Poor idle

[awiedie]

After completely rebuilding my whole engine compartment several years ago, including carbs of course, with all new parts, I made the great mistake of not starting the car up for several more years, at which time the gas had all evaporated from the tanks, filter bowl and float bowls. I know, kind of stupid but I was caught up in the restoration of a 1956 MGA which took years longer than expected.

Anyway, back to the S Type. I drained what small amount of gas was in each tank; removed and cleaned tank filters (they were fine); looked into the tanks which looked very clean; cleaned the filter bowl and brass screen; removed the float bowl lids and inspected the needles and seats (all looked fine); added a couple of gallons of fresh high test to each tank; removed the fuel feed line at the filter bowl and ran a hose from it's end to a bucket while running fuel from each tank until it ran clean; and then set the slow idle and mixture screws to initial start settings.

The car will start right up and run under choke at about 1000rpm but drop down to 0rpm when the choke kicks out. I can start it right up and keep it running as long as it stays above 600rpm or so, but the moment I take my foot off the pedal, rpms drop to 0. I have cleaned and adjusted slow idle screws and at the suggestion of Joe Curto moved the needles up in the pistons a small amount to enrich the mixture, all to no avail. I really, really don't want to take those carbs back off the car and start from scratch, but if it comes to that, I'll do it. Any suggestions at all will be greatly appreciated.

[Orlando St.R]

Hi Alan. Well, I'm no expert, but it sounds to me as if there can't be much wrong if it will run on choke and idle smoothly on the throttle. Those two factors suggest the problem lies in the idle circuit, so if it were me, the things I would look at are:

1. Have you genuinely been adjusting the idle screws? And not the mixture screws. (Apologies for this insulting suggestion, but it is worth checking).

2. Have you turned them to three complete turns out from the closed position?

3. While it's on choke, if you turn the idle screws out enough, will it increase the idle speed? (If this works, the idle circuit is working).

4. Remove the idle screws and check for condition, gum and any obvious blockages.

5. Push compressed air through the idle drillings to be sure they are clear.

Those are a few things to try while I sleep on it to see if anything else occurs.

[Glyn Ruck]

+1^ ~ I'm still learning SU constant depression carbs being a twin sidedraught Weber & Dellorto man. Had a carb boffin from the club around this weekend to teach me and he has achieved beautiful stable idle at 500 RPM hot on my newly rebuilt engine (& carbs). Slight run-on problem also gone. (pistons & rods weight matched & crank & flywheel balanced with clutch)

He started by removing the piston/suction chamber units & setting the main jet (that the needle goes into) to be level with the centre of carb body/housing as a starting point to get the mixture roughly right. (Mine were too low & hopelessly rich). He put the piston chambers back & then adjusted the jet up to weaken or down to enrich the mixture until perfect. (excuse me if I use the wrong terminology ~ I'm still learning ~ SU's have been a mystery to me until now)

Suggest you run a bottle of Techron in your next tank of gas. It will make everything clean as a whistle. (after you have done primary cleaning suggested by Orlando.)

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[awiedie]

Thanks Orlando and Glyn for the good suggestions. My responses are:

1., 2. Yes, it's the slow running screws I turn out two turns as the manual states for initial setup but I will try three.
3. I will try this.
4. I pulled these screws and cleaned them with steel wool as they were somewhat dark. Bore and well looked good.
5. Hopefully I can blow out the idle drillings down through the slow running bores? Have to check construction of the carbs. You can see I'm trying really hard to not have to pull those carbs.

I'll see if I can rustle up some Techron. One note, Glyn. Book says to set jets level with jet bridge and then turn down adjusting screws 2 1/2 turns for initial setup. Guess it doesn't matter where they start as you have to adjust them anyway.

[Glyn Ruck]

Yes Alan ~ I have read the book & he was probably just demonstrating to me what exactly was happening internally & what roughly correct looked like internally. He was very patient & I was asking questions. On reflection they were probably very slightly below the surrounding body prior to him remounting the chambers. He was pleased that we had fitted new diaphragms. Apparently they can mess you around royally if not in good fettle.

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[Orlando St.R]

Hi again, Alan

Yes, I just meant push compressed air down the slow running screw holes.

Did you happen to take a note of the existing screw settings (assuming the car ran okay previously) before re-setting to the start settings?

A couple of other thoughts emerged from my night time slumbers:

1. You could crack open the throttles slightly using the adjuster screw on the throttle mechanism. That would be easier than trying to make adjustments in the short time the choke is on.

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2. How does it run once the choke has turned off? Is it smooth? If no, the mixture may be too weak anyway, which might be why Joe Curto suggested adjusting the needle height. Or, of course, the carbs could be significantly out of balance, now. If yes, then the mixture is probably near enough.

The start settings are really just to get the car running. Having adopted these settings, you will have to go through the full tuning procedure in order to get to settings that run acceptably. The most reliable way to do this is with:

• two Gunson Colortune - one in number 2 and one in number 5 spark plug holes

• a suitable carb balancer (I don't think listening to the hiss gives accurate results)

• a home gas analyser, eg Gunson Gastester. (Lifting the piston never really worked for me)

Forgive me again if you know all this stuff already.

[awiedie]

Orlando,

Sorry for the late response to your last post. It seems that sometimes I don't get notified of new comments but I may have that straightened out now. Anyway, good suggestion about using the throttle stop screw to increase rpm instead of the slow running screws. When the weather warms up a bit here (-2F) I'll give it another go. I do have a UniSyn and the SU tuning needles, so when I can get it to run long enough, I'll be able to tune it. The frustrating thing about all this is that the car ran pretty well until I let it sit for a couple of years (probably longer)and my fuel all evaporated in the system. Dumb

[David Reilly]

The following was received with the registration of the 697th ‘S’-type to the Register.
Cheers,
David

Currently I am experiencing trouble trying to register on the S class web site and I have forwarded an e-mail to your web master in that respect.

Meanwhile I think the suggestions I have listed below may help the register member in the current thread running on the web site called “poor idling”.

Providing it is possible, and you agree, I have no objection to you listing my answer in full to assist that register member. I would add that I spent 3 weeks in the mid 60’s on training courses for S.U Carburettor and Pumps that were all held at Morris Motor, Cowley, England and have worked in over 50 countries on automotive and plant problems.

Norman Geeson I Eng. MSOE. MIRTE
Jaguar 3.4 S Type MOD chassis IB4722 DN

Answer to poor idling as described on the thread......
Awiedie

Although your description of the problem could point to a number of causes, I think you would find it more productive to systematically working through the trouble shooting.
Your description of the car history, the fault and the long standing period leads me to believe you may have a massive air leak. I would suggest you start by eliminating the entire brake vacuum servo system by blanking off the vacuum supply pipe adjacent to the bulkhead cylinder. (Whether you call that No1 or No 6 cylinder is up to you). It is probable that you are experiencing an air leak from a faulty brake vacuum pipe or coupling that has deteriorated.
It is important that you blank off at that point to eliminate the pipework, flex pipes, vacuum valves, tank and servo. Once you have tried eliminating the servo vacuum supply check the following points in order.
Remove the dashpots and damper valves and check that each piston takes the same time to slide down the dashpot when the dashpots are inverted.
Ensure the dashpot needles are correct and the needle shoulders are level with the steel section of dashpot pistons.
Using the mixture adjusting screws, screw them in and out some distance to check that they are raising and lowering the jets. This is to ensure that the diaphragms’ have not hardened too much with standing and are in fact moving in sympathy with the adjusting screws.
Use a vernier and check and set the main jets 0.065 inch below the jet bridge on each carburettor.
Switch on the ignition and wait for the pumps to stop. Look down the main jet from the top, with the dashpots still removed, and check that you can see the fuel level approximately 0.125 inch below the jet bridge. If the fuel level is out of sight, or flooding over, then remove the float chamber lid and adjust the float level after checking for fuel inside of the floats. This can be checked by removing the floats and shaking them, best to replace the floats with the new types that are not affected by modern fuels.
If you need to adjust the fuel levels then set the float levels at 0.500 inch instead of the manual setting of 0.430 inch. Note, whatever you are told, S.U Carbs are not particularly sensitive to float levels, but a 0.500 inch setting does help with modern fuels.
While the float chambers are exposed, test the float arm movement up and down with either fuel pressure (pumps on) or blowing by mouth through the inlet to see that the actual needle valve does not jamb like a sprag at the bottom of the float arm travel. This problem is the main reason for flooding, which operators blame on sticking valves. If necessary replace the float arms with later units that have a tang to prevent the arms dropping too far, or if the arm has a tang adjust it by bending the tang to set the arms to an absolute minimal drop distance to just admit fuel.
With the dashpots still removed, view the throttle plates down the carburettor bores, and if necessary adjust the interconnection links to ensure both throttles close fully and simultaneously. On HD type carbs in particular owners and mechanics do not pay sufficient attention to ensuring that the throttle plates are centred in the body bore as the HD’s rely on the dedicated idle volume circuit with no mixture passing the throttle plates at idle. If the air intake horn has been removed from the carburettors it is possible to view the throttle plates squarely by using a mirror.
At that point replace the dashpots with pistons, but no springs, then lift and drop the pistons ensuring that each produces a clean “clonk” as it contacts the bridge, proving that the jets are correctly centred.
Remove the dashpots and refit the springs and pistons then replace those assemblies back on the carburettors leaving off the dampers.
Lightly fully screw in the volume control (idling) screws, and then back each of them off two full turns.
At that point if the engine is potentially runnable (ignition, choke, valve settings good) it should start and run. The above checks should at least get you mobile, keep us informed of progress.

[Glyn Ruck]

This is a very good post. For clarification ~ No. 1 cylinder is located closest to the firewall with a firing order of 1-5-3-6-2-4.

[awiedie]

Thanks very much, Norman, for the comprehensive article. Your experience is valuable.

I will definitely check the vacuum first by disconnecting and plugging the manifold port. Having just restored the complete system not too long ago, that may not be the problem but there is that one-way valve on the canister that could be the culprit.

I have already gone through most of the carburetor checks that you list and found things OK but am not against doing it again, following your step by step recommendations. I keep thinking that since the car runs fine at rpms higher than say 600, the problem may lay in the slow idle circuit, especially considering the car sat for several years and all the fuel in the carb system had evaporated, possible leaving varnish behind in small passages. My one thought is to spray carb cleaner down the slow idle port, let it sit for a while and then blow compressed air down the needle tube.

My problem is at the moment, the S is in the front of the garage in my basement with an E type and MGA behind which have to be backed out into the drive where the temperatures are now in the low single digits. As soon as we get a break in the weather, I'll be able to experiment.