McMuttley Posted November 7, 2015 Report Share Posted November 7, 2015 (edited) So i cleaned the carbs up, scruffy but nothing terrible, needles shiney, piston drops with an acceptable dink - no sluggishness on either. Synchrometer showed they were balanced. Gunson showed red all round and no adjustment of the jet nut changed this (wish i'd remembered the starting point before turning flats !) However, as the attached u-tube video shows, the piston for the carb serving cylinders 1 and 2 (right) moves earlier, higher and returns quicker than the rear carb. Same amount of damper oil in both. Could this be sluggish linkage? Oddly engine now runs, despite last weekends tow job - and I havent yet changed coil or condenser! http://youtu.be/Daea-6TyywM Edited November 7, 2015 by McMuttley Quote Link to post Share on other sites
peejay4A Posted November 7, 2015 Report Share Posted November 7, 2015 If it's running without the air cleaners attached it tends to validate Peter Cobbold's theory about blocked atmospheric vents. Do the air cleaners have the requisite holes? Did you use the balancer at idle or off idle? If at idle then try balancing them at say 2000 rpm then see if the Pistons lift together. Quote Link to post Share on other sites
Peter Cobbold Posted November 7, 2015 Report Share Posted November 7, 2015 Austin, "" Oddly engine now runs, despite last weekends tow job - and I havent yet changed coil or condenser! " So it does run fine after removing the filters. Probably the filters block the atmospheric air bleeds to the underside of the SU piston. These are the two holes above the filter mounting holes/bolts on each SU. Check also a gasket is not blocking the holes. If atmos air cant get in there the carbs will run stinking rich -enough to prevent the engine from running............ as I've been telling you - see the Doretti tale!! ========== The different piston lifts: The synchro tool shows the air flows are the same. So the different lifts may not be because the butterflies are out of step. Did the synchro tool report the same flows when blippingthe throttle? If it does, there are several other possible causes: a damper rod is slightly bent and binding - test by removing both dampers (and opening throttle gradually) spring wrong or missing on one piston- do they feel the same weight to the finger? air leaking too fast between one piston and dome Peter Quote Link to post Share on other sites
Peter Cobbold Posted November 7, 2015 Report Share Posted November 7, 2015 (edited) If it's running without the air cleaners attached it tends to validate Peter Cobbold's theory about blocked atmospheric vents. Do the air cleaners have the requisite holes? Pete, thanks ! Peter Edited November 7, 2015 by Peter Cobbold Quote Link to post Share on other sites
McMuttley Posted November 7, 2015 Author Report Share Posted November 7, 2015 Thanks Gents The holes are there and the (somewhat soggy) gasket is not blocking I did think the damper rod on carb 3/4 looked a tad wonky, will check again tomorrow. I only have the one synchrometer, so will need to get the wife to be accurate with her foot! Will try it with the choke too. Both float chambers were v clean, no sign of any sediment Quote Link to post Share on other sites
Peter Cobbold Posted November 7, 2015 Report Share Posted November 7, 2015 (edited) Austin, Puzzling.. And you've marked the filter backplate to ensure they go back the right way up. So I cant suggest the bleed holes were at the bottom of the carb mouth Does removing the fliters let it run on the road without being towed? A static garage test doesn't get much air flowing at all, just a couple of horsepowers-worth. Peter Edited November 7, 2015 by Peter Cobbold Quote Link to post Share on other sites
Lebro Posted November 8, 2015 Report Share Posted November 8, 2015 (edited) Are both piston springs present, & are they of the same strength ? Bob. EDIT Just noticed Peter has already suggested this - sorry ! Edited November 8, 2015 by Lebro Quote Link to post Share on other sites
RobH Posted November 8, 2015 Report Share Posted November 8, 2015 This carb thing, interesting as it is, may be nothing to do with the engine conking out. If its a mechanical derangement of the piston/spring it will have been there a long time and would not have caused the engine to stop working. The fact the engine runs with the air cleaners off may be a red herring; after all it was running before with them on until you got to the motorway..... The real fault may still be temperature related and if you have not had the engine fully hot since it will not manifest itself. Rob Quote Link to post Share on other sites
McMuttley Posted November 8, 2015 Author Report Share Posted November 8, 2015 Noted Rob, indeed, it ran well until the conk last week, has been to france and back, Malvern Silverstone etc all since May There is a repeated issue with it running rich and sooting up the plugs, which obviously will not help, but as well as changing the coil and condenser, I wanted to try and get the mixture a little better before sending them off for re-build. Maybe a new coil and stronger spark might assist. Inside the dizzy all looks fine, but will change the condenser today - as well as the coil. The engine ran even with the cable out of the top of the coil, but with the spark arcing into the dizzy How do I tell if a spark is good or not ? I am using this as my guide for the carbs..... although have been watching the moss video, u-tube and reading every other web reference there is til late into the night ! http://www.them-g-c.com/index_files/Page2858.htm Quote Link to post Share on other sites
RogerH Posted November 8, 2015 Report Share Posted November 8, 2015 Hi Austin, some good advice given so far. To see if the piston springs are the same swap them from carb to carb. If there is a difference then the rear piston should rise quicker than the front. Or you could simply buy two new identical springs. Have you checked the throttle linkage?. Is the rear butterfly lagging the front? Roger Quote Link to post Share on other sites
McMuttley Posted November 8, 2015 Author Report Share Posted November 8, 2015 Are both piston springs present, & are they of the same strength ? Bob. EDIT Just noticed Peter has already suggested this - sorry ! Hi Bob, having just checked, no the springs are not identical, one is newer (why didn't they replace both) - one is taped the other isn't Quote Link to post Share on other sites
McMuttley Posted November 8, 2015 Author Report Share Posted November 8, 2015 Hi Austin, some good advice given so far. To see if the piston springs are the same swap them from carb to carb. If there is a difference then the rear piston should rise quicker than the front. Or you could simply buy two new identical springs. Have you checked the throttle linkage?. Is the rear butterfly lagging the front? Roger Rog - the linkage is next on my growing list - cheers Waiting for the nasty neighbour to wake up before starting again this morning - alway an issue getting an engine to a temp to tinker when you suspect the environmental health officer will get there before it gets to anything workable ! Quote Link to post Share on other sites
RogerH Posted November 8, 2015 Report Share Posted November 8, 2015 Hi Austin, re - the spark. If you have a spark then don't bother trying to make it bigger. It only starts the fire, the petrol does the rest. Also TRy to change only one thing at a time to cure the possible conking out fault. Sort the carbs first. TRy to get a decent mixture. Stop the plugs sooting up. Roger Quote Link to post Share on other sites
Paul Down Posted November 8, 2015 Report Share Posted November 8, 2015 (edited) Roger beat me to it regarding only changing one item at a time to see if it cures the problem otherwise you will never isolate the culprit. A tip when checking the condenser: Turn on the ignition with the distributor cap removed Flick the points open and observe the spark (better done at night) If the spark is compact and blue the condenser is usually OK. If the spark is flashy orange yellow this usually indicates a faulty condenser (open circuit) If there is no detectable or an extremely weak spark the condenser has short circuit, a rare occurance Edited November 8, 2015 by Paul Down Quote Link to post Share on other sites
Peter Cobbold Posted November 8, 2015 Report Share Posted November 8, 2015 (edited) It only conks at motorway speeds? Hi Bob, having just checked, no the springs are not identical, one is newer (why didn't they replace both) - one is taped the other isn't Taped? You can check the spring force on kitchen scales, if the paint that identifies them has worn off. But easier to replace both with standard new ones - someone will know the rating. Peter Edited November 8, 2015 by Peter Cobbold Quote Link to post Share on other sites
Peter Cobbold Posted November 8, 2015 Report Share Posted November 8, 2015 Looks like the rich running fault might have ben found - carb spring(s). But the fault is new and it only conks at motorway speeds and then wont restart it might be getting too hot because of retarded spark. Maybe the centrifugal advance seized? test the disy shaft springs back when twisted after removing the rotor arm. Or perhaps the vac advance capsule has punctured or seized. Peter Quote Link to post Share on other sites
McMuttley Posted November 8, 2015 Author Report Share Posted November 8, 2015 sorry peter 'tapred'. One spring is, one isn't I am currently fabricating a chopstick/light washer spring compression/strength testing device. My racey setup doesnt have vacuum advance Quote Link to post Share on other sites
peejay4A Posted November 8, 2015 Report Share Posted November 8, 2015 If you do the flicking the points trick don't use a naked finger. You will get a belt. Quote Link to post Share on other sites
Fireman049 Posted November 8, 2015 Report Share Posted November 8, 2015 (edited) For my 3A I know that the Red piston springs are fitted ~ (both new) with SM needles and H6 carbs 2.2 L engine. My plugs are always pitch Black although I've set-up the correct (Bunsen blue) flame with Colortune. What advantage, if any, If I fit Blue springs and TW needles? Tom. Edited November 8, 2015 by Fireman049 Quote Link to post Share on other sites
Peter Cobbold Posted November 8, 2015 Report Share Posted November 8, 2015 (edited) For my 3A I know that the Red piston springs are fitted ~ (both new) with SM needles and H6 carbs 2.2 L engine. What advantage, if any, If I fit Blue springs and TW needles? Tom. Tom, I would cure the richness with leaner needle. Changing the spring opens up a can of worms., as folllows: Blue spring is 2 1/2 oz Red spring is 4 1/2 oz Swapping to blue will give slightly less 'constant depression' than red. But you have to add the weight of the piston to both before making the calculation of the ratio. What is the weight of a piston? The piston will lift fractionally more with the blue spring. BUT the mass air flow will not be significnanty raised by going to a lighter spring, only by the change in constant depression that allows at the most 0.1psi more intake pressure at wot. Tell me the weight of the piston and the outer and inner diameter of the piston rim, and I'll calculate the change in the constant depression. With a blue spring the increase in piston lift at a given throttle setting means a new needle will be needed with a different profile. To calculate the extra lift I need the dimensions of the choke tube and bridge., and the bhp...............it gets much more complicated**. Even then if lift is known calculating the profile is not possible: AFR has to be measured The extra piston lift means the air particle velocity across the jet slows, and that will impair fuel atomisation at full throttle ( at part throttle the butterlfy smashes the droplets ) The air mass velocity changes very little, it follows that "0.1psi" change above. In short not a lot to be gained by changing. This is because the SU piston only appears to be an obstruction to air flow. In reality it drops very little pressure, and hence drops very little flow. Peter ** my exhaustive analysis of 2" SU fluid mechanics here: https://supertrarged.wordpress.com/2015/05/04/the-operating-limits-of-an-su-carburettor-2/ ......with many ramifications Edited November 8, 2015 by Peter Cobbold Quote Link to post Share on other sites
Lebro Posted November 8, 2015 Report Share Posted November 8, 2015 Easy way to see it springs are matched is to find bar, or tube over which the springs will be a nice sliding fit. then with both springs on, touching each other (ooo er missus) compress them into each other. If matched they will compress at the same rate, if not the one will become fully compressed before the other. They should also be the same uncompressed length to start with. Bob. Quote Link to post Share on other sites
Fireman049 Posted November 8, 2015 Report Share Posted November 8, 2015 Thank you Peter ~ I'll stick with what I've got and experiment with different needles. As far as needles are concerned. When I had my Mini-Cooper in 1966 replacement needles were 4/6d each!!! Tom. Quote Link to post Share on other sites
Lebro Posted November 8, 2015 Report Share Posted November 8, 2015 I think you will find they have gone up ! Bob Quote Link to post Share on other sites
McMuttley Posted November 8, 2015 Author Report Share Posted November 8, 2015 Hi Bob Only had chance to check springs, The left spring is from 3/4, the carb which is tardy. It is circa 5-8mm shorter and 0.5mm narrower ? Both have circa 34 turns. Despite differing dimensions, they end up fully compressed at the same time ! Quote Link to post Share on other sites
Lebro Posted November 9, 2015 Report Share Posted November 9, 2015 OK springs probably not the cause then. Just watched your video, I see the RH piston rising further than LH one. Going back to basics, balance of throttles is usually carried out at slow tickover, & is adjusted with the small screws on each carb individualy. But, when you depress the throttle pedal this rotates the ¼" rod nearest the engine (between the two carbs) & that in turn moves the two arms which should only touch the throttle levers on the carbs after a small amount of free play. It is important that when this free play is taken up that both carb levers start to move together. Otherwise one throttle will start to open before the other. Good luck with tracking this down. Bob. Quote Link to post Share on other sites
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