MGB Trouble with adjusting the front SU carb- 74 MGB, HS4 carbs [long]

[wkilleffer]

Hey everyone,

After alot of work and downtime, I finally got my 1974 MGB to run and drive again. It smelled like it was running rich and suffered from a richening mixture and rough idle whenever the temp gauge got to N or slightly above.

I read that the carbs need to be adjusted semi-independently, both here and in other forums. In my case, the car may be a 74, but the engine is closer to 72 and equipped with HS-type carbs. The floats in the bowls are plastic and show no sign of leakage, and the jets are grosse jets. Before the downtime due to starter failure, I got the timing set and adjusted the valve clearances, so those should not be issues.

The needles are spring-loaded.

What I have noticed is that when lifting the pistons and letting them fall, the rear carb seems to let them fall faster. The sound may be more definite, even though they both fall and hit bottom. Front carb seems slightly slower. I did this after emptying the oil from the pistons and giving everything a wipe-down with carb cleaner.

Got started by slackening the interconnection between the carbs and adjusting the idle screws 1.5 turns open. I then set the mixture screws down 10 flats from the bridge. Something I had noticed when doing this is that at a point near the bridge, the adjustment nut did not seem to be doing much. So, I gently pushed upward on the bottom of the jet, and it popped up to a point slightly above the bridge. So, I backed it down to bridge level and ten flats down (lean) from there.

The car fired right up and reached 2000 RPM, and did not take long to hit N on the temp gauge. I backed off the throttle screws half a turn at first in an orderly way until I got the idle to a point between 500 and 1000. I then used the Carbbalancer to check their breathing, and after some adjustments got them to breathe at the same rate while preserving the idle speed. The idle was a little rough and the exhaust smelled pretty rich.

At this time, I decided to use the lifting pin to check mixture on the front carb. It seemed to always be producing an indication of a rich mixture, even when I turned it almost back up to the bridge, where it came close to stalling. I backed it down 10 flats and tried the rear carb. It was rich at first, but after turning it two flats lean, I got a textbook response for the correct mixture. A slight upward idle blip, then a fall-off. I set both carbs at the same place and drove the car. It drove better, but still seemed a little off and smelled rich, though not as much as before. The rough idle with rising engine temps was less of a problem and the car would start right up when hot. Previously, starting it when it was hot could be a bit of an ordeal, like it was getting heat soaked.

Could the front carb have a needle and jet issue, like being off-center? I have an SU tool kit with a small jet wrench and a jet-centering tool, but the instructions for using that tool are not very intuitive. Also, someone on here once said that the spring-loaded needles do not need jet centering.

I am concerned about the fact that the piston will not fall as fast on the front carb, and also wonder if the adjustment problem could be caused by dirt in the front float chamber.

Any ideas or suggestions would be appreciated.

Thank you,
-William

 

[Sarastro]

The rate at which the pistons fall isn't a concern when you're setting them up, as they're stationary. Did you check the floats? If the float valve is sticking, or if the fuel pump pressure is too high and is blowing fuel past the float valve, you'll get a rich mixture that you can't adjust out. Look down the throat, and if you see liquid fuel all over the place, that is probably the problem. I also don't quite understand the bit about the jet popping up above the bridge, but it sounds like the jet might be sticking--it has to move smoothly in response to adjustment or the choke.

 

[JPSmit]

Confirm that the needles match. It is amazing to me how often they don't

 

[wkilleffer]

Thank you both for your replies.

Sarastro, the business about the jet popping up above the bridge: while turning the jet adjustment nut in the lean direction, it did not seem to be having an effect on the level of the jet. So, I gently pushed upward, and it popped up, higher than I thought it would. From that point on, it seemed to respond as designed. So yeah, a sticking jet could be an issue. I will check the floats and valves again, especially since the car was sitting since November and may have more dirt in the fuel than typical. It may also need a new fuel filter. The fuel pump is a genuine SU with points.

JPSmit, I will check the needles today. I have had the carb rebuilt within the past ten years and will try to find the sheet that the rebuilder sent back, see if anything was said about the needles then.

Thanks again,
-William

 

[Sarastro]

Ah, OK, I get it. I can't visualize MGB carbs, but usually there is a spring somewhere that keeps the jet all the way up. In my Bugeye, it was part of the choke mechanism; probably the same for MGBs. If it is sticking, or the spring is missing, you might not be able to adjust the mixture--as you move the adjustment, the jet isn't moving.

And, yes, dirt in the fuel system could lodge in the needle valve--a classic cause of needle valves failing to close properly.

If you are concerned about the dampers dropping at different rates, you can try switching them--it might improve things. Also might make it worse, but it's worth a try. Best to do this before tuning the carbs.

Good point about the needles, JP. When I first took a close look at the carbs in my Bugeye Sprite, I found that it had dissimilar needles--and the PO had rebuilt the carbs! Obviously with a kit that didn't include jets and needles.

 

[wkilleffer]

Ok, an update:

The needles were the same when I had the carbs rebuilt. Neither the rebuilder nor I changed them.

The possibility of dirt in the front carb float chamber lead me to take it apart and have a look inside. The plastic float was in good shape and no leaks. I took the grosse jet off and blew carb cleaner through it. When it was off, I noticed a flat steel washer between the chamber lid and the jet's rubber washer. I took the rear carb's float chamber apart and did not see any such washer. So, I took the washer out and reassembled the float chamber.

I then did the whole tune and balance thing, setting a good idle speed and making sure the carbs breathed at the same rate.

Now, they behaved differently this time. I had the jets set lean 10 flats from the carb bridge on both of them. When I raised the lifting pin on the front carb, it made no change. I have been told that that's a sign that no adjustment is needed. This time, when raising the pin on the rear carb, there was a slight upward blip in RPM followed by a fall-off, which once again is supposed to suggest that nothing further is needed. I went ahead and leaned them half a flat to make allowances for the air filters, and may make that a whole flat after a test drive.

Is it too good to be true to think that they need so little in the way of adjustment? It's a nice thought, for sure.

Thank you,
-William