Should I change my main Jet?

[Fire-medic]

Okay you have proven to yourself that the main jet that you have without knowing whether or not it was altered is significantly leaner than the stock jetting. I think the best thing to do is to install the standard 152 and a half, or a 150 or a 147 and a half. I suspect that if you change your stage one needle for an original needle that the bike is going to behave better. If the pilot air Jets have been changed then you probably will need to swap them out as well. I bet that after you make the switch to 152 and a half, 150, or 147 and a half, that your air screws will be able to be screwed out less then the three and a half to four that you said you have now.

 

[dannymax]

I believe DJ directions call for drilling the slides...not the end of the world if they are as long as the diaphragms aren't compromised.
Have you sync'd the carbs?
Also it'd be interesting to see what clip the Stg 1 needle is on.

 

[Fire-medic]

An old technique for modifying the slides' function, was to use some safety wire to tie up a few coils of the slide spring to lessen the return 'push.' That coupled with drilling the slides' hole larger to allow more air movement and less restriction of its range of motion changed the throttle response characteristics.

 

[Cyrotronix]

I have synced the carbs. And the clip position is the second slot from the top.

Also, the diaphragms are in good condition.

I'll have a set of #150 Main Jets and a set of #170 PAJ2s tomorrow. I'll just have to get my hands on some OEM needles and I can give it a try.

 

[Fire-medic]

I suspect the Y will also be needed for proper airflow management. Maybe some duct tape to mimic the sq. inches opening of the Y?

 

[one2dmax]

I've got plenty of cheap y's here if needed.

 

[Fire-medic]

Returning to stock jetting, I think you will have the best results with a stock Y in-place. Sean has them.

 

[Cyrotronix]

I've got everything ordered up. Once I get it all I'll put it together and post the results.

 

[Cyrotronix]

I'm still waiting on the needles, but I got everything else. I put the Y-Duct on and I'm guessing I'll probably need the other piece of the cover too, as the PO cut parts of it out.

 

[Fire-medic]

I'll probably need the other piece of the cover too

If you get the needles first, then just use some duct tape for the cut-out area, temporarily. See how that thing runs.

Do you have parts # 53 & 54? the brackets for the hoses off the individual carbs to allow the float bowl to be pressurized properly as speeds climb? From the (part #1 air vent pipe, section view, carburetor) to part # 53 or 54, which attach to the airbox lid screws at the front-side screws of the airbox (one pair on each side).

 

[Cyrotronix]

Right on. Needles will be here tomorrow. I'll put them in Sunday and see how it goes.

 

[Cyrotronix]

Do you have parts # 53 & 54? the brackets for the hoses off the individual carbs to allow the float bowl to be pressurized properly as speeds climb? From the (part #1 air vent pipe, section view, carburetor) to part # 53 or 54, which attach to the airbox lid screws at the front-side screws of the airbox (one pair on each side).

Yes, those are present.

 

[Cyrotronix]

This morning I put the carbs all back together. I installed the OEM needles (Thanks to dannymax for getting those to me so quickly), a Mikuni #150 Main Jet and a Mikuni #170 PAJ2. l also put the Y-duct on (Thanks to Sean for getting me that) and used duct tape for the openings that remained. I adjusted the idle screws to 2 1/4 turn out and synced the carbs.

These are the results.

Idles like a champ. But it always idled good, so I'm not really sure if I can say it idles any better? But it doesn't idle worse.
Runs great, cruises great. The one issue I've comes across is when it go WOT from a cruising speed, it dogs down between 5-6.5k RPM. This happens only at WOT and only from a cruising speed. From a dead stop there is no hesitation when I'm jamming through the gears. If I use between 50-80% throttle she picks up speed with no issues at all.

So, on whim I took the Y-duct back off, and left it open as it had been. It still dogs down, but not nearly as much. And the bike flies. Pretty sure it's never been that fast since I've owned it, because wow.

And that's what I've found.

 

[99YAMAHAVMAX]

This morning I put the carbs all back together. I installed the OEM needles (Thanks to dannymax for getting those to me so quickly), a Mikuni #150 Main Jet and a Mikuni #170 PAJ2. l also put the Y-duct on (Thanks to Sean for getting me that) and used duct tape for the openings that remained. I adjusted the idle screws to 2 1/4 turn out and synced the carbs.

These are the results.

Idles like a champ. But it always idled good, so I'm not really sure if I can say it idles any better? But it doesn't idle worse.
Runs great, cruises great. The one issue I've comes across is when it go WOT from a cruising speed, it dogs down between 5-6.5k RPM. This happens only at WOT and only from a cruising speed. From a dead stop there is no hesitation when I'm jamming through the gears. If I use between 50-80% throttle she picks up speed with no issues at all.

So, on whim I took the Y-duct back off, and left it open as it had been. It still dogs down, but not nearly as much. And the bike flies. Pretty sure it's never been that fast since I've owned it, because wow.

And that's what I've found.

If you put the Y back on the openings on the sides where the hoses mount are to be left opened and not taped over.

 

[Cyrotronix]

If you put the Y back on the openings on the sides where the hoses mount are to be left opened and not taped over.

Oh yeah, I was aware of that, and they were left open. What I taped over was the part of the lid that the previous owner had cut out, as can be seen in a picture I posted with a couple of posts back.

 

[99YAMAHAVMAX]

I have a 2001 Vmax, that I am still learning about as I go. I bought it last year and have been exploring the things that have been done to it. Recently, I took the carbs apart for cleaning, and curiosity had me inspect what jets were in it. I found that my Main Jet is a 134, and the PAJ2 is a 190 on all four carbs, while everything else matches stock sizes. The bike has stock headers (I believe) with 4-2-2 Kerker slip-ons. The duct has been eliminated from the airbox, and it has a K&N air filter. For the most part the bike seems to run well, at least in my opinion. When I warm it up with the choke on I do get a pop from the right side exhaust until I take the choke off. And my spark plugs show signs of running lean. So this leaves me wondering if I should adjust the jets, and if so, how far? Any input here is appreciated.

Edit: Also, the pilot screws are set around 3.5-4 turns out. Which to my understanding indicates the jets need to go bigger.

Keihin jets will have a stamped K with a 5 point star both leaning to the right-Mikuni will have a box with a smaller box in the inside corner

 

[Cyrotronix]

The ones that were in it had no markings that match either Keihin or Mikuni. I figure they ware either Dynojets or no name randoms.

 

[MaxMidnight]

This may help identify the jets...or not.

 

[Cyrotronix]

Thanks! That's pretty informative. Looking at those pictures they closely resemble the Dynojet talls. Which, I think would make sense considering it had a Dynojet Stage 1 in it.

 

[Fire-medic]

An observation, bogging is now 5K-6.5K rpm, from cruise, no bogging from idle-redline, running thru the gears.

The VBoost starts opening about 5.7K rpm so close to where your issue is. You might look at chap. 5, p.13-15 of the service manual for info on adjusting the VBoost to ensure you're properly set-up there.
https://dn790009.ca.archive.org/0/i...ice-manual/yamahavmaxvmx12-service-manual.pdf