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Hi guys, been a long time but I'm back to getting my 1959 BE running again. I'm having carb trouble. The 1275 runs and I have synced the carbs to draw equally about '12' on a vacuum style synchronizing tool at idle---8-900rpm. But when I advance the idle speed to about 2000 rpm, the rear carb draws about '35' and the front carb draws only '18-20'. I can drive it but it seems sluggish. Not too bad, but enough for me to think that the carbs are still not in sync correctly. So next I did a compression test and ALL cylinders are at or above 170 PSI. I tried to check for vacuum leaks around the carbs by spraying starter fluid but got no increase in rpm. I can't figure it out.....does anyone have any ideas? Thanks for your help...Don
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H2 SU carb issue
- Thread starter DRH
- Start date
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SD Bugeye
Jedi Warrior
Offline
So are these self centering needles in the carb if so they would have a little spring on the top of them.
you may have a needle that's dragging .
also you may have a pot that is dragging
could the dashpots have been inadvertantly swapped try swapping them.
were the dashpot housings polished?
if this was done on a wheel you well warp them causing them stick.
CHECK PISTON FALL
Remove the three small screws holding the dashpot to the body of the carb. Be careful not to lose the screws as they are almost impossible to find in your engine bay. I also lay them out in the same relative position on the bench.
The dashpot can be removed by hand but take care to hold the piston inside it and lift the whole assembly out vertically, so as not to bend the needle. Note how there is only one way for each dashpot to go (one of the screw holes dictates this) and make sure you know which is the front and which is the rear carb dashpot, as well as which piston goes with what dashpot (by convention the front carb is the carb nearest the radiator). I tend to rest the piston, with the dashpots over them, on the edge of the opening of a long glass coffee jar (long enough so that the needle is not resting on the bottom of the jar).
Clean the piston carefully and clean it right to the top inside. Don’t whatever you do use wire wool or a wire brush and any sandpaper OR ANY OTHER ABRASIVE MATERIAL. SU pistons and dashpots (also known as suction chambers) are matched assemblies with a defined and specific clearance between the piston and the suction chamber. Anything abrasive will ruin this and make tuning impossible. I use a degreaser followed by Brasso to do this. Ensure everything is meticulously clean and dry and then lightly coat the rod in the centre of the piston with WD40.
Don’t stretch the spring inside the piston and make sure you put it back on when reassembling (easy to forget). With both pistons cleaned, reassemble and tighten the small screws gently and evenly. The piston locates in only one orientation inside the carb due to the groove mating with a corresponding section on the piston. The piston should fall freely within the dashpot. If you tighten the dashpot screws too much or unevenly, then the piston may get stuck.
Having removed the air filters, it is possible to see the end of the piston through the front facing air inlet hole. With the engine turned off, gently insert a finger into each and pull the piston upwards to the same height, holding both pistons with separate hands. Release each piston at the same time. As the piston hits the bridge inside the carb at the end of its travel it will make a dull clunk.
If both pistons hit the bridge at the same time, this is perfect. If the pistons hit within a fraction of a second of each other, this is also probably okay. More than a second and I would investigate why they drop at different rates. Below sets out what to do if the pistons are not falling in sync.
the link to the rest good stuff here
https://them-g-c.com/index_files/Page2858.htm
you may have a needle that's dragging .
also you may have a pot that is dragging
could the dashpots have been inadvertantly swapped try swapping them.
were the dashpot housings polished?
if this was done on a wheel you well warp them causing them stick.
CHECK PISTON FALL
Remove the three small screws holding the dashpot to the body of the carb. Be careful not to lose the screws as they are almost impossible to find in your engine bay. I also lay them out in the same relative position on the bench.
The dashpot can be removed by hand but take care to hold the piston inside it and lift the whole assembly out vertically, so as not to bend the needle. Note how there is only one way for each dashpot to go (one of the screw holes dictates this) and make sure you know which is the front and which is the rear carb dashpot, as well as which piston goes with what dashpot (by convention the front carb is the carb nearest the radiator). I tend to rest the piston, with the dashpots over them, on the edge of the opening of a long glass coffee jar (long enough so that the needle is not resting on the bottom of the jar).
Clean the piston carefully and clean it right to the top inside. Don’t whatever you do use wire wool or a wire brush and any sandpaper OR ANY OTHER ABRASIVE MATERIAL. SU pistons and dashpots (also known as suction chambers) are matched assemblies with a defined and specific clearance between the piston and the suction chamber. Anything abrasive will ruin this and make tuning impossible. I use a degreaser followed by Brasso to do this. Ensure everything is meticulously clean and dry and then lightly coat the rod in the centre of the piston with WD40.
Don’t stretch the spring inside the piston and make sure you put it back on when reassembling (easy to forget). With both pistons cleaned, reassemble and tighten the small screws gently and evenly. The piston locates in only one orientation inside the carb due to the groove mating with a corresponding section on the piston. The piston should fall freely within the dashpot. If you tighten the dashpot screws too much or unevenly, then the piston may get stuck.
Having removed the air filters, it is possible to see the end of the piston through the front facing air inlet hole. With the engine turned off, gently insert a finger into each and pull the piston upwards to the same height, holding both pistons with separate hands. Release each piston at the same time. As the piston hits the bridge inside the carb at the end of its travel it will make a dull clunk.
If both pistons hit the bridge at the same time, this is perfect. If the pistons hit within a fraction of a second of each other, this is also probably okay. More than a second and I would investigate why they drop at different rates. Below sets out what to do if the pistons are not falling in sync.
the link to the rest good stuff here
https://them-g-c.com/index_files/Page2858.htm
Gerard
Luke Skywalker
Offline
Don,
The first thing to check would be to make sure the butterflies are in perfect sync. They need to be locked down exactly in the same closed position with the idle screws completely backed off. Next, engage the idle mixture screws the same number of turns. I start with one revolution each from the point of contact. Next check that when the throttle cable is pulled, that the engage both butterflies at precisely the same moment. If it's still off, with the engine off, manually check the fall of each piston to see the fall rate is the same. You could also switch dashpot springs and see if the behavior is the same. I have many times found mismatched springs. I would also check for vacuum leaks around the throttle shafts with a volatile aerosol and see if idle changes. HS2s are notorious for worn throttle shafts.
The first thing to check would be to make sure the butterflies are in perfect sync. They need to be locked down exactly in the same closed position with the idle screws completely backed off. Next, engage the idle mixture screws the same number of turns. I start with one revolution each from the point of contact. Next check that when the throttle cable is pulled, that the engage both butterflies at precisely the same moment. If it's still off, with the engine off, manually check the fall of each piston to see the fall rate is the same. You could also switch dashpot springs and see if the behavior is the same. I have many times found mismatched springs. I would also check for vacuum leaks around the throttle shafts with a volatile aerosol and see if idle changes. HS2s are notorious for worn throttle shafts.
This sounds a bit like the issues I was having, but my compression numbers are far lower on my rather worn 1275 that's due for a rebuild at the end of this driving season. Gerard's advice definitely works. It's still not perfect (probably due to internal issues), but the car does drive well enough that I can make it through the rest of the season.