[soconnoriv]

Hi, finally done with my CB900C project build. Goes great and rides real smooth. Was going to post it in the project builds thread as soon as I can find a matching left side cover. I currently have a red one, while the tank and right cover are blue. I'll probably end up getting the whole thing painted if I can't find a left cover that's blue.

Anyway, I didn't like the sound or the look of the stock exhaust, so I picked up a 4-2 sport exhaust for cheap. It's straight through with no baffles, fits on pretty well and sounds good just from letting it run for a few seconds. I'm aware that most people re-jet their bikes to compensate for the extra flow, so I bought a dynojet kit for it. Before I bought it, I assumed that stage 1 was just a re-jetting and didn't require any drilling. But after reading the instructions, I realized both stage 1 requires drilling as well as stage 3.

Other than the exhaust, I haven't upgraded anything else engine related, and don't plan on it.

I wanted to try and avoid permanently modifying the carbs, such as drilling them. So my question is: has anyone gotten away with going up a few jet sizes without drilling out their carbs, after installing an aftermarket exhaust? Or is it completely necessary to do so? I'm more worried about keeping the engine lasting than performance, so if drilling doesn't do anything more than increase response, then I don't really care for it.

Thanks, here's some before and after pics:

Before

After

 

[Geeto67]

Bike looks nice.

On the subject of rejetting: what is your bike telling you? Usually if the bike needs a change in fueling the plugs will tell you, also you will have flat spots, surging, etc... instead of smooth power delivery. Don't just jet for the sake of jetting, make sure you really need it.

I dont know know what drilling is required to use dynojet jets, can you post up the instructions? Often when people talk about drilling the stock carbs it is the air bleeds since the emissions era carbs saw these sealed over.

 

[soconnoriv]

Bike looks nice.

On the subject of rejetting: what is your bike telling you? Usually if the bike needs a change in fueling the plugs will tell you, also you will have flat spots, surging, etc... instead of smooth power delivery. Don't just jet for the sake of jetting, make sure you really need it.

I dont know know what drilling is required to use dynojet jets, can you post up the instructions? Often when people talk about drilling the stock carbs it is the air bleeds since the emissions era carbs saw these sealed over.

Thanks. The engine sputters or hangs a little just after twisting the throttle, before it starts revving normally. Setting the choke on doesn't make it run any better, but pulling it slightly makes it more responsive from idle.

Step two says to enlarge the slide lift holes with a 5/32" drill bit. Here's a picture of the instructions for stage one:

 

[DohcBikes]

yes, drill the holes

 

[Geeto67]

Thanks. The engine sputters or hangs a little just after twisting the throttle, before it starts revving normally. Setting the choke on doesn't make it run any better, but pulling it slightly makes it more responsive from idle.

Is that cold or warm? Cold is within about the first 30 minutes of running, warm would be up to operating temp which usually takes 30 minuted of riding an aircooled cb to fully come up to temp. If it goes away the hotter the bike gets then it might not require jetting (it all depends on what your plugs are telling you).

Step two says to enlarge the slide lift holes with a 5/32" drill bit. Here's a picture of the instructions for stage one:
View attachment 37490

you can try just jetting without drilling and see what it does. The slide is lifted by vacuum pressure, so enlarging the holes will change the rate at which the slide lifts (in this case it makes them come up faster. CB900s in general don't exactly have the fastest throttle response and this will improve it slightly. If you do decide to drill make sure you use all pieces of the dynojet kit - it's a specific system all meant to work together.

 

[8ball]

Most DynoJet kits require you drilling the slides, which is one of the main reasons I am not a big fan of their kits. They give you lighter springs and have you drill the slides to open the slides faster, but in some cases they can open so fast they bounce off the top. If there is a Factory Pro kit available I always choose FP. The type you would need, they call recalibration kits. Unfortunately for a bike that old, and a model that doesn't have a race history, I'd be surprised if FP has one for you.

Their website sucks.....but that's because they spend their time on the products not the web site So you'd be better served calling them.

carb10series.html, Factory Pro, superSport, carb, Carb Recalibration Kit

 

[soconnoriv]

Is that cold or warm? Cold is within about the first 30 minutes of running, warm would be up to operating temp which usually takes 30 minuted of riding an aircooled cb to fully come up to temp. If it goes away the hotter the bike gets then it might not require jetting (it all depends on what your plugs are telling you).

Warm enough to the point where it didn't need choke, but still cold, probably running no longer than 5 minutes. I'll buy some new plugs for it and take it for a ride this week to find out.

 

[soconnoriv]

Most DynoJet kits require you drilling the slides, which is one of the main reasons I am not a big fan of their kits. They give you lighter springs and have you drill the slides to open the slides faster, but in some cases they can open so fast they bounce off the top. If there is a Factory Pro kit available I always choose FP. The type you would need, they call recalibration kits. Unfortunately for a bike that old, and a model that doesn't have a race history, I'd be surprised if FP has one for you.

Their website sucks.....but that's because they spend their time on the products not the web site So you'd be better served calling them.

carb10series.html, Factory Pro, superSport, carb, Carb Recalibration Kit

Just gave em a call. He told me he doesn't really carry anything for bikes older than 1985 unfortunately. What he did tell me though, is that drilling only increases the snap response. Kinda like what geeto was saying, it doesn't have anything to do with mixture, which I'm not too worried about. Thanks for the link