Lurching and bogging down accellerating at low revs

[Brenjacques]

Need some thoughts from the hivemind on this one - some of you might recall the saga last year of my 1.8 GTS playing up, turned out the carb was faulty and Weber replaced it with a new unit. From then on it's been okay, not had any issues and the car has probably done about 4k miles since then. I've noticed however that with the new carb the engine always felt a bit more 'flat' at the bottom end than it did before.

This week I started getting a new issue where the car lurches/bogs down at low revs, like if you lift the clutch a bit too quick it'll really lurch and drop a lot of power before suddenly shooting forward, whilst it's been quite flat at the bottom end before it's never had this issue. The rest of the rev range is fine and the car otherwise drives normally with no other issues.

As a side note I did remove the king lead from the dizzy cap this weekend passed when tidying up the engine bay and only noticed the problem on the monday after this - is it likely that the king lead could cause this if it's became damaged? I'd have expected problems across the whole rev range rather than just at low revs.

Otherwise... Any pointers to look out for or any theories? I'll check all the usual, fuel lines, filter, vacuum hoses and any possible air leaks. I did do the ignition timing and set the carb up when I fitted it last year.

Also, due to the difference in power at the bottom end I want to double check that the carb is in fact for the 1.8 so if anybody has the jet sizes for the 1.8 32/34 then I can give these a check also.

[GT_II]

Being the issue is at low revs, I would suspect it may be due to an incorrect idle mixture. It may be that it is running too weak at idle. This would also cause a weak mixture at low throttle openings. Ideally you need to set up the idle mixture with the aid of a CO meter. Did you get an emissions printout at MOT? With a Pierburg 2E2 and manual gearbox you should be getting between 0.5% and 1.5% CO at an idle speed of between 700 and 800 rpm. This is with the crankcase breather hose disconnected and all electrical consumers turned off. The settings may be a little higher for the Weber. The idle CO limit to pass the MOT is 3.5% (4.5% on C-reg or earlier cars).

In the past, I have aimed to set up the idle mixture on my Pierburg "by ear". Anxious to pass the MOT, I have always erred on the side of a weaker mixture, and last autumn got a CO figure of 0.17%. This year, I noticed the power delivery slightly jerky at low speed, so suspecting this might he down to the mixture, decided to splash out a stupid amount of money on a professional exhaust gas analyser, frustrated at always having to set the carb up blind. Having set up the idle mixture to 1.0% CO, which involved a surprising number of turns on the Pierburg's mixture screw, the issue of slight jerkiness at low speed went completely. The idle CO figure at MOT was a bit higher at 1.25%, which I put down to the crankcase breather being connected, and the twin headlamps, hazards and fog lamp being left on throughout the test (why do they always do that?!)

There's a very useful guide to setting up the Weber 32/34 DMTL on the MK1 Golf forum, which appears to be copied from a Haynes Land Rover manual, presumably as it was fitted as standard on some Land Rover models:

https://vwgolfmk1.org.uk/forum/index.ph ... ting-guide

[Brenjacques]

Thanks for the info GT_II, I think idle mixture makes good sense from your description. The last time I set up the carb I did it with a Colortune so not incredibly accurate but I set it somewhere in the mid range between too rich and too lean, perhaps it's still a bit too lean and with the colder weather coming it's starting to have an added affect.

Strangely I can't recall them even testing the emissions at the last mot, it passed the test but I didn't get a print out if they did. I remember when I last had it tuned by a proper specialist he set CO at 2% (from memory) which he said from experience is where they like to be at.

I certainly don't think it's a fuel supply issue or blockage as was the problem last year, it really is right at the bottom of the rev range, if you're past sort of 1500 rpm then the problem doesn't exist at all, nor does it exist if a gradual load is put on the engine at low revs, but too quick off the clutch and you'll defintiely feel it.

Will have a play around this weekend and see if I can solve it, I'll take a look at the plugs too and see if they offer any insight as I have a new set to go in anyway. I'll also book in with the carb tuner again so he can come and do it on the analyser just so it's all set up nicely.

[GT_II]

Just a thought, I'm guessing either too-lean or too-rich idle mixture could cause lumpiness at low throttle.

[Brenjacques]

Thinking about it, I reckon it may be lean. When I leave work in the evenings I usually join straight onto heavy, slow moving traffic and the choke is out just so that the engine idles at 1k - I can't say that I've ever noticed it when this is the case and it's a fairly long stretch of stop/start traffic. It only seems to be when the engine is up to temp and the choke fully in that it happens.

[Brenjacques]

Found the problem - split carb flange!

Surprised really it was only a problem at low revs, it's in a pretty bad way, easily fixed though. Just wish I kept that used spare I had now!

[GT_II]

Glad you found it - I guess the split opens up momentarily when the engine rocks as drive is engaged, then settles back down in normal driving