Steve,

Just a quick what if.

I currently have a Mikuni 40 pilot and pms at 3.5 turns out. Would I be better off to go up 1 size and not have the pms set so far out?

Just thinking out loud while reading the different posts.

Later, David

Your thinking along the right lines David. 3 1/2 turns out is the whole circuit. But if your not having a problem (backfire on decel) you don't need to change it.

Just a thought though. Why does your bike need so much of its Pilot circuit? Have you dialed in your PMS? Or was this a chosen setting?

If it is dialed in, and required that much PMS, I would love to know more about your bikes accessories, ie: pipes, manifold, air kit, needle clip, and main.

GRAM

Steve,

You threw the perfect question out there in regards to my question. After spending several hours pondering that over, I had the carb tuning special for lunch. Make adjustment. Ride a little. Repeat as needed. Nice way to spend lunch.

You were right. I am not sure why I had the screw so far out. I am now at 2 1/8 turns. Good throttle response. Real sutle blub blub blub on hard decel. No popping or backfiring. I did have to set the idle up as it was very slow idling.

Also, how much power or response is gained in porting the manifold?

Thanks,
David

I have heard as high as 7 hp, but I think thats optimistic and assumes some pretty awesome aftermarket accessories.

Good job on the "tune up" ride. Thats how I like to do it too. And sounds like it got you right to where you needed to be.

The best way to understand what you will gain from manifold porting is to understand what it inhibits. We put awesome aftermarket pipes and air kits on, but they are both still limited to the absolute capacity of the manifold's flow. The stock "t" junction in it acts like a restrictor plate, and literally keeps you from enjoying the benefit of those great performance add on parts.

I think that a gain of three to five horsepower would probably be in the range for most guys out there. It will depend on how freely their air kits and pipes flow. The more these things flow, the higher the gain.

It also smooths acceleration. Its a difference you will notice.

GRAM

Gram,

I have a question. It was suggested to me to use a #40 pilot. I bought a #40 mikuni for hsr carb. It looks the same as the stock, but it is significantly smaller bore than the stock 35. Have you run into this before? I think at the moment, I am going to stick with the stocker and just adjust the PMS, but I hate that I had to buy a 4 pack of pilots and they seem to be wrong.

Thanks!

a number #40 pilot should have a larger bore than the stock #35 Chiro.

Not sure what to tell you about this. Are they the same size and shape externally? If not, they might not be the right ones.

Be aware, that there is a very small venturi in the pilot jet. If they located it at the end of the jet your looking at, that might be what you are thinking of as the bore. It is the critical part of the jet, but need to be sure you are looking at the venturi bore for the stock pilot too.

GRAM

Gram,

They are the same size and shape, but the end with the 8 holes on it is much larger around on the 35 than on the 40. I don't have a way to get my digital caliper into the end where the screwdriver slot is, but the other end is clearly a larger bore on the 35. They are both Mikuni and have identical markings except the stocker says 35 and the other says 40.

I seem to remember in my motocross days that yamaha oem carbs used different jets than standard Mikuni. Is this still the case?

I can tell you for sure that the main is the same for a Mikuni HSR42 and Yamaha's 40mm carb. I had understood that the pilots were interchangable as well.

Mikuni's numbers are flow rates, any jet, from any carb with the same flow numbers is designed to flow at the same rate.

If the shape is the same, I would suggest it will probably work. If you get a chance, and no one else pops in here with the answer, go over to Baron Custom Accessories' technical forum and ask Bulldog that question. He replaces a lot of pilot jets on stock 40mm's and should know the answer definitively.

FWIW... The Mikuni carbs on Yamaha's dirt bikes use a rather unusual jet that I have never seen in any other carb.

GRAM

Just an update. Although Bulldog says not, I still believe that Yamaha has mikuni make jets specific to them. The yamaha pilot jets have a line around the base of the thick part of the jet to identify them.

But, the main difference is the tube that has the 8 holes in it. The yamaha ones are significantly larger bore than the ones bought from mikuni(as measured with my digital calipers). The hole down inside the pilot is very similar in size and I couldn't eyeball much difference between the 35, 40 or 50. Since the flow will be determined by the area of greatest restriction, I now believe these jets are compatable and am going to try the 40 instead of the 50 I have been running. The part number was KVM28/486-40 or something like that which is the part number for the HSR carbs.

Gram, if you have any pilot jets laying around, take a look and see if you see the same. What does that tube with the 8 holes do anyway?