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A friend's TR3 with recently rebuilt carbs is having a problem getting them to run right. They only seem to function properly when the adjustment nut is turned down "25" flats! He tried an RH needle with no change. Wonder if this is one of those "90% of SU carb problems are ignition problems" type of situation? Any thoughts?
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TR2/3/3A TR3 SU Carb Mystery?
- Thread starter karls59tr
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TR3driver
Great Pumpkin - R.I.P
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Doesn't sound like ignition, to me. Does it run OK with the nuts down that far?
Are the springs inside the domes? Do the pistons move absolutely freely within the domes (not even a rubbing noise)? If you drop the piston (with the domes installed on the carbs), does it fall smoothly and land with a distinct click? Even with the mixture nuts all the way up?
Any vacuum leaks?
Are the springs inside the domes? Do the pistons move absolutely freely within the domes (not even a rubbing noise)? If you drop the piston (with the domes installed on the carbs), does it fall smoothly and land with a distinct click? Even with the mixture nuts all the way up?
Any vacuum leaks?
Apparently it runs OK with nuts turned down 25 flats. I saw the car on our annual Spring garage tour and I lifted the pistons up with my finger. To me it seemed like it took a lot of effort to lift them,more so than my SU's. The pistons came down down slowly and didn't have that distinctive clink! I didn't try it with the nuts all the way up.What would that indicate?
TR3driver
Great Pumpkin - R.I.P
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When installed, it does take a fair amount of force to lift the pistons as you are working against the damper. How much force depends on what fluid in the dampers; 20 wt motor oil will cause more resistance than ATF or whatever.
But they should fall freely (the damper has a one-way valve built into the plunger). Although it is possible for the piston to bind against the dome (due to varnish buildup or the dome being dented, etc), the most common cause is not having the jet centered properly. As the tapered needle comes down into the off-center jet, it rubs against the side and slows or stops the piston. Since raising the piston leans out the mixture, having the jet off-center can make the idle mixture appear too lean.
Having the mixture nut all the way up just reduces the clearance and makes it more obvious the jet isn't perfectly centered. And getting it centered is about the hardest part of the rebuild, IMO. I usually wind up having to use trial and error to get it just right, as the jet always moves a bit as the nut is tightened. I've tried the centering tool from the SU tool kit, and it didn't work for me.
Leaving the jet off-center can also eventually lead to an insidious problem, where the inside of the jet wears from the needle rubbing against it. This causes the mixture to go rich at idle. Not a big deal, except that then when you adjust the mixture at idle, it is now too lean at cruise; which leads to a lack of power and overheating. In retrospect, I believe this was the reason my Dad's TR3A would always overheat at freeway speeds. And likely was also the reason that a friend's MGA was so gutless at freeway speeds.
But they should fall freely (the damper has a one-way valve built into the plunger). Although it is possible for the piston to bind against the dome (due to varnish buildup or the dome being dented, etc), the most common cause is not having the jet centered properly. As the tapered needle comes down into the off-center jet, it rubs against the side and slows or stops the piston. Since raising the piston leans out the mixture, having the jet off-center can make the idle mixture appear too lean.
Having the mixture nut all the way up just reduces the clearance and makes it more obvious the jet isn't perfectly centered. And getting it centered is about the hardest part of the rebuild, IMO. I usually wind up having to use trial and error to get it just right, as the jet always moves a bit as the nut is tightened. I've tried the centering tool from the SU tool kit, and it didn't work for me.
Leaving the jet off-center can also eventually lead to an insidious problem, where the inside of the jet wears from the needle rubbing against it. This causes the mixture to go rich at idle. Not a big deal, except that then when you adjust the mixture at idle, it is now too lean at cruise; which leads to a lack of power and overheating. In retrospect, I believe this was the reason my Dad's TR3A would always overheat at freeway speeds. And likely was also the reason that a friend's MGA was so gutless at freeway speeds.
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Just a question about checking the free fall of the piston. I remove the cap and plunger to test mine. Does this harm the integrity of the test? I always make sure to replace the plungers into the same carb from which they were taken. It just makes the piston's easier to lift.
Geo Hahn
Yoda
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There is a John Twist video on this.
Somewhere there is also a Twist video on a drop test where you cover the air holes and time the fall of the piston from a loose dome set-up. I don't know that you need oil & damper for either drop test.
Okay -- this is the video I was thinking of. Thought it an interesting test --
Matching the Pistons
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It all raises in my mind the possibly (un)related problem of whether pistons and domes might have been mixed up?
vivdownunder
Jedi Warrior
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It's usually the very last fractional turn of the big nut on the jet assembly that pulls the jet off centre relative to the needle. That last turn needs to be made really slowly with no jerk of the spanner.
Viv.
Viv.
TR3driver
Great Pumpkin - R.I.P
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The problem was just that the jet was not centered accurately enough after using the tool. If you look at it, it locates to the bore in the carb body; which may or may not be accurately located relative to the dome that guides the piston. Apparently yours is and mine isn't.
I get the jet pretty close with just the needle (jet all the way up, install dome with jet housing nut loose), then try to keep it there while I snug up the nut. If it doesn't click, I try to see which way it needs to go, then loosen the nut, tap it that way a bit, and snug the nut up again. Usually takes me 4 or 5 tries before I'm happy with it.
TR3driver
Great Pumpkin - R.I.P
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Should be fine. I check sometimes with them installed, just to make sure the plunger hasn't gotten bent enough to cause binding, but I've never found a problem.
TR3driver
Great Pumpkin - R.I.P
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Could be, but if they are so badly mismatched that one of the pistons drags against the dome, that should have been caught after cleaning them and checking for free movement at that point. Usually the sizes are close enough that neither piston will drag on either dome; they just don't leak at the same rate.
glemon
Yoda
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Oh Randall you are being too nice, the reason the MGA felt gutless at freeway speeds is because you were comparing it to a powerful TRactor engined car
martx-5
Yoda
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Oh Randall you are being too nice, the reason the MGA felt gutless at freeway speeds is because you were comparing it to a powerful TRactor engined car
Uh, oh...A fight's gonna break out...even if it's true...
