Das nightmare has returned lol

[DigitalArts]

So I fixed the throttle cable sync between both sides and that made a huge improvement, then I turned all the air screws in and back out 3/4 turn, that also made a huge improvement.

After that, I finally brought her out for some road testing.

Symptoms:

- Sometimes it will come off idle with tons of power and seem to be perfect

- If I cruise at 4k and pull clutch in, slow down, down shift, then let it out, she will fall on her face and sometimes totally shut off

- If I'm anywhere between 3k and up and go to *slowly* accelerate, it literally drops in rpm at least 100 before actually starting to accelerate, but if I continue to slowly turn the throttle she eventually "catches up" to whatever is missing from the mix and starts to take off.

- Alternatively, when I go to accelerate from 3k and up, sometimes it never makes it to the point where it catches up and just continues to drop rpm until it stalls.

- In addition, I think that to maintain rpm I actually end up slowly turning the throttle. Like, if I get to 4k and leave the throttle where it is, I think it slowly drops.

I don't even know where to begin. New oem size battery on the way, any links to budget spark plugs?

 

[Traumahawk]

Well the NGK's that fit the Vmax are about 10 bucks for all 4. I dont know any more budget than that.

 

[DigitalArts]

Well the NGK's that fit the Vmax are about 10 bucks for all 4. I dont know any more budget than that.

Sounds right up my ally If I ask auto zone for ones that fit should they pull right up?

 

[Traumahawk]

Should be.....I always get mine from Oreillys....but same principle.

 

[DigitalArts]

Should be.....I always get mine from Oreillys....but same principle.

Do you think they're the culprit?

 

[sdt354]

3/4 turn is way to lean. 2- 1/2 will usually get any stock motor to run ok, though it's not uncommon to go richer with mods. Tell Auto Zone to give you the NGK's that the manual states or you risk getting, whatever. The Yamaha manual is free, on this forum, and the Haynes I believe. Gap plugs to .035 and forget them for now. I never met anyone on this forum that wouldn't help out a another Vmax rider. If I was within reach I'd come help u out. Check who's in your area and ask for a hand. It will probably save you a lot of aggravation. Sync has got to be right. https://dl.dropboxusercontent.com/u/55030067/VMX12%20Service%20Manual.pdf

 

[DigitalArts]

3/4 turn is way to lean. 2- 1/2 will usually get any stock motor to run ok, though it's not uncommon to go richer with mods. Tell Auto Zone them to give you the NGK's that the manual states or you risk getting, whatever. The Yamaha manual is free, on this forum, and the Haynes I believe. Gap plugs to .035 and forget them for now. I never met anyone on this forum that wouldn't help out a another Vmax rider. If I was within reach I'd come help u out. Check who's in your area and ask for a hand. It will probably save you a lot of aggravation. Sync has got to be right. https://dl.dropboxusercontent.com/u/55030067/VMX12 Service Manual.pdf

2 turns out on the air screw for the fcr carbs? I'll give that a try tonight and see what happens!

As for the spark plugs, will do. Any hint on which page I can find them on? My pc is really slow at loading the pdf for some reason

I'd hate to get in someone's way asking them to come help, but if anyone needs a reason to leave the house I'd certainly appreciate it!

How do I check the sync without the vacuum tool?

I have a new Apex battery on the way as of now!

 

[Traumahawk]

Well the screw is for A/F mixture from idle at part throttle up to 4000 rpm (with the stock mikunis).

When we talking about syncing the carbs....we mean getting all 4 carbs to open and close at the same time. If the sync is off....at part throttle.....you might have one carb working....then 2 more kick in....and then the last one kicks in. Thats the reason why a sync is VERY important on a Vmax.

 

[DigitalArts]

Well the screw is for A/F mixture from idle at part throttle up to 4000 rpm (with the stock mikunis).

When we talking about syncing the carbs....we mean getting all 4 carbs to open and close at the same time. If the sync is off....at part throttle.....you might have one carb working....then 2 more kick in....and then the last one kicks in. Thats the reason why a sync is VERY important on a Vmax.

Is that also the case for the Vgas FCR carbs? Because I believe these have an air screw on the front and a fuel screw towards the back of the bottom of the float bowls.

Is there a precise way to check sync besides looking at them? How can I adjust it on the vgas carbs?

Thanks for all the help, guys.

Update: When I roll onto throttle slowly between idle - 1.5k, it comes on fine until I can physically see a short burst of flames (backfire?) coming out of one of the velocity stacks (two in particular did it more than others)

When that happens, that's when it begins to fall on its face and if I don't let off the throttle it stalls.

Based on the sound, if I had to guess - this is also what is happening at higher rpms (4k and up) so is it lean or rich? How to fix? :bang head:

Thanks again! unk:

 

[Traumahawk]

On....the mikunis (Ive never messed with the Vgas).....a backfire thru the carbs......or even out the exhaust is because its too lean.

 

[Traumahawk]

Well the Vgas carbs are either 39 mm keihin or FCR's. You could call Sean....or maybe he will post some good settings to get all 4 carbs the same.

 

[D-Max2012]

Try this simple little test.. Take some duct tape and cover up one side of your airbox. Take note of how the bike responds during the test. Write it down and post results.

Then again, you might be running another type of setup that does not have a standard airbox.

 

[DigitalArts]

They are the 39mm Keihin, but I think that is a type of FCR downdraft carb (might be wrong here)

And I called up UFO (finally got a hold of them awhile ago) and he told me some suggestions, but it's still happening.

 

[DigitalArts]

Try this simple little test.. Take some duct tape and cover up one side of your airbox. Take note of how the bike responds during the test. Write it down and post results.

No air box, velocity stacks - 2 on left, 2 on right

 

[sdt354]

2 turns out on the air screw for the fcr carbs? I'll give that a try tonight and see what happens!

As for the spark plugs, will do. Any hint on which page I can find them on? My pc is really slow at loading the pdf for some reason

I'd hate to get in someone's way asking them to come help, but if anyone needs a reason to leave the house I'd certainly appreciate it!

How do I check the sync without the vacuum tool?

I have a new Apex battery on the way as of now!

Sorry, forgot you are running different carbs.

 

[DigitalArts]

Sorry, forgot you are running different carbs.

No problem man, thanks for the help anyway unk:

 

[D-Max2012]

No air box, velocity stacks - 2 on left, 2 on right

Just thinking out loud here..
Wouldn't the velocity stacks still lend itself to the same air / fuel mixture issues. weather that that be good or bad with the air/ fuel mix?

Reason for asking is this.
Ever since I picked up my used 91, I've been trying to knock out this annoying stumble around 4.5 - 5.0 K. As soon as I throttle past that point, she takes off like a beast.. I've changed everything that could leak air. Cleaned everything that could be cleaned multiple times. Synced up carbs so many times I could do it blindfolded. I tried that duct tape trick and noticed that the bike responds way better, not perfect, but a BIG difference in response around those RPM's. Waiting for the weather to break to try a different set of needles I got from Sean. Going by that simple test, it looks like mine may be too rich or lean, as the needles extract around that RPM range for the stock mufflers I have.

Just food for thought.. Hope you get that ironed out..

 

[jagco1]

I run the carbs 2 turns out on my bike. v-boost hook up when syc them.

Carb syc pics

You may have dirt in the carbs just a thought.

Just me doing this - if you have the air box on the bike each black hose motor running - help clean the carbs - i run wd-40 through each carb 1 at a time. Doing the winter install a little wd-40 in the carbs mix with the gas through black hose all 4. I have not had any carbs gumming up like i had in the pass. Not saying this is for you - this is me doing it on my vmax.

Sorry seen you not running the stock vmax carbs.

 

[DigitalArts]

I run the carbs 2 turns out on my bike. v-boost hook up when syc them.

Carb syc pics

You may have dirt in the carbs just a thought.

Just me doing this - if you have the air box on the bike each black hose motor running - help clean the carbs - i run wd-40 through each carb 1 at a time. Doing the winter install a little wd-40 in the carbs mix with the gas through black hose all 4. I have not had any carbs gumming up like i had in the pass. Not saying this is for you - this is me doing it on my vmax.

Thanks for the tip! I might try that.

Update after more troubleshooting:

- If I start her up for a minute and rev it between 1-3k, then shut her off, I can twist the throttle to all the way open and see white fumes coming out of all four velocity stacks

- Also, sometimes when I rev to 2-3k and twist the throttle back down to closed, the revs stay up around 1.5k. And no it's not the throttle linkage because I looked and both things were sitting on the idle screw :ummm:

Haha any help is appreciated!

 

[jagco1]

If the carbs like on my kz, you could have jetting problem.

But would say some dirt in the carbs,, if everything was find then you start having problems - these are 40s