3.9 New Build - Carb Issues

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vaultsman
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3.9 New Build - Carb Issues

Post by vaultsman »

Hi,

Although I did a top-end rebuild 8 years ago on the original 10.5:1 3.5 in my '73 P6B, the rest of the engine has been getting pretty tired since.

Top-end work at that time ended up with '89 Range Rover heads, 3.9EFi cam, JP timing set, K&N's. Still on twin HIF6...set up and dyno at Peter Burgess with BCA needles gave 146HP at the wheels, which works great for me and the occasional road use I give the car.

I've now built up and just installed a 3.9 with nearly-new 10-bolt block, Crower 50229, and Cloyes Street set from Paul at V8 Tuner. Same heads. HIF6's retained (for now?) but setting the jet heights to the book-plus-a-tad (2.5-3 turns of the screws from jets level with bridge), I couldn't get the tickover speed down below 1100 even with the screws off their stops. Suspecting air leak?....not found it yet though.

Car MOT'd OK yesterday (my usual friendly guy), but I asked him to put the probe up to check CO......1.0% ??? :? Does this say air-leak?

I'll get the car back up to Peter Burgess once I've got it set up somewhere near.

Thoughts on the HIF6's also welcomed!

Stan



DaveEFI
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Re: 3.9 New Build - Carb Issues

Post by DaveEFI »

Setting the carb jets to a level below the bridge is only an initial setting. At idle, lift each piston in turn a couple of mm. HIF carbs have a lifting pin to do this IIRC. The engine speed should speed up slightly, then return to normal. If it slows down richen the mixture (lower the jet) If it speeds up, weaken it. Repeat after setting them. 1% CO says it's too weak at idle - think with those carbs it should be more like 4.5%

Also check both carbs are balanced at idle. A stethoscope or bit of tube to one ear, and listen to each carb input (you'll need to remove the air filter) Position the tube to the same place on each carb intake. Slacken the linking rod, then adjust each idle screw to achieve the same sound on both, and the correct idle speed.

You'll likely have to repeat both processes to get it spot on.

If the carbs are in good condition, and have the correct needles etc for your engine, the tune should now be correct overall. But when you change a cam etc, you could well need different needles.
Dave
London SW
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vaultsman
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Re: 3.9 New Build - Carb Issues

Post by vaultsman »

Thanks Dave.

Air leak found at RH manifold to downpipe joint. Now sorted, and all good....dyno soon. :D

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