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anessen

Weber Woes - DMTL on Mk2

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anessen

Hi,

I've been struggling with my Weber carb for a while now, and I'm getting a bit bored of it.

I have a DMTL 32/34 on a Mk2 Polo 2G engine, which is stock.

Jetted as (from left to right, as seen from the front of the car, looking towards the back, with the throttle cable on the left):

70 | 185 - F30 - 120 | 155 - F22 - 100 | 45

 

Problems:

All of these are on a hot engine with no choke.

At Idle: No idle unless the mixture screw is 2.5 turns out. It's a little unstable but it's not obviously hunting (could be normal?)

Coming off idle: Poor throttle response - 0.5 second delay, followed by a noticable lurch forward. Throttle feels like an on-off switch in low gears.

Slowly opening to full throttle: Loss of power as second stage opens up, but no obvious misfire.

Quickly opening to full throttle:  Car hesitates, then lurches forward.

 

I have tried:

Searching for vacuum leaks. I have been spraying carb cleaner at the base of the carb, and at the hose connections. Nothing obvious.

Timing. Ignition timing has been set and checked, with the vac advance unplugged and capped off.

Twiddling the idle mixture screw. Highest idle speed is about 2.5-3 turns out.

Cleaning. I have had the entire carb apart and thoroughly sprayed everything with carb cleaner. Float valve works. No obvious blockages. Mixture screw is air tight. Accelerator pump seems to work correctly and doesn't leak.

 

Questions...

At what point am I looking at jetting issues rather than something else?

I have the Pierburg's old swirl pot (?) connected up and cable tied in place. So far I have the fuel going in at the top, the return in the middle, and the line off to the carb at the bottom. What's the proper way to connect it up?

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bastibum

Does the swirl pot look something like this? This an aftermarket version of a golf mk2 swirl pot. This brand was smart enough to engrave the connections. It applies to the original swirl pot as well I believe.

spacer.png

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anessen

I found a picture of the one I'm using. sadly, it has no markings. The diagrams I'm seeing online have conflicting information on how these things work. Some are saying that the return should be the top?
image.png.257ccdb3bc08ecfe8a4519da728e3ed3.png

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bastibum

Do you have an OEM-number on it? That could get you the correct diagram

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anessen

I've reconnected the thing with the return on top, the intake in the middle, and the line out to the carb on the bottom. It seems to have helped a bit, but still having problems as soon as the throttle opens into the second stage

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dvderlm

Long shot but the pump diaphragm might have split. it covers the weakening which occurs when open throttle because petrol is heavier than air so takes a while to catch up.

 

But you see it squirt down throat when operate the accel cable lever by hand?

So not pump.

 

hm

 

Can you blow air through from mixture screw hole into the well and hear it bubble?

Edited by dvderlm

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dvderlm

Mechanical fuel pump might be giving up?

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dvderlm

Haynes Golf 1.6 carb shows swirl pot like this.

WIN_20200530_18_15_17_Pro.thumb.jpg.8bcdd7c5e4195932c967b1d14980035f.jpg

That's not how I remember.

 

I thought it went in at highest side and to carb from lowest point and return was the central top because bubbles rise in the centre, but fuel is swirled around the outside.

That's certainly how the Pierburg pot is plumbed, so I suspect Haynes may be wrong for the 1.6 plumbing.

Compare

WIN_20200530_18_19_38_Pro.thumb.jpg.6e164d7cf1ab87dea665672b91531e27.jpg

 

This is on the same page 4A-6 as previous pic, Haynes Golf and Jetta 84 to 92. 

 

And I'm fairly sure feed from pump is 7.9mm tube and return is 5.9mm so it is hard to get it wrong.

 

But the carb well also removes bubbles, so it only matters when fuel prefers to drain back to the tank than flow into carb when it's needle valve reopens.

This will happen with reduced flow from pump. Try clamping the return line partially?

 

 

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dvderlm

IMG_20200531_100609_4.thumb.jpg.a877b605352c8915591f69d21f813d29.jpg

My memory is wrong.  Found my ancient swirl pot from mk2 Golf. In and return both high on side. Out to carb is on top. There is no outlet down low.

I did fit a 6mm check valve in the return line to reduce flow away from carb  back to tank even more than 6mm tube versus 8mm.      When I switched to Huco electric pump which pumps on demand like  a motorbike pump I reversed the check valve to stop all flow back to tank. 

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bastibum

Aha so those connections are the same as on my aftermarket swirl pot. Then the stock swirl pot in a MK2 polo differs from a MK2 golf's right?  BTW: Did you find any neat way of mounting the pot in the engine bay?

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dvderlm

See that 6mm Allen headed bolt on my pot. . I drilled a hole in the bulkhead near the chassis number, and attached it with that bolt and nut.

Higher than the carb.

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anessen

Reconnecting the swirl pot has helped a lot.

The mixture screw comes out quite far (3+ turns) to get maximum RPM with the throttle adjustment screw at about minimum (gets about 750rpm) in this configuration.

The good:

It holds idle speed OK. It has good low end torque. High RPM open throttle is good.

The bad:

Jumpy acceleration coming up from idle.

Accelerates slightly when reducing throttle through a certain point about ~30% of travel.

Hesitation when opening the second throttle stage (but no misfire).

 

The mixture screw situation tells me that the idle jet might be wrong. Is it worth going up a size on both?

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dvderlm

Might be surging lean, if everything else is in order.

 

Only change one thing at a time

 

45 -> 50 might be overkill.

 

Sure, try 46,  47 or 50 primary idle.

Or get a spare 45 idle and a tapered needle file to cut a fractionally larger hole.

 

Idle jet is misnamed.

 

It actually covers a lot of the engine's use up to maybe 3500rpm.

 

To prove this remove primary main  jet stack and see how far  it will go driving gently not far from home.

 

or to check primary main see if partially activating choke  (with engine warm) improves how it accelerates

or experimentally move the 120 to primary and see if that overcomes the hesitation.

 

then try 100->105.

or 100->110.

 

Or even raise the float 0.5 to 1mm making it richer overall.. 

 

I think ethanol has reduced the energy density of petrol by about 2 to 3% so it needs a bit more. An injection car will compensate automatically.

 

 

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anessen

So far, I have tried the following things:

Bigger idle 1 and idle 2 jets. Going up to 50 on the primary side has enabled me to have the mixture screw in a more sensible position (it's around 2 turns iirc). After that, I tried adding a larger secondary idle too. The larger secondary idle doesn't seem to have done much.

 

Swapping the main stacks around (1 in 2, 2 in 1) fixed the hesitation issues at low RPM, but made opening up the second stage much worse.

 

So to be clear, with this set up (as looking at the carb from the front):

75---[185/F30/120]---[155/F22/100]---50

I get a good mixture screw, slight hesitation down low, a lot of hesitation on opening the second stage throttle, and weak power between 2k - 3.5k.

 

With this:

75---[155/F22/100]---[185/F30/120]---50

I get a good mixture screw, no hesitation down low, very strong hestiation/misfire when opening second stage, and decent power between 2k-3.5k.

 

I verified the engine timing, too.


I'm going to return the stacks to where they should be, obviously. At the moment, I am thinking of trying a larger primary main jet tomorrow. I have a 105 in my stash. I don't know what to do about the secondary side issues.

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dvderlm

What is the float height?

7mm from float to gasket just pushing on needle valve I think.

(for about 18mm droop)

 

You have 75---[185/F30/120]---[155/F22/100]---50 (and the bridge to venturi in barrel with choke flap heads to the side with 50 idle).

You intend to try

75---[185/F30/120]---[155/F22/105]---50.

Okay.

 

If that improves down low enough then try bigger secondary main like 125 or 127 or 130.

75---[185/F30/125]---[155/F22/105]---50

 

If 105 doesn't help enough  try 110 primary main.

 

if still poor transition check the diaphragm and spring of the pump. If diaphragm has split it won't pump sufficient volume.

If spring is broken it won't refill enough after each squirt.

 

To test pump you can place carb over a container with fuel in the well and operate throttle lever 10 times, weigh or measure the fuel and see if it matches the spec.

(a Pierburg 2e3 delivers 10 to 11 ml in 10 strokes through a 0.3mm jet so each squirt is quite long duration to overcome weakness as secondary opens.

 

DMTL usually have 40 or 45 pump jet so squirt duration is shorter than Pierburg, but it should be much the same volume of fuel.

On racing Webers you can adjust how much is returned rather than squirted with a bleed and pick different rod/spring strengths for greater volumes.

It has to last to cover the drop in vacuum as secondary opens.

 

Make sure to measure the rod length of pump diaphragm if you do need a replacement. Too long and it won't fit.

 

There's also a ball bearing in the pump jet as a simple valve. If it does not rattle when removed and gently shaken you need a new pump jet.

 

 

I ended up using F33 emulsions in both stages as I went to ever larger venturis . These are thin walled so have more reserve.

 

 

 

 

 

 

Edited by dvderlm
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anessen

Swapped the stacks back to where they should be, now with a 105 on the primary side.

It's a bit better again, still not perfect, you can feel a little surge as the throttle closes.

I'm not sure what the secondary idle is meant to do, it doesn't seem to have had much effect, I'm debating dropping it down again.

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anessen

I swapped the emulsion tubes, and upped the primary side to a 110. It drives much, much better now. There is a slight stumble at mid RPM when the second stage is opened, but no other obvious problems! There's no jumpiness at lower RPM, decent low end torque, and it's got a lot more power at open throttle.

Next, I will look into adjusting the accelerator pump, just to see if I can tune out that little hiccup.

Current configuration:-

75---[185/F22/120]---[155/F30/110]---50

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