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carburetor question (non-Lotus)


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  • Gold FFM

A friend recently bought a '77 Triumph Spitfire 1500. I agreed to help sort the car, and it has proven to be a pretty good choice. Most items that needed attention were normal wear and tear: fuel lines, suspension bushes, brakes and some electrical gremlins. Over the winter we will remove the gearbox and rear diff to sort a few oil leaks. She enjoys the car a lot and has driven it more in a couple of months of ownership than the PO over several years.

There is one thing that has me puzzled: the engine always fires up first time, but only if you give it enough (full) choke. Idle is nice and stable and can be set as it should. But when up to temperature, the throttle seems to stick, causing a high idle. The cable and linkage has been checked and greased and doesn't seem to be the cause. Even with a slack cable and the levers against the stops, idle sometimes remains high. If you blip the throttle it settles to the preset low rpm. Carburetors are Zenith Stromberg. I've cleaned the butterflies and made sure the damper oil is at the correct level. No effect. When I drive the car, I can get around it by heel-toeing (not as easy as in a Lotus I must add). This is not an option for her and shouldn't be necessary. If you just release the throttle when coming to a stop, you're likely to end up with a very annoying 1500rpm idle. Not always, but most of the time.

Carbs are not my forte, but I do want to help her. I've gone through some topics on Triumph forums, but didn't get much wiser. So I'm hoping somebody on here can point me in the right direction.

Thanks,

Filip

Edited to add there is nothing on the engine to adjust fueling in relation to temperature, no cold enrichment devices or such.

I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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Strombergs are a CV type carburettor and a daiphragm acts upon the vacuum suplied by the butterfly.  Assuming the butterfly is operating freely and returning to its stop, as a matter of course, I'd try replacing the diaphragm(s)

Unless its a very old stromberg, there is a plastic appendage on the side of the carb with a bimetallic strip that compensates for temperature  

 

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  • Gold FFM

Yes, I set the idle with the engine at operating temp. It runs well, until you play with the throttle (either pedal or directly on the linkage), then it will more often than not sit too high.

Steve, I'll check to see if I've missed anything.

Filip

I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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Correct viscosity oil in the damper?  That is a pure guess as its 30 years since I had a spitfire and they were  su carbs.

 

 

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  • Gold FFM

Hi Filip,

There's a rubber diaphragm under the dashpot which is a weakspot on these, but take off the dashpots and give the slides bores and needles and good clean first, they may be sticking on carbon deposits.

Steve 

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  • Gold FFM

Good tips everyone! I'll check the diaphragms when I get to the car. I used 20W50 engine oil for the dahspots, worked well on my Stag. When cleaning the carbs I checked the dampers move up and down without sticking. Obviously that was not at operating temperature, so it could still be the cause.

I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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