All,
I have just completed a tune-up on my MK10 (Coil, timing, plugs etc.) and adjusted the carbs with Uni-syn and and Livingston’s method (used color-tune as well).
The fires up nicely when cold, but is rich at idle when warm and has a cruise surge.
I am considering looking into the Jet needles. Not sure what is in there now (supposed to be UM’s), but would like to know if anyone knows of a good source for the alternate jet needles for the SU HD8 Carbs (preferably in the US).
Thank You
Usuals have them as does Joe Curto in NYC, I’ve bought from both.
I had a similar issue with my car which was adjusted to run well cold but overly rich when warm. It was an issue with adjustment more than needles.
Is the engine rich at idle and lean under acceleration or some other combination?
Before I worked on the tune-up, the car was ridiculously rich at idle, and would surge at cruise speeds.
Part of the issue was the Enrichment device. I have not been able to tune that properly, but turned the screw (jet) most of the way down to get it not to flood the engine as it is pretty warm here and realistically I won’t be driving it in the winter (I will correctly tune the enrichment once the normal operation is sorted out).
I adjusted the carbs as indicated above, and thought everything was just fine. Starting was great, idle speed when warm was 800 (car has A/C). I let the car cool off, and put the intake and tube back on. The next day I started it. Again, very good but when it warmed up it would not hold the idle to the point of stalling. I had to richen the mixture and adjust air to get it to hold the idle. Now it smells rich at idle. I have not taken it out on the road (still on lift checking the front wheel hub bearing play) I have not checked the float level (don’t really want to take the carbs off right now) but was just starting to research where I could get needles if needed and only found the UM on some sites.
I would be curious about your experiences and what you ended up having to fix/adjust.
I really wish these carbs had vacuum ports! It would make tuning better to have all the parts of the intake in place for sync and fuel adjustment.
Thanks
I’m not an expert in diagnosing these carbs but I wonder if it’s floats or something else making it hard to adjust. You can set the floats without removing the carbs, just loosen the caps and take them off. You adjust them by bending the little arm and then reinstalling them.
I never used my enrichment device as I live in CA and it’s never cold. But I had my carbs set to start and run cold but then WAY rich when warm. I didn’t know the difference other than getting horrid milage. I put O2 sensors in the pipes to know for sure. I ended up having to go up to richer needles as I set the idle to be in a good range when warm the engine was lean under load. Then I had to adjust the floats because under heavy load they went lean even with richer needles. So it’s a few different things that contributed to getting it close to perfect (still not perfect but close).
Thank you for the info, it sounds very familiar to what I am dealing with. I will check the float level before I try to tune again. Just out of curiosity, what elevation are you normally driving, and what jet needles did you end up using? Do you call Joe Curto? I tried to find a to order things, but seems to be limited on-line interaction.
Thank you again for sharing your experience.
If UM is stock for the 3.8 MK10, (check FSM), I doubt changing your needles will rectify the issue
The MKX throttle mechanism is very complex…check the return spring situation, and also the external “jaws” that activate each butterfly are all the same…outlined in FSM…but not easy to achieve
I live in a warm climate, similar to coastal California…If you set you carbs rich enough to not need the ASC at all, they will be too rich imo.
It is very debatable how long the ASC should be run, and therefore how it should be set
…and how the car is used…lots of short trips do not warm up the engine, I have found it takes 5-10min for the engine to warm up, more when ambient is cold
I live in So Cal, so sea level and warm all the time.
I ended up with UB in the front carb and UE in the rear carb but that’s because for some reason my rear carb runs lean under hard acceleration. I think it’s because the engine tilts back slightly so when the car is accelerating the fuel shifts back into the bowl on the rear carb, meaning I need a slightly richer needle.
I also have open exhaust and open intake / cold air box so it gets more air than stock.
On my 3.4L Mk 2, I had a similar problem. After I had carefully set the carb mixtures at operating temperature (water at 70C), the engine would not idle well after a cold start. I checked that the thermal switch opened at 35C as specified. Above 35C and until reaching at least 50C, the idle speed was low, around 300RPM.
The engine is a new rebuild and running an EDIS system with carefully timed ignition.
I fitted a manual switch to operate the enrichment solenoid and while this does take care of things, I still was not happy. So I made up a replacement thermal switch that opens at 50C and now do not need to use the manual switch.
I suppose the Jaguar method involves running the carb’s with a rich idle when at temperature so that idling can be maintained during warm-up. Since I do not like a splashy exhaust I tend to set the carbs so that as soon as the piston is lifted, the engine dies.
BTW I checked my temperature gauge readings using a thermocouple under one of the screws holding the Otter switch and I was pleased at how accurate the gauge was.
I have previously balanced the carbs using a UniSyn but its readings are jumpy so it is difficult to be precise.
I just bought a Synchrometer by STE and it gives very nice steady readings. However for use on the Mk2 you have to remove its rubber cone and make up an adapter so that it will fit in the restricted space.
OK, so I haven’t had a chance to check the float levels yet, but took a bit of time looking at all the linkages and carb assemblies tonight and noticed a few things. On the front Carb intake, the air/vacuum tube sealing gasket is misset. This may explain why I could not really turn the idle down very far to do the tune initially, and may explain some of the issues I have noticed with the vacuum controls of the heat/air system. I also noticed that the throttle linkage arm top clip was misset causing the rod to be engaged at the bend, and not the flat portion. This would cause uneven throttle engagement to the other two carbs.
I am going to order the gaskets, clean the linkages, and adjust the floats before attempting to adjust the mixtures again.
Thanks to all for your insight,
Double_A
The 420G…and presumably MKX, have a system of vacuum control that has a “reservac”, and one way valve. (for the heater valve and plenum). It serves no useful purpose…check FSM for description
This setup should be removed entirely and replaced with a one way valve…your system will work better, and less clutter in the engine bay
(chuck the OEM gear in the boot)
Tony, could you elaborate? If I understand, you are suggesting removal of the vacuum tank, otherwise leave the system the same. This same system carries forward to the XJ6 BTW. Why do you suggest this? Without the tank, the plenum (by which I believe you mean the scuttle vent) will open and close as the car switches from acceleration (no vacuum) and cruising (vacuum). Windshield wipers used to do that years ago on USA cars. So will the heater valve, although this would have no effect on average. I’m probably missing the point?
I went thru this with a gent, and yes I believe the S1 XJ is the same
the (plenum) reservoir serves one function (as per FSM)…it allows plenum operation for ~30secs after the engine is switched off…who needs that?
In addition the one way valve becomes ineffective due to age, and leaks, last I checked they were either NLA or expensive
You will note I said replace the one way valve…I bought a pack of 10 off Ebay for about $5, suitable for aquariums
Insert that in the line instead of all the other junk
You will be rewarded by a plenum that positively jumps to attention.
The one way valve holds vacuum from the manifold, preventing the plenum rising and falling
(just as the old one way valve did)
The expert doubted me, but I have proven its operation over the last 5 years or so
Removing the resevac tank is an advantage, as it is right in the way on a 420G
note we are clearly not talking about the brake resevac
Tony,
Thank you for the info. I have a '62 MK10, and it has the separate vacuum accumulator on it. For others, it is mounted to the right hand inner fender under the front carburetor. The vacuum line feeding it is the vertical line coming off of the vacuum manifold on the carb. assembly. I agree with your feeling that there is no need for heat/air controls after the engine is stopped. I will remove the bits and replace with a check valve to see if that helps with the controls.
All Series Jags have the reservac tank, I had it disconnected for a while and since I have a little leak somewhere, it would lead to the recirc vents opening on acceleration - cold feet. If there is no leaks at all it should work, unless you want to change a setting on an uphill stretch or so and the system loses some or all vacuum. It does have a purpose.
Still I wonder why not to keep it and add another one-way valve in line with the original one?
Unless the tank is leaking, which would be quick to fix.
The engine seems to maintain enough vacuum to keep the plenum door open at all times.
Admittedly, perhaps I haven’t run it at WOT for more than 30secs…(my test run hill is a good one, but I have to back off a tad on the sweeping curves)
The main reason to get rid of it, on a MKX/420G, (or S1 XJ with tri-carbs @Robert_Wilkinson ) is the reservac tank is in a location that makes some maintenance tasks difficult, and , as I said, the system seems to work perfectly without it.
I haven’t checked recently, but the OEM style valve was NLA as far as I can remember when I did this, meaning a substitute needed to be found…not sure if that has changed…the OEM valve has a rubber diaphragm…dont know if it is available now?
I do not advocate throwing the parts away. They should be retained so that all may be returned to original. I attempted to repair the original valve by making a new diaphragm, but it did not work
