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dvderlm

Weber 34dmtr 2nd stage stumble

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dvderlm

So having a play with a 34DMTR - as used on big Lancia.

It was £5 plus postage.

I spent £1.50 on a 4mm grub screw for the throttle linkage and I had spare idle mixture screw already as old one was seized and I used 1/3rd can of brake cleaner to clean up the filthy outside,

I've not bothered dismantling it yet. The pull down unit is stoved in from postage in a plastic bag but those UFOs are indestructible so starting opens the choke flap and all is fine. Need to wire in the idle cut-off solenoid properly - in parallel with fuel pump post inertia switch I suppose.

It's 25/26 venturis (I normally use a DMTL with 24/25 no problems and the original Pierburg GK was 22/26).

I'm using a 48 idle that I made myself with solder and fine drill. Idle and first stage upto 2800rpm is really nice. Top-end 4000rpm+ is scarily good. But I've got progression phase blues. Stamp on the load pedal and it stumbles before roaring.

Accelerator pump works fine - no idea what size or how much it is squirting, but it's a good stream.

The jetting is currently

48/115/F30/200

55/135/F30/210

So I plan to up the secondary idle, then try my beloved F33 emulsions, then play with the pump jet.

I think it might have a slight leaking needle valve from the fuel pump pulses (I blocked the return line on the carb) - so I'll look into that and then maybe adjust the float height too.

(I know I should be doing my piston rings, but I can maybe use this carb if end up going the oversize rebore route.)

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dvderlm

With 70 secondary idle it improved a bit, no more kangaroo past half accelerator pedal travel: but not as urgent as the DMTL.

So I swapped all the jets but primary idle from my DMTL 24/25 and stumble is gone.

It sounded noisy not crisp at full chat and felt like it could manage bit more so upped the sec main to 140

48/112/F33/230
70/140/F33/220

One day I'll use an AFR gauge...

Edited by dvderlm

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dvderlm

Well I used a vacuum gauge as my home made AFR reads 1.0 no matter what I do with the choke flap!

Idle was a bit lumpy. Mixture screw was 1 and 2/3rds turns from bottom. Vacuum gauge said it was only 15inHg @ 650rpm when hot. Or it needed to be at 1100rpm on the speed screw to smooth it out.

But vacuum would hit nearly 25 when throttle snapped closed and cruise at 15 to 16 so not too bad.

Swapped the 48 for a 45 idle and used Redline's lean best idle method and reduced speed and opened mixture to 2 and a smidge turns. That got to 20inHg at 900rpm. Still suspect the needle valve is leaking.

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anessen

Hi, I'm sorry to bump this, but...

How do you use a vacuum gauge to tune the carb?

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dvderlm

More to fault find than tune, but essentially highest manifold vacuum is greatest suction from the pistons descending since an engine is just an air pump.

You need a take off point below throttle plate.. 

 

Try

www.classiccarhub.co.uk/articles/tuning-and-fault-finding-using-a-vacuum-gauge.html

 

or

 

https://www.onallcylinders.com/2015/05/08/quick-tech-how-to-read-a-vacuum-gauge-to-pinpoint-engine-problems/

 

Edited by dvderlm
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