main jet replacement '67 911S

[chapron67]

Rookie question: Where are the main jets on the original Weber carbs on a '67 911S? The schematic on Pelican Parts seems to show them 'on the side' of the carbs. But I think they are accessible from the top once the velocity stacks are removed... are they 'down' in the hole on the top of the carbs? If so, can you get them out to replace them withOUT dismantling the carbs? Thanks!

Greg in Seattle

PS just trying to reduce the jet size as it seems to be be running rich and 'missed' when you push on it around 3,000 to 4,000k rpm.

[edmayo]

Main jets screw into side of float bowl. Remove the holder, then the jet will unscrew from the holder. There are some series of Webers that access from the top, but the I D A's aren't it.

[chapron67]

Thanks Ed... yup, couldn't be more visible... ; > )

[chapron67]

so.. I changed out the 65's for 55's and the car now 'misses' and stumbles' at lower rpm's but pulls quite nicely from 3,000 (where it use to stumble before 'catching') ... so, is the answer for me to find 60 main jets? And can I just change out the main jets by themselves or are there other components I need to also change at the same time (i.e. is there a mixed pair of components?) Thanks!

[chapron67]

So I'm clearly confused... I guess the pink circle is the idle jet which I've changed from 65s to 55s... can someone tell me which colored circle is the main jet (holder) so I can check their current size. Thanks.

[1QuickS]

• pink with slot is idle jet holder
• orange with 8mm hex is grub screw securing main venturis
• yellow with 10mm hex is fuel drain bolt
• green with 10mm hex is main jet holder
• blue with slot is port plug that covers recess where progression circuit holes are located

Idle jets for your engine should be 55s and possibly as large as 60s. Larger jet requirements indicate air leakage past throttle shafts during slow speed operation with resultant erratic idle. Main circuit is mostly 3000 RPM & above and is not significantly affected by low speed circuit.

I offer a link to my site regarding discussion of these circuits: http://www.performanceoriented.com/c...r-operation-1/

[chapron67]

Thanks so much for all this! Great article!

Couple questions: so I have 55s idle jets installed which now cause a stumble at low revs; whereas the 65's caused a stumble above 3500. The main jets currently in the car are 130's... i have a set of 125's that came with the car too... do you think I should buy '60's and try those? ANd/or does changing the main jets to 125 potentially helpful? Accelerator pumps seem to be working in each throat.

Thanks!

[edmayo]

While I agree that 55's may be too lean, and 65's were probably too rich, and the 125 or 130 main should be about right, you can chase your tail going about this haphazardly. The golden rule is that 90% of your carburetion problems are in the ignition system. Obviously whomever changed the idle jets originally was trying to solve a problem, which evidently didn't get solved,,,,,or you wouldn't be doing this! So make your changes and if you resolve your problem,,,then great,,,,but if you don't you need to approach this in a more logical approach. Right now we don't know how well you're setting the synchronization and idle mixture settings. We don't know if the float levels are correct,,,,we don't know if the ignition timing is correct, we don't know the condition or heat range of the spark plugs,,,we don't know condition of spark plug wires,,,point gap,,,etc. etc. Try your changes,,,,,might work,,,but if not you'll need to practice your diagnostics.

[chapron67]

Thanks Ed.

Yes, I was first trying to see if a simple jet change would remedy the issue. I guess you're saying that 60's might be a silver bullet but doubt it. I have read through the pelican parts carb overhaul/ adjustments and was going to attack that next. I've tested the timing, and it has electronic ignition. I'm really interested in learning about how this all works and doing the work myself -- I just don't want to take it to a shop and have them do it if I can help it. I'd love to watch and learn from an expert work through the full tuneup -- anyone know of such a person in the Seattle area?

[edmayo]

I still wouldn't jump into rebuilding the carbs. See if you can feel play when trying to move the throttle shaft up and down. If so, as Paul suggests it might be shaft wear letting in extra air. More important is to measure float level for instance,,,which you'll need to do after a rebuild anyway. What kind of ignition do you have, and how exactly did you check the timing, and has it had this problem since you've had the car? (I know, its a recent purchase)

Try Chris Powell,,,Chris's German Auto Bremerton I think.