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SU Carburettor Piston Drop Test
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- čas přidán 26. 02. 2020
- When you have multiple SU carbs on your engine it is vital that the right piston goes to the right dash pot. This test is commonly knows as the drop test and in this video I will show you you can test it yourself.
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Adam, I was VERY concerned to see your drop test with the piston pointing down, and you having to stop it with your finger. Dropping it onto the floor can have disastrous effects.
I would refer you to the SU Reference Catalogue 1930-1980, which details this procedure.
For the HS8 carburetter, p170 of the catalogue, it refers you to the procedure for the HS2, HS4, and HS6, on p 168. I quote the relevant paragraphs (31 - 37).
NOTE : The following timing check applies only to standard Suction Chambers, and need only be carried out if the cause of the carburetter malfunction which necessitated the dismantling has not been located.
31. Temporarily plug the piston transfer holes. (I actually use plugs of wet tissue paper)
32. Fit the piston into the chamber, without its spring.
33. Fit a nut and screw, with a large flat washer under the nut, into one of the suction chamber fixing holes, positioning the washer so that it overlaps the chamber bore.
34. Fit the damper and washer, if fitted. (I note that you did NOT fit the damper in your test).
35. Check that the piston is fully home in the chamber, INVERT (my exclamation) the assembly, to allow the chamber to fall away until the piston contacts the washer.
36. Check the time taken for the chamber to fall the full extent of the piston travel. For carburetters 38mm (1.5in) to 47.6mm (1 7/8in) bore, the time taken should be 5 to 7 seconds.
37 If the times are exceeded check the piston and chamber for the presence of oil, foreign matter and damage. If after rechecking, the time is still not within these limits, renew the suction chamber assembly. (I usually clean the inside of the chamber with a paper towel moistened with Isopropanol Alcohol).
I have an HD6 on my car, and the procedure is the same apart from the use of the bolt, washer and nut, (which limits the travel of the piston). I usually count, 1000, 2000, 3000, etc., upto 8000, (it really takes a second to say thousand), before catching the suction cover in my hand, but that actually allows an ''extra'' thickness of the piston to exit the chamber, thus increasing the time taken.
I also mark the 'keyway slot' on the suction cover so that I 'know' that this is the exact same location as in the operating position.
I hope that this was not too long winded, if you want a copy of page 168, or any other page, then let me know !
Please comment !
Great video, found it just in time to see that the pistons had been switched on my SU hs6 pair. Thanks!
Thank you Adam. I have never seen this test. 🙏
Thanks for your video's they are not to long each and clearly to the point.
Glad you enjoyed it!
"Swap the pistons over".... I had 2 moving at very different speeds. So I saw this video and swapped them over.
Jesus it fixed everything. I rarely comment on videos but this one was 100% bang on accurate. Thank you.
Liked and Subscribed.
I'm really glad I could help Trevor! Hope your car is running well now
Never come across that test before, I'll have to try that on mine. Many thanks for the info.
Glad you found the test useful. Thank you for watching!
Great video. thanks for posting...
Thanks great easy test I’ll try it on my MGA
Very nice test and thank you. I will try this on a pair l recently purchased for a future project.
So far they have been mismatched on all my carbureted cars. They run a lot better when they are correct
Great tip thanks
Hey just found you channel awsome. Thank you all this great info to get me old girl running better. 75 s1 xj6 4.2
Very good information! Thank you from Argentina!
Thanks for watching 😊
Fascinating, great tip, thank you
Thanks for watching!
Going one step further, what do you do if you can't resolve a big difference in the drop rate?
A great tip 👍 Thanks.
Thank you for watching!
Great tip.
Glad you enjoyed it!
Tested on my son´s -71 and found it mixed, switched and now they fall at same speed. Super tip.
Great that you found that! Hope it runs even better now 😊
Good info and good way of explanation... please do the tuning video on car as well. Good work.
I will 😊 Thank you for watching!
Thank you good information.
You’re welcome
And what to do if they're still not synchronised after interchanging the pistons?
Good video, I have a massive difference with mine, whether I swap them around either way???
Awesome info! Thanks so much!
Thanks! Pretty sure the ones on my 72 MG Midget were swapped. They fall at the same speed now.
I have a 1952 mg TD carbs didn't have tag # the kit that came had slightly thicker metering rods. I don't think I tightened all the way seats car only runs with choke on
Yes, to get the best sync of multiple SU carbs, you'll want to match piston drop rate as much as possible. John Twist showed this to us in 2007. Look up his MANY vids on MG's and other British cars. Drop test is nothing new - SU documentation has described it for many decades.
John Twist has som great videos! Thank you for watching 😊
Thanks Adam. I've never seen that test before. I will try it on mine. When I saw the title I assumed you would be doing the "drop" test with the carbs together which checks that the needle is not catching on the jet (as detailed in my Mark 2 service manual). Wouldn't you need to also perform that test if, as with your result, you ended up switching the pistons over?
On these "later" carbs the needles are spring loaded and self center. On earlier carbs like on the MK2 or my 3.8S one would need to check if the needles are centered.
Thanks for watching!
@@LivingWithAClassic oh I didn't know that either! Thanks for the response.
What oil must be used in the carbs
Can You explain strongberg carb ? I have a Range Rover 1973, 3.9 engine and i want make a tune the carbs
I have a rebuild and rune series on Strombergs. I think you’ll find them if you search for Stromberg and my channel name
If you swap them over, you need to reposition the jets?
On the older style carbs yes but the HS and newer have self centering jets
Wait a minute here! How did you keep the piston from falling to the floor? What stops the piston at the edge of the suction chamber (dashpot), plugging the holes with tape only? If suction fails would the piston fall to the floor damaging your jet needle?
I have 2 fingers under that keep the piston from falling out
@@LivingWithAClassic Oh I see them now. I need to be more observant. Maybe I'll try that test on my HS6 S.U's on my '67 1800S Volvo. Thanks for the video and response.
@@LivingWithAClassic I agree with Stuart. I would at least do it over the bench with a big pillow (my wife's) to catch a mishap.
You can use a small screw and nut to hold a washer on the bottom of the pot that will keep the piston from dropping out. I've seen a diagram for that, but I don't have a copy I can post. If I can find it I'll add it.
tried this today and neither piston wants to drop. I have lightly oiled the inside of the dash pots and outside of each piston where applicable but they are sticking. They don't seem (too) dirty although they seem to be rubbing inside the dashpots.. This may be a stupid question but could they be tightening up due to the colder weather (snowing today) and should I perhaps bring them inside to bring then at least to room temperature? Thanks for any comments or email.
I would try it at room temperature and also it should not be liked. Clean with break clean and let it dry. Make sure all surfaces are nice, clean and free from grease
@@LivingWithAClassic "not be liked"? not sure what this means, or did you mean "should not be lightly oiled" ??
Sorry I was typing fast. It should be that it’s not suppose to be oiled
What if the third carbie is out ?
You then have three carbs to swap between, and find out which of the combinations show the least difference in drop speed. With some luck one of the combinations will stand out as clearly the best. But one cannot know for certain the history of part swaps some earlier owner might have undertaken. And regardless of number of carbs..that taping up the holes... make sure holes are really covered off so no leaks there will spoil test results. And as cautioned; take care to not drop parts to the floor to avoid damage.
What's with the weird hand gestures?
A secret code