Cruising at 4,500 rpm...

[alaric]

Hi. Are the Strombergs like SUs where the mixture is altered by a venturi courtesy of monsieur Bernoulli; the pressure seen by the fuel needle valve is controlled by raising and lowering a piston with an oil damper? If all's working correctly, then as you start putting your foot down the mixture is smoothly richened by an amount proportional to the rate of depression of foot. The damper slows the piston movement down, so makes the mixture richer for a particular foot press. Lack of oil or dampers not working properly - they're usually like a one way valve moving up and down a bore, so they can get choked up. If it wasn't working your mixture would be temporarily lean. If the oil level is too low it could be fine at lower rpm where the flow rate is lower, and start to fall apart when the piston is higher up the bore.

On the other hand it's surprising that this would be happening at these revs and not be evident at lower revs.

I love these old mechanical solutions though. Before the days of computers someone worked all this stuff out by hand.

Sean.

[69S4]

My S4 has Strombergs and I've experienced just about every fault with them that's ever been posted here - except this one. The carbs ice up (this doesn't sound like carb icing), the jets are worn out, the needles have been replaced but lack of use meant they grew some sort of brass fungus that I scraped off and the temp compensators are set at random, I have spindle air leaks and oil leaks past the damper o rings in about 1 hr of driving, but the car cruises quite happily at any speed over about 2000rpm and will accelerate without missing or hesitating.

There are a couple if things that might be worth mentioning. My car goes through a slight resonance period at about 4200rpm. It's not wheel balance but definitely comes from the engine and I did think about about fuel frothing if the carbs are too loose / tight. Clutching at straws maybe but 4.2k is close enough to 4.5k to make me wonder. The other thing I wondered about is the float heights. Too low and you will get high speed misfires but in my experience the Strombergs are not hugely sensitive to it. Worth checking though when you've next got the carbs apart.

Incidentally (and there's probably a huge number of archive posts about it (haven't checked)) I've always used 20/50 engine oil in the dampers. It does make a difference in that I can definitely tell when its all leaked out and I can just about feel the difference using 10/40 (apart from it leaking out quicker!). ATF is so thin surely it would feel like nothing in there. Any wisdom on the subject gratefully received.

Stuart

[collins_dan]

Frothing is certainly possible. I was reading about the weber soft mounts. My strombergs certainly do not have any kind of soft mount, they're about as stiff/tight a mount as I can imagine.

Float level sounds worth looking into.

I used to use 20/50 before I had the engine rebuilt. The guy that did the work suggested atf so that the engine would run as lean as possible. I've checked the plugs and they are as pretty as plugs can be, nice and light brown, so it seems to be working.

I was just driving back from an autocross on the highway and I didn't so much notice the hesitancy as slight misfires and rough, almost like when the car is first warming up. A friend was driving behind and he noticed occasional puffs of smoke every couple of minutes. I was fairly steady on the car, so acceleration wouldn't the smoke.

This one has me very stumped. I'm starting to consider sending the carbs out for a professional refurb. The only other thing left would be to replace the distributor.

Thanks for your thoughts. Dan

[reb53]

Can't remember what I used in the Stromberg equipped car I first had, but recently spent some time looking around the net to find what a friend should be using in his Stag and the overwhelming consensus was 20/50.

Wouldn't be hard to stick some in would it, and see what happens ?

Cheers
Ralph.

[69S4]

The only other thing left would be to replace the distributor.

Regarding the distributor, I suppose there is a chance that there is some sort of mechanical or electrical fault in there that's giving rise to your symptoms and a close inspection should reveal if there is anything amiss but the other query would be whether the advance curve is in some way responsible and that's not so easy to check short of putting the car on a rolling road.

A new distributor off the shelf from the "usual suspects" in the UK afaik comes with an advance curve more suited to the Weber engined cars and that's what I had to fit some years ago. I have noticed that with it the car is flatter at the top end than it used to be - presumably because max advance is less than the engine needs, but it doesn't play up in any other way - no misfiring etc.

From what you've described I'd still be giving the carbs some "counselling". Try some 20/50 in the dampers. How's your airfilter btw?

Stuart

[collins_dan]

I agree, the carbs need attention and the advance as well. Unfortunately, rolling roads don't seem to be as common in my area. I did a brief investigation some time ago and came up with nothing. Any suggestions from other folks in the US mid-atlantic area, specifically Baltimore, but Philadelphia is not that far and neither is Washington DC/Northern Virginia. Thanks, Dan