Hi all, can anyone shed some light on why my engine drops from 900 RPM in neutral to just under 500 RPM in drive ? I set the idle at 750 when warm but when the engine gets hot it rises to around 900 in park or neutral and a very noticeable jerk when engaging drive or reverse. I've got the motor in reasonable tune but anything less and the car would stall at idle in gear. I've fitted a 123 Tune + ignition and recently fitted a brand new AED on the HIF 7's which at the moment seems to be ok apart from said problem. Had this issue amongst others before fitting the AED and 123. Engine is in good shape as far as I can tell so i'm a bit baffled on this one.

 

As long as you had vacuum removed from distributor, and the line to throttle body capped for the initial timing, then it seems that you have a lean mixture at idle. Either mixture adjustment or vacuum leak.
1. At initial 750 RPM, spray some carb cleaner at the usual places around carb and manifold, to see if a vacuum leak is the issue.
2. When engine is hot at 900 RPM reduce idle speed on each carb a 1/2 turn at a time to get the 750 RPM, then adjust mixture equal amounts to get back to 900 RPM, then adjust idle setting back to 750 RPM.
I think these two quick tests, may shed some light on the situation.
Rgds
David

 

I think Dave is on the right track. I have my HS8s set quite lean for good fuel consumption. The result is (when engine hot) an idle 850-900rpm in N and around 600 and a bit lumpy in D. I could smooth it out if I went a bit richer, but the car goes extremely well and gets good mileage.

 

Many thanks to both of you for your advice. Fitting the new AED & 123 bluetooth distributor has made a huge difference other than idle issues as stated. If the car had a maual box I'd live with it as performance beyond idle is good. However the big drop in RPM, stumbling and erratic idle in drive is a nuisance especially the odd occasion when it stalls and I do think the idle was better before these replacements. I've sprayed carb cleaner around obvious points and no noticeable difference in revs. I've also double checked static timing at 8 degrees BTDC which is spot on. I believe older 123's had to be set at 0 BTDC but newer versions have the option to include static timing in the app. I have to say that instructions and info on the 123 are a bit vague, lacking detail and not entirely perfect English. Regards vacuum I've checked the pipes to distributor and also brake vacuum lines etc. which seem ok. I've also checked and adjusted the carbs with colourtune so that idle is blue, revs increase is yellow and steady higher revs is blue.-- No real difference, although I know this isn't the most accurate test. I've also checked and adjusted idle balance on carbs. It sounds like erratically missing a spark at low rpm, and as mentioned there is a big drop between neutral and drive, however this has always been the case and I'm not sure why. I'm fairly certain the engine is in good order, has only covered 55k miles with documented history and sounds good. There's no pinking or run on and starts well cold or hot. Despite having a brand new AED I do wonder if this notorious device might still cause a problem, although it seems to work well in so much as initial start sits around 1000 rpm and gently lowers as it should plus the steel inlet pipe goes from cold to warm within a few minutes. I fitted new NGK plugs same time as the AED and I've also tried the previous Champions but no noticeable difference. Coil is a new Remax Sport and HT carbon leads were replaced a couple of months ago. One thing that does spring to mind is I didn't fit a new cork seal to the 123, although I think this is mainly to keep things out of the distrbutor shaft and probably not relevant unless its absence creates wobble in the shaft ? Any thoughts appreciated

 

Remax ES3 Ignition 12V Sports Coil Push In MADE IN ENGLAND
Ignition coil Remax ES3 Made In England. 12V Sports coil
Primary resistance Ohm 2.9 Secondary resistance K Ohm 9.2 Replaces Lucas DLB105


Complements the needs of the 123ignition high performance ignition system:
3.0 Ohms for 4-cylinders up to 6,000 rpm
1.5 Ohms for 4-cylinders > 6,000 rpm and all 6-cylinders
1.5 Ohms for 8-cylinders up to 5,000 rpm

1. The optimum coil for 123 system is 1.0 - 1.5 Ohms. If you have a 2.9 Ohms coil you will have weak spark. You want the lowest Ohm coil, that will not over heat your distributor electronics. 123 web-site says not to go below 1.0 Ohm.
I use a DLB198 with a Pertronix module with my S3 OEM distributor, which is .8 Ohms, and allows me to open plugs .005 in over book for hotter spark.

2. you may want to try 10 degrees static to see if any improvement.

3. For the SU HIF7 carbs, I have no experience and will let other members comment.
4. If you can check compression, one low cylinder would give you similar symptoms at low RPM.
Rgds
David

 

Hi, I’m a bit late to this post, but if you still have the idle speed drop issue when selecting ‘D’ and you’ve eliminated all other potential inlet manifold air leaks, then try this.

Remove the SU carb housing chambers, piston springs and pistons. This is so you can easily see the carb throttle butterflies. When looking at the butterfly, is there a small poppet valve & spring visible on the throttle disc? (See pic below) If so this is the emission control valve. It was originally designed to allow additional air into the manifold on the overrun, to weaken the mixture for emission purposes. However with age the springs soften and allow the valve to open at a lower point of inlet manifold depression. Hence the increase in idle speed when selecting drive. Over time the symptoms get steadily worse.

New throttle butterflies with valves are available from SU Burlen. However the discs are also available from Burlen without the valves and it’s these I’d recommend. Plus they’re much cheaper.

 

Hi David, thanks for the info. What I've fitted is a ballast coil (Remax ES4B) with 1.4 ohms primary resistance. I'm not geat with electrics but assume with the ballast resistor this should be ok ?

Regards, Rob

 

Yes, that is the recommended coil, I didn't see that one listed. The 2.9 Ohm coil would be for points distributor.
You should not need the ballast resister with this coil and 123 distributor. The ballast resister was to improve performance of coil without overheating the points of standard distributor.

Rgds
David

 

Hi XJJK, it's a frustrating scenario. After fitting the 123 ignition and new AED the engine ran faultless for around 100 miles. Very pleased with myself I said to the wife fancy a drive out ....she's running really well. Kiss of death, for no apparent reason backfire through carbs and wondered if we'd get back home. I think my next step is take apart the suction chambers and pistons, clean things up, level up the jets, adjust accordingly and see what happens, although I've done this before !

 

Hi Rob,
I feel your pain! My experience was similar and I too have a 123 Ignition fitted and I did have a new AED, but ended up removing it completely and fitting a manual choke linkage.

Good luck!

 

Hi Gentlemen,
Today I've removed the dashpots, pistons etc. and cleaned them thoroughly with carb cleaner. I then wound the jets up to level (HIF7) and then wound both in two turns of the screws which I believe is the starting point for these carbs. Unlike other SU's winding in HIF7's enriches the mixture. Re-assembled everything and the car started fine. Checked the mixture with colortune and looked ok with blue at idle, yellow when revved and blue at steady revs and yellow with further revs then settling to blue. One thing I did discover is the vacuum pipe that connects to the hidden underside of the black inlet manifold is badly split so i've snipped the end off. This pipe was connected to the air intake and the vacuum pipe from the the carb inlet was connected to the distributor. I've swapped them around and it doesn't seem to make much difference, although they are now quite taught. Do you know which one should be where ? Again, the engine runs well through the rev range and idle in neutral, but it still drops 400 RPM when D, R, or 1&2 are engaged and is likely to stall. It seems to bog down under load on tickover but has plenty of power on the move, doesn't burn oil of any significance and no nasty noises. I will blank off the new AED as my next step and will see what happens. Many thanks in anticipation of your thoughts

 

To isolate idle mixture from initial timing as to possibly causing the low power at idle.
1. Richen mixture 1/2 turn on both carbs and see if you get the same RPM drop when shifting to Drive. Then try again with 1/2 turn towards lean (from original setting) and see if the RPM drop changes.
2. Move mixtures back to original settings, and advance static timing by 4 degrees, again to see what the RPM does when you go from Idle to Drive. Don't rev past 2000 until you get back to recommended timing.

Overall, I think you have a pre-smog aggressive distributor curve(s) at higher RPM and a post-smog lean carburetor. Lean burning or late timing will move max combustion pressure past best power at idle.
Rgds
David
PS: do you have the P/N and or model number from the OEM distributor you removed? I will see if I have the curves for that distributor to compare idle advance with 123 unit.

 

Hi Rob,

In answer to your question concerning the two vacuum pipes, it sounds like they were already connected correctly. In other words the take-off below the inlet manifold runs to the air cleaner valve & the one from the rear carb body goes to the distributor. However with a 123 Ignition this is not normally required and can be blanked off, because the mapping curve in the distributor can compensate.

I have a question though; does the idle speed in neutral increase significantly once the engine is up to full running temperature?

 

Hi David
According to a reciept I have for a full re-build in 2012 the distributor model number is 45D6 41616. I've tried adjusting the fuel mixture as suggested and advanced the timing (via both the timing light & the 123 app) and no real difference, Running ok-ish under normal driving conditions but bogging down / stalling in drive & reverse.tickover which is a pain

 

Hi Jules,
As you suggest, I think the vacuum pipes were in the right configuration but despite fixing the split pipe didn't make any difference. Yes the idle speed does increase significantly when fully warm and then drops more than it should in D or R to the point of stalling I've isolated the AED when warm which makes no difference at all so in that respect I'm pleased this is working as it should given the bad reputation. As mentioned before, after fitting the 123 and brand new AED it ran fine but things suddenly deteriorated. Occasionally I can hear a sucking sound when the engine is about to stall.

 

Hi Rob,
It does sound very much like the emission valves in your throttle discs have had their day.
From my experience with this, you can set the idle just right. You then go for a drive and get the engine nice and warm and when you return and drop it into park, the idle slowly creeps up towards 1500-2000 rpm. If you put it back into drive it’ll drop to about 500 or so and feels sluggish.
Another item worth checking is the length and strength of the carb piston springs. Again overtime these get compressed and weak allowing the pistons to rise more readily.
There’s a simple process to check these, but you’ll need the base data for your carbs. If you want further info just say.

 

[QUOTE=roberlar;2663119]
Hi David According to a reciept I have for a full re-build in 2012 the distributor model number is 45D6 41616. I've tried adjusting the fuel mixture as suggested and advanced the timing (via both the timing light & the 123 app) and no real difference, Running ok-ish under normal driving conditions but bogging down / stalling in drive & reverse.tickover which is a pain[/QUOTE

From a recent post on Mark 2 forum.
Jaguar Mk2 1961 ……” My distributor is marked „Jaguar XK 15-09-10“ (from 2010). Curve „0“ was set on the internal micro-switch. Contact with the manufacturer revealed none of the curves is optimal since they all start advancing the timing at a much higher rpm than the original mechanical distributor. They recommend using Curve „0“ and set the timing at 8deg BTDC.I have done this but have the feeling that the engine is not running as well as it did with 5deg BTDC. I will try to optimize.
Hope this helps “

Excerpts from “website” ( https://sucarb.co.uk/technical-carbu...e-settings-hif }
Type HIF Carburetor Introduction: The type HIF (Horizontal Integral Float Chamber) carburetter has been designed primarily to meet the requirements of exhaust emission control carburation systems……
Mixture Adjustment The jet tube of the HUF type carburetter is moved in the vertical plane to provide mixture adjustment only. Unlike previous types of S.U. carburetter the jet tube is not lowered down the jet needle to provide cold start enrichment. The jet adjustment assembly is comprised of a right-angled adjusting lever of unequal length arms riveted to a bi-metal blade The blade is cut out to accept the jet head (3) and the shape of the jet head is formed so that any movement of the bi-metal blade is transmitted to the jet head, moving it in the vertical plane…..

Fuel Temperature Compensation (Viscosity Compensator) This device alters the jet position in relation to the metering needle to compensate for changes in fuel viscosity which takes place with changes in fuel temperature. When the fuel temperature rises, the viscosity is lowered, and in an uncompensated assembly, this would allow more fuel to flow for a given jet/needle relationship. In the HIF jet assembly the jet head is attached to a bi-metal blade (1). This bi-metal is immersed in the fuel in the float chamber and will move in the vertical plane in response to changes in fuel temperature. The jet will be raised to a weaker position on the jet needle when the fuel temperature rises and will be lowered to a richer position when the temperature falls...

1. I checked my data base for Lucas distributors. there was no internal timing data for 45D6 # 41616 distributor listed, listed for use on Jaguar–Daimler 4.2 Limo.
2. The two bi-metal blade mechanisms would be suspect to me, as well as internal carb-leaks associated with rising temperatures.

Rgds
David

 

Hi David,

I double checked and re-set everything including the 123 ignition settings and carb mixture. Starts fine but not running well.. Previous plugs i've used are Champion N7YC & NGK BP5ES.
Tried Champion RN11 YC and a significant improvement however revs still drop from 750 rpm in park to 350 rpm in drive, although not stalling at such low revs so can only assume carbs and timing are good but something is dragging engine power down in gear? Double checked all vacuum ports and replaced one way valve and pipes (which were bad) to T junction at brake servo but no difference. Generally better with a different choice of plugs but things are still not right. As ever, it's the drop in revs in drive which I can't fathom out.

Regards,

Rob

 

Rob
I don't have a new idea, but I did check 123 website for the Tune+ distributor. Most of the issues in the past have been with the switch unit where you picked a curve. The tune+ model actually wants you to establish base line from OEM data. I down loaded software, but it won't work with out a distributor USB'd or Blue tooth'd. The info is in good European tech speak. You have their charts in advance staring at zero in the distributor as apposed to crank degrees starting at top dead center. Standard vacuum references have to be converted to absolute pressure.
Here are 4 curves that are the best of Jaguar and Pertronix for the XK engine. The issue with smog engines and carburation is: Pre-Smog carbs ran rich of Best Power and there were many needles 3-5% apart for getting mixture right at idle as well as top end. Smog engines run lean of Best Power and have leaning mechanisms to allow more air as engine and carb warm up. Very few needle options, most later needles, were for more emissions reduction, not performance.
When you get this sorted we all will be looking for how you tune the curves for mech and vacuum advance.
Note: I would not hesitate to take the Pertronix D171618 up to 10 degrees static advance. And move the 41912A down to 14 degrees static advance for non EFI engine.
Rgds
David

 

Rob,
Some theory on why power is low at idle.

Just a thought: Idling at 750 RPM, move to trans to Drive and momentarily the RPM drops below 500. Below 500 the idle Advance immediately drops out, back to static. This drop of 2-3 degrees at idle is enough, that the engine doesn't recover back to 600-650 ??? A mechanical distributor advance probably could not react that fast??
Rgds
David