Engine bogging only at 6500rpm??

[loopsandlogic]

My thread title is pretty much explanatory!!

I have an 84' VF500F, and when I'm just putting around town or the backroads, as soon as I reach the 6500rpm spot on the gauge, I feel there's like a pair of cylinders shutting down slightly.

Its not a big deal, but it's noticeable! And I've pulled the plugs, later to find out there NGK 7's which is too hot for my 60's weather.

So, I've put NGK 8's in hopping this would fix my problem, but it hasn't.....

Plugs look nice and brown!!! Not black or white.

Questions:
Could my pilot jets affect this rev range??
My jets are perfectly clean, but could just one speck upset the motor?
Could it be my coils? Or my CDI??

I'm at a lost, because the bike performs flawlessly!!

And as a side note, I have put new OEM head gaskets in the bike which only has 7k original miles, but if I'm one tooth off on the cam sprocket, would this problem I'm having indicate this mechanical flaw??

Thank you for any help

LL

[RVFR]

Boy I'm just guessing, but from what I can read into this it sounds like a fuel starvation area. I'm pretty sure if you where one tooth off you'd have more problems from the get go. Did this come about right after the head gasket replacement? Did it run right before? Sorry to say but this is one of those touchy feeling things to witness. I'd also say it's not in the ignition. Good place to start, but if she's running good below, it's either fuel or maybe spark advance? Reason I say this, I had small block that ran great down below, but once you put a load on it and it was in the mid rpm range it stopped pulling, come to find out it was the ignition wasn't advancing, but with these Vfs I'm thinking that's something you can't mess with, so I'd lean more towards the fuel being the issue. Again just what pops in to my mind as I read this.

[Jimtt]

I agree it seems you are not getting enough fuel in the mixture; will it rev OK past 6500 after the stumble?

[loopsandlogic]

This bike never ran when I bought it. The gentleman mixed up the cams and I had to reset them the best I could per the OEM shop manual.

I'm not sure, but the cams looked like new...he could have replaced them with new cams for all I know.

I agree, I don't think it's the cam sprocket, because it would be off at every rev range.

So, you think its the fuel flow? Could it be the pilot screws?
Would slightly larger gaps on the valves pose this problem I'm having??
Could a tiny air leak cause this


I kinda knew someone was going to ask me that, but I really don't take any of my bikes past that on the tach.....
6500prm on the tack is pretty much my passing range.


So....I should see if it goes away around 7k.... I will try to get that ride in by tomorrow.

Any other suggestions??

Thank you

[loopsandlogic]

Well, it seems when ever I really give it gas it stumbles right after 5k on the tach.

And if I slowly rev the bike up to 5500rpm, it starts it's stumbling.

I've finally gotten the proper word to describe this condition I have and
after coming home today, I'm doing some testing on the pump.

And hopefully, I'll have some info to post tomorrow.

This couldn't be a case of an air leak or manifold boot leak??

Help please

LL

[masonv45]

Put an unlit propane torch near the carb boots and see if the idle raises. If it does, then you have an air leak.

You could have the main jets plugged, float height set incorrectly, cams 180 degrees out of phase.

This could help: Carburetors/Tuning - SabMagFAQ Be sure the read the links at the bottom.

[rc24rc51]

Sounds like a jetting issue to me. You may have a mid range lean condition. Does anyone have a dyno in your area? put that sucker on a load control dyno with a exhaust gas sniffer and you'll know right away. It'll tell you wether you need to richen the mixture or raise or lower your needles. A little trick i learned would be to ride the bike to duplicate the problem and open the choke a little bit at a time. If it improves the condition you're too lean.

[loopsandlogic]

I thought the choke only primes, as in squirts a rich mixture of fuel into the cylinders. Or is this a continuous shot of rich fuel???

Isn't the needle jets set permanently from Honda??
I don't remember seeing slots on the needles......

And for me to have it dyno'd is way to expensive. All the shops charge big
money because were I live, Sport bikes are Huge!!!

Especially drag racing.

And I definitely agree it's a lean problem!!!!
I even placed a cover over half of the intake boot on the air box, and it
sure did help.

You can feel mid-range is lacking, because when you open her up to almost 1/2 to 3/4 throttle, you can feel the main jet starting to work when reaching WOT.

Well, it's snowing here in Ohio, so I guess spring will be her time to shine :)

I still have to get her winterized

Thanks for the help,-LL

[rc24rc51]

the choke will add additional fuel as long as it's open. because riding with the choke open or partially open too long will foul your plugs. if your bike is bone stock (jetting, exhaust, etc.) and you're stll lean in the mid range you'll need to shim the stock needles AND OR adjust the mixture screws. Have the carbs properly synced as well. the air filter. what kind is it? A K&N will flow more air and possibly cause or contribute to the problem. Does it stumble only under load or can you make it do it in nuetral?

[squirrelman]

you may have one or more bad diaphrams causing inadequate high speed enrichment....

also inspect air filter for holes or tears

drain float bowls to remove water and rust

forget about pilot screws as this is way ouy of their effective range....

[JamieDaugherty]

+1 on the possibility of a damaged carb diaphragm. It's not the pilot circuit at that rpm range but more likely the slide/needle. That's why the diaphragm could be the culprit. Or it could simply be a case of really bad jetting. Here are some thoughts and questions:

1) The previous owner hosed up the cam installation so it stands to reason that they could have also hosed up the carbs while they were off.
2) Why were they replacing the cams? I wonder if they installed a set of Megacycle cams? Those can be identified by the grind number being stamped on the end. If they are Megacycle cams then it would explain the poor jetting in the midrange!

[jamey2466]

yes it is in the carbs. I havea 1986 vfr 700f and it did the same thing. I just hade to sinc the carbs. and no more it ran like hell good luck.

[Maggot]

Here's the part that scares me. The previous guy had the cams off. I don't know anything about this motor but if the guy cracked the valve springs or installed too weak of springs, the valves will not fully close at high RPM. This condition is called valve float. The valve does not have enough spring tension to follow the cam and dances across the top of the cam lobe. Once the valves begin to float at a specific RPM you will not be able to achieve any higher RPM.

Just another angle to the problem, but I hope it is a fuel starvation problem.

Good Luck!

[loopsandlogic]

Hi,
Sorry for the late response....I've been swamped.

But here it goes.

rc24rc51,
the bike is bone stock!! But the filter is an aftermarket filter that's unknown. It looks just like the oem one.
I'm pretty sure I checked the jet numbers in the spring...but it's been to long to remember......
Bike will rev just fine in neutral under NO load.
And the your choke idea sounds like a good one....I will have to try it in spring!

squirrelman,
A torn diagram sounds very likely!!!! Carbs were drained, and your right about the pilot screw.

jamey2466,
Are you saying that after syncing your carbs the bike ran great or it ran like Hell??

JamieDaugherty,
The cams are OEM honda. And I would definitely know if they were Megacycle cams

Maggot,
Your idea sounds good, but like I've said, I can continue all the way up to WOT if I wanted to, but this stumbling is quite noticable.
And the fuel pump is working top notch. Pumping out its required amount.

But the issue could still be the float height : (

Once I get my garage heater going, I'll be going at the carbs, but this time, from Top to Bottom.
This XR600R SM project is getting in my way

Thanks guys for the help....VFR owners always help out one of there own, no doubt,

-LL

[squirrelman]

if your bike runs ok until problem rpm, most likely not a fuel level problem, however if you want to check float height:

it's helpful to make a gauge to set the float height, helps set all 4 the same....

use an old plastic credit card cut so that each end rests against the carb and with a section in the middle cut to float height......

(simply destroying the credit card could save u lotsa money!!)

like this:

VFRD -> Gallery -> Viewing image -> float height gauge

[loopsandlogic]

Originally Posted by squirrelman

if you want to check float height:

it's helpful to make a gauge to set the float height, helps set all 4 the same....

use an old plastic credit card cut so that each end rests against the carb and with a section in the middle cut to float height......

(simply destroying the credit card could save u lotsa money!!)

LOL!!!

I'll give my local shop a quick call for carb bowl gaskets....cause the last time, they didn't look that good....I'm surprised there not leaking

Cheers mate,

LL

[jamey2466]

yes the bike ran great like new

[loopsandlogic]

Well, that's good news.

You know, with all of these tips and advice you guys are giving me, I've started making one big list, so when I get one day off, I'm tackling this VF all at once.

Thanks guys!

LL