Alright guys,

Thought it might be a good idea to get a thread up of peoples set-ups who are running double barrel Weber’s? DMTR’s and DMTL’s etc,

The people running motorbike carburettor’s have made one and it can be quite interesting and useful, this might just fall flat on its face because I bet there’s only a hand full of people.

Currently my set-up consists of;

Engine code/capacity : MH

Tuning modifications: 268*/268* Newman camshaft, Polished + Ported cylinder head, port matched MH inlet manifold, Mk3 standard exhaust manifold and exhaust system (soon to be replaced with GT bore and 4 branch)

Carburettor: Weber DMTL with 24mm/25mm venturi’s, Land rover item.

Jet set-up: Currently

IDLE: 55 60

MAIN: 110 115

EMULSION TUBE: F30 F30

AIR : 210 190

Still needs setting up properly as my second stage is a bit flat and upto around 2500rpm is pretty flat.

Get some set-ups please!

Edit: My mistake - reopened and FAQ'd

Cheers Steve,

Please can this be kept as a set-up thread and not installation.

Engine code/capacity : GK / 1272

(I think it says 1263 cm3 on the Zulassungsbrief)

Tuning modifications: Raceland 4 branch exhaust manifold, 3F centre box, custom rear box, K&N panel filter, Beru UX56 plugs, Hotwires leads.

Carburettor: Weber DMTR with 22mm/22mm venturi's, Golf item reverse mounted.

Jet set-up:

IDLE: 50 70

MAIN: 100 125

EMULSION TUBE: F33 F33

AIR : 220 185

PUMP JET : 50

I also drove with

45/105/F30/210

70/117/F30/195

(50 pump jet)

for a long time which was really smooth. The F33's have faster pickup like the thin walled F22 used as standard in the primary.

Edited 2 July, 201115 yr by dvderlm

Cheers for the set-up dude.

Anyone else?

Mine is flat 2000-3000 rpm on stage 2 throttle I reckon.. But then it really takes off above that!

Can anyone put up a diagram of what jets are what? Mine seriously needs sorting but cant seem to get anywhere with it...

It would be easier for you to post up a pic of the top of your carb and go from there, models vary.

It would be easier for you to post up a pic of the top of your carb and go from there, models vary.

The main problem is pulling away. There is a big gap between idle and pulling away so you have to be very careful not to stall it. Its nowhere near as smooth as it should be. Could be a problem with a jet? Not sure...

Any help would be great.

Thanks.

Far right = Primary idle jet

Middle right= Primary Air Jet, Emulsion tube and main jet, Air jet being at the top of the stack

Middle left= Secondary "" ""

Far left= Secondary idle jet

Problem with pulling away, stubbling etc could be one of either;

Pump jet not working correctly, this is the tube hanging over in the primary choke, near the middle, on the right in the pic (the top choke). An easy way to check this is to open the throttle up by hand, and see if the aformentioned pipe (pump jet) squirts in a shot of fuel.

Primary idle jet too small, or mixture too weak. Don't forget the mixture screw effects mixture up to about 1800rpm, if its not issuing enough fuel it will cause a lean stumble. Try adjusting it half a turn out, and see if your problem is solved.

Try the two above and get back.

Edited 4 February, 201214 yr by MaxMax

Far right = Primary idle jet

Middle right= Primary Air Jet, Emulsion tube and main jet, Air jet being at the top of the stack

Middle left= Secondary "" ""

Far left= Secondary idle jet

Problem with pulling away, stubbling etc could be one of either;

Pump jet not working correctly, this is the tube hanging over in the primary choke, near the middle, on the right in the pic (the top choke). An easy way to check this is to open the throttle up by hand, and see if the aformentioned pipe (pump jet) squirts in a shot of fuel.

Primary idle jet too small, or mixture too weak. Don't forget the mixture screw effects mixture up to about 1800rpm, if its not issuing enough fuel it will cause a lean stumble. Try adjusting it half a turn out, and see if your problem is solved.

Try the two above and get back.

Brilliant, thanks for that. Ill have a look at what jets I'm running as my set up is very similar to yours. Ill richen it half a turn too, any idea where the mixture screw is? Ill probably be ok finding it anyway but just in case..

Thanks.

Right, had a look this morning and this is what i have.

primary idle - 45

primary air- 155

emulsion tube - F22

main jet - 100

secondary air - 185

emulsion tube - F30

main jet - 120

secondary idle - 70

not too sure what to make of these figures but they seem pretty different to yours (maxmax). Anything you would suggest changing?

Also, half a turn richer on the mixture screw has sorted that gap right out, much smoother now. Thanks a lot!

Edited 5 February, 201214 yr by azza46

Thats good news!

People under estimate just how important the fuel mixture is, more important the mixture screw!

Mine--------------------------------------Yours

IDLE: 55 60------------------------------45 70

MAIN: 110 115---------------------------100 120

EMULSION TUBE: F30 F30---------------f22 F30

AIR : 210 190----------------------------155 185

What exactly is your current engine spec? Looking at the jet configurations above, yours is fueled much richer on the second choke, something i found to be very detremental to performance.

Is yours a DMTL or DMTR?

Edited 6 February, 201214 yr by MaxMax

Thats good news!

People under estimate just how important the fuel mixture is, more important the mixture screw!

Mine Yours

IDLE: 55 60 45 70

MAIN: 110 115 100 120

EMULSION TUBE: F30 F30 f22 F30

AIR : 210 190 155 185

What exactly is your current engine spec? Looking at the jet configurations above, yours is fueled much richer on the second choke, something i found to be very detremental to performance.

Is yours a DMTL or DMTR?

Yea seems leaner on the first stage and richer on the second.

Its a DMTL carb, almost new condition.

Engine spec is:

originally a 1300 MH

new GT pistons machined to 10:1 compression ratio

all lightened and balanced

DMTL carb as said

GK inlet

ported and polished

newman fast road cam

currently running a standard 1.0 litre exhaust but i have a 4 branch ready to go on when i can get an exhaust

Think thats about it really.

Also, is there any way of getting the second stage to kick in sooner?

Thanks.

I've edited the jet comparison post to make it easier to picture...

Your current engine spec sounds very much like what i was running, i always found it very, very quick on part throttle, from say 2,000 right up to 5,000.

But flooring it from the same engine speed caused it to accelerate slightly "flatter" at first, soon getting onto its stride, though.

What's the reason for machining the GT3F pistons down to a compression ratio of 10:1? They give a slightly higher 10.8:1 in standard form, something you would have benefited from a high rpm? Do they not fit or something?

I noticed your carb came with a primary F22 emulsion tube, if i remember correctly, these are used if you desire a "quick" pick up in acceleration, at part half - full throttle openings, which reflects in the performance you're receiving? I found that the emulsion tube choice gave you the overall characteristics of the drive, i used an F30 which is tailored to give a much less aggressive AFR whilst accelerating.

If your happy with how the first stage is working for you at the moment, then leave it alone. No two engines perform the same on the same jet configuration, even more so with differently port and polished heads, something that creates more or less turbulence in different places which will have its own effect on cylinder fuel mixture supply.

You cant get the second stage to "kick in" quicker, by the way in which the carburetor is designed to work (Air pressure). The problem you're having is that in theory the engine is struggling to create a big enough pressure differential between each side of the venturi, thus giving poor fuel atomization at low RPM which completely kills engine performance and response. The DMTL's choke/venturi size is more suited to a bigger engine such as a 1.6-1.8 , although it can be made to work very well on a smaller engine, better than the slightly smaller DMTR, it just requires more thought and work.

The second stage "kicks in" when you have opened up its throttle plate, open it slightly and it meter's in fuel from the secondary idle circuit, opening it more brings on the main jet circuit. If i was you, I would maybe think about limiting how much the second stage can actually open, i think mine was limited to about 2/3 open. This leaves the air entering the second stage choke a smaller gap to pass through, increasing its velocity and maximizing fuel vaporization, something that's highly important. I also tested a few different intake designs into the two throttles, making the inlet to the second stage quite narrow, which proved to have its benefits.

Thats weird, i thought the pistons were less than 10:1 as standard. To be honest, a friend of mine did all the detailed measurements and I'm sure we machined mine down to give 10:1. Obviosuly as the combustion chamber is in the piston itself, machining it down raises the compression. If they are 10.8:1 standard, then I'm running slightly higher.

Id quite like to have a play with the jets as the initial pulling away still isnt as smooth as id like, still tries to catch you out a bit. Although I've got some bike carbs going on soon so might not be a problem anyway.

Finally, my second stage isnt vacuum operated, it is operated with a linkage from the first stage. But if its not adjustable then its not an issue.

Thats weird, i thought the pistons were less than 10:1 as standard. To be honest, a friend of mine did all the detailed measurements and I'm sure we machined mine down to give 10:1. Obviosuly as the combustion chamber is in the piston itself, machining it down raises the compression. If they are 10.8:1 standard, then I'm running slightly higher.

Id quite like to have a play with the jets as the initial pulling away still isnt as smooth as id like, still tries to catch you out a bit. Although I've got some bike carbs going on soon so might not be a problem anyway.

Finally, my second stage isnt vacuum operated, it is operated with a linkage from the first stage. But if its not adjustable then its not an issue.

Nope, 3F pistons give a CR of 10.8:1 as standard.

Also, i think you've got abit mixed up regarding increasing the CR of a heron design engine by skimming the pistons down...

If that's the method you choose, the amount taken (machined) off the piston tops must be also removed from the top of the block. i.e. I machined 2.2mm off the piston tops, to reduce the size of the combustion chamber, but then skimmed the same amount off the block.

Otherwise skimming the pistons will infact reduce the CR as all your doing is removing material that is filling up the combustion chamber initally.

Have you had a read of this? http://www.redlineweber.com/html/Tech/carb..._lean_best_.htm

Follow that to make sure you've got the idle speed and mixture screw settings correct, because i'm not convinced. Pulling off should be nice and smooth, definetly with them jet settings anyway...

I didn't mean the second stage was vacuum operated, but how it atomizes fuel. Indeed your second stage uses a mechanical linkage from the first stage to open, IIRC the first stage opens 2/3 before the second stage begins to open, there is a bit of adjustment on getting it to open up earlier or slightly later.

Yea, same amount was taken off the block.

Ill have a read through and try and set the carb up.

Thanks.

EDIT: Just followed that web page and its running much better now. I leaned it off loooaaads too so hopefully my fuel economy will be better. It hasnt been as good as id hope, i get around 60 miles to a tenner so its not been great. Hopefully it will be a bit better now.

Thanks for all the help.

Edited 9 February, 201214 yr by azza46

Thought I might upload my Main jets:

100, 120

I got it rolling roaded at BD engineering in sittingbourne and thats the result. I'll upload the rest later as I'm pretty sure its a good starting point for those who are buying new DMTLs or second hand ones with standard jetting.

As it stands though I'm now running an ABD head (skinnier valve stems) and have matched the inlet runners of my MH manifold to the head. Also I've smoothed out the valve bowl area of the head. The result is I'm now running pretty lean I think. My g40 gearbox shows this up with my inability to pull away from 60mph at 1900rpm haha.

I think, because of the rolling road tune, all my jets will be 'in proportion' with each other if that makes any sense.. I was wondering, which jets do I change if I want the whole thing to be a bit richer? main, idle.. I'm not sure I fully understand the Weber yet (though I'm told its dead simple) and I'd like to..

Smaller idle jet(s) will lean out the mixture when fueling from the progression holes,

Basically, and generally speaking;

Mixture screw controls fuel mixture at idle, up to around 1200rpm , (unless idle speed screw is incorrectly adjusted, too far in.)

Idle jet sets a maximum possible amount of fuel allowed to travel to the idle progression holes, and the above. The progression holes come into play when the throttle is opened up slightly from idle, before the main circuit is in operation.

Main jet sets the maximum amount of fuel that can travel through the emulsion tube to the main discharge nozzle, so this only effect fueling when the main circuit is initiated.

The thing is with this design of carburetor, the idle circuit ceases operation once the main circuit starts. So you can't up size in one particular jet and have a richer AFR on offer throughout the range, if thats what your after, i would go up a size in both primary idle and main jet.

That or do some research on which emulsion tubes offer what kind of enrichment. Different emulsion tubes have different effect on fueling, for example i think the f22 offers more part throttle enrichment than a F30, so if your wanting a richer AFR throughout, this may be a good way to do it.

I have a lot of literature on this sort of thing if you'd like me to scan it in?

thanks for the info, and yep the literature would be great if you can! thanks

Heres my full spec:

Now running a slightly (but effectively, I think) flowed ABD head with 7mm valve stems, of course. i have a K&N air element also. Other than that standard MH with a DMTL... may not sound much but I know I'm pulling more air through since the head work, as it feels pretty lean. So I want to get richer jets (hoping people could make suggestions). Heres my Jet Spec:

Primary idle jet: 45

Primary Air Jet: 155. Emulsion tube: F22. main jet: 100

Secondary Air Jet: 185. Emulsion Tube: F30. main jet: 120

Secondary idle jet:70

I'm having most problem pulling away (stalling), Could this be down to the gap between my 45 and 70 idle jets (it seems 70 is the largest one can get..?)

Again, a generally richer jetting suggestion would be great! thanks.

That shouldn't stall pulling away from standstill,

Check the accelerator pump squirts fuel down the throat.

I would more likely expect a midrange bog if you plant the accelerator at 2000rpm because the 25mm secondary doesn't get as much signal.

Hows the idle mixture screw adjusted? If that's not optimised then it doesn't really matter what idle jets are in.

Thinking about it, pulling away is usually done at about 1000-1500rpm ish?

So its your primary idle jet thats fuelling i'd say, which again points to incorrect idle mixture screw setting.

A 45 idle sounds correct for your set-up by the way.

replaced the 45 for a 55 and the problem is solved, tho by the redline guide it is now too rich (not more than 1 turn out on the mixture screw). I've also been fooling around with the main jets as well. I mean fooling ha. I bought a load of jets off fast road cars.

I tried (dont laugh)

130, 150. was fine untill I restarted from cold on choke.. haha

115, 130 feels much nicer than my previous lean jetting and most of the flatness has been removed. Since I never really use the 2nd stage throttle other than when I'm really ragging it, it may as well be nice and rich. Fantastic acceleration, really making use of the ATV ratios (bearing in mind its now significantly more potent than the 70BHP I got on the rolling road).

Edited 19 April, 201214 yr by Breadvan Princess