Why '66 fhc temp diff between headers + exhaust pipes?

(David Ahlers) #21

And of course your butterflies have to be completely closed at idle for your vacuum balancing, bypass screw and mixture adjustment to be meaningful. Just saying that most all tuning procedures depend on the previous step being done properly. A concept that I myself ignore from time to time. Leading me to sit down in the garage and contemplate how great a cold beer can taste.

(Puddinhead) #22

Removed plugs, all dry except #6, it was quite wet. Doubt #6 was firing

Edit: put new plug in #6, it starts to wet up again.
Adjusted front carb leaner. Cleaned plug, Tad leaner now. Result #6 not wet but dry soot, still too rich.


'66 fhc

(69 FHC ) #23

Could it be that the plug wire or spark plug itself was the problem?

(Puddinhead) #24

John, I replaced fouled plug #6 with new plug. Strange symptoms now, even with front carb slow idle fully CW closed, and all (fast idle) butterfly closed (at idle of course) I can’t reduce front carb vac pull enough to match center and rear carbs (using Unison vac tool). Since large canister air filter not installed yet, engine crank cast pipe open, could that leak cause my symptoms ? Why does Jaguar term the butterfly screws as fast idle?

'66 fhc

(Michael P Moore) #25

When the butterfly valves are closed, you ought not see a gap between the butterfly and the carb throat.I had a problem traced back to not spending enough time centering the plates when I kitted the carbs.

(Ray Livingston) #26

Only a few possibilities:

  1. Fast idle screws too far down - back them out until there is clearance between tips of screws and fast idle push rods.
  2. Throttle linkage incorrectly adjusted - DISCONNECT the vertical links from the three throttle arms to the throttles.
  3. Throttle plates not properly centered - Verifying this requires removing the carbs.
  4. Vacuum leaks DOWNSTREAM of the carbs
  5. Throttle return springs not properly adjusted, or throttles sticky.

The crank breather plays NO part in this.

Ray L.

(D Barnes) #27

In case it might help when I put the adjustable needles in the ZS carbs on my '68 I found this Colortune tool helpful in getting the mixture set up. I was running too rich but nervous about setting things too lean. Seems like could help figuring out if you have a bad spark plug wire too since you can see the spark in each cylinder. (it is the most fun tool that I have )

68 E-type FHC


(Puddinhead) #28

Thanks David,

Yesterday I ohm tested the plugs and all wires were conducting ok. I also made sure the plug wires were correctly wired (plugs show they were burning fuel). Today I will take each SU piston out and eyeball each jet bearing to height of fuel relative to the flush of bridge. Here on the JL others have noted around .070" drop from the bridge is good ideal, with fuel showing in the jet; as if fuel not showing then fuel bowl problems exist. And a YouTuber displayed a tool which measures the relative difference between the bridge and the jet (when no choke). Regarding the term 'fast idle ’ screw, I always wondered why it was termed such as the fast idle screws at no choke I have set at 0.015" free play before any choke action. Apparently some tuners adjust the fast idle during choke action but I’m dense how to accomplish that task at this point. As if one did adjust fast idle during choke action, then with no choke the fast idle screws could end up resting too close to butterfly shaft or above shaft; and everything I’ve read notes the fast idle screws should equally be tad off the bf shaft,at idle (no choke). I’ve got about 160 compression, plenty of oil pressure, no smoke but can’t get it idle stable.

'66 fhc

(Ray Livingston) #29

That is a complete and utter waste of time, and a meaningless measure. Verify the float heights using a 7/16" test rod.

Ray L.

(Puddinhead) #30

Thanks Ray,

I’ve read on JL that there were problems with some fuel bowl shut off levers that caused problems with fuel arriving to it’s respective jet. Here on JL, it was often noted that the fuel showing height should be same for each SU.

'66 fhc

(Ray Livingston) #31

No, it will not. Fuel height in the jet is a completely unreliable measure.

Ray L.

(69 FHC ) #32

Well, I think it should, but as Ray points out the only reliable way to achieve that is to have the floats correctly set, and the only way to do that is to physically measure them.

(Puddinhead) #33

Thanks John

I was hoping that if I could eyeball the fuel level relative to the jet and the bridge I could rule out problems with the float bowl operations.

'66 fhc

(Tom Rutherford) #34

Maybe try a new plug wire. The ohm meter uses low current. Or check for a blue spark while cranking with the plug out. (don’t hold it, just lean it in the head valley)

(Puddinhead) #35

Tom, I have a glass plug tester, and it lights up when firing.

I just eyeballed the fuel level at the bridge / jet when jet mixture is 2 CW turns from flush with the bridge. Results showed fuel. Results showed front carb fuel level quite low, relative to the other two carbs…so I cleaned cross ball type valve and reinstalled, note the small washer that’s mates to the valve and reset lever setting at 7/16" and results showed fuel level visible but too high relative to the other carbs. Tomorrow I’ll fix that.

It fires easily now but idle still rough. Found all three quad screws were loose from vibration.

'66 fhc


float level looks low, I think we installed ours without the thick fibre washer?

(William H Wayman 1970 S2 DHC) #37

The float arm ideally wants to be 90 degrees to the valve, otherwise it can jam. Try removing the thick fiber washer. Then bend where the curve meets the straight to get 7/16" float setting.

(David Ahlers) #38

If you want to know what your fuel level is, take the cover off the bowl and measure the fuel level from the top. Getting them all close to the same will take the guess work out of whats bent & isn’t. I had a 7/16 rod, I used to set the floats, actual fuel level varied from carb to carb more than I thought it ought to be. Bit of trial and error to get it equal in all three. An SU carb while elegant in its simplicity and working is a pretty crude piece of whittled metal.

(Puddinhead) #39

Thanks David,

I just removed all three bowl cut-off valve washers; so angle is closer to 90 degrees as poster ‘dirt’ noted. I re checked the 7/16" shut off levers (after I took its washer off) and all were set too low; so fuel in bowls was less than it should have been and thus less fuel was showing at their respective jets. Now it fires great with some choke and warms nice, and with choke slowly decreased rpms drop from 2k to 1k but if I don’t nurse the throttle she will fall to 500rpm and shut off with a gurgle. I hope my new alloy flywheel isn’t the culprit? Damper is original but balanced with crank. Plugs look fine, not wet or sooty. While on choke, rpms are smooth and exhaust note is perfect. I have SU air vac consistent with Unison tool.

'66 fhc

(Robin O'Connor) #40

I would surmise that with an ally flywheel the idle may need to be slightly higher than spec due to the lower inertia.