• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

SU Rebuild

Share this page

Patton

Patton

Jedi Warrior
Offline
I have my extra set of carbs apart for rebuild and am trying to figure out if one of the dashpots is worth saving.

Here are the pictures of the carbs before I did anything to them. I picked up a cheap ultrasonic cleaner from Harbor Freight for the little stuff. I'll report on how much I like that after I play with it some more (initially I'll give it a "B")




Both dashpots pistons were frozen in their cylinders. I had to soak for a couple of days in PB blaster then gently persuade them loose with a rubber mallet. After soaking them in Berrymans, one looks great. The other still has "corrosion" marks on the inside bore. Most notably is the spot where the piston was stuck in the bore.

I am concerned about two things. First, cleaning the corrosion off without damaging the bore. What should I use? Second, is there very fact that that this bore has had this happen mean that I need to find a replacement?

Here are the pictures of the problem dashpot.



Thanks for your thoughts

Patton
 
Hello Patton,

anything that alters the piston to dashpot bore clearance affects the rate of lift, which is why abrasive cleaners are not recommended.
Corrosion will probably have eroded the body of the dashpot but how badly is difficult to assess from the photographs. There is a test which entails blocking the air holes in the piston, insert the piston into the dashpot and then timing how long it takes the piston to drop out of the dashpot with gravity. If a pair (or more) of dashpot\piston assemblies have differing drop rates then they will not work so well on the car.
However, the corrosion is at the idle position, so it may not adversely affect the running of the car significantly, all you can do is try.
Dashpots are probably available new if you want to go that route.

Alec
 
HI Patton, You might try polishing up the bores with Mothers Mag polish used gently.To determine the usability of the matching parts after clean up I suggest you refer to the factory manual's timed drop test for the piston when installed in the its bell.---Fwiw--Keoke
 
Hey Piman, Have you seen Holdens new webb site its really neat and user friendly.---Keoke
 
Like piman says, it's hard to tell. Some of the corrosion looks like harmless staining.
When I was younger and poorer I have cleaned up pre-wwII SU dashpots with a piston, kerosene and toothpaste. I and even matched a pair so thay pass the drop test. I always held the pistons and let the dashpots fall. In the old SUs you could put your thumb over the hole in the piston, and a dashpot falling can hurt itself less than a piston.
 
Hello Keoke,

no I haven't yet. I had an advisory E-Mail from them but haven't got round to looking yet. I suppose I should with Christmas so close.

Alec
 
When the carbs are built the piston & the housing are fitted as a matched set. Just replacing the piston may or may not result in what you want. Also on multi carbs, they "should" also all match. As roger said, they can be fine tuned by polishing the tightest set to match the others but proceed very carefully. Sometimes things can be improved by interchanging the pistons in their housings. It's very common to unintentionally "mix" the parts when working on the carbs.
D
 
[ QUOTE ]
When the carbs are built the piston & the housing are fitted as a matched set.

[/ QUOTE ]

Sorry Dave, I left out the "/" between dashpot and piston. I am looking for a set. The different between the two pairs seems to be to much to match them as the faster is the one with the pitting and scoring. The bore on the other one looks perfect.

Patton
 
Hi Patton, Probably best to locate another carb in better shape and use its piston/bell .---Keoke
 
Sorry to hear that Patton.However, that 815 number does not ring a bell with me aren't they of the HS or HD 6 type carb??---Keoke?
 
For an update, with my impending move to Dallas, I am doing what I swore I wouldn't and am sending the car and assorted parts out for work since my company won't move a non running car.

I sent my carbs out to Jim Taylor of Oklahoma at the recommendation of several I trust, and I need to share how happy I am with the service. The price was VERY reasonable as well.

Here are a couple of pics, I got a parts carb to replace the broken bowl! Note I didn't send the cross pipe or the linkage between carbs, so they still need to be cleaned!

IMG_0006.jpg


IMG_7.jpg


I can't wait till I get to hear the car fire up!
 
Back
Top