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Series 1 SU Carb Choke

Series 1 SU Carb Choke

Larry Lee

David McBride wrote:

Has anyone sorted out the problem of the thermostatic control on the HD8 SU Carb, ie a manual choke conversion?

You did not say, but I presume you have a Thermostatic Starting Carburettor (TSC, also called Thermo Carburettor, Auxiliary Enrichment Carburettor, or Auxiliary Starting Carburettor), as I do not think any of the completely different Auxiliary Enrichment Devices (AED) were fitted to HD carbs (though somebody probably has one, somewhere, and will point out my error). The TSC is the earlier of two goofy things SU did to enable cold starts. Why they never developed an automatic choke--they had manual chokes--or an automatic actuator for their own internal enrichment device (in some HIFs) is beyond me, but they didn't.

The TSC is basically an ON-OFF device. It has a solenoid that is energized by a thermostat switch mounted on the engine block when the engine (meaning coolant, since that's what the switch's probe is exposed to) is below 35C (95F). With the solenoid energized, a valve in the TSC opens, so it draws fuel from one of the HD8s, through a metering orifice/needle, mixes it with air, and feeds the mixture to the manifold downstream of the throttle disks. There is no significant variation in the enrichment as the engine warms up, but engine vacuum can pull the solenoid valve closed during idle to reduce over-enrichment.

I'm assuming your main carbs are properly set. Be they are before tinkering with the TSC.

The metering needle is adjustable, but this is usually not the problem. The biggest problem is failure of the thermostat switch on the block. Even if it works as advertised, problems can occur after short trips. The switch can be too cool to open and de-energize the TSC, but the engine be warm enough to start without it, leading to flooding and failure to start. The car sitting for a good while on a hot day can cause a failure to provided needed starting enrichment, too. And there are, no doubt, other problem situations.

There is an easy test to see if the thermostat switch is the culprit. With the engine warmed up, pull the wire off the switch. If the switch is faulty (stuck closed in this case), the TSC should shut down and the engine run properly. If you are still running rich, the problem is in the TSC itself.

Changing the thermostat switch is not difficult, but, as noted above, this may not completely solve your problem. Many people convert to manual operation by wiring the TSC through a switch on the dash instead of the thermostat switch. The wire from the TSC to the thermostat switch is a ground, so wire your dash switch accordingly. Adding an indicator light is a good idea, too.

If your TSC is faulty, you can either rebuild or replace it, depending on what you are comfortable with.

Hope this helps.

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