Hesitation on pulling away from stop

I have a slight problem of the car wanting to stall when I let the clutch in, if I do not give it some revs.
which seems very odd for an engine that should have a lot of initial torque. Once driving it appear to pull ok.
It is an XK120 with the timing set at 10 BTDC with vacuum and about 700RPM, HD8 Carbs with UO needles, Jets set about 1/16 to 1/8th down. It has the Rob Beere special mild camshaft designed for running with SU carbs. It has an electronic ignition sold by Denis Welch, which has the gentle curve from 10 degrees up to a max of 34 degrees at 4,000 RPM. It does have a slightly lightened Flywheel and first gear ratio is 2.95 on the Tremec T5 gearbox. I have checked the cam timing, ignition timing,
tried the jets up and down. Manifold reads about 14”. Also if I let the engine idle with the temp at 80C it soon climbs up to over 90c unless I put the electric fan on. This is with ambient air temp of 20-25c.

Is there anything obvious I am missing or not tried. Car does not use oil or water !!! Leaks a bit oil.

Not a mechanic, but my gut feeling is that it's a fuel issue. Could it be down to some bad petrol? There are a few members more experienced than I am who will no doubt weigh in on the matter.

Hi Steve, not bad fuel as drove 300 miles this weekend, also filled up twice and it has been like this sometime.

Does it pop out the carbbies when it bogs?
14" isn't much....my 2.4 had that, ended up being 30 degrees of inlet and exhaust cam advance.
Piston springs is a thought...when did this start doing it and what was the last thing you changed before it occurred?
What dashpot oil...and I didn't read that you had checked that....as far as leaking a bit of oil..can't help you there.

Hello TOC

No poping out of the carbs.
The springs are the recommended Blue/Black for the HD8
oil is the correct SU SAE20

The car has always been hesitant.

Always. Well, let me say this. You change cams and all bets are off.
Manifold vacuum drops, and guess what happens to the pistons with "correct" springs with less manifold vacuum?
Factory was SU H8, if I recall on 120's...so a special cam to run SU's is a different way of describing the bump stick.

Sometimes...you have to just use the "book values" for things like needles and springs as a starting point. I have boxes of "track kits" from the factory for track tuning carbs.

Oils....sometimes....I have gone lighter (ATF) or heavier (30WT) to see if there is any change.

I'd start with a fuel issue first.

Here in the states, we have 10% to 15% alcohol added to our gas and it plays havoc tuning an older engine. And they change the percentage summer and winter. Before you start changing things, if you have the same alcohol problem, see if you can find some alcohol free gas. We have local auto stations that have it special and boat marinas use it for outboard motors.

Phil.

except he says:
The car has always been hesitant.

Therefore.....one might surmise variation in fuel have been dealt with

Long time, ongoing problem, with cam changes and possible different carbs (H8 to HD8, and I can't recall), low vacuum, yeah, we can dump different fuels into the tank for a long time......

Interesting comments about fuel. I have been using Shell V power and have tried BP Ultimate and also Tesco 99. I have investigated the amounts of Ethanol and i believe the BP and Tesco do not have Ethanol, but it is hard to find out if the Shell VP does or doesnt?.

What does interest me me is the comment from TOC about carb spring and carb oil.

I am now wondering if it is worth trying another strength of spring. The blue/black is the recomended one, but i will call up Burlen who are the suppliers/manufactures of the SUs and ask for their opinion.

The hesitation or flat spot on take off could be caused by the piston raising too quickly or not enough ??? Which way ???

Here's the deal on springs and tuning (remember I haven't done this in a LONG time):

The smaller Brit car crowd has all sorts of tuning tricks for SU's....and since piston pull is generated by engine vacuum on throttle opening, if you have all of 14" of vacuum, and the spring are for 18" of vacuum, the piston will not pull up right away..and you get a bog.
That is how I am remembering it, but again, been a long time. Too much up and it pops, too little up, not enough fuel for the requirement, and it bogs.

Remember what was it, my first question? Does it pop out the carbbies?

That's too weak a spring (often, but not always) and the piston comes up too fast.

There should be a colour on the spring showing rate...find a chart showing springs for an SU H8 and an HD8, try one tension lighter, do NOT cut your old springs.....or a buddy who has some lighter springs.

If I remember....winter and summer took different springs if tuned to the edge.

Now, the Jag carbs I am using now (stock part numbers and tags) have totally different characteristics than yours...and probably none of them 64 years later have the same jets in them anywhere (and there are...remember...5 jets per, and two carbs....and you never know if someone has drilled one out until you check....so idle, mixture, bleed, bypass, and choke enrichment can be anywhere...and nobody puts stock jets into one of these and expects it to actually run any more.

Ad to that, when is the last time you saw accelerator pumps on an XK engine powered Jag?

Someone else will correct me if I remembered that wrong....no problem. Just getting you to think outside the box as far as "original springs" with other mechanical issues changed.

I think TOC is on it. Low vacuum. Need to check the springs. Fuel mix is set,timing set, ignition set. Might start figuring why you have to blip throttle to release clutch. Sounds over rich and blipping throttle clears the fuel.

Hello TOC, not sure what you mean by the five jets ? My HD8’s have one 0.125 main jet. There is a separate passage to to the volume screw for idle as the HD8 does not have idle adjustable stops. The butterflys are totally closed on idle. The mixture is adjusted by a screw on a side lever. The EType has manually adjustable choke, but i have the separate electric cold start that was on most other Jaguars and works ok. The HD8s are different to the H6’s which have screw idle stops and nut adjustable mixture. The standard carbs on the 120,140 were H6’s but with some with H8’s. Unfortunately i have two HD8’s, so i am a sort of cross between standard and an E type, but with two carbs not three !!!!!

The spring i am using in the HD8 is the blue/black 4.5 ounce. The only other alternatives are thicker at 11 and 18.
Both Burlen and Southern Carbs say i am using the correct one for the HD8

i have done a compression check and they were all about 170lbs, BUT it has been suggested that i do a leakdown test on all cylinders to try and find out why i have low vacuum.

further to Larrys comment, it is not blipping the throttle, but having to give it a few revs on pulling away. It will not just move off by letting up the clutch. What i will just mention again is that i have a slightly ligthened flywheel.

i really appreciate all these comments and sugestions as it is helping me to think about other alternatives ideas.

I took it you were using the throttle to release. My bad. Seems like a lightened clutch car has to be revved on take off or raise idle. I have 2 with idle at 850. I know tires might squeal but works. Vacuum with a hotter cam is usually lower, tinker with idle might be able to overcome it. I have had to redo thinking on stock and do a whole curve of timing, fuel and carb intakes to bring the vacuum up. But, I think you have to overcome the lightened flywheel, as you don't have the massive weight in centrifugal force anymore.

OWD724 said:

Hello TOC, not sure what you mean by the five jets ? My HD8’s have one 0.125 main jet. There is a separate passage to to the volume screw for idle as the HD8 does not have idle adjustable stops. The butterflys are totally closed on idle. The mixture is adjusted by a screw on a side lever.
i really appreciate all these comments and suggestions as it is helping me to think about other alternatives ideas.

Click to expand...

The purpose was an example, if you read it again. None of the extant examples appear to retain factory jets, either numerically or size.
Plus accelerator pumps.

And downdraught.

But as easy as the SU's are, when you start any changes (cam, ignition profile, exhaust restriction) you have to start over on settings and hardware.

Larry and TOC,

yes, agree with what you are saying.

The engine in my car was rebuilt about 10 years ago with D type spec cams which were 280 degree duration and 375 or 385 lift. They were great at 4,000 rpm but terrible around town. Now have the Rob Beere SU cams that are similar duration to original XK cams but very slghtly higher lift of 410. The torque low down has certainly increased in the 1500 to 3000 area, but feel less power above 4000.

This weekend, i am going to try thinner and thicker oil in the SU dashpots, advancing timing to 12 and retarding to 5.
also have a set of slghtly richer UVO needles to try. All good fun experiments. But i bet none of this changes the manifold pressure. Thus the next exercise will be the leak down test, the equipment i dont have so will take to friendly Jag garage to see if air coming out of either exhaust, carbs, oil filler cap or radiator. If it is any of those, its going to be expensive !!!!!!

There are more things to look at. Have you done piston drop tests? Is there a drag in the bores? Is the needle centered or dragging on the jet?
I don't think bog will be addressed by jets unless you throw two handfuls of sizes at it, and then you'll end up with runability.

Oil is basically a shock absorber. No oil or too thin, piston will jump (along with attached needle) and warble with any load change. Too thick an oil will cause long delays in piston actuation.

Drop test.
Oil.
Springs.

You can change timing all over, but I don't think you are going to overcome bog with timing.

Especially since vacuum and mechanical advance are nil off idle.

Yup, done the drop test - ok
oil is the standard sae 20 supplied by SU.
cannot get lighter spring than the current one of 4.5oz - they are not crushed or stretched. Wire diameter 0.04”
the thicker spring is 11.5oz and wire diameter of 0.048”
jet size is standard 0.125”
jet is down 1.25mm below base

Just a WAG; could the throttle shafts be leaking air and need new bushes?

Any number of things. I still can't quite figure out what a cam for SU's is...since they came with SU's.
If one sets the cam position without making certain your are TDC, you can (and will) end up with low manifold vacuum.
On my XK engine, the front damper pointer is adjustable...but the flywheel pointer (if installed correctly) is not.

Having to blip the throttle to get the pistons to pop up is indicative.

The piston springs for OTHER marques using SU's should be available.

You can pull the air cleaners, start the engine, and slowly raise the RPM's and see that the piston is doing. If it pops up, weak spring. If it takes a lot to get it to move, stiff spring.

Shaft leaks...yes....low float levels, one other thing.

Just doesn't sound like ignition. You can disconnect the vacuum advance line and see if it makes any difference.

What is the intensity of the spark at the plugs? Not that it would make any difference....but one never knows.

Yes, i am going to look at the SU pistons to see how they move up. No aircleaners, just open gause trumpets.
spark plugs are perfect colour of light brown. I have checked for shaft leaks, but also trying to find someone with a stethascope to check for other air leaks ? Maybe i should go to see doctor and borrow a pair !!!!!

The cam timing has been checked twice, by taking off the rocker covers, taking out spark plugs and using a dial gauge to find exact TDC. The sprockets on the camshaft end wheels were adjusted and set with the correct XK engine plate with the notch. The camshaft in the car is pretty standard, but with very slightly higher lift, so that should not be the cause of the ‘boging down’ .

Other than the things i said i am going to try this weekend — although in typical UK spring weather it is going to rain all weekend ? —- i am thinking more to the low manifold pressure as some sort of major engine problem. So, i am going to get a leak down test done.

What i can say, is the engine uses very little oil, no water and sounds ok. Oil pressure hot on freeway at 70mph is 50lbs.
fuel consumption in town is fairly bad but it is a heavy 120FHC. On the freeway at 60 -70mph it is getting 22mpg in top gear. It has a Tremec 5 speed.

i hope you all have good weekends. We in UK are all getting ready for the big XK70 celebrations at Shelsely Walsh hill climb. They are going to get 700 XK’s together for the weekend. Cars are coming from all over the world. The meeting is in June, so need to get the car ready by then. My XK120 FHC is one of only 194 right hand drive cars built and one of only 24 in dark green, so pretty rare compared to all the over 2,000 LHD cars sent to USA in 50’s.