Idle Jet Differences

[sixpipe98]

Hi All,
I recently had a go at balancing my carbs using the "lean best idle mixture" method as outlined on http://www.performanceoriented.com/. During the procedure I couldn't get the idle mixture screw on cylinder one to have any effect on engine speed/smoothness (the plug on this cylinder also fouls roughly every six months). As a result, I pulled all the idle jets and used some carb cleaner on them which unfortunately didn't have any effect. However, I did notice that there seemed to be two types of idle jet. Some had two parts, the screw head and an insert, whereas some were one whole part. Does this mean I'm using different types of idle jets throughout the carbs, or is this not an issue as long as they are all the same size. Does anyone think this could be contributing to the lack of sensitivity from the mixture screw on number one or is it simply a red-herring.
Some mixture screws do seem to be more sensitive to change than others (or perhaps my ear isn't good enough to detect more acute changes), trying to decide whether its just worth getting the carbs rebuilt as I don't think this has been done in about 10 years.
Cheers, Sam

P.S car is a 73T with S pistons, E cams, MSD, Pertronix, recent set of plugs, cap and rotor

[mycar]

Hello, what type of carbs do you have ?

[sixpipe98]

Weber 40IDA 3C. Sorry I should have mentioned that, oops!

[Lightweight_911]

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I've only ever seen the 2-piece idle jet set-up - the jet itself which pushes into a separate threaded brass 'jet carrier' (which has a slotted head).

Can you post a picture of the one-piece idle jet you have ?

.

[sixpipe98]

Yes will do now, give me 10.

[sixpipe98]

the one on the left in image 2 doesn't come apart. Can't remember which way round in image1. I also noticed that some have 50 stamped and some have 55 which can't be right.

[Lightweight_911]

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They should all obviously be the same size - I would say that '50' idle jets are too small for your engine - that size was only fitted to the 2.0 911L model , anything else had either 55 or 60 idle jets.

I would have thought that your engine spec would warrant '60' idle jets & see what effect that has.

However, it's always making one change at a time so I'd be tempted to swap any '50' jets currently fitted to '55' jets.

As to your original question, they appear to be identical apart from 2-piece vs one-piece so I wouldn't think that aspect makes any difference.

.

[wildtexas]

Probably worth blowing your idle passages and ports through with compressed air just in case there is gunge in there. That can make an idle screw seem ineffective.

[sixpipe98]

Thanks Andy, I think I'll order a set of 60's as well as completing my set of 55's, see which works best and get back to you.
Wildtexas, can this be done to the carb on the car or does it require some disassembly? cheers

[wildtexas]

I don't have a 6 pot but on the twin solexes on my car i'd get my air gun on my compressor and blow them through from where the idle jets screw in, just be careful with your threads in the carb body.

A can of carb cleaner with a straw would probably help too.

Perhaps someone with 6 cylinder experience might like to chime in.

Matt

[sixpipe98]

I just had another look at all the jets and I managed to find marks on them ranging all the way from 45 to 60. Very odd ! Some of them look like they've been drilled so my guess is that someone has just used odd bits from here and there and drilled them all out to 60? For the sake of continuity I plan to order a complete new set of holders and jets as well as do some cleaning with compressed air and go from there.

[sixpipe98]

Ok thanks Matt, will give that a go!

[neilbardsley]

I don't think that the size of an idle jet is as sensitive as main to size because you can adjust via the mixture screws? If it isn't responding apart from checking blockages as mentioned have you checked valve and plug gap?

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[sixpipe98]

Yes, I recently gapped the plugs to MSD spec and did the valve clearances using the kirk engines tool.

[neilbardsley]

Yes, I recently gapped the plugs to MSD spec and did the valve clearances using the kirk engines tool.

So you are ahead of me

I once, by accident, put a double washer/gasket on one of my idle jets and it responded completely differently to the others.

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