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03 CR250R Slight Bog After Full Rebuild

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Title says it all. Fresh engine rebuild. Everything new. Added a MX bonz airbox kit.

 

Vforce 3 reed cage, it has a slight gap in the reeds. I've read mixed opinions on whether or not this is okay. Some say the pressure from the crankcase will close the reeds. But I am a little skeptical. Should I start here?

 

I also have a Keihin Air striker carb. Jetted 175 and I believe 38 but will need to double check the pilot. I did not use this carb before the rebuild.

 

Just rebuilt it and did a heat cycle. Then test rode it in my yard. It has significantly more power than before. But it does have a slight bog off the bottom that I would like to clean up.

 

Just looking for some advice on where to start.

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I just replaced the reeds on an older CR, a 1983. When I held them up to a light, I could not see a light gap at the end of the reeds, but the sides were bowed, and there was a gap on each reed. OEM reeds are not available for my bike, so I got the Boyesen Pro-Series reeds.

It was obvious that the throttle response was more crisp and it idled better. Seemed peppier, more power at low rpms.

However, your condition could be jetting, too, since it sounds like you installed a different carb. I would address both. I would choose to install a new reed first, then address Pilot, Needle, and main jets.

I'm new to 2-strokes. I had same question. Won't crankcase pressure close the gaps? Based on my one case, the answer is yes...the reeds will close and the bike can run pretty well, but installing a new reed will improve throttle response and idling.

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I would start by adjusting your air/fuel mixture if you haven't​ done so already. Just make sure when adjust it that you check it by shutting the fuel off and letting it die out, if the mixture is correct you will get a slight RPM increase just before the engine dies.

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18 hours ago, tjs514 said:

I would start by adjusting your air/fuel mixture if you haven't done so already. Just make sure when adjust it that you check it by shutting the fuel off and letting it die out, if the mixture is correct you will get a slight RPM increase just before the engine dies.

I messed with the air screw yesterday. Tried it at 1.5 turns out and 2 turns out but neither really made a difference. It runs well up top but spits and sputters when getting there.

I have a stock reed cage with boyesen power reeds, so I am going to swap out the Vforce for now and see if that helps. If not, I will be ordering a needle and some different jets to mess around with.

 

Thanks!

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On 6/7/2017 at 7:27 PM, lerea3 said:

38 pilot sound lean. Put a 42 in to start

It's actually a 45 pilot. I think it's a little rich. Would dropping to a 42 make that big of a change or should I try a 40?

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It's actually a 45 pilot. I think it's a little rich. Would dropping to a 42 make that big of a change or should I try a 40?

I would start with a 42 then play with your air screw again

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Sputtering/bog  then cleans out up top... 9 times out of 10.... Is jetting, reeds or power valve problem. You have to make sure everything is on point before you jet. Start with new plug, clean air filter (not over oiled), no air leaks, new reeds, properly adjusted power valve cables and make sure PV opens and closes all the way and that the stopper pin is in place, ( I've had them fall out during rebuild) Repack the silencer and make sure the coil and the ground wire is properly grounded to the frame ( all need to be super clean) On my 02, a few rebuilds ago.. I put the bike back together and it sputtered on the bottom. I tried everything to fix it with nothing but failure. Last thing I did was check the coil to frame ground and the ground wire.... I found a break in the ground wire. Fixed it, and problem solved. Note: The ground wire eye has two wires that connect to the eye. Mine was broke up inside the sheathing about an inch from the eye. Two strokes are simple, but if you want it to run perfect everything must be perfect.  GOOD LUCK

Edited by mxsteve1968

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On ‎6‎/‎9‎/‎2017 at 8:13 PM, mxsteve1968 said:

Sputtering/bog  then cleans out up top... 9 times out of 10.... Is jetting, reeds or power valve problem. You have to make sure everything is on point before you jet. Start with new plug, clean air filter (not over oiled), no air leaks, new reeds, properly adjusted power valve cables and make sure PV opens and closes all the way and that the stopper pin is in place, ( I've had them fall out during rebuild) Repack the silencer and make sure the coil and the ground wire is properly grounded to the frame ( all need to be super clean) On my 02, a few rebuilds ago.. I put the bike back together and it sputtered on the bottom. I tried everything to fix it with nothing but failure. Last thing I did was check the coil to frame ground and the ground wire.... I found a break in the ground wire. Fixed it, and problem solved. Note: The ground wire eye has two wires that connect to the eye. Mine was broke up inside the sheathing about an inch from the eye. Two strokes are simple, but if you want it to run perfect everything must be perfect.  GOOD LUCK

Thanks for the detailed response.

I swapped reeds blocks and it still spits and sputters down low but runs fine up top. Haven't messed with jetting yet. Going to play with it some more this evening. I might take my buddies electronics and swap it over to my bike to check that variable.

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I think you said the VForce reeds have gaps? Can you see light through the gaps? Issue is probably jetting, but without new reeds, its tough to get jetted correctly. You said you swapped reeds? What's the condition of the other reeds? Light gaps?

When you say it misses down low, do you mean low rpm or small throttle opening?

I also mark my throttle grip and put a small piece of tape on the housing to mark closed (but no play). Mark the WOT position and put a few marks in between. Helps me when I take it down dirt road to look down and see throttle position when it misses.IMG_2018.thumb.JPG.9baa10cd7ebdae16d7a53a11f155fa34.JPG

 

 

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25 minutes ago, Sofiedog said:

I think you said the VForce reeds have gaps? Can you see light through the gaps? Issue is probably jetting, but without new reeds, its tough to get jetted correctly. You said you swapped reeds? What's the condition of the other reeds? Light gaps?

When you say it misses down low, do you mean low rpm or small throttle opening?

I also mark my throttle grip and put a small piece of tape on the housing to mark closed (but no play). Mark the WOT position and put a few marks in between. Helps me when I take it down dirt road to look down and see throttle position when it misses.IMG_2018.thumb.JPG.9baa10cd7ebdae16d7a53a11f155fa34.JPG

 

 

Thanks for the reply. I swapped to an OEM Reed cage with boyesen power reeds. No light gap with those. Still running 175 and 45 jetting. 

 

It spits and sputters at low speeds when you are trying to get on the throttle hard. Then it cleans up after a bit and runs great. Hoping to do some more testing this evening. 

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Keep at it...you'll get there...
Next time you are out, don't focus on what speed you are going, determine what throttle position and rpm. Jetting is easier to diagnose that way.
Probably what you mean is you crack the throttle (~1/8 throttle) it blubbers initially, then as the rpm builds, it clears out?
There's an overlap between the Pilot circuit and the Needle circuit at 1/8 throttle. If your plug is oily and black, you're starting on the rich side.
I try to get my Pilot close first by how well it idles and starts, then address the needle position.

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What is the proper procedure for checking float height? What should it be on the air stryker carb? Round top. No electronics. I am starting to wonder if it could be off.

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Does the carb leak fuel when the gas is left on and the bike is off? If not then the float would not be the issue

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3 hours ago, tjs514 said:

Does the carb leak fuel when the gas is left on and the bike is off? If not then the float would not be the issue

This bike did not, but the bike that I got the AS carb from did leak once when slightly tilted over to the side (Same carb, different bikes). It was temporarily leaned up against my wall while I was rearranging the shop. This bike ran poorly but I didn't keep it long or try to ride it very much.

 

 

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2 hours ago, DCBmx119 said:

This bike did not, but the bike that I got the AS carb from did leak once when slightly tilted over to the side (Same carb, different bikes). It was temporarily leaned up against my wall while I was rearranging the shop. This bike ran poorly but I didn't keep it long or try to ride it very much.

 

 

What needle are you running?  Also did you do a leakdown test after the rebuild.  

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2 minutes ago, c-slak said:

What needle are you running?  Also did you do a leakdown test after the rebuild.  

Not sure on the needle. I'll verify once I pull the tail section off again.

 

No it was not leakdown tested. I don't think it is burning any trans oil though. And it doesn't seem lean. It's a sputter not really a bog that would kill the engine.

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Just now, DCBmx119 said:

Not sure on the needle. I'll verify once I pull the tail section off again.

 

No it was not leakdown tested. I don't think it is burning any trans oil though. And it doesn't seem lean. It's a sputter not really a bog that would kill the engine.

Leakdown test after every rebuild every time in my opinion.  Since you have a new carb it can be jetted rich and still lean because of an air leak.  If you are comfortable not doing one, that's your call.  

Needle choice is the key here then.  The pilot and the needle's straight section control the idle to 1/4 throttle position which is where the bike sounds off.  You should be able to pull the needle and slide without taking anything else apart.  It should take you a couple minutes at most.  Once you have the needle code report it here.  

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 Here is a short video of the issue. The bike stays at a friends shop so can't work on it everyday. Hopefully I will have some free time this weekend to check  it out more. The AS carb is a tight fit so it is just easier for me to pull the tail section, or at least pivot it, to access the carb. 

 

 

Edited by DCBmx119

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