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Discussion Starter · #1 ·
My Suzuki Katana 750 started dying when I cracked the throttle open and RPM's went above 4000. It would still chug along, but continued to get worse. I sprayed carb cleaner into the air intake while engine was running and the issue got a little better with dying (seems like not enough fuel) occurring closer to 5500 rpm. Problem wouldn't go away so I took carb off and cleaned it. Reassembled and started her up last night and now problem is worse (always a confidence booster when thing gets worse after you've "fixed" it).

Here are current symptoms: start fine, idles fine, will maintain 3000 rpm with choke engaged and I can ride in first gear with choke engaged up to 5000 rpm and throttle open a bit, once I open throttle (choke not engaged) she'll go up to 3000 rpm then start dying.

I have a question about carb mechanics, and would also like to know folks thoughts on what the issue might be, beyond my own theory.

1) Seems to me like a fuel starvation issue. She runs at 5000 rpm with choke engaged, but can't get close to that when choke is off, so I'm thinking that maybe it's not getting enough fuel because of a possible fuel filter clog. However, for this to make sense my assumption about how the carb meters fuel must be correct. Here's my question on carb mechanics - I'm thinking there is a difference between amount of fuel v.s. richness of air-fuel mixture. When the choke is on and rpm = 3000 it enriches the fuel/air mixture (let's just say 10:1), but that the air going through the carb only picks up X quantity of fuel. When choke is off and throttle is opened wider to get 3000 rpm let's say air-fuel ratio is 5:1, but because more air is passing the amount of actual fuel picked up is 4X. This would mean to get the same rpm at the leaner ratio you need twice as much actual gas. If the carb works like this perhaps it is an issue of fuel not getting from the tank to the bowls fast enough, and maybe a clogged filter is the problem.

I may be way off here and any wisdom is welcome.

Thanks
 

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Retired twice: Navy and as a govt contractor
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How did you clean the carb?

Check the fuel line, the valve on the tank, and the fuel filter.
 

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Discussion Starter · #3 ·
Took everything apart: slides, diaphrams, jets, needles, bowls, soaked in carb cleaner (not diaphrams), spayed with carb cleaner, pushed a sewing needle into all the holes in the needles. The only thing I didn't do was take the floats and float needles off - which now I'm wishing I did - and I didn't have a compressor, so I could blow everything out. Cleaned the in tank fuel filter. Tank valve seemed clear.

Is my theory on how the carb works correct, or am I missing a piece in my reasoning?

My next step was to replace the fuel filter.
 

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Discussion Starter · #4 ·
Also, forgot to mention on the float needles, I did spray carb cleaner down the fuel line when i had the bowls off and carb cleaner seemed to flow out past the float needles fine (at least to my non-expert eye) .
 

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Retired twice: Navy and as a govt contractor
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Have you checked the float levels? Yes you should probably the float needles and adjust the floats.

The fuel air mixture should ideally be around 14.7 to 1. Under acceleration it should richen to 12-13 or so to 1. Then level back out.

Did you push fine wire throught all the passages in the carb body?
 

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Discussion Starter · #6 ·
Didn't do the fine needle on the idle or choke passages, but those seem to be operating correctly, in their own capacities when I'm idling and choking the engine. The main needle on my carbs (through an additional fitting) go directly into the throat of the carb and I was able to remove and soak those, and can see plenty of daylight through them.

With regard to how the carb meters under different conditions, my original uncertainty is given a set rpm of say 4000, is it possible that with the choke on the carb is actually metering less fuel than if the choke was off at 4000 rpm. That is the engine can run at the same rpm but needs less fuel when the fuel to air mixture is richer?

Thanks very much for you help, it's greatly appreciated!
 

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The fuel petcock may have its own filter or screen on it. If the fuel is being used at high RPMs faster than the gas can get from the tank to the carbs, you will experience these problems.
 
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