Lotus Elan

Stromberg damper spring differences

PostPost by: JohnCh » Mon Jan 14, 2019 4:21 pm

Has anyone experimented with damper springs on Strombergs? Looking at the specs, the Federal and non-Federal versions use different color-coded springs (blue/black and natural) so I'm curious if the non-Federal springs are a little stiffer and offer some improvement to throttle response?

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PostPost by: SENC » Tue Jan 15, 2019 1:58 am

I haven't experimented, but the federal emissions spring (B18277) is stiffer of the 2 versions used on Elans. According to the current Zenith and CD Reference Catalogue (from Burlen), the light spring (B18278) was used on 2 of the carb series used on Elans, 3296F/R (69-70) and 3322F/R (1971-72). Following find the other part numbers for each series from the same catalogue, might be useful.

stromberg-table_1.jpg and


As to throttle response, I wouldn't think the piston spring would impact that to any significance. I think it primarily serves to help speed the close of the piston after lift off of throttle.
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PostPost by: JohnCh » Tue Jan 15, 2019 2:40 am

Thanks, that's very helpful. My understanding is that rather than use a pump jet to provide immediate enrichment when the throttle is aggressively opened, Stromberg's use the oil to slow the initial upward movement of the piston, which increases air velocity and subsequently pulls more fuel from the jet. When I saw that different springs were used, I thought perhaps this also played a part and assumed Federal carbs would have received softer springs and worse throttle response. Obviously that's not correct.

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John
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PostPost by: 69S4 » Tue Jan 15, 2019 7:56 am

As I understood it (and I admit it may only be a partial understanding) the function of the spring was to help determine the ‘balance’ point where the piston stops under a given vacuum. The damper + oil determines the rate at which it moves but it’s the piston weight + the strength of the spring that eventually stops it. Fit a stronger spring and the piston won’t rise as far under steady state conditions. That means higher air flow speed so a richer mixture, which in turn will need needle adjustments to compensate.

My Haynes Stromberg book lists three spring colours (= strengths), red, blue and ‘natural’ for the 175s, the natural being the lightest followed by blue and red. Quite what determines which spring was chosen originally and why is not something I’ve ever seen explained.
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PostPost by: SENC » Tue Jan 15, 2019 11:58 am

Stuart's thoughts on function seem logical to me, and I particularly agree with the idea that modifying original spring strength would require adjusting the metering needle. Indeed, looking at the chart above, the medium spring is used with carb series that used needle B1G, whereas the light spring was used with B1Y and B2AR.
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PostPost by: rgh0 » Tue Jan 15, 2019 12:07 pm

The spring (and piston weight) needs to be selected so the carb piston progressively opens with air flow and is fully open at the maximum airflow through the carb that the engine needs, so a stiffer spring is required for a bigger engine with fewer carbs. Then the needle profile needs to be matched to the airflow and piston opening to get the right steady state mixture.

Enrichment during acceleration is handled by the oil damper on the piston as other have said.

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PostPost by: USA64 » Tue Jan 15, 2019 1:13 pm

Carburetor is a French word meaning -leave it alone-
We are supposed to be having fun, are we not?
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PostPost by: JohnCh » Tue Jan 15, 2019 2:05 pm

I've seen a number of threads where people have converted their Federal spec engine to non-Federal specs by changing the inlet manifold, distributor and needles, but no mention of the springs. Given the explanations above, shouldn't that also be part of the process to ensure proper progression?
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PostPost by: JohnCh » Wed Jan 23, 2019 4:26 pm

To follow up on this thread. I just discovered that a previous owner changed the carbs to adjustable needles and it appears he may have also changed the damper springs to the non-Federal version (there are no paint markings on the springs). When investigating this, I noticed that he had used very thin ATF for the damper well, which I subsequently replaced with thicker 20w50 as per Miles Wilkins. It has made a noticeable difference to throttle response. Although it still has that two stage progression which feels similar to a modern turbo (initial wave of acceleration upon large throttle opening, followed a 1/2 beat later by a second, stronger surge), the initial surge is slightly stronger, which lessens the delta between the two. Definitely not Weber-crisp, but an improvement.

-John
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