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Spark plugs - Andrew
I drive a Triumph 1500TC 1975 model (essentially a Dolomite). I've had the engine overhauled, a replacement distributor fitted and new carburretor parts. There seems to be a slight dead spot on pick up - overcome by pulling out the choke - but the engine then soon chokes up. The handbook recommends Champion N12 plugs. An old lag says use N9s - they get hotter quicker and should overcome the dead spot. Any comments?
Re: Spark plugs - Piers
Sounds like it's running too weak - do you have an accelerator pump with the carb you are using - when opening the throttle sharply it pumps more fuel in to overcome this lack of response.

Hotter plugs might help keep the electrodes clean but the choke will be giving you a richer mixture - if this cures it then probably something to do with the carb.

Piers
Re: Spark plugs - John Slaughter
Andrew

I'd agree with Piers - sounds like the accelerator pump on the carb isn't doing its bit, assuming this isn't an SU or Stromberg carb on this model. Check it out, and make sure that corrosion outside the carb isn't preventing the lever making its full travel.

If it's an SU or Stromberg, then ensure the basic mixture isn't too weak, and that the piston damper is full with oil. The seals can go and lose oil very quickly on the Strombergs.

Regards

John
Re: Spark plugs - Ian Cook
Andrew

This car probably has twin SUs or Strombergs. They both work on the same principle, but neither has an accelerator pump. They achieve enrichment on transfer from "idle to main" by means of the dashpot damper (oil filled) and it is very important that this is full of oil.

Strombergs are notorious for perished diaphragms. Did they replace those with the new carburettor parts?

Are you sure you haven't got an air leak? Have you checked the vacuum advance retard on the distributor - an air leak here will cause a weak mixture around idle, when manifold vacuum is at its highest.

I really don't think plug grade would cause the symptoms you state on an engine of this type - it's only a 30 year old cooking engine with twin carbs.

Finally, does this problem ahppen all the time, or only when the engine is cold?

Ian
Re: Spark plugs - Michael
I agree with John and think this model has SU carbs, so no accelerator pump.
Assuming the carbs are set up properly as John describes, check also that the vaccum advance unit on the distributor is working properly. The best way to check this is with a strobe light. But you can also check by making sure that the vacuum tube from the manifold is intact and that you can feel the vacuum (disconnect tube from distributor, put your finger over the end of the tube, blip the throttle, you should feel the vacuum). Also check that the vacuum capsule on the side of the distributor is ok and not perforated. Have encountered many cars where the capsule is faulty and the initial timing advance is compromised. I would stick to the standard plugs, although I prefer NGK's to other brands.
Re: Spark plugs - Andrew Moorey (Tune-Up Ltd.)
It is essential that the carbs are balanced, the diaphragms (Strombergs) are NEW and the correct grade of oil is in the pistons. I always use a straight 30 grade oil but really any engine oil will suffice. If the diaphragms are new are they orientated correctly? There is a moulded lip on the inner edge of the diaphragm which locates in a slot on the piston and another lip which locates the piston correcxtly in the body of the carb. Check that the jets are the same height in the body and the needles are correctly located in the pistons. If you can get your hands on a CO meter set the mixtures up to give around 2% at 2000 rpm ands see how you go from there. Regarding plugs I would only fit NGK plugs, BPR5 ES at around 32thou' gap.
Re: Spark plugs - Bob Jeffery
I am not familiar with this car but if it has SU or Stromberg carbs, and you have had them rebuilt, have the correct metering needles been fitted? I've had a similar problem with a Rover V8 engine in a kit car (using Stromberg carbs) and had the same effect in that it would perform much better if I pulled out the choke. I eventually found, after a tortuous trial and error session that different (more tapered and hence richer) needles were required. There are a great many different needle profiles which can be fitted.
 

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