Won’t run smoothly at idle and up to 1/4 throttle

[Krizer]

I have a 1998 vmax. It has 25k miles and has slip on exhauat and sat for years before I purchased it . First thing I did was remove the carbs and clean, removed jet block and cleaned all jets, ultra sonically cleaned all carbs as well. Installed a dyna jet stage one kit (160) mains. I then replaced all rubber between carbs and intake including all rubber on vboost just to narrow out any leaks as it had sat for years. Installed new plugs and put it back together, used sync gauges to sync the carbs all together. It starts up great and will idle but does not sound like it is idling on all cylinders . If I adjust the carb sync screw you can hear the cylinders pickup but at that point it’s out of sync . When leaving the carbs “synced” it tends to fall on its face between idle and 1l4 throttle. It also backfires a lot as well. If you open it wide open it picks up and screams but it’s really annoying when you’re running 45-60 mph and it is cutting out and Ali don’t like s***. I am not an expert mechanic by no means but have been working on motors my whole life and know my way around them. But this animal has me feeling like a noob

 

[02GF74]

If you suspect it's not running on all cylinders, measuring the temperature of the headers will confirm this and identify which cylinder it is.

Regarding cleaning the jets, you removed them from the jet block and ran wire through them so you know the hole is not blocked.

You put on a stage 1 kit; I would say get it back to standard, once you have it running spot on, that's when the tuning starts.

Did fit NGK plugs as per OEM spec?

Is the vboost functioning?

 

[Krizer]

If you suspect it's not running on all cylinders, measuring the temperature of the headers will confirm this and identify which cylinder it is.

Regarding cleaning the jets, you removed them from the jet block and ran wire through them so you know the hole is not blocked.

You put on a stage 1 kit; I would say get it back to standard, once you have it running spot on, that's when the tuning starts.

Did fit NGK plugs as per OEM spec?

Is the vboost functioning?

The vboost operates when the key is turned on, I replaced the plugs with the oem NgK plugs . I will go back to the 152.5 jets today when I get home. Yes I ran wire through every jet to ensure they were clear of krud.

 

[Fire-medic]

I don't think the DJ Stage 1 kit is 'the one to have.' Stage 7 is what the guys using Dynojet have the most success in tuning. You can also tune it to run with the VBoost, which in the DJ instructions they tell you to remove. I suggest if you go with the DJ Stage 7, you keep the VBoost.

Sean Morley also has a jet kit, 'Morley's Muscle,' contact him at [email protected]

Get rid of the Stage 1 jetting changes from stock, and try 150 or 147.5 Mikuni main jets, it should run better.


I suspect if you find one or more cyl's aren't hot as the others at idle, you still have an obstruction in the carb circuit for idle and just-off idle for those cyl's.

 

[Sniffbooger]

I agree with FireMedic, if it's not firing on one or two cylinders intermittently it's probably a carb issue... backfiring is likely a lean condition or blocked pilot jet issue...even the type of jet block gaskets you used can affect how it runs! (stay away from the Chinese rubber ones, they have less than precision holes cut in them and they expand when soaked in the ethanol gasoline for a day!) .....I've been where you are at, I purchased my 2000 Vmax non running after sitting for years, I replaced all the rubber boots, cleaned the carbs, rejetted them because upon closer inspection the prior owner had different size jets in all 4 carbs...and synched them...I've taken them on and off so many times I have the whole procedure down to less than ten mins from removing the airbox to splitting the carbs apart...I'm currently waiting for my new Factory Pro emulsion tubes to arrive, as my OEM ones are showing wear and seem to be contributing to a rich idle condition...and poor mpg...and one of my float needles keeps sticking so I'm going to replace that seat and needle...off come the carbs again! If this bike wasn't so much fun I'd sell it and ride something fuel injected!!! Grief....!

 

[dannymax]

The vboost operates when the key is turned on, I replaced the plugs with the oem NgK plugs . I will go back to the 152.5 jets today when I get home. Yes I ran wire through every jet to ensure they were clear of krud.

The wire helps are the holes sized correctly? PFJ especially

 

[Jack Hammer]

Stock jets in the carbs will help as I can see you have no mods. Other thing is replacing NGK with Champion plugs. Make sure your filters are clean and fuel pump is really doing the job.

 

[Sniffbooger]

In 40 years of messing with engines, cars, trucks, street bikes, dirt bikes, I have found that spark plugs make an extremely minimal difference when it comes to engine performance. Many plugs that are expensive like iridium, platinum, dual electrode and quad electrode can actually detract from engine performance. Just get those carbs fixed right, make sure you have good spark, and it'll runnnnn!

 

[Jack Hammer]

You have about the same amount of experience as me. I totally disagree with you. That's OK. Sounds like you are used to run engines not on the limit. I agree with you about iridium and such, just the basic heat range and brand makes a big difference. According to me. Try it yourself to know for sure if you really need to see how much power your basic parts like block and heads will hold.

 

[Sniffbooger]

Haha, cool, love to meet others who have been messing and tinkering with engines since they were 10 years old lol...

I have read that many people think the copper plugs are best for Vmax, but I don't want to steer this thread too far off course...

I just put some NGK non resistor plugs in my bike, and it runs exactly the same as it did with the 10 year old rusty corroded plugs that were in it... lol

But yes, I don't usually drive my Vmax in the Vboost range or above 6k rpms...so you probably have a valid point!

cheers,
Jacob

 

[Jack Hammer]

To each their own. I like certain plugs for certain reasons according to what I have seen and experienced along the years.

 

[Fire-medic]

KLG, Bosch, NGK, Champion, Pretty-sure those are the brands I've used over the years, on British, Spanish, German, USA, and Japanese two and four-wheeled machinery. I usually have followed the factory recommendations on heat range and size, as well as thread reach. That usually allows good consistent performance. In all my years, I've had one defective spark plug NIB. It really threw me for awhile, but a friend, a pro mechanic, steered me to correct my oversight.

 

[Jack Hammer]

When we are talking about stock or close to stock engines then recommended plugs tend to work no matter what the brand. When you add forced induction and change a lot then new things come to play. That's the scenery where small things start to matter a lot.

For example, a decade ago I built a 496 cubic inch big block Mopar for myself. Twin turbo with homemade fully programmable fuel injection. That thing hated certain plugs and stayed alive with others. Ten years ago things were different. Life goes on and you adapt.