Mystery mixture

[keithmac]

I am stumped...
1967 E-Type, triple carb
rebushed spindles last year and rebuild kits. Just getting to drive it now ( rebuilt engine ) Spark plugs show extreme lean ( white ) on 3-4 cylinders, yet main jet is 0.150” ( three times lower than the 0.055” of other two ) Suspect an air leak after middle carb
Man am i frustrated
Propane could not find an air leak. Two hours later , I am dizzy from carbon monoxide inhalation!
Get this, you know how you can lift the piston to check for mixture. Not a fan of it but I tried it for fun. Lift front or rear carb 1/16” engine slows, lift the centre piston to the top NOTHING HAPPENS AT ALL
No change in RPM !
Cover the centre carb , it stalls. Air draw is equal ( unisyn ) on all at idle with similar idle screw settings !!!!
Checked float -OK, checked needle valve =OK not sticking, checked overflow tube-clear, checked compression 160psi
Needles all UM, calliper matched lengths. All O-rings checked for position. No fuel leaks. float settings double checked @ 7/16”. Piston drops all 7 secs.
When you move the jet with dash pot off the fuel squirts up 2 inches in the air…….
Fuel starvation above idle?? Still an air leak after carb???
This car hates me ;~)
Thanks,
Keith

[bitsobrits]

I assume you have tested the piston function per the manual, and properly centered the jet assembly and the needle is not hanging up or dragging on the jet. When fully raised (using a finger inserted into the carb opening), the piston should fall back with a soft but distant "clink".

Once you have exhausted all other possibilities: I once received a new needle (in a set of 3) that was either incorrectly machined or mis-labeled. Drove me nuts for a bit.

If these are new needles, try swapping the needle (or the entire vacuum chamber and piston) with another carb and see it the problem moves.

Of course always carb keep pistons with their original, matched vacuum chambers.

[Tom W]

Is your firing order right? You don’t have the middle 2 plug leads transposed?

[tinworm]

I had a Bee in the fuel line just short of the carb - could only have got in there when the pipe was off for a week (awaiting parts I think - it was a long time ago!) Made the middle two cylinders run lean.

Barr

[keithmac]

Hi Steve
All pistons clunk and are free moving, no marks on needle just check ( again ) OK.
Tom,
Just verified firing order traced to each cylinder per manual from dist cap now, all OK.
Barr,
I was thinking fuel starvation too ( a bee? really ? wow ! ), as in possibly an obstruction in the fuel to middle carb too. How else can you lift the piston to the top and see NO change in RPM??
How else would I get near white spark plugs on 0.150" setting on the jet when the other two carbs are 0.055" ??
Next step -check fuel line to carb, then strip the middle carb, although looking at the diagrams , I cannot see for the life of me what could block fuel in the lower part of the carb...
And , as always , thanks for the suggestions ( keep em' cumin' )

[Tom W]

How is the choke set? Is the mechanism set so it’s not still on when the engine’s warm? Are all the idle speed screws set approximately the same? I’d run through the basics again before pulling too much of the carbs apart.

[keithmac]

Hi Tom.
Yes choke has been set and double checked as per shop manual and visual for main jet operation and fast idle clearance set to .002" on the screws .
I have gone over everything twice prior to posting not wishing to waste members time ( not to mention trolling the archives for a few hours ! )
Fuel to carb manifold is off and one small flake of debris in middle feed , but not enough to block or slow flow I think. I was really hoping for an insect of any type !
I now suspect a blockage from the float bowl to the carb as the brass filter in the float bowl is missing on this carb and not the others. Also when I got the car the fuel pick up in the tank was utterly disintegrated.
Taking the carb off is no big deal so I will get at it in the morning and report.
Cheers

[mystery type]

Piston dampers stopping the needle lifting?

[christopher storey]

You haven't been doing the plasticine trick to check the piston fall time have you ? People do this and then forget to remove the plasticine plugs ! personally, before you take the carb off, I would unscrew all the piston dampers but leave them in position, then start the engine and see how much each rises when the throttle is opened. If the middle one is not rising, it is a suction problem. If it is rising, then it is likely to be a fuel problem. Another check is to run the engine at a fair speed e.g. 2500 rpm, and then suddenly block the centre carb with the heel of you hand and see what happens. Colourtune would be a great help if you can borrow one. So would a vacuum gauge, which can be a mine of information

[abowie]

Another air leak possibility is the slow idle screw. I have one on one of my cars that even fully screwed down still leaks air.

Have you checked the idle airflow through the carbies with a gauge?

[keithmac]

Yup all carbs checked with gauge at idle. All idle screw washers checked .What I did find today was an oddly taller fuel bowl float and a sticky new needle valve in the middle carb ( after removing the carb of course ). These things may have lead to fuel starvation/lean mixture despite the lower/richer jet setting. I had a spare new float for my MG TD which Joe Curto tells me is identical to the Jag. I will start up tomorrow and check all again and report.
Thanks for the input

[keithmac]

[keithmac]

So as a test I switched jet needles between carbs. The problem followed the center jet needle to the rear carb where I placed it . Here is why

The problem jet needle is on the left. Both needles clearly marked UM. The rebuild kits were ordered from an extremely reputable north American specialist in late 2017, I have been trying to diagnose and solve the problem ever since. I do not blame the specialist , as he likely does not manufacture the parts. I will let him know.
The lesson here for all of us is to inspect all replacement parts. NO MATTER WHAT THEY ARE. I have paid dearly in time and frustration more than once for trusting new parts. I have had this car for three years now and have only 500 miles on it as a result , first faulty brake parts that mimic air in the lines, then this. I await a new jet needle....

[Tom W]

That’s progress. It’s difficult to see in that photo, are the calipers gripping both needles, so for a given diameter, the distance down the needle from the reference line varies?

[keithmac]

Yes Tom,
If the needles are set correctly in the pistons , the one on the left would result in a very lean setting as mentioned earlier in my post. I also have some old UH needles the car came with and they are identical at the level you see in the above photo and get thinner ( richer ) farther towards the tip as you compare with a UM.