Author Topic: Jap B400 VC carb jetting question  (Read 339 times)

Offline Ivandit

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Jap B400 VC carb jetting question
« on: July 01, 2019, 04:59:33 AM »
Hi there!

So last year I got a 6Sigma jet kit for my '94 VC, installed and running (apparently) well with a K&n air filter and stock exhaust.

The fitting instructions tell you to replace the stock jets with 105 on #2 and #3 and 102,5 on 1# and #4, which kinda made sense to me after reading about how that difference compensates the different cooling conditions on inner and outer cylinders, adding a bit of extra fuel to help #2 and #3 cooling:

But then...last week I was reading the japanese service manual prior to a valve clearance check and got a bit shocked when reading the carb specs:

So, apart from them being the same size (what's the point with the kit then??)...they are supposed to be fitted completely the other way around!! That is 105 on #1 and #4, 102,5 on #2 and #3.

Any thoughts/experience on this?

So far I've had different opinions, from "stick to the manual" (why the outer cylinders are set to be richer then??) to "the manual info is wrong" (hard to believe IMO...).

I just want to go back to stock jets, but I just don't know which is the right way...any help is much appreciated!! :beers:

Offline T2098

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Re: Jap B400 VC carb jetting question
« Reply #1 on: August 29, 2019, 02:33:25 PM »
When I jetted my 400VC I followed the factory service manual and did it as they specified, with the larger jets on cylinders 1 and 4.

The Bandit engine is liquid cooled and pretty cool-running at that from the factory with the 170 degree thermostat.   The two cooler cylinders likely have cooler cylinder heads and intake ports as well, and the cooler an intake port and its associated valves are, the less it atomizes the incoming fuel.

This lack of complete atomization is one of the main reasons why a cold engine needs to be run richer, both with fuel injection and carbs.   The cool intake port walls and intake valves leave an awful lot of little liquid fuel droplets un-vaporized, which just get pushed out the exhaust port as unburned hydrocarbons.

So the answer as to what the correct jets are for which cylinders depends on whether you're looking for nice even air/fuel ratios across cylinders (like Suzuki did with the Bandit, since it has zero issues with keeping itself cool) or whether you're trying to use the excess fuel to keep hot running cylinders cool and from overheating.  On a 70s-80s air cooled 4-cylinder it wasn't uncommon to see richer jets used on the hotter running inner cylinders to allow them to run a bit cooler - even if that meant the air/fuel ratio was perhaps richer than ideal for combustion and it cost you a little bit of fuel economy and some higher hydrocarbon emissions.