On older SU carburettors, which have a vent hole in the screw-in plastic top of the dashpot, the oil level should be i in. (13 mm) above the top of the piston tube. On dustproof SUs, which have a hole in the dashpot neck instead of the top, the oil should be i in. (13 mm) below the piston top. SU Dashpot oil - manufactured for the specific application of SUs in STANDARD applications. That is on engines that are completely standard as the manufacturer produced them. It is a little on the thin side, but the whole engine tuning data for each engine is based on using this oil. SU carbs are not that sensitive to oil level in the dashpots, and you've got the proper weight oil in there. All the oil does is regulate the carb pistons' movement during acceleration and prevent them from chattering during idle and cruise. If there's oil in there, you're OK. I have a 1972 MGB roadster. I had the carbs (SU HIF4's) rebuilt in April and have been working to get them adjusted (I'm having a little trouble with the mixture). In between efforts, I have found that the oil in the damper tubes is leaking down into the carb throats and fouling the pistons. This is happening in both carbs to some extent, but the rear one is much worse. SU DAMPER OIL is a Mineral based oil containing anti-corrosion and anti-wear additives for the lubrication of pistons and dampers in SU and Stromberg carburettors. Application SU DAMPER OIL is suitable for post 1940 vehicles using SU and Stromberg carburettors incorporating a damper in the dashpot assembly. 3 in 1 is pretty similar to SU damper oil. The oil isn't going to get very warm in the dashpot, the carb is essentially refrigerated. As for oil level, it should be 1/2" below the top of the damper tube. That's the inner tube that you can see inside the dashpot. Check also for vacuum leaks, a vacuum leak will lean the mixture. Check the level of the dashpot oil – use 20w oil or specific SU dashpot oil. The piston rising without restriction prevents enrichment necessary for acceleration. Ignition too far advanced or retarded – ignition occurs while valve is still open.
Today I will discuss tuning of SU carburetors. Tuning includes mixture adjustment, idle speed adjustment, and synchronization. It is assumed that your carbs have been properly rebuilt and the jets centered; if not, stop here, because your carbs are likely not tune-able. Instead, go study one or more of my posts on SU rebuilding, starting…
The bottle shown in the test is labelled “SU Damper oil” so it should flow like any other SAE 20 at normal temperature – say 70 degrees Fahrenheit. So… it could be that the oil inside the bottle is not genuine SU Damper oil as supplied by SU Carb Co or maybe the ambient temperature was hot – say over 100 degrees F. SU Dashpot oil - manufactured for the specific application of SUs in STANDARD applications. That is on engines that are completely standard as the manufacturer produced them. It is a little on the thin side, but the whole engine tuning data for each engine is based on using this oil. Most Automatic Transmission Fluid is 20wt oil. And several people use it in the carbs. I do and have no problems. The fill line is on the shaft. Don't worry if you over fill. It will get sucked into the engine and burn off. Fill it to the top of the piston hollow and let it back down, then put in the plunger. This will displace a small amount of oil onto the piston shaft to lubricate it and keep things going nicely (some even have bearings here that really enjoy being lubricated as such). It's important that the dashpot on your carburettor is filled with oil, preventing fluctuations in fuel/air mixture under acceleration. SU damper oil is designed specifically for this job. The second type is SU Dashpot Oil, which is for use in earlier, pre-war, SU carburetters where no damper is used. In the case of carburetters fitted with dampers the oil should be filled to a level just below the top of the hollow piston rod, so that when the damper is fitted it does not displace the oil any higher than the top of the piston.
How To Repair, Overhaul, Strip Down and Re-Assemble the SU Carburettor. Fill the damper with 20W50 grade oil to about 13 mm above the top of the moving
It's important that the dashpot on your carburettor is filled with oil, preventing fluctuations in fuel/air mixture under acceleration. SU damper oil is designed specifically for this job. The second type is SU Dashpot Oil, which is for use in earlier, pre-war, SU carburetters where no damper is used. In the case of carburetters fitted with dampers the oil should be filled to a level just below the top of the hollow piston rod, so that when the damper is fitted it does not displace the oil any higher than the top of the piston. In my Haynes SU Carburettor book for the HIF Type it says: Fill the piston rod with oil. On standard suction chambers (ie without a damper rod guide clip, or upper circlip on the piston rod), the level must be 0.5in (13mm) above the top of the hollow piston rod. Oil level was still way down below top of reservoir area. It took many more drops to bring it up to top, then I slowly took oil out drop by drop until it was 1/4" below inner reservoir top. Plunger stays up 1/2" plus and takes some pressure to push it down, matched level in both and tightened caps. On older SU carburettors, which have a vent hole in the screw-in plastic top of the dashpot, the oil level should be i in. (13 mm) above the top of the piston tube. On dustproof SUs, which have a hole in the dashpot neck instead of the top, the oil should be i in. (13 mm) below the piston top.
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Buy Penrite SU & Stromberg Carburettor Damper Oil 150ml at Amazon UK. Free delivery on eligible orders. SU Carb Dashpot Oil The purpose of the SU damper is to retard the rapid upward movement of the piston on the rapid throttle opening associated with acceleration. This delay in piston movement causes a momentary decrease in pressure at the throat, thus achieving a momentary increase in richness much the same as with an accelerator pump. The bottle shown in the test is labelled “SU Damper oil” so it should flow like any other SAE 20 at normal temperature – say 70 degrees Fahrenheit. So… it could be that the oil inside the bottle is not genuine SU Damper oil as supplied by SU Carb Co or maybe the ambient temperature was hot – say over 100 degrees F. SU Dashpot oil - manufactured for the specific application of SUs in STANDARD applications. That is on engines that are completely standard as the manufacturer produced them. It is a little on the thin side, but the whole engine tuning data for each engine is based on using this oil. Most Automatic Transmission Fluid is 20wt oil. And several people use it in the carbs. I do and have no problems. The fill line is on the shaft. Don't worry if you over fill. It will get sucked into the engine and burn off. Fill it to the top of the piston hollow and let it back down, then put in the plunger. This will displace a small amount of oil onto the piston shaft to lubricate it and keep things going nicely (some even have bearings here that really enjoy being lubricated as such). It's important that the dashpot on your carburettor is filled with oil, preventing fluctuations in fuel/air mixture under acceleration. SU damper oil is designed specifically for this job.
SU carbs are not that sensitive to oil level in the dashpots, and you've got the proper weight oil in there. All the oil does is regulate the carb pistons' movement during acceleration and prevent them from chattering during idle and cruise. If there's oil in there, you're OK.
Many of the SU carbs I see have had dampers replaced with the wrong type. If it still doesn't want to turn. squirt an aerosol penetrating oil through the float The proper procedure is to fill to 1/2″ below the top of the hollow piston rod. not Fill piston damper bowl is filling and that there are no fuel with SU carburettor leaks from the carburetors oil or oil with similar. 5. Start car and check again for fuel This web page on the Hitachi-SU carburetor is copied from a Service Manual Total oil volume is approximately 3 cc (0.18 cu in). Slowly fill damper with oil to