Xk 120 modified engine poor idle

XK-Engine
1

I’m hoping that someone might have a suggestion for my poor idle problem, as I’ve done about everything I can think of.

The engine is the original from my 53xk120 fhc project, mounted on the restored frame, so not a drive-able vehicle. It came to me over-bored to 3.8 L. Sonic check was ok, and I added .020 to clean up. New pistons are 9:1, cylinder head modified with big valves, xj6 size, mild porting (compared to my spare head .050 out board, zero mid, .080 deep), Bill Terry’s WAD street cam, manifold inlet opened to accept 2" HD8s. Carbs are rebuilt (added upgraded spindle seals yesterday) and jets are centered, UM and UO needles tried, float levels correct. Mallory dual point (recently removed one set of points with no improvement), new condensor. Pertronix coil (no change with Lucas coil). Standard plugs and gaps, new wires. Stock exhaust manifold, dual SS exhaust. Rebuilt square body fuel pump with modern internals.

So, the engine starts pretty easily, but doesn’t like to idle without the throttle cracked open. Bypass screws pretty much wide open, and timing advanced to 16 BTDC to maintain a lumpy idle between 650 and 750 on fresh, premium no alcohol fuel. Manifold vacuum 12-14, very erratic! Idle shoots up to 1600 with the starter carb in maximum lean. I’ve had the intake off twice, and the surfaces are dead flat - I don’t think there is a leak. And plugs are always sooty! If I lean the carbs until the engine stumbles, they are still black. I’ve never had trouble tuning SUs for idle, and I can find nothing wrong with these, but changes in settings don’t seem to affect idle that much. On the other hand, it revs great, runs cool, carries great oil pressure on the original pump (modified to xj6 spin-on with external oil cooler). I’ve quadruple verified TDC on the 420G vibration damper, and confirmed that cam timing is dead on. Spark is strong at all plugs. Head retorqued, compression even across at 165psi.

Bill Terry has been very kind with suggestions - he tells me that in his shop, engines with his cams and with similar mods all idle well. They have more trouble getting them to be rich enough under load. I was thinking that the engine might be overported, but I don’t think my measurements suggest that. My next step is to try the original SE cams on the engine, as the engine behaves like I’d expect with a high lift, long duration cam - this cam is not supposed to be that aggressive. Seems like it wants more air. Just wondering if anyone else has an idea here. I’ve run 10 gallons of fuel through it so far, and I’m not finding it. What am I missing?
Ron

2

You haven’t mentioned checking the throttle plate fit in the venturi? Jet height? Spring type?

3

Similar to my problem after port work and cam change, things got better with the Edis, but you have to expect a stumble when moving away from the original cam and valve spec, let alone the bigger carbs…UOs will always be rich at tickover I have found, but allowing them to drop a fraction from the correct setting can sometimes help…if you have the annular groove as a setting mark on your needle , expose the groove rather than set it to the shoulder as the book dictates, then try boosting your ignition set up for a fatter spark at those low rpms

4

Throttle plate fit is perfect. I’ve tried jet height all over the place, but what idle I can get seems to be about 2 turns down, probably about .070. I have not changed the damper springs. Should I? If so, which springs?

5

I’ve had the needles dropped with no improvement. Also dropped the floats down 1/16th without improvement. Which cam did you use and how much porting? As I mentioned before, Bill Terry says their cams should idle fine. My cam has 0.436 lift, 261 duration, 215 at .050.

6

Dwell is at 35, on spec according to the Mallory tech guy. How do I get a fatter spark?

7

Hi Ron,
I have a modified 4.2L in the replica D-Type - cam (Rob Beere full race) is a bit higher lift and quite a lot longer duration. Also run Webers and the exhaust manifold goes over the frame so breathes a lot easier and only 1.8" inlet valves. It will idle when cold around 600rpm or even less until it warms up then will sit around 750-800 but able to be adjusted. Hot idle RPM is more a function of it idling when cold. It has always surprised me how smooth it is - less than stock but not horrific.
Assuming that static at 16 BTDC may be a Bill Terry spec but also wondering if you have tried backing the advance off from 16 BTDC. I am running about half of that. My apologies in that when I switched to a Mallory distributor I didn’t really take a lot of notice what the idle speed was at higher advance but I think the idle speed was out of the range to adjust with the carb idle setting on the Webers. Your setup may vary but advance of 30 is all in at 2000 rpm.
As noted, the Webers and running slightly cooler plugs with stock gap, there is no way I can get the plugs not to be sooty at idle. I use colortunes to get a sense of what is going on with the mixture. No insights about the SU’s so this latter information may be irrelevant.

Regards
Keith

8

It seems to me that 165 psi compression is a bit low for a fresh 9:1 engine,
since Jaguar says it should be 180 psi. Could your cams be out of time?

Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

9

A more radical cam will have a lower compression reading because of the later intake closing event. My fresh 8:1 3.4 shows about 150 psi with stock cam(s), but it is not broken-in yet AND I have advanced the intake about 5 degrees, which would increase cranking compression.

10

Thanks for the multiple replies. Let me try to address some of this.

My initial cam setting was exactly right on the exhaust side, but about 3 degrees advanced on the intake side on recheck. I corrected it, but it didn’t help (in fact, might have been a tad worse). Bill Terry says that his cam has a higher lift, but uses a stock duration, so it shouldn’t have the idle problem of a more radical, overlapping cam.

16 BTDC is not Bill Terry’s timing. It’s what I have to do to make this thing idle at all. I started with 10 degrees and added the least amount that would make it stay running without a hand on the throttle.

As to the compression, I’ll recheck, but it was very even. I suppose you could postulate bad valve grind, but it was done by the owner of a busy local shop with a good reputation. I’ve had good experience with him on multiple engines.

Could the grind be off on the cam? Bill says unlikely. If I put the stock SE .375 lift cam in it, and it’s not improved, it would rule that out. This morning, he did suggest I talk to Joe Curto about it, and Joe suggested that I could try adding a hole to the throttle butterfly, which he has had to do on some Rovers. I might try it. I can always plug it if it doesn’t help. The other suggestion Joe had was to change the spark plug ends from the screw in type to crimp on. I’m using a 7mm Pertronix flame thrower wire that I got in bulk roll from Summit - It’s not solid, and not described as spiral, so it must be standard carbon core. Maybe I should put some solid wire in? Joe also gave me some data about SU damper springs, and I’m going to check the strength.

11

I don’t see how adding a hole to the butterflies would “cure” the problem, though it MIGHT help as a band aid…I’ve see this done on big V-8s w/wild cams. But a stock-ish duration cam should not be noticeably different from stone-stock one when it comes to the idle circuit. I would lean toward trying lighter dashpot springs myself…FWIW.

12

Yes, I agree with you about the butterfly. I’ll order some more dashpot springs and see what that does, but I’m skeptical that it will solve the problem. I can manually raise both pistons, which is the same effect of using a weaker spring, and it doesn’t fix the issue.

I’m going to change the terminals on my plug wires, and re-recheck for a manifold leak. If that doesn’t do anything, I think I’ll change the cams. Since I don’t have a giant stock of shims, I’ll have to order the correct ones, so I’ll report back after that is done. I appreciate the suggestions put forward here. This is the 4th xk engine I’ve built, the first modified, and the first I’ve had any trouble with.

13

Sounds like you have good access to the engine since it is on a stand/running rig of some type? Assuming this, I would check intake and exhaust centerlines (max valve lift) to confirm the cams ARE timed/ground correctly…really not hard to do if you have a dial gauge and easier than changing cams. The low vacuum is the big concern to me…should be 3-5 inches higher. I was gonna suggest you lift the carb pistons to lean out the idle, but looks like you did that without positive effect…that SHOULD have leaned the engine out…confusing. The little brass nubs are still in place on the bottom of the pistons? IIRC, they provide about 25 to 30 thou of gap between the piston and carb throat. Now I am wondering if you really ARE at 16* BTDC on the timing since generally, advancing the timing will show much improved vacuum at idle. Grasping at straws, the newer ignitions are pretty powerful and allow very wide plug gaps so you might wanna open those babies up to 35, 40, 45 thou.

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Yes, I have good access - the engine is mated to a T-5 transmission on the restored frame. The body is ready to go on, but not before I sort this out.

I did a quick check of the intake centerline and I thought it was ok, but I’ll recheck. I thought about checking the lift, but haven’t - will do that. The cam id is under the serrated fitting and I didn’t think to look when I re-timed the intake cam. I’ll try uping the spark gap - I think they are N12ycs.

I agree that the low vacuum is a primary concern. I will mention again that I have the air bleed screws wide open to make it run. It does have higher vacuum at 16 BTDC. Plugs have always been sooty, even when I lean the needles to the point of stumbling. Yes, confusing. I know I’m missing something. Will have some time to work on it tomorrow.
Thanks.

15

The 2" carbs when running in pairs instead of triple setup use .100" needles, not .125" like the UM & UO ones mentioned. You might try running the needles that the Euro XJ6 cars use along with the appropriate jets.

16

I’d hate to send you on a wild goose chase w/regard to the installed cams cos I just don’t see them as the problem. What about fuel pump pressure? Are we good there? Can you see the fuel level with dashpots removed and fuel pump on?

The mismatching on needles and jets Mr. Maury mentioned is interesting…though I must admit I’m confused as to why one fewer carbs requires LESS fuel flow capacity per carb when the air flow thru each of two carbs must equal the total flow thru 3 carbs given the same engine size. UNLESS he is saying the .1" needles mate with the .125" jets??? (Or maybe I’m just missing something basic here). I know that 4 cylinder cars generally use .1" needle/jet combos, but those carb throats are from 1.5 to 1.75 inches in my experience…far smaller than those big 2 inchers.

I will have to look again at my (spare)HD carbs to see how the idle circuit is supposed to work given the fact that my 140’s H carbs do not have any kind of idle bypass circuit…which would make for a significant operational difference between the two styles.

17

The needles and jets always need to match size wise. Larger size jets and seats do not mean more gas. You had mentioned that your spark plugs were black which is to much gas. Why would you think that going to a smaller needle and seat with less fuel would be a bad think since you are running rich? Best advice is to get a SU jet book and look at applications where dual 2" SU carbs were used and see which needles were used. I would use that as a starting point. The XK140 carbs do not use a idle bypass circuit. They just use a screw to open up the throttle plate. The idle bypass is a bit more accurate but accomplishes the same thing. I have also had to add a hole in the throttle plate on the dual SU 2" carbs running on a 4.2 car to get the idle bypass circuit to come into adjustment range.

18

This is very interesting, Dick. I’ve never heard of the .10 needles. The UO needles (0.125) were provided by Joe Curto, and I carefully described the application to him when I bought them, and then again yesterday. He said nothing about .10 needles and jets. Does a .100 needle have a .125 shoulder? If not, it won’t work in my carbs and all the other HD8s I’ve seen, as you couldn’t center the jet. I’ll see what I can find out about it.

Do you recall what size hole your throttle plate has? Joe suggested I start with .075", larger if needed. I believe I will try this before messing with the cams. Easy enough to solder closed if it’s not helpful.

My premise, and I’ve had numerous others agree with this, is that I should be able to make this engine idle with any needle in reasonable range. It won’t necessarily be the right needle for any other phase of operation. Bill Terry has a problem getting engines with my specs to run rich enough. Additionally, Walt Osborne made the comment to me that sooty plugs aren’t necessarily due to over richness. I think that applies to low idle vacuum situations like mine, but I can’t explain it.

Lee, the original fuel pump was rebuilt by a reputable specialist, Dave Dubois in Bremerton, WA, and has modern internals, flows good volume. Fuel levels seem appropriate, but I’ll recheck. The engine revs well, which I don’t think it would do if it was fuel starved or if there was an ignition issue. BTW, the manifold vacuum will blip up to 25 on revving, settling between 20 and 22 at sustained 2000rpm. The root of the problem seems to be low vacuum at idle, leading me to suspect the cams or possibly over-porting, but I would love to find something simple that I’ve missed. Bill is going to weigh in on the porting specs when he has time.

19

.075 is approx. 1/16" which is a good starting point. As you said, you can always solder it up but you can drill it larger with it on the car so go in little steps. As far as needle sizes and application, get a SU rebuild book with all the needles listed and the fitment for them. The .100 needles have a .100 jet. You should be able to get it to idle with .100 or .125 needles if the jets match in size. Centering the jet is not going to greatly effect mixture unless it is binding and not letting the damper float up and down smoothly. ( still needs to be centered but don’t chase your tail on this one) BTW: the European XJ6 4.2 with dual 2" SU carbs use a .100 AAW needle. On my racing XKE, I was using the strongest cams Bill Terry had short of Billet ones. I also ported out the head rather extremely, even removing the valve guide bosses. It was a FI head so had the larger valves and ran long tube headers. The Triple SU carbs did not lend themselves to a very good idle. Pretty much around 1100 and rough. I ended up switching to Webers and got it smooth at 800. Ran very strong all the way off the end of the tach and would smoke the 295/30-18 Hoosier slicks. So with a mild cam and some porting, you should have a decent idle. 12 inches of vacuum is way to low for all but a radical engine. Are you sure your plugs are hot enough to burn off the residue? I was also running a MSD unit with a switch to go back to stock as the unit died at a race and that was that. I could switch between stock and MSD and the MSD really helped. It all depends on the car as once you start down the road with modifications, they build on each other and cause other needs.

20

All good information for me. I’ll post again when I have more data. Dick, what spark plug are you suggesting?