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Old 21-12-2010   #1
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Carb jetting and fuel economy

My 81 1500 uses fuel like its going out of fashion. About 14mpg in old money round town driven pretty sedately.

She starts very rich on choke but settles down and runs well after that. Warm idle is a bit fast at about 1100rpm and is pretty sensitive the idle screw adjustment. You can often smell petrol after putting it in the garage. There are no fuel leaks - honest

Jetting is not per the book.

Primary main jet 1.15, air correction 180 and idle 0.47
Secondary main 1.35, air correction also 180 and idle 0.70
Emulsion tubes are F30s and the needle valve is a 1.75.

The larger air correction jets have been done to lean the mix out, even though it is running a foam high-flow air filter. I was running a 150 air correction on the secondary as per the book but the 180 seems better and I read that with the high-flow air filter you should increase main and air jets by 2 sizes anyhow (and I had them).

Float level and choke position and operation all check out.

Comments welcome
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Old 28-01-2011   #2
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Re: Carb jetting and fuel economy

I had a lot of problems with fuelling and mixture when I got my car, and my advice would be check for air leaking from the base of the carb around the heat shield. If the carb has been on and off the car a few times (mine definitely had!) it may be worth replacing the gaskets - they dont cost much. It also may be worth stripping and cleaning the carb itself - rebuild kits are available on ebay etc.

But first of all I would check that the ignition timing is set up properly- mine was off and the mixture had been adjusted to compensate, leading to all sorts of problems with fouled spark plugs, wrong jets being used, etc, etc.

Best of luck!
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Old 29-01-2011   #3
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Re: Carb jetting and fuel economy

like turbo jim says, check for air leaks, spray carb cleaner round the inlet manifold and the carb base, and if the revs don't change or you can't see any being sucked in, rebuild the carb then take it to a rolling road, it'll probably cost you about 100 depending on the firm you take it to. Word of caution though, find a firm that has a lot of experience with classic cars as some knobheads would rev your car to the red!
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Old 05-02-2011   #4
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Re: Carb jetting and fuel economy

Thanks Turbo Jim and Littlephil. I'm in the UK on business at the moment so will pick up a kit and gaskets - it looks as though eurosport-uk have these for the best price.
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