Mikuni vs Amal

Hi fellows,i have my MK3 cafe racer Mikuni VM36 carburettor and it seems that cannot adjustable it right.There is PW3 camshaft and cylinder head has made bigger channels,now i think do i must buy twin Mikuni carburettor set or original Amals,what do youn think?,the problem is that engine cannot give enough power out.

hi mr norton if its power you,re after i think you will need 2 carbs either mikuni or amal you say you have a pw3 cam and the head has been ported but you didnt mention compression

Hi Mr Norton,

You also didn't say if the head had been PROPERLY flowed. I had one individual who makes big claims, but can't back them up. He "modified" one of my race heads, and without touching anything else in the motor the power went from 68bhp at the rear wheel down to 44bhp! it can't have been easy to make such a mess, but he managed it! the wrong person flowing your head can be a disaster!!!

I'm no expert with mikuni's but is it possible that 36mm is too big?

Does it idle OK? What is the rpm range where it falls flat?

You need to run it for a while in the rpm range that it runs well in then bring it up to the rpm range where it falls flat and kill the engine and inspect the plugs to see if it gets too much gas or not enough. Then change jets too make improvements. Start with clean plugs.

You might want to swap carbs off your other 850 and test ride 1st. just to make sure it is carberation at fault.

Lack of power in a modified motor can be more than the carb. I run a VM34 on a stock 850 Mk2 motor with Dunstall exhaust. It makes as much power as a standard Mk2, so the twin carbs benefit is very insignificant.

if you run a 22 tooth gear drive sprocket, you should be able to hit a top speed of 190 - 200Kph without over-revving the motor.

The main jet should be up around 250 with a free flow airfilter as large as possible.The exhaust system should be near the stock length and internal diameter with minimal internal baffles. The carb needs to be jetted to suit any exhaust particulary if it is less restrictive than the stock pipes and mufflers.

I suggest you need to check the valve timing. That is going to the most likely source of lower power. Next check the ignition timing, you should be using an electronic system?

Mick

Thank´s

The engine start very easy,running idle speed really good,easy to start hot but....every time when i increase rounds more than about 5000rpm the engine start "cutting",seems like it get enough fuel oil.
I have chanced or tested more than 15 size of main jets between 135-500
also moved needle between top&bottom and always same happens-feel that the top of power is lost...

Re: Thank´s

MrNorton said:

The engine start very easy,running idle speed really good,easy to start hot but....every time when i increase rounds more than about 5000rpm the engine start "cutting",seems like it get enough fuel oil.

When saying the above, could it just be something as simple as a partially blocked petrol filter or tap reducing flow required at higher revs, or even an occasional vacuum in the tank.?

Click to expand...

Hi

I tryed drive with petrol tank cap was open-same problem...
But maybe in fuel oil lines are something dirty,the filter is plastic and it seems at least clean,who knows,only Him,myabe the problem is in that part of system...

All motor tuning starts with cam selection. All part selection after the cam choice are a slave to the cam. Here in the states a decent old set of Amal's can be had for as little as 25.00 each to re-sleeve about 75 each. Seems like this is a good way to start, put on a well sorted pair and tune them. We have a good vintage racing program with lots of Norton's. A trip to the pits at Mid-Ohio showed that many winner's were running a pair of Mark 2 Amal's so that's the way I choose. Now it's not impossible to get your single carb to work but still the dual set up is known to work start here to work back. That being said having a powerful ING system unlocks a lot of power as well. It takes a hot spark to set off the lean mixtures used for max power kind of confusing you see. As Mick say's I ended up with 250 mains 1 5/8 exhaust and hot plugs to get my Commando to do 118 sitting up in the wind near the red line.

Okay

I try next chance over two Amals,during next spring,now here in winter time so time will tell what will happening...

When going over 5000 RPM, try running with both fuel taps open.

A duff coil and weak spark can do the same thing.

Cash

yeap

Ok brothers,i believe soon or later she will run like hell..