SU Carb Lean at Part Throttle

(Cackle) #1

Hello, looking for advise. I am having a issue with my 1963 MKX with the 3.8 and triple HD8s. I am getting a surge at part throttle and full throttle. I have verified it is a fuel issue because I can engage the starting carb solenoid and the car positively responds and runs great.

The engine has good compression, timing has been checked, floats adjusted to just below jets. I can get the the engine to idle good.

I have raised the jets needles above the grooves in the pistons and the problem is resolved but the idle is too rich and wont run good. I have looked for vacuum leaks with starting fluid around gaskets and seal and have found none.

The needles are UM.

This is the first time I have ran this car so I do not have a baseline.

My question is: All this time I have run the car without the air filter plenum. Would this cause the engine to run lean at highway speeds under load? Should I try some UO needles?

Thank you in advance.

(Mike Eck) #2

The engine will run lean without the proper air cleaner attached. The needle type was determined taking into consideration the pressure drop across the air cleaner.

Mike Eck

New Jersey, USA

'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

(Cackle) #3

I will need to replace the studs on plenum and see if it affects the mixture.

(Douglass Harroun) #4

If you go to richer needles, try UE rather than OU ( need engine mods-cam, headers for UO)

(Mike Spoelker) #5

I don’t have a solution, but I’m giving you the full 10 points for your description of the problem and your methodical approach toward a resolution.

(Cackle) #6

I will try the UE needles if fitting the plenum doesnt solve the issue.

(Roger McWilliams) #7

One area to examine is fuel level in the jets. You mention above “floats adjusted to just below jets”. The paragraph below may apply to your circumstance of running rich at idle. Note that this 1956 SU workshop manual direction is different from what many people do. I’ve been happy following the SU direction and had occasional problems when having fuel level higher in the jets.

In the event of using Skinners Union carburetters, S.U. Workshop Manual Carburetters and Electric Fuel Pumps (6/56) says (assuming correct float bowl is installed) on page B.4 under Section B.5 titled Flooding From Float-Chamber or Over Jet: ‘‘the position of the fuel level in an S.U. carburetter need not be exact and to a critical dimension, but the standard level is 3/8 in. (10 mm.) below the level of the rectangular facing known as the jet bridge. If it is higher and only just short of the jet bridge, then leaving the car idling on a fairly steep drive may cause slow flooding, and in this circumstance resetting to a lower level is desirable.’’ Hope that helps for your circumstances.–

(Cackle) #8

Thank you but the issue of rich idle was when I had adjusted the needle higher in the piston in an attempt to correct the lean cruise surge.

(Roger McWilliams) #9

Here is the line of thought I was pursuing for the rich at idle issue. If the needle is higher in the piston than normal then the needle taper allows more fuel to pass at idle because a narrower part of the needle now is towards the top of the jet at idle than if the needle is seated normally. If the fuel mixture also is higher than 3/8" below the level of the rectangular facing then the combination of less needle constriction on fuel flow and higher fuel level each seems likely to lead to rich idle mixture.

(Lee140FHC) #10

And one would think that that high fuel level/high needle position would normally cause richness during any low-load/high vacuum situation, (high vacuum at idle and high vacuum at cruise). But the OP sez lean at cruise so my inclination would be to try stronger springs in the dashpots. After all, the SU is designed to richen the mixture during acceleration not by increasing the size of the fuel-flow orifice,(jet diameter vs. needle diameter) but by increasing the vacuum the fuel “sees” thru the use of the dashpot damper system. Indeed, the dashpot system actually reduces the “room” for fuel to flow by retarding the rise of the needle within the jet…the increased vacuum resulting from this operation thereby pulling more fuel thru the jet than one would normally expect.

If it was me, assuming no stronger dashpot springs were available, I would simply “weight” the carb pistons by installing a washer or such(a weight) around the base of the center slide(bellow the spring’s lower end) to simulate a stronger spring. I’ve done this with my Datsun Roadster, which uses SU “clones”.

(Cackle) #11

Early on that was my line of thought also…weak springs since the might have been new in 1963. So I ordered new springs from Burlen and that made no noticable differance. I have ordered a set of UE needles and will report when they get installed. I have seen a glipse of the performance from the little 3.8 and now Im on a mission😂

(Robert Wilkinson) #12

I believe that the piston spring is not primarily associated with damping the piston (which provides the same function as an accelerator pump) but rather with the steady state position of the piston whilst cruising or otherwise under steady load. Its force, combined with the weight of the piston, provides a downward force that is opposed by the upward force of the venturi vacuum applied to the bell above the piston. Older SUs had heavier brass pistons and no springs. The damping force is independently controlled via damper oil viscosity.

More in this recent thread:

(Robert Laughton) #13

What Mike Eck said. Had that issue with a 420.


(Lee140FHC) #14

Correct…as I say, the spring is there mainly to handle steady-state vacuum conditions…both at idle and at cruise…which is the OP’s concern…

(tony) #15
  • Yes, the car needs plenum & air filter hooked up

  • what oil viscosity in SU damper ?..try heavier

  • setting jet height…baseline 1.25mm to 1.75mm down from top of bridge with vernier tail.
    this makes them all the same, and should work for triple SU with UM

  • as you have plenum off…rev motor and observe SU pistons…do they all rise at the same rate?

  • use some airflow measuring device. The tri-carbs can be a devil to get right, all aspects need to be matched

  • it takes a fair while for these motors to warm up, and the ASC setting, and timing can confuse tuning. In cold conditions, imo 10mins for motor to be warm minimum

(Cackle) #16

I contacted Curto and he like others here advised that the UE needle would work. I am happy to report that the UE needle worked for me. The surging at highway speeds is gone, power is available from low end through the high speeds. I know that the idle stations are similar between UM and UE but this thing is idleing smoothly. The air plenum does infact richen the mixture but it was not enough to cure the surging at speed. Plugs are a nice tan color. This boat moves now!

(j limongelli) #17

Cackle, With the air cleaner removed and the engine up to temp.
Start the car and raise each piston up an 1/8th or a finger height, the engine will stumble if lean , load up if rich or if just right will rise a hair then settle down.
Your float height is probably set to rich, your cheating with the needles
Once the float height is right the car will be better than fuel injected.
Youll get the hang of it.
I start with the middle one , say its lean and it stumbles turn the screw a half turn clock wise for more gas , then pull up pistin again, its easy, then do all three back and forth.