Jump to content
Sign in to follow this  

1980 YZ250 38mm Mikunki round carb problems?

Recommended Posts

Okay everyone got my new carburetor today.  Installed it after about 30 mins of persuasion and fired it up.  Well took awhile, turns out the only way I can get it to start is if I hold throttle open some.  It will now start with two kicks and up and running.  However when I give it throttle it doesn't seem like it wants to take the gas or have the power.  It won't stall out when I gas it just bogs, however when I let up on throttle with choke on or open whichever it means with these bikes it stalls.  I can start it back up no problem but again holding throttle open.

 

What should my air screw be set to?

 

After bikes warms up to temp then I will adjust the idle screw right?  But for the sake of getting it to start what should my settings be for both air screw and idle?

Share this post


Link to post
Share on other sites

Okay everyone got my new carburetor today.  Installed it after about 30 mins of persuasion and fired it up.  Well took awhile, turns out the only way I can get it to start is if I hold throttle open some.  It will now start with two kicks and up and running.  However when I give it throttle it doesn't seem like it wants to take the gas or have the power.  It won't stall out when I gas it just bogs, however when I let up on throttle with choke on or open whichever it means with these bikes it stalls.  I can start it back up no problem but again holding throttle open.

 

What should my air screw be set to?

 

After bikes warms up to temp then I will adjust the idle screw right?  But for the sake of getting it to start what should my settings be for both air screw and idle?

 

Did you put the jets from the other carb in it first before installing this carb?? Also did you check to see if it has the correct needle and seat?? Does it have the same slide cutaway?? FYI Dirt bikes use a different needle and seat than a snowmobile carb.

 

Bogs are from a lean condition!!

Edited by Zrt1200

Share this post


Link to post
Share on other sites

Did you put the jets from the other carb in it first before installing this carb?? Also did you check to see if it has the correct needle and seat?? Does it have the same slide cutaway?? FYI Dirt bikes use a different needle and seat than a snowmobile carb.

 

Bogs are from a lean condition!!

 

No I did not use the old jets, I left the new carburetor the way it was when I got it except the needle and slide.  That was a different setup so I used old one that was still intact to my throttle cable rather then try to figure out how to use the one that came with the new carburetor.  Again though everything in the new carburetor is the way I got it from the seller.  Suppose it could be a snowmobile carburetor huh?  So will it be best for me to change out the jets and use what was in mine instead?

Share this post


Link to post
Share on other sites

Did you put the jets from the other carb in it first before installing this carb?? Also did you check to see if it has the correct needle and seat?? Does it have the same slide cutaway?? FYI Dirt bikes use a different needle and seat than a snowmobile carb.

 

Bogs are from a lean condition!!

 

Also I don't know if my reference of bog is correct but rather when you give it throttle it acted like it wasn't warmed up if you know what I mean.  Still smoking good so it appears to be getting the correct oil/gas but that doesn't mean anything I guess.

Share this post


Link to post
Share on other sites

OK, Is the new carb for you bike or just a random 38mm carb?? If random Here's what you do. Take the new carb off and tear it apart. Check every thing according to what is supposed to be in the OEM carb. Lets say the OEM call's out for a 3.3 needle and seat and the new carb has a 1.5 needle and seat. The 1.5 will not flow enough gas so it could burn the engine up on top end. Lets say your OEM spec call's for a 6dh3 needle and you have a 6dh4 needle. It will be leaner and so on through the carb. If you do not have the correct jets it will not run correct. The factory's spend a lot of time getting jetting correct for most riding conditions. Here's a link to check for your carb factory specs. If it dont tell you the number click on the part and it will tell you.

 

http://www.boats.net/parts/search/Yamaha/Motorcycle/1980/YZ250G/CARBURETOR%20YZ250G%20-%20H/parts.html

Share this post


Link to post
Share on other sites

OK, Is the new carb for you bike or just a random 38mm carb?? If random Here's what you do. Take the new carb off and tear it apart. Check every thing according to what is supposed to be in the OEM carb. Lets say the OEM call's out for a 3.3 needle and seat and the new carb has a 1.5 needle and seat. The 1.5 will not flow enough gas so it could burn the engine up on top end. Lets say your OEM spec call's for a 6dh3 needle and you have a 6dh4 needle. It will be leaner and so on through the carb. If you do not have the correct jets it will not run correct. The factory's spend a lot of time getting jetting correct for most riding conditions. Here's a link to check for your carb factory specs. If it dont tell you the number click on the part and it will tell you.

http://www.boats.net/parts/search/Yamaha/Motorcycle/1980/YZ250G/CARBURETOR%20YZ250G%20-%20H/parts.html

Okay well it appears that the new carburetor is in fact different. It has different float as well which is two individual rubber floats instead of the one piece float. And since the main jet calls for at least a 360 I think new carburetor has a 330-335 something like that. So is this carburetor no good for my bike? Meaning not going to be able to make it work and my best approach is fix my old one instead?

Or is it possible I can just swap parts around like the jets and make it work?

Share this post


Link to post
Share on other sites

I would hate to say my new carburetor is useless now :(.....hopefully with any kind of luck you guys will be able to assist me and make this work. Suppose I would need to change needle, main jet, and pilot jet to what my original carb has in it and should work right?

Edited by irishhawk81

Share this post


Link to post
Share on other sites

Yes to your last post about swapping parts!!

 

Here's what you do. Disassemble the old carb. Use carb cleaner and make sure every jet is clean and fluid goes through every passage of every jet. If need to use some small wire and push through the holes to help clean them out. If you have to do that let them sit over night in carb clean!! Then put your old parts (being Pilot jet, Main jet, needle jet, and possible the needle and seat) in the new carb. The needle and seat will have a number stamped on it. Make sure they are the same number!! Like 3.5 or 3.3, 3.0, 2.0, 1.5. etc. It will probable be a 3.5 or 3.3 in your stock carb. As for the floats the new carbs floats will work just fine. Just make sure you do not mix them up as to which way they go in as one side will have a pin coming out of it. That side push's against the brass that close's off the needle and seat. It should be on the bottom of the float when looking down into the float bowl. As for setting float level with the top of the carb off, Set the main body on a flat work area, The brass piece that touch's the needle & seat should be level (parrell) with the bottom of the float bowl flange.

Edited by Zrt1200

Share this post


Link to post
Share on other sites

I just want to thank everyone who has helped!!!! Turns out the jetting was part of the equation!!!! However little did I realize the bike wouldn't start without holding throttle open some because the throttle cable was out of adjustment!!! Well I fixed that put all my original parts back I to my old carburetor adjusted float and it's working like a charm!!!

It takes 1-2 kicks and she fires up with no throttle and only choke! I can also adjust choke to on and off once warm it and idles fine! No bog issues and takes gas very well.

My next order of business is this...What type of plug is best and what should my gap be set to? NGK B8EG is what I have now according to Yamaha dealer but wanted to know if anyone uses anything different and what your gap is set to.

Share this post


Link to post
Share on other sites

Any plug that is a direct swap for what you are using now should work fine.  You may get slightly better life out of the more expensive plugs, but you won't notice any improved performance (regardless of what the plug claims).

 

Set your gap to 0.035" as that is standard for many of the older ignitions.

Share this post


Link to post
Share on other sites

Alright so I'm back to square one.......turns out I had it running seemed to take the gas no problems.  Then I left it alone put plastics back on it and walked away.  Came back after about a hour started up again no issues.  Then I walk away again and came back another hour later and nothing.  Popped a few times and that was it no start not even trying.  So I decided to take old carburetor off and build the new one with all the old parts once again.  This time to include swapping needle and seat, all jets, etc so that it has what my old carburetor had in it from the get go.  Try to start I get nothing, not a single thing.  If I hit throttle again it would start. 

 

So after about two hours adjusting the cable I finally got it at a sweet spot and she starts again with no throttle, choke works, (which if choke is down does that mean off or on?  Or open or closed?  While running I give it some gas and she takes it but doesn't quite seem right though.  I did not get a chance to ride it around to see the throttle response but she seems boggy again. 

 

Seems earlier she wasn't smoking nearly as bad with old carb, but now she is smoking a lot more!

 

Main Jet: 360

Pilot: 60

Seat or Needle: 30

 

Are these correct or I missing something?

Edited by irishhawk81

Share this post


Link to post
Share on other sites

Thanks for plug sizes and gap!

Okay also since my old carb had a 3.0 needle and seat but when I ordered a new one from Yamaha they gave me a brand new one that was a 3.5, should I put this in the new carburetor since it says on the microfiche that I should have a Main Jet: 360, Pilot Jet: 60, Needle and Seat: 3.5

Again right now my new carb has everything except the 3.5 it has 3.0

Share this post


Link to post
Share on other sites

Would not having correct needle and seat affect my boggy?

Edited by irishhawk81

Share this post


Link to post
Share on other sites

I went back to old carburetor again! This time using all parts including the 3.5 Needle and Seat that I purchased from Yamaha dealer.  Hooked it all up, I kicked it ever so slightly just to see if it would give me the put put sound that it was ready to fire, and damn it fired right up!  It was late so I had to shut off quick.  Will be testing it tomorrow to include a small ride if all goes well.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×