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MG MGB Technical - SU Idle

Over the winter I rebuilt the SUs on my 1963 B. The jets and needles were replaced along with all of the gaskets. I replaced the shafts but left the bushes as they were. I eliminated most of the play in the shafts. In trying to set them up this weekend I cannot get them to idle below 2000 RPM. All of the idle screws (fast and slow) are backed off completely. I have played with mixture to no avail. The butterflies appear to be closing properly. I did not adjust the ignition before the SU rebuild when it idled ok at 900 RPMs. The car runs very well, doesn't overheat has plenty of zip. Any ideas on what is not correct? Thanks.
Jim H

Having had the same problem, although I use the later HIF4 carburetters, you more than likely have a vacuum leak somewhere in the line. Try to block the vacuum hose that is connected from the distributer to the manifold and see what haapens. I also had a problem with my idling due to use of improper oil in the dampers, as well as my butterflies not properly closing due to the shaft blocking on the choke shaft causing them to stay open. Once all the was settled I now have a perfect idle at 800-850 r.p.m.

Cheers-
Stephan

Jim--
Even though it looked like your butterflies were seated properly, they may not be. Unfortunatly, the carbs will have to come off the car to prove my theory. If you decide to accept this mission, hold the carb up to bright light. If you can see light, air can also pass. If this proves to be the case, undo the split screws and snap the throttle shaft briskly untill the disks seat completly. This may take some finagiling but they will eventually cooperate. It's also worth mentioning that the disks are angle cut on the edges and it is possible to install backwards. This will also produce your symptons. I'd leave nothing unchecked. When you achieve satisfactory idle, check your timing.

Good luck,

Paul
Paul Hanley

Thank you Stephan and Paul. Your thoughts are similar to mine. I can't see how I would have a vacuum leak - I was pretty careful putting the setup back on the car, but I will check that first. I think it's likely I have to fiddle with the butteflies. I could have one backwards.........
Thanks again
Jim
Jim H

The engine *cannot* idle if the butterflies are fully shut, and a vacuum leak on its own cannot cause a high idle that cannot be 'cured' by adjusting the idle screws.

Either the butterflies are not seating properly, or if they have poppet valves one of those may not be seating properly, or the linkages are misaligned such that one butterfly fully shut is keeping the other one open, or the butterflies are hanging on the cable i.e. there is not the required amount of free play in the cable when the throttle is closed, or the fast idle screws are holding the throttle open, or any combination of these.
Paul Hunt

Paul,
I loosened the connection that links the carbs together and I have slack in the cable. The front carb seems to be the culprit when trying to balance the system. No poppet valves and the fast idle screws are fully backed off. Will get them off this weekend to have a look.
Thanks,
Jim
Jim H

Jim,
If your carbs are the originals on your 63 B, then they are at least 41 yrs old. That is not necessarily a death sentence for SUs if they have been properly maintained and worked on over the years. However, wear is very common on any mechanical device used over time and you may find that at least part of your problem is worn linkage pieces, and/or worn carb pieces (sloppy fitting idle cams, etc). Any or all of these are possible problem sources between your carbs and also as they affect the carbs' function with your engine. Some things, like these, are indeed rare possibilities, but they have surfaced many times and driven folks nuts trying to track them down.

My experience has shown that when you set up your carbs, you cannot assume that everything is hunky dory just because it LOOKS OK or even measures or feels OK. I have found that you need to check your syncronization between carbs at more than just idle - the normal setup point for sync as per the shop manual. By using the little kit of SU Tools (Moss, others sell this little pouch of tools) that mount in the air piston damper holes, I have discovered that linkage and wear problems that DO NOT show up at an idle sync setting can often manifest themselves as you go up in RPMs from the idle stop. This is because the overall linkage slack gets taken up by the forces generated by the throttle cable as you press on the gas pedal. What was in sync at idle can suddenly go out of sync as you press the pedal. This is not quite your problem, perhaps, but it may well be one you are not aware of yet. But it can be an additional problem caused by any other problems you are having with your carb linkages. Hopefully this is not true, and perhaps it is not, but on a car of your's age, it could be. FWIW.
Bob Muenchausen

Bob,
Thanks for the input. I haven't even gotten that far yet. I've had one of those SU kits for 30 years and will give it a go when I get the idle squared away. As an aside, while the car may be 41 years old I believe the actual mileage is only about 60,000 based on DMV records, discussions with the previous owner and general observation. She sat for 27 years as a restoration project with virtually no miles. I have pictures of the car taken in 1977 with the same Pirelli P3 tires. There is no appreciable wear on them (too bad I've been told they won't last long at this age). Here's hoping the SU's aren't hiding more "surprises"!
Safety Fast
Jim
Jim H

Bob,
Forgot to add the carbs are dated "11 63" which would make them March 1963 date. This corresponds well to the car number of 9150.
Jim
Jim H

Jim,

I heard of this odd month/year code just recently. Until then, I aways thought 11 63 meant Nov. 63. How does the system work for dating parts?

Paul
Paul Hanley

I bought a set of almost NOS HS6s from a guy who had the same problem w/his car. After getting them home and examining them the factory didn't center the butterfly on one of the carbs thus causing a high idle. His loss was my gain.
Mike MaGee

Mike,

You have pulled this thread nicely back on course. If cable is slack, Idle and fast idle screws are backed off the butterflie/s are not centralised. Slacken the butterfly screws and move the spindle such that butterfly centralsies and almost sticks in the venturi. You should not be able to see light through the choke tube. Tighten the screws and refit carbs. Open each butterfly two turns of the idle screws from this position start engine, reduce speed and synchronise.
Iain MacKintosh

Bob & Jim,

You both mention SU tools. Are these some kind of matched flow gauges. If so can you say a little more about them and how they operate.

I use a gunsons Carb Balance. I find it a little hit and miss. The best thing you can say about it is that it was cheap.

My brother had a pair of matched vacume gauges that he used on a motor bike. I tried attaching these to the SU HIF6s via the breather holes but there was not enough suck for them to work.

Thanks

David
David Witham

As far as date codes go manufacturers often use week numbers rather than months, so '11 63' would be week 11 of 1963 i.e. some time in March not November. No idea if this was the case with SU.
Paul Hunt

Thank you Mr. Hunt. Sometimes little bits a minutia like this crop up in a thread and I think most folks enjoy the trivia. Cheers!
Paul Hanley

David,
The SU Tool Kit is a small collection of pieces which fit together to produce a method for visualizing air piston syncronization, and included are also a rudimentary jet centering tool and usually, an SU pattern Jet Nut wrench. In the US, they are sold by Moss and others and come in a small plastic pouch and are usually advertized in the same area as UniSyns and Colortune air flow gauges.

I will give you a recap of something I have stated here before about using the "SU Kit of Tools":

"To determine balance you can also use the small SU Tools Kit which has an extender tubes to be fitted into the damper oil reservoir for each carb’s air piston and L-shaped wire "indicators' which are inserted into each of the plastic extender tubes reveal the height of each carb’s air piston in relation to the other. The two wires are long enough to nearly touch each other and can be adjusted with the engine OFF so that they coincide in your line of sight.

The one advantage that this type of balancing tool has over other balancing meters is that each carb now has an indicator {the wire} which will allow you to see the relationship of each air piston to the other all the way up the RPM range in one continuous sweep. This can be a useful indicator for showing up problems with the linkage which can cause the two carbs to APPEAR to be synchronized at idle but which cause the carbs to go OUT of sync once the linkages are actually pulled into engagement by tension on the accelerator. Remember, if your carb was worn enough to warrant a rebuild and possibly a new throttle shaft, the cams, adjustment screws and linkage pieces may be worn enough to not work smoothly as they did when new."
Bob Muenchausen

Thanks to all for their terrific input. Misaligned butterfly was the problem. 20 minute fix with some subsequent tuning and she runs like a watch.
Jim
Jim H

This thread was discussed between 22/03/2004 and 30/03/2004

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