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

I actually mentioned it in my reply that i started out by asking if the CDI Boxes from an '85 bike would be the same as for the '84. I think i am narrowing in on it though, i will be soooo relieved to have it figured out when i finally get that far! haha...

 

Quick Check: swap the timing light inductive clamp.

Take the timing light inductive clamp off #1 or #3 wire and put it on #2 or 4. Get access to timeing marks (remove plug in RHS cover). Look at #2 or #4 firing marks with timing light. If light is dead steady and timing is on the mark you have located a problem in the 1-3 ignition circuit. Before you rush out to buy a CDI unit try taking the tach out of the circuit. CID units are expensive, even used. Most Junkers know they cost hundreds and are not repairable by most shops. Junkers will ask $100 or more for a used one (half price?). You should eliminate the tach as a source of the problem before you commit to buying a used one.

If the tach is a problem it may cause the CDI unit to fail (blow a transistor). It might take some time for the transistor to go bad. Then you have no ability to return the used CDI unit. Make sure that the junker will let you return the one you buy to replace the one that you know is bad. Their's may be bad too. They should let you take it home and try it with a 1 or 2 day return. Anyway, electrical parts are usually difficult to return. Take your old one with you for comparison with the replacement.
DKC

 

Thanks for the advice DKC, exactly what you have just said is what my next step is. I will report back and let you know. Thanks again! I really appreciate all the help.

 

The '84 and '85 model CDI are the same.

But there are 2 separate CDI modules. They are different sizes. The longer CDI box is for Cylinders 2-4 (Blue). The shorter CDI box is for the 1-3 cylinders (yellow) . They would seem to be the same. But consult your wiring diagram if you try to run two of the same kind. The blue wires and yellow wires need to be swapped if you are going to use a 2-4 box in the 1-3 place. At that I would still take the tach out of the circuit just in case there's some special circuitry inside one or the other to run the tach or run the pump cut off relay. You are on your own if you try to run a 2-4 in place of a 1-3. I'm not saying it won't work. In fact it should work, but I am leary of differences between them because one is longer than the other. This might have simply been Honda making certain that no confusion could cause problems between long and short. If you try it let us all knoe how it turns out. PS one of the CDI units is no longer available. A new relplacement is $320 plus shipping and tax.

 

Ok guys, sorry its been so long without an update, but life is pretty busy these days and i haven't had much time to work on the bike. Here's where im at with it... I tried everything that you guys have said so far, and ive replaced the CDI Ignitor Modules without any luck. The carbs are now totally and utterly clean! I completely stripped them down and boiled the crap out of them in lemon juice and managed to get them back together and the bike ran exactly the same, but at least now i know its NOT the carbs for sure! What i did find though, was that while i had the bike running with the airbox off, i noticed that the vacuum slides are slapping opened and closed rapidly, regardless of whether the bike is idling or revved up. I know this is not normal so i checked the source vacuum and noticed that its fluctuating rapidly on the vacuum gauge as well, in turn causing the slides to open and close rapidly. This to me is obviously why the bike seems like its running out of fuel when i go to ride it away. My question to you guys, is what would cause the vacuum to fluctuate so much like this? Im scared to ask, as i think its probably a valve guide or valve springs, but you tell me! At least im finally getting somewhere with this headache... thanks a million guys, let me know what your thoughts are on this...

 

Any more help here guys and gals, i know that this isn't normal, im just not sure what would cause the vacuum to be all over the place, my cv slides are snapping open and closed and idle and when revved up and the vacuum guage on the vacuum source for the fuel petcock shows the vacuum to be all over the place too?

 

Looks like you're struggling Bob :frown: but it sounds like you've looked at all the basics

I've just taken a quick look through all the posts ! and the one thing I didn't see (but I may have missed it) was a cylinder compression test. If you can get in there that should show any inetrnal problems.

Secondly, if it only manifests itself when you load up the engine (pulling away for example) then is something stretching, touching, shorting getting choked , etc as the engine twists.

Finally, as its an older bike, you havent got a lump of rust or other crap choking off the fuel feed inside the tank have you.

Thtats the best I can do, so good luck and keep the faith :smile:

Tris

 

Long thread here..

...are you sure all the diaphrams have no small holes or cracks and are properly seated ???

 

my cb750k did exactly the same thing when we put the engine back in and found out about a month later that the the small wires going to the coils were on backwards. the positive was on the negative visa versa. i couldn't believe it. i wouldn't think the coils would work backwards but they did. (not the sparkplug wires but the small wires that power them up) so i would check your wiring just to make sure. maybe this will help

 

Those are all excellent replies guys, thank you! I actually have done a compression test but it was quite sometime ago, im pretty sure it was all ok, but ive brought my compression gauge home with me tonight so that i can re-check it again this weekend and be sure. I will let you guys know the results. I am 99 percent sure that its not the carbs anymore (aside from the slides bouncing around and throwing off my fuel mixture) as i have completely stripped them down right off the intake plenum and everything and boiled them in lemon juice for about an hour. They are SPOTLESS now. I have also checked and rechecked and rechecked that slide diaphrams, they are all good and actually in really good condition. Horseiron1, thank you for that excellent experience post. I will be sure to check that very first, as that is a possibility. Thanks again to all... i will keep you posted. This has just turned into a nightmare.

 

Has there been any progress or a resolution to this problem? I have the same story with my '85. I just replaced the plug wires and caps tonight, but it's too late to piss the neighbors off. I'll give it a go in the morning. But I am very interested to hear how this has panned out.

 

Hey there, no there hasn't been a fix to this problem yet or i would have been sure to post it. I got the bike fully restored and never did figure out this problem, ive tried literally everything!! SOoooo frustrating. If you figure yours out PLEASE let me know what you found. I had to move and renovate the new house so the bike took the back burner and i havent been able to look at it now for many months... although im out of ideas now anyways... ugh..

-Bob

 

What an amazing thread. So much information and it has obviously been frustrating to everyone. Your bike has one heck of a gremlin and I hope a solution is found. With only 10k km it sure would be nice to have it running. Good luck.

 

With all due disrespect, I can assure you there is a reason and when I worked in a motorcycle shop often we would get a motorcycle to repair when the owner and all his friends had already tried to fix the motorcycle.

You are on the right track though by going back to the beginning and doing a compression check.
When you say, "im pretty sure it was all ok, but ive brought my compression gauge home with me tonight so that i can re-", what this tells me is that each step was not done correctly or completely.

If your answer was.... I did the compression check and you are sure it was done properly and you had the compression for each cylinder with the plugs out and a good battery OR the method described in the workshop manual you would NOT be doing this step again.

Just go back through all the steps again and have a good WORKSHOP manual.

Think about this.... Is there any part that you would rather not deal with or do not know how to test, then that may be the problem.

Hope this helps.

ONE MORE THING....
Will not rev up......... Disconnect the Tachometer wires and see what happens.
The tachometer is the rev limiter. Get it?
It is the yellow wire on most VF's
But I would just unplug the whole thing temporarily.

 

Absolutely correct. Carbs LAST.

 

Donald is right, there IS a reason. I am happy to report that the reason for mine not being able to get out of it's own way, was as simple as a faulty spark plug wire. As was mentioned earlier in this thread about using a timing light to verify that spark is being delivered to each plug, this is what clued me in to the real problem. I was in the process of checking the timing to make sure that the CDI boxes were functioning properly, when I noticed that the light went out on one cylinder when I began to rev it up (I wanted to see that the advance was working). From there, I replaced all of the plug wires and caps with new, and wow, the bike is running better than it has since we bought it a couple of months ago. This has been the challenge through a couple of restorations that I have done in the past. You fix the obvious, only to find that it still doesn't run right, and you ask yourself, "is there ANOTHER problem, or did I just screw something up"? In my case, when I bought the bike, there were a couple of electrical issues to deal with, and a set of dirty, clogged carbs. This thread has been such a help in keeping my head with this thing in the midst of the frustration, and realize that there IS a solution, and it can't be too far removed from the fundamentals of proper air/fuel mixture, compression, and a spark at the right time.

 

Donald is right, there IS a reason. I am happy to report that the reason for mine not being able to get out of it's own way, was as simple as a faulty spark plug wire. As was mentioned earlier in this thread about using a timing light to verify that spark is being delivered to each plug, this is what clued me in to the real problem. I was in the process of checking the timing to make sure that the CDI boxes were functioning properly, when I noticed that the light went out on one cylinder when I began to rev it up (I wanted to see that the advance was working). From there, I replaced all of the plug wires and caps with new, and wow, the bike is running better than it has since we bought it a couple of months ago. This has been the challenge through a couple of restorations that I have done in the past. You fix the obvious, only to find that it still doesn't run right, and you ask yourself, "is there ANOTHER problem, or did I just screw something up"? In my case, when I bought the bike, there were a couple of electrical issues to deal with, and a set of dirty, clogged carbs. This thread has been such a help in keeping my head with this thing in the midst of the frustration, and realize that there IS a solution, and it can't be too far removed from the fundamentals of proper air/fuel mixture, compression, and a spark at the right time.