125 starts, backfires, quits

Hi all! I just bought a Honda RS125 but now that I've got it home, it will only run for like, 30 seconds and then it backfires and dies . When I first fired it up today i jetted conservatively (1 jet fatter than the 'conservative' setting from the www.fatbaq.com calculator) and it started first bump and ran well. Me and my buddies took turns running up and down the street in front of the house and then it wouldn't rev and died. I then remembered I had left the choke on, so I tried again with the choke off and from then on it's started easily, run for 15 or 20 seconds, let out an almighty backfire and stalled... the failure pattern is SO consistent I thought maybe someone might have heard of it before and know what the problem is???? Thanks for your help!

 

How come the guy with 'I need help from the 2 stroke gurus!!!!' for a subject line gets 19 responses and I get none? Is this one too tough? Not enough description? Wasn't I polite enough, or complimentary enough to the '2 stroke gurus'?

 

The only time that my bike ever back fired is when it was getting way to much fuel. We then proceeded to force the issue and kept trying to start it and hydro locked a brand new crank and top end. We later found that we had a small fuel leak that was filling the bike up with gas which is why now turn the fuel off AND take the fuel line off when trailering.

However, when this happened to my bike, it would fire and start for like a second. It never kept running for 30 seconds and then quit, so I don't think that is what is happening with yours. Although it does sound like some sort of fuel delivery problem.


You may want to go to this link and join the 125 list serve and post your question there.

http://www.two-stroke.com/mailing-lists/gp125/

Mary

 

DIES

Did you assemble the engine ? IS there a rag, towel or mouse nest in the pipe ? Is the piston installed with the arrow facing the exhaust port. Check for broken reeds Check the timing

 

TZRusty,
No, I haven't torn it down yet... Considering it was running fine 4 days ago (when I bought it) and it still happily starts, I wasn't initially considering that it was something catastrophic because I didn't think it would run at all with a plugged exhaust, broken reeds and the piston installed backwards... Perhaps I am being naïve, but if it was a major mechanical problem like that, surely it wouldn't start and run at all?

 

Overdraft, There are a lot of basic things to do when your bike will not start. Since you are new I would suspect that it "may" be user error. This happens all the time. Starting proceedures are simple but must be followed. Here are some tips. And it is almost always something very simple as to why you are having this problem.

1. You noted that you ran it for a while with the choke on.. On a 125 you take the choke off immediatly after starting.
2. The backfire was most likely a result of the engine being loaded up with fuel.
3. If you start a 125 let it run till it is warm.
4. When you start it hold it at 4-5 grand and give it a little blip every once in a while. Holding them at 2 grand just loads them up.
5. Change the spark plug with a new or a good dry one.
6. When you start a 125 never.. never.. give it gas. Some have a problem with this as it doesnt have an idle circuit. People think it isnt getting gas. It is getting plenty.
7. Jetting is not an issue till you are riding it and it is under load. You didnt state what year it is and if it has a PJ carb. If it is a 98 or newer put a 160 in it and the 1270 needle on the second clip position from the top. Second one down.
8. Open the air screw 1 3/4 turns.
9. Choke on
10. Push and push and push in 2nd gear till it starts to pop.
11. At this point you can start to catch the motor with the throttle and pull in the clutch.
12. Neutral and choke off.
13. Rev it up to 5,000 and hold it giving it a few blips to clean it out.
14. When it is at 50c temp you can rev it to clean it out.
15. It will not totally clean out till you actually ride it at the track.

Information that would be helpfull would be fuel. Leaded or unleaded, Premix and ratio and year and powerjet carb or not.

When you get it running take the plug out and put a new one in or dry it out.

Before you do this proceedure I would do the following. Remove the carb and look down the intake. You will see the reeds. Make sure that they are complete and sealed to the manifold. The backfire may have broken one.

If this does not work.. and it will take some time. I would look at the stator. Ensure that the bolt that holds the rotor on is tight and that the keyway isnt broken. This would cause an eratic timing fire (backfire) A backfire is VERY rare in a 125 so I am leaning toward it was loaded up.

Post your results.

 

I see from one of your other posts that this bike is an old style. I would reach out for someone with evperiance in setting the needle and jetting. For the ones giving the recomendations set this guy up LEAN. Give him a 100deg. day with 75% humidity so that it will clean itself out.

Do not be concerned about this lean jetting draft. Run it till the temp goes up then change to something maybe 2-3 sizes richer. For example we run a 158 main on the 98 when we normally run a 162-165 as a norm.

 

Change the spark plug with a new or a good dry one. first guess

 

Thanks for the responses all! Once again, the thing starts fine, just won't run for any length of time... I'll let you know if I get any results when I've worked through some of your suggestions!

 

Dies

one more try Check the carb needle seat, to see if is sticking and running the bowl dry

 

New Plug...

kneedraggin129, Same technology as my 1954 Seagull Outboard

 

So it does run... That is good.. It is running dry for some reason.. Check the fuel system obviously. Take the petcock from the tank to ensure that the screen isnt all mucked up.. clear fuel line and certainly check/clean the seat. May be helpful to hook the carb to the tank off the bike with the jet access bolt out of the bowl and turn on the fuel and see if it has good flow.

 

Bill cole,
how do I go fast???

 

Moto.... that is an easy one.. Commitment, focus, mentoring and a few $$.. With this combination it will come. Some faster some slower.. But that is the fun isnt it.. ?

 

Thanks all! I got the little bastard running last night finally... not sure which thing fixed it and the troubleshooting process was interupted by the rotor coming off and flying through the neighbourhood... (the guy I bought it off said he put a new stator in... you need a tool to hold the rotor so you can torque it correctly so I'm guessing he just put a wrench on it while holding the back wheel... not good enough). Anyway, had too fat of a jet in and I forgot the wrench to take the plug out of the float bowl so it didn't rev that well, but it definitely ran, so I'm goin' racing next weekend!

 

That is great news.. Please check the keyway for the Rotor. If that bolt wasnt tight I would conclude that your backfire was from that rotor moving to an unknown area of the crank. Ensure that the keyway is in the crank that that it is in good shape.. If not repeat process..

Glad you are going racing this weekend.

 

Let go of the lever on the right handle bar or remove it

 

???s On starting 125

Bill,

You mention NEVER giving a 125 gas...

This seems to me to be the only way to get mine going...

I have a 94 TZ 125 Stock Jetting per Manual...
Location Florida

Brand new engine - 4 heat cycles, New Plug, Carb Clean...

I get pushing in first gear - Choke on - Drop the clutch nothing...Push again Nothing...

Then I figure I am flooding the engine with the choke on so I take the choke off...Push nothing...

Then I push and crack the gas, little farther, little farther...Pop, pop, Then it starts to grab and I pull in the clutch and roll the gas....

I probably open the gas atleast a 1/4 turn...

At the track I was starting my bike on the stand virtually the same way....My buddy would relase the clutch in first and crack open the throttle-just a tad- then I would put my foot on the stand and roll the rear wheel and usually the first compression stroke It would pop and light up...

Could I possibly be pushing the 125 too fast at home?? I start from virtually a Full run?? Thus flooding the motor and requiring me to open the throttle??

I can get it going, but it seems ridiculous for me to have to have to use throttle when everyone else adamently Prohibits open throttle when starting...

Thanks,
Ron

 

Ron, That is interesting.. may I ask what pilot you run and where the air screw is.. ? It may be that your low circuits are rich making it nessesary to open the throttle a bit to get the needed air. Let me know if you dont run 15 pilot and air 1 3/4.

My comments on opening the throttle are more dont keep killing it with throttle usually out of frustration. Just a touch is..... OK. Basically if your carb is set correctly for the current conditions you shouldnt need any.

 

Bill,

When the engine was rebuilt, my buddy disassembled the carb and told me he put in stock jetting all around...

My book is in Japanese, so I ahve no clue what stock is!!

I will check...

Ron