Trijet carbs on LC engine - pros and cons

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Alan_M
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Trijet carbs on LC engine - pros and cons

Post by Alan_M »

Ok, giving my new 1982 Excel acquisition a service I noted the carbs fitted are the later powerjet/trijet carbs from the later SE spec.
The distributor is still the non vac version as originally fitted.
The engine is still LC spec, albeit with later HTD pulleys fitted. It was rebuilt 7k miles ago, but also over 20 years ago. There is a pretty comprehensive list of parts used with no pistons being in the list that would suggest it was upgraded to a HC engine.

So what are the pros and cons of these later carbs meant for a HC engine being fitted to the LC engine?

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Re: Trijet carbs on LC engine - pros and cons

Post by Lotus-e-Clan »

Alan_M wrote:
Tue Jan 03, 2023 10:33
Ok, giving my new 1982 Excel acquisition a service I noted the carbs fitted are the later powerjet/trijet carbs from the later SE spec.
The distributor is still the non vac version as originally fitted.
The engine is still LC spec, albeit with later HTD pulleys fitted. It was rebuilt 7k miles ago, but also over 20 years ago. There is a pretty comprehensive list of parts used with no pistons being in the list that would suggest it was upgraded to a HC engine.

So what are the pros and cons of these later carbs meant for a HC engine being fitted to the LC engine?
The powerjet supplements (adds to) the fuel at high engine speeds that is normally metered only by the emulsion tube/main jet/air correction jets in ordinary DHLAs.

For example, when tuning a normal (non-powerjet) DHLA you choose the emulsion tubes to roughly match the cam. So for high revving competition cams where the power band is 4000 rpm - 9000rpm, the emulsion tube should deliver weak fuelling at low rpm (off cam) and rich fuelling at high rpm (on cam). This is achieved by the size, position and number of holes drilled along the emulsion tube. The holes allow air to mix with the fuel delivered by the main jet at the bottom of the emulsion tube. More holes/bigger holes = weaker mixture. For a sporty cam with a power band from 2000 rpm to 6500 rpm, you would choose an emulsion tube with a richer bottom end and a weaker top end compared to the competition cam. For example, the competition cam might need No 6, and the sport cam No 10 emulsion tubes from the picture below. The hole at the very top of No 6 means a weaker bottom end compared to no holes at the top of No10 etc.
Image
With the SE cams, the emulsion tube, mains and air corrector jets are chosen to compliment the extra top end fuelling delivered by the powerjet. In other words, without the powerjet, the emulsion tube, main, air jet would be too weak at the top end on their own for an SE engine, and not quite right for the LC engine if you opted to block-off the powerjet circuit to rely only on the SE emulsion, main and air jets.

So with the lower powered non-SE, having different cams, the powerjet carbs will be too rich at the top end.

The ignition advance curve complements the fuel delivery. For the weaker bottom end of a competition cam, the ignition needs more advance at the bottom end compared to a sporty cam. At the top end, the ignition curve is unimportant because the advance is 'all-in' and at its maximum beyond about 3500 rpm, so you can simply set the required advance by the radial position of the dizzy. In other words, if your engine is running richer at the top end above 4000 rpm with the tri-jet carb then it probably needs less advance. But if you retarded the top end by rotating the dizzy body, then the advance would be too retarded at the bottom, which means you'd get a misfire at part throttle - especially if you have no vacuum advance to compensate.
Peter K

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Re: Trijet carbs on LC engine - pros and cons

Post by Alan_M »

Many thanks Peter for that info. Helps me understand it better.

As it seems cams will make a difference here I need to investigate the engine more and see what’s in there.
I don’t recall seeing anything on the invoice about different cams, but it did have a new cylinder head listed, but no part number. This was done in 2007, so maybe had an SE head fitted?
It was also then set up on a rolling road with mention of changing jets etc, so I will delve into carbs and see what’s currently fitted.

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Re: Trijet carbs on LC engine - pros and cons

Post by Marten »

Alan_M wrote:
Tue Jan 03, 2023 13:41
so maybe had an SE head fitted?
red paint is cheap, but isn't the shape of the valve covers a clue?

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Re: Trijet carbs on LC engine - pros and cons

Post by Alan_M »

Marten wrote:
Tue Jan 03, 2023 22:35
Alan_M wrote:
Tue Jan 03, 2023 13:41
so maybe had an SE head fitted?
red paint is cheap, but isn't the shape of the valve covers a clue?
Doh :oops: obviously didn’t have brain engaged when I wrote that. It has the saddle type cam covers that were never used in the later SE head. It is actually painted pearlescent red to match the body, but I know never to go by cam cover colour !
I will still check what cams it has installed.

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