YZ250 Surges When Hot?

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6/4/2018 4:58 AM

This was my 2nd ride on the bike (10’ Yz250) since a complete rebuild. It did it the 1st actual ride after break in. It got hot and started doing it then. So when we got back home, I checked over the carb boots, reed cage bolts, ignition cover, etc and all were tight.

So yesterday we were riding, 10-15 minutes into the ride it started doing it AGAIN. Wtf?

FYI: We use highest rated octane pump gas mixing between 32:1-36:1. Plenty of oil.

It could be my hearing in my helmet and when the bike is doing it, it sounds almost like a “knocking” type of sound.

The bike while riding I also notice has kind of a lag to it as well. I ride woods and technical stuff so most of my riding is slow cruising/lugging through stuff and up rocks, etc. but when i open it up it “feels” like it doesnt explode like a new bike should especially for a 2 stroke.

What do I do here? Im lost as far as carb and jetting goes and depending on engine stuff...that too.

I have research the surging issue all over google through other known forums and no one seems to have the same problem as me or know of a sure answer as what it is.

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6/4/2018 5:06 AM

Ignition coil can do that. Can also cause knocking if theres misfires. Often times after the bike gets hot it will start being problematic until it cools down.

Grab a multimeter and test the resistence on it.

Im going through the same kind of issues on one of my bikes. It sucks.

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6/4/2018 5:12 AM

kb228 wrote:

Ignition coil can do that. Can also cause knocking if theres misfires. Often times after the bike gets hot it will start being problematic until it cools down.

Grab a multimeter and test the resistence on it.

Im going through the same kind of issues on one of my bikes. It sucks.

Never thought of that. It didnt do it before the rebuild and we kept the same carb settings as well. Ill have my dad test it since he has a meter. What should the meter read when you test it?

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6/4/2018 6:27 AM

kb228 wrote:

Ignition coil can do that. Can also cause knocking if theres misfires. Often times after the bike gets hot it will start being problematic until it cools down.

Grab a multimeter and test the resistence on it.

Im going through the same kind of issues on one of my bikes. It sucks.

Alec138 wrote:

Never thought of that. It didnt do it before the rebuild and we kept the same carb settings as well. Ill have my dad test it since he has a meter. What should the meter read when you test it?

Youll have to look in the service manual for the spec.

Im not sure how coils go out. Mine went out while i was riding. They just go bad i guess.

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6/4/2018 8:13 AM

Sounds more like detonation to me. Ride it again, when it starts to do it turn your air screw in. If that does not make the problem go away, you may need a larger pilot.

Or you’ve got crank seal issues

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Tomac and/or Anderson for 2020.....

6/4/2018 8:22 AM

BobPA wrote:

Sounds more like detonation to me. Ride it again, when it starts to do it turn your air screw in. If that does not make the problem go away, you may need a larger pilot.

Or you’ve got crank seal issues

The 1st ride we messed with the air screw and it went away for like 10 minutes butnonce we started riding again and getting on it, it started doing it again.

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6/4/2018 12:16 PM

I’ve heard that many YZ250’s are right on the edge of detonation stock. Add high load, a pipe, and/or extra heat and it tips over the line. You might try a tank with some race gas added (you could go 50-50 so you’d only have to buy a gallon of race gas if cost is an issue) and see if it goes away.

If that fixes it, you could either keep running race gas or have the head re-cut for pump.

Are you the original owner? If not, do you know for sure that the head hasn’t been cut for race gas by the previous owner?

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6/4/2018 12:33 PM

omalley wrote:

I’ve heard that many YZ250’s are right on the edge of detonation stock. Add high load, a pipe, and/or extra heat and it tips over the line. You might try a tank with some race gas added (you could go 50-50 so you’d only have to buy a gallon of race gas if cost is an issue) and see if it goes away.

If that fixes it, you could either keep running race gas or have the head re-cut for pump.

Are you the original owner? If not, do you know for sure that the head hasn’t been cut for race gas by the previous owner?

Im not. No idea. The mechanic didnt say anything about it being cut for race gas. The PO got it on a trade from a GNCC b/c class rider. PO said he didnt like the yz and would rather of rode his xr250. Im thinking the jets are too lean. But why would it have not done this before the rebuild?

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6/4/2018 12:41 PM

omalley wrote:

I’ve heard that many YZ250’s are right on the edge of detonation stock. Add high load, a pipe, and/or extra heat and it tips over the line. You might try a tank with some race gas added (you could go 50-50 so you’d only have to buy a gallon of race gas if cost is an issue) and see if it goes away.

If that fixes it, you could either keep running race gas or have the head re-cut for pump.

Are you the original owner? If not, do you know for sure that the head hasn’t been cut for race gas by the previous owner?

Alec138 wrote:

Im not. No idea. The mechanic didnt say anything about it being cut for race gas. The PO got it on a trade from a GNCC b/c class rider. PO said he didnt like the yz and would rather of rode his xr250. Im thinking the jets are too lean. But why would it have not done this before the rebuild?

Is your timing right?

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6/4/2018 12:43 PM

omalley wrote:

I’ve heard that many YZ250’s are right on the edge of detonation stock. Add high load, a pipe, and/or extra heat and it tips over the line. You might try a tank with some race gas added (you could go 50-50 so you’d only have to buy a gallon of race gas if cost is an issue) and see if it goes away.

If that fixes it, you could either keep running race gas or have the head re-cut for pump.

Are you the original owner? If not, do you know for sure that the head hasn’t been cut for race gas by the previous owner?

Alec138 wrote:

Im not. No idea. The mechanic didnt say anything about it being cut for race gas. The PO got it on a trade from a GNCC b/c class rider. PO said he didnt like the yz and would rather of rode his xr250. Im thinking the jets are too lean. But why would it have not done this before the rebuild?

kb228 wrote:

Is your timing right?

Would have to check it. Dont see why it wouldnt be. Wouldnt the bike run like garbage all the time if it was advanced or retarded?

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6/4/2018 12:49 PM

Alec138 wrote:

Im not. No idea. The mechanic didnt say anything about it being cut for race gas. The PO got it on a trade from a GNCC b/c class rider. PO said he didnt like the yz and would rather of rode his xr250. Im thinking the jets are too lean. But why would it have not done this before the rebuild?

kb228 wrote:

Is your timing right?

Alec138 wrote:

Would have to check it. Dont see why it wouldnt be. Wouldnt the bike run like garbage all the time if it was advanced or retarded?

No but if its off just enough it will cause detonation

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6/4/2018 12:51 PM

Alec138 wrote:

Im not. No idea. The mechanic didnt say anything about it being cut for race gas. The PO got it on a trade from a GNCC b/c class rider. PO said he didnt like the yz and would rather of rode his xr250. Im thinking the jets are too lean. But why would it have not done this before the rebuild?

kb228 wrote:

Is your timing right?

Alec138 wrote:

Would have to check it. Dont see why it wouldnt be. Wouldnt the bike run like garbage all the time if it was advanced or retarded?

Yes, if it’s too far advanced or retarded, it’ll either not start or will knock terribly. I’m guessing your bike is too far advanced by a degree or two. If it’s advanced too far from 0, it will detonate and knock unless you are running race gas. If it’s retarded, it won’t do much except take some “hit” away.

Get a dial gauge indicator and retard the timing a degree or two. If you don’t want to do that right away, drain your gas and put in a gallon of straight race gas. If it stops, it was detonating. Seeing as it was a race bike, I’d imagine the PO ran race gas and may have had the head cut for it.

Was the dome on the head cut when you rebuilt it?

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If you're not mixing gas, you're not haulin ass.

6/4/2018 1:05 PM

kb228 wrote:

Is your timing right?

Alec138 wrote:

Would have to check it. Dont see why it wouldnt be. Wouldnt the bike run like garbage all the time if it was advanced or retarded?

TrailDigger wrote:

Yes, if it’s too far advanced or retarded, it’ll either not start or will knock terribly. I’m guessing your bike is too far advanced by a degree or two. If it’s advanced too far from 0, it will detonate and knock unless you are running race gas. If it’s retarded, it won’t do much except take some “hit” away.

Get a dial gauge indicator and retard the timing a degree or two. If you don’t want to do that right away, drain your gas and put in a gallon of straight race gas. If it stops, it was detonating. Seeing as it was a race bike, I’d imagine the PO ran race gas and may have had the head cut for it.

Was the dome on the head cut when you rebuilt it?

I didnt rebuild it. Had a shop do it. He didnt say anything to me about it being cut for race gas. Im probably going to have him look it over again and check it out anyways.

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6/4/2018 1:27 PM

Surging is usually caused by lean/too much air. If you're running extra oil in gas, that can make it run lean. Alternatively, if you have a crank seal leak or base gasket leak, those could all cause it.
A simple thing to give the bike slightly more fuel is the airscrew, as others suggested. The other thing is to switch from 32:1 to 50:1. This will give the bike more fuel throughout the rev range, whereas the airscrew is really the first 10% of the throttle, only.
Note that 50:1 is not enough oil (in my experience) for desert racing/pro mx/running on dirt roads). If you're riding on/off throttle as you said, 50:1 will be fine for a test.

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KTM250SX YZ125 YZ85 PW80 ZX6 FZX700

6/4/2018 1:30 PM

YZed250 wrote:

Surging is usually caused by lean/too much air. If you're running extra oil in gas, that can make it run lean. Alternatively, if you have a crank seal leak or base gasket leak, those could all cause it.
A simple thing to give the bike slightly more fuel is the airscrew, as others suggested. The other thing is to switch from 32:1 to 50:1. This will give the bike more fuel throughout the rev range, whereas the airscrew is really the first 10% of the throttle, only.
Note that 50:1 is not enough oil (in my experience) for desert racing/pro mx/running on dirt roads). If you're riding on/off throttle as you said, 50:1 will be fine for a test.

May go back to 40:1 or 45:1 like we previously ran. We changed oil ratios due to the bike being freshly rebuilt wanting to give it enough oil.

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6/4/2018 3:38 PM

omalley wrote:

I’ve heard that many YZ250’s are right on the edge of detonation stock. Add high load, a pipe, and/or extra heat and it tips over the line. You might try a tank with some race gas added (you could go 50-50 so you’d only have to buy a gallon of race gas if cost is an issue) and see if it goes away.

If that fixes it, you could either keep running race gas or have the head re-cut for pump.

Are you the original owner? If not, do you know for sure that the head hasn’t been cut for race gas by the previous owner?

Alec138 wrote:

Im not. No idea. The mechanic didnt say anything about it being cut for race gas. The PO got it on a trade from a GNCC b/c class rider. PO said he didnt like the yz and would rather of rode his xr250. Im thinking the jets are too lean. But why would it have not done this before the rebuild?

kb228 wrote:

Is your timing right?

This was my first thought, pull the left cover and check the advance. Could have easily changed it accidentally when putting the motor back together.

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6/4/2018 4:36 PM

If you're getting a lot of smoke then you might consider changing the primary side crank seal.

Smoke would be from oil leaking in and the surge would be from sucking a little air once warmed up and the oil is burnt off.

A lot of times you don't get a full on air leak from the primary side like you would the ignition side, since it's bathed in oil. It just gives you some strange symptoms like that, yet seems to run pretty good, overall.

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6/4/2018 4:49 PM

Mr. Robinson wrote:

If you're getting a lot of smoke then you might consider changing the primary side crank seal.

Smoke would be from oil leaking in and the surge would be from sucking a little air once warmed up and the oil is burnt off.

A lot of times you don't get a full on air leak from the primary side like you would the ignition side, since it's bathed in oil. It just gives you some strange symptoms like that, yet seems to run pretty good, overall.

Im taking it to the mechanic who built it tomorrow and let him look everything over. Im not good with chasing problems honestly and i have heard like 900 different things from too rich of an oil mixture to crank seals and plugs.

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6/4/2018 6:17 PM

MXA recommends mixing some c12 with pump gas, even with stock motors. Your new top end bumped up compression some. Can't hurt to try it.

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The older I get, the faster I was.

6/4/2018 7:43 PM

Stock YZ250s detonate on pump gas. Thats the knocking you are describing. I run 20% race gas mixed with pump, and that stops mine from doing it. You can also get the head modified to run on pump gas by someone like RB Designs.

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6/4/2018 9:55 PM

Straight c12 or vp110 mixed 40:1 is right on the money. Stock jetting and all stock motor and my 05 rips.

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @hammerfamily_4 & @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

6/5/2018 12:10 AM

omalley wrote:

I’ve heard that many YZ250’s are right on the edge of detonation stock. Add high load, a pipe, and/or extra heat and it tips over the line. You might try a tank with some race gas added (you could go 50-50 so you’d only have to buy a gallon of race gas if cost is an issue) and see if it goes away.

If that fixes it, you could either keep running race gas or have the head re-cut for pump.

Are you the original owner? If not, do you know for sure that the head hasn’t been cut for race gas by the previous owner?

Alec138 wrote:

Im not. No idea. The mechanic didnt say anything about it being cut for race gas. The PO got it on a trade from a GNCC b/c class rider. PO said he didnt like the yz and would rather of rode his xr250. Im thinking the jets are too lean. But why would it have not done this before the rebuild?

The new top end likely restored compression that was lost when the old one wore.

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6/5/2018 5:58 AM

Check your squish.

Paw Paw

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