Tuning question....

[ChrisC]

It was sunny today so I took the car out for a spin following an initial try at tuning the carbs.

In normal driving up to 2800 revs the car is lovely but it seems to be reluctant to want to pull past 3000 revs and the carbs spit back. I doubt I have the carbs right yet but as I am running in the engine I cant put it under a heavy load...

Its also slightly reluctant to start when hot unless I use the choke at the halfway position... my gut feels is that all this pointing towards a weak mixture?

Tuning engines is not really my forte so I am open to suggestions

Cheers.

[christopher storey]

Chris : I assume you are on UM needles . This is the first thing to check if you have not already done so . Also, before checking the needle itself , see that it is positioned in the piston with the shoulder of the needle flush with the base of the piston . Whilst doing these checks ( the needle designator is on the shank of the needle at its very top, so you have to remove the needles from the pistons to see it, and a magnifying glass is also a help ) check the jet positions with no choke on . I start with the jets about 65 thou down from the bridge and then tune from there . At that level, you should be able to see fuel in the jets, but not overflowing the jets , if the float levels are set correctly. This datum should give a setting with even running throughout the rev and throttle range, but it may be slightly on the rich side . The symptoms you describe , however, may also ( indeed may more probably ) be connected with inadequate ignition advance . I cannot now remember what system you are running , but at 3000 crank rpm , total advance excluding any vacuum advance should be of the order of 30 to 34 degrees before TDC for a 3.8 and about 27 to 30 degs for a 4.2

If you are using a 123 , particularly a 6 cylinder generic rather than a Jaguar dedicated 123 , it is common in my experience for the curve used to be very much less advanced than the standard distributor i.e you need to use a more aggressive curve than you think

[ChrisC]

Thanks Chris - so for the fuel to be visible inside the jet, logically the float level must dictate how high the fuel is within that jet.

So I first need to ensure that with all jets 65 thou down from the bridge - the fuel is also at the same level in each of them.

I will check all teh fuel side first.

Anyone used a colour tune successfully on an XK engine? I know I have one somewhere....

[Heuer]

Yes, I have used a Colourtune but you have to use it three times e.g. cylinders 1, 3 and 5 or 2, 4 and 6 to get each of the carbs correct.

[christopher storey]

Chris : I have just edited my post so I suggest you have another look at it. Don't get too excited about fuel level in the jet - it is indeed governed by a. the float level and b. the jet setting, because the lower the jet, the higher the fuel level is in it. The important thing is to ensure that it is not flooding by spilling over the edge of the jet into the throat of the carb . Also , my 65 thou down is just a suggestion - some people go slightly more down than this, but I would think that 70 thou down is about as far as is desirable for an initial setting . I have used colortune , and it is useful as a rough guide for idle setting - it certainly shows you if there is a gross discrepancy, with overrich being a very definite yellow and weak being a bluey white colour - what you are looking for at idle when hot is a bunsen burner blue flame , and to be honest I have never found this all that easy to achieve without ending up with mixture further up the range being a bit weak, , but you can come close to it. Something else I should warn you about is that I have found that the jet adjusting screws can be inconsistent in their effect - a quarter turn on 1 carb might alter the setting by say 5 thou, whereas it might be 2 thou or 10 thou on another carb , and it is perhaps wise to check this with your depth gauge before you start to tune

One further point connected with David's post above : because there is quite a significant balance pipe effect between the carbs ( perhaps more so on a 4.2 - I am not so familiar with 3.8 setups which have different manifolds ) , each time you alter one carb's setting, there is also a fractional effect on the other cylinders ; hence it is better to make small adjustments on each carburetter in turn

[ChrisC]

Okay - float level forks adjusted to 11mm. Jet depth set to 65 thou and I can report that the fuel levels within the jets varied from flooding over the top of the jet to about 4mm down. None of the floats appear to have fuel in them (indicating a hole) but I am guessing they are floating at slightly different heights to cause this issue. So I have re-adjusted the floats so that the fuel level is now about 2mm from the top in all 3 jets.

Starting the car was definitely better and the engine idles with a nice smooth beat but pressing the lifting pins on any carb stalls the engine which implies the mixture is too weak.

Sadly my colortune shows yellow on all 3 carbs regardless of where the jets are set which in theory means the mixture is always too rich but the lifting pins say its too weak...

Not oo sure why but I cant actually get the colortune to show a shade of blue for any carb regardless of the mixture setting so for me that tool is not working.

Aaaaaaaargh this tuning malarky is frustrating.

Ignition is set to 8 degrees with TDC having set with a depth gauge (not sure what the tools real real name is).

I also cant work out why the rear carb is sucking in more air than the middle and front carb. Shutting the the idle screw right down on the rear and opening the mid and front ones quite a lot gets it close but not perfectly equal but the idle speed is then too high. Ho-hum.

Wish I knew what i was doing on this stuff...

Not too much of a problem whilst I am running the engine in - but one day I would like to use the whole rev range

[tinworm]

Hi , it sounds as if you have an air leak (maybe more than one) as the engine is drawing in air while the throttle is shut and idle screws closed down . Most likely the intake manifold to head joint and carburettor to manifold joints need looking at - another less likely candidate is valve guide wear as air can be drawn down at this point (amazing but true) and this will upset the idle. You need to get all this right as a starting point and then go on from there. As regards your colour tune this cannot be wrong as it shows only the colour of your engines combustion flame. It is entirely possible for an engine to be rich and weak depending on engine speed and air fuel settings of either adjustment or incorrect parts . I had a year or two of fun with the Weber carbs on a Ferrari of mine that would not idle properly and had a stumbling pickup - I eventually found that the idle jets which were correct to the parts book were too small and I needed to go up a size or two- this solved the problem. Ferrari used to send cars out with the carburation settings as a guide - it was up to your mechanic to get it to run correctly !
regards Tinworm

[tinworm]

Forgot to say in my post another likely SU problem is worn carburettor spindles and seals allowing air in. You can determine any wear by trying to move the spindle end from side to side or up and down. Any movement means wear I am afraid and the carbs will need to come out for remedial treatment.

Tinworm

[Peter B.]

Hello,
try to throw in my personal experience here. In my case missing carb damper oil was as well playing it?s part. With the same symptoms you have described, but maybe that is too easy.
Regards
Peter

[christopher storey]

Chris : if you suspect an air leak from eg spindles or indeed from any manifold joint, it can be confirmed by using an unlit butane torch played near the suspect areas ( do it outdoors so there is no build up of gas on the floor ) . If the idle changes - usually by speeding up - you can be pretty sure that there is a leak where you were playing the torch at the time the variation occurred

[ChrisC]

Ah - thats an interesting idea Chris - the butane obviously being nice and flammable and will increase revs if sucked in...

For clarity I notice the additional airflow when I put a meter on the mouth of the carb - i.e. the reading is higher than the other 2 carbs indicating more air being sucked THROUGH the carb inlet as apposed to more air sneaking in from spindles or elsewhere. Its almost like there is better air flow through this one carb for some reason.

I'll get there plenty of time to sort it over winter.