Should I change my main Jet?

[Cyrotronix]

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.

 

[Fire-medic]

Boy that main jet sizing is beyond me! I am not familiar with any Mikuni sizing for a VMax like that.

 

[MaxMidnight]

If you like messing about and dismantling and re-assembling your bike then go ahead, but...
You may notice a difference which would beg the question should I go one higher...or it may feel the same ...or it may feel worse.
Either way you will be tempted to tear things down again. As I have said, if that is your 'thing' then fair enough.

IMO a far better route would put the bike on a dyno and get factual data to base any changes on.
That would:

• Save you time and possibly money (if you keep having to buy jets that aren't suitable).
• Get the carburation near to optimum with the minimum of fuss.
• Give you more time to ride.

 

[Cyrotronix]

Curiosity often leads me down a path of over thinking. But considering the bike has been running good has me leaning towards leaving well enough alone rather than chasing ghosts down a rabbit hole.

What spurned me to ask about it, was how small the main jets I have seem to be, mixed with the condition of my spark plugs. From the other forums I've read, I hadn't seen anyone running that small of a main jet. And with such limited knowledge of what the previous owner had been messing with on it, I thought it best to see what others have to say. Jetting carbs is something I haven't messed with before and while I understand the concept to it, I'd prefer to take the simple route and not become a professional at taking the carburetors off.

Getting the bike onto a dyno to actually see what it's doing sounds like a good route to go.

 

[MaxMidnight]

IMO if it seems to be running OK then it probably is unless you have indications to the contrary.

Assuming the dyno run doesn't show up any issues then will have a baseline to work with should you decide to change anything in the future.

 

[Fire-medic]

OTOH, swapping that odd-sized jet out for stock jetting seems like a good use of your time and is much-less $ than going to the dyno. See how it operates with a return to OEM, and if you want to dyno that I believe that will be more-fruitful.

The USA jetting is 152.5, and many of us have gone a bit leaner, to 150 or (my current ride) 147.5. It has a Dynojet Stage 7, A K&N filter in the stock airbox.

Another suggestion, take a close look at the slide needle, and match it up to one of these, What do you have? The second one is courtesy of Sean Morley.




I for one would be interested to see what the dimension of the '135' main jet hole is. If you have a good set of ddrill bits, try seeing what the two drill bits are for a 'go/no-go' measurement.

 

[maleko89]

Mikuni didn't have a size 134. Either not a Mikuni jet or someone spent time drilling/modifying own jets? Mikuni goes in increments of 2.5 and so from 130 it goes to 132.5 to 135, etc.

Mark

 

[Tom Determan]

What's your elevation and where did the bike come from ?

And the ambient temp range for you and the previous owner.

I'm not familiar with the jetting chart but hot thin air will bring that MJ size down.

 

[Fire-medic]

What's your elevation and where did the bike come from ?

His listed location is MI, so getting to the Sault Ste. Marie bridge peak elevation may be all he needs to account for, that or a visit to Sleeping Bear Dunes. Hey, maybe it came from Denver.

That 135 main jet sounds like someone's proprietary sizing, or maybe some cheap bastard drilled-out a smaller jet because he didn't want to spend a few bucks for an OEM set of jets.

Let's see if he can get some info for us on which slide needles, and the main jet's actual inner diameter.

 

[Cyrotronix]

In looking at my slide needle, it would appear to be the Dynojet Stage 1.

As for checking the size of the main jet I have, I'll have to take it to work tomorrow and try various drill bits.

And I do live in Michigan. And I bought the bike from someone in the State, but where it has been before that I couldn't say.

 

[Fire-medic]

Sh__can the DJ Stage 1. Put it back to stock/OEM slide needles and OEM 152.5 main jet, or 150 or 147.5 main jet as an alternative. The manufacturers set bikes up a bit rich, which helps to prevent engine damage should the bike develop an intake air leak and run lean. In the days of two-strokes, leaky crank seals or somewhere in the intake side would cause a seizure when the bike 'went-lean.' A quick hand on the clutch lever was needed to possibly avoid a slide on the asphalt if the engine locked-up. Kevin Cameron has a story about sliding along the pavement after an engine seized, and trying to switch position to keep a layer of leather between him and the asphalt, before he lost momentum, and came to a stop.

Let us know what the '135' mains actually are measured to be.

The Stage 1 is hard to make work as-good as the stock carburetor setup. I suspect if you go back to a stock setup, you're gonna enjoy the bike's performance much-more. You have a K&N filter in place of the stock air filter, and no lid on the airbox (#45 in the diagram), or you have the lid but not the 'Y'? #51 in the diagram, called the 'duct.'

https://www.ronayers.com/oemparts/a/yam/50045c0ef8700209bc7942f2/intake

 

[Cyrotronix]

I have the lid, but not the Y.

 

[Fire-medic]

If you search about modifying the airbox on here, the removal of the Y causes more problems than it solves. Shimming it up a bit with specific thickness flat washers is what many riders do. Removing the Y altogether causes issues in proper airflow to the engine.

The Dynojet Stage 1 isn't easy to get right, and the Stage 7 is only effective if you run a complete exhaust, w/larger tubing in the headers, and do the jetting to eliminate flat spots.

https://www.dynojet.com/amfile/file/download/file/1170/product/3785/ Stage 7

https://www.dynojet.com/amfile/file/download/file/1032/product/3713/ Stage 1

 

[Cyrotronix]

I've been wanting to get the Y duct anyways. It bugs me not having a "complete" bike, even if I don't intend to use them I would prefer to have all the parts to it.

Looking at those Dynojet kits, I notice the instructions indicate to do a bit of drilling. Are those modifications anything I'm going to have to worry about when changing the jets and needles back to stock?

 

[MaxMidnight]

Looking at those Dynojet kits, I notice the instructions indicate to do a bit of drilling. Are those modifications anything I'm going to have to worry about when changing the jets and needles back to stock?

Perhaps I'm missing something here but you say the bike runs well as it is. Before doing anything it would be prudent to put it up against a stock Max to see how it performs.
If it is as good or better then taking it back to OE isn't going to bring any benefit and will cost time and money.
If it is significantly worse then there may be some benefit in the exercise.

IMO just throwing needles and jets at it without knowing what or if it will make the bike run better is not a good investment.

As you have noted Dynojet instruct to enlarge the slide lift hole with a #19 drill so it would be worthwhile checking to see if that has been done. I'm sure someone can advise the OE size.
Should you want to go back to OE the plug the hole with an epoxy glue and re-drill to the correct size.
Also note the DJ slide springs are different.

 

[Cyrotronix]

Right. I want to learn and understand what has been done to my bike. I don't intend to go running rampant buying all sorts of different jets for trial and error. But I also don't think there is much harm in having the stock stuff on hand. Especially if I get the bug to tinker with it.

I guess my main concern here is that the spark plugs look like I have a lean condition. Maybe I'm focusing too hard on that?


I checked the 134 main jet with drill bits. A #55 (.0520) goes through. A #54 (.0550) does not. A chart I found says the Mikuni 152.5 has a diameter of .070, does that sound right?

 

[dannymax]

Those may very well be Kehin #134 mains drilled out...although individuals who drill out jets tend to file off the original size numbering to eliminate confusion such as we are now having.
I'd go with Mr Midnight's suggestion...compare with another Max & check your mileage.
BTW, what size are the pfj's?

 

[Cyrotronix]

For fuel milage I get what seems to be pretty average, 30-33mpg depending on how heavy I am on the throttle.

PAJ1: 90
PAJ2: 190
PJ: 37.5

 

[dannymax]

Keihin does not have a #134 so that doesn't work. Dynojet does although a #134 would be very small. Would PAJ2 bumped from #170 to #190 compensate? Don't know.

Plug looks ok, mileage about average...maybe a little low.

Stg 1 Dynojet set-up, sounds like. Be interesting to see what a #170 PAJ2, #150 Main and a stock needle would do

 

[Cyrotronix]

I was looking at a conversion chart of different jet manufactures, and the drill bit test falls just a little short of what the chart lists as the ID of the Dynojet 134.

https://www.jetsrus.com/FAQs/FAQ_mikuni_vs_dynojet_vs_keihin_sizes.htm
I'm going to check the lift hole in the slide when I get home, I have a #19 drill bit so I'll be able to see if it's been drilled out.

I'm curious myself at what that setup would do. If nothing else it would be a learning experience.