Setting the mixture on Carbs by ear?

[neilbardsley]

I tried to have a go at adjusting my weber carbs on my Targa on Sunday. I believe that I'm running a little lean and that perhaps my idle is a little too low about about 600/700rpm. The carbs are only a few months old. I have the colour tune thing but that is in Daventry and the car lives in London. My understanding is that at you increase the fuel in the mixture the car revs higher but then gets to a point where it is too rich and rev lower? I adjusted a single screw, No 4, about 1/4 of the turn but didn't hear the engine rev any higher so I tried 1/2 but still no real difference. I then tried adjusted back to about 1/4/ to 1/2 leaner then it was before but the revs didn't seem to slow. I tried briefly with cylinder 3 but no real change in the revs? Now assuming that the mix screws are ok since the cars are new it must be my ear? I ended up moving each screw about 1/4 turn richer and adjusting the idle screws in a little but didn't really heard much of a difference.

What am I doing wrong?

The only reason that I'm doing this is because when slowing down, from speed, above 3k rpm if I take my foot completely off the the accelerator then occasionally it backfires. Some had suggested this was because it was switching to the idle circuit while others and thought it might of been leak in the exhaust system.


PS I set the timing about 1-2k about, but maybe that is worth checking, and it's running an optical dizzy so no need to check the points gap.

[Nige]

Personally I'd take it to a proper old school carb tuner. Even though I've tried many times in the past it's all black magic where proper experience counts. No use for you but I found a local guy who tweaks MGs and allsorts of multi carb stuff. He listened to my carb running 912e and knew within seconds what was lean, rich or out of balance. An hour or so with him and £50 had it running perfectly.

Back firing when off throttle is usual an air leak in the exhaust

[MikeB]

You need a carb balancer, or if you want to be really old school a piece of tubing to listen to each choke.

Follow the PMO installation guide, it's good enough to apply to the Webbers.

http://www.pmocarb.com/installation.htm

if it is backfiring, it could be because one or more cylinders are running weak, sometimes the offending cylinder might spit back thru the carb when idling.

[911hillclimber]

10 x 10 for the above.

Balancer, PMO instructions and Halfords colour tune (yes) and the job is done.

That though will cost you about £5 more than going to a real carb man, but DIY is good.
Did my 3.2 on carbs this way and it is great.

[mycar]

Personally I'd take it to a proper old school carb tuner. Even though I've tried many times in the past it's all black magic where proper experience counts. No use for you but I found a local guy who tweaks MGs and allsorts of multi carb stuff. He listened to my carb running 912e and knew within seconds what was lean, rich or out of balance. An hour or so with him and £50 had it running perfectly.

Back firing when off throttle is usual an air leak in the exhaust

Personally, I agree with Nige, I think you need professional help.

I messed about with my carbs for seven years and just got to the point where I was getting a feel for them.... but for the first five years I needed professional help too.

The tuners I used, Bob Watson and Mr Bogg, had not only had the ear but also the rolling road and electronics to measure Co2 and hydrocarbons throughout the rev range. It's all very well listening intently to a quarter turn of a mixture screw but you really need to know how the engine is burning the fuel as it goes up to the red line.

The tricky bit is finding an old school tuner within striking distance. Bob Watson was 180 miles away from me, Mr Bogg a more comfortable 60 miles away, have a look around and see who you can find or ideally take a recommendation.

Oh and you can only really tune a carburetor when you've stripped it down to it's component parts and built it back up again with your own bare hands.

Apparently.

Regards, Mike.

[911hillclimber]

Just seen where you live.
Get the car to HiTech in West Midlands, Mick has the old schooling and a good rolling road.

www.hitechmotorsport.co.uk

mick@hitechmotorsport.co.uk

I've been there and used him (and Bob of course)

Jets for some webers are hard to find and most carb tuners will not have them to hand.
Tripple choke down-draught carbs are not an everyday sight...

ps
my mixtures were spot-on after using the colour tune.

[mycar]

Colourtune is an excellent tool but it won't diagnose unresponsive mixture screws, blocked jets, plugged passages, air leaks, badly adjusted float levels, fuel leaks, loose throttle shafts, loose or cracked manifolds and all those other things that can make tuning carbs a belt biting nightmare.

Taking the car to a pro will give you a base line from which to work.

What Webers are these Neil ?

[Nige]

If it's any use this is where I took my twin Dellorto carbed 912e
http://www.burgesstuning.free-online.co.uk/

The guy is really into his cars, engines, etc and was very friendly.

[911hillclimber]

Yes, Colour tune works when the carbs are in 'good condition' but a tuner will have a fight if the carbs need some TLC
If they need a rebuild kit then that must come a first action.

I assumed the carbs were good, just need a skilful tweek.

[mycar]

Back firing when off throttle is usual an air leak in the exhaust

This is most likely your problem though, a leak anywhere between the tailpipe and the head will cause popping on lift off.

[neilbardsley]

I should have a pretty good baseline to work from. The carbs are nearly new and setup when the top end engine work was done. I just wasn't happy with them when I first drove it. Unfortunately Mrs B drove it the 100 miles home but when I got in the car I knew I had to change it.

I'm now very happy about how it handles through the rev range but sometimes backfiring. The chap that set it up has offered to look but I haven't made it over there and to be honest I'm keen to have a play myself. I think the answer is to bring the colourtune down to London. Unfortunately my garage doesn't have lights which will make it interesting but I will try to have a go.

I did take the no 3 & 2 plug (I couldn't be asked to do 1 & 4) out of the car the other day and I can't say that they looked particular dry or as you would expect from a lean cylinder.

[911hillclimber]

Rough rule but after a good warming and a crisp healthy run pull the plugs (do not allow to idle too long) and check the colour.

Light grey/biscuit is good, sooty is of course bad and time for action.

The colour tune is surprisingly sensitive and the colours are vivid.
Petrol blue is the target, black = no combustion (too weak/too rich) and yellow is just too rich.
Give the engine time to adjust to a carb setting change, ie 5 seconds minimum.

All good fun and interesting to get right.

[butzsi]

1/2 or 1/4 turns will not make that much difference unless that particular choke is already too weak/rich. Get it to full operating temperature and then turn the mixture adjusting screws one by one in and out until you can hear the change in rpm and feel the increase in smoothness of the idle - if no difference then that idle circuit /jet is probably blocked or compromised, finally when you are happy with the results rev the engine and observe if the idle comes back to what you had set it at . If it falls below normal idle speed and then rises back to normal after a few seconds then the fuel mixture is too rich and you need too weaken off the idle mixture screws a little at a time until this no longer happens.

Ard of hearing/ Appy to Adjust NW10

[neilbardsley]

1/2 or 1/4 turns will not make that much difference unless that particular choke is already too weak/rich. Get it to full operating temperature and then turn the mixture adjusting screws one by one in and out until you can hear the change in rpm and feel the increase in smoothness of the idle - if no difference then that idle circuit /jet is probably blocked or compromised, finally when you are happy with the results rev the engine and observe if the idle comes back to what you had set it at . If it falls below normal idle speed and then rises back to normal after a few seconds then the fuel mixture is too rich and you need too weaken off the idle mixture screws a little at a time until this no longer happens.

Ard of hearing/ Appy to Adjust NW10

Thank Chris

[neilbardsley]

Following on from chris' where we balanced and set the carbs I got in the car today and it was running strangely. Coughs whenever I started off. Pretty fail but there and more backfiring when slow down. Foot off the gas. When I got to Daventry I had another quick go. Carb 2 was very responsive to the idle mix 1 wasn't at all! It didn't react at all! Blocked jet. The carbs are new so hopefully not the idle circuit? Should I check the value gap?