A little while ago I changed from bike carbs back to a Weber DGV one, for the sake of simplicity. I got it tuned on a rolling road after fitting, and this worked really well; I ended up with 114 BHP and it pulled through the rev range like a good 'un.

After a few weeks it's started to misbehave - tickover is rough, it hesitates/coughs when I put my foot down sharply (probably when the second choke opens) and it isn't pulling as well as it used to. When I had bike carbs (for many years) it would pull through the whole rev range without problems - that was only a month or so ago. I don't believe that the problem is in the engine itself. However, it runs pretty well on the first choke, and my mpg is much better than on the bike carbs when driving sensibly (a rare event!). The ignition map hasn't been touched since the rolling road session. Oh - the engine seems to be running slightly hotter (maybe 5 degrees more) and it sometimes runs on when I turn off the ignition.

I've done some investigation: the float level is correct, the jets I can get at are clean, and the carb's intake filter is also clean. Also, the plugs are the colour I'd expect them to be (I know that's not an absolute indicator, but it is a guide). The carb is tight on the manifold, there's the usual plastic heat insulator & gaskets between the carb and the manifold, and the manifold is tight to the head.

Can anyone suggest where I should look next? (apart from putting the bike carbs back on!)

[Edited on 19/8/16 by David Jenkins]

Accelerator pump diapham split, do you see a squirt of fuel when jerking open the throttle?

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Originally posted by mark chandler
Accelerator pump diapham split, do you see a squirt of fuel when jerking open the throttle?

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Does it have an electronic solenoid operated idle valve thingy? Designed to cut fuel supply when ignition is switched off (operate from a switched live).

They are prone to going up the Swanny apparently. I've disconnected mine and fixed the valve open with some epoxy. I'm going to fit an electric pump which will cut out anyway when the ignition is switched off.

I had had some strange intermittent mis-firing so thought I'd get rid of it to rule it out as I was fitting the electronic pump anyway.

Just a thought

Sounds like the mixture has gone weak, have you checked any blanking plugs in the manifold? Servo take off etc...

I'll be really interested in the result of this. I switched from a nearly new 32/36 some years ago for exactly the same reason. I never could get rid of that stutter. But my old knackered and much fettled twin 40s are no better!
I 've spent hours and hours fiddling with linkages and flow meter and not yet got
it to a state where I think it's worth spending money on a rolling road

Clean the slow running (idle) jets on both carb venturis and check the accelerator pump is squirting.

[Edited on 20/8/16 by britishtrident]

As already suggested David, try cleaning the idle jets . If that cures the problem it might be a good idea to fit an in line fuel filter ? One little tip I was shown years ago
When dealing with blocked jets by the side of the road was to get the revs up with the air filter off and whilst holding the revs place your hand over the carb inlet to block it off for a couple of seconds but don't allow the engine to stall . The vacuum often clears the obstruction, I've used this method on several occasions with success

Idle jet holders can accessed from outside of carb whel air filter is removed.


Description


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[Edited on 20/8/16 by britishtrident]

It's OK B.T. - I knew where they were thanks to a YouTube video I'd watched first thing this morning... Thanks for taking the time though.

I've blown them through with compressed air, so the next time I can get on the road I'll give it another test (we're getting blustery showers ATM).

The accel pump is working OK, and the progression mechanism seems to be working OK.

[Edited on 20/8/16 by David Jenkins]

This is why I like this forum so much!

I took all the advice given here, plus a few tips I picked up in some YouTube videos, and the engine's now pulling as hard as it did straight after tuning. Apart from blowing the jets through, I checked all the mechanisms, tightness of fastenings, etc.

One significant change I made was where the MAP vacuum was taken from; I used to use MAP advance when I first built the car (before the bike carbs) and at that time I took the vacuum from the outlet on the side of the carb, where the pipe used to go to the distributor. This used to work OK, but gave odd readings when I plotted the revs/vacuum/advance curves output from Megajolt - everyone told me that this was not a good thing as the carb outlet was above the throttle butterflies leading to zero vacuum (i.e. normal atmospheric pressure) when the throttle was closed. So, as an experiment for today's outing, I swapped the MAP connection from the manifold back to the carb - plugging the original connection of course - and now I have a decent idle tick-over, instead of the coughing and spluttering I was getting previously. It wouldn't have made any difference to performance when the throttle was open, but I think it must have been trying to idle with a totally wrong ignition advance. I'll check the technicalities later on to see what advance it would have been getting in each case, but for the moment I plan to leave it as it is.

So, all these actions seemed to have put everything right apart from a very slight hesitation when I stomp on the accelerator pedal - and I think I know the answer to that as well, after watching a few YT videos. Basically, their theory is that the DGV carb is designed for daily driving in an ordinary car rather than a fast sporty one, and it really doesn't like the driver stomping on the pedal. When I was out just now I tried stomping (and got the hesitation) and then I tried flooring the pedal a little slower - maybe a second or two to floor it - and the car just pulled away like a train. So it looks like that all that's required is a slightly different driving technique.

So, at the moment, I'm a happy bunny. The only remaining task is to fit a fuel filter - I've got one of these, and I was wondering if it would be good enough? It claims to be good enough for up to 2000cc engines, but that may be a load of cobblers.



My fuel pump is a low-pressure, high-volume one so I'm unlikely to burst it. I'd rather avoid fitting a fancy/expensive one unless it was really necessary, but I will if that's what's needed.

Anyway - thanks to all! And thanks to BT for that Weber diagram - it's much easier to understand than my Weber-supplied exploded diagram.


[Edited on 20/8/16 by David Jenkins]

That type of fuel filter is fine, they were fitted to VWs with carbs as standard on most of the water cooled engines.

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Originally posted by rusty nuts
That type of fuel filter is fine, they were fitted to VWs with carbs as standard on most of the water cooled engines.

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And so the story continues... and it's a saga...

A couple of days ago I was out and about, planning to get some fuel then drive around for a while. Got a couple of miles from home and the engine began to get very rough, so I decided to turn around and head for home sharpish - but when I got to the roundabout the engine stalled and wouldn't re-start. A passing builder parked his wagon and helped me to push my car off the roundabout and into a lay-by (I couldn't have done it by myself). Rang Adrian Flux Rescue - and waited for 2 or 3 hours to get rescued (not really their fault - one of their agents refused the job because he thought he couldn't get a kit car on his wagon, and there had been a huge number of breakdowns due to the hot weather).

While I was waiting I took the filter off and the carb was full of petrol - a couple of inches of it over the throttle butterflies. I took the lid off the carb, fiddled with the float valve (which seemed OK) and waited for the rescue man. He took the plugs out (I didn't have a plug spanner with me - now ordered) and we spun the engine to clear the cylinders. Dried the plugs and replaced them, and the engine started with the help of some jump leads as by this time my battery was knackered. All seemed fine, so I headed for home - and barely made it as the problem re-appeared.

Today I stripped everything down - all the jets and passageways looked clear and reasonably clean apart from a few particles in the float chamber, which I cleaned out. Checked the float height again, reassembled the carb, fitted a fuel filter and tried again - and the carb immediately flooded. Stripped it down again, held the float up and turned on the ignition - and the petrol flow was stopped correctly. Had an 'Aha!' moment and put the float into the now-full float chamber - and one side sank like a stone.

This explained why the car was fine to start with, but flooded after a while - I guess one side of the float was slowly filling up until it couldn't close the valve any more. It may also explain the other problems I described earlier in this thread, i.e. coughing and spluttering when I floored the pedal, as float chamber fuel level is critical in these carbs.

Now I have sent an email to the carb supplier to see if I can get a new float...

...sigh.