Hi, just installed a new Weber DMTL using a kit (from fastroadcars). Standard jets, which according to the topics I've read should be fairly ok for this OK.

 

Not getting any idle, with the choke pulled out all the way it surges up to 3k and almost stalls, then back again.

 

Had a little fiddle with the idle mixture adjustment screw, not doing much.

 

The rubber flange (?) on the inlet/base of the carb is pretty crusty. I'm thinking that's a possible culprit. Does anyone have any other ideas or things to try?

 

 

vacuum tubes in good nick, connected right?

From distributor capsule to the brass take off that is higher, and from lower take off to the thermo switch for hot/cold air flap.

 

 

Yeah they're fine, the pipes aren't leaking air, neither is the advance unit. I'm running it with the air box pipe blocked off and air tight (tested).

New base mount then. You could try some thin blue hylomar to seal it as an experiment, but there's 50 vacuum cleaners suction from an engine so it won't last.  

If you still have have same symptoms after you have replaced the carburettor base seal, you may wall have a blocked or partially blocked pilot jet (idle jet or slow running jet if you prefer).

Being the smallest jet in the carb, it doesn't take much to block it.

With an obstruction in this jet, the idle adjusting screw will have very little effect.

Clean the jet by blowing through it DO NOT 'prod' it with wire. - If you don't have compressed air, a can of  Fellows Air Duster may do the job.

 

The recommended 'starting point' for this screw's setting, is one-and-a-half turns OUT. - turn the screw clockwise 'till you JUST feel it contact its seat DO NOT overtighten, then turn it 1 1/2 turns anti-clockwise.

Once the engine has reached normal operating temperature, try turning the screw in 90° stages, noting any increase or decrease in engine speed, if the engine slows down, turn the screw 180° in the opposite direction. If the engine speeds up, slow it down to normal idle speed on the throttle stop screw, and again try the 'mixture' screw in 90° increments 'till you find the optimum position.

Finally set the idle speed by the throttle stop screw.

 

Regards

Cheers 🙂

Working on getting a new base for the carb.

Am I right in thinking that with the idle mix screw 1.5-2 turns out, it should hold an idle (even if it's running lean/rich)?

It might chug like a tractor but 2 turns out should let it idle.

http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm

 

Even with a new carb the pilot/idle jet could have become clogged from the oil breather on block or its hose as the jet is only about 0.4mm diameter.

 

 

I think I've found one problem. The allen head screws that come with the kit are about 2mm too long, if my caliper's depth gauge is correct. I thought I could feel them bottoming out, and I noticed it visually on the new carb flange.

 

Gonna try and cut them down (all I have is a hack saw at this time so that's fun).

new shorter bolts will be pennys at a fasterner shop if you dont have the tools to cut yours

Screw a correct sized nut on before hacksawing, then unscrew once shortened and the nut will tidy the thread up. Scrape rough end on a brick wall.

 

Sorted. 

I STRONGLY recommend that you have something (pliers?) to hold the screw whilst you cut it - Your post brought back memories from long ago, when I cut a screw whilst holding it - finished up with two stitches in my finger!

 

Regards

Yeah! I have a little vice. I just took a couple of mm off each one (if that) with a hack saw, just so it's not bottoming out. Added a cardboard gasket in between the adaptor and the rubber flange. Seems to do the job. The car drives, just got to set it all up properly.

I am having some issues getting the partial throttle to be consistently good. Sometimes it is ok, sometimes not. The problem is that, when warmed up and off choke, the car will sometimes (but not always) hesitate when pulling away, creating a kangaroo type effect. I think it's also sometimes weak under acceleration higher up in the rev range, but it's not as obvious. I've also noticed the engine RPM drop substantially below idle when coming off throttle sometimes. There is also a smell of petrol.

 

I have plugged off a suspect vacuum hose. I am planning on going around the carb base with easy start to look for leaks. Also heard a tip about listening to leaks using a tube as a stethoscope.

 

Any other things to try?

The following are only 'could-be's'  I am afraid, as it is always difficult sorting things like this out remotely.

 

The speed drop and smell of petrol when decelerating could be due to the fuel height in the float chamber being too high. Possible causes of this include a leaking float (take out the float, give it a shake near to your ear - if it 'rattles', it has petrol in it due to a leak.)  The fuel pressure to the carb may be too high - If you are using a mechanical pump, you could try placing thicker (or extra) gaskets between the pump and the camshaft cover. It is also possible that dirt has entered with the petrol and is stuck under the needle valve, holding it open. It is just possible that the fuel height was set incorrectly in the first place.

 

This intermittent hesitation when pulling away is a bit of a puzzle, if it were consistent, it would be easier to get to grips with. A vacuum leak between the carb and the manifold, or between the manifold and the cylinder head are possibilities - Maybe the new carb base will overcome this, so it might be as well to do that before attending to other areas. A VERY slight compromise between the best idle and smoother pulling away might be the answer - try richening the idle mixture slightly (turning the screw anti-clockwise)

 

Let us know how you go on.

 

Regards

Thanks, I'll give this a go. Out there just now, it wasn't too bad (sod's law). Just sat waiting for it to cool down so I can go in and have another go at the bolts.

 

How are these carbs meant to react to having the throttle stomped on? I know the thing is that they don't have all the vacuum gubbins on the Pierburg that does... what ever it does, I assumed it was to smooth it out. At the moment it seems to react OK to sharp throttle input (while out of gear, anyway, although there is a tiny little hesitation).

 

I'm also wondering if those Gunson-whatsit emissions testers are worth getting? I am a little but suspect that there's precisely one model on the market. Makes me think it might not be any good.

id say take a good look around it again , make sure its fitted as per the intructions , everything clean and tight and vac hoses where they should be

 

mine was very easy to tune by ear..did exactly what it was sposed to 

 

the gunsons things arent very accurate , i got mine tuned by and easily passed the mot

When accelerating, you need a very brief richening of the mixture, this is achieved in two ways (1) By the 'capacity well' (emulsion tube or equivalent in some carbs) discharging and (2) By a shot of neat petrol from the accelerator pump - You would probably see this if you were to look into the carb and open the throttle WITHOUT the engine running. - It can be dangerous to look into a carburettor when the engine IS running, especially if the throttle is opened briskly, as there is a chance of a back-fire, which could do much more than just singe your eyebrows!!

 

Regards

 

I wasn't able to find a leak. I did tighten the connections I could get to a little bit, though.

 

Leaning out the mixture seems to have helped a bit. Which is weird because driving with the choke ever so slightly on makes it go like you-know-what.

 

What I was going to say was, it feels like it has very little torque at low RPM. But that isn't quite right. It will pull away with the throttle at idle if you're gentle on the clutch. But if you try to pull away at a more normal pace (lift clutch through bite point while accelerating) it quite easily bogs down. Hmm. It's really annoying having a little problem that is hard to describe and even film, although I might have a go at that.

 

The current plan is to fiddle with it a bit more of the next couple of days.

 

At the moment I am using the little T-piece thing that came with the kit (restricted end going to the return pipe). Would it be better to use the Pierburg's old (metal) pot? I'm not 100% sure what connections would go where anyway.

The fact that you have weakened the mixture and there is an improvement with the choke slightly closed isn't all that strange really.

Think of the slow running side as a separate carburettor - You seem to have set this to the optimum setting.

Now the 'main' carburettor is calling for a richer mixture, this has to be due to either too little petrol or too much air.

 

I still have that dodgy carb base at the back of my mind - Have you tried spraying Carb Cleaner around the base whilst the engine is idling? If you do this and there is any change in engine speed, it is probably due to the Carb Cleaner being drawn in via the dodgy base.

Failing vacuum leaks, you are probably back to the possibility of a partially blocked jet. - If the problem were down to the fuel height being too low, I wouldn't imagine that pulling the choke  would have much effect, as there probably wouldn't be sufficient fuel in there for it to make any difference.

 

I'm not sure what this 'T' piece is or should be connected to - maybe you could post a picture of the set-up, also, if you have an exploded view of the carburettor that you could post, it might help to identify the set up.

 

Regards

 

the T peice is a crude way of making a feed and return on the car ...the feed goes down normal diameter and one part of the T is restricted as a return

smell of petrol.  wipe the hoses from fuel pump to carb and see if any are damp or dirty.  Cloth braided hose can hide a leak.

I do prefer a swirl pot to t-piece for constant pressure.

So here's the thing so far:

 

The car goes really well on slight choke.

Misfires with no choke when accelerating.

 

I have tightened the whole assembly down a couple of times (allowing for heat cycling) (not over-tightening though).

I have sprayed the jets through with carb cleaner. No blockages that I can see. I have given the jet wells (?) a spray with carb cleaner too.

I have replaced the t-piece with the pot from the Pierburg.

The mixture is set to highest engine RPM for a given idle screw speed (I think this is correct?).

 

Basically, I can make the car accelerate smoothly as low as 1,500rpm in third with a little choke. Without choke it misfires and bogs down.

 

Today the mission is to go around the base with carb cleaner again. Is it possible to unscrew the mixture screw and spray all in there as well? If I take the top off the carb for cleaning, is there anything special I need to do? IIRC there's just the cardboard gasket, right?

Highest RPM might be a little too lean, (especially if using choke improves). Go anti-clockwise 1/4 (or less) of a turn on the mixture screw and see if it does not surge.

 

Is that using a 40 idle jet?

Done any other tuning such as better flow air filter?

 

Read the Simple rules for low speed calibration in this

http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm

 

 

 

 

Primary Idle - 45
Primary Main - 155/F22/95
Secondary Main - 185/F30/120
Secondary Idle - 70

Got spare jets 115, 125 and air corrector 175

 

And this is a new DMTL 32/34 carb as sold on FastRoadCars, on a 2G engine.

 

I have enriched the mixture (now about 5 turns out?) and it seems better but still bogs down at partial throttle sometimes. I'll do some more fiddling tomorrow, going to follow that guide again.