Splashy idle troubleshooting. Possible pre vapourlock?

[Dennismo]

I have a splashy idle with the odd intermittent lean misfires on triple SUs. Using colourtune or an air fuel ratio meter, it shows the mixture is not stable and is varying. The mixture varies more at idle and less at speed. It seems to be more splashy when warm and occurs on all cylinders.

I have done everything possible to trouble shoot. Ignition components all replaced. No vacuum leaks found, all carb components replaced etc. Float bowl vents to atmosphere ok

It occurred to me that perhaps the carb insulating block gaskets may not be doing their job and perhaps the gas is slightly vapourizing causing a surge of fuel in the jets that could explain why the mixture is varying and is more pronounced when warm.

Might anyone have suggestions how I can troubleshoot this idea? Or any other troubleshooting ideas? This is the first problem I have not been able to easily solve.

Thanks from the Colonies

[Heuer]

Could be wear on the SU spindles allowing air in. Check for play in the bushes.

[christopher storey]

As David says, a check for leaks at the spindles may be useful - an unlit butane torch applied to each end of a spindle helps - if the engine picks up speed, you have a leak . If you have a vacuum gauge, this can be very revealing . Are the needles correct ( usually UM) and are they set correctly with the shank level with the base of the piston? Are the float levels correct? Are all the jets equally down - I usually start with them set about 65 - 70 thou down from the bridge and adjust from there . Vapourlock problems usually occur before the carburetters and show themselves by the fuel filter glass bowl not being full . Also, have you made sure that the idle is being governed by the air bleed screws and not by the throttles?

I am sorry if this is a bit of a shopping list but it may give you some further ideas. Incidentally, these engines prefer richness - I usually set mine to idle at about 5% CO or something like 12.5:1 stoichiometric ratio

[Dennismo]

I have put rubber o-rings on the throttle spindles as the cork seals do not last. I have played with the float levels in various positions with no luck, needles are flush in the pistons.

I can set the mixture with my air/fuel meter and if i set it at 13.5 to 1, the exhaust note is splashy and the meter shows the mixture is varying from about 11 to 1 to 14.5 to 1.

If I ultra richen the idle mixture, the splashiness goes away but the engine bogs down

Dennis

[Heuer]

You have an air leak somewhere.

[Dennismo]

Hi David,

I have looked for air leaks as follows. Manifold off, pressure tested and surfaced. Reinstalled with petro-proof sealant between manifold and head.

Plug brake booster and distributor advance take offs, new carb to manifold gaskets. Intake manifold vacuum is steady, not varying.

Rubber o-rings used instead of cork seals on throttle spindles. New idle screw rubber seals.

I leaned out the carbs and put propane on all outside areas where leaks could happen to see if engine speeds up.

The strange thing is that the varying mixture is on all cylinders as evidenced by Colourtune spark plug

I am perplexed and out of things to try!

Dennis

[Heuer]

Have you had the carbs rebuilt?

[Dennismo]

Hi David,

The carbs totally redone with o-rig spindle seals, new floats, needle valves, needles, jets, idle screw seal, throttle butterflies etc.
The piston drop times are all within spec.

Dennis

[christopher storey]

Have you checked the fuel pressure? it should never exceed 3 lbs/sq.in and preferably should be in the region of 2.5 lbs/sq.in . Also, as some wise old bird once said, 95% of SU troubles are ignition related! Another nasty possibility : have you checked the cam timing ( and are you using standard cams)?

[Dennismo]

I took the cam covers off and checked the timing. The cams are standard. The fuel pump is a standard SU and I have replaced all its parts.

If I fill the float bowls with gas, let the car idle and turn off the pump, the splashy misfire still occurs

Dennis

[christopher storey]

A compression test is desirable to exclude some internal failure e.g. valve or head gasket. Check tappet clearances. Also , and this may sound strange, check each cam lobe to see that it is not worn - I have not seen this on an E or indeed any XK engine, but I have seen iton several Rover V8s , and it produced precisely this type of problem

[Dennismo]

Compression is good. Valve clearance not checked but I figure it must be good. vacuum text shows no sticking valves.

[christopher storey]

With all the checks you have made , it is becoming apparent that something is not as it appears . A possibility is ignition timing which presumably you have checked . The markers for this can often be misleading. A common problem if you have the left hand side timing pointer is that it has moved on its fixing ( the bolt holes usually are slotted) and thus points to TDC or whatever when it is up to 10 degrees adrift. This is easily corrected with the help of a dial gauge ( or less accurately a piece of rod) put down nos 1 or 6 cylinders to reveal true TDC . A more serious fault which can mislead is if the crankshaft damper has suffered derangement of the rubber which connects its two halves together. The outer ring of the damper then moves on its bond relative to the inner ring , and thus the markings move relative to true TDC. This also can be checked with a dial gauge

[Dennismo]

Hi Christopher,

Thanks for your ideas. I have checked the timing and advanced it until I get knocking and backed it off so I am sure it is correct.

Dennis

[Heuer]

I have a splashy idle with the odd intermittent lean misfires on triple SUs. Using colourtune or an air fuel ratio meter, it shows the mixture is not stable and is varying. The mixture varies more at idle and less at speed. It seems to be more splashy when warm and occurs on all cylinders.

Based on your earlier comments it has to be a fuelling issue. Have you tried disconnecting the throttle/choke linkages? One of the problems with the standard setup (and why several of us are keen to have a cable operated throttle system) is the engine can rock at idle and that movement can be transmitted through the throttle linkages. Which also begs the question - have you changed the engine mounts? If those are worn you will get the engine rocking and rolling at idle, it will get worse as things warm up and will be self perpetuating in a vicious circle.

[Dennismo]

I am pretty convinced it is a fuelling issue. There is play on the levers before the throttles start to open the throttles. The mixture is also varying when on the road as evidenced by the air fuel ratio meter I have hooked up and I think it is independent of engine mounts and linkage.

But the rpm is not varying at idle and if it did, I can;t see why the air fuel mixture would vary like it does, going all the way to lean.

I am thinking I will try to find someone in Canada with a spare set of SUs (not likely!) and try them out.

Dennis

[Heuer]

To be clear - have the SU's been professionally refurbished or have they been overhauled locally? I had a similar issue with my 'rebuilt' SU carbs (although in my case it was also the inability to get a stable tick-over as well as the mixture being all over the place) and in the end, on the advice of CMC, I sent them to David Lonsdale who did the job properly. Since then I have had no problems so maybe worth biting the bullet and sending them off to a reputable and recommended company.

[Dennismo]

Hi David,

I very much appreciate you taking the time to think about my problem.

I redid the SUs myself. I have worked on these for 40 years! Replaced throttle discs (and centred them using a light source behind them to make sure they are even), spindle seals, jets, needles, floats, float needles, idle screw seals etc. Throttle spindle seals are replaced with o-rings which are a little stiff but will not degrade as quickly as the the cork do.

Checked piston drop in seconds and all are good. I use 20W oil in dampers.

Needles are centred in jets (ie no rubbing). Nice metallic clicks when dropping pistons onto the bridge with the jets screwed all the way up

I have adjusted the float chamber needle shutoff lever so there is no side to side sloppiness and so that the float does not drop too low when the valve is open.

Carbs are synched with a uni-syn. I have vacuumed leak tested by leaning out the carbs and delivering propane to all possible areas. Manifold has just been off, inspected, planed and reinstalled with a gas proof sealant between the cylinder head and manifold

I called "the" North America rebuild expert named Joe Curto to ask what "flow testing" is as some advertise this. He said there is no such thing as there is no rebuild set up adjustment one can make based on flow and that the mixture adjusting screw compensates for everything variable. I told him of my problem and he said I have done everything he could think of.

If I put a Colourtune spark plug on each cylinder, ALL have varying mixture with flame colour going from yellow to blue to blue-white. This tells me that it cannot be a vacuum leak (although I have checked) as a single leak could not affect every cylinder. I have plugged the brake booster and distributer vacuum advance take-off.

Therefore I conclude that either;

1) The air velocity over the jet is wrong say due to possible wrong springs keeping the piston too high or too low at idle. I don't know what the effect of wrong spring rate would be but I have asked Burlen if there is more than one spring type that fits the carb. Perhaps there is a different rate spring in my carbs and air velocity does make a difference. I would think that if there was a spring that was too weak, the piston would rise higher at idle, and one would compensate for lower velocity if needed with the mixture screw if there was any mixture variation. If you have the time, might you measure with a set of calipers, how far down in the dashpots each of your pistons are when engine is off, and then how far they are down when the engine is idling? I could compare the amount yours have risen at idle to mine at the same idle speed and see if there is a difference.

2) OR the fuel level in the jet is pulsating up and down all the time which would explain why the mixture is varying. I have checked that the carb vents are working. If they were not venting, there would be pressure buildup in the float chamber forcing fuel up through the jet.
I have adjusted the fuel levels way up and down with no improvement. There is no flooding ever. Although I have replaced the jets and needles, I thought that perhaps the jet holder (that the jet slides up and down in when adjusting mixture or choking) might leak air up into the jet area. But the fit seems good and putting propane under that area during vacuum leak check resulting in nothing. I think that since the carbs are constant velocity, any air bleed from this area would be consistent for all rpm and compensated for by the mixture adjusting screw.

3) In searching for a cause I undid the fuel lines after the car had been turned off 10 minutes on a hot day. Fuel did spurt out of the line. But float bowls were not flooding so I believe the float needle valves were doing their job. I have a fuel pump switch and if I let the engine idle and turn off the pump, the splashy idle continues until the engine starves of gas so I don't think it is fuel pressure.

Absolutely perplexed on this side of the water!

Dennis Vancouver

[Heuer]

There are various types of SU springs differentiated by a splash of paint colour on the ends:

AUC2107 is specified for the E-Type so worth checking someone has not tried to 'improve' the car by swapping springs. Could be they used the lighter loaded AUC4587 (light blue) mistaking it for 'light blue and black'? Do you know which needles are installed?

[Dennismo]

Thanks for the charts.

I just checked the piston spring force at the specified length by compressing them against our kitchen gram scale to the correct length (as drawn on a vertical piece of paper.) All are plus or minus 5 grams to the 128 gram spec at roughly 3.875 inches. My wife was not pleased as the springs brought that workshop smell into the kitchen!

I have the same SU tuning book as you with the troubleshooting page. My pages are now very yellow as I got the book in the 1970's!

Dennis