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SU HS2 Piston assy movement-- question

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58Custom

58Custom

Jedi Warrior
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The piston assy on the forward SU HS2 moves further than the one in the rear carb. Always has, even before I rebuilt the engine and before I went through the carbs and installed new jets. It's a little higher at idle as well. I was just out there setting idle speed and mixture and it sounds OK but there seems to be nothing I can do about the pistons operating differently.

Any suggestions?
 
Piston lift is caused by air. Pure and simple air passing through the carb. One of yours may have a bad throttle shaft or gasket on the manifold side. Easy to check. While running at idle spray a bit of carb cleaner at those places, if idle increases you found the prob.

Could also be caused by the carbs not balanced, a whole other desertation but something to think about. They should be the same. In fact they must if they are adjusted correctly.

Balance first then adjust seat.
 
Look closely at the linkages, one may be opening up the throttle before the other one is, they are easy to adjust bu loosening the nuts on the links and setting them so both come in contact at the same time.
 
As stated above the pistons should be close to the same height in operation. Before you go through the tuning steps, make sure the shoulder on each needle is flush with the bottom of the piston. Likewise, make sure each jet is flush with the bridge before you lower them by the 2-full turns of the adjusting nut. Before making any adjustments to the throttle linkage, remember that you must also loosen and back-off the fast idle screw and choke linkage. If the choke is not completely out of the equation you will not be able to balance the air flow and set the mixture adjustment properly.
 
It is important that the piston return springs are balanced/matched. Same tension at the same weight & length. Springs can be easily tested.

Other causes of unbalance could be different clearances between pistons & suction chambers. As in the two assemblies interchanged. There is a test to measure piston drop timing to check for matched clearances.
D
 
You need a copy of "Tuning S.U. Carburetters.

One of our folks has it on his web site for download. Chris you listening?
 
Luke_Healey said:
Here's the little helpful video that gave me the idea of how to best determine how to match suction chambers on my HS2 carbs:

https://youtube.com/watch?v=IfU47Oqq9wA

There's a good bit of SU information contained in many of his videos. All are well worth the watch.
Click to expand...

Hazzah! That's good stuff. Very helpful, thanks!
 
May I suggest that all of us need to watch all the John Twist videos from University Motors on Utube.

This guy has got his act together and is doing a real service for LBC owners.
 
:iagree: I have saved his videos in my Youtube account! They make a nice referrence file! Where is University Motors located?
 
Up in Minnisota some place I beleive.
 
Is a two parter utube on his shop.
 
The vids were very helpful.

I did the piston drop test and it turned out that switching the pistons made for a more equal drop. I worked on getting the opening points for the throttle as close as possible. After that, they piston opening positions at various engine speeds were very close. Better than before.

The pistons in my carbs had vacuum holes different than in the vid. You guys are probably familiar with them, but I just want to put this into the forum record. The vacuum holes on mine are recessed into a relief that extends to the sides so that a simple piece of tape will not plug them. So I grabbed an old set of foam hearing protector ear plugs. The yellow cylindrical types. I quarterd them lenghtwise, compressed them and put them in the holes. They expanded in the holes and plugged them nicely. Poifect! They are now a part of my carb parts & tools box.
 
So, how does it run now?
 
I took a quick run to In&Out for a nutritious snack and it seemed to run nice and smooth. I'll need a longer run with some hills to know for sure, and then I'll check the plugs. It seems very sensitive to mixture, like it runs better when it's set a little lean by the idle test and I have it set per the book.
 
My journey to HS2 Nirvana is still not complete after a year with this car. The reason being that I had the same Sprite with the same carbs sitting next to it last year that whooped up on it performance-wise, so I KNOW what the carbs are capable of if they work right.

As per plugging the holes, you need two pieces of tape to plug each piston. Do just like John did in the video in order to plug the recesses on the side and top.

Also, my dashpots are slightly out of round it seems. Depending on the orientation of the pistons, they fall at different rates, so I lined them both up how they would sit when assembled on the carburetor and did the test that way to determine the match quality.
 
They are sensitive to mixture, like a quarter of a flat from close to the happy spot. Actually you can hear the happy spot, it sings.
 
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