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Idle Mixture Jet Size - Dell Orto Carbs

BobFletch

Forum Website User
Non-VOC Member
Can anyone please help? I am struggling to get the idle jets correct on my Egli's 1330cc engine with Dell Orto PHF 36mm carbs. All other jets and needles are now very close according to the Dyno readings last week but it continues to run rich at idle.

Current settings are
Accel pumps set to only 50%
Main jet 150
Slides 60/3 with K3 needle
Needle jet 262
Idle 40 - the smallest they do is 30!

Or could there be another problem I haven't yet sussed?
Thanks in anticipation
Bob
 

vince998

Well Known and Active Forum Website User
VOC Member
Hi Bob,

Taking as read that you´ve checked float height, maybe the below passage will help, or failing that, follow the link.

http://bmwmotorcycletech.info/dell.htm


Selection of the needle valve size

For a motorcycle with gravity feed from a fuel tank, the fuel inlet valve size, stamped on the seat of the needle-valve itself, should always be 30 % greater than the main jet size.

In case of malfunctioning, you may find that the needle valve size is too small when running the engine at full throttle for a long stretch and that the engine rpm falls, due to the progressive weakening of the carburation.

Conversely, you may get repeated flooding in use where the needle valve seat size is too large.
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Bob I am running, 55 Idle jets on my 998 special, And I have had to take a file to the slides to make it weeker, I have taken out the pump levers, In the 1970s I ran 12 to 1 pistons on 5 star fuel but now only 9 to 1s so I find the pump too much, Also use the best petrol you can buy at the pumps. Have you checked the ign timing ?. Good Luck, Bill
 

BobFletch

Forum Website User
Non-VOC Member
Thanks very much Bill - your response and Vince's have been very helpful - got some further checks to do and will probably disable the accel pumps. It's really slow progress with these Dell Orto Carbs I don't have any reference points so have now nearly spent as much on jets as I did on the carbs - they are pretty though! - regards Bob
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Bob, Just a thought, I see you have a big motor, I don't know what you are going to do with it but I found when going for full power,( I did a lot of speed events with mine ) It would not go over 110 mph, which is why i went to 36 dellorto carbs in the first place, 32 ml amals would not pass enough petrol, even with lots of mod's, anyway I had to change the float needle jets for the bigger ones off the 40ml Dellortos, This was 1975, ( I don't know if they changed the later ones) After that she flew, All the Best Bill.
 

BobFletch

Forum Website User
Non-VOC Member
Thanks again Bill - I hope to progress to worrying about max speed and fuel flow once I've got the botttom and mid range sorted. I'll let you know how it goes. This business seems incredibly iterative. So once I've got the main jets nailed down I'll probably have to start with idle and needles/slide again - ho hum!! One thing I have discovvered though is that the engine prefers different size main jets in the front and rear cylinders. Seems you have to treat them as 2 separate 660cc singles to cope with different airflows, exhaust lengths etc.

Anyway I now have a 155 jet in the front and a 145 in the rear and the mixtures as measured on the dyno are spot on - I live and learn.

best wishes
Bob

Hello Bob, Just a thought, I see you have a big motor, I don't know what you are going to do with it but I found when going for full power,( I did a lot of speed events with mine ) It would not go over 110 mph, which is why i went to 36 dellorto carbs in the first place, 32 ml amals would not pass enough petrol, even with lots of mod's, anyway I had to change the float needle jets for the bigger ones off the 40ml Dellortos, This was 1975, ( I don't know if they changed the later ones) After that she flew, All the Best Bill.
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Bob, Those jets should be the other way round !!, Clev Trev and a lot of other people think on the road the rear cyl' runs colder ? I don't know how you can do that on a Dyno. I have never used a Dyno, Wish I had it would have saved me a lot of time and money, But I think you still have to fine tune on the road. I used to run 60 idle jets befor they messed up the petrol a year or two ago. I am thinking of doing a 560cc Comet, I bet that will drive me mad !!. I have just dug out an E.I. carb from the U.S.34ml flat slide, I am going to try it on my 500 just for fun, to adjust the fuel you just turn the needle in or out !! Cheers Bill.
 
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