hi all i have a issues i want to sort out after tuning my car. i origianally thought a nice tune up would get rid of the lumpy 'about to stall' feeling i had whilst cruising in 4th which only occured with slight throttle not acceleration or deacceling. so i checked my dwell (60 dead on) timing about 8 or 9 degrees (i think, mainly cos i couldnt see any other mark). colourtuned end spark plugs and checked centres got them blue with a tiny bit of orange. spark gaps at 25 tho. i had a sticking carby 1.25 twin Su so i got some dry glide teflon lube in them (i have gravity not spring carbs) i noticed also my idle is changing from 500 to 800 to 1000 at different times. i also noticed the little disk for the vac advance on the top of the dizzy is loose and can move up and down. i dont know what a vaccum leak is or if its the cause but now after all this the lumpy fourth has dissappeared and first gear is bogging down on take off to the point of stalling occasionally so i have to rev her up to take off??? anyone have ideas i suddenly am spending alot of time at home so i can probably sought it out and check anything people suggest. thanks for reading

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Enjoy Chris

no ideas?

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Enjoy Chris

can you explain what you mean by "gravity" as opposed to "spring" carbs. All SU carbs should have a spring inside the dashpot pushing the piston down to the bridge.

Also, do you have oil in them?



can't you tighten that up with the distributor cap off? while you're fiddling with that, pull the hose off the carb end of the hose that goes to the vacuum unit and suck on it (with your mouth) - it should be sealed up, (and if you suck really hard, it'll move the rod that goes into the distributor)



Simply put, a vacuum leak is where there is an air leak on one of the tubes coming off the inlet manifold. Engines suck air into the cylinders as they run, when the throttle(s) are closed, the suction produced by the pistons produce a vacuum in the manifold (which can be used by various things - boosters, carb advances), if there is a vacuum leak, then it means there's air getting in to the motor when there shouldn't be.... if the leak is variable (blocking and unblocking), then the idle will go up and down. It will have a higher idle when there is air leaking in to the manifold, until the point where there is more air getting in there than is coming through the carbs, and it leans out to the point the motor stalls



on face value of what you've said, you might find that the vacuum unit is stuffed and leaking a lot..... take it off completely, put some tape over the hole in the distributor, and block off the hole in the distributor where the hose goes

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did I tell you that I won a trophy?

VERY old SUs used on MGs etc back in the 40s had no spring- they made the piston from brass instead (it weoghed a friggin' ton).
My dad had one stock on his 1948 Morris 10, he junked the SU in favour of a `Smiths'? one and it went harder. Used to blow away his brother's Austin A40 Devon...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Why was the dashpost sticky in the first place ??? Teflon lube is a bandaid not a fix. Try re-centering the jet and check the needle for damage.

Mike

thanks so much for your halp. i think i have what the doctor has described. the pistons drop by gravity rather then spring although this could be another cause of the problem. ill try to tighten up the vac advance and suck it and see what effects i get. might try backing my timing of a tad if that fails. i think my carby suck as i hadnt oiled it in ages but next chance i get i will pull it apart and take photos, it may be a MG Su as we have twin 1 inch Sus lying around which im pretty sure arent a mini item. thanks again for ur help ill let you know how it goes

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Enjoy Chris