For me and the bikes I have worked on - the stock CV carb works best with a #150 main and a #50 pilot.  Only 1 engine that I worked on wanted a #52.5 pilot.  If you regularly ride over 4,000' msl......you may need smaller jets.

A #155 main is likely going to be waaaaaay too rich and the engine will be sluggish and lose the crispness that proper jetting provides.

The bottom line here is that the carb needs to deliver the proper air/fuel mixture.  

Anytime the carb pulls more air, it will require more fuel.  This is the reason pilot jets are smaller than main jets.  There is less air being consumed off idle than at high speeds.  

We often say the pilot jet only affects the low end.   That's not completely accurate.  The fuel delivered by the pilot jet doesn't go away as we pull from the main.  The main jet is simply adding more fuel than the pilot can provide on its own.

So, in theory, a 50/150 pilot/main combo is the same as a 45/155 combo at WOT in the same carb.  If the ideal total of the jets is 200 at WOT, the performance will be the same at WOT because we are delivering the same air/fuel ratio. 

If switch to a larger bore carb, we still need to achieve the same air-to-fuel ratio.  Since the larger bore will bring more air to the table at WOT we would expect that we will need to increase the fuel delivered to achieve the same air/fuel ratio.  

ThumperPaul wrote on 11/23/23 at 05:54:09:


Differences in how well the different carbs atomize fuel at various throttle openings could account for that too. Carb A may work best with leaner part throttle mix and richer full throttle mix, while carb B may require the opposite.

Dave and Fast 650 bring up good points.  

My example is comparing jets in say a stock carb at WOT, just to make a point.  And, I oversimplified the larger bore example.  The larger bore isn't the only factor at play here.

I honestly don't know exactly how these carbs work in detail, but I don't think you can assume anything about the air/fuel ratio based on jet sizes across different carbs. I don't think it's that simple.

Sheet 2

I believe Dave already mentioned this, but I will reiterate.  To make any sort of comparison between the Murray Carb setup and Lancer’s carb setup, you need all the pertinent data.  They are both VM carbs, but there are 10 separate components/adjustments that must be considered.

Main Jet
Pilot Jet
Needle Jet
Needle
Needle Clip Setting
Main Air Bleed
Slide
Float Level
Air Screw Setting
Needle & Seat

So, you just can’t say throw a #165 main jet in there because the main air bleed has a huge effect on the main jet’s fuel delivery curve.  The #165 main jet might work fine on a carb with a #0.5 main air bleed, but if it has a #2.0 main air bleed the #165 main jet will be grossly lean.  If one of the carbs has a really small needle jet (for instance a N-5), it might run too lean no matter how large a main jet is installed.  Same sort of guidance pertains to the needle.

I advise you to look at every one of the ten items in the list above (not 3, not 6, all 10), and record the appropriate information (size designator, setting, etc.).  Then post that information and see if Lancer or someone else with a VM36 can verify if you have a good setup.

To drive the point home, let’s look at the VM38 and the PWK38 to see how the main air bleed affects the WOT mixture.  They both have 38mm venturis.  At WOT, the venturi vacuum should be about the same.  On my bike with a hotrod motor, the VM needed a 210 main jet.  That’s about .051”.  On the same engine, the PWK liked a #140 main jet.  That’s about .055”.  In the world of jets, .004” is a whole bunch.  Why did the 38mm PWK need such a large main jet?  It’s because the main air bleed on the PWK is .071” while the main air bleed on the VM is only .020”.

You need to know the size and/or setting of each component.  You need to know how one component affects the other (i.e main jet vs main air bleed, slide cutaway vs needle jet, air screw vs pilot jet, needle vs needle jet, etc.).

One last comment, right out of the box these carbs should be set up grossly rich.  The vendor doesn’t want you to melt your engine.

"EDIT in your comparison question: “Why did the VM38 (not PWK) need such a large jet?”"

Actually Paul, no edit is necessary.  That question is correct just as I posted it.  While the VM #210 jet does have a larger number designator, the orifice is actually smaller than the PWK #145.  The point I was trying to make is that you would think that the two 38mm carbs would probably require main jets with orifices about the same size.  But that quick & dirty assessment doesn't take into account the main air bleed.  The PWK has a much larger main air bleed that necessitates a larger main jet orifice.

I found some good info in the Mikuni Tuning Guide.  It may be of use to you.  A standard VM36 comes delivered from Sudco with the following components.

Main Jet:   310
Pilot Jet       35
Needle Jet   159 Q-5
Needle        6FJ6
Needle Clip Setting   Mid
Main Air Bleed   2.0
Slide  2.5
Float Level   17-19
Air Screw Setting   1.5
Needle & Seat   3.3

I suspect as long as your needle jet, needle, main air bleed, slide, float level, and needle & seat agree with the as delivered data, you should be able to install a main jet and pilot jet recommended by Lancer and it should run just fine.  You will have to adjust the needle clip position and idle air screw to suit.