Mikuni Carbs Type2, Type3, and Type4

[S Allen]

Dave P drug out some Mikuni carbs to show me the difference between the type2, type3 and type4 Mikuni carburetors. Thought I would share the pics.

mikuni-carbs1.jpg

mikuni-carbs2.jpg

mikuni-carbs3.jpg

Direct any questions to Dave P.

Steve

[Gregs672000]

I note the type 4 have externally adjustable floats. Not often needed but nice to have. Fuel level influences how the emulsion tubes operate.

[Alvin]

Thanks for posting these Steve and Premo!

[SLOroadster]

There is a 4th variation that looks identical to the type 4, but has a slightly different jet cover and a couple other small details. They are late production models that say Mikuni on the jet cover rather than SOLEX or Mikuni SOLEX. They use slightly different gaskets (I've got two sets of gaskets that don't fit my type 4s. I didn't realize there was a difference until I got the gaskets and found they didn't fit anything.

Will

[2mAn]

If I get lucky and upgrade from Type2s to a 3 or 4 is it as simple, functionally, to just R&R the carbs or will I need to modify something to make them work?

[SLOroadster]

If I get lucky and upgrade from Type2s to a 3 or 4 is it as simple, functionally, to just R&R the carbs or will I need to modify something to make them work?

Everything should swap over, but there might be a slight difference in the linkage as well as how the fuel lines are run. One other thing will be whether or not the new carbs are homogeneous or independent type (Type 2s are independent. If you have the other type you will need a set of #8 jet blocks.)

Will

[david premo]

If the type 2’s still have the original throttle shafts then you will need new linkage. Other than that everything else will work.

[2mAn]

If the type 2’s still have the original throttle shafts then you will need new linkage. Other than that everything else will work.

Can you show me the difference between the linkages? I have to assume mine are still the original ones.

Thanks

[david premo]

Simon,
the difference is in the throttle shaft of the early carbs, they are clocked differently than the latter units. So if you switch to the latter style 3 or 4 carbs, you will need new linkage.
Dave

[2mAn]

Simon,
the difference is in the throttle shaft of the early carbs, they are clocked differently than the latter units. So if you switch to the latter style 3 or 4 carbs, you will need new linkage.
Dave

Isn’t that the same thing I’d have to do if I were to get Webers or nearly any other carb for that matter? That’s a bit disappointing

[cromgnman]

I have twin Mikuni HSR carbs. http://www.mikunipower.com/HSR01.htm
I am attempting to tune the twin carbs. At 5500 RPMs in 5th gear (approx 95 MPH) the
engine stutters as if it is not receiving enough fuel.

I have changed the main jets from 160 all the way to 200 in increments of 10.
Finally at 200 I did not see any change from the 190 jets.

There are 2 possibilities:
1. carb is not opening completely (linkage)
2. fuel pump is not delivering enough fuel. (electric fuel pump - mechanical not working)

P1010729.JPG

[Gregs672000]

Ain't tuning carbs fun? I'm afraid I know nothing about this model of carbs, but from years of tuning I can tell you that a wide band air/fuel ratio meter is THE way to go. Yes, there's some expense and hassle involved in this, namely the cost of the unit (which has dropped considerably for what you would need), getting a bung welded into the collector at an acceptable angle and finding a place to mount the gauge (even temporarily) but this takes away a lot of the guess work. You will KNOW exactly what the ratio is and what direction to go with jetting/adjustments EVERYWHERE.
The engine could also be too rich. On Thursday we went into the mountains in cold weather. I already know I'm running a bit rich on top, and the recent addition of much bigger pump jets combined with the cold weather and altitude would create some bog if I hit it hard, especially in 5th. My meter confirmed that with readings in the 10 to 1 range. I also noted that i could probably lean the slow speed system a step down now that I have an effective accelerator pump system, as I had been compensating for it before with a richer overall mix (ratio was showing in the high 12s to low 13s at 3200rpms in 5th, light load. Too rich for that demand, wastes fuel and pollutes more). I could write a book on how much the wide band has helped me, so as a fellow tinkerer I encourage you to consider one.
Regarding the throttle being fully open, I hear you on That! I recently discovered that I was only getting 80% throttle due to wear on the accelerator pedal... I even ran it on the dyno like that! It's easy enough to check that by locking the pedal in place (or having a friend hold the pedal down) while looking down the throats. Can't tell you how much difference full throttle makes... can hardly wait to run it on the dyno again. On the dyno it would still hit over 6000rpm in 4th with 80% throttle before he would cut it off, but now it hits 7000+rpm, and much more quickly, so do check.
I know this may not help you much, but you really need objective data to sort things out, as otherwise you're just guessing. I have a box full of Dellorto jets that I'll never use from all that guessing!
Does it rev to redline otherwise? Ignition system working well? What cam are you running?

[eastmedia]

Just put on dual weber 40s. Seem to be running well, but I am having difficulty getting the full range of throttle, as the roadster gas pedal does not provide enough travel to fully activate the linkage. Any ideas how I can get more travel? A pully system or something?

[nismou20]

The HSR carbs have 3 or 5 increment adjustments on the needles I believe. Also since they were originally gravity fed carbs for bikes they don’t need a whole lot of pressure. My quad RS40 only needed less than 1 psi. Make sure your slides open completely at full throttle and adjust the limit screw for that.

[Gregs672000]

Just put on dual weber 40s. Seem to be running well, but I am having difficulty getting the full range of throttle, as the roadster gas pedal does not provide enough travel to fully activate the linkage. Any ideas how I can get more travel? A pully system or something?

What I had to do for a temporary fix is to move the cable mount (the arm that bolts to the passenger side of the trans tunnel) forward to allow more travel for the pedal. It has two bolts holding it in, I just moved it one forward and it's solid with one bolt holding it. You lose the hand throttle as there won't be enough cable to allow it to hook up, but this allows the arm that pulls the throttle to move toward the engine and give you more range of motion. You will need to adjust the cable where it attaches at the carb. Look down the throats, actuate the carb linkage and look to see at what point the plates are fully open, and adjust things accordingly making sure you 1) get full throttle, 2) you don't over-clock the throttle plates (i.e. go past fully open, they probably have a stop), and check that you're pedal is to the floor and movement is being stopped by the pedal hitting the floor (there's an adjustment bolt on the floor) and not relying on the carbs to stop motion which puts stress on the linkage (adjust your cable where it attaches to the carb).

As noted, this is a temporary fix. I plan to modify my cable mount so as to allow more forward motion but being bolted to the tunnel correctly with two bolts. You may also want to take a close look at where your pedal bolts to the arm, as that area wears and creates slop. I was not able to improve this enough to get full throw.