Setting up MGB carbs with Colourtune

[AceOfHearts]

My MGB has been running lean since I have had it (a few weeks). I discovered this due to the white plugs and also by it being barely driveable when cold (clears after a few minutes though and the engine is warm).

Today I tackled the setup with my trusty colourtune, but have a few questions since this is my first car with twin carbs. As per manual I set the tickover as best I could using a piece of tubing in the inlets to make sure both carbs were roughly equal (listenening to the hiss), and loosened the clamps to make each carb independant.

Colourtune in cylinder 1, and it was burning very blue, and took a good amount of turns in to get it to run rich (red). I then backed it off again to get it to the recommended blue with a flick of red but the adjustment was not that precise. Considering it says do a 1/4 turn at a time, it could take 1 1/2 turns to make any discernable difference. I set it as best I could though and turned my attention to the rear carb.

Colourtune in cylinder 4, and this one was slightly lean but not as bad as the front one, again I set it as best I could and readjusted the tickover for both carbs to make it roughly equal.

My question is that does it make much difference the exact setting on the SU HIF carb? Would half a turn in or out make any real difference? It seems to run well but I won't get a chance to run it from cold until the morning. Also are there any good gauges out there for balancing SU's instead of just doing it by ear?

I guess being an old, understressed engine it is probably not too critical, and if it feels alright it probably is, its just I want to get it at its optimum. Is the only way on a dyno or with an exhaust gas analyser?

Maybe I should just leave it where it is (roughly where it should be) and just keep an eye on the plugs, see what they are telling me and adjust from there

[v8250]

Colourtunes are very good kit for this job - I too have HIF4's on my 'B. You've set up correctly as per instructions for best idle speed and fuel. You should also check off-idle and mid range revs with the Colourtune but only once the engine's warm. Once done, and providing the ignition is set up, you'll notice a very clean/smooth pull throughout the gears with the little 'B.

If you have access to an exhaust analyser, shove one up the exhaust for final check...but only after taking the 'B for a good blatt to clear any over-fuelling during set-up.

Re: carb balancers - Gunson do an air/carb balancer that works very well. Readily available from good suppliers - though do shop around as prices vary significantly. Well priced units can often be found from Amazon vendors. Good luck with tune up

Edited by v8250 on Wednesday 27th March 20:55

[Slidingpillar]

Some engines 'bark' through the carburettors as they start to come on cam. Have a listen to a carb as you raise the revs. If the MGB does this, it's a simple adjust balance to get both carbs to flow the same - just raise the revs on the throttle screw to provide a fixed point and when done, reset the idle.

[V8 FOU]

Here's a better way.
Raise the dashpot using the little sprung plunger underneath. Raise it by approx 1/32" the revs should rise slightly, then fall. If the revs just rise = too rich. If they drop = too lean. Very simple - been doing that as a basic method of setting for decades! The adjustment is quite critical - one flat or two at a time.
If they are "waxstat" jets, change them to the old type as these will weaken off the mixture as the car gets hotter.

[Richie Howard]

sorry to hijack, but i am interested in v8fou's comment.

my car leans right out when hot (i assume it is leaning out...). it happens really badly after a motorway run, when stopped in traffic and the heat has had a chance to build up. so much so that i need a bit of choke to idle. i have just lived with it, but if is easy to swap the jets, i would love to do it, as i will be driving to tuscany and back in august and i am a bit worried about the hot weather. i have read about bi-metallic strips and the like, but like i said, i have just lived with pulling the choke out until the inevitable k&n/exhaust upgrade/carb rebuild is on the cards.

[Motown Junk]

v8250 said:

Re: carb balancers - Gunson do an air/carb balancer that works very well.

Edited by v8250 on Wednesday 27th March 20:55

Obv not an advertisment, but I do have an unused, boxed one of these gathering dust in the garage....

[shambollic]

Richie Howard said:

sorry to hijack, but i am interested in v8fou's comment.

my car leans right out when hot (i assume it is leaning out...). it happens really badly after a motorway run, when stopped in traffic and the heat has had a chance to build up. so much so that i need a bit of choke to idle. i have just lived with it, but if is easy to swap the jets, i would love to do it, as i will be driving to tuscany and back in august and i am a bit worried about the hot weather. i have read about bi-metallic strips and the like, but like i said, i have just lived with pulling the choke out until the inevitable k&n/exhaust upgrade/carb rebuild is on the cards.

If you have the waxstat jets then these can be a nightmare. Better to change to fixed jets for starters. Also if you do change to k+n's it is best to richen the needle (AAA I think) and swap the springs to yellow. Do you have HIF's or HS4's?

[Richie Howard]

hif4s

it would be nice to fix, as apart from this, the car is running as well as it has in my ownership

• awaits lightning bolt*

[davepoth]

Richie Howard said:

hif4s

it would be nice to fix, as apart from this, the car is running as well as it has in my ownership

• awaits lightning bolt*

I think I know what this is.

The jets on a HIF 4 will be Waxstats. They're designed to lean out the engine when it gets hot. That's not especially desirable, especially since when they get old they appear to lean the engine out too much. I had this problem on my Dolomite 1850HL.

I took the jets off, carefully prised off the waxstat cover on each side, and replaced the waxstat with two pennies. It's a bit of a fiddly job, but only took an hour or so and when it's done it stops the leaning issue.

[AceOfHearts]

Motown Junk said:

v8250 said:

Re: carb balancers - Gunson do an air/carb balancer that works very well.

Edited by v8250 on Wednesday 27th March 20:55

Obv not an advertisment, but I do have an unused, boxed one of these gathering dust in the garage....

PM Sent

[AceOfHearts]

Thanks for the replies everyone

I tried moving the slide up slightly but really did'nt notice much difference and really didn't understand what I was looking for, hence just went with the colourtune as its quite easy and I understand the theory behind it. I will report back with my plug colour and cold running tomorrow, and maybe have another check.

[nta16]

bear in mind the engine don't just run on the carbs (or the rest of the car for that matter)

carb fiddling is a pub wisdom game

check and adjust (clean) if required, in this order

tappets - CB points - plugs - timing - and mixture last

above are best done as part of or just after a service (don't forget servicing the rest of the car is more important that the engine)

with full and proper servicing, maintenance and repair and frequent driving then subject to the dissy, carbs and other parts not being too worn you shouldn't need to fiddle with the carbs other than possibly at the annual service or when replacing components or parts that effect the chain

here's two John Twist videos that might help with (the principles of) setting the carbs up -

147 MG Carburetter Tuning - http://www.youtube.com/watch?v=4nvGLgO6pj0&fea...

35 Tuning HIF Carburetters - http://www.youtube.com/watch?v=ASeMfXfjNpw&fea...

here are another 5 useful videos that explain about them and what to watch for if they've been taken apart before (by a PO perhaps)

44 Secrets of the SU Carburettor Part 1 of 2 - http://www.youtube.com/watch?v=7GRAcqDySog&fea...

43 Secrets of the SU Part 2 of 2 - http://www.youtube.com/watch?v=60Bj_2cZQnc&fea...

29 SU H-Type Carburetters - http://www.youtube.com/watch?v=GHNlT5yHDxk&fea...

41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

123MG Carburetors & Doors (first part needles and seats, springs and carb oil) –
http://www.youtube.com/watch?v=v9QI3NlvwiY&fea...

ETA: when you get the engine/carbs set up right it will sound right and as you diviate away from that point and back to it you'll know the difference

personally I don't like the Colourtune but it'll easily get you near enough until the next big service and set up by which time you'll have settled the car in more with use

Edited by nta16 on Thursday 28th March 02:12

[AceOfHearts]

Thats a lot of reading :lol:

I have just given it a full 30,000 miles service (first thing I did) so the tappets are all set, plugs are new, distributor is electronic and timing is spot on

Looks like i have some bits to keep me busy tomorrow

[nta16]

AceOfHearts said:

Thats a lot of reading :lol:

you can boil it down to two vids for setting carbs but those others might help highlight something that may throw up

AceOfHearts said:

I have just given it a full 30,000 miles service (first thing I did) so the tappets are all set, plugs are new, distributor is electronic and timing is spot on

great you'll have cleaned and lubed the carbs, linkages, cables and replaced/cleaned air filters then

did you check engine breather pipes and oil cap to be clear and clean?

AceOfHearts said:

Looks like i have some bits to keep me busy tomorrow

get engine fully warmed up and keep blowing the carbs out as per vids, overfilling the dashpots with oil is just a waste as the excess will just blow out, try using 90 weight gear oil (not as thick as you might think) as the man says

good luck, I'd still suggest you go by sound more than colour, if parts or components are worn you could spend forever trying to get it just right and trying to keep it there - get it to where you're happy with it or as near as you can then drive the car for another month and reset again if required, after that leave until next annual service to check or you'll be fiddling with it every change of weather and driving conditions, carbs are always a bit of a compromise, get them to a consensus then leave them there

Edited by nta16 on Thursday 28th March 05:43

[V8 FOU]

Richie Howard said:

sorry to hijack, but i am interested in v8fou's comment.

my car leans right out when hot (i assume it is leaning out...). it happens really badly after a motorway run, when stopped in traffic and the heat has had a chance to build up. so much so that i need a bit of choke to idle. i have just lived with it, but if is easy to swap the jets, i would love to do it, as i will be driving to tuscany and back in august and i am a bit worried about the hot weather. i have read about bi-metallic strips and the like, but like i said, i have just lived with pulling the choke out until the inevitable k&n/exhaust upgrade/carb rebuild is on the cards.

As i said, replace the jets with conventional ones. I would get a full service kit for both carbs - about £65 and replace the float jets too as they can also cause problems.
Back to the OP, have you tested the compressions?
Also, some later MGB's have the diaphragm thing on top of the inlet manifold for emission purposes. Either remove it and blank off, or replace the actual diaphragm and clean out the goo from inside.
Last point! Keep to original air filters, as those shiney pancake things are very good for pulling in loads of warm air and restricting the flow. Not good.....

[paulhemming999]

V8 FOU said:

Here's a better way.
Raise the dashpot using the little sprung plunger underneath. Raise it by approx 1/32" the revs should rise slightly, then fall. If the revs just rise = too rich. If they drop = too lean. Very simple - been doing that as a basic method of setting for decades! The adjustment is quite critical - one flat or two at a time.
If they are "waxstat" jets, change them to the old type as these will weaken off the mixture as the car gets hotter.

I think that you are bang on with that advice chap-good to be cast back all those years to the "old" way. Another thought-- Is the old SU carb service centre still alive in Birmingham??