Gunson Colortune

[Highroller]

Hey guy's,
Has anyone successfully used the Gunson colortune to tune a set of twin carbs on their bus. I currently have a webber ICT34 twin carb setup on my DJ 2.1 which i feel could do with a tune up. The engine runs a bit lumpy on idle with a slight misfire noticeable on acceleration . I've checked that the timing is correct at 10 Deg BTDC and it has recently had new plugs, leads, rotor arm and dizzy cap. There are no vacuum leaks and the advance on the distributor operates when sucked on. I have always felt that the left hand bank has ran slightly too lean. The engine was rebuilt for the previous owner almost 4 years ago by a very reputable VW engine builder but I'm not 100% certain if he set the carbs up.

[mr_nunn]

I just picked up a Colortune to try and do the same on a CU engine. Will let you know how it goes.

Suspect it will get you roughly there but would need a garage/gas analyzer to set exactly to spec. Gunson also do a CO tester but it's a hundred quid. (And only does CO not HC).

It does seem like the Colortune could have some diagnostic uses as you can view individual cylinders. Eg if one bank only is not behaving.. where an exhaust analyzer only gives you an average.

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[Highroller]

Thanks mr_nunn. If you could keep me updated that would be great.

[mr_nunn]

I had a go with the Colortune over the weekend.

Firstly - it is very fiddly if your plugs are buried (as on the CU) - the Colortune plug itself, the long thread adapter, and two washers, all have to join together to go in, but none of it is designed to stay together when you unscrew the plug from the head - so to cut a long story short I spent a lot of time retrieving the various bits and fishing for washers with a bit of wire in the area around the spark plug holes ... (NB the Colortune is also too small for a standard spark plug socket, so my nice magnetic socket was of no help...)

The tuning itself - well, one trivial point is it's quite fun to see the burn! I didn't get a massive amount of variation in colour - couldn't get the rich yellow at all except when pumping the accelerator, even with mixture screws way out - but this might be something odd going on with my setup - the yellow seems to come up in various Youtube videos. I'm pretty sure there are no air leaks left but perhaps I have some other issue. I guess that's a plus point - it hase given me a useful warning sign of a possible issue.

It isn't that obvious to distinguish the correct blue from the paler/whiteish lean blue, but you can sort of see it.

Going from one bank to the other is a pain because of the aforementioned access issues - two Colortunes, one for each bank, would probably be better.

It does seem to be running ok after my effort, so I'd say it has probably helped to get a rough tune but I won't feel totally confident about the tune till it's checked by garage.

Interested to know if you decide to try it highroller and if so how you get on!

[mrhutch]

weld (or get one welded) a lambda boss in the exhaust(s)

then tune by electricity?

[silverbullet]

Running lean on left hand bank? Leaky brake servo hose or servo itself?

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[mr_nunn]

Running lean on left hand bank? Leaky brake servo hose or servo itself?

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I'll double check but thought I had excluded a brake servo hose leak when dealing with my stalling at idle (which now seems better - see other thread).

To be more specific, here's what I did/found:

Haynes said to set the lower individual carb mix screws, you disconnect the idle air solenoid (i.e. cutting out the central idle circuit) and pull/block the retard hose, then set to 3-5% CO. So I disconnected solenoid/blocked hose, and figured in this state each individual mixture screw should give a good range from yellow to blue on the Colortune on that side; I planned to richen it to the edge of yellow on each side and then turn back in a little from there... but it didn't go to plan as could not get it rich enough to see yellow on either side (swapping the colortune between banks). So just had to settle for a few turns out each and a reasonably bright blue. Still not sure why it was refusing to go rich.

With the central idle/retard back in play, Haynes says 0.3-0.9% so here you are in pale blue territory on the Colortune - which isn't as easy to detect as the yellow colour change - I'm not sure it's any easier than just weakening the central idle mix screw till the idle drops and then coming out a touch, which is the other rough method that people seem to use.