Carb Piston Won't Fully Drop

[Steeman01]

Hey there,

First and foremost, I was able to finally take my "newly" purchased SV this past week for the first time since I bought it last September! What a blast!

That being said, I am still working on setting the idle and one issue I am having is that the carb closest to the firewall (I still have the stock Stromberg 150's), the piston moves well, but won't freely settle to the bottom on the last 1/4" or so of the drop/plunge. I rebuilt them both, and even when I take the top off (air chamber) it all looks good, nothing binding and the piston runs smoothly up and down. The piston drops when the top is on and in place and being held in place by hand, but once I tighten the 4 screws on top, the problem appears... I can't leave the top loose as the oil will seep out of the top (dash pots)... I am lost... Shall I polish the piston to make sure it is nice and smooth?

Thoughts??

I also had sure all location tabs were in place. Also, the piston will go 100% down if I gently slid it the rest of the way down with my finger with very little effort.

 

[bernd_st]

Did you center the needle on the rear carb properly ? You need to loosen the lower main jet holder ( the big outer brass thingy) while dropping the piston down completely. Give it a few piston strokes up and down to make sure the needle is properly centered in the main jet. When you are sure the piston moves freely and completely down without any binding retighten the brass jet body. If there are still issues you may also try to swap the dashpots of front and rear carb...

 

[rixter]

You need to determine if it is the piston binding or misalignment of the needle valve in the seat. The needle has a set screw to fix it in place and that screw must be on the flat side of the needle. Assuming that is ok, you'll want to be sure the jet is centered. This document describes jet centralization among other helpful information: https://tecb.eu/onewebmedia/Tuning_Stromberg_CD_Carburetters.pdf

Rick

 

[Steeman01]

Thank you all, I did centralize the jet on both when I rebuilt them.

The needle doesn't have a flat spot for the set screw...

The weird thing is that it sets/falls all the way down when the 4 top screws are out of the carb top (aka lid), but as soon as I tighten the 4 screws it seems to bind on the last 1/4"....

So strange.

I tried the suggestion of swapping the dash pots and the issue was the same on the back carb, even with the front dash pot (even though it works fine on the front one)....

I don't get it...

 

[rixter]

So you are using the same diaphragm and piston/air valve in the rear carb? So you still aren't sure if it is the piston, the carb body bore or something else. If you are using cheap diaphragms, I've seen issues with ones that are not flexible enough. Note difference with Victoria British diaphragm on the left and one from SS on the right.

Rick

 

[Steeman01]

Thanks @rixter, I am using SS parts, I read on the forum when I first got the car and understood SS was far superior to VB... So I am good there....

It's almost like the tightening of the lid puts a small title on the piston... Causing it to slightly rub on the last 1/4" or so. The needle looks and acts perfectly straight (hence it runs up and down smoothly and completely if I don't have the 4 screws fully tightened...

 

[hartmandm]

Did you center the jet with the carb top installed?

I'd pull the metering needle and make sure it isn't bent. I had one that appeared ok, but once I spun it in my drill press it was obvious to see the wobble.

 

[RootesRacer]

Thank you all, I did centralize the jet on both when I rebuilt them.

The needle doesn't have a flat spot for the set screw...

The weird thing is that it sets/falls all the way down when the 4 top screws are out of the carb top (aka lid), but as soon as I tighten the 4 screws it seems to bind on the last 1/4"....

So strange.

I tried the suggestion of swapping the dash pots and the issue was the same on the back carb, even with the front dash pot (even though it works fine on the front one)....

I don't get it...

Your problem is definitely tied to the jet centralization proc. Try Try again... Be sure the jet can actually move when the holder is loose.

 

[RootesRooter]

It can be just trial-and-error.

I recommend tightening the mixture screw all the way in so the jet is pushed up as high as it can go. With the piston out, even if the jet appears centered, experiment by moving the jet just a HAIR to the right, etc. Reassemble and back the mixture screw out 2 turns. Still stuck? Open it up and move the jet adjustment 90 degrees. Etc.

On really stubborn ones, I go back to the School of Hammer Mechanics. I shake them like I'm mixing a giant martini so the piston pounds up and down just enough to jiggle the jet. I've tried this with the mixture screw both all the way in or backed way out. And it sometimes works.

Finally, the four screws holding the cap don't have to be super tight to be secure. Try alternating tightness in different combinations. eg. two mildly loose, two VERY tight. It may enough to keep the needle from tilting the wrong direction.

 

[husky drvr]

If you don't have a keyed flat on your needle, it might be as simple as trying the needle in a different orientation or two.

 

[bernd_st]

Additional hints which may help your problem:

- once the rear brass jet holder is loose give the dashpot a few soft strokes with a rubber mallet. From front and rear side. Sometimes it works wonders.

- if it still binds give the dashpot inside a slight polish with very fine emery

 

[Steeman01]

Thanks everyone for the suggestions, I have a free evening ahead of me and I plan on trying some (or all) of these suggestions out! I will keep you posted!

 

[beamdream]

Yep, all of the above.

I've often gone round and round in circles with these and SUs.

Its the last few flats on the jet whilst continually raising and dropping the piston, easing off the cover screws and a few light taps with a screwdriver handle, all in a well orchestrated sequence that gets the job done; be patient.

It might come to checking the needle hasn't been bent (easily done). There's a grub screw on the bottom of the piston holding it in place, I like to roll the needle on a flat surface to check it, and you may find the needle won't want to come free of the piston so gently does it.

Does it need to be said that, lastly, both pistons should drop at the same rate?

 

[bernd_st]

That's the next challenge then

 

[Steeman01]

Hi all, I think the issue was a mild bend in the needle, which I noticed using @hartmandm suggestion with a drill... It had a slight wobble. I put the needle in the middle of a thick notepad and pushed on the back with a flat piece of wood and spun it with the drill and it straightened it right out! I also messed around with the 4 top screws as well.

So now it seats properly but now I'm having to work on dialing in the fuel mixture!

Thanks again for all the suggestions!

 

[RootesRooter]

A Colortune kit really helps.

Keep in mind that if your throttle rod has noticeable play in the carb body, a common affliction of well-used Strombergs, you might not be able to get the mixture right.