VF500F won't rev up while riding

well for me i got a new set off ebay,swapped in the parts and that confirmed it when the everything got better. Only one box is the revlimit box and, the other one is just a normal spark box that will go to infinity. I believe the big one is the rev limit box its been a good while since i pulled mine out and checked. I nvr could find a way to bench test the units sadly as this would help make this problem alot simpler to solve. If you have a vf500 buddy in your area it takes about 5 mins to swap the boxes and you could have an answer then. to test the coils any motorcycle coil without a built in ignitor can be subbed for the stock unit. (I believe mine cost about 45 dollars apeice new from the local harley dealer, back in 2004 they are each rated at 7k rpm but you must remember each coil only fires every other stroke. Making them viable to 14k rpm in a v4 like ours)

 

One is larger than the other because it has the circuits to drive the tach. While others have suggested that only one does the rev limit, I believe they both have this function.

 

nope only one does cause my tack works just fine and i have spun 13-14k with no cut compared to the 10500-11000 i got before. This mod makes 3rd gear last till almost 100 mph vs the 90 i got before the mod, for those times when you realy need to get going. I didnt realize you where that close jamie i might have to stop in your area and check out your stuff the biggest draw back of my bike ever since i have owned it has been my suspension and, well my fat ass. These things where designed with a 160-180 lbs rider in mind after all. I didnt mind it when i was 17 but now that 30 mile comute kinda sucks at 25 lol.

 

I just remembered one time after i did a valve adjustment and took the bike for a spin it had 0 power and didnt wanna run for me. So i limped it back home and rechecked my adjustment seems i had set the valves abit to tight. But if you have apreciable power before the cut off then this will not be the problem, who knows it might come in handy someday when you do the valves though.

 

this might help.............http://users.rcn.com/kochc/moto/spark/ig_repair.html

 

Interesting, I've never heard of anyone actually trying this before. That would explain why you can power through the limiter sometimes, you still have two cylinders running at 100%. It's funny how the more you understand the more things start to make sense.

 

yep i got it from a fellow vf500r owner who i found in my epic search for parts to put on my bike to improve it.

 

I've also read somewhere that you can swap in 2 small CDI boxes, leaving the larger one out, to completely do away with the rev limiter.

 

yep it works 100%

 

You could have a clogged exhaust believe it or not.
Check the directions that came with your vacuum gage.
If no air or restricted air is going or trying to go out the exhaust then no intake can come in.
The revs will be limited.
The vacuum gage instructions are VERY important as a diagnostic tool.
IT can tell you about a bent valve or low compression or clogged exhaust.
Many things.
BTW did you perform a compression test first?

 

Sorry i havent been online in a bit guys, looks like lots of talk going back and fourth in this thread, im glad to see it! Donald, i have actually just recently done a compression test and its good. I will have to bring home my good vacuum guage from work and give it another test but im starting to think its either plugged exhaust or electrical. The problem only exists in gear under load, its fine in neutral. I have a motorcycle mechanic tell me it sounds like worn valve springs, to me this kind of makes sense for the lack of revving, but i think if it were valve springs it would not rev in neutral either, am i correct?

 

clean the slick50 port................

 

It could be valve float, if so you are ready to drop a valve at any time. Be VERY careful with this on the VF500's because it's been the fate of many o' them over the years. Dropped valves is the #1 problem with those bikes, in my opinion.

It doesn't take much to rev in neutral. You could be running on a two cylinders and still get to redline. It wouldn't rev like that under load though. If the valves are floating then your performance is way down, that would explain the lack of rev ability.

Hey, that's a new term I just made up - revability.

 

Revving up an engine in neutral is not good. Better to have it under a load.
Just be reasonable.If you are floating valves its time to lower the revs.I don't care what the plastic digital gage says.

 

Checking the firing pulses is a good thing to do but if it has a smooth steady light at idle it is probably good. Idleing is the hardest thing for an engine to do. I would start with a compression check first THEN a vacuum gage test on each cylinder. How many miles on it?

 

Something to check.
I know it sound s simple but...
Look into the carbs with air cleaner OFF and with engine OFF turn the throttle and make butterflies open all the way. Cables could be adjusted incorrectly?
What size ARE the main jets?

 

Why?

I've heard this mentioned before but nobody seems to know why this would be a problem. The engine doesn't care, providing everything is operating properly it will be just fine.

 

The reason it should not be revved up a lot when not in gear(any reciprocating engine) is that the vibration and forces are not countered by any other forces,such as the transmission, being under a load and it is connected to the crank and clutch etc. Rotating masses being counterbalanced and providing something to work against to dampen vibrations.

 

How do you know if you're about to drop a valve? How do you know if you have valve float? And if so, how is this resolved?

 

If you are revving it up so high the valves are floating then be ready for a bad sound. Have a rag handy.