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#1 Huw


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Posted 11 September 2006 - 05:06 PM

well here we go....

Set out to tune y twin Su's, has been very smoky of late...
anyway, balanced with the air/flow meter....
screwed the fuel adjust all the way up and then down a 1/2 turn...
began adjusting until seemed right...however....

when i rev to 3.5k+ starts to sputter and fire off a bit of black smoke....(rich?)
BUT when i lift EITHER of the pistons in the carby throats the engine dies (lean?)
furthermore, after a bit of playing would rev slightly higher but then backfire fuel out of carby (flames once!)....that was when i stopped.

Any advice would be great...have been through the haynes manual, my SU manual and any others i had but to no avail.


#2 Huw


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Posted 20 September 2006 - 05:37 PM


I've removed the fuel pump and having that looked at at the moment...but any other suggestions?

#3 Toecutter


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Posted 20 September 2006 - 08:04 PM

I had the same problem when I first bought my car and installed early 240 SU's. I just kept playing around till I got them right. Sorry I can't help you out but that was about 5 years ago and I have'nt touched em since. They run fine.

#4 gav240z



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Posted 20 September 2006 - 08:15 PM

Hi Huw

What condition are the carbs in? Have you recently rebuilt them?

If it has been a while since you kicked her over it may be possible for a few issues to arise.

The Achilles Heal of the SU carbs is the throttle shaft. Overtime they tend to wear out and require a rebushing. Once they are worn they leak air and make it virtually impossible to balance.

Another problem could be the needle and seat. The seat tends to dry out and crack over time, I believe they are rubber from memory. This causes an inconsistent amount of fuel to be delivered to the engine.

Again this would make tuning impossible.

If you are running an L28 it would pay to check that the needles and seats in the carbs are correct for the displacement of the engine otherwise it will run lean.

Best of luck! There is a B&M fuel systems in Richmond who cater to the SU carb I believe you can find contact details in the links area.


#5 Huw


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Posted 08 October 2006 - 10:45 AM

all right...looks like i have it largely solved....

petrol pump was only slightly disfunctional....
haven't been able to find an air leak (although there may still be one)
carby's were out of tune...thus plugs blocked up on back 3
and most importantly....fuel filter and line was obstructed....

anyway, these are all fixed and it seems to be idling nicely...when i drive it down to the shop on monday I'll have a better idea but fingers crossed.


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