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

Hey Guys,

As most of you know im restoring this bike for my dad. Im dealing with a very frustrating problem right now and i just can't seem to get it figured out. Whats happening is the bike will rev up fine in neutral/no load, but the second you try to ride it, it will rev up nicely to about 5000 or 6000 rpm and just stop revving. It feels very simular to the rev limiter on my R6. This makes the bike very unrideable. Ive had the carbs off twice now to clean them, first time i found a few clogged jets and passages. Second time everything appeared to be clean and clear, except i did find a small tear in one of the slide diaphrams so i replaced it with a good spare one i had. I thought for sure this would fix it, but still no luck. The problem is still there. The other thing worth mentioning is that when i hooked a vacuum gauge up to the vacuum port on the side of the head and ran it, the vaccum gauge needle seems to fluctuate rapidly between about 5 in/hg vacuum and about 1 or 2 psi pressure?? I don't know if this is related to the not revving or not, which is my main concern, but i wouldn't think so? Does anyone have any ideas as to what to check next? ANY HELP PLEASE, its greatly appreciated. Thanks!

 

can you say carb problem! mybe the carb you've got a torn carb slide

 

My brother's '86 VF500F just had a similar problem. It would rev fine up to 8000 rpm then it acted as if a rev limiter had kicked in. We swapped carbs with my bike and it did not resolve it. It ended up being electrical. He had a frayed wire leading from the upper ignition coil to the harness. If you think it is not fuel related, then I'd trace the wiring.

 

Thanks Stewart, that actually crossed my mind that it might be something electrical. Im not really sure where to start with that though. I just had the whole bike torn apart and everything went back together looking ok. I will look it over again though. Thanks for that tip. Was it the sparkplug lead that was frayed on your bros bike?

 

It was a wire that connects directly to the upper ignition coil. The sheathing was worn away and the wire was exposed. From there, the wire goes to a harness that connects to the pulse generators and the spark units. I would go over and check all of that wiring in addition to the spark plug wires.

 

Ask these guys on this Yahoo site I belong. Lots of help here and they're exclusively 1st Gen owners. . .

interceptors : The Honda VF Interceptor

 

Matt, i don't have a torn slide diaphram, as i just checked them all today, and replaced the one that was torn. There was no change in the way the bike ran.

Alex, thank you for the yahoo link. I tried to sign up for yahoo to make my post but for some reason it wont accept my postal code?

Maybe someone could correct me if im wrong, but if the bike runs good while in neutral, idles, and revs up normally, then could it still be carb problems if its only a problem while riding the bike/under load? It just seems odd to me. I was going to pull the carbs again and recheck my float levels but i can't see that making much sense with my given symptoms?

 

it sounds electrical....check all your connections.. check you coils outputs.

 

Thanks Crusty, how would I go about testing the coil outputs? The only tests for the coils in the manual were resistance tests, which are pretty useless really.

 

if not a carb problem then i would say get a voltmeter and check both of your coil's

 

Im not totally ruling out the carbs just yet, actually i might even pull them off again today at some point and check the float level to be sure. It just seems a little odd to me that it would rev up fine in neutral? The problem only exists when you are riding it. Does this still sound like it could be float level or other carb issues to anyone else?? How would i go about testing the coils with a volt meter? I have a voltmeter here. I also forgot to mention earlier that this bike has the fuel pump delete done on it as the fuel pump was dead. Thanks guys.

 

Just an update... so i just pulled the carbs off and checked the float height adjustments. 2 of the float levels were off by 1mm but thats negligable. I also managed to do a coil resistance test on both the coil packs. My primary resistance is 3.2 ohms on both coil packs, and my secondary resistance between coil outputs is about 15k ohms. According to the manual, my primary should be about 2.8 ohms and the secondary should be about 21k-28k ohms. Are my readings going to be close enough or is there some kind of breakdown inside the coils going on that could cause my problems? Thanks guys... any thoughts or ideas are appreciated!

 

that is a good possibility, as a coil wears out it wont produce as great of a voltage increase across the coil. this means that at higher loads it wont create as good of a spark, so also check your plug gaps.

 

make sure the vaccum line is connected to the fuel petcock. if it is not then you will not recieve sufficent gas flow and the bike will not rev high or make good power

 

Thanks for the replies airgreesemonkey and Pcohen. The test that i did on the coils was a resistance test, so im not sure that it proves that the coil is actually worn out (as a voltage test might)? The vacuum line is connected to the fuel petcock, although as i mentioned earlier my vacuum signal seems to be bouncing all over the place and im not sure why? So what i thought about today, is maybe due to the erratic vacuum signal, the fuel petcock is not letting enough fuel into the fuel line to the carbs? Im not sure... once i get the carbs back on the bike im going to try riding it with a vacuum pump on the petcock and the vacuum line plugged off and see if it is any better. Thanks for the ideas.

 

make sure the vacuum line is plugged into the middle nipple on the petcock

 

You mean the one on the end cap of the valve, right?

 

Hey guys, yeah its plugged into the one on the end cap so no worries. If you suck on the other one with a vacuum pump nothing happens so i know ive got the right one. I noticed that the bike seems to rev to about 7000 rpm or so as a pose to only 5000 rpm if i use my vacuum pump on the fuel petcock instead of the engine vacuum source.

Jamie, i was actually meaning to ask you, do you think that the gas cap vent could be causing a lack of fuel if its plugged up and giving me these symptoms? If so.. where is the vent and can it be cleared up? Thanks.

 

No, that's probably not it. When the cap vent is the problem the engine has trouble after some number of miles. It's because you must consume enough fuel to draw a negative pressure inside the tank - that's what keeps the fuel from coming out. I'd say there is still something wrong with the carbs. You might check all of your ignition system as well. It's not a bad idea to read the plugs to see what they tell you.

You know, it almost sounds to me like your cams are not in correctly. I did that once and it had a similar effect. The bike ran great and you could ride it, you just couldn't get it to rev very high. Has your engine been apart recently?

 

The thing with this engine is that its been sitting since 1988. But otherwise it only has 10,000 original kilometers on it. The only thing i had apart on it was the one side cover to install the oiling kit that i bought off you, and i had the waterpump off to make sure that it was all still going to be working ok when i got it running. Other than that though, i dont believe this engine has ever been apart. Its my dads bike and he has owned it since new. But it ran perfect when he parked it?? Ive had the carbs off soo many times now and made sure everything is clean and adjusted. The float height is set to 6mm and everything?? Im not sure, its very frustrating though. The odd thing is.. is if i put my vacuum pump on the fuel petcock in place of the vacuum line from the engine.. then it will rev to about 7000 before it does the same thing. So it must be a fuel source problem im guessing now.. im going to go play with it more tonight after dinner so i will keep you guys updated. Thanks again for all the help, this site rules.. i might have to get my own vfr one day