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Discussion Starter · #1 ·
I am asking some questions about my son's bike that we ran at Bonneville. This might not be the correct place to post this and feel free to move it where it needs to go.

My son has a Suzuki 400 custom with an overbored KZ400 to about 460cc. The Mikuni's are 34s. I am not sure of the slide number. We started out with a 250 main and the needle all the way down. We only had less than 2 weeks to break the race engine in and find some happy medium on the carbs.

We ran into a couple of issues while out on the salt that we could not seem to overcome. One is that we could not run with a wide-open throttle. When we did the engine fell on its face and picked up when we backed off. We kept dropping main jets to lean it out as we were running at a density altitude in the afternoons around 7,000 ft. We got some speed out of it (85 mph) and the plugs looked great. I was told this is a rich/lean condition.

We tried to lift the needle one notch and it ran terrible at 76 mph. What is the rule of thumb on carbs? If you lift one notch you drop 2 jets sizes?

We finally removed the baffles from the mufflers and found 10 mph. Our top speed on the last day was 96 mph. We met all of our goals short of going 100.

The sprockets are a 15/40 setup. We tried another set of sprockets but we couldn't get over 7000 so we went back to the stock set up of 15/40.

What would cause the engine to fall on its face and how can we get more speed out of the bike?
 

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Welcome.
I wish you luck in solving your problems.

It would be nice if we still had someone here with the training and experience to help but I doubt that we do.
I think you need to find a custom bike shop near you with a "real" mechanic to help you get a good tune.
 

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Discussion Starter · #3 ·
I truly appreciate your response. You are correct, as it is going to take someone with the expertise to tune this bike to maximize the power and speed that we are searching for. Thank you again.
 
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