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Carb lifting pin

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tahoe healey

tahoe healey

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When lifting the carb piston/needle, how high does it raise the piston when fully pushed? I've been getting no change on the rear carb and significant change on the forward carb with any mixture adjustment made. (BJ8)
Thanks
TH
 
tahoe healey said:
When lifting the carb piston/needle, how high does it raise the piston when fully pushed? I've been getting no change on the rear carb and significant change on the forward carb with any mixture adjustment made. (BJ8)
Thanks
TH
Click to expand...
A "skosh" which equals 1/16".
 
Sounds like the carbs are not balanced TH
 
To test the mixture using the lifting pin method the shop manual says to raise the pin 1/32" (ideally, test one carb at a time with the others at closed throttle). You should see a momentary, slight rise in RPM then a return to previous speed.

You can do it by feel, but I like to use the wire pointers that come in the cheap tuning kits; set two wires to point at each other then lift one piston the width of a wire.

If you get no change on a carb lifting all the way something is seriously out of whack.
 
Bob_Spidell said:
To test the mixture using the lifting pin method the shop manual says to raise the pin 1/32" (ideally, test one carb at a time with the others at closed throttle). You should see a momentary, slight rise in RPM then a return to previous speed.

You can do it by feel, but I like to use the wire pointers that come in the cheap tuning kits; set two wires to point at each other then lift one piston the width of a wire.

If you get no change on a carb lifting all the way something is seriously out of whack.
Click to expand...
I usually stick a large screwdriver under the piston and lift it up that way. Seems to be more precise(for me). I shut off the carb not being checked/adjusted by raising the piston "high" and screwing the jet to flat/flush(may car is a BJ8), so no fuel gets past and then separating the connecting linkage between the carbs.

A fast idle of 1200 rpm or so also helps to keep the engine running smooth enough to make adjustments or to check them. If you have a good tach that is portable that you can set on top of the engine is good and most important a tach one that the needle is dampened is a big plus and you can actually see the rise and fall as you lift the piston.
 
Basically you're just lifting the carb needle slightly which allows a little more fuel to flow into the engine, thus a rise in rpm's.
 
Sorry, guys. I mislead you. I have been working on SU carbs since 1970. My question is how high does the piston rise when the pin is pushed all the way up? I don't think the rear one is moving much if at all.
 
WELL clean it UP----Keoke
 
tahoe healey said:
Sorry, guys. I mislead you. I have been working on SU carbs since 1970. My question is how high does the piston rise when the pin is pushed all the way up? I don't think the rear one is moving much if at all.
Click to expand...

If you remove the air filter you can see how far it lifts, BTW there should be a small spring in the opening that forces the pin back down where it belongs.

If you don't care to remove the air filter you can also just remove the top of the SU carb exposing the large spring and piston and then you can easily see the lifting pin.
 
tahoe healey said:
Thanks all. I will use the "wire" synchronizer set I have to lift.
Click to expand...

Have you considered checking the pin? I'd confirm the pin is not the problem first. It maybe something as simple as a gummed up pin. You should be able to R&R the pin without having to remove the carburetor. The back carb has easier access to pushing the pin up then the front. It's held in place with a press on clip. Once out, clean the pin, the pin path and make sure the correct spring is being used. There should be a washer maybe two under the spring clip.
 
Pins work fine. I'm thinking the rear one is not as long as the front one and therefore not raising the piston as high. I will remove the domes and measure each pin although the front one is almost impossible to measure while mounted on a BJ8. That is why I started this topic.
 
You don't need to remove the dashpots, the easy way is to just take the damper cap off and insert a small diameter rod (like 1/16" or 1/8" welding rod) down in the hole where the damper was. Don't use too big a diameter rod or you will displace the damper oil. With the wire all the way down place a mark even with the top of the dashpot and then you can see the amount of rise when you lift the pin.
 
tahoe healey said:
Pins work fine. I'm thinking the rear one is not as long as the front one and therefore not raising the piston as high. I will remove the domes and measure each pin although the front one is almost impossible to measure while mounted on a BJ8. That is why I started this topic.
Click to expand...

Interesting. Shouldn't both pins be the same size? Makes one wonder about the rest of the parts in both carbs being correct.
 
OH, a paint can opener works a treat to lift those pins up.
 
Lifting the piston approx. 1/32" actually decreases the depression over the jet, although the effective jet area is increased, less fuel is introduced into the airstream.
 
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