Bad rev issue or idle issue please help

5Caleb9

Member
Jul 17, 2018
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Pennsylvania
Hello, I’m Caleb, I’m a 16 year old who went to a trade school for diesel technology, it’s nice to meet you all. I was given a (believed to be) 1991 Yamaha Blaster 200cc as, I guess, a summer project. When it was given to me the previous owner of the gifter had bored it out and installed a bigger piston, enlengthed the axles, and put on an after market Keihin carburetor, not sure of any of the specs. The one who gifted this to me said he flipped it And only ran it a few times after but when he did it took him sometimes a half an hour to start it, so when I got it the first thing I did was use engine degreaser (a safe brand not bad for rubber or plastics) to clean the whole quad itself, then I proceeded to remove the carburetor, I was told it originally had an oil injector but it was deleted from the quad, and the TORS was also removed. Once I cleaned the carb (removed the bowl, float, float needle, and the main and idle jets, cleaned it all thoroughly, put it back together and put the carb back on the quad, I cleaned the old several year old gas out of the lines and tank, flushed the lines and tank to remove the varnish created by the gas, I then added fresh mixed gas (ratio of 32:1) I then proceeded to attempt to start the quad. It started on the first kick every time started but the issue hasn’t started yet, the next day went by and I went to the lake to fish and swim, then the following day I started up the quad and let it idle for a bit and out of nowhere it began revving itself to max, the throttle wasn’t touched, the throttle doesn’t seem to be stuck, there’s no TORS, and the one who gifted it to me said he had it running a time or 2 after he flipped it and it didn’t do this, so I caused this by messing something up when either cleaning the engine or the carb. Please help! (If you need part numbers, or specs please provide info on how to find the information needed, such as the location of the part number)
 
Hey Caleb

with the bike not running, can you hear the carb slide hitting bottom when letting off of full throttle ?
air filter cleaned and re-oiled ?
what carb ? (get us a pic) and what pilot, main jet and needle #'s ?

check.......

for airleaks, it could be crank seals under the flywheel or on the clutch side, intake or boot, head or base gasket
a correct leakdown tester is the only way to find and correct them, I have testers for $40 shipped if you need one (every 2 stroke owner should)

re-clean the carb if this isn't how you did it or or missed any steps: https://www.blasterforum.com/threads/how-to-clean-your-carbureutor.254/

float height: https://www.blasterforum.com/threads/float-heights-and-how-to-adjust-them.50565/


my best advice is to not run it again until you get this figured out, no sense in frying it over something simple.
 
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Hey Caleb

with the bike not running, can you hear the carb slide hitting bottom when letting off of full throttle ?
air filter cleaned and re-oiled ?
what carb ? (get us a pic) and what pilot, main jet and needle #'s ?

check.......

for airleaks, it could be crank seals under the flywheel or on the clutch side, intake or boot, head or base gasket
a correct leakdown tester is the only way to find and correct them, I have testers for $40 shipped if you need one (every 2 stroke owner should)

re-clean the carb if this isn't how you did it or or missed any steps: https://www.blasterforum.com/threads/how-to-clean-your-carbureutor.254/

float height: https://www.blasterforum.com/threads/float-heights-and-how-to-adjust-them.50565/


my best advice is to not run it again until you get this figured out, no sense in frying it over something simple.
Can I have a link and pictures of the tester you mentioned? I’ll provide the info of the carb once I get to the quad later on tonight
 
4C0680D8-6377-4CA7-BC75-238E4B0379DA.jpeg
Hey Caleb

with the bike not running, can you hear the carb slide hitting bottom when letting off of full throttle ?
air filter cleaned and re-oiled ?
what carb ? (get us a pic) and what pilot, main jet and needle #'s ?

check.......

for airleaks, it could be crank seals under the flywheel or on the clutch side, intake or boot, head or base gasket
a correct leakdown tester is the only way to find and correct them, I have testers for $40 shipped if you need one (every 2 stroke owner should)

re-clean the carb if this isn't how you did it or or missed any steps: https://www.blasterforum.com/threads/how-to-clean-your-carbureutor.254/

float height: https://www.blasterforum.com/threads/float-heights-and-how-to-adjust-them.50565/


my best advice is to not run it again until you get this figured out, no sense in frying it over something simple.
Ok here’s the pictures of the carb and jets you asked for, I got the best pictures I could, I have no idea what half the numbers on these parts mean so I took a picture of everything I saw.
 

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Looks like a Keihin PWK airstryker or quadvent 33/34/35/36mm ? 38mm ?
measure the inner throat of the carb side and let us know in MM's ?

typically these carbs are best left for heavily modified blaster engines
stock to lightly modded/piped engines are better off with the stock carb....they are easily tuned and will handle most bolt on mods and moderate porting.
I'd figure out what size you have there, and trade someone looking to go big, for a stock carb and intake

I see a 60 pilot and 170 main in your pics, that's way too rich for anything but a heavily modded or drag 240cc and stroked engine

the jetting required to get these carbs to run on a blaster are .....
45 48 50 pilots
#6 slide (a must have )
DGH or CEL needle (must have)
145 thru 155 mains (200cc cylinders)


here's my leakdown testers....
https://www.blasterforum.com/threads/leakdown-testers.55015/
 
Looks like a Keihin PWK airstryker or quadvent 33/34/35/36mm ? 38mm ?
measure the inner throat of the carb side and let us know in MM's ?

typically these carbs are best left for heavily modified blaster engines
stock to lightly modded/piped engines are better off with the stock carb....they are easily tuned and will handle most bolt on mods and moderate porting.
I'd figure out what size you have there, and trade someone looking to go big, for a stock carb and intake

I see a 60 pilot and 170 main in your pics, that's way too rich for anything but a heavily modded or drag 240cc and stroked engine

the jetting required to get these carbs to run on a blaster are .....
45 48 50 pilots
#6 slide (a must have )
DGH or CEL needle (must have)
145 thru 155 mains (200cc cylinders)


here's my leakdown testers....
https://www.blasterforum.com/threads/leakdown-testers.55015/
I was under the impression from the previous owner that this is heavily modded, with a bigger piston and more. But he didn’t go into complete detail, what would happen if I put a stock carb on a this quad and it is heavily modded?
 
a bigger piston isn't modded and adds zero performance, the stock carb is capable of feeding most every mod a stock cylinder can have.
 
a bigger piston isn't modded and adds zero performance, the stock carb is capable of feeding most every mod a stock cylinder can have.
Ok I’ll try the float and jetting you informed me about with this carb and if that doesn’t work I will order a stock carb, thanks again, I leave for vacation this Monday so I will post my results next Sunday when I am home and have done the work, can you please also send me a private message so I can purchase a leakdown tester from you, I will send you the MM for the carb on there once I return home
 
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As mentioned, fix the air leak first. A runaway engine can self destruct in short order, measured in seconds.

IF this happens and the kill switch doesn't stop it, pull cluch, shift in gear, hold brake, dump the clutch.

At this point it has gone into "autoignition" and even pulling the plug wire won't stop it.

Either find the PDF on the forum or get a Clymers repair manual.

Usually heavily modded refers to the engine such as porting, re-chambered head, pipe, and bigger carb last. It's too easy to over carburate an engine. It will run but may have issues at certain rpms or throttle openings, including lean conditions, and poor driveability.
 
If you get a new carb, make sure it's a genuine Mikuni, not some cheap eBay junk.
Thank you, and I worked on diesels for 2 years, I know that 2 stroke diesels can do what’s called a “Runaway engine” where it runs out of control, so they had special plates to shut off the air supply, is that what can happen? and why you informed me on dumping the clutch? Because I’m not trying to have it launch me in my 1/4 acre backyard LOL!
 
Not 100% sure, but I'm pretty sure I've seen 4 stroke diesels runaway also.

Just went to YouTube, lots of runaways, and all were to new to be a two stoke .

Not sure about your comment about the clutch. My point is to not play around stalling the engine.
 
Looks like a Keihin PWK airstryker or quadvent 33/34/35/36mm ? 38mm ?
measure the inner throat of the carb side and let us know in MM's ?

typically these carbs are best left for heavily modified blaster engines
stock to lightly modded/piped engines are better off with the stock carb....they are easily tuned and will handle most bolt on mods and moderate porting.
I'd figure out what size you have there, and trade someone looking to go big, for a stock carb and intake

I see a 60 pilot and 170 main in your pics, that's way too rich for anything but a heavily modded or drag 240cc and stroked engine

the jetting required to get these carbs to run on a blaster are .....
45 48 50 pilots
#6 slide (a must have )
DGH or CEL needle (must have)
145 thru 155 mains (200cc cylinders)


here's my leakdown testers....
https://www.blasterforum.com/threads/leakdown-testers.55015/
Ok I adjusted the float level to the 16mm as your post said about, I did find out my carb is a 36MM in the carb side you had me measure, I’ll add a picture as well, I also adjusted the Carb with the pilot air screw I think it’s called and nothings working, I’m sending you the 40$ for your leakdown tester today.
 

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I’m about out of things to try other than the leak down testing which Awk is shipping me one. Would it help anyone help me if I posted a video of myself starting the quad and letting it idle until it redlines itself? Then kill switching it. Because I’m about out of ideas, and everything, I’m scared there won’t be an air leak, and I’ll end up buying a new carb and that won’t fix anything. So would a video help?
 
No sense letting it run away. No sense blowing the engine. It can happen VERY QUICKLY !! :(:mad:
True, I’ll leak down test it when the tool arrives, then if there’s no leak, I’m out of ideas and I’ll just buy a new carb because awk did say my jetting was way too rich
 
There should be no need to buy a new carb, one can replace jets..

If you do buy a new one it will possibly come with incorrect jets for your engine and may need resetting.
 
There should be no need to buy a new carb, one can replace jets..

If you do buy a new one it will possibly come with incorrect jets for your engine and may need resetting.
I have 170 main jetting with the only known mod is a bored motor with a bigger piston, so I could buy new jetting you’re right, I guess I’ll try that first. I’m gonna goto a 155 main jet, and a 50 idle jet, just the highest jetting to work properly with the blaster, I’ll attempt that after I leak down test the quad
 
170 is WAY lean, totally bone stock is like 220-230, depending on year.

A bored cylinder isn't considered a mod. It adds very little displacement or power.