[Dave'sBlade]

Anyone ever experiences extreme lean condition on the middle two cylinders of a 900 CBR? I've richened the needles all the way and they still run lean. Carbs have recently been cleaned and runs good otherwise but bike cuts out at mid to WOT. #118 Main jets Factory Pro, stock air filter, Factory Pro timing advance, Jardine slip-on on aftermarket headers.

 

[Geoffm]

Hi you don't mention if this was happening before the carbs were cleaned or just since, either way I would say it sounds like fuel starvation on the centre carbs, check the jets don't have dirt trapped somewhere, and check the float heights, also the fuel feed, filters etc, check the floats valves didn't get dirt in on reassembly, if it was running fine before cleaning something has changed on reassembly (float height?) Or has dirt got from the tank into the carbs on refilling for start up, if it was doing it before cleaning the cleaning has not been thorough enough, this all assumes you havent changed anything other than cleaning the carbs, if you have changed jets or fitted a new exhaust then that is where you need to start as a less restrictive exhaust will cause it to run lean and require re jetting good luck 🖒

 

[IanDoohan]

The Canadian model used a larger set of mains on 2 and 3, and I know some kits also richened up 2 and 3 believing that heat soak stole some fuel from them.

Why do you believe 2 and 3 are leaner than 1 and 4?

 

[Dave'sBlade]

The Canadian model used a larger set of mains on 2 and 3, and I know some kits also richened up 2 and 3 believing that heat soak stole some fuel from them.

Why do you believe 2 and 3 are leaner than 1 and 4?

Because I dropped the clip on the needle to the last position on all cylinders and 2 and 3 cylinder's plugs were very white while 1 fouled out and 4 carboned up heavily.

 

[Dave'sBlade]

Hi you don't mention if this was happening before the carbs were cleaned or just since, either way I would say it sounds like fuel starvation on the centre carbs, check the jets don't have dirt trapped somewhere, and check the float heights, also the fuel feed, filters etc, check the floats valves didn't get dirt in on reassembly, if it was running fine before cleaning something has changed on reassembly (float height?) Or has dirt got from the tank into the carbs on refilling for start up, if it was doing it before cleaning the cleaning has not been thorough enough, this all assumes you havent changed anything other than cleaning the carbs, if you have changed jets or fitted a new exhaust then that is where you need to start as a less restrictive exhaust will cause it to run lean and require re jetting good luck 🖒

This all happened after cleaning but the bike didn't run before so I have nothing to compare it to.

 

[Geoffm]

Hi well if thats the case check everything on the centre carbs is definitely clean , take out the jets and blow them through , check pilot jets are correctly set, and float heights are correct, if it all checks out it looks as if it needs jetting up to suit the exhaust , and I would also think it needs the outer carbs jetting as well, whilst lifting the needles may make it appear rich as it runs in the mid range it may well run lean at higher revs