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MG MGB Technical - Carb slow to rise

Normal 72 carbs no mods

With engine oil in the dashpot damper chamber, and the damping piston screwed down the carb piston is very difficult to lift and very slow to come down.

Without the oil and just with the damper piston in its chamber, the carb piston does lift better and comes down with a bit of a clunk. Everything cleaned and ok - whats going on?

Ta

iwan
Iwan Jones

Too heavy an oil. Try around 20.

Herb
Herb Adler

Herb, engine oil usually is 20 when cold in the UK.

Iwan, "Without the oil and just with the damper piston in its chamber, the carb piston does lift better and comes down with a bit of a clunk". Do you mean it lifts without any effort and slides freely? If not there is something wrong. Is the spring fitted between piston and dash pot? Does the little brass damper move slightly on the centre rod?

Sorry for silly questions to confirm the status but without asking we won't know....
MGmike
M McAndrew

Has everything been cleaned thoroughly with carby cleaner? The piston edges sometimes accumalate crud and even the inside of the dashpot. with all clean, the piston should slide easily, if not there has been distortion somehow. needle correctly placed? Mike
J.M. Doust

Hi, yep been using engine oil. Carbs have been cleaned, nice and bright no crud - no abrasives have been used in cleaning nothing snagging, nothing rubbing.

Without the damping piston in, the carb piston rises easily and clunks down.

With the damping piston in position and no engine oil, it lifts ok and clunks - a bit like the carbs on my other car (this one has oil in the and damper piston screwed down).

When I put engine oil in the dampening chamber, up to about half an inch from the top of the dashpot (as in the other car), it changes completely! I need to put great pressure up on the carb piston to lift it up and it comes down very slowly indeed with no clunk - if it comes down at all!!

Yep the springs are in place (have tried 2 sets) and the brass bit does waggle. There is an air hole in the top screw down part of the damper, this is clear. This top screw oart is made of brass not black plastic like the other one.

Ta for the replies already.

Iwan
Iwan Jones

The damper should cause the piston to rise slowly, but allow it to drop sharply. If it only drops slowly then it is binding on something, or the damper is faulty. The little piston at the end of the damper rod should be free to move up and down a couple of mm. When the main piston starts to rise the oil in the chamber pushes the damper piston up against a flange to limits how fast oil can get past it, and hence how fast the main piston can rise. When the piston stops rising, or at the very least starts to fall, suction should pull the damper piston down against a circlip which allows oil above the damper piston to flow easily back to the bottom of the chamber.

One brass and one plastic damper cap does not sound good at all.
PaulH Solihull

FWIW Only time I've seen brass damper caps was on older MGs (TC I think). Wrong bit maybe?
Michael Beswick

Sounds like a P O has been mixing parts up. The dampers should be AUC 8114. And not brass topped.
Allan Reeling

Iwan,
Eliminate the dampers by using the ones from the "other car".
FYI it is possible to get after market brass caps but these will need a small hole in (as you say yours have) to let the air out/in as the damper rises/falls (assuming you have the older carb's without the closed circuit tube in the dash pot cover).

MGmike
M McAndrew

Hi, Will clean up everything again and also will try out the different dampers. Will do seperately to see what difference and when and report back!

Ta,

Iwan
Iwan Jones

If both carbs have been dismantled at the same time dashpot covers, pistons and dampers may have got jumbled up so you may have to mix and match to see what gives the best results. There is a drop-test which should be done, see XII here http://www.jcna.com/library/tech/tech0006.html. Also if you have fixed needles and the jet centering is off that will affect how the piston falls. Centering needs to be done with the jet wound up to be flush with the bridge, so that you can be certain it won't bind when the jet is back down in its normal running position. However if you do that then you will have to balance them for air flow and mixture balance from first principles, which they will probably need anyway.
PaulH Solihull

Hi Paul, thanks for that link.

The important point for me was......

'The damper rods can occasionally get bent which will force the damper piston off center. This will create a drag or side force on the hollow piston rod and restrict normal piston movement. Remove the suction chamber and look at the end of the damper rod with cap screwed snugly into the chamber neck. If the end of the rod appears to be in the center of the bore. everything's OK. If the rod is noticeably offset in one direction. mark the cap with a magic marker in the direction the rod needs to be bent. Remove damper and gently bend rod in the indicated direction. Several passes may be required to get it right. Visual centering is adequate as there is some lateral float in the damper piston. Just as a matter of interest. the new SU plastic capped dampers have a ball socket in the cap which allows the rod to be self aligning'

Yep, the black plastic capped ones worked better.

Turning the dashpot to look down the chamber as I screwed in the brass topped damper rods showed the ends 'circling' off centre in the damper chamber. Also, not that easy to straighten out though.

One of the sides of the damper chamber column had a nick, probably dirt caused it, this could easily 'catch' now and again. All seem to work fine now.
Iwan Jones

This thread was discussed between 16/04/2012 and 23/04/2012

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