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

The needle goes up and down THROUGH the MAINJET so raising the needle will make SURE the mainjet is getting full flow. You will know you have a big enough main jet when it is in high gear and accelerating - IT WILL JUST KEEP ACCELERATING.

 

I understand that you float a valve by over-revving the motor. My point was that the OP was saying he couldn't rev past a certain rpm - and it was possible his valves were starting to float at a lower RPM than normal and he may be about to drop a valve.

I was asking why that scenario would happen - what would cause something like this situation, and how would it be resolved.

 

You don't know if you are having valve float, there is absolutely no way to tell. For the VF500's, if you hear a loud ticking sound then you need to pull the cam covers and visually inspect the valve stem tips for damage. If the valve(s) is floating then the rocker is basically hammering on the tip which causes it to break away. The valve spring retainer keeper also does a slide-hammer job on the keeper groove. Eventually is the keeper that strips off the end of the valve, allowing it to drop into the cylinder.

The resolution is to buy new valve springs. To date the only option for that was new Honda parts........ but those are now discontinued. It's been on my list of upcoming projects but I'm not when I'd ever get to it. Maybe I should bump it up in the priority list?

 

In the case of the VF500's, the valve float occurs at rpm's below redline. In fact, since it has a rev limiter you can't really 'over-rev' it. On my personal VF500's I keep the revs below 9000 in an effort to prevent float. I'm not sure if that's a magical number, but so far it's worked for me [hard knocking on wood sound].

 

Thanks for a good explanation! I pretty much knew this (being an automotive tech for years) but didn't quite know how to put it into words!

 

First off, let's define valve float. Valve float is when the valve spring cannot hold the valve closed all the way above a certain RPM due to the velocity of it opening, and the spring not being strong enough.

Sometimes the valve will actually bounce off the valve seat when it closes, which I believe is why a broken valve spring will make a vacuum gauge needle fluctuate rapidly at lower rpm's.

From what I've seen on car engines, when the valves float it acts somewhat like a rev limiter. I odn't think this is always the case, but the engine will definitely not be able to perform very well with the valves not closing all the way.

Maybe someone can chime in and clarify what I may not have worded very well?

 

Actually, valve float does not have anything to do with the valve staying against the seat. It's when the valve won't stay tight against the rocker arm. When the cam ramps open the valve it generates a lot of momentum by putting into motion the valve, keeper, retainer, some portion of the rocker arm mass and some portion of the spring mass (everything that is moving). At the end of the lobe lift the spring must slow this mass down in order for the rocker to stay in contact with the cam lobe. What basically happens during valve float is that the valve continues to move, think of it as the cam throws the valve away from the lobe. That makes the valve open a little more and for a little longer duration. That's not the problem, the problem is when the spring finally does slow the valve mass down - now it's resting in free air. It then slams back against the cam lobe. This action works like a slide hammer on the valve stem tip and the keeper. That's when the damage is occurring.


Edit: The reason valve float happens at high rpm is because the momentum of the valve that causes the float is proportional to the velocity squared. That means a small decrease in engine speed (i.e. not revving over a certain rpm) makes a significant impact on the the momentum. It makes you appreciate the newer bikes with sky-high redlines.

 

Ok, gotcha. I read up some more on it, and what I described is actually Valve Bounce. From what I can tell, valve bounce will uusally occur before float though, reducing power, making it tough for the RPM's to increase. From what I understand, both are very damaging to the valvetrain components.

 

I've not heard that about bouce, everything I've read/seen states that float will happen first. Keep in mind that automotive engines are different - they don't see nearly the rpms of motorcycle engines. Maybe in a car engine the valves would bounce before they would float, but the VF500 is different.

 

Not sure, but doesn't really matter either way I suppose! They both will cause damage, and the inability to rev much higher.

 

Well there is certainly some good food for thought in this topic. Thank you for all the answers and things to check guys. Unfortunately i havent been online much or looking at the bike much, as other things have come up. Im hoping to get back on it soon though so im starting to pay some more attention to this thread. It looks like valve float is a definate possibility. I am in need of a new starter button assembly now for the vf500 before i can do much more testing so if anyone knows where i can get one of those cheap let me know. Otherwise, the bike only has JUST over 10,000km on it, but was sitting for 20+ years, which brings me to my next question. Being such low mileage, is it even possible for these valve springs to wear out just from sitting?

 

Why do you need a new start buton? Describe the symptoms most likely its just the selonoid that sits beside the battery that is crapped out. you can hotwire it with a screw driver to get the bike started in a pinch.

 

When I first started rebuilding my VF700F ('84) It would get "stuck" at about 4000rpm and not go any higher. If I pushed it gently it would just stall. I found that if I kept twisting the throttle to full, briefly, and then letting off it would start to "break through" this barrier. Eventually it would rev just fine.

I don't know if this helps or not for your situation. I found that it does this fairly consistantly when the carbs are out of fuel. (it did the exact same thing after I ran it out of gas)

 

Magnavmx5, i need a new start button because mine melted down on me. The bike sat forever and so i think now that i finally had it running and was using the switch after 20+ years i think there was some metallic cobwebs in there or something and it must have shorted out. It started smoking and then it wouldn't crank anymore. I took the switch all apart and its just a big pile of melted plastic. Anyone have any other thoughts on weak valve springs?? And if its even possible for mine to be weak at only about 10,000km but keeping in mind it was sitting for a long time!

 

ahhh damn 1983Bob i have never even concieved that to be possible, thats one heck of a problem. I do not think your valve springs are weak, but maybe just set improperly checking the clearance wont hurt you any and from there you can go into all the other venues we discussed in this thread before. If you are unsure how, to do this then just ask and or do some poking around on here and the net. And one of us can help walk you through it if need be.

 

Yeah, well once i get a new starter switch, i will be removing the exhaust and checking that out first. I have reason to believe the manifold collector box is partially restricted, probably due to rust from sitting?? We will see...

 

meh its all stainless but gl.

 

HERE'S what i shook out of a VFR muffler that had been sitting on a shelf for 2 years, blasted mouses !!

 

My turn.

I have not read all three pages of this thread. Today is the first I have looked at it. My immediate reaction to the symptoms was that you have electrical problems. 20 years sitting without being in one of those vacuum bags can cause some strange things to happen. But to stop firing under load just hits me as an electrical problem. I think the melt down of the starter switch is the tell. Why was the bike parked in the first place? Was there a problem with the way it ran? Ask your dad why he stopped riding altogether at the time he parked the bike?

In my life I have seen problems like the one you began this thred with maybe 3 times. I was very skeptical of a faulty coil in the first instance. I did the resistance tests and I couldn't get the coil to test bad. But the same sort of problem kept occuring. The car would start and idle. I could make it go but as the revolutions increased the plug fire would become erratic and finally the motor would refuse to rev higher. I think it would back fire if I pressured it to keep accelerating. After exhausting every other avenue I broke down, bought and installed a junk yard replacement coil. End of problem. I later found out that this sort of break down in the coil has to do with shorting of the windings inside the coil under load. this was back in the days when coils had PCB's in the oil that they used as a dielectric coolant.

The second and third instances of failure to rev occur at night when the vehicle has a faulty or failed charging system. What occurs is a slow drain on the battery that slowly comes up on me while driving cross country (across a lonely desert in both cases). Remember that this was back when head lamps were under the riders control. So if your charging system went south and your head lamp was off during the day, when dusk came and you turned on the headlamp the bike or car as it was in each of the two cases the bike and the car immediately started to exhibit the sort of behavior you described. Inability to rev past a certain limit. The reason for this is that there was not sufficient voltage( analog to electromagnetic pressure) to saturate the coil(s) before calling on the coils to collapse their respective magnetic fields in order to create a spark of sufficient strength to ignite the air fuel mixture. The limiting factor is time. It takes time to saturate a coil. The time it takes is related to voltage. So, at low engine speeds the coil(s) were sufficiently saturated to create a spark sufficient to explode the mixture. As the revolutions increase the time to saturate becomes problematic. The plug fails to fire at certain engines speeds. Eventually the battery voltage drops your motor speed to nothing as it drops into the 9 volt range and you stop going anywhere. Try that on a Triumph 650 in the middle of a freezing night in the middle of Indian country (Eastern Oklahoma) about 150 miles from Oklahoma city and the next Triumph Dealearship. You get to spend the night sleeping (if you want to call it that) on the side of the road. Then you get to take a bus ride to Oklahoma city and leave the bike in a one stop light town until you make the round trip. Better be prepared. The tell is the fact that your head lamps have been growing dimmer at the same time your vehicle was slowing down and not allowing you to rev to highway speed.

The third time I had my coil fail me was in South New Mexico. I had the headlight diming/failure to rev up blues again. I was too many miles from nowhere and I did not want to sleep in the car. I figured the only way I was going to get to the next town was to turn off my lights and drive dark. To do this I turned off the 10 and went to shortcut 2 laner that I had traveled before. It had no traffic and it was a clear moonlit desert night. I made it to lordsberg on the juice in the battery. I could have made it to Tucson if I had just had time to recharge the battery every 40 miles and I drove what the discharging battery allowed me to drive.

The tell is a little messed up for 83Bob because he has only ridden during the daylight hours, when he can't see the headlight, it stays on during daylight because someone decided that riding was safer that way and made them so they stay on. If you have a new battery (not the ressurected one that is 20 years old) try charging it with a slow charge over night and see if the bike runs different with a fully charged battery. The battery will discharge rapidly during the day because the headlight is on. This will bring the voltage down quickly, so if the bike runs better when its just started and run on a fresh full charge and say in 5 minutes it starts to do the same old thing.......well don't let the bike sit at idle for 5 minutes to warm up, get on and ride immediatly. If it runs to higher RPM's and then starts to crap out after 5 minutes, you have your finger on the problem. There is a reason why the starter button melted down and it wasn't your heavy hand on the button. You have some sort of short in the electrical system and the fact that you melted down your start button is the tell.

If you can get the motor to rev in the garage with the battery on a charger when you are blipping it and then take the charger off and 5 minutes later get on the bike and ride.....well, you take it from there.

I don't know how an 80's Honda with electronic ignition will respond to coil failure. I'd check the charging system, and the battery, and make sure that everything in the charging system is 'tight and right'. I left the ground or hot lead to the battery loose on my '84 Magna and ended up buying a voltage regulator after only about a 20 mile ride.

If you are using a battery charger to start the bike be careful

 

An excellent, well-reasoned analysis, Perfessor !! Nice work.