ZS air piston sticking

[rdssdi]

I took my Elan +2 on a shake down drive. The car ran well until, at the end of the 15 mile drive, I ran it up to the red line in one gear. The car shortly after began running somewhat rough and down on power.

I removed distributor cap and all appears normal.

I did find one of the air pistons in the Zenith Stromberg carb was stuck in an "up" position. The other piston operates rather "sticky" and difficult to push up but it does slowly return.

I recently replaced these air pistons with "used" pistons with the adjustable needle feature. I also replaced the needles with the proper Lotus profile adjustable needles. I used new engine oil to top up the piston reservoirs.

The pistons take a bit of effort to push home. It feels as it is a hydraulic resistance as opposed to something binding. The needles appear to traverse the jet with no snags.
Is it possible the pistons that were original to the old air pistons do not fit properly? They do press in and I can tighten the knob.

Any suggestions?

Thanks

Bob
1969 Elan +2

[rdssdi]

It appears to be metal burrs and poor cover alignment. The piston tolerances are close. Probably exasperated by the "new" air piston.

I removed the burrs cleaned and it is free but a small rough spot in the travel remains.

I may have to send it to be rebuilt again. To someone who understands the alignment issue. I will try Jeff Palya at Paltech if I am to have it rebuilt again. He did a weber 32/32 for a Cortina and it looks great.

Bob

[miked]

Bob,

I have ZS and my understanding from all the things that I have read is that the body, the slide and the lid are a match set. I don't know whether it is bull sh*t to make us buy new kit but I tried changing slides and found binding. I then robbed the lids that came with the other slides but found some slight misalignment. I gave up. There is also some text about air leakage and an embedded (then sealed) compensation screw (front of carb) that is set to allow matching of mulitple carbs systems to make the leakage around the slide identical.

Re piston damper. It is quite hard to push aginst the damper but they should come down quite progressively. I would not have thought there would be a problem with the damper piston with the right grade oil. How are the air slides with no dampers in?

There were some articles on the net on Triumph sites showing how to convert your fixed needle slide into and adjustable one by some drilling and plugging metal works etc etc. I should have it on hard copy in my carb file.

Regards Mike

[rdssdi]

Mike

That seems to make some sense. The tolerance of all parts is rather tight. I purchased the adjustable air pistons from a brit car supplier in California. He told me he has sold many of these salvaged adjustable pistons to people to convert their fixed needle carbs to adjustable metering needles. He said it was a "drop in" procedure. As I wrote I replaced the metering needles with the adjustable type. I have removed some metal burrs and the piston moves without binding although when I push the piston up by hand I can feel a minor rough spot. I will polish the air piston "piston" and use it as is.

I will adjust with a uni syn and set the mixture. If it runs as it did prior to the problem it will be fine with me.

I have the old fixed metering needle air pistons to compare dimensions. This is a lot like fitting parts in a firearm. Few parts offer direct replacement. Although as manufacturing standards have improved fitting is less of a factor.

I will post my results. Hoping it all works!

Bob

[Rob_LaMoreaux]

I replaced the pistons on my stromberg engine with adjustable needle ones and had no issues at all.

SU carbs are definitely matched to the pistons, but the ZS with the diaphragm do not need the tight tolerances since the seal is not piston to cylinder.

I ended up getitng it to run quite nicely, and it would still be in the car except for the right hand turn oil pickup problem. I added an Accusump to take care of the pickup problem on the weber engine, which I now need to tune right.

Rob

[leifanten]

I have found that the tolerances are so fine, that when tightening the screws on the "lid" over the diaphragm, you need to do this whilst feeling the effect on the air valve. Mine were sticking until I evened out the slight torque on the screws, and it then came loose and moved freely when the torque was balanced while moving the piston up and down with my finger

Leif