I've got a 89 CBR600F engine with a Dynatek ignition module on my micro sprint and I'm having a terrible time trying to get it to run right.

The engine idles fine but has a terrible bog in almost all of the midrange.

I think what is happening is that when I get on the throttle coming off the corner the slide diaphragms that lift the needle are opening partially with the butterfly wide open and they are kinda oscillating as the motor stutters. Once in a great while the engine will catch and go like a bat out of hell right at the end of the straight.

I have seen the slide diaphragm's do this even just revving it in neutral before. The engine just will sit there and stutter and not rev up.

I've been through the carburetors a few times cleaned them changed jetting back to stock specs except for the main Jet where I'm running a 115 jet on 89 octane pump gas.

I have completely dumped the fuel and started over with new gas too.

Any ideas?

 

is it naked? lol thats always the first thing i ask.

 

First, let me tell you, I have no experience with sprints and engine-modification to same.
So, this is just, guess-timating, based on your post. ;-)

Are you using the 'whole' package on your sprint, including the fuel pump? Or are you using a gravity-feed? Even if you are using the pump, check pressure out-put. If it only happens at speed, you might think about centrifugal force causing a low-pressure in the fuel delivery. Is the fuel pick-up mounted on the out-side of the tank? Sprints do run on just a circle track, correct?

If it's doing it even sitting still, your jetting may still be at fault. What kind of exhaust do you run with a set-up like that? A high-performance, low-restriction is going to want something different from the stock settings. The actual carbs are designed to work with side forces. I have a hard time thinking, that the that if they are properly adjusted, they would be the issue.

Hope, I've at least, given you some new avenues to explore.

Ern

 

Is it a wet rythmic bog or is it a sharp stutter? It would help if we could see a photo of how your airflow is set up.

 

I'll try and answer all the questions:

The motor is naked. Air tubes have been cut and pinched closed by previous owner.

The fuel system is using a pump with a pressure regulator and a return line up next to the carbs. The return line runs back to the tank and has a restriction in it at the fuel tank to ensure back pressure at the carbs. I tried just a regulator but due to bleed of the regulator and low fuel consumption I constantly got drips from the overflow tubes. This system has worked well on other gravity system engines we have run on CR500 etc.

The bog appears to be wet and a fast rythmic pattern. When I observed it at in neutral you could see the slides chattering and almost a back fog of fuel going in and out of the carbs.

Here are some pics.