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Stromberg Dash Pots

Hodee

Donation Time
What weight of oil should be used in the Stromberg dash pots? The manual is a bit confusing on the issue. I always thought a light 20 wt or so but now I’m not sure. Thanks
 

Alpine 1789

SAOCA President
Diamond Level Sponsor
I don't think there is any real consensus on this. I've heard some people swear by Marvel Mystery Oil, but I just used ATF when I ran Strombergs. Of course, I could never get them properly tuned or synchronized and switched to a Weber 20 years ago, so what do I know?
 

puff4

Platinum Level Sponsor
Another touch at the “third rail” of LBC ownership... proper lubricants.

OK, I’ll bite.

Here’s what’s happening...

That dashpot performs the function of an accelerator pump. When you hit the throttle, the air valve inside the bell chamber receives a dose of engine vacuum to the top side of it and starts rising. The oil in that dashpot slightly delays the air valve’s rise, and in so doing temporarily increases the vacuum over the fuel jet, which in turn enriches the mixture to provide extra power for acceleration.

So... the heavier the oil, the more accelerator pump you get, and the lighter the oil, the less you get. Light oil will therefore often lead to a flat spot on accelerating, and a single weight oil will get thinner as engine temperatures increase, similarly leading to a flat spot.

I recommend using the same engine oil you’re using in the car. By using a multi weight oil you’ll gain a more consistent “pump” over varying engine temperatures, and the oil will be heavier and give you a nice shot of petrol when you need it most.
 

bernd_st

Bronze Level Sponsor
Use 20W50 in the dashpots same as in the engine. Tried the dashpots mod as in the service bulletin, but it made no difference. The Stroms still consume dashpot oil. So don't waste any time on it, just check and top up oil level regularly...
 

rixter

Gold Level Sponsor
I've tried various fluids in mine including Marvel, engine oils and SU carb oil. Also heard ATF works well but have not tried. As Bernd says, these carbs seem to "consume" the fluid over time despite the fixes and thus I am regularly checking and topping up when necessary. I will usually get a bogging down on acceleration when the level has dropped as low level will result in similar action as too thin oil in that the air valve is permitted to rise too rapidly when you snap the throttle. I liken the dash pot function as sort of a shock absorber, but only in the up direction.

Rick
 

RootesRooter

Donation Time
I've never read of any SV racers in the day running heavier oil in the dashpots to gain better acceleration. To the contrary, they ran stock, ultra-thin viscosity 'sewing machine' oil - or none at all.
 

puff4

Platinum Level Sponsor
I've never read of any SV racers in the day running heavier oil in the dashpots to gain better acceleration. To the contrary, they ran stock, ultra-thin viscosity 'sewing machine' oil - or none at all.

Because when you’re racing you’re never really coming off low RPMs, where you’d need a pump (and feel the flat spot the most), and at high RPMs you’d want that air valve to pop up quickly for best throttle response. Plus, the cam is of an entirely different configuration such that bottom end performance isn’t emphasized. Racing setups are very different than Street.
 

husky drvr

Platinum Level Sponsor
Don:
I can't open the image.

Thor,

Interesting, it's not an image but a page link for a page on Tiger East/ Alpine East web site.

Search TE/AE website home page. On the orange site address bar at top of page, drop down menu under "Tech Tips", click on "Factory Service Bulletins.

On that page's listings under section "D - Fuel", look for Bulletin 68-14: Zenith C.D. Carbs Loss Of Oil From Dash Pots.

This info is on the web in several places, so you could just search the bulletin title to get the information. The TE/AE page seems to have a better copy of the one illustration in the bulletin.

Hope this helps,
 

Shannon Boal

Platinum Level Sponsor
I add motor oil to carbs every time I drive it. I think an O-ring inside the slide has failed. I also get o-ring failures at the fuel concentric jet, (leads to rich running at idle), but am switching to Alpine innovations down-draft Weber conversion. If you want to replace ALL the hidden o-rings with Viton, Buna-N, or other fuel-resistant material, those carbs might work as intended. It would be advantageous, before undertaking that job, to have a background in clock repair....
 

RootesRooter

Donation Time
Because when you’re racing you’re never really coming off low RPMs, where you’d need a pump (and feel the flat spot the most), and at high RPMs you’d want that air valve to pop up quickly for best throttle response. Plus, the cam is of an entirely different configuration such that bottom end performance isn’t emphasized. Racing setups are very different than Street.

Why was the factory recommended oil ultra light-weight?
 

puff4

Platinum Level Sponsor
OK, I give up. I'm not gonna comment on this one ever again. I should have known better.

As someone who has taught automotive engineering in the past, I've explained the theory behind the Zenith-Stromberg CD design as best I can. I can do no better, and further elucidation on the subject would be at best Sisyphean.

As my final word on the subject, I'll simply leave you to read the factory manual... please feel free to write to the publisher with your questions and quibbles.
C_58.gif
 

Shannon Boal

Platinum Level Sponsor
Sisyphean....Good word, meaning futile. I can make many carburetors sing.... four carb corvairs, Olds and Cadillac, Ford and Dodge, Solex, Weber, Carter, Rochester, SU; yes to all of those. These Zenith-Stromberg, if they were ever right, did not stay that way. Constant depression is truly what these carbs have meant to me!
 

hartmandm

Moderator
Diamond Level Sponsor
Kevin,

I appreciate the detailed explanation you have provided and the WSM 145 reference is useful as well. Thanks for taking the time to provide this information.

Don't be too quick to give up. :)

Thanks,
Mike
 

Mike O'D

Gold Level Sponsor
Thanks for the technical, science based explanation Kevin. Some of us really appreciate learning how/why things work. The WSM is very clear on what not to use (light weight oil), but doesn't have the "why".
 

rixter

Gold Level Sponsor
This is how I understand it. If there was no dash pot implemented, or there was no oil in the dash pot, or if the oil was super thin weight, the air valve would want to rise up too quickly when the throttle was snapped. This would create an over lean condition because the amount of fuel that could come up from the jet couldn't match that surge of air being sucked in. Using a heavier grade of fluid than super thin in the dash pot slows the rise of the valve to avoid this situation. Most discussions center around the best weight and type of oil to get the optimal performance for the operating conditions. As mentioned above, racing conditions can call for different weight fluids than suggested by the manual. For normal conditions, using the recommended fluid is certainly a good starting place. But to my knowledge there doesn't seem to be a down side to "some" experimentation to see what works best for your set up.

Rick
 

Thor 1211

Silver Level Sponsor
Thor,nks

Interesting, it's not an image but a page link for a page on Tiger East/ Alpine East web site.

Search TE/AE website home page. On the orange site address bar at top of page, drop down menu under "Tech Tips", click on "Factory Service Bulletins.

On that page's listings under section "D - Fuel", look for Bulletin 68-14: Zenith C.D. Carbs Loss Of Oil From Dash Pots.

This info is on the web in several places, so you could just search the bulletin title to get the information. The TE/AE page seems to have a better copy of the one illustration in the bulletin.

Hope this helps,
Thanks Don.

I found a copy of the service bulletin in an old copy of the Horn which is the UK Alpine Club magazine. The writer of the article regarding the bulletin said he made the recommended modification but it didn't cure the oil use problem.
 
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