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1993 Suzuki 300 king quad…bogs in gear

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You should check the plug as Gw says. And, you need to be a bit more specific about how much throttle you get to before it starts to die. And, whether it starts to die as soon as you get to that throttle position, or whether it will start pulling at that throttle then die after a second, or more like five or more seconds.. The throttle setting, if it dies as soon as you get to that position, will indicate what jet or setting is at play, and the time indicates low float level or lack of fuel getting into the bowl past the float needle, or a fuel pump problem. Probably a good idea as a first check would be to undo the drain screw on the bottom of the carb and make sure there is plenty of fuel in there, then crank the engine over and make sure it pumps plenty through..

And, if it ran ok before with the needle set higher, then you should put it back where it was..  Changing things indiscriminately will just get you into more problems and confuse the issue. Diagnose carefully, then change one thing at a time and retry.

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  • 2 weeks later...

So I fully choke and it starts reasonably.  Runs ok and throttles up ok when choked.  -2 Celsius here btw.  Once I warmed  up a bit and take off choke…idles ok but bogs when move the throttle off idle…checked fuel pump and and is getting good fuel pressure.  I now am gonna take carb off again and reinspect/clean but blew with compressed air and cleaned with carb cleaner before…certain jets of carb should look at better?  Diaphragm/needle looks good too.   

took apart carb again and physically removed jets rather than blow just compressed air…think I got it.  Found bit of crap and then blew.  Going to put a small new fuel filter on the vent hoses to see if prevents anything going in to carb…thanks guys


think next I’d like to adjust valves…

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Oh yeah we always need to remove the jets to clean them, especially that emulsion tube, which is the brass tube the main jet screws into and that runs up to where the slide needle drops down into it. That emulsion tube has small holes in the sides of it that need cleaning out, and the cavity it fits into in the carb body needs cleaning out too. The float needle's seat needs taking out and checking for dirt and that it doesn't have a bad sealing O ring or washer.

If it still plays up after a real careful clean then you should check the fuel pump is working correctly, and that the charging system is working and regulating..

While you have the carby off you need to check the rubber manifold it pushes into for splits too.


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Every chinese carb I have put on had the wrong jets in it. Too Small. Always had to either takes the jets out of the old carb or drill them out to the proper size. 

Beyond that I usually have to shim the needle because the mid range was lacking. 

Hope you got it. 

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What Tiha says is generally right.. most aftermarket carbs do need re tuning. But.. you say it's "just started" running bad, so the question has to be.. how long has the chinese carby been on there, and did it use to run alright with the chinese carb.. or have you just fitted it and now it's running bad ?

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Turn the headlights on and see if that helps.

Anyone who owns an old suzuki 300 or 250 4x4 should always check the regulator.

The regulator goes bad allowing the voltage to spike up to 16-18 volts with rpms which prevents the cdi from functioning and you will be chasing your tail forever trying to find the problem.  Turning on the headlights lowers the voltages and helps the engine rev higher.

If the intake valve were the problem then it would be very difficult to start the engine when cold.  Since it starts ok then it's probably the regulator.


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  • 2 weeks later...

I’ve tried posting two replies and they just disappear…

I’ll try again…thought I had it with the last carb clean but it burped and started to have same symptoms.  Sits and idles reasonably but bogs as soon as try to give it gas.  So I tried the headlight trick and it appears to help…ran fine without headlights on the other day so I imagine it depends on when it overcharges.  Is it worth changing that part?  Or just flick on lights when burps again?  

Now it’s having a hard time starting though…cleaned plug, bluish spark, so definitely fuel related…stupid thing is once it does start it runs reasonable and has no issue restarting…

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Do you have a voltmeter? 

Like Just Randy said that is the next option. If you have a bad regulator it will be charging over 14 volts and that causes problems. I can't remember the exact number but like 14.4 volts. 

Mine was the other way, I was only achieving 13.6 volts and was having trouble. New rectifier fixed it for me. Now it is charging properly. 

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You need to check the charging, that it does charge and that it regulates. Check all the wiring between the battery and the regulator, and the earths, for broken wires that loose contact intermittently, which you can do by letting it idle and wriggling wires listening for changes or observing changes in the battery voltage, and you need to check the wires don't have bad/dirty contacts or resistance in them. 

If the regulator is defective causing the running problem, it will be consistent. It won't matter whether it's in gear or driving, or sitting stationary running, when you rev it up the motor will either strain or start to cut in and out. Since it's only doing this when you try to ride it, I'd suspect a wire problem.


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16 minutes ago, BuddyLee said:

Is it worth changing that part?  Or just flick on lights when burps again?

All those regulators from the 90s are assumed to be bad.  The net is littered with threads about it.  The issue is so common it really should be a sticky at the top.


The regulator is by the top of the handlebar stem on the frame.  You can pop it off and look at the epoxy on the back.  If it looks swollen or cracked then there is your problem.

Or you could put a voltmeter on the battery and see if the voltage goes up to 16 or higher when the engine revs.  If it goes higher than 14.4 then your regulator is bad.

23 minutes ago, BuddyLee said:

Now it’s having a hard time starting though…cleaned plug, bluish spark, so definitely fuel related…stupid thing is once it does start it runs reasonable and has no issue restarting…

That sounds like your intake valve is not closing.  Hard starting when cold, but starting fine when warm, means the intake valve isn't closing.  It means there is no gap between the rocker and the valve stem.  You can wiggle the rocker and see if there is any gap.  You can either find TDC on the compression stroke or just pull the cord while randomly looking for any gap anywhere.

If there is no gap then back the lash screw off some and see if that helps your starting issues.  It doesn't have to be exact.  The spec is .001 - .003 but I've run as high as 1/8 inch or more.   You just need to see if that is the problem so setting it perfect doesn't matter at this point.  Just make it loose so you know for sure the valve is closing.

If that is the problem then your intake valve is toast and you will need to re-cut the valve seat and buy a new valve.  Otherwise you will constantly be resetting the lash as the valve wears down.  I've gone about a year doing that, but eventually the valve will pop through the seat.  Replacing the valve won't help because you have to re-cut the seat or the new valve will wear down also.

Both the regulator and intake valve are common problems on these machines.

I'd also want to peek at the cam lobes through the valve lash caps and see if there is any obvious wear.  If the engine has ever run out of oil then it doesn't take long to destroy a cam.  Been there, done that.

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Hey there BuddyLee, just thot I'd share what finally fixed mine. I have the same quad and had the same issues.

Replaced regulator-didnt fix it 

Adjusted valves-no better 

Cleaned carb-no difference

Then when I was putting the carb back on I noticed a small hole in the intake tube between the carb and the head🤨(looks like it got too hot once and melted a little) so I got some high heat gasket maker and smeared the hole shut and perfect, throttles all the way up, lots of power and running sweet! 

Idk if this is any help just thot I'd share






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7 hours ago, Mel22 said:

small hole in the intake tube between the carb and the head

I drilled a hole in mine then installed a small bolt with a hole drilled through it then slide a tube over it and that's what powers my fuel pump.  I don't have the stock carb so I needed to tap in somewhere.

Manifold leaks are definitely something to look for.  Every now n then mine wont start and after popping off the seat I find the carb fell off lol.

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  • 2 weeks later...
  • Solution

Everyone is right…lol. 

Firstly.  Had the opportunity to run it around and once it burped and started to bog I turned on the headlight and started to run fine…thank god for that one…I’d never would have figured that one out.  That’s why this forum is so essential…we have the opportunity to learn from others hardships who have gone through the agony.

Secondly.  I used an unlit propane torch to test for a manifold leak and it kicked so I investigated that next…turns out the intake manifold was cracked!  Gooped that all up with gasket maker for the time being.  Hesitant to buy Amazon products and wonder if best to buy a used original piece…(had bought an Amazon maintenance kit…air filter, plug, whatever else…and the air filter came apart and allowed dirt into the carb/motor.  Went back to a cleaned original.  So in other words you have to be careful what buy from Amazon).

Thirdly, haven’t checked yet but will check the inlet valve lash…since it still doesn’t start the way I’d like…but it is an older bike and all.  

thanks to all who have read and future enthusiasts who will need to read…lol.  

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That's great you're making progress.  If the air filter let dirt pass then it could have acted as an abrasive on the intake valve and once the hardness is worn off the valve surface it wears down pretty quick.  And since the gap is only .001 there doesn't need to be much wear to start producing symptoms.

If you do get a new valve make sure it's made in Japan, like Vesrah.  http://www.vesrah.tokyo/product-line/engine-valves/

I think you can get new OEM intake boots too, but used may be ok.  With air filters I stick with foam because paper falls apart, especially if it gets wet.

Another guy on here bought an amazon regulator and it fixed his problem for cheap but I wouldn't recommend buying another OEM regulator because they all are prone to the same problem.  I bought one from Rick's https://ricksmotorsportelectrics.com/  But I guess it depends on your plans for the quad and your budget.  I have 2 winches and bunch of lights so I told Rick's I wanted the biggest they had.

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The regulators don't have any current regulation Randy, they just regulate on voltage alone. The original ones are fine till they get old, or, the wiring or battery deteriorates.

You should check your charging system Buddy as the book suggests you do. Check the wiring for breaks or bad connections,  on both the live wires and the earths. Check your battery has acid to the right level too if it's a servicable one. The regulator might be fine still.

And if the choice is between a years old manifold and a new one I think I'd take the new aftermarket one. They normally only get damaged when people are taking the carb in and out. The damage happens because the old rubber has gone still and inflexible.

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When mine first failed I replaced it with another from a LT4WD and it lasted a couple months before also failing.  If Buddy wants to go down that road then it's his choice, so long as he's making an informed decision knowing the reliability issues of OEM regulators.

I decided to fix it for good with a lifetime warrantied Rick's electronics one that can handle 50 amps.  It cost $150 but I wasn't going to spare any expense considering the work I had already put into the rest of the quad.  There are many cheaper options to consider.  The one I bought was intended for a Triumph motorcycle so I suppose he could buy any old regulator for a street bike and consider it an upgrade.

The manifolds can be found both as new aftermarket and new OEM, as well as used OEM.  I agree that new aftermarket is better than used OEM, but the option exists to buy new OEM if he wants.

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