Further to carb cleaning ...careful with SU domes.

[karls59tr]

Did someone recommend using a Scotchbrite pad on the interior of the dome! I seem to remember from Bob Schallers book that it's a good way to turn your carbs into junk. Ruins the factory clearance between dome and piston if you remove too much.

 

[HerronScott]

I would hope not! They might have mentioned it for the outside.

Scott

 

[MGTF1250Dave]

Aloha Karl,

Here is an excerpt from an SU manual addressing cleaning the piston and interior of the dome specifically warning against using abrasives:

"Examine the piston assembly for damage on the piston rod and
the outside surface of the piston. The piston assembly must be
scrupulously clean. Use petrol or methylated spirit as a cleaning agent. Do not use abrasives. Lightly oil the outside of the piston rod.

Clean inside the suction chamber and piston rod guide using
petrol or methylated spirit. Refit the damper assembly and
washer. Seal the transfer holes In the piston assembly with
rubber plugs or Plasticine and fit the assembly to the suction
chamber. Invert the complete assembly and allow the suction
chamber to fall away from the piston. Check the time this takes, which should be 3 to 5 seconds for HS2-type carburetters of in. (31•75 mm.) bore. or 5 to 7 seconds for larger carburetters. If the time taken Is in excess of that quoted, the cause will be thick oil on the piston rod, or an oil film on the piston or inside the suction chamber. Remove the oil from the points indicated and re-check."

 

[dklawson]

You may have seen Hap Waldrop or me mention using abrasives including ScotchBrite on the OUTSIDE of the vacuum chamber prior to polishing. Hopefully no one misunderstood that advice as applying to the inside.

 

[AEW]

Do any of you have a way to test an SU vacuum chamber to see if the piston lifts at the proper rate? (bench test I mean) In other words, is there a specific level of vacuum for say 20%,40%,100% piston travel, and what about WOT?

 

[Darrell_Walker]

Do any of you have a way to test an SU vacuum chamber to see if the piston lifts at the proper rate? (bench test I mean) In other words, is there a specific level of vacuum for say 20%,40%,100% piston travel, and what about WOT?

Actually is it flow that raises the piston, via the pressure differential cause by the venturi.

I've not seen any specific test spec or procedure. The spring (and the weight of the piston) control the height at a specific flow, and the dampener (via the viscosity of the fluid you use) controls the rate of the rise (and drop).

Ideally you would tune all of this on a dyno, selecting a spring that allows the piston to fully rise at full flow, and a dampener that keeps the mixture reasonable during acceleration.

 

[dklawson]

As an extension of Darrell's comments, go back to Dave's post and note the section of the factory procedure talks about how quickly the piston "drops" when the holes on the piston are plugged. That is a method of determining the fit and friction between the piston and vacuum chamber. (BTW, the holes on the piston can be sealed equally well by tape. John Twist has a video showing this on YouTube).

Once you know the piston fit is "right" based on the drop rate, as Darrell mentions, look at the rate of the spring inside the carb and lastly, the amount and viscosity of the oil in the dashpot. Those complete the variables that affect the mechanical issues affecting the rate the piston moves at.

 

[AEW]

OK, I'll look for the youTube video. It would be nice to have a method to quantify wear and 'cleaning damage' to know what will be irrevocable.