81' Suzuki GN400 power issues

I recently picked up my first bike about a month ago, so relatively new to all of this. Bare with me.

The bike does not rev past 3.5k rpm and also tops out at 35mph..

I know the bike is much faster than this because 250's even hit the 60mph+ mark

Was thinking about the following:

Remove gas tank and remove rust within
New fuel filter - what filter? Universal?
Clean fuel lines
Replacing spark plug - what's the plug gap?
Replace plug wire/coil wire?
Clean carb
Re-jet carb if i absolutely must..


Just listing possible things it could be, but honestly don't have any idea.
If you need any more information please let me know.

Whats been done to it? Pod? Straight through exhaust / no muffler?


We need more info / pics to see what we're dealing with.


But, if you say there's rust in the tank, then getting rid of that and all the rust that's gone into your carb is a good idea anyway

Sorry about that, Pod filter with stock exhaust.

Other than that I don't know. Guy I bought it from was kinda an idiot so I don't know if he messed with the carb at all.

You'd be best trying to find a stock air box and bringing it back to standard tune. Otherwise you'll be in a world of pain with wrenching, plug chops and guessing trying to get this thing to run right. And it's your first bike - you want to be riding, not wrenching.


The guy you bought it off no doubt slapped a pod on there for "the look" and then found out it ran like shit.

Well I bought the bike as a project bike.

When I got it, it didn't even have a pod filter on it. He just removed the stock airbox and left the rubber hose on the carb..
I'm turning the bike into a cafe racer so I'm definitely going the pod filter route.

Pod filters and CV carbs do not make great bed mates. There are lots of posts about it around. Those carbs can be rejetted to run with pods but to get it running a stock airbox would make life much easier.

Start with a service manual which you can probably find on line but I prefer hard copy. I would then check and/or replace the plugs and check for a spark.

If he sparks are at home and come out to play, next stop is the fuel system. Most petcocks (fuel taps) have a filter on top of them inside the tank. Drain the old stale fuel and remove the petcock and clean it out. Next remove and strip and clean the carbs. The outside isn't important but the jets and tiny drillings are and they should be cleaned with carb cleaner and never wires or drill bits.

When you are 110% sure the carbs are OK, check that the wheels spin freely and the brakes are not binding. When you spin the rear wheel you will also get to check how bad the chain is. A rusty chain can make it really hard to rotate the real wheel.

Test compression would be a good idea while you have the tank off and plugs out. You can borrow a compression tester from your local auto store. Avoid buying one from Harbor freight. I bought one for a friend and it was so inaccurate I just told him to throw it away. It was worse than useless.

The one thing to avoid at all cost is the temptation to start stripping it and "making it your own" because the basic problems will still be there after you spend all your time and cash. Nothing worse than making a bike all pretty just to find that the motor is junk.

How would I find out the spark plug gap? I tried doing some research but I came up with nothing.

Edit: found it, gap is 0.028!

I have to make a decision on whether to bear the headache that is tuning/rejetting my carb to work with the pod filter and achieve the look of a cafe racer or to go back to a stock airbox with all it's ugliness yet runs much better (maybe).

For a compression test what would be a good reading?

Just started tuning in. I have a GN400 from 81 or 82, also as a newbie. I have the complete stock bike running well. The stock airbag looks awful, but super inexpensive. i think anyone with experience will say its best to start clean from stock, they have with me. I'd say go for it to make your life easier. But thats me, and I admittedly don't know much.

Good luck, and I'm looking forward to learning from your journey.