Methanol - jetting

I've just had my flywheels pushed together by a speedway engine builder and got talking about jetting for methanol. He says the speedway 500's are around 210 main jet and he could not believe that on petrol I run the same size (220 actually) for a road Vincent comet. When racing in the late 80's with a TT carb on a 350 my main jet was around 700 which I think was to big but thats what I had and it ran well.
Checking speedway engine specs they are all around 210 main jet - is this just a carb difference? Can anyone shed light on this as I need to jet my 350 model 50 with a Amal Mk2 for hill climbs and the odd race.

Must be a different carb or a different way of measuring the jet.

An Amal main jet is measured in cc of fuel flow per min. When I swapped my Amal Mk1 concentrics from petrol to methanol I would increase the main jet around 2.3 to 2.5 times. I beleive that is about the theoretical ratio petrol to methanol.


I did a lot of research on this about 20 years ago and did report it in the forum. But unfortunately the search engine doesn't seem to go back far enough for me to locate the thread.

johnm said:

Air fuel ratio – x-engineer.org

Tutorial on what is the air-fuel mixture, stoichiometric ratio and its influence on the performance of an internal combustion engine

x-engineer.org

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Thanks for the link - I've been looking for this info and it appears very complete, easy to understand andauthoratative!

I used to swap a 500 twin Dominator back and forth from New Zealand clubmans class rules to open class rules. From 98 octane RON fuel to methanol.

I used twin 30 mm Amal Mk 1 concentric carbs. The swap required methanol needles and I think needle jets - I can't remember sorry. Plus chrome slides to prevent sticking due to the drier nature of methanol and a change of main jet from around 200 to around 460 to 500.

Ignition timing advance changed from 28 to 32 degrees.

On a 1 minute 12 second lap the times would drop by about 1 second. Old Pukekohe circuit.

I should also add because the fuel volume is much higher you must also check and increase if necessary fuel tap internal size, fuel lines, any filters plus the internal feed lines in the carb bowl. On a big single it's a challenge to get the volume through the system and starvation at the end of longer straights is pretty common due to lack of methanol supply through the plumbing.

Main jet size is not as critical as needle jet and needle. In my 500cc Triumph twin, I used 700 main jets. In my Seeley 850, I use 670 main jets. In both cases, my comp. ratio was about 10 to 1. With the needle jets - if you lower the needles, you should be able to cause the motor to miss when you ride the bike. When you get that - raise the needles one notch. That is as lean as you can run, when accelerating. I use 34mm Amal carbs with 6D Mikuni needles and feed the throttle on in a controlled fashion. Needle jet sizes are in thou of an inch - not flow. If your main jets are too small, you can cause damage - at full throttle, the main jets need to be slightly rich. Some guys get around corners - get the bike straight, then whack the throttle open. For me the next straight begins just after I have braked into the previous corner. I brake to just inside the corner, then accelerate all the way around it and onwards.
Getting the needles and needle jets right is of critical importance. I make my needle jets out of brass hex, using a mix of metric and number drills to get the sizes. The current size in my Seeley 850 is 0.1165 inch. The half thou is important. I use 32 deg. ignition advance. I always add 4 degrees to the petrol ignition advance when using methanol - then jet to it. Ignition advance, comp. ratio and exhaust system are constants. The adjustment is always in the jetting.

johnm said:

I used to swap a 500 twin Dominator back and forth from New Zealand clubmans class rules to open class rules. From 98 octane RON fuel to methanol.

I used twin 30 mm Amal Mk 1 concentric carbs. The swap required methanol needles and I think needle jets - I can't remember sorry. Plus chrome slides to prevent sticking due to the drier nature of methanol and a change of main jet from around 200 to around 460 to 500.

Ignition timing advance changed from 28 to 32 degrees.

On a 1 minute 12 second lap the times would drop by about 1 second. Old Pukekohe circuit.

I should also add because the fuel volume is much higher you must also check and increase if necessary fuel tap internal size, fuel lines, any filters plus the internal feed lines in the carb bowl. On a big single it's a challenge to get the volume through the system and starvation at the end of longer straights is pretty common due to lack of methanol supply through the plumbing.

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Check the breather on your fuel tank. A vacuum in the tank can cause fuel starvation on big circuits.

If I remember correctly, my JAWA 500cc speedway bike had a 35mm dell'Orto carb fitted with a 300 main jet. Dell'Orto numbering was jet bore diameter, so a 3 mm bore. Quite a big hole. My previous JAP speedway had a smaller TT carb with dual float chambers. Not sure of jet size (55 years later) but I think it was in the region between 850 to 1000.
I have a note about 1938 Manx 500cc jetting. Main jet petrol 420, alcohol 1000. The 350 was 390 and 800.
So I think that the 210 jet size you mentioned is jet bore, not the flow used by AMAL and Mikuni.

Mike40M said:

If I remember correctly, my JAWA 500cc speedway bike had a 35mm dell'Orto carb fitted with a 300 main jet. Dell'Orto numbering was jet bore diameter, so a 3 mm bore. Quite a big hole. My previous JAP speedway had a smaller TT carb with dual float chambers. Not sure of jet size (55 years later) but I think it was in the region between 850 to 1000.
I have a note about 1938 Manx 500cc jetting. Main jet petrol 420, alcohol 1000. The 350 was 390 and 800.
So I think that the 210 jet size you mentioned is jet bore, not the flow used by AMAL and Mikuni.

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That makes sense. Bore size not flow rate in cc.

johnm said:

That makes sense. Bore size not flow rate in cc.

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My brother uses Dell Orto carbs on an 880JAP. He has fitted needles into them - they had none, The running rich idiocy began with Phil Irving. If you run rich, itis true that you still get good power - but we do not need to be stupid - lean as you can everywhere without doing damage - it is much better. Even the 6D Mikuni needles in my MK2 Amals make the bike faster. It is like riding a two-stroke, and the principle is the same. If you are rich on the needle with a two-stroke, the motor will not even start.

On my 850 Commando motor, my needle jets are 0.1165 inch ID, and I use 6D Mikuni needles with a number 3 slide in 34mm Amals on 30mm ports - no step. I use straight methanol - no acetone.
Try it.
I hope this helps you to fly the flag.

acotrel said:

My brother uses Dell Orto carbs on an 880JAP. He has fitted needles into them - they had none, The running rich idiocy began with Phil Irving. If you run rich, itis true that you still get good power - but we do not need to be stupid - lean as you can everywhere without doing damage - it is much better. Even the 6D Mikuni needles in my MK2 Amals make the bike faster. It is like riding a two-stroke, and the principle is the same. If you are rich on the needle with a two-stroke, the motor will not even start.

On my 850 Commando motor, my needle jets are 0.1165 inch ID, and I use 6D Mikuni needles with a number 3 slide in 34mm Amals on 30mm ports - no step. I use straight methanol - no acetone.
Try it.
I hope this helps you to fly the flag.

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I just bought this conversion kit.


Changed main jet size, used a chrome slide, increased flow rate through pipes taps etc, 4 degree extra advance all as described above. Bought methanol from a local stock car place and raced.

To be frank it was the simplest and easiest conversion I ever did. Just followed published info from credible sources.

Bike got second in the NZ Classic senior TT at Pukekohe behind Paul Dobbs in its first race on methanol. Which tells you it was some years ago. 2009 I think.

Easy as. If the bike is running well on petrol make the appropriate changes as described and it will run perfectly well on methanol.

Be sure to clean out the carbs, tank and lines and put excess methanol in sealed containers. Do not leave it in tanks or carbs.

The only thing I found a bit fiddly was making needle jets. I used a mix of metric and number drills and measured the tops of the drills with a micrometer. I converted metric drill sizes into imperial. When the jetting is really right, it makes a very significant difference to performance. I think that is the main difference between using petrol and methanol. Methanol is easier to get right. The errors in jet sizes are less significant. In my case half a thou of an inch in needle jet ID, made a big difference. Normally needle jets go up in one thou steps - too big.
Have you tried Mikuni 6D needles ?

My brother has a jeweller friend who can actually measure the internal diameter of needle jets. I think a flow meter and gas bottle might be a better way of calibrating needle jets. It is really critical.

acotrel said:

The only thing I found a bit fiddly was making needle jets. I used a mix of metric and number drills and measured the tops of the drills with a micrometer. I converted metric drill sizes into imperial. When the jetting is really right, it makes a very significant difference to performance. I think that is the main difference between using petrol and methanol. Methanol is easier to get right. The errors in jet sizes are less significant. In my case half a thou of an inch in needle jet ID, made a big difference. Normally needle jets go up in one thou steps - too big.
Have you tried Mikuni 6D needles ?

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I detailed carefully the steps I took.

I have never used a Mikuni needle of any kind.

from johnm