Lack of power, hesitaiton, lean running problem

[alan12666]

Evening everyone,

I'm seeking advice on a poor running issue; I've had the engine out to refurbish box and overdrive; now it's all back in I am struggling to get the engine running right again. Car is a 1961 3.4 MOD with SU HD6's.

Symptoms: idles smoothly and without a problem for a long period; 30 minutes plus just sitting on the drive. When driving, will not pull much beyond about 65mph in higher gears (2500rpm ish), is very hesitant and juddering, and generally does not have the power it previously had.

What I've checked:
- idle speed very smooth and stable c 650-700rpm; idle CO reliably at 4.5% both carbs.
- off idle (c. 1500 rpm stationary in neutral) the CO drops to <1% which makes me think lean running is the cause of the hesitation.
- plug tips are also going white after a short drive; again makes me think lean running
- ignition all seems OK; timing spot on 7 deg BTDC, advances when engine revved in neutral as expected.
- float heights spot on
- fuel level just below top of jets in carbs
- jets setting nicely a small distance below the carb bridge, moving when adjusted.
- pistons all cleaned and moving freely, no sticking
- fuel feed OK after filter, to carbs; delivered 250ml in about 20 seconds into a jug so seems fine
- no apparent intake leaks; spraying solvent in various locations does not give any appreciable rise in revs

Current thinking (I'm running out of ideas):
- ignition component failing (coil, condenser?) but I don't think it would produce these symptoms.
- leak somewhere I can't trace (brake servo hose is about the only thing left ... but brakes working fine therefore don't think that can be leaking)
- could fuel be leaking past the cold start device at idle, so I'm setting the jets mixture extremely lean to compensate; then when the throttles open and let more air via that route the main jets have a more significant effect and it leans out as is being experienced?

Other than this I'm running out of ideas ... any other suggestions gratefully received!

Cheers

Alan

[piman]

Hello Alan,

try adjusting the mixture. The mixture screw's nominal setting is 2.5 turns down from flush with the bridge but if it still seems weak at that setting screw down a little more. If you can get the plugs a better colour also try some dynamic timing adjustment, ideally it should be retarded a small amount from the position that causes it to just pink. Do the mixture first as that will affect the correct timing. (A weak mixture takes longer to burn)

Alec

[PPV]

I would question what components did you disturb during the work? It does sound like fuel starvation at higher revs. In addition to fuelling issues what about the distributor, maybe sticking advance/retard mechanism?
I have never had plugs show white on the tips, this does suggest weak mixture though but I would have thought very light brown really as white is extremely lean. Have you dented or bent any part of the fuel pipe. I would have thought the flow rate should be a few pints per minute from the pump (depending which type is fitted) and yours gives about 3x .43pints = 1.3pints which seems a little low. I do not have the exact figure for a 3.4 but a 3.8 E type uses about 2.4 pints per min at max power if I recall.
Hope this helps a bit.
Regards,

[Ronniebiggs]

Many years ago I had similar problems with mine.

Problem traced to an extra filter, fitted upstream of the pump, which was clogged with so much crud it belied belief.

I concur your measured flow rate seems a bit low, but likewise don't have figures to hand. About 15 gallons per hour, 2 pints per minute would seem about the right amount.

BTW, having the extra filter seems a good idea as it stops sediment from the tank (which can be your tank's rust, or (frequently) debris from a forecourt), getting into the pump.

Jay

[340jag]

We spent a long time tuning my HS8 cards on my MK2 , engine ran smooth revved up freely , but on the first road trip , it acted just like yours , we adjusted the mixture (enriched) , and the engine was running 100% better on the road .

I got myself a Colourtune kit , and was surprised just home much the mixture was still out !


All adjusted up now and the engine pulls like a train

[alan12666]

Thanks all for the replies, some interesting info. I don't suppose any of you know what % CO setting you are setting to at idle?

Maybe the fuel flow could be an issue at full power but surely not cruising at 50-60mph ... I'm not doing full load runs at present, I daren't in case plugs and pistons start melting.

I've used Colourtune and CO meter and both suggest idle mixture is right.
I also believe the ignition is OK: stripped the distributor on Sunday and checked vacuum and mechanical are free to move, appear to be in good working order and timing light check shows good increase in advance as engine revs are increased.

So, plan for Wednesday / Thursday work:
- blank off the cold start device feed to inlet to eliminate that.
- richen idle mixture a few %, see what difference that makes off idle during driving
- check in more detail for possible air leaks
- start looking at ignition components (coil, condenser, leads) and timing again.

Logic and evidence to date convinces me the problem lies within that list somewhere. I hope logic is valid!

[PPV]

I know that you say that the advance etc seems Ok with the distributor as the weights move but have you tried sucking on the vacuum line to it and see if that moves the baseplate of distributor as the centifugal effect on it is only part of the story. Also have you checked that the advance is correct at say 3,000 revs?
The stoichiometric CO level should be just under 1%. However, nearly all cars run at above stoichiometric ratio as at higher revs there needs to be slightly more fuel available and therefore between 1 and 3% CO is generally acceptable in older cars. The Gunson kit I have identifies this range as 'normal'.
Looking at your plan of attack then I am not sure what adjusting the idle mixture will do as it seems that it runs ok then but it is either a fuelling issue or sparking issue at higher revs/loads.
Regards,

[piman]

Hello Alan,

S.U.s do not use an idle mixture screw, you may be mistaking the idle screw as a mixture screw. Do adjust the main jet adjusting screw to see if you can richen the mixture (there is no reason why you should not be able to do that?). Do you have a manual for your car?

Alec

[PPV]

Hi, the two screws available on the 3.4/3.8 carbs are the slow running volume screw and the mixture adjusting screw.
The first alters the amount of air that effectively by-passes the throttle butterfly and hence does affect the slow running mixture at tickover. The second one raises or lowers the main jet and this also adjusts the mixture as it moves relative to the jet needle which is shaped to allow different amount of fuel in depending on the engine vacuum which lifts/lowers the piston the jet needle is fixed to. Once both these are set the only thing which then operates is the throttle butterfly and this is what causes the vacuum to lift the piston and the jet needle. I don't mean to teach anyone how to suck eggs but if all passageways are clear, all settings ok and their are no leaks it is a fairly simple robust system. Of course you need the correct needle and piston return spring. Sometimes if someone has polished inside the suction chamber (top dome) then it will mess up the vacuum operation but otherwise it is a fairly tolerant system which takes quite a bit of abuse.
On a small point it is important that the distributor vacuum advance/ retard pipe is connected to the carburetter connection and not to somewhere on the inlet manifold so it may be worth checking this also.
Regards,

[alan12666]

Folks,

Thanks for the further input. I'm ok with the function of all the various screws and jets on the carbs (slow running vs main jet etc.) I'm just not explaining it too well in my posts; doing them quickly during lunch break whilst at the day job doesn't help. The day job is testing engine management systems on engine and vehicle so I can definitely get my head round all this(!), it's just a case of learning what the XK engine is tolerant too in terms of lean, rich etc. I've got the Haynes manual but not a Jaguar manual, and the Haynes is a bit lean in terms of exact technical spec on some points hence the learning needed; for example I don't know what correct advance at 3000 rpm is (Jaguar service manual now on Christmas present list, but I am hoping to take the car to Goodwood this weekend - at this rate it's not happening!).

Regarding vacuum advance: simply sucking on the pipe doesn't create any noticeable change of advance (running) or movement of the plate (non-running), but neither is there any noticeable air leak. From this I would judge the diaphragm is presumably intact but possibly not moving internally; everything else is definitely free to move. Anyone know how much advance change the vac advance should be able to invoke when active please?

Cheers

Alan

[piman]

Hello Alan,

assuming you have the correct vacuum module on your engine, there are three sets of numbers stamped on it, maximum advance, start vacuum and maximum vacuum.

Do you understand the princile behind the vacuum advance? Simply it is used as an economy device, with the carburettors set to provide a leaner mixture at cruise which requires more advance for smooth running. It does not work at idle\low throttle positions or wide open throttle driving.

Alec

[alan12666]

Thanks for the clue re the numbers. They are 8-18-7 followed by 54412438.
This morning I rechecked vac advance. I definitely cannot get it to move by sucking on the pipe. I therefore tried adding some static advance, and the driveability was improved at part throttle which is hopeful.
Other stuff to do for the rest of the day now so will have to wait till tonight for further work; wondering where I can get hold of a vac advance unit if poss before Friday.

[alan12666]

Also attached is a pic of the plugs after 10 mile drive this morning.

[piman]

Hello alan,

what I forgot to mention, vacuum advance is quoted as engine degrees unlike distributors which are quoted as distributor degrees with one distributor degree equalling two engine degrees, a bit confusing. As is Jaguars cylinder numbering sequence of that period with number one being the rear cylinder.

Alec

[XJR308]

I assume you have checked the fuel filters?
Bruce