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SU carbs
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PostPosted: Sun Jan 14, 2007 1:58 pm    Post subject: SU carbs Reply with quote

with a new HS SU carb, approx how many turns from fully 'up' should I need to turn the jet adjuster down in order for it to be close to correct?

ta
Rick
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UKdave2002
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PostPosted: Sun Jan 14, 2007 2:05 pm    Post subject: Reply with quote

Rick

Turning 12 "Flats" of the nut will be the approx starting point.

Dave
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PostPosted: Sun Jan 14, 2007 2:07 pm    Post subject: Reply with quote

thanks Dave, so thats approx 2 full turns if memory serves..

Rick
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UKdave2002
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PostPosted: Sun Jan 14, 2007 2:10 pm    Post subject: Reply with quote

Rick
Sound about right, the rule of thumb was turn all the way in and then 12 flats back, I can't remember how many flats on the nut ! when the carb is on the car in some where inaccessible like one of Buzzy’s mini’s counting flats was easier than counting full turns!

dave
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Uncle Joe
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PostPosted: Sun Jan 14, 2007 2:13 pm    Post subject: Reply with quote

Yes, thats correct, two full turns. But note that this is two full turns from the jet being flush with the bridge of the carburettor.
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PostPosted: Sun Jan 14, 2007 2:14 pm    Post subject: Reply with quote

yeah, this is on a Prefect with brand new Aquaplane gear on it (£££), but I don't think its been set up right. Accessibility isnt brilliant on this either Rolling Eyes

I want to get it something like before selling, but at the rate I'm going at it may have to go 'as is' for someone else to tune and fettle!!

Rick
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Uncle Joe
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PostPosted: Sun Jan 14, 2007 2:27 pm    Post subject: Reply with quote

In that case, you also need to know that the throttle adjusting screw is 1 1/2 turns out from just clearing its stop.

Do you want a full carb. tuning procedure? That should start a discussion! Laughing
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Uncle Joe
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PostPosted: Sun Jan 14, 2007 2:29 pm    Post subject: Reply with quote

Sorry, that should read of course 1 1/2 turns in.... Embarassed
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PostPosted: Sun Jan 14, 2007 2:57 pm    Post subject: Reply with quote

Hi UJ

It was the initial setting of the mixture I wasn't sure about, the rest I'm not too bad on I think but a full procedure would always be handy for reference Smile

Rick
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Uncle Joe
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PostPosted: Sun Jan 14, 2007 3:50 pm    Post subject: Reply with quote

Hi Rick, here it is then! Now then, who can spot any deliberate mistakes?

SU H and HS carburettors.

1: Unscrew throttle adjusting screw until the face just clears the stop. Open by screwing in 1 1/2 turns.

2: Remove Piston and Suction Chamber, disconnect mixture control wire, and screw jet adjusting screw until jet is flush with carburettor bridge (or full up if you cant). Replace parts, and check Piston falls freely on the bridge (by using piston lift pin). Turn jet adjusting screw 2 complete turns.

3: Start engine, and adjust to desired idling speed (eg, warning light glowing).

4: Turn jet adjusting screw to obtain fastest idling speed WITH EVEN FIRING.

5: As the engine speed may have increased, readjust Throttle Adjusting Screw.

6: Check mixture strength. Use Lifting Pin to lift piston about 1/32”. A) If the engine speed increases, and continues to increase, the mixture is to RICH. B) If engine speed decreases, the mixture is to WEAK. C) If engine speed momentarily increases slightly, the mixture is CORRECT.

7: The exhaust note should be regular and even. If it sounds ‘splashy,’ the mixture is to weak. If there is a rhythmical misfire, together with blackish smoke, it’s to rich.

8: Reconnect mixture control wire with about 1/16 inch free movement before it starts to pull on the cam lever.

9: Set Choke knob (on dash) to max. movement (about 5/8 inch) without moving the carburettor jet, and adjust the fast idle cam to give a fast tickover, about 1000RPM.
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PostPosted: Sun Jan 21, 2007 6:58 pm    Post subject: Reply with quote

Had a look at the Prefect carbs today. Took the tops off and wound up both mixture adjusters to the top of their travel, and dropped them down equally (they were quite a way apart previously). Set the idle to something close, and once it pulled the fuel up from the tank it sprung into life. There's still a gremlin somewhere - I think the fuel pump isn't up to the job of supplying twin carbs, so that wants changing. The carb settings I think are fairly close, the plugs look a decent colour so I think the pump wants refurb/replacement as occasionally it peters out at idle, then comes back, then splutters, then comes back again to an ok idle, and repeat once again.

Sure its fuel related. I notice the previous owner who bought all the new stuff didn't replace the pump, so I'm sure that wants a looking at. Other than that it seems to run well!

Rick
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admin



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PostPosted: Thu Feb 08, 2007 10:04 am    Post subject: Reply with quote

UJ, what were the 'deliberate mistakes' that you mention, havent had time to re-read your 'how to' but if you could point out where changes need making, I'll put up a page on the main site for SU carb fiddling

ta
Rick
Smile
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Uncle Joe
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PostPosted: Thu Feb 08, 2007 11:50 am    Post subject: Reply with quote

Rick, the reference to 'deliberate mistakes' was just in case anybody had a few comments as to my explanation. As nobody has, then its OK to put it on the main site.

Are you going to do the tips section with direct links to the forum in case anyone has questions? You might have already said that you are going to do this - my grey cell isn't working today, due to the weather. Very Happy

UJ
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admin



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PostPosted: Thu Feb 08, 2007 12:55 pm    Post subject: Reply with quote

ah right ta, yeah I'd link back to here in case anyone has some input
Smile

cheers
RJ
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admin



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PostPosted: Thu Feb 08, 2007 2:53 pm    Post subject: Reply with quote

Chaps, I've put the SU info on this new page, and linked back to here in case anyone who reads it has anything else to add

http://www.oldclassiccar.co.uk/balancecarbs.htm

ta
Rick
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