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Help Diagnosing 2015 YZ250 Engine Knock and over rev off throttle.

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notice how having to check the crank seals is the last thing people want to do.
do they think they last forever?

I was under the impression a bad crank seal would darken the trans oil? Trans oil looked new when drained.

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5 minutes ago, LorenUSSTR said:


I was under the impression a bad crank seal would darken the trans oil? Trans oil looked new when drained.

Left side crank seal will suck in too much air causing high idle. Right side crank seal will suck in trans fluid and smoke a lot more than usual.

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I was under the impression a bad crank seal would darken the trans oil? Trans oil looked new when drained.

Exactly mine were just replaced as well as all the seals in the bottom end. What do you think it is man? All ive heard so far is “crank seals” and no solutions.

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Left side crank seal will suck in too much air causing high idle. Right side crank seal will suck in trans fluid and smoke a lot more than usual.

My bike smokes a lot but when its cold and because im mixing 32:1. I had spooge rolling down my pipe. But it doesnt smoke once its warm

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52 minutes ago, RidingWithStyle said:


My bike smokes a lot but when its cold and because im mixing 32:1. I had spooge rolling down my pipe. But it doesnt smoke once its warm

Spooge is typically normal during cold starts plus the 32:1 will make it a little rich but shouldn’t hurt it. Have you tried leak down testing? Simple process of removing expansion chamber plugging it and somehow adding pressure through the intake boot. Monitor pressure loss over 10 mins and spray with soapy water. Pretty easy and cheap to do. 

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Spooge is typically normal during cold starts plus the 32:1 will make it a little rich but shouldn’t hurt it. Have you tried leak down testing? Simple process of removing expansion chamber plugging it and somehow adding pressure through the intake boot. Monitor pressure loss over 10 mins and spray with soapy water. Pretty easy and cheap to do. 

I dont know how and or a way to

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The Honda 2-stroke sub-forum has a helpful sticky thread titled something like 'pressure testing a 2-stroke'.

About 25$ worth of parts from your local hardware and auto parts store gets you the required items to

build your own test fixture and can save you $$$ from a blown engine.

Tightly sealed (intake, exhaust, powervalve vent etc.) your crankcase must hold 6psi for a

minimum of 15 minutes with zero drop in pressure but, ideally hold it for longer.

 

A 2-stroke ingesting 'un-metered' air (elsewhere than thru the carburetor) from say from common locations like

a worn LH crank seal or around the reed block will run leaner, hotter and risk a seizure.

Wrongly diagnosed some may be tempted to jet richer to compensate but, a temporary band-air fix at best.

Despite installing a new LH crank seal, if the crank bearings are worn the new seal won't be able to cope with the extra radial play.

Edited by mlatour
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44 minutes ago, RidingWithStyle said:


I dont know how and or a way to

Here is a video I made just to help people with your exact situation 

 

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Exactly mine were just replaced as well as all the seals in the bottom end. What do you think it is man? All ive heard so far is “crank seals” and no solutions.

A smoke machine for diagnosing EVAP system leaks on cars is what you need. We had one at an auto shop I used to work at and that tool is AWESOME! If I had to find your leak I would remove the carb and the silencer and plug the exhaust at the end of the expansion chamber. The machine had a variety of sizes of rubber cone plugs. There are solid ones for plugging one end and ones with a hole in the middle to feed the smoke from the other end. Use a plug with a hole in it that fits the intake boot and attach a small tube the feeds the smoke from the smoke machine. The smoke is cool and harmless. Some machines are a little different from others but they're all electric and pump the smoke through a small tube at low psi. There's a cheapish one I saw on ebay below. 

In less then 60 seconds any air leaks between the 2 plugged ends will show up. I would also open the trans fluid fill hole so you can see the smoke if the crank seal on that side is leaking. The machines can run anywhere from $75 to $2500+ depending on brand and quality. If I didn't have room in the budget or didn't want to deal with that I would call a few bike shops and ask them if they can smoke test my machine from the intake boot to the end of the expansion chamber. If they said no or didn't know what I was talking about I would call an auto repair shop that specializes in emissions control and has ASE certified drivability techs. In CA they're called SMOG shops. I'd tell them what my symptom is and ask if I could bring my bike in with the carb/airbox and silencer removed for them to smoke test it. I would make sure they would allow me to be there with the tech so I could look for the leaks myself once the machine was hooked up.

 

When I was a young smog tech I had an older Camaro that kept failing emissions tests from a lean condition. Spent 3 days on that damn car before the boss came over and said 'I'd smoke test it if it was me' and grinned as he walked off. I hooked up the machine and in less then a minute there was smoke coming from multiple cracks in the exhaust headers which were sucking air back in. I had to use a mirror to see the cracks from under the car and never would have found them without the smoke. I jb welded the holes and tested the car just to confirm that was the cause. Sure enough it passed emissions and we told the owner the manifolds needed to be replaced. After that the smoke machine became the first test I did when I had a lean condition on a car.

 

http:// https://www.ebay.com/i/282111999443?chn=ps&ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F711-117182-37290-0%252F2%253Fmpre%253Dhttps%25253A%25252F%25252Fwww.ebay.com%25252Fi%25252F282111999443%25253Fchn%25253Dps%2526itemid%253D282111999443%2526targetid%253D400149263513%2526device%253Dm%2526adtype%253Dpla%2526googleloc%253D9031565%2526poi%253D%2526campaignid%253D1028603671%2526adgroupid%253D48124585622%2526rlsatarget%253Dpla-400149263513%2526abcId%253D1130046%2526merchantid%253D6296724%2526gclid%253DCjwKCAjwjZjZBRAZEiwAPeLSK-BHeV-uS_ttUZ1e5_zpAiHeBcweAk1UCbkOkfcMvtgBzd3zNQSeLxoCsk0QAvD_BwE%2526srcrot%253D711-117182-37290-0%2526rvr_id%253D1568853331792%2526rvr_ts%253D1059c8151640a991e3065b39fff60584

 

 

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I had a few big bore two strokes and they would do the same exact thing.   We called it over run.  Sounds terrible on a yz250.... tried the watch the above leak down video ..... after 2 minutes I had to go jog around the block in the rain.... help!!!

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I had a few big bore two strokes and they would do the same exact thing.   We called it over run.  Sounds terrible on a yz250.... tried the watch the above leak down video ..... after 2 minutes I had to go jog around the block in the rain.... help!!!

Try this video. This guy made my laugh.

http://
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"a strange engine knock and over revving when backing off the throttle."

says jetting is right on, maybe yes maybe no. some people wouldn't know good jetting if their life depended on it.

check the left crank seal with soapy water, turn engine over and check for bubbles.

pull the head and check for excessive carbon buildup which can increase compression ratio which with lean jetting can cause "knocking".

make sure the right plug is installed, not one that's too hot.

raise the needle one notch at a time till it's obvious it's too much. letting off the throttle means you got off the main and are now on the needle jetting.

Make sure idle jetting is right. Here's my page describing how to jet right: http://www.dragonfly75.com/moto/carbtuning.html

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Here is a video I made just to help people with your exact situation 
 

I'm thinking put a cheap smoke machine between the tire pump and your adapter and the leaks will jump right out at you.

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13 hours ago, kurbe397 said:


Try this video. This guy made my laugh.

http://

That guys is hilarious...I love how he ends each sentence by TURING IT UP A NOTCH!

I think this is how Land Rover guys replace the smoke in their wiring harnesses too...

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Like others have said, it is lean surging. Lean jetting or air leak. Did your air screw by chance back out on you? Easiest way to check for a air leak it so spray WD40 at suspect areas while the motor is idling, if you get a spike in your idle then you've found the leak, pull the ignition cover off and do there also. Could very well be that suspect base gasket.  
 

Edited by TommyB 754
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Possibly a warped head , has the same symptoms myself , easy fix if it is , flat surface like granite and 400 wet and dry paper and figure 8 maybe 20 times

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sounds similar to a problem I had. ended up being the clutch side main seal had blown and caused some lean. do a pressure test...-luke

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That’s either a bent needle jet hanging up, worn out/ damaged power jet plunger seal( little black rubber piece breaks off at the end) or maybe ever sticky power valve. When mine did it it was a bent needle jet I used out of a different carburetor.

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