13 replies to this topic

[Minigirl]

can the dashpot vibrate? I noticed a vibrating noise when putting my foot down so far

[Gremlin]

It's screwed down tight, can you get a video of the sound?

[HarrysMini]

Do you have oil in the dashpot?

[Austin mini 30]

Make sure its not worn engine mounts causing it to vibrate off anything

[Minigirl]

its down tight and last time checked it has oil, I topped it up with engine oil the other day

 

Its just when the throttle is at a certain position when driving at about 50mph

 

The mounts are all new orange pollybush mounts

[Ethel]

It is directly plumbed to 4 cylinders with moving pistons inside. That's partly why you need an oil filled damper. If it still seems out of the ordinary, or the car doesn't run smoothly, you could do a piston drop test.   Sure there'll be details online, probably on this forum.

[HarrysMini]

For the piston drop test, I believe you need to take off the air filter so you can see the piston, raise it to the top and let it drop. It should do so in 1 second and with a metallic 'clunk'. That's what I read on here anyway. 

[Ethel]

Pinched off a Jag site, who could've pinched it from elsewhere as the same wording is used on several sites.

 

It probably isn't necessary here, and the drop rates may be a bit quicker for hif carbs, with their roller bearings. Aside from the throttle, the piston is the only significant moving part in a SU, so it is a useful test for assessing if one is serviceable or restorable.

 

............................................

 

SU carburetor suction chambers and pistons are furnished as matched assemblies from the factory. There is a controlled clearance and thus controlled air leakage between the piston and suction chamber bore. A convenient means of checking this is a vacuum drop test. The proper damper should be in place. Chamber and piston should be clean and dry .Check piston for any spots of drag or interference over full travel. Spray piston rod lightly with WD-40. Turn assembly upside down with piston against top of chamber. Plug holes in the bottom of piston with windshield caulking (commonly referred to as Dum-Dum). Measure the time for the chamber to slide down and fall off the piston. Specified times are as follows:

 

1 1/4" and smaller carb = 3-5 seconds 

1 1/2" and 1 3/4" carb = 5-7 seconds 

2" carb = 7-10 seconds

 

If vacuum drop time is too fast, carb will tend to run rich. If drop time is too slow carb will tend to run lean. If both carbs are too fast there is nothing you can do to correct. Chances are someone has sanded inside of chambers to clean them which is a no-no. If they are too slow you can polish the chambers or pistons or both. very lightly and recheck frequently for compliance. If one carburetor is fast and one is slow there is a good chance pistons have been interchanged. Try switching them. I recently had a pair of  1 1/2" TF carbs where the front was 5 seconds and the rear was 8 In seconds. Switching pistons gave me 7 seconds on the front and 6 1/2 seconds on the rear -just lovely. It is ideal if both are alike and right in the middle of specification. It doesn't happen very often. Minor disparities from specified drop times can be accommodated by tuning adjustments. If I had a 1 1/2" carb with a drop time of 4 seconds I wouldn't fret about it. If it was 1 second or 1/2 second (which I've seen) I'd hunt for some different pans. It should be noted that if you do interchange pistons. it will be necessary to re-center both jet assemblies. Ideally this type checking should be done at the time of a major rebuild. 

 

 

drop time is too fast, carb will tend to run rich. If drop time is too slow carb will tend to run lean. If both carbs are too fast there is nothing you can do to correct. Chances are someone has sanded inside of chambers to clean them which is a no-no. If they are too slow you can polish the chambers or pistons or both. very lightly and recheck frequently for compliance. If one carburetor is fast and one is slow there is a good chance pistons have been interchanged. Try switching them. I recently had a pair of  1 1/2" TF carbs where the front was 5 seconds and the rear was 8 In seconds. Switching pistons gave me 7 seconds on the front and 6 1/2 seconds on the rear -just lovely. It is ideal if both are alike and right in the middle of specification. It doesn't happen very often. Minor disparities from specified drop times can be accommodated by tuning adjustments. If I had a 1 1/2" carb with a drop time of 4 seconds I wouldn't fret about it. If it was 1 second or 1/2 second (which I've seen) I'd hunt for some different pans. It should be noted that if you do interchange pistons. it will be necessary to re-center both jet assemblies. Ideally this type checking should be done at the time of a major rebuild.

[colinu]

The large spring inside the suction chamber also needs to be removed when performing the drop test (as does the oil in the damper... which is kind of implied from the above text).

However...it's never been clear to me exactly how long it should take a roller bearing HIF piston to drop. Apart from the details that Ethel provided above, info about this on the web etc is sparse to say the least (and 99% of it is the above text plagiarized/reused). I've been trying to figure out if my HIF44's piston/chamber is a little too worn. It drops in 4 seconds (i.e. outside the 5-7 second rate shown above)... but so do 4 other HIF44s that I've tried. OK so like most HIF out there they are all a little old hence there will be some wear in them, but is 4 seconds too far out of spec???

Edited by colinu, 13 April 2014 - 05:38 PM.

[Steve-O 2014]

Thanks. The carb had a green over cover on it removed this (it uses the plastic dashpot nut thing to hold it down) and it seems to have stopped

[Captain Mainwaring]

It is directly plumbed to 4 cylinders with moving pistons inside. That's partly why you need an oil filled damper. If it still seems out of the ordinary, or the car doesn't run smoothly, you could do a piston drop test.   Sure there'll be details online, probably on this forum.

 

Nooo, other vV carbs don't have oil damping - the induction pulses are quite well damped by the weight of the piston and the spring.

[Steve-O 2014]

aahhhh thought that was the problem but it wasn't. 

 

It seems to happen when your cruising along above 40 and you put your foot to say 60% throttle but as soon as you push further to say 70% throttle it stops

 

so above 40 at 0% - 59% theres no noise, 60% - 69% it makes a strange like air surging whir noise, 70% - 100% throttle no noise

[Ethel]

Have you got a sports air filter (bolt on K&N)? They get noticeably noisier when the throttle opens enough to let the  induction pulses bounce off it.

[Steve-O 2014]

yea it has a K&N Cone fitted to it

 

its just strange it does it at that certain throttle position but past or before that position it stops. 

 

Also when you instantly let off the pedal as soon as it goes back past that point you quickly hear it (sort of like a recirc dump valve on a car with a filter kit)