Stromberg Air piston - sometimes stuck, always fluttering

[leifanten]

OK,
so here I am at the watering hole of the collective wisdom again. I have the car running very well now. Throttle response is excellent, good torque on low revs, snappy acceleration - just like it is supposed to. Except for one curious matter (possibly two, but I think they are related) .

The rear carb "flutters" when throttle is released. By that I mean that the air valve piston does not drop straight down to its idle position from wherever it came, it "bounces" in a damped sinusoidal fashion. This can be heard as a fluttering sound as it comes to rest and can be viewed quite clearly when running sans air filters.

An (in my theory) related issue is that when the engine is at operating temperature, the same carb occasionally sticks leaving the engine idling at 2500 rpm after the throttle (butterfly valve) is completely closed. It drops to proper idle either by me using the damper piston to manually push the air piston down (with the help of the viscosity of the oil) or if I carefully release the clutch so the engine sees load again.

Now, I have compared the two carbs and they both open and close their air valve piston equally.
I have balanced the carbs with the help of two (no less) calibration sucker-measuring-tools
The butterfly closes as it should
The needles and jets sit at equal depth in their respective holders
There are no vacuum leaks
Sticky piston only happens when hot
Flutter is on at all temps

Thanks for any wild, clever, unwild and even unclever tips that could lead me closer to solving this mystery


The facts:
Stromberg carbs, newly rebuilt with everything possible replaced (some of you may recall my previous thread about those strange little plastic rings around the needles that caused me some headaches)
All the emission control plumbing out and properly blanked off at the balancing "piece" (for lack of a better word)
Electronic ignition - brand new dizzy and all from pertronix
new cylinder head gasket
new water pump (not relevant to this, but I had to put that here since I am soooo pleased that it is a long while until that needs doing again...I hope)

[69S4]

Hi. Off the top of my head while I think through what might be causing your probs - I noticed that uneven tightening of the four screws that hold the suction chamber cover (the large domed bit on top!) on caused my pistons to stick. Maybe the diaphragm's not seated properly or something like that. Also, have a look at the rear of the pistons. On one of mine there are some rubbing marks where the piston has contacted the narrow bridging strip on the body. That piston has periodically given me sticking problems. The other carb has no marks and doesn't stick.

Just to check the obvious - you have put the springs in, you're sure the piston dampers are damping, you're using something like engine oil in the dampers, the air vent in the suction chamber cover is free?

[oldelanman]

Hi Lief, Some more thoughts.........

I presume when you say that the emission plumbing is removed that you no longer have the throttle bypass valves? Also you mention blanking off at the "balancing piece" - not quite sure what you mean but you do still have the balance pipe between the carb mountings I take it ?

As both your problems are in one carb only you may be able to pinpoint the cause by swapping parts between the two carbs.
Swap one thing at a time and see if the condition moves to the front carb, if it has no effect swap it back before trying the next thing.

Start with the dampers, then piston and cover (obviously your set up will not be correct but the engine should run well enough to check if you still have the problem or not) Also swap the temperature compensators.

All the best.
Regards,

[ppnelan]

Also swap the temperature compensators.

...Or remove them & blank off the holes. This will eliminate one possible source of the problem!

Matthew

[leifanten]

Stuart, Roger and Matthew. First of all, thanks to you all for taking the time to reply

the even torquing is something that I have in mind as well. I always carefully and evenly torque down the screws with a stub screw driver whilst moving the air valve piston up and down to ensure smooth operation. You are quite right that it will make the piston stick if you dont get it right. I was thinking of trying to retorque when hot to see if the piston drops then.

I must check the air vent in the suction chamber, that could be it...

I think you could be onto something with regards to where it could be sticking. When moving up and down there is a very very slight feeling of "drag" on the one piston. It drops like a champ from top to bottom position both when hot and cold, but I am suspecting the change in temperature could cause it to expand enough so that dropping from 1/8 throttle to idle proves difficult. I am going to swap the internals around to see if that moves the problem (or indeed cures it, but I dont expect that kind of luck)

With regards to the emission system, what I meant was that the cross over tubes have been removed (I also have steel headers, so the pre-heating chamber deal in the original manifold is not there) and the inlet and outlet points for the cross over tubes have been properly blanked off. Balancing plate/Assembly is in place.