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Tips

SU Carb Rebuild

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Dadandson

Dadandson

Jedi Trainee
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Please be patient with this lengthy conundrum. I bought a set of SU carbs on eBay. I have completely rebuilt them. New jets, throttle shaft, needle valves in the bowls and all gaskets. I have cleaned and glass beaded all the parts and have cleaned the meticulously before I reassembled. I need to mention that the car has a set of SU's that seem to work pretty well. In fact the car runs great. All I did was try my new rebuilt carbs. I put them on the car and she fired right up. I adjust the vacuum and the mixture to what I thought was correct. Everything seemed fine. My son took it for a test ride and after a few minutes it started to chug violently. I have tried every adjustment and it still chugs. What should I look for? Dirt, bad parts or hopefully it just needs adjustment. I would appreciate any and all help. This is my virgin experience with these type of carbs. Thanks
 
I would put the old carbs back on and see if the problem goes away. That would confirm that it is the new carbs fault and not something else (it could be a total coincidence that it started chugging after installing the new carbs).

Maybe I'm getting this wrong, but I remember hearing something like 90% of all carb problems are ignition related. Assuming that the car still chugs with the old carbs, I would look at everything in the ignition system and go from there.

Assuming that the chugging goes away with the old carbs, then I would look very closely at what you did to the new ones. Disassemble again and make sure there are no leaks in any gaskets, etc. Check the floats for holes. Make sure you have fluid in the dashpots (might have overlooked it). Then just keep tweaking the mixture and get both carbs balanced. Use a Uni-Syn or similar device......the old rubber hose trick is reasonably accurate.

That's all I can think of. I am no SU expert, but I've fiddled with mine enough to think I am! :smile:
 
I will put the old ones back on tonight. Maybe I fouled a plug or something.
 
I vote for either gunk that got mixed or air getting in the system - can't understand how timing would be different if you haven't changed it
 
Spoke with my Local MG Shop and he confirmed that the initial carb mixture setting may have been way too rich and probably fouled a plug or two. That seems to be were I will start tonight. I will also take John's advice and look for a gross air leak. The funny thing about this is it ran great with the new carbs but only for a short time. I did not change the timing, points, rotor or condenser, which are fairly new. The cap and wires are about three weeks old and were working fine with the old carbs. I am really hoping it is the plugs. I will update later tonight with my success or failure.
 
New carb setting open 12 flats.
 
Please Jack, "12 flats"?
 
Starting with the top of the jets level with the bridge in the carb throat turn them down twelve flats (two complete revolutions on a hex head).
 
Trevor Jessie said:
Starting with the top of the jets level with the bridge in the carb throat turn them down twelve flats (two complete revolutions on a hex head).
Click to expand...

Or .070-.075" down from the bridge measured with a dial caliper, with the dashpot and piston removed.
 
Got it. Wish me luck. Thanks and I still think this is fun!
 
Update!!! Put in new plugs and the same violent Chugging. I put the old carbs back on and it purred like a kitten. I opened the front float bowl on the rebuilt carbs and it was full of fuel. I opened the rear bowl and it was empty! It seems the ball style needle valve my local MG guy sold had stuck closed! I will order a pair of the viton tipped needle valves and try it again. Live and learn. At least the car is running for the Labor Day show in downtown Phoenix. Have a great weekend and thanks for all your help.
 
Gesh, good on you. Least you found the problem and it was not of your makeing.
 
I hate to be a "I told ya so!" but those GrosseJet thingies are junk.


Viton tip needles are TATS.

:thumbsup:
 
By ball style needle valve is that the Groose Jet? I hate those things! I installed a set on HIF carbs for my 73 BGT. Stuck wide open allowing fuel to get pushed and gush out the charcoal canister. Threw them away and installed proper ones.
 
This happened to me last time I rebuilt a set of carbs. Started the car, got the chugging you describe--really impressively bad running. The needle valve's tip was some kind of red material, maybe silicon rubber? But after a little fiddling, it stopped sticking (perhaps the fuel washed off some sticky crud on it), and then it wouldn't seal well.

Just got a delivery of needle valves from British Parts Northwest. These have a black tip, which might be viton or maybe buna-n. Dunno. If it works, I don't care.
 
Sarastro said:
Just got a delivery of needle valves from British Parts Northwest. These have a black tip, which might be viton or maybe buna-n. Dunno. If it works, I don't care.
Click to expand...

That should be the viton tipped unit, should gold plated in color, with a black rubber looking tip, sounds like that you got the good ones.
 
:iagree:
 
Over the years, the single-most source of aggravation has come from dealing with faulty/worn float valves!!!! IMHO the cause of a majority of smooth-running problems! (Grosse jets are JUST what their name implies!!!)
 
Black tips have been ok in my experience.
 
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