Amazon: Carb reassembly help

[AidanC]

Hi,

I have twin SU HS6 carbs on my B18 engine. As the car hasn't seen much use over the last few years, I decided to take them apart and clean them out. I replaced the jets, needles and gaskets but have run into a problem on reassembly.

I refitted and centered the jets and fitted new needles. However, on one carb, the dashpot will not go down fully as the needle is fouling the jet. The dashpot from the other carb will sit happily in either carb so I don't think that there's an issue with jet centering. The old needle when refitted to that dashpot has the same issue on both carbs so I don't think that its the new needle either.

Is it possible for the dashpot to misalign the needle in some way? I've tried rotating the needle in the offending dashpot through 360 degrees in 90 degree increments but have the same problem every time.

Both dashpots have the same part number - AUC1332

I'm probably missing something obvious but as I've eliminated the new needles and jets as suspects I'm now wondering about the dashpot itself.

I vaguely remember having a similar issue when I fitted the newly refurbished carbs approx ten years ago but was able to fiddle around with everything so that it eventually worked but that's not the case now. I thought that the area into which the needle fits used to 'float' slightly before the holding screw was tightened but neither dashpot does this now. That said, I could be confusing them with other carbs that I worked on before.

So, to recap, one dashpot and needle works fine in both carbs, the other dashpot doesn't, regardless of whether the new or old needle is fitted.

Where am I going wrong?

[Burdekin]

Pics would be helpful.

One thing is you want to ensure the piston is matched to the correct dashpot. Have a look on YouTube for SU piston drop test. Doesn’t sound like this is the issue though.

Check if the piston is fouling the edge of the bridge cast opening. Once centred the piston should drop onto the bridge with a clunk.

Is the choke pushed all the way up? How have you got the jet adjusted? Is the jet tube bent? Is their oil in the dash pot?

John Twist has good videos, centring the needle: https://youtu.be/sxVFCY_1aSA

[Ron Kwas]

Aidan;

Presuming you have not mixed Dome and Dashpot, as these are matched and selected at time of assembly, (so should not be interchanged), the things I can think of which will prevent Piston from falling completely to bridge/venturi would be:
1. Jet position is no longer concentric with Metering Needle (causing MN contact and interference).
2. Bent or out of proper position Metering Needle (causing same)
3. Jet is installed at an angle to its mounting surface (causing interference at some insertion depth)
4. Dome is not cleanly on its mounting surface on carb frame (again causing non-concentricity)
5. Debris in Jet is not allowing full MN insertion (also causing interference)

Since you have one carb which functions as expected, it can be used as a model to help find the issue with the non-functional carb...you could even try some swapping/interchanging of Dome, Dashpot, MN, but (only!) for the purpose of troubleshooting!

See also: https://www.sw-em.com/SU%20Carb%20Se...-Contact_Areas
https://www.sw-em.com/su_carbs.htm

Good Hunting!

[blueosprey90]

The MGB used a SU HS4 and maybe the MGC used a SU-HS6. The only real difference is the size of the carbs, 1 1/2' vs 1 3/4".

So if you go to the MGExp.com and search the forums, you may come up with something. Use the advanced search function.

I agree with Ron's assessment. I suppose it is also possible that the set screw that holds the needle is not recessed in the piston and therefore binding against the inside of the dashpot. It would be #15 on the parts list for these instructions.

https://www.mgexp.com/article/su-hs4...ction-sheet.44

Centering the jets:

https://youtu.be/sxVFCY_1aSA

Here is a video to rematch your piston to dashpot.

https://youtu.be/IfU47Oqq9wA

[AidanC]

Thank you all very much for your help and suggestions. The videos were very interesting.

I spent a few hours today going though everything from the start and still had the same issue. I swapped everything between the two carbs one piece at a time as per Ron's suggestion and found that the issue followed the jet assembly (as you'd expect). Once I had the carbs back together with their original pieces, it finally occurred to me to slacken the jet adjustment nut slightly (two flats) while I was centering the jet. The needle stopped binding immediately. Once I tightened the jet locking nut, I was able to retighten the jet adjustment nut and the needle was still free and the dashpot hit the bridge with a satisfying clunk. So it looks like it was #3 on Ron's list.

Thanks again, everyone.

[northNH]

FWIW Although the warning is well-known I have never found an undamaged piston which had any binding/sealing problems with any randomly selected undamaged dome.

Have I just been lucky?
You’alls past PERSONAL experience?

What’s the currently acceptable term for “Old Wive’s Tale”?

[Derek UK]

Yes the pistons will all fit the dashpot OK but the drop time has to be slow not fast. Hence the need to match the two together. Having a slow pot working with a fast pot will cause one to run richer or weaker than the other. They shoild be very close to having an equal drop time. Mark the piston and pot with a dab of paint if you are doing a complete strip down. Clean the pistons and insides of the pots with petrol or carb cleaner only. No polish or abrasives.

[JP 1800]

Hi Aiden, I sent you a pm.