Which size carburettor to use

[Colesy]

I am running a Sporster in the 1350 MPG class and it has Thunderstorm Heads, Redshift 635 cams and a 48mm Mikuni carb with a 190 main jet and a 100 Thunder jet. It still leaned out on front cylinder. The salt is 110 metres above sea level. Pre speed week on the dyno it showed very good hp and AFR. My guess at this time is that velocity/venturi is not correct and the Thunder jet did not work as expected, therefore leaning out on the 190 main jet. After rebuild my thoughts are to run 48mm with large main and no Thunder jet. Or drop down in size to a 45mm to increase velocity with large main and no Thunder jet. Any ideas appreciated.

[Stainless1]

How is the carb vented?  Which way does the carb face in relationship to the airflow?

[Colesy]

Vented through the OEM tank cap but with a hole drilled in the underside to increase venting. Carb faces in the normal direction out the R/H side with large high flow air cleaner.

[wobblywalrus]

The folks at www.rbracing-rsr.com understand Harley carburetion and land speed racing.   

[kiwi belly tank]

I know virtually nothing about Harley's but logic would tell me due to their firing configuration that would probably be a common problem with a single carb, especially high in the rpm range due to the leading cylinder robbing the trailing cylinder. I suspect the answer won't be in the jetting because you will be effecting both cylinders, Harley race people should be able to help you.
  Sid.   

[wobblywalrus]

A lot of the newer Triumphs twins (made since 2000) have an uneven firing order.  Triumph went to cams with no overlap to get them running good.  Otherwise, the acoustics from one cylinder mess up the flow at the other when they "talk" to each other.  This is in addition to the starvation issue that Sid is mentioning.  The two carbs need to be completely independent from each other to work with racing cams.  They can not share the same plenum.

[panic]

First you have to eliminate any chance of a vacuum leak in the front cylinder: head gasket, valve guide, intake nipple, etc.
What manifold are you using, and have you done anything to it?

[Sporty Dan]

What size of a vent are you using for the Thunder Jet? By changing it's size you can adjust when it starts to take effect. I can't remember which way to adjust the vent size to go higher or lower in the RPM range when the T-Jet kicks in. Also, what RPM's were you at when it started to go lean?

[Colesy]

First you have to eliminate any chance of a vacuum leak in the front cylinder: head gasket, valve guide, intake nipple, etc.
What manifold are you using, and have you done anything to it?

S&S and no vacuum leaks

[Colesy]

What size of a vent are you using for the Thunder Jet? By changing it's size you can adjust when it starts to take effect. I can't remember which way to adjust the vent size to go higher or lower in the RPM range when the T-Jet kicks in. Also, what RPM's were you at when it started to go lean?

Didn't realise there were different vent sizes. Leaned out at 5 1/2 in top.

[Bookfla]

There are a lot of good little portable Air Fuel Ratio units on the market that have a lambda sensor that can connect to your exhaust pipe to get an accurate reading of your fuel mixture. When using additional fuel circuits like Thunderjets they will make your jetting much easier to understand. Nice thing is when you are done with it you put it in your tool box and save it for the next project.

[wolfeman28]

When you say 'thunder jet' do you mean one of the powerjets from Thunder Products? Which system are you using? I was chasing demons with the Thunder PowerJet and made a switch to the dual Intelajet setup from them, works perfect. Move one knob near my bars and both jets get lean or rich. At WOT I can watch the change on my wideband instantly. Can even adjust it during a run.

As said above me, buying a wideband meter will be one of the best investments you can get.

[wobblywalrus]

This might help.http:neverworld.net/socalnetscum/files/tech/CarbShoot.pdf

[wobblywalrus]

I did it wrong.  I am not a geeky guy.  http://neverworld.net/socalnetscum/files/tech/CarbShoot.pdf