Ian Vincent Posted January 21, 2021 Author Report Share Posted January 21, 2021 18 hours ago, Peter Cobbold said: Are the oil-filler caps drilled with a vent hole. It is possible with bitsa SUs to have no vent holr and no cross drilling at the top of the damper tube under the inside of the dome. That combination traps air in the tube that compresses wehn the engine is underload and will richen the mixture at cruise, lower mpg. Thanks for the tought Peter, but yes they are drilled with the vent hole. Rgds Ian Quote Link to post Share on other sites
Lebro Posted January 21, 2021 Report Share Posted January 21, 2021 All this has caused me to check my springs, as they have no colour, & have been in the carbs from new, when they were bought from Cox & Buckles very cheaply ! the AUD No. indicates that they were originaly intended for the Allegro Sport !!! Anyway, took them out, & checked the force required to push them down to 2.635" both were very close to 4.5 oz, so they are the correct red springs. I have now applied some red paint to them. I have also just ordered a pair of RH needles to try, based on all that I have read above. Bob Quote Link to post Share on other sites
Peter Cobbold Posted January 21, 2021 Report Share Posted January 21, 2021 The effect of say doubling spring force is surprsingly small. The needle will work at a lower position but the constant depression rises, so fuelling changes only marginally. What does happen is atomisation fo the mixture improves with the stiifer spring. improivng distribution in the manifold, combustion,smoothness and mileage. https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/ Peter Quote Link to post Share on other sites
Lebro Posted January 21, 2021 Report Share Posted January 21, 2021 I did attend your seminar on the SU carbs, I think I understood at least some of it !! Bob. Quote Link to post Share on other sites
Peter Cobbold Posted January 21, 2021 Report Share Posted January 21, 2021 2 minutes ago, Lebro said: I did attend your seminar on the SU carbs, I think I understood at least some of it !! Bob. Bob, From my perspective the seminar was a disaster, voice failing, louspeakers outside.. So glad you caught some of it. Am always willing to elucidate the difficult bits on the forum. It was not easy to fathom how it works from the very few articles, so I worked from first principles of fluid mechanics. I really should write it up as an article some day. Peter Quote Link to post Share on other sites
Lebro Posted January 21, 2021 Report Share Posted January 21, 2021 Yes, the P.A. (& music from somewhere too I seem to remember ?) was a pain. Quote Link to post Share on other sites
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