1981 suzuki gn400 won't go above 50 miles per hour

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Deryck · #1 Unread post by **Deryck** » Tue Sep 11, 2007 8:58 am

Sorry if this is the wrong forum, I tried my best though!

Ugh, I don't even know where to start. I have a 1981 suzuki gn400. My bike seems to idle fine, and I can rev it up to higher rpms no problem while not moving, but it becomes a different story when I am moving. It seems to be able to get to somewhat higher rpms (5500-5700 approximately) in 1st gear, then I will shift up to 2nd gear, and my rpms will hardly drop down (probably from revving the bike up so high in 1st, I'm no mechanic though, and new to manual transmission, so if this is a possible problem, let me know

) Then I will shift to 3rd, and I'm not quite sure on my rpms here. Still a new rider, so when I'm going this quick, I'm focusing more on control than staring at my tach, and I try to go more off sound of my engine.

Once I get into 4th gear, my acceleration will begin to slow, and I can get, roughly, my rpms up to about 5000, and then when I shift into 5th, acceleration basically stops. I have my throttle opened all the way, I try shutting my throttle and re-opening. Still, it barely accelerates, and I can get up to about 47-MAYBE 50 mph and my tach will be at about 3500-3700.

My bike had some electrical problems, and my carb was a bit busted up, and I had a mechanic go over both of those, so I sure would hope that neither of these things would be a problem.

If any one has any other ideas or tests I could run to try to discover the problem, or if I need to give more information, I would completely appreciate it if you would respond. Thanks and good riding!

mgdavis · #2 Unread post by **mgdavis** » Tue Sep 11, 2007 9:54 am

You got the right forum.

Maybe your carb(s) isn't tuned or balanced quite right, or it might be junked up. It sounds like you might not be getting enough fuel? I'm nowhere near 100% on that, hopefully someone will be able to get you closer. What do you mean when you say your carb was "busted up"?

Deryck · #3 Unread post by **Deryck** » Tue Sep 11, 2007 9:56 am

I suppose I was being a little over dramatic when I said busted up; I was leaking fuel out of my overflow, so since I had to take my bike to the shop anyways, I had the mechanic take a look at it for me. He charged me for fixing the float and "general labor" on it, and for the price a motorcycle mechanic runs, I'd sure hope the carb is in tip top shape now

Deryck · #4 Unread post by **Deryck** » Tue Sep 11, 2007 2:37 pm

I missed this before, but you mentioned that I may need to balance my carb....I believe my bike is a 1 cylinder bike, so unless I misunderstand the meaning of balancing the carb (which is very likely since I'm not mechanically inclined at all) wouldn't it basically be automatically balanced, or have nothing to balance it against?

Ok, so re-reading that, it comes out really confusing to me, so let me explain MY understanding of balancing the carb is balancing the output or input of one of the carbs vs the other. Is this correct or am I off?

While I'm here, any other suggestions as to what it may be?

I've checked my air filter since my initial post, hoping it might be that....My air filter SEEMS to be fine and clean, according to my untrained eye at least

Deryck · #5 Unread post by **Deryck** » Tue Sep 11, 2007 4:04 pm

After reading around a bit, I've seen that people that have had similar problems with their GNs have had bad CDIs. A CDI appears to be a pretty spendy part, so is their a way to test my CDI before I throw some money at it? Thanks in advance for any responses!

mgdavis · #6 Unread post by **mgdavis** » Tue Sep 11, 2007 5:37 pm

You're right about not needing to balance a single carb. I'm not familiar with that specific bike, so I was just throwing out the normal stuff. Sorry if I confused you.

Deryck · #7 Unread post by **Deryck** » Tue Sep 11, 2007 5:44 pm

Oh no, its more than fine; no apologies needed. I appreciate your suggestions, and I was just trying to fully understand. Thanks for all your suggestions so far!

Deryck · #8 Unread post by **Deryck** » Wed Sep 12, 2007 4:25 pm

Anybody else have any ideas I could try? I'm getting pretty desperate here! Thanks in advance for any advice.

flynrider · Thu Sep 13, 2007 12:52 pm

Your bike is showing signs of fuel starvation. Your bike's fuel burn is directly related to the amount of HP the engine is generating. Which happens to be closely related to how fast you're going.

It certainly sounds like you're not getting enough fuel flow to maintain higher power output/higher speeds. I'd start going through every fuel system component from the tank to the carb. You could have an obstruction in the tank fuel pickup, the petcock screen, fuel line, or even inside the carb itself (improper float setting or clogged main jet).

If your petcock is vacuum operated, also check that you don't have a vacuum leak. Insufficient vacuum can cause insufficient fuel flow.

I wouldn't suspect the ignition system for one reason. You have no problem revving it up when it's not moving. If the ignition were messing up at high rpms, it would do so whether you were moving or standing still. It works the same either way.

Deryck · #10 Unread post by **Deryck** » Thu Sep 13, 2007 6:12 pm

Great! Thanks a ton for you reply. I'll absolutely go through as much of that stuff as I can tomorrow and let everyone knows how it goes. Keep your fingers crossed for me haha