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Carb settings Dirt Track Douglas engine running on Methanol?

Started by Buzzie, 17 Sep 2021 at 21:58

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I'm setting up my DT500 engine to run on Methanol, and need some help on carburetor settings. Its running on standard type 6 carbs

I've got the engine running on methanol (well ticking over at least...) I've fitted 600 jets. I start the engine after tickling the float chambers but it starts to die after a bit of running. If I tickle the front float chamber again the engine picks up again and continues running smoothly. I therefore assume the fuel level is too low when left to the float to set the level.

On checking the specific gravities of petrol and methanol I find that petrol is lighter, and therefore the float will ride higher in methanol, causing the fuel level to be lower. Did Amal use different spec floats or fuel valve needles, and if so what is their spec and are there any markings to distinguish them? One of my floats is stamped with "30"

Thinking about the engine running on methanol with wide throttle openings, will the standard apperture in the float bowl lid feed union be big enough to supply enough fuel? If not how large should it be, and are larger diameter fuel valve needles available to suit?

Any info to help me move on would be much appreciated, TIA Douglas

PS this is my Buzzie engine!!


Any use? From Pittman's Motor Cycle Overhaul 1947

Image converted to linked file - Dave, 19Sep2021


Quote from: Buzzie on 17 Sep 2021 at 21:58
On checking the specific gravities of petrol and methanol I find that petrol is lighter, and therefore the float will ride higher in methanol, causing the fuel level to be lower.

Disclaimer: I know nothing about alcohol-fueled Douglases!! However:

If I google the densities of methanol and petrol, I find 0.792 g per cubic cm for methanol and "about 0.8" for petrol. I'm surprised that you found the petrol to be "lighter", but maybe so for modern fuels. If we take the numbers from the interweb, the key thing is that the densities are only about 1% different, and, as the fraction of the float above the fuel level probably depends on something like the ratio of the density of the fuel to the mean density of the float, the fuel level when the float valve closes is probably almost unchanged with petrol or methanol.

So I reckon that the fuel level in the jet will stay about the same when you switch from petrol to alcohol.

Amal's alcohol carb was the Type 27 - this came standard with no throttle needle and twin float bowls. They recommended at least 1/4" inside diameter for the fuel lines, and special high-flow fuel taps. Of course the Douglas with two carbs will need less flow for each, presumably why most racing Douggies can get away with single bowls.



That shows the benefit of sharing things on here  :D

I too googled the specific gravities, but clearly got them mixed up  :( . You are of course right and so the float height cant be the issue!

So any other ideas, and of course I still need info on flow rates etc. All info gratefully received!

Daren W Australia

I was involved with Liter Sprint cars in Australia in the 80's/90's when I asked the engine guy for some info he said the Methanol was used at twice the rate of Petrol for the same engine (the car we had was powered by a Kawasaki 900 bored and stroked to 1180cc). But I was not involved in that part of the car I did fiberglass and steel fab on race cars all the cars I raced ran on petrol. I hope this helps ta Daren
too many dougli not enough time!


Thank you Alan and Daren, both posts really helpful.

Today I have tried resetting the front float level. It was a bit better but not right yet. The fuel level is now, if anything, a bit too high, as it weeps out of the idle air holes when the engine is not running

I then reset the front idle mixture screw, and then increased the diameter of the feed hole in the float chamber top to increase the rate of flow into the chamber. That made little difference.

My next trick will be to buy new floats and needle valves - which I think are still available from Amal - and to check the spec and mounting of the fuel metering needles in each carb......