Half throttle sputter/hiccup when vboost hits

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Aug 15, 2023
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Long Beach
Hey everybody about a month ago I picked up a 2000 VMAX that had some issues for a great deal!
So I went to look at it under the impression it needed a carb clean, it would bog and fall on it's face over half throttle (which seemed odd because mains take a long time to clog) was told it was all stock, had a brand new stock air filter the stock exhaust. I'm actually a motorcycle tech and have been working on bikes for close to 10 years now. A few years back I worked on a VMAX and was hooked after the test ride.
As soon as I rode it I could tell it was extremely rich over about half throttle so I took it home! I pulled the top of the airbox off and took it for a quick test ride with open stacks and it absolutely ripped. I figured I'll just put some pods on it and it should be pretty close, and thought that it may have been jetted for mods that had since been removed to sell the bike as stock.
The slides have been drilled and it had stock needles and what I believe to be stock slide springs. So I shimmed up the needles a bit with the pods because at that point just under half throttle (right where it was running great before) was lean and it seemed to be running quite well but I could tell wide open it was not as strong as with the pods vs a wide open airbox. I had since done a slip on with cheap parts... A midpipe from Amazon and some cheap straight through style mufflers, it looks and sounds great considering the amount of money I put into that. After doing some digging now I have taken the stacks out of the airbox and made them just over 3 inches long to the top of the carbs and the pods are on the end of the stacks and just barely fit under the faux cover. Low and behold my wide open hesitation at around 7500 seems to be gone and it pulls very hard all the way through the rev range, the front end comes right off the ground about a foot in first before coming back down but now at around 50% throttle on the transition into vboost right at 6000 to 6200 I get what is feeling like a cylinder or two dropping in and out. If I roll into the throttle a little bit more it clears right up. It does not appear to have the main air restrictors and I'm hesitant to change that considering how strong it feels wide open. I feel as though it needs different needles or possibly make the PAJ2 smaller. The plugs read very lean. Here's the kicker with vboost unplugged wide open or fully shut it seems to pull right through that range with no issue or surge at all. I have had an fzr600 and a katana 750 that I had pods on and could get those running great fairly easily, but the max is proving to be a finicky beast. Fuel levels are all in spec according to the fsm. I have NOT looked at what size the mains are or taken the bowls off because I don't have access to a sync tool I can currently use. I'm hesitant to lift the needles even more because I have 3 shims under them and have already trimmed down the spring holding them down inside the slide, but that is what it seems like it wants. If anybody has any input on what may solve this I'd be stoked to hear it 😀
Congratulations on tackling the carburetors.

You can probably get stock springs for the slides, and then just tie-up a few coils w/safety wire, and experiment w/what length seems to work best.

You can buy an inexpensive set of bourdon tube gauges for synching off amazon.
VMax carb synch tool.01.jpgVMax carb synch tool.02.jpg

I have both of these, and the blue one is probably 40 years old. Still works well.

It's possible that you need to do a thorough cleaning on the carbs. I prefer ultrasonic cleaning. You need to remove the jet block and the two jets behind the rubber plugs to ensure their passages are clean. The pilot jet, because of its small orifice, clogs easily, and while it isn't probably the one causing you issues, at low throttle openings, and idle, it does bollox things up if not clear/open.

Splitting the carbs into two left and right will allow float bowl, and then jet block removal. Check the float level while you're in there. Here are some pics to help you.

VMax carb float area.pngVMax carb floatlevel bowl off.jpgVMax carb kit.jpgVMax carb kit K-L labelled.pngVMax carbs disassembled diaphragm side.jpgVMax carburetor needles-Sean.jpg
Chart courtesy of Sean Morley

Do Not Buy the '4-in-1' jet sets online! The ones which come with all the brass, and gaskets, for all 4 carbs. They will not work for you! If you want OEM, be prepared to pay for them. I use K&L sets like shown. Use the search function to look for CaptainKyle's recommendation of where to order the K&L online. He and I both use that vendor. It's peakmoto for the name of the vendor.

Yamaha VMX12 VMAX V-Max 85-07 Basic Carburetor Repair Kit K&L18-2879

Sean Morley [email protected] and dannymax [email protected] sell parts and rebuild carbs. CaptainKyle [email protected] does repairs of all sorts also.

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That's a lot of great info! Yeah I was thinking of getting one of the cheap sets of vacuum guages but was a little hesitant because I was unsure of the quality. The bike seems to idle near perfect with the mixture screws set at 3 turns out and doing a fuel level test with the engine idling all are very even and right about at the lower screws on the vacuum chambers. The fsm shows 15-17mm below the line on the vacuum chamber and it seems to be very close. No issues up to just over 1/4 throttle at any rpm it's just that 1/2 throttle range at 6000 and just over. Mainly under more load such as 5th gear ripping down the freeway, in the low gears it's barely noticable. The sizing and taper on all those needles look VERY different and I think sourcing a different set might be my best bet. The springs I had shortened are the ones inside the slide under the plastic screw that just hold the needle
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Just an observation from my c. 1yr on this group. I may be wrong.
The people who seen to have most problems with this sort of thing are ones who have gone 'off piste' from OEM in regard to exhausts, airbox and carb needles/jets or have bought bikes where the previous owner has or with preexisting problems. My impression is you really really have to know what your doing (with the Vmax) if you want to start messin' away from OEM.
My trike was no exception. Carbs were full of crap & had been 'adjusted' so the engine would just run regardless of performance. Exhaust was cobbled together from an 850 twin. I had to strip down the carbs, thoroughly clean, replace gaskets, carb to engine rubbers (+ complete 2nd hand airbox for those rubbers), reset float levels & pilot jet settings & synched with one of the cheapy dial gauge ones (which worked fine & made huge dif. Tubing was sht though & kept ripping).
Replaced exhaust with OEM headers & back box - also huge improvement.
My only probs now are with rust in the fuel from the tank, which I'm working on. Otherwise goes like stink.
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rust in the fuel from the tank
What are you doing for that? Plenty of posts on it. I use 6% 'cleaning' vinegar, after removing the fuel reserve electrical sensor. If you do not remove it, the vinegar will destroy it as it's potmetal. Vinegar can be stored for re-use. It's also easy to dispose of. You can get up to 30% cleaning vinegar at Big Box stores like Home Depot. I would not use that unless it was diluted! Electrolysis is another method, but I wouldn't do that in a garage unmonitored, or overnight in a garage. Evaporust is expensive, but works. I've heard of molasses being used, but never have tried it. Sounds like a bug-attracting smorgasbord, and I read that it's very pungent/odoriferous.
What are you doing for that? Plenty of posts on it.
I'll be flushing out the rust to start with time permitting, then 2nd fuel filter, then monitoring it. I don't have time to remove it for thorough de-rusting & coating atm esp as I want to fit a much bigger tank at some point.
I am aware there are plenty of posts on this. Just mentioned it as something to check on.
So I found a cheap set of venture needles on eBay and figured I'd order them to give it a shot and play with the height of those a bit. The way it is now it seems like it snaps out of the issue area when rolling on the throttle In a very abrupt way. I figure maybe the sharp taper on the vmax needles are just coming out of the emulsion tube far enough at that point that it suddenly gets enough fuel. My other thought is maybe my stock slide springs combined with the drilled slides are actually causing the slides to slightly fall when it loses the intake velocity when vboost cracks open. I think this might be the case because how it's almost non existent with vboost unplugged fully open or closed. More tinkering to come I'll update when I try out the other needles.
Just wanted to update that the venture needles solved my problem 100%! The plastic spacers that came with them are actually slightly thinner than the original VMAX ones and it was still slightly too rich at low rpm and throttle so I took them out and used some washers to space it to what would be about a clip position lowering the needle one spot if it had adjustable positions and it's running perfect! 😎IMG_20230827_181852161.jpg