Suzuki GN 125/250/400 Tech, Service and Repair Tips

Posted in Blog
06/23/2017 admin

In this Article we are going to talk in depth about Suzuki GN 125/250/400 Tech, Service and Repair Tips.

It’s fair to say that for most people, it becomes a hustle to find the right repair shop which on one hand will fulfill thier demands and on the other just requires you to spend a few bucks to get everything fixed.  Unfortunately to good to be true!

Therefore we would love to share our experience. Keep reading an we’ll help you to understand more about ride. Be aware this post presents tips in no particular order!




If you have recently bought a Suzuki GN model or you already own it a while and you figure the bike takes a long time to start, but does eventually, and the battery is draining incredibly quick, if You stall it you cannot get the bike started again without having to recharge the battery?

What would be the causes:

  • knackered battery or faulty charging system for the battery
  • simple knackered sparkplug/plug cap


How to resolve the issue:

  • charge the battery proberly with the right equipmet, proper chargers that can automatically stop when the battery is fully charged
  • if the battery is charging on a charger “fine” but you figure it isnt holding the charge, there will be no way around and to buy a new battery


Tip: If you buy a new battery keep in mind to buy 9A+ battery




For the Suzuki GN Owners having trouble with thier carburetors please consider the following. There are know issues about the carburetor, but know worries we gathered up quite a lot of information to make life easier and help fixing them.

If you feel the bike will turn over but will not start without some ether sprayed into the air intake? Or it was just a long winter break and the bike has been sitting long ( maybe you did not even drain the carb before storing your moto for the long winter break?)

Here is a solution:

  • The carb is probably gummed up from having gas sitting in it. You have to take the float bowl off, remove the float, the float needle, the jets & emulsion tube. Flush every passageway and every jet with carb cleaner. Lots of carb cleaner. Then blow everything out with compressed air. Put everything back together and it should work fine.


Next time you store it, drain the carb!


Note: For the ones who do not feel save and would like to have a little guideance, watch this youtube video on “how to clean and rebuild a motorcycle carburetor”.
Many Thanks to Ichiban Moto to publish the video!



The carb is Mikuni vacuum type, mod.:BS26SSMikuni VM26 Rebuild Kit

Also it’s a realibe bike, as you told, but I would warn you, GN 125 users, that there are a typical error around the carb ! The diaphragm certanly goes wrong by the fuel sold in the EU. Dealers sent back the diaphragm to Japan, so Mikuni re-designed it.


The symptoms are:

  • lack of power
  • hard to take of
  • hard moto starting etc.


How to fix:

  • You have to disassemble the carb, remove the diaphragm, go online order a new one and put erverything back to gether. After all it should work just fine.


General carburetor adjustment

Keep in mind! Increased breathing needs bigger jetting, decreased breathing needs smaller jetting… Then… it’s up to where you live and ride…

Adjust the carb the following:

  • thinner air = less oxygen = go leaner
  • hotter  air = go leaner
  • more humidity = go leaner
  • higher altitude = go leaner


  • thicker air = more oxygen = go richer
  • cooler air = go richer
  • less humid = go richer
  • lower altitude = go richer


Finde here some repair parts to fix  your Mikuni Carbureator





Engine – Valve Clearance

In case you face the problem, like some others already did,  and you hear your engine making a tapping noise. To give you an example watch the video below, it is pretty obvious and you probably won’t miss the the tapping sound the engine is making.


If it occurs to you and your engine is generating the tapping noise, it is most likely that you need to adjust your valve clearance. Make sure you rotate your engine in the right position, otherwise it is getting worse and afterwards you’ll hear some loud ticking!


How to find the right position?
Rotate the engine to the correct position make sure you align the mark on the flywheel with the other mark on the crank case cover. Check the rockers for free play, the rockers should be loose when you find top dead center (TDC) correctly.  If you are lined up with TDC but there seems to be some pressure on the valves, you’re probably at top dead center on the exhaust stroke. A second way to check for the right position is to look into the cylinder with a flashlight. You should be able to see the top of the piston.


We’ll list you the intake and exhaust valve clearances below, but you should also be able to find them in the repair manual (free download). Make sure that you have all tools available to access the valves and a proper feeler gauge that fit’s your needs in this regard.


Adjust valve clreance the following:

  • intake = .08 mm to .13mm (Suzuki GN 125)
  • exhaust = .08 mm to .13mm (Suzuki GN 125)



Engine – Cam chain tensioner

(to be continued…)



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This post is to be continued. Be aware that every information prestend in this post is based up on experience and best practise. We are not an offical repair shop and therefore unliable for any damaged occured to your vehicle.

Comments (2)

  1. Brian Dalrymple

    I have bought a Herald 250 Classic which I am told has the Suzuki GN engine. The bike came with a very, very basic owners handbook in Chinese English. When asking Herald (3 times) some of what I consider to be basic technical questions they refuse to answer me. Checking with some of their distributors they also tell me they do not get supplied with any repair manuals from Herald and they use generic information and suggest I try and find some of the old Suzuki maintenance manuals for sale. My other bikes are older Brit ones which I have manuals for and just surprised at Herald who claim they are passionate about motorcycles and quick enough to sell you one but then leave you to your own devises. I would appreciate any info where I can purchase a manual, the downloads on the internet apparently are not very clear and I copied a couple of pages that were offered and some of the diagrams you could barely see any details. Many thanks, Brian

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