Author Topic: Advice needed. Pre Monoblock Amal Carburettors for Methanol  (Read 282 times)

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Offline Buzzie

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Those of you who have been following my thread about rebuilding my DT 500 engine will know that I have managed to get the engine into my 1947 single seat racing car (Buzzzie) and running on petrol. I now need to get it running on methanol.

The engine is a “standard” period DT 500 engine running on magneto and 1” Amal 6 carbs. It has Douglas dome topped pistons which give a CR of around 10:1 fitted. The carbs are the variants without a second control cable and have four air intake holes at the bottom of the mixing chamber. The cutaways are marked 6/5 ,the floats are marked 30 and the needles are on the second groove. The needle jets are unmarked.

I also have the parts to make another engine, including a second pair of carbs. That engine has low compression pistons and appears to be an engine designed to run on petrol. Those carbs have 100 jets, 6/3 cutaways fitted and also floats marked 30.

The engine drove well on petrol with 150 main jets and standard float level settings, although it was running a bit rich.

In preparation for running on methanol I changed the main jets to 600s as suggested on here. However with these settings the engine wouldn't fire at all. So I changed the main jets to 150 as mentioned in the Douglas 1935 datasheet. After a bit of fettling the engine fired and ran evenly on both  cylinders for short length of time before stopping. I also got the impression that the engine was harder to kick over, feeling as if the cylinders may have some fuel in them. However that didnt stop it firing!

I tried resetting the the idle speed on the throttle adjustment valve and and the engine ran a little bit better but still stopped after a short while. At this point I noticed there was fuel dripping from the bottom of the carburettors and I assumed this was due to leaks round the the float bowl mounting bolts or or the throttle block retaining nut. I tried replacing these seals and refitting carefully but it's still leaked on both carburettors.

Then I noticed that fuel was coming out of the four air holes at the bottom of the mixing chamber so I assumed this was due to to the fuel level being too high. I couldn't find any information about what the float chamber fuel level should be. I checked the needle valve by blowing through the fuel inlet at the top of the float chamber and and noticed that it appeared to seal when pressure on the the needle valve was released but the seal returned with a little bit of gentle pressure on the needle valve. I then noticed that the carburettors, which were apparently adjusted for methanol, had some spacers fitted underneath the needle to raise the needle valve higher and therefore cut the fuel off  at a lower level. The other pair of carburettors which I believe to be set up for petrol were without the spaces.

Increasing the number of spaces to drastically raise the needle further didn't seem to stop the leaks from the air holes. By reducing the the idle speed setting and and increasing the fuel flow through the pilot chamber the engine started and ran morel slowly for a little bit longer but eventually stopped again. I have tried a variety of throttle and and pilot screw settings without improving matters and the carburettors still leak fuel from the the air holes.

I feel I've got to the point now now where I don't know what to do next and really need some advice from someone with experience of tuning and running these carburettors on methanol!

My suspicion is the engine starts and runs on fuel that has flooded the carburettor by operation of  tickle valve prior to starting the engine,  and it is not actually running on fuel air mixture from the carburettor operating in the way it's meant to. I also feel that the main jet hasn't yet come into the equation as the engine should idle quite happily on the pilot circuit. But without the engine running without cutting out I cant start the tuning procedures recommended by Amal.

So anyone who's got any words of wisdom to send will be very much appreciated!
« Last Edit: 21 Apr 2020 at 18:17 by Buzzie »

Offline EW-Ron

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Re: Advice needed. Pre Monoblock Amal Carburettors for Methanol
« Reply #1 on: 21 Apr 2020 at 22:17 »
I can't recall where I saw this being discussed, but engines running on methanol need to be bigger EVERYWHERE in the fuel supply dept.
This includes fuel taps, fuel hoses, a bigger needle and seat in the float chambers, much bigger needle jet and needle, idle jets, the whole works.
Methanol requires 2 to 2.5 times as much flow as petrol...
I don't have actual experience with this, other than little aero engines, so can't help further. 
Amal might be a good starting point, they'd have customers who know this stuff ?

It also helps if you can drain off all fuel when you are done.
Methanol is famous for exuding a drop of water or 2 when its left standing, and this can rot alloy components if left.
It also makes starting hard next time, if all it is pulling is water...


Offline Alan

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Re: Advice needed. Pre Monoblock Amal Carburettors for Methanol
« Reply #2 on: 22 Apr 2020 at 04:10 »
If of any help....from the 1947 Pitman’s Motor Cycle Overhaul book...as said, it would appear bigger is better.

Offline cardan

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Re: Advice needed. Pre Monoblock Amal Carburettors for Methanol
« Reply #3 on: 22 Apr 2020 at 06:23 »
AMAL recommended using its Track Racing Carburettor for racing; the model for alcohol fuels had a smooth bore with no needle. The following might give you some ideas of the standard and racing settings.

Leon

Offline cardan

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Re: Advice needed. Pre Monoblock Amal Carburettors for Methanol
« Reply #4 on: 22 Apr 2020 at 06:48 »
From yet another 1930 AMAL document, this time List 254. This table is about setting up the standard carb for alcohol, unlike the second table above that applies to Track Racing Carburettors.

Leon

Offline eddie

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Re: Advice needed. Pre Monoblock Amal Carburettors for Methanol
« Reply #5 on: 22 Apr 2020 at 09:20 »
When you say 'pre monobloc carbs', I presume you mean Amal '76's' or '276's' - in which case, they will be normal road going carbs with needles in the slides. That is what I have on both my 500 DT engine and the 600 DT. They are on methanol and both are running 600 main jet, .109 needle jet (with a trimmed down needle), and a 3 cutaway slide. There is no provision for altering the slow running jet, so the mixture can only be controlled with the pilot air screw.
   The main consideration when running on methanol and in anger is that the mixture should be on the rich side ( lean mixture will result in holed pistons!). Having said that, the magneto needs to give a good,strong spark to reliably ignite it! The mag timing is fixed at 36 degrees BTDC.
   As has already been said, the whole fuel system has to be opened up to cater for the increased consumption with methanol. If you have top feed float chambers, they may not be getting enough fuel through the float needle to keep the carb supplied - bottom feed chambers usually feed more fuel.
  When the DT models were produced, for speedway use, there was the option of 'track carbs' - they were similar to the standard carb but had modified choke blocks and ran without a needle in the slide - the idea being that initially the slide cutaway controlled the mixture until 3/4 throttle, then the main jet took over - the throttle was more like a tap (on or off!).
  Going back to the fuel leaking from the carbs - the fuel level should be just below the level of the bleed hole in the side of the choke block.

  Hope some of this helps,
                                          Regards,
                                                         Eddie.

Offline Buzzie

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Re: Advice needed. Pre Monoblock Amal Carburettors for Methanol
« Reply #6 on: 22 Apr 2020 at 21:59 »
Thank you everybody for the really useful information. Quite a few things for me to look at there!

My carbs are top fed bronze bodied no 6 carbs, which I believe were the standard road going DT carbs. I have 600 main jets I can try again, and the carbs already have no. 3 cutoffs. I will need to take a needle jet off to see if it has any markings on it, The ones on the spare carbs are only marked "Amal", which I understand means they are the standard size.

I think the magneto spark is good, the engine fires up quite easily on a kick start, it then runs cleanly on both cylinders for about 15 secs before stuttering to a stop, as if it had run out of fuel.

Eddie do you know by how much your needles have been shortened? Presumably they have a thinner profile at the point they enter the needle jet?
« Last Edit: 22 Apr 2020 at 22:11 by Buzzie »