H2 carb setup

Discuss mechanical problems here.
Forum rules
By using this site, you agree to our rules. Please see: Terms of Use
Post Reply
brambler
Minor Friendly
Posts: 16
Joined: Wed Mar 27, 2013 9:04 pm
MMOC Member: No

H2 carb setup

Post by brambler »

Hello All,

I have an H2 carb, model AUC924 on my '59 948 Tourer.

I recently stripped and cleaned it and fitted new gaskets but nothing else was replaced as all seemed in good condition. Pretty sure the needle is standard.

When trying to set it up on the car I’ve found that the only time I can get the lift pin to increase revs then drop back is with the bottom nut screwed right in against the spring/body. Even then the increase in revs happens very near to the top of the pin’s travel.

As background, the engine has been previously rebored and fitted with 998 (+30 thou) dished pistons. Compression is good and equal on all cylinders. Tickover is smooth at around 700 rpm, timing is 4 degrees BTDC, fuel pump is working well, there’s oil in the dashpot and as far as I can tell there are no air or fuel leaks.

The car pulls fine on the road, attached photo of plugs after a run. Previously when the nut was set to 12 flats the plugs were black.

I don’t want to fiddle unnecessarily but why would it be that I can’t get the carb to adjust in the “normal” range?
Attachments
39C77E91-D48E-4082-B924-1C796A9E879F.jpeg
39C77E91-D48E-4082-B924-1C796A9E879F.jpeg (2.76 MiB) Viewed 338 times

oliver90owner
Minor Legend
Posts: 1653
Joined: Thu May 12, 2016 6:33 am
MMOC Member: No

Re: H2 carb setup

Post by oliver90owner »

why would it be that I can’t get the carb to adjust in the “normal” range?


Possibly because the jet and/or needle have been changed/altered or are very worn. The float chamber level may also be incorrect.

Plugs look fairly light coloured, so perhaps a little lean with the mixture.

Nothing specific, but maybe something to get you thinking about it.
kennatt
Minor Legend
Posts: 2625
Joined: Fri Feb 02, 2007 9:11 pm
MMOC Member: No

Re: H2 carb setup

Post by kennatt »

try dropping the ile speed (How do you know its now at 700) if too high it will lift the piston and move the contact of the pin higher up. Ideal tickover is to be just high enough to put the ign light out,500 to 600 is the average I believe,depending on condition of the engine etc.
brambler
Minor Friendly
Posts: 16
Joined: Wed Mar 27, 2013 9:04 pm
MMOC Member: No

Re: H2 carb setup

Post by brambler »

Thanks both, yes I wondered if the jet and/or needle were worn but I’m out of my depth as far as knowing so since the needle was straight and it all fitted back together I didn’t look further. Is there any way of checking? I couldn’t find any references on the SU Carb site.

My multimeter has a setting to read rpm that hooks to the distributor (multiply x 10). I’ve adjusted it down to just before the ignition light comes on and can get the lift pin to raise & lower the revs with the nut wound out one flat so will try that on a run and see how it goes.

Many thanks.
Attachments
6D1CCDDF-A90E-411E-8CC4-75FEA0CED31E.jpeg
6D1CCDDF-A90E-411E-8CC4-75FEA0CED31E.jpeg (4.17 MiB) Viewed 276 times

Post Reply