Spitting Strombergs

[david.g.chapman]

I have had an issue with my Stromberg carbs for some years.

When the carb bodies are cold (for the first 20 mins of driving at ambient temperatures below 13 degrees C or so), The engine bogs down on initial acceleration under load, and the carbs sometimes then spit back. The choke helps with this, but this bogging down persists beyond the point when the temperature gauge reaches normal, and only stops when the carb bodies have warmed up.

When the carbs are warm, the engine runs well under all conditions, possibly eliminating issues like float levels, mixture, air leaks etc.? Note that raising or lowering the needles has no effect on this, just altering the colour of the plugs.

I use 20-50 engine oil in the dashpots and the pistons drop freely with a clear click when the carbs are warm, and more slowly with a soft click when cold.

Any ideas?

Dave Chapman.

[USA64]

You didn't mention the dash-pot springs; are they correct?

[rgh0]

I thought the dash pots used a light machine oil. 20-50 may be too thick until it warms up??

cheers
Rohan

[elans3]

I'll second that, Rohan. I always used 3 in 1 thin oil in the dashpots on my previous SU and Stromberg equipped cars. I too, found that 20/50 was a touch too heavy .

[david.g.chapman]

Thanks very much for your replies.

I replaced my original springs with silver coloured ones some years ago. I can't remember if they were stiffer or not. Could be worth going back to the original springs and using light oil in the dash pots.

Dave Chapman.

[dereksharpuk]

I had problems with Strombergs. They suffer from carb icing which can occure up to 10 deg C

[david.g.chapman]

Yes, I can get icing in my Strombergs in winter after a few minutes running. All part of the fun. I have thought of a carburetor heating system that some other classic cars have, such as routing the air intake over the exhaust with a summer/winter position. Not easy to do though if you want to go back to original quickly.

Another option could be to fit aluminium resistors to the carb bodies using a convenient existing screw hole(s). You would then heat the carbs for 10 mins via a switch in winter before setting off. Something like flying a light aircraft! Also easy to remove resistors and wiring to get back to original.

Using ATF oil in the dampers and fitting the original springs (which are about 10% less stiff), did help. However, the weather has warmed up here in sunny Essex so everything is better anyway.

If I get anywhere with carb heating I will post it on here......

Cheers,
Dave Chapman.

[david.g.chapman]

Thread Update!

The car had been running OK when hot, but more recently it has been spitting occasionally when hot - after a period of overrun when you get on the throttle again. The car is also bogging below idle speed after a blip on the throttle before stabilising on the correct idle speed. All the symptoms of a lean mixture/intake air leak? I wonder.....

So I've just bought yet another gadget from ebay - a US PRO Fuel-O-Vac combined fuel pressure gauge and vacuum gauge. Me, I love a gauge....

I first of all teed it into the fuel line just before my Strombergs. I have a mechanical fuel pump with a Petrol King regulator before the take-off. I get a steady reading of 1.5 psi at all engine speeds, so that's OK.

I then connected it to the intake manifold instead of the servo, to measure intake vacuum. I get 15 inches of Hg at idle, which according to the gauge is a bit low, and below the green zone of 17 to 22 inches of Hg. Plugging the gauge in the headlight vacuum port gives 16 inches of Hg - still a bit low. Squirting oil over the throttle spindle bushes does nothing, and I can't see any other source of an air leak.

Would this slightly low vacuum level be responsible for the spitting back? Or is it just normal for a federal low compression head or a twink in general?

Otherwise I'll just raise the needles a bit and try that....

Dave Chapman.

[Donels]

I would suggest an air leak or the float level is too low or the float is leaking. Spitting back is usually the mixture is too weak, ie too much air or not enough fuel.

[oldelanman]

A couple of areas to check for air leaks if you've not done so already ..... the balance pipe O rings where they enter the adaptor blocks and the seals between temperature compensator and carburettor body, if I recall correctly there are 2 on each compensator. Also worth checking the compensators are working properly and not sticking open.

[david.g.chapman]

Thanks for the responses.

I had the carbs apart recently and checked the good points raised. Nothing definitive was found.

It's not that affected by mixture setting - I raised the needles and there was no effect, but if the choke is pulled out a bit there is an improvement, so you would think it's weak mixture. I am now trying going back to points from electronic ignition, just to see if it makes a difference. Note that its not affected by timing.

Just to explain the symptoms a bit more, from a cold start on the choke with carbs exposed (no induction chamber) the engine runs fine. After the choke is closed and the engine is half warm the engine continues to idle steadily, but bogs down when the throttle is snapped open, occasionally spitting back through both carbs at once. After this happens the engine threatens to stall from idle, and can stop completely. Starting again and blipping the throttle sets the idle again and the engine can then rev quite happily for a few cycles, until it spits again. The intake flooding from the spitting back might be upsetting the idle, so it might be all the same issue.

As the carb bodies warm the spitting improves, but recently its been occurring even then, as mentioned above.

I will try points ignition tomorrow. If I find anything or if it runs OK I will post it.

Cheers,

Dave Chapman.

[SENC]

You mention the needles a few times, but not the jets. The adjustable needles give a very limited range of adjustment, and require the jets to be set correctly in the first place. Are they set per the service bulletins? And what needles are you using? I'm not sure the above is the problem since you're idling ok, but worth confirming.

In my experience your vacuum numbers aren't a surprise - and particularly if your car has more radical than standard cams.

So, what I think I'm inferring from your posts is that at idle you're good, then when you hit the accelerator the butterflies open allowing the vacuum upstream and raising the pistons - too quickly, presumably, creating the lean condition? Or is it too slowly and going richer and flooding? That would bog, too.

Are the carbs balanced? Ie., can you confirm butterflies across the carbs are opening the same amount and at the same time?

Just a few other things to consider. Happy hunting!

[69S4]

I get similar spitting back issues on my S4. If I start the engine from cold using the choke it runs fine but if you push the cable back before the engine is completely warm (4-5 miles) it'll spit back on acceleration. I've noticed it spits more severely if I top up the dashpots (usually 20-50 oil - which is too viscous). If the dashpots are empty it doesn't spit back anything like as much. After the engine is warm it runs fine without spit back which is different to your experience but I suspect something similar is going on.

[david.g.chapman]

Tried points ignition again and better when warm, though not really OK. I had a closer look at how the air valves drop unto the bridge, and tried to get a clear click when the valves drop. The slightest misalignment and the valve bind a bit, which will affect things.

Freeing up the valves to get some sort of click and trying again - and it's better still and virtually OK when the carb bodies start warming up (from one run only though).

I didn't mention that I have CPL2 cams - nice one SENC!

I am now referring to this excellent source from the web:

https://www.buckeyetriumphs.org/carburetors

I will also try out different oils in the dashpots and I have a stronger set of springs to try as well. Going out again this Sunday hopefully.

Dave Chapman.

[david.g.chapman]

Good news! I think I've fixed the spitting completely! I went out on a 20 mile run from cold this morning and not one spit back, including when the choke was out for the first 4 miles or so!

What did I do?

1. Cleaned the air valves. There was soot on them, probably from all the spitting back?
2. Aligned the top covers carefully to ensure that the air valves drop cleanly when cold and with engine oil in the dashpots. The valves make a soft click on the bridges. If the dampers are removed then there is a louder click and the valves drop quickly. I kept going until I got the soft click with the dampers fitted.
3. I had some heavier new springs in my stock so I fitted those. This lowers the equilibrium point for the air valves and gives a slightly richer mixture at all throttle openings I believe.
4. I filled up the fuel tank! This may be important but shouldn't be as I've checked the fuel pressure on a low tank. I will see ...

All other settings, including jet positions (I have fixed jets), mixture, float condition and height, fuel valve condition have been checked recently and are nominal as per spec.

So I'm a happy bunny. Hopefully this is some help to others....

Cheers,

Dave Chapman.