Amazon: Carb change on auto?

[woodman]

Hello,

Thinking of changing from single Stromberg to twin SU on my Amazon auto. I'm just concerned about connecting to the kickdown linkage: does anyone know if this is a straightforward job? I guess it depends if there were any twin carb/BW35 combos built.

Cheers,

Andy.

[swedishandgerman]

I have a question! What are you hoping to achieve by putting on twin carbs?

[woodman]

I've just never had any joy out of the Stromberg. I changed the original for a recon, which has improved the emissions etc, and it accelerates ok, but it's hypersensitive to dashpot oil level, doesn't give a progressive choke/fast idle, is crap at hot starting and I still feel is holding back about 10% of potential driving fun. I had three Triumphs on twin SU set-ups and was always really happy with them.

[agent_strangelove]

There were 122 autos with SU's. I've driven one got the past 20 years. The linkage is slightly different as it has an ear for the kick down cable to attach. I've got an extra I could part with. Happy to provide photos but out of town till Tuesday.

Chris

[swedishandgerman]

I'm not sure I completely understand the benefit of running an auto on twin SU's would be

These comments are based on "Adam Theory" and I have no statistical information to back this up but.... Twins provide better breathability, for want of a better expression, so extent the usable rev range. In other words, increase the power at the high end of the rev range

In an auto, when do you use that bit? When do you drive your car with the engine at full revs? I would consider that question before you make the change

Twins do not provide increased torque at a given engine speed throughout low and mid range, but might improve power when you've wound it up a bit

In combination with a different cam (like a 122s would have had) is a different story, where breathability is further improved by how much air can be drawn through the valves which can be opened for longer and wider. But again, my "Adam theory" makes me query how much benefit you'll enjoy with an auto box where a standard A-cam provides a nice smooth torque delivery at low engine speeds

I really don't think you're going to get that extra 10% you're looking for by the switch of carbs alone

If I was you, I'd pay a little more attention to where your dashpot oil is going and why it's not starting when hot. You can get a new fast idle cam from Burlen for not much money. Are you certain it has the correct needle in there that hasn't been worn or damaged? Is you're ignition set up absolutely properly?

[woodman]

Many thanks for your detailed thoughts, Adam, and also Chris for your kind offer of the spare parts.

My options are to persevere with the Stromberg or swap to SUs. I'll try and deal with some of the issues in a bit more detail....

Performance: I have had REALLY good performance out of this engine, with the single Stromberg, when running in cold weather with the Speedograph pancake air filter. Nice quick pick-up, good starting, great acceleration. On a good run I was getting 27-28mpg. Nice noise too! As soon as summer came, the fuel consumption went down badly (21ish) and we put this down to the pancake filter picking up too much hot air from around the engine. I went back to the standard, sealed for 20,000 miles type paper filter, which has never given quite such good oomph and I've never got back above 24mpg. My planned solution has been to revert to the pancake filter but build a bespoke airbox for it with a removable intake hose running through the front slam panel and pulling cooler air in for better summer performance. This is still an option. The hot starting problem is now in full swing, and seems to be getting gradually worse. I'm assuming a vapour lock, but cold starting is also deteriorating.

Ignition: I've swapped from points and condenser to Accuspark with no change at all, for better or worse, in starting, performance or fuel economy. I'm running a second-hand distributor which may or may not be in top nick, but the cap and rotor are good, HT leads are recent, timing was set up correctly about 2000 miles ago and there's no discernible waggle in the dizzy shaft. I checked the plugs last week and they all looked as near to spot on as you'd wish for. If I could source a brand new distributor it might be worth a try, but if I'm doing that I'd probably go down the 123 route. I'm not convinced the ignition side has much room for improvement, although I may have an experiment to see if a slight tweak of the timing improves the starting. If so it might indicate that the timing is drifting, which in turn might suggest more distributor wear than I thought.

Carb needles: My mechanic thinks this is an issue. The car has spent several hours on his gas analyser over the last couple of years! We're running a recon carb from the place in London whose name escapes me right now, and they assured me there's only one needle profile for this engine/carb combination. The needle was new with the recon carb, and it looks and feels fine. But we really struggle to get a good mixture setting: if it's right at idle it goes too rich at higher revs, and if it's right at higher revs it goes all weak and lumpy at idle. The only thing I could think here would be to experiment with alternative needles and see if one comes up with the goods.

All of which presents me with two general options: either keep going with the Stromberg and experiment with its needles, air filters, choke cams etc in the hope of getting somewhere; or swap to twin SUs and THEN optimise their behaviour. A major factor here is personal bias, after many happy years with twin SU set-ups. I'm not proposing the swap because I think 2 carbs will be better than one, but basically because I think SUs are better than Strombergs, in my experience, and because if I'm going to put time and money into perfecting the way this engine runs, I have a gut feeling that I'll get better results doing that with SUs than with the Stromberg.

[222s]

Originally Posted by swedishandgerman

In combination with a different cam (like a 122s would have had) is a different story, where breathability is further improved by how much air can be drawn through the valves which can be opened for longer and wider.

I really don't think you're going to get that extra 10% you're looking for by the switch of carbs alone

^^This^^

Twin SUs without the appropriate cam won't be worth the hassle imho.

[swedishandgerman]

For the cost of a new needle, I would experiment! Same with the choke cam

Another thought, do you have the heat sheild in place? Also, I have seen somewhere some kind of a plastic gasket thing, about 10mm thick placed between the heat shield and the carb which I presume is some kind of a heat insulator

I have also read on Amazoncars website sometime ago Rob Henchoz's conversion on a 140 from a single Stromberg to a single SU of some description

[woodman]

Originally Posted by swedishandgerman

For the cost of a new needle, I would experiment! Same with the choke cam

Another thought, do you have the heat sheild in place? Also, I have seen somewhere some kind of a plastic gasket thing, about 10mm thick placed between the heat shield and the carb which I presume is some kind of a heat insulator

I have also read on Amazoncars website sometime ago Rob Henchoz's conversion on a 140 from a single Stromberg to a single SU of some description

Hmm, that's interesting. There's no heat shield and no 10mm gasket. I know the type of gasket you mean, I've seen them on other cars I've owned. The carb is straight onto the manifold with a paper gasket only. The only thing is there's certainly not enough stud length coming out of the manifold for there to have been a 10mm gasket. I could fit such a gasket but I'd need to change the studs for longer ones.

Is the float level adjustable on the Stromberg? I have a manual for Stromberg carbs but it doesn't say anything about adjusting float level. Someone on this forum suggested checking this before but I couldn't see any way to influence the float level.

Cheers,
Andy.

[Derek UK]

If you've never changed the diaphragm, get a new one. You can take the old one out to check for splits and holes. These often aren't noticeable until you take them off. Check/clean/adjust the temperature compensator. Problems with the latter possibly the reason it runs differently summer and winter. It can also affect consumption.
+1 to fitting the proper heat shield. That has a gasket both sides but there isn't a spacer as standard. You shouldn't need one.