3.8 wont idle under 2K – Help with Carbs (pics)

HI Guys,

My newly re-built 64 FHC 3.8 with 3 rebuilt SU’s won’t run under 2000 RPM. Ihave Ray’s electronic ignition system and it starts quickly with the choke on but once warm and I reduce the choke the engine dies. Choke lever is set properly with first third or so just increasing idle before engaging the choke with the rest of the travel.

I’ve set the whole thing up per all the manuals and online tips, funny ho so many start, “set idle to 600”, but never HOW. Car runs and drives great but just wont idle. I can’t seem to find anything on setting the “Slow volume running screw”, Sounds like it’s the one I want but I’m not even sure which it is and I don’t want to twiddle anything I don’t understand. See my pics Below, is it C or D? What are C and D?

A is idle? Opens the butterfly anyway. B is the Choke on idle speed. There is room to close the butterfly but the car dies when I turn it down.

What am I doing wrong?


“A"is your fast idle screw. With choke off you should have clearance between it and the stop. Try .010”
I’d bet your linkage is holding a butterfly open, though. Try setting the carbs up with the connecting linkages independant of one another.
If you go back to all stock bypass settings “D” and you have space under “A”, then your butterflies are not closing properly…or you have an airleak.
“C” is just a screw holding 2 parts together, it should be tight.

C looks to be connected to the bottom of the jet? There is a spring under the ‘Z’ piece of steel which would appear to work in conjunction with the idle screw?

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I think the problem could be that D is closed (clockwise as far as it will go)

Turn all the three D’s clockwise until tight, then unscrew counterclockwise two full turns

I suspect your car will then idle
Dennis 69 OTS

A is the fast idle screw. This opens the throttle when the choke is actuated. With your choke completely off set these so a 2 thou feeler gauge will just go through.

D is the slow idle screw. This allows a small amount of air to bypass the closed throttle butterfly and controls idle when the car is up to temperature.Wind these completely closed then open all 3 of them 2 1/2 turns.

C as marked is not an adjusting screw.

B adjusts the jet height. Ignore what the manual says and use a vernier caliper to set the jet height 60 thou below the bridge. You do this by removing the bell and piston. Don’t mix the sets of bell/piston up.

Now before you try anything else:

  1. ensure your car is static timed to 10 degrees BTDC
  2. ensure that all 3 carb butterflies are COMPLETELY closed and correctly synchronised so that they open at exactly the same time. Make sure that there is an equal amount of slack in the throttle actuation arms one each carb.
  3. ensure that the float height is set correctly at 7/16".
  4. disconnect the vacuum line from the front carb and plug the manifold. You can reconnect it once you’ve got it idling.
    Assuming this is all done, and that there is no misassembly or damage to any of your carbs or induction system, start the car with full choke. Allow it to come up to temperature, closing the choke gradually to keep idle below 1500 rpm.

Now gradually close the slow idle screws, D, until you have about 700 rpm.

Now you can adjust the mixtures.

I followed every step above laid out by Andrew, except timing as I have no control over that (Ray’s electronic ignition) No Change. I opened up the slow idle screw another 1.5 turns and it runs with the choke completely in the down position but any attempt to idle under 2K the engine stalls.


I suspect there is an air leak. Have you checked all the brake vacuum hose that plugs into the intake manifold?



When you check the brake vacuum hose, check it all the way to the vacuum reservoir tank and follow the hose from the tank to the brake booster etc

I just plugged the hose at the manifold. No change.
I should mention this is a fresh build up and this engine in it’s current configuration and has never idled properly.


I think I can help. Can we speak on phone off line?
I will send you a private message

Brian it sounds like you have a too lean condition at idle - that it’s lean enough that it won’t run. A simple check would be to screw the mixture screws B down a turn or two and see if that helps. If it now runs you need to go through the procedure for setting the needles, jets, and float level and ensure it’s all correct. The photos don’t show any filter, trumpets etc attached. That alone will cause leanness. Both the owners and operator’s manual contain detailed instructions on tuning triple SU’s. I think you need to refer to that - those identify the screws, and much more.

Were the carbs rebuilt? Make sure the butterflies are centered and close completely.

Leaks… I found this on one on one of my vac hoses when my S1 arrived. I had to feel it to find that it was broken.

Thanks to the list for all the help, Special thanks to Dennismo for talking me through all the adjustments.
I think it was 2 things:

  1. The new throttle cable setup has no off stop and the cables may have been too tight leaving a butterfly or two slightly cracked. Added some slack to the cables. Double checked closed butterflies.
  2. No air filters and too lean, I added a full enriching turn on the mixture screw and bingo.
    Now Idles at 800-900 RPM, good enough until I get the air filters and vacuum lines all back on.
    Thanks again,
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Mixture screws should be adjusted to the highest rpm for proper mixture. I think you’ve gone too rich which is slowing the idle down.

Ummm, How would I do that? The car wont idle under 2K with the jets at .060"?

With a properly tuned SU, when you raise the piston ever so slightly (I think 1/32" with a screwdriver or pin in the carb) the rpms should rise slightly and then fall back to normal. If rpms rise and stay, too rich. If rpms fall and stumble, too lean.

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Another test is to listen to the exhaust at idle. From the Driver’s handbook, page 67:

“When the mixture is correct, the exhaust note should be regular and even. If it is irregular with a splashy type of misfire and colorless exhaust, the mixture is too weak. If there is a regular or rhythmical type of misfire in the exhaust beat, together with a blackish exhaust, then the mixture is too rich.”

When i put adjustable needles in my ZS carbs I bought one of these Colortune tools to help make sure I set them properly…especially not too lean. I figured it might be handy too in identifying spark mis-fires.

68 E-type FHC