MGB & GT Forum

So, just for the heck of it I removed the brand new Bosch plug wires and put the old Bosch wires back on .................................. SHAZAM! The car is almost back to normal!

Now, having said that, it still has a bit of a bog if I mash the throttle. If I ease on the gas she will climb to 60+ easily. No popping, no spitting.

Now, while cracking the throttle and watching the carbs, I see that the damper on the rear carb begins to raise immediately while the damper on the front carb does not move unless I really crack the throttle. I have just had them apart, they are clean, have proper oil level, and, by the "Twist" method, they are matched.

Again, no popping, no spitting.

Reading through your first post, two things bother me. First, the high spot on one vacuum chamber. With the piston/chamber fitted to the carb but with the damper removed, does the piston now move up and down its full travel with no hesitation at all? If so, it's OK. Second, the two different damper piston lengths will likely lead to uneven initial piston movement when the throttle is opened. Find out which one is correct for your model of SUs and get a second one of that. If the pistons still rise unevenly remove the piston that rises slower and pour out a bit of the oil and top it off with 3 in 1 household oil or an equivalent. If the pistons still don't rise evenly repeat until they do.

Finally, I can't possibly tell you what amount of resistance is right when you lift the piston. It's a matter of feel you develop from experience. I always used 20 wt oil in the damper chambers and when I tried 20-50 wt recently, as is commonly recommended today, the resistance felt way too firm and the AFR meter showed the mixture going way rich longer than necessary (3-4 seconds) for clean acceleration. I did the oil thinning described above until the resistance felt correct and the AFR meter showed the mixture going rich only momentarily (1 second) and acceleration was crisp and clean.

Silas

Hi, the pistons move smoothly in the chambers when lifted manually. They seem to lift fairly easily. As for the damper piston lengths, I was wondering about that. I should probably get a matched pair.

So both dampers should rise roughly the same under acceleration?



Edited 1 time(s). Last edit at 2024-04-10 07:05 PM by Red Guy.

So, education needed;

What is the function of these dampers and what impact will the different sizes have on performance?

"What is the function of these dampers and what impact will the different sizes have on performance?"

They are one way valves, that resist movement on one direction and very little in the other. Slow to rise, fast to drop. Something like the valves in a shock absorber.

No way to know if the different lengths have any differing effect at all. It isn't their length that makes them work, it is the internal damping. If they don't meter oil the same, they won't respond the same. If they meter the same, their length isn't relevant.

It may be that the difference in length correlates to a difference in damping due to changing specs over the years, or different applications that used the SU carbs. Don't know. Perhaps someone who has been rebuilding these carbs for the past half century can say. I would not make any assumptions.

Take the dampers out and see if your pistons react in synch without them. At this point you don't know that one pistons reluctance to lift is due to the damper. Could be sticking. Could have a dragging needle not centered in the jet. Could be blocked air ports on the carbs and pistons. Could be different rate springs on the pistons.


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Greg x2.
Test piston moving speed w/o dampers, change dampers front to rear, check holes, check springs, put in spindle oil or similar.
If statically everything is sound, synchronise teh carbs using synchronising tool, its easier than with the SU wire needles. I guess the acceleration at full throttle will give you a grin!
Cheers
Bernd

New does not always mean good.

Does the piston on the right have some "play", i.e. is it fixed to the shaft, or can it be moved up and down on the shaft? It it moves on the shaft, that piston would have a delayed reaction. If it doesn't move, and the diameter is the same as the other piston, the difference in lenght doesn't matter, since the pistons would both be immersed in damper oil throughout the acceleration cycle, as pointed out by Greg McC. The pistons delay the increase in venturi size when the throttle is opened suddenly, increasing air flow velocity (see Bernoulli) and enriching the mixture, somewhat like the accelerator pump in a fixed venturi carb, or the primary and secondary throats in the Weber DGV. The SU and Zenith carbs are elegant devices when sized and tuned correctly. But that "tuned correctly" part is important. Joe Curto can supply what you need. Or Burlen UK :SU damper assemblies

I once addressed a mystery similar to this (1976 retro-fitted with SU's and a points set-up) by switching in and out, different dampers. If memory serves, these dampers are numbered. The stampings may indicate specific characteristics, and although this is how I began my analysis, I just collected several different ones and did a trial and error experiment.

In my case, my SU's were tuned and the car was idling great but 3000rpm was the power limit, My mixing and matching eventually got me a noticeable power bump, but frankly, it was never where it should have been. So I would say these dampers are a variable in the equation, at least. I never found anything else on the car, ignition related or fuel related that made a difference, including the pistons. BTW, I was recently told by an verifiable expert that the oil in these SU's, although recommended by the maker to be 20/50, doesn't in fact mean much when the system is set up properly. I use something in the neighborhood of 10/30 now.

Someone mentioned Joe Curto in this thread. I would give him a call and see what he thinks!

DD

I have 10-30 in them as well, although, Twist mentioned that the viscosity can alter the amount the pistons move under acceleration. My situation right now is that the piston in the rear carb rises as soon as the throttle is opened even a bit while the piston in the front carb does not rise at all except under WOT. Both pistons move easily when lifted manually.

The jets need tweaking.
Try changing the jet on the front carb 2 flat at a time, to see if you can get the front carb piston to rise earlier.

I'm going to throw out an idea. What causes the pistons to rise? Opening the butterfly, causing a drop in pressure in the venturi, lowering the pressure above the piston and causing it to move up. There are several reasons the piston isn't moving as fast, a couple have been mentioned.
Other problems not mentioned.... bad gasket on the carb allowing air to leak in instead of lowering the pressure in the venturi. Check by using a flow meter when reving the engine. Both carbs should show the same reading.
Another issue could be the piston itself is not providing an "seal" and allowing the pressure to equalize between the top and bottom of the piston. A drop test will help. See Twist video
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Another test which I thought I saw mentioned earlier would be to remove the dampers, tape over the top and run the engine. See how fast the pistons rise and drop when reving the engine.

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Jay in La Marque, Galveston Texas.
In God we Trust. All others bring data.
Trust, but Verify

How about this, Just for the heck of it, I stuck my finger in the rear carb while opening the throttle and prevented the piston from rising too quickly ............................ the engine did not bog. Is it possible the rear carb is getting too much air too soon?

Jon S.

Have you properly tuned and balanced the carbs? If not you are just pounding sand and wasting time.

BH

They were properly tuned and balanced, the car was running great, I pulled away from a stop sign and the car bogged at about 2500 rpm. Prior to that, no issues.