The 'E' Type Forum

Chaps,

The engine tuning continues and looking at the plugs - I think I am running a little rich as they have a dark, dry residue on them which in modern engines would imply the fuel is not 100% burned.

Trouble is if I weaken the mixture the engine doesn't run as well. I have a high output coil and electronic distributor so the spark is pretty decent...

Does anyone on the forum achieve the light grey/brown colour that spark plug charts show?

XK engines need to run richer than most, with a stoichiometric ratio of about 12.5 to 1 rather than the 14.7 which is usually promulgated as ideal. If you have a CO meter, I should aim for something of the order of 4.5% to 5.5% CO at idle, which should show very considerably weaker readings ( circa 1% ) if the engine is run light at say 2000 - 2500rpm. Modern fuels, by the way, rarely produce the colours which used to be expected with leaded fuel

Thanks Chris - I miss the tail pipe colour you used to get with leaded fuel...

the "lift the pin and watch the revs" doesn't work for me either so perhaps I will simply stick to the "seems to run best" setting :)

Raising the piston a little doesn't affect the revs at all for me. A BIG lift wil stal the engine down, but nothing else has an effect whether I lean or enrich the engine beforehand.
I've just got a Gunson colortune. I don't know if others here have tried it, or if it's any good, but I'm going to give it a go next week.

No affiliation; I have spent 30 years trying to tune SU's using the manual, but one of these is a game changer.

http://www.ebay.co.uk/itm/Gunson-Gastes ... 3f17acc9c7

abowie wrote:No affiliation; I have spent 30 years trying to tune SU's using the manual, but one of these is a game changer.

http://www.ebay.co.uk/itm/Gunson-Gastes ... 3f17acc9c7

did look at these, but was told it only looks at total output and thus doesn't tell you how the inmdividual carbs are set up. Is that not right?

I use it in both exhausts which gives you front and rear.
Because the middle carb isn't measured you have to extrapolate a bit but this isn't impossible.
My approach is to carefully set all 3 carbs back to the starting position in the manual and go from there, trying not to let them get too out of sync. I use a flowmeter to set the slow idle screws to get them all as close as possible to each other.
The assumption you are making is that all 3 are pretty much identical; with carbs in good condition that are properly set to srtart with you should be reasonably close. If you're not convinced you can measure the jet height with a micrometer and get them very close.
I've only had my CO meter for about 12 months but it really has made a huge difference. For example setting up the newly rebuilt carbs on my rebuilt 3.8 only took me about 15 min and it seems to run perfectly. This was not the case with my 4.2.
I haven't used a Colortune yet to check the 3.8; it's still not complete and I can't run it ATM.

I have used an earlier Gunson infrared CO device for years. As Andrew says you put it in both exhaust pipes alternately. It does in fact take mixture from the centre carb in both cases because the centre carb feeds the rear port on the front manifold and vice versa. Also, on the 4.2 at any rate it is always taking some mixture from all 3 carbs irrespective of which pipe it is in, because of the large balance port in the manifold . Thus all 3 carbs really should be altered together. I set mine with the jets about 65 thou down from the bridge and start from there ( and note that because of inaccuracies in pitch of the screws, a one-eighth turn on one screw may produce a variance of e.g 5 thou , whereas on another it may only be 2 thou ! )

Thanks. Looking forward to having a go with the Gunson, equipped with some of your tips. Also got a flow meter on order so will wait for that before the Grand plan.

abowie wrote: Because the middle carb isn't measured you have to extrapolate a bit but this isn't impossible.

I should have said "measured independently"

Thanks Andrew. If the engine was burning a little oil, or if it was a bit carbonised, could this lead to high exhaust readings even where the carbs are properly adjusted?

Many thanks Gents - I will search out a fuel/air meter, I am sure it will pay for itself eventually :D

malcolm wrote:Thanks Andrew. If the engine was burning a little oil, or if it was a bit carbonised, could this lead to high exhaust readings even where the carbs are properly adjusted?

I'm no expert, but I'd guess that burning oil might well increase the CO independent of carb adjustment...

Well, got the kit and had a go. I was very pleased with the SU flowmeter; showed the differences where I thought I had them perfectly synchronised. Easy to read, so got all 3 carbs breathing the same. Interestingly, although they tick over the same, the flow meter shows that the front carb sucks less air than the other two when you run at a steady, say, 2000rpm.
Anyone know if that's normal? The throttle linkages all start to open at the same time when you increase revs.

Not quite so pleased with the color tune. It certainly worked, and told me I was running rich (which I expected) Adjusting the mixture (leaning), you saw the colour change. Got them all running nice and blue, and although car runs well, it doesn't tick over so well. Getting the mixture right slowed the tickover (should be the other way?) and made it a bit lumpy. Suggests the car likes running richer than normal, although as I said, it drives well; just the tickover that's a bit up and down. Any ideas on this as well?

It always seems to be a compromise Malcolm, we leaned the mixture off on the 4.2 the other day by about 1/4 of a turn and the engine wasn't quite as nice on tickover, but still good. It depends how long you spend at idle really