Just looking at them I'd say the oil was congealed more on the left one than the right. I've gone throught the same carbs twice, 25 years apart. They were almost glued solid. Cleaned em out and used straight 30 weight. I had the manual and I'm pretty sure it had the amount of oil. Unfortunately the guy who owned the car died 12 years ago and no access to the manual. The carbs were on an MGA 1959. Square cut gears in the tranny and you had to double clutch to downshift. Fun to drive once I got the hang of it. Felt a little like Sterling Moss. Rupp rupp rupp-
@D3Sshooter has extensive, informative videos on the SU carbs. He mentions that the pistons and dashpots are mated, so they shouldn't be swapped when working on them. It could be that yours had been during a prior repair. Thanks for the video. Every little bit helps.
You could use new washers on the float shaft. Both show signs of leaking. Observation, ATF is way too thin and it will cause lean conditions at higher RPMs. And finally SU carbs only need to be filled where you're a good inch from the top and maybe even a little more than that. I fill mine by resting my finger on the lip of the piston, the level is matching the top of my finger. People over fill them constantly. You want to know the true level, fill it as high as you want, take it for a drive then look, you'll see it is over an inch from the top after sucking all the extra out. That's how high you should fill it. And to really understand how well you're running get a AFR gauge installed, that is how I found and fixed all my fault issues with my HIF4s. P.S. 20-50 works best for me. It is 20 weight in the winter when you want it thinner and 50 in the summer when you want it thicker. ATF does not react anything close to that and is even thinner than 20 weight. Remember in an SU it's there mainly for dampening, meaning stopping the pistons from micro movements, the secondary effect is also to slow the rise of the pistons in concert when the spring weight used. You mess with one it throws the other off.
I think they look bent in the video too, but I am thinking that is because the brass part at the bottom is a little loose and cocked to one side. I'll double check them for sure when I get home, because that could be it! Good catch!
@@MyMGBJourney If I were you I'd get new ones anyway, there are two types for HIFs so make sure you get the style you have which. You can see the difference in the pictures on the Moss sight. They are slightly different in size on the barrel by a .002 or so in size. That relates to the specific model of HIFs, there are two different part numbers used in the 4 years the appeared. I would also get a set of new Red Springs as well, they will wear out and lose tension causing you to also run way too lean at cruise etc.
I think they look bent in the video too, but I am thinking that is because the brass part at the bottom is a little loose and cocked to one side. I'll double check them for sure when I get home, because that could be it! Good catch!
I agree! As carburetors go, they are very simple. They are much closer in function to a motorcycle carburetor than a traditional 4 barrel, and most of the methods of tuning a "traditional" carburetor have no place with SUs. Now that my SU's are freshly rebuilt, they are super simple to tune. I'm just talking about all those little SU specific things that can trip people up.
You are kind of there. All the fluids you mentioned will work, but the viscosity of the fluid controls the rise of the piston which is how you can tune whether the carburetor goes lean on acceleration.This is a Black Art. If you are getting popping and spitting you are going to lean go heaver. SU's do not have a accelerator pump which is what the piston is compensating for. Be careful to not mix up the pistons and the dashpots as they are matched at the factory. Sounds like theo PO mixed up the dashpots. I typically use the same oil as the engine or MMO. The amount is to the top ring. BTW the spring on top comes in different spring rates which can throw you a curve and the plunger at the end of the screw on cap is a check valve which also controls the rise of the piston. BTW looks like you have vented dashpots and vented screw caps, which would give you two vents per carburetor should only have one. Check you throttle shafts, they are most likely worn and leaking vacuum and need to be rebushed. Check with carb cleaner of starter fluid, spray around the shaft and if the engine speedup they are leaking. Start watching John Twist videos or put on as set of Mikuni carburetors. Mikuni ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-30bXQr4oGmI.html John Twist ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-4nvGLgO6pj0.html More than you will ever want to know about SU carbs: mgaguru.com/mgtech/carbs/cb130.htm John Twist Site: universitymotorsltd.com/technical-help/
Thanks Jim! Fortunately, this is after a fresh rebuild so I know my throttle shafts are in good shape. I had no idea that pistons and dashpots were matched at the factory though. Based on my result, I'd say you're spot on.
It's oil viscosity, you can tune them to tempatures, it calls for standard 30w oil. Thinner for warm, thicker for cold. Why? Because cold air is denser, warm air is thinner. BTW, look closely at the rod, there is a hash mark for the proper level. 20 wt in the cold 40 wt in the warm.. 30 wt for the average. And measure the diameter and look at the grooves to make sure they are the same.
@michaelannen4168 yeah, it should be the 20wt for the summer and 40 for extreme cold. Air will ram the pistons up too fast if it were thin in the winter and too slow in the summer..
There is a small vent hole in each cap and they weren’t blocked. The resistance comes from the fluid moving around the small piston hanging off the cap and not the cap itself. I guess the interference between that little piston and the bore was off by just enough!
Hopefully someone can answer this. What is the pipe that loops over the top of the carbs and it looked like it tees off somewhere. If someone can let me know would be much appreciated
Hey There! That T is supposed to go over to the charcoal canister if you still have it fitted. If not, it can simply be capped off. This diagram might shed a little light: mossmotors.com/mgb-017-emission-system-controls-1970-74
20 weight is correct! Your dash pots are incorrectly fitted looking at your he video. The casting boss at the top of the pot should point away from the engine! Your first carb is correct. Helpful?
Oh wow! I didn't even notice that 🤦♂! Could that really make a difference? No matter, If I can just spin it, I definitely will, if for no other reason than the symmetry. And yes, I could always use this stuff mossmotors.com/220-226-dynolite-carburetor-dashpot-oil-125ml which is 20 Wt.
Oh I definitely agree! I would never put anything that heavy in the dashpots. I have heard the argument, however, that because the parts are old and have inevitably worn, the heavier oil option takes up the slack. As slow as my air pistons move with 20w50, I would never use anything heavier than that. I also agree with your second point, but after a full rebuild, this is still part of my "set them up correctly" process. Thanks for your input!
@@MyMGBJourney My 63 MGB has the original carbs, not piston wobble, no sticking , no evidence of some moron sand blasting them.. worn throttle shafts just renewed. Overhaul them, or have a pro like Curto do it..
@@jeffhildreth9244 I just rebuilt mine with a kit from sucarb.co.uk and it made a huge difference. I think that sand/vapor blasting the bodies is just fine as long as all the holes and passages are completely sealed up. I can't imagine trying to tune a carb with grains of sand grinding around inside. 😖
