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Hello.
Can anyone give me a guide to where my carb jettings should go after increasing the bores from 900 to 1050cc. The 900 was running AB265 needle jets with K4 needles, 155 Main jest and a 65 idle jets in a pair of Dellorto PHM 40's. I know these are probably oversize but it ran well before and I ain't paying out for a new pair!
Cheers

chasbmw has the 1050 and dells, so I expect he'll have the answer.

Nourish wrote:Hello.
Can anyone give me a guide to where my carb jettings should go after increasing the bores from 900 to 1050cc. The 900 was running AB265 needle jets with K4 needles, 155 Main jest and a 65 idle jets in a pair of Dellorto PHM 40's. I know these are probably oversize but it ran well before and I ain't paying out for a new pair!
Cheers

What kind of heads are you using? Dual plug? Big Valves, size? Did your 900 have 9.0 or 9.5:1 compression? What is it now?

The 900 was a 90S and I've used those twin plugged heads, I machined them to create a squish band to match the pistons. STD exhaust valves and +2mm inlets (44mm) were fitted. I made the head gaskets out of 1mm thick copper to give me a C.R. close to 10:1 - I think - I built this motor 3 years ago and looking back at my paper work I realise my notes are sh*te!

Nourish wrote:The 900 was a 90S and I've used those twin plugged heads, I machined them to create a squish band to match the pistons. STD exhaust valves and +2mm inlets (44mm) were fitted. I made the head gaskets out of 1mm thick copper to give me a C.R. close to 10:1 - I think - I built this motor 3 years ago and looking back at my paper work I realise my notes are sh*te!

Sounds like a screamer. If you are sticking with a stock 308 cam, then the jetting that worked for you with the 9.5:1 900cc's
should be pretty close. The bigger displacement doesn't really need a richer mixture unless breathing is seriously modified (ie cam, exhaust, intake flow, etc.). It justs sucks more of the same mixture. The slight upgrade in compression and the 44mm valves may up the need for a richer mixture though. Maybe go up 1 or 2 sizes on the main jets. The pumpers on the Dels may also need to be screwed out a little. It's really just trial and error. make sure the bike is good and hot when you make carb adjustments and check the plugs often.

Nourish,

sorry for the delay in getting back to you,

My bike is fitted with 165 mains, 60 idle jet standard slide with needle at lowest position. This is on 38mm Dells, pump fitted.

Heads are ported, twin plugged, straight through exhaust, K&N filter+ opened up filter cover.

Thanks for the replies Chaps,
Chas I see that your settings aren't far from the Standard 90S setup backing up Matt's thoughts, I think I'll try my original settings
Matt - why should a higher compression require a richer mixture?
Cheers

Nourish wrote: Matt - why should a higher compression require a richer mixture?
Cheers

Higher compression does not necessarily need more fuel, but it's more apt to be able to do something with it. Also high compression tends to heat things up more so a lean condition will be more apparent and more potentially damaging.
That all said flow modifications (cams, big valves, porting, polishing, exhaust mods, intake mods, etc.) can create the real need for jetting changes.

Thanks Matt