engine not revving and sputtering at 6000rpm and above

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Well-Known Member
Dec 25, 2017
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Hy guys,
first of all I would like to apologize for the bother, but recently I had a series of annoying issues with my Max that are really going to make me desire to sell it.... although I really love this bike from when I was 13 Y. Old

A friend of mine tested my bike ( 1996 EU VMAX 1 gen with the vboost and every limitation removed ) . He told me that even if it has a great low RPM torque it is not as "powerful" as everyone say. In his opinion a standard Honda hornet 600 have the same "pull".
Since then I start to think about what is wrong with my bike, all in all it should have at least 145HP! and to tell you the truth I never felt "my arms to be ripped off for the acceleration".

What I've done is that from that moment I tried to gain some more HP with some mod ( COP, better oil, better sparks ,ecc,ecc ) but still very little ( or nothing ) changes.

since the issue regards the motor I will not list the mods that I've made that are not related, such like suspensions , clutch or brakes.

Standard EU motor ( 1198CC )
DynoJet stage 1 kit ( installed from previous owner )
--dynojet stage 1 needles
--lighter springs
--enlarged holes on carburetor slides
--mikuni 152.5 size main jet
Modified air box ( top cover removed and a single large flat filter installed and sealed )
Iridium spark plugs
rapid gas throttle from DOMINO
Dyna 3000 digital ECU ( no change in configuration were made )
Complete V-boost system with new servo cables , vboost unit ECU, and flasher relay mod to be fully operational.
custom modified carbon fiber muffler ( no mainfold tubes ) installed from previous owner with no dbkiller and larger internal tube .

Some months ago I found a bargain for a flat K&N filter ( paid 30$ new ) so I decided to modify my airbox to be something like the Stage 1 mod from Mr. Morley.
From that moment on the bike starts to run poorly, but I was expecting it, because I know that should also take care of a lot of other parts such like exhaust, jets , needles and so on .
The issue is clearly a lack of power from midrange to top range.

with the new air filter the bike stops revving at 4000RPM , so I move the carb needles 1 groove inward from the original position
result: slight better mid-range , but when I hit 5000RPM the bige goes from "VROOOOM" to "WWWOOOOO" and stop pulling.
Take off the carburetors and clean them in every orifice without taking them apart, then swap the original mikuni jet 152,5 with dynojet 170 jets.
mount carbs and filter back and test.
result: slight better, but up to 6000RPM then the bike stops pulling and starts to sputtering.
At that RPM I expected the vboost to come into play, but I feel nothing.... just a sputtering bike .
It doesn't want to go over that.

The mechanic in my city told me that the bike is running lean and suggest me to install larger jets, but I found this very strange since I already have a 170 ! What jets should I put inside the bike ? a 180????? Is this possible?

Is there something I can do without taking out the carbs from the bike ?
I still have original needles , springs and jets , but please remember that European needles are slightly different from US needles. My original needles have 5 grooves and are pretty similar to the dynojet needles, just more stained from age.
First Dynojet and Mikuni use different numbers for the same size orifice. Use our search function for a comparison chart. Mikuni 152.5 is the standard OEM for USA VBoost bikes.

Second, Stage 1 Dynojet needles are probably contributing to the problem. 'Stage 7 or OEM,' my advice. The Stage 1 kit doesn't work well, and not as well as an OEM setup for throttle smoothness. I use DJ Stage 7 and keep my VBoost, and do not remove the butterflies like the Stage 7 instructions say to do, where they provide replacement tubing to replace the butterflies. Keeping the VBoost als greatly simplifies synchronizing the carbs.

VMax carburetor needles.jpg

Sean Morley's chart:


If you want 'open-tubes in the VBoost,' simply unplug the round electrical connector under the L side scoop when you turn on the ignition key, and the VBoost cycles open; unplug it before the VBoost butterflies close, and you get that SBC idle that drives the Harley guys crazy with jealousy. "I wish my V-Twin made an idle sound like that!"

Third, do you still have the OEM airbox lid? Try reinstalling that.

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First off if you still have the stock front header pipes you are wasting time with the flat filter and you did not mention putting air correctors in which are needed with the modified air box but again this mod should not be done with stock header pipes. The best thing you can do is put the stock needles and air box back in and go back to the stock jetting or one step below. These bikes are rich from the factory already .
IMO you can spend a lot of time, effort an potentially money trying to fix what would appear to be a fueling issue. There is no guarantee that you will sort it as to a certain extent you will be guessing where the problem lies and what the air - fuel ratios are.
We can all make educated guesses as to what you need to do but whitout the bike in front of us the advice may well be wide of the mark.
You will save yourself a lot of time (and probably money) by getting a dyno baseline so you know exactly what the a.f.r. is throughout the rev range. You may well also get advice as to whatchanges you need to do to set you in the right direction.

To save
first of all, thank you very much for your suggestions. I really appreciate them.
So , to summarize, tomorrow morning I will clean the old original needles and install them.
I'm thinking about posting a pic to show you the differences between those 2 needles types.
At my eyes they seems pretty similar in the shape and in the pointing.
Also , my original needles also have grooves, so I will install them at about the same position, since when I change the groove on the dynojet needles I had some little benefits in the mid-range.

Regarding the main jet I will for sure search for the topic on the forum and see the comparison between them.before taking out again the carbs I would like to have a clean idea of what I should put inside.

Unfortunately the top cover of the original air filter is not usable anymore, but I will look for a good deal on ebay to see if I can get one to eventually use for a roll back of the modified air filter.
Luckily I still have the original K&N air filter to use just in case.

A visit to a mechanic with the dyno is something I've been thinking about for a long time , but near my place there aren't any available.
The nearest is a150Km trip from my place, not to mention the price of this test in Italy..... but you are right, this seems to be the best way to know what direction to take for optimal results.

A full exhaust system is something that in Italy is very difficult ( if not impossible ) to do . We have a strict regulation about exhaust. If swapped with another exhaust system the new exhaust must be approved from the law and must be for this specific type of motorcycle ( brand , model and frame numbers ) .
So, to have a 4 in 1 or a full open header for an old bike like the vmax it is not an easy task.
The same is true for the tire sizes...but that's another story.
I will post my results as soon as I can test the bike with the original needles.
thank you again to all of you!
This is what happens when you try to DIY a jet kit without paying the cost to get the real deal. Tape off about 90-95% of the filter you just installed and that should restore at least somewhat decent running. Then find the OEM needles and junk the S1's. At some point you can buy the air correctors and or rest of the kit so you can get it installed properly.
The big thing you need to also consider at some point is a complete aftermarket header. Not just slip ons or stock exhaust. Brand of the full header doesn't matter. They all make similar power.
thank you Sean, I'm going to swap the needles right now.
Here they are, polished and ready to be installed.
As you can see they are pretty similar to stage 1 needles, so you will forgive my ignorance if I ask you what are the difference between my original needles and the stage 1?
I've always believed that the the hate for the dynojet needles was related to the difference between the US needles and the Stage 1 , and not between the EU needles and the stage 1.

Regarding the mod I've choose this road because I already have the "air breather" tube and filter and , most important, because I've found a good deal for the filter ( 30$ new ) so I decided to give it a try for this reason.
To be honest I have to tell you that with the new air filter I've felt a good improvement in the low range of RPM form the beginning, the troubles are from mid-range to high- range RPM.
After swapping the main jets and changing the groove on the needles the bike goes fine up to 6000/6500 RPM, and that's the reason why I'm asking for help.
I had a call with a friend and I hope he can send me the original air filter top cover to restore the air filter if I will not sort out the issue and resolve it.
Regarding the full header exhaust I would like to know if I can use my muffler ( or the original muffler ) or if it is needed to change also the mufflers. As I wrote there are some serious difficoulties to do this , and an aftermarket exhaust not compliant with italian laws could ( and will ) invalidate the bike insurance and be the reason for a motorcycle seizure.
there is a little symbol of a square with another little square inside of it in the low left corner, and next to it the writing : 5DZ19
VMax carburetor USA and CALIF..png
For purpose of comparison. Might your 'D' be an 'E'?

If it's impossible to find an exhaust system that is compliant with Italian laws, then the answer is simple: revert to OEM, and be happy with a time-proven setup which guarantees you no 'hassles from The Man.'

There are other ways to improve your ride, like going to radial tires, which requires at least a new rear wheel. The difference in stability especially at higher speeds is very noticeable, especially as the tires wear.
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Watch this on exhausts (from 4:00 min). Exhaust are tuned to give the negative pressure (part vacuum) at certain rpm (usually high) to draw out exhaust at overlap (this known as scavaging). As explained in the vid this means it can be out of phase at lower rpm (usually mid range). In V-max case the collector is designed to mitigate this along the lines he states. Header length & bore dirrectly affect how this achieved (along with collector config and muffler design).

Watch this on resonance airbox intakes (as we have on V-max). Gen 2 use variable length intake (velocity) tubes to alter the resonance frequency/phase.

The engine has the most money spent on it's development by manufacturers. The next most money is spent on intake & exhaust development to give best performance esp in view of various countries emissions regulations. If you start messing with this without thoroughly knowing what you're doing the chances are you'll end up losing power & performance not gaining any.
Up to you where you go with this. In your situation with all the legal restrictions I recommend going back to stock as much as possible & see what that feels like.
Kevin Cameron, a great resource for learning about engines, suspension, tires, electrical systems, and almost anything you can think of having to do with our motorcycling interests, and more. His blending of historical knowledge with engineering advancements makes for a great way to learn about our sport, and many other engineering topics.

Another great writer about motorsports, and motorcycling in particular, is Matt Oxley. One of his books, below.

How are WW II and the Third Reich's quest to develop war machines like the V1 'cruise missile' buzz-bomb, and the first ballistic missile, the German V-2, which served as the basis for the American post-war rocket research as directed by ex-Nazi Werner Von Braun, related to one of the most significant advancements in motorcycle exhausts?

A great read about how the expansion chamber exhaust for two-strokes resulted in industrial espionage that changed motorcycling 'bigly.' I have a copy of this in my library. If you've never tried Amazon Kindle e-books, this is a good way to try it.
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For purpose of comparison. Might your 'D' be an 'E'?
I've found an old topic on this forum regardind the needles, and can confirm that I have the adjustable stock needles. here's the link to the post

Stok needles are on the bike right now, but it's raining a lot here in Italy, so I have to wait for a test.
I ask for the air correctors to Sean ( I really didn't know about this ) .

Right now I'm searching for a used exhaust that could help me to solve the problem, but for the reason stated in my previous post I have to stick with 4 in 2 system and my actual mufflers wich are omologated for the vmax.

If I have to swap even the mufflers with something not compliant with the law I'm afraid that I have to roll back everything.

Fortunately the bike is running, so for the sake of knowledge I will call a dyno center to book an appointment and see what's happening inside the bike. All in all I believe that with some tuning on the ECU I can at least have some benefits.

There are other ways to improve your ride, like going to radial tires, which requires at least a new rear wheel. The difference in stability especially at higher speeds is very noticeable, especially as the tires wear.

belive me , this is the FIRST mod I've ever think about since I purchase the vmax.....unfortunately In Italy the only way to do this is to take the bike in Germany, sell the bike to a dealer, ask him to omologate the bike in Germany with the new wider tires , purchase the bike back and make new Italian papers for this bike.
The price for this mod ( excluded the new rims , new tires and the work ) is far beyond the price of the bike itself.
Most of the people simply put the wider tires and didn't care about the laws, but I prefer not to be in any trouble .A routine check from the police will lead to the immediate seizure of the bike. To have it back you have to put the original exhaust system back , pay the ticket and take the bike to a specialized center where the police check that the original exhaust is correctly installed. Also, If something happens it could be very annoying hear the insurance say that they will not pay for any damage to people or items.
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We don't have a 'tuning on the ECU'-capable bike for the Generation 1 ignition, other than to replace the entire ECU with either a DYNA or an Ignitech replacement ignition. Then you have the whole homologation issue that Italy apparently has.

Sounds to me that your first attempt to rectify things should be to return it to OEM specs for the intake, and then see how it runs.
Put it back to stock, shim the airbox "y" center section, set the needle adjustment (likely to the 3rd clip from the blunt end), and ride it. Until you get a FULL header you're going to be going backwards in performance. If you do not have the Vboost system you would also want to get that installed.
thank you all. I've learned the lesson in the hard way....
I'm going to do the "roll-back" and let you know.
the good news is that I only lost 30$ for the flat air filter ( other parts were already in my spare part box ).
The bad news is that I have to take down the carbs again.

my last question is about the full header exhaust:
I've found some good used open header systems, but to avoid any issue I must replace the mufflers with the original parts or at least an omologated mufflers for the vmax 1200

Is it possible to do this?
I can modify the internal parts of the mufflers, replacing the glass wool or the internal pipe, but the outside of the muffler has to be original.
Given the punitive penalties you could face (confiscation/possible loss of the motorcycle, and a monetary fine ($)) I think the way to mental health and 'ease of mind' is to go back to stock/OEM for the exhaust.

I have no experience with the Marving though it does look like the OEM exhaust, superficially, though the OEM has a much-larger resonator box than the cross-tube of the Marving. In prior posts about people looking for a replacement double-megaphone exhaust, I have mentioned the Marving system. Be aware that this system will not increase your performance! To accomplish that, you need larger headers and canisters of whatever type you choose, along with tuning of the carburetors to take advantage of the lower restriction and greater gas flow over a stock intake/exhaust.

If I was facing the penalties you say are going to happen to you if the Italian police inspect your bike, and find it non-compliant, it would be a no-brainer: put the bike to OEM specifications and stop worrying. If you can find an acceptable legal approved in Italy exhaust, a complete exhaust from header pipes to megaphone/canister end(s) and then you are able to tune the intake to take advantage of it, well, there you go. Have at it.

As it sounds from your prior messaging, returning to OEM for now will mean you don't need to worry about the loss of your motorcycle for non-compliance with Italian equipment laws.
that's right Fire-Medic, sometimes in Italy the police check also for the license plate canting.....and belive it or not, this is a good reason for the seizure of the bike.... license plate MUST be 30° , at a specific height and in the middle of the bike ( no side plate ). Sometimes I really really envy your freedom of choice for the mods.
Since the police can only check the mufflers ( they have no means to check the internal size of the exhaust ) the only "trick" I can think of is an open header (4 in 2) with some mufflers like the Marving or another omologated mufflers ( like the one I already have installed )

Coming back to the issue I would like to have your opinion about the performance of the bike with the mods I made.
As I wrote the bike goes bad after 6000rpm, but until this rpm range it seems to me that it goes better than stock and have a faster acceleration . the issue occurs only after 6000/6500 RPM. Does this make sense to you?
I'm asking this because I would like to have this kind of acceleration even swapping parts back to original.

Maybe I can leave the stage 1 shorter/lighter sliding tubes spring on the bike?
Coming back to the issue I would like to have your opinion about the performance of the bike with the mods I made.
As I wrote the bike goes bad after 6000rpm, but until this rpm range it seems to me that it goes better than stock and have a faster acceleration . the issue occurs only after 6000/6500 RPM. Does this make sense to you?
I'm asking this because I would like to have this kind of acceleration even swapping parts back to original.

Maybe I can leave the stage 1 shorter/lighter sliding tubes spring on the bike?
I think the only way you could confirm the above is to go against a stock bike with the riders swapping bikes after several runs to eliminate rider ability.
Any changes you make will affect how the bike performs so it isn't possible to predict how it will affect acceleration.
As you have found some modifications have a positive result and others are detrimental.

IMO taking the bike back to standard first is the best starting point; at least you will be dealing with a known baseline.
You can then start to introduce changes one at a time and evaluate what difference it has made. I stress only doing one change at a time; do more than one and you won't know which has given the benefit or detriment to the performance.

To answer your last point, I wouldn't.
I've no experience with a S1 set-up but don't recall ever reading positive comments.