Mikuni needle jet

Hi again,has anyone single Mikuni VM36 on MK3?,what size should be needle jet.I have my own MK3 PW3 camshaft and ported cylinder head and seems that engine get not enough fuel over 5000rpm.

Hello MrNorton,
There could be lots of problems, but here's a start.
I don't have personal experience with this particular setup ( 36mm single Mik ) but a good friend of mine does.
He ran this set up on Moto Cross Sidecar with warm 828cc motor.
He says this should be close.
Needle Jet = # 159.... P-6 or P-8.
Needle = # 6DP17.
Air jet = # .7
He did hasten to say... He eventually went to twin carbs when he improved the motor more.
Make sure your Main jet is big enough, too big does not hurt when tuning Mid and Upper mid range, you can fix it later.
Air jet is crucial to make Needle jet work.
Are you sure your float level, fuel taps, fuel filters and float needle and seat are all OK.,
Hope this helps. Let us know how you get on.
AC.

MrNorton,

If you're looking for life above 5000 rpm, I'm afraid a single Mikuni won't deliver, whatever the jetting. My VM34 will flow only a maximum of 5-6 fluid oz per minute to the bowl, which limits the maximum horesepower of the 850 engine to 35 HP or so. I don't know the flow specs on your VM36, but you might want to run this quick test yourself: remove the bowl plug, place a measurement cup beneath the bowl, and turn on both petcocks for 30 seconds. Measure the fuel collected, multiply by 2, and obtain the flow rate in fluid oz per minute. You'll need upwards of 9-10 fluid oz per minute to allow for 50 HP or so, which is obtainable with dual carbs. At the lower flow rates of a single Mikuni, you'll have plenty of grunt and performance in the lower rpm range. The performance limitation will show up at the higher rpms, where the higher HP is developed. This is NOT a petcock limit, but rather a limit in the internal piping and valving of the Mikuni. Those of us enjoying the ease of adjustment and tuning of a single Mikuni have to pay this performance price.

Rick

Rick,

Very interesting- I have a VM 34 on my MK III -35 bhp??!!

Is this the same on a single MKI Concentric? Or to out it another way, is this why Norton never offered a single carb from new?

Yeah, I think it's a pretty universal result with single carbs. Jim Comstock reported measuring an 850 with a single Mikuni on his dyno on a similar thread last year: he got 35-38 HP out of it. Keep in mind that the calculation is a bit rough and assumes perfect aspiration; YRMV.

I thought of doing the same experiment on my Victor, but the Concentric isn't set up with the convenient bowl plug, so the short answer is "I don't know".

Thanks Rick (sorry for hijackinng your thread MrNorton),

I've been totally smitten with the general improvements the Mikuni made to the bike- no tickling, lighter throttle, better fuel consumtption, lasts forever and starts/runs like a clock. I s'pse you can debate forever about which carb is best, but riding around on a 35bhp 850 makes going back to twin Amals look good!

MrNorton said:

Hi again,has anyone single Mikuni VM36 on MK3?,what size should be needle jet.I have my own MK3 PW3 camshaft and ported cylinder head and seems that engine get not enough fuel over 5000rpm.

Click to expand...

PW3 and a ported head....hhmmm.

I have a 750 Combat with an equivalent JS stage one and a 1 1/2" exhaust.

This bike demands flow as in breath in and breath out. I have run the VM36 and is was good before my cam upgrade. Before I ran it with a stock cam and a compression plate the detune the head (now removed). I tinkered with the TM34 and was a replacement for the VM incorporating the jetting from the VM. This is a great setup up for a stock bike but those day are gone for now.

After the rebuild with the new cam, I went around and around trying to get the single carb to work and came to the conclusion that a single carb was just not going to allow this motor to run as designed. The single carb became the weakest link in the chain.

You may get it to run OK but in my honest opinion you will never see the benefits of that great cam and the porting job with a single carb. You got that great cam and ya just strangle it by the neck before it has a chance to live.

This is my experience with the hot street cam setup. You will always be yearning for what it could be with a single carb.
I am running the JS Twin 32mm setup and now I need a proper seat to keep me aboard. I about slid off the back bringing it to red line in second gear. I wish this feeling for you.

All that being said, I've thought about one of those Easy Kits for HD's. 40 or 42mm pumper, but besause after a cam timing issue was resolved, the performance light came on and there was no looking back.

I know a bloke with a 38 Flat slide for sale. or a pair of 32 vm.
Mrnorton, check the size of your Needle and Seat.... They go upto # 3.0 at least, that I know of...
Thats 3 (three) mm. A lot of fuel.
I would assume that the Carby has a Dual taper needle... #6 (60mm) D ( first taper, shallow) P (second taper, quite steep)
That second letter is the one you need to look at... P. It is the angle of the second taper... M,N O, are less than P or Q.
It will help give a bit more Jooce.
You might be flogging a dead horse, but try everything....
Then fit Twin Mikunis' or those other things that seem to work.
That's my $1.50 worth.
AC.

I have air jet 2,0#,what do you thik about that?.

Wow thats big. But I wouldn't change it yet.
Your problem starts at upper mid range, The 'air correction jet' (at 6 o'clock on the bell mouth) will only affect higher revs.
It might not be the right size, but don't confuse things by playing with another circuit.
Try the suggestions to get a better result, hopefully clean crisp running to 7/8 throttle.
Then go to Main jet and Correction jet.
As Rick and I said previously,... Be sure you are getting plenty of fuel through the carby. (float, needle and seat)
Did you understand what I said about the Needle and Seat inside the Float bowl.? It has to be big enough or nothing will work at revs.
If you can't get a satisfying result, Take the main jet out and leave it out, do some testing...
It is possible to get the carby almost totally sorted with out it.
Hopefully you can get it to run properly up to 7/8 throttle, then, when the Main jet should take over it will hopefully be way too rich.
Then you can restrict the flow at top revs, with a Main jet and Correction jet.
Don't be disheartened, Carbies can be a pain. Keep going.
Exhaust every avenue before you give up.
Possible high lean out problems... Air vent in petrol cap partially blocked. Taps. Fuel filters. Float level too low. Needle and seat too small.
Needle jet too small. Air filter too small or dirty, take it off for testing.
Hope you get it right.
AC.

Ok,thanks for many good winks.When i bought that Norton about 8 years ago the Mikuni was already installed by owner before.
Now it seems that almost all parts(needle,jets ) are changed by him,so back to basic:i try "factory settings"and look what will happening.

What type of airfilter are you using? If its too small it will restrict the airflow and stunt performance.

Eample - I got a 850 with VM34 with a narrow K&N airfilter that could just fit into under the top frame tube. It struggled to get over 5,000 in 4th gear. Remove the airfiler and up to 6,000 no problem. I modified the filter backplate to increase the airflow area and now its good.

Mick

Ok,i will chack taht too.

MrNorton , Have you done a plug reading yet.
This will show if it's rich or lean.
Take the filter off and do a reading.
AC.

Not yet,because gearbox is still under ovehauling-but last when i have driving teh plugs was quite light.so it seems that mixture is too weak.

Does anyone for example PW3 cam?,what are jets size if you have VM36 single Mikuni on Commando?.

Are you sure it's a 36mm, most conversions were 34mm.

Yeap,the carburetor is VM36.

I tryed R4 needle jet and it´s too big,even idle speed coming black smoke and it was very poor power when i was driving .Next step,i have ordered Q5 needle jet and time will tell what happens.

MrNorton said:

I tryed R4 needle jet and it´s too big,even idle speed coming black smoke and it was very poor power when i was driving .Next step,i have ordered Q5 needle jet and time will tell what happens.

Click to expand...

P8 is standard. Q4 might be a good compromise.