Hi,
It's been a long time since I dealt with SU carbs, looking over one for a friend at the weekend I noticed that the piston lifts nearly 1/4" before the damper has any effect. Is this normal or should the resistance from the damper be present as soon as the piston starts to lift? To me it seems that the early part of the travel would be the most vital being it affects idle and cruise running when there is little throttle opening.
Any ideas?
Alan
SU Carburetors
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Hi Al
really it depends on vintage of the carb and it's original use , some dampers were set up to not have an effect before about 1/8 to 1/4"of lift, this was so that the mixture would remain lean at idle even with pulsing from the manifold caused the piston to rise and fall at idle these carbs tended to be fitted in pairs like on MGs BL sports cars and used when BL were more worried about emissions. Where one carb was fitted to feed 4 or more cylinders then the damper would come in earlier so pick up would be better and as there were more pulses at idle then there was less up and down motion on the carb and so it was easier to get the mixture lean and stable at idle. on twin SU installations on minis we used to add washers to pack the "c" clip on the damper to make it more effective as you opened the throttle and use gearbox oil in the dashpot to get more enrichment and improve pickup.
best regards
Mike
really it depends on vintage of the carb and it's original use , some dampers were set up to not have an effect before about 1/8 to 1/4"of lift, this was so that the mixture would remain lean at idle even with pulsing from the manifold caused the piston to rise and fall at idle these carbs tended to be fitted in pairs like on MGs BL sports cars and used when BL were more worried about emissions. Where one carb was fitted to feed 4 or more cylinders then the damper would come in earlier so pick up would be better and as there were more pulses at idle then there was less up and down motion on the carb and so it was easier to get the mixture lean and stable at idle. on twin SU installations on minis we used to add washers to pack the "c" clip on the damper to make it more effective as you opened the throttle and use gearbox oil in the dashpot to get more enrichment and improve pickup.
best regards
Mike
poppet valves rule!
Cheers Mike, make sense, this is on a mini and believe the carb is from a much later Metro, car starts and idles for 30 secs to a minute and then dies, appears to be dead rich but it surges at idle like a blower motor. He has another carb coming wednesday so told him to fit it and run it, then come see me so we can fine tune things. Didn't feel right to me with all that play and when idling you could feel the piston around 1/4" up.
Thanks
Alan
Thanks
Alan
Hi Al
has the carb got an over run poppet valve in the butterfly? From the symptoms you described the spring that holds the poppet valve shut at idle has gone weak and it holds open close to idle. The mixture is then set so the engine will run with it open the mixture is now right but the idle is too fast (1000 to 1400 revs). the next thing that happens is that you set the idle screw in a bit to reduce the idle speed and the manifold vacuum drops, the poppet valve closes and it dies way over rich.
Another symptom that would confirm this is that on the over run the engine does not want to slow properly and as you slow down it holds a fast idle for about 5 to 10 sec then dies away and won't pick up on the throttle, in fact blipping the throttle kills it even quicker. the best answer it to solder the poppet valve shut and reset idle speed and the mixture. Check the needle is held in on the right indent as often in an attempt to get any idle someone may have moved the needle up into the piston to get the mixture rich enough for it to run with the poppet valve open.
the surging would also be consistent with the poppet valve opening and closing at idle due to vacuum and the mixture having been set for it open.
Best regards
Mike
has the carb got an over run poppet valve in the butterfly? From the symptoms you described the spring that holds the poppet valve shut at idle has gone weak and it holds open close to idle. The mixture is then set so the engine will run with it open the mixture is now right but the idle is too fast (1000 to 1400 revs). the next thing that happens is that you set the idle screw in a bit to reduce the idle speed and the manifold vacuum drops, the poppet valve closes and it dies way over rich.
Another symptom that would confirm this is that on the over run the engine does not want to slow properly and as you slow down it holds a fast idle for about 5 to 10 sec then dies away and won't pick up on the throttle, in fact blipping the throttle kills it even quicker. the best answer it to solder the poppet valve shut and reset idle speed and the mixture. Check the needle is held in on the right indent as often in an attempt to get any idle someone may have moved the needle up into the piston to get the mixture rich enough for it to run with the poppet valve open.
the surging would also be consistent with the poppet valve opening and closing at idle due to vacuum and the mixture having been set for it open.
Best regards
Mike
poppet valves rule!
Hi again
don't get too worried about the piston bouncing up and down too much, you probably have the wrong spring in there, I suspect there is also quite a bit of wear in the carb as well which will make the bouncing up and down worse. Once you have confirmed it there is a poppet valve issue and fixed it have a look at the spring, once you have a stable idle look at how the piston moves, it should move up and down around 1/8 to 3/16 of an inch, a quarter is a bit too much. to reduce this you need to increase the preload on the piston, so take the spring out from above the piston and pull it so as to lengthen it a bit, put it back in reset the idle and idle mixture and see if it still bounces up and down too much, if yes stretch the spring a bit more and retry. You should be able to get it down to about 1/8" of movement with a nice stable idle. if you can't then take the damper out and take the cir clip off the end and add a thin washer, put the clip back on and see if that helps. If it still is no good put gearbox oil (hypoid 90) in the dashpot! You can also radius the leading edge of the piston, that is a mod that improves flow at full throttle and high revs, but it also can stabilize the piston a bit by reducing turbulence at low throttle openings, it can increase droplet size at idle a bit, that is not so good.
Best regards
Mike
don't get too worried about the piston bouncing up and down too much, you probably have the wrong spring in there, I suspect there is also quite a bit of wear in the carb as well which will make the bouncing up and down worse. Once you have confirmed it there is a poppet valve issue and fixed it have a look at the spring, once you have a stable idle look at how the piston moves, it should move up and down around 1/8 to 3/16 of an inch, a quarter is a bit too much. to reduce this you need to increase the preload on the piston, so take the spring out from above the piston and pull it so as to lengthen it a bit, put it back in reset the idle and idle mixture and see if it still bounces up and down too much, if yes stretch the spring a bit more and retry. You should be able to get it down to about 1/8" of movement with a nice stable idle. if you can't then take the damper out and take the cir clip off the end and add a thin washer, put the clip back on and see if that helps. If it still is no good put gearbox oil (hypoid 90) in the dashpot! You can also radius the leading edge of the piston, that is a mod that improves flow at full throttle and high revs, but it also can stabilize the piston a bit by reducing turbulence at low throttle openings, it can increase droplet size at idle a bit, that is not so good.
Best regards
Mike
poppet valves rule!
