MGA Forum

After dinner today I decided to tackle my carbs. The car ran, but not smoothly or sweetly. It now runs very well. Maybe my learning curve can help someone. It's not as complicated or as hard as it seems. This is the second time I've tackled this in as many months, bu this time I had a lot more prep and some practice, and some help from Barney and John Twist, and others. God love the internet.

I first set the dwell and timing - all good. I set the point gap at .015 and confirmed the dwell at 58 degrees and set the timing (at 10* BTDC per John Twist) statically rather than dynamically. The only thing I didn't do was re-check the valve lash, but I did that a year or more ago and haven't run the engine until this week, so I'll let them go for a while before re-checking them (and they're not making any noise).

I then took the carbs off (fun!). It's a lot easier to disconnect them and remove the front carb before trying to get at the bottom nut on the rear carb.

I had two major problems - 1) the air pistons wouldn't slide down and make a "clunk" when you lifted them - they just got hung up, and 2) the jets wouldn't move in the jet bearing without major effort (meaning pliers).

The first problem was solved by lapping the air pistons and the chambers using lots of rubbing compound and elbow grease. (From the A-Antics manual, it works really, really well.) One was so bad that it literally stuck hard in one position - but both are now free and smooth. It only took about 10 minutes per carb, so is well worth doing if you are having issues.. Be sure to wash and rinse all the compound off before reassembling. I tried first swapping the pistons and air chambers, but that didn't do it. It might be that they get that way with age, or that mine are a marriage and were never lapped into each other previously.

The second problem was fixed by deburring and then polishing the jets with 240, then 400 and then 1000 grit paper until they were mirror bright. I reassembled them with the teflon o-rings that Barney recommends, putting a drop of oil on the jet and working it manually to make sure it was free to move. (I had to order 50 o-rings from McMaster Carr - for $14 plus freight. Anyone want a set? I can sell some of the left overs. These should last almost forever.)

*Note. The carb kits I ordered from Basil that I used the first time I did the carbs are very complete, but the jets themselves that come with the kit are too large in diameter to fit into the carb jet bearings that are on my carbs! I had to reuse my old jets - so don't throw anything away. (No knock on Basil here - it might just be that my carbs are not 100% correct or that there are/were differences over the years...?) Again, even the jets I have had to be polished and made a wee bit smaller to really fit in the carbs and move smoothly.

For reassembly of the jets, I centered them by using the needle itself as the centering pin. Push the air piston down inside the jet as you tighten the large nut on the carb bottom. When it is tight, check that the piston moves freely up and down with the carb held upright. It should bottom out with a clunk, and not bind on the needle at all. If not, loosen, turn the jet bearing a bit, and try again. Pull the jet, put the spring and mixture adjusting nut back on and reassemble everything, again checking for free movement of the air cylinder. Set the mixture nut to 12 flats (two turns) back off from seated.

I checked the butterflies at the carbs' rear throats - one was a bit cocked and not seated evenly all the way around. I loosened the two screws, wiggled the butterfly into place, and then retightened the screws. The first time I did the carbs a while back, I took these out completely to remove the throttle shafts. If you do this as part of a complete tear down and cleaning, mark them with a sharpie - there are four ways to put each one in, and you don't want to swap them either. If you work on one carb at a time, you won't mix up the parts, and if you forget how one goes together, you have the other to look at for reference.

For the float chambers, make sure the rubber connections are not perished. Like with the trunion rubbers, mine are only less than a year old and are already cracking so that will have to be attended to next time, though they are not yet leaking. I use Grose jets which I picked up last year at the show in Vermont, and they seem to work great. Check that your forks are set correctly using the SU tool or the old drill bit trick.

Assembly is the reverse of disassembly - put the rear carb on first. Get a small child to put the bottom rear carb nut on for you. This takes longer than the rest of the job put together if you have large hands. Get yourself a "stubby" 9/16" or 14mm open ended wrench and this wil be easier...or cut a wrench in half. You could also grind a bit of meat off the sides of the wrench "cheeks" to allow better access. When both carbs are on, reconnect the choke linkage, but leave one of the 1/4" nuts on the throttle shaft linkage loose. Replumb everything, but leave the air cleaners off for now. Turn the slow speed idle adjustment screws out until they are not making contact. Then turn them in until the arm just starts to move. Turn each in another 1/2 turns to start. I added 20W/50 engine oil to the dash pots - I don't think special oil here is necessary.

Car started right up and ran smoothly, though was idling a bit fast. I hooked up my tachometer/dwell meter (great tool - go get one if you don't have one) and tried to set the idles screws down to 850 RPM, but they stopped adjusting at a bit less than 1000 RPM's (screws not touching the carb body anymore). Too high idle = too lean. To verify, lifting the carb pin on the front carb dropped the idle speed. The tach makes this easy to check. I backed off on the mixture nuts two flats each, and tried again. This time, I was able to set them to 850 RPM no problem.

My SU tool kit is still in the mail, so I tried to sync the carbs by ear using a piece of rubber hose as a stethoscope. To me, the rear carb is "whooshing" louder than the front, but I had to increase the idle set screw on the front carb a ridiculous amount to get them closer, so I abandoned the idea. Instead, I had the idea of using the tach and setting each carb to the point at which when turning it in any more caused the RPM's to rise above 850. Going back and forth, I got them both pretty spot on and then tightened the clamp on the throttle shaft again. This procedure is not documented anywhere that I know of and might be completely wrong...but I'll recheck the synchronization when my SU kit gets here. Barney says to do something similar - loosen the clamp, and adjust each idle screw in 1/2 turn past just making contact with the carb body, then tighten the clamp. Then turn each the same number of turns as you make your idle adjustment. In fact, using the tach as described above, the resulting positions of the screws was about the same as using Barney's set-it-and-forget-it method.

The car sounds really good. Before, it sounded like a motorboat - very put-putty or lopey even at speed, and was idling at about 1100 RPM or so. Now it is smooth and quiet, and idling nicely at 850. It will even idle lower if I wanted to. The one issue is that I get a bit of run-on after cutting it out, except the last time I started it and revved it up, it stopped when I turned off the key almost without any run-on, so this may be a non-issue. I won't get to test drive it and get it really hot until tomorrow. Somewhere on Barney's site he talks about this, and I think he said that a bit is normal. Also, I am running "regular" New England gas with 10% Ethanol...so I can try using hi-test, and I can probably drop my plugs one notch from NGK B7 to B6 as well. (Mine are NGK BPR7ES or some such alphabet soup.)

The choke action works now - I can actually move the jets in their bearings, though the cable is too slack and it's really hard to get two wrenches on the attachment and hold the cable tight unless you have tiny hands or a helper with tiny hands and arms. It's a hot night here though (40*F last weekend, now in the 90's...summer has arrived) and when I added any choke, it actually made things worse (too rich) - so the car is either warm enough, or I need to go back one flat on my mixture adjustment. I would rather be a bit rich than too lean, initially. I will drive it a bit, do the spark plug test, and see what the mixture looks like. It was definitely too rich before when the carbs were hanging up and such, so I'll have to burn off some of the soot on the plugs.

I'll post round two when I get the synch sticks. Question - I have a mercury manometer that was used for synching motorcycle carbs. Is there some kind of attachment that can be used for these SU's? Since there are no fittings on the carb body for this, I was thinking of a simple wooden disk that slaps over the carb throat with a 1/2 hole drilled through it for free air flow and a 1/8 hole to receive the fitting for the manometer tube (without the 1/2" hole, you just starve the carb for air completely, I think). I guess you could also drill and tap holes in the carb body to accept manometer fittings and plug them with machine screws when not in use.

Cheers,

JIM in NH



Edited 3 time(s). Last edit at 2011-06-09 10:14 AM by JimNH.

Nice. if you lived closer, I'd be offering cases of beer if you'd come by and help me get mine tuned.

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http://1956MGA.com
geoff@1956mga.com

My tuning kit came today. I am going to try to see if the synchronization is at all even close...should be interesting.

Got the fuel gauge fixed - want to go for a test drive, but it's pouring out right now

UPDATE - I checked the synch with the SU carb tools - and it was really, really spot on. So either my RPM method worked or I got lucky!

I put some high-test in the car today to see if that has an impact on the running-on after turning off the key. It really is running well now

The transmission isn't getting any better, though - reverse and downshifting into second are a bit dodgy. Rebuild in my future there I think.

~JIM



Edited 1 time(s). Last edit at 2011-06-09 06:29 PM by JimNH.

Jim,
Why did you sand down the chamber and piston assembly??? If they were dirty and keeping the piston from dropping. I usually just clean them. Did you try the drop test with both chambers and pistons to see if they are matched after you did the sanding? Pistons and chambers are machined matched. And yes they do get switched around over the years. That is one reason you do the test..

For the synch you can use a rubber hose and listen to match them up.

It seems I missing something on your repairing the SUs. I love the SU and I believe they are one of the best set up there is. I'll take it over almost anything else.

John,

They were very clean - they just didn't fit. They were getting hung up to the point where a drop test was meaningless- they would get stuck and wouldnt' drop.

Lapping them together is how they are originally prepared one to the other so that is what it takes to get them to that condition given where I started.

Lapping isn't really "sanding"...it's more like polishing and the amount of material removed was incredibly small - and it removes it only where needed to allow a good fit.

I should take one apart again and take pictures...but if your SU's won't pass the drop test then you might try this!

Listening with the hose was a non-starter for me - guess I don't have a good enough ear or something - or my hose was defective, but I don't think I'll be able to synch the carbs that way with good result.

I have used a Dremel tool with a polishing disc and polishing compound to clean and polish the cylinders.

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Gary

Murphy's law

Tell a man there are 300 billion stars in the universe, and he'll believe you. Tell him a bench has wet paint on it, and he'll have to touch it to be sure.

Jim,
I'd be interested in two sets (qty:4) of your teflon "O" rings if they haven't all already been spoken for ;-). If the offer still holds can you PM me at king_alex@msndotcom.
Thanks,
John
1957 MGA
1958 Sprite
1959 Sprite
1959 Mini

OK - need help now.

I drove the car for a while today. The carb mixture nuts were at 14 "flats" back from all the way in, and the engine was idling at 850 RPM when I finished tuning the other night.

Now the idle has risen. I tried putting them back to 13 flats, which I now think was the wrong way. If I press the pin in on the side of the air chamber, the engine speed drops a lot. I can't imagine that I have to richen them up to 15 flats...seems excessive.

Does this mean I'm getting air coming in from somewhere else that is causing the high idle? I am thinking that once the engine warmed up, it started pulling in air from somewhere else...like the carb manifold gaskets or something. I have some carb cleaner, so I will try spraying that around to see if the idle drops when I spray near the manifolds.

I will also hook up the dwell/tach again, since I don't know if I trust the tachometer (I cleaned the tach and the speedo, and they are working much better now - though the speedo reads high - not calibrated for this car, IMHO.)

It is so hard to reach the mixture nuts, that I am loathe to mess around too much without knowing what my plan of attack should be!

Thanks.

JIM

I used to have a sticking carb problem. Try flapping the gas pedal. Push down a little and then slip you foot off and let it pop back. I fixed mine by increasing the tension on the shaft springs.

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Gary

Murphy's law

Tell a man there are 300 billion stars in the universe, and he'll believe you. Tell him a bench has wet paint on it, and he'll have to touch it to be sure.

The tach in the car is reading really high as it turns out. It seems to work well, but it reading 1500+ RPM when the car is really idling at 1000 RPM.

I can see whey a speedo would be off due to calibration factor, but RPM is RPM, so I thought the tach would at least work.

I did reset my float heights - they were a bit low. The rubber bowl connection under the rear carb is in fact leaking now, so the rubber there doesn't last long either.

I am going to put the mixture screws back to 14 flats.

JIM in NH