Mixture control screw on 34PICT (duals)

aggri1 Insano Dub Head

824 posts · joined Dec 2003 · 20 Jun 2004, 2:22 pm

#189548

Hi there!

This is about stock carbies on a 74 bus: 34 pict 3-4's (is that right - I know they're the stock ones).

If I understand the situation correctly, the two screws on top of the left carby control the engine at idle. The big screw says how much air goes through, the little one controls the ratio air:fuel.

Is this right?

Secondly, what does it mean when the little screw has no effect any longer on engine speed? I am guessing here, but if the ratio a:f can be varied, the engine should run fastest at (near?) stoich', lower as the mixture is leaned out. Does it run slower if it's rich too? When the mixture screw doesn't affect engine speed, are there some little tiny holes that have become blocked up somewhere?

Any help appreciated. I think I might end up getting a garage to tune the carbies, I have no CO meter or nothing. I'm not even sure how to balance the carbies. I would, though, like to understand what's going on. Anyway, thanks heaps guys, sorry this is kinda long...

Regards,
Aurel

1303Steve Scirocco Rare

8,650 posts · joined Aug 2002 · 20 Jun 2004, 9:52 pm

#189777

Hi

I wrote an article on this for our club magazine, which I can’t find at the moment.

The idea is that you have 3 idle systems on your motor, L&R carbs contribute a portion on the idle speed & mixture, and the central idle circuit gives idle speed & mixture to both sides.

The Bentley manual explains the tunning process using a gas analyser. What I find is the best way to field tune is,
1 disable the central system by removing the wire from the solenoid on the top of the left-hand carb. If the solenoid is not working clamping the pipe it feeds into works as well.
2 then tune them like any other dual carb but keep the idle speed around 650 rpm, use a carb synchroniser or do the plug lead removal trick, get an even drop in revs side to side.
3 hook up the central idle circuit again and adjust idle speed & mixture.

1302Steve

aggri1 Insano Dub Head

824 posts · joined Dec 2003 · 21 Jun 2004, 10:27 am

#189941

Aha! That helps. Not having an exhaust analyser though... I'll see if they're cheap little things or big expensive machines.

Thanks,
Aurel

1303Steve Scirocco Rare

8,650 posts · joined Aug 2002 · 21 Jun 2004, 10:35 pm

#190357

Hi

I forgot to add in step 3 to get the central idle circuit back working again you need to blip the throttle to get the solenoid to release.

1302Steve