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crf450r Misfire totally stumped

By Rossi 46,
in CRF450R/RS/RWE & RX

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Rossi 46

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We run a CRF450R in a Laeger Quad. The engine is a 2007 with ported head, 12.5:1 piston Stage 1 Hotcam and 2006 de-compressor setup. The quad was running well but has suddenly developed a severe misfire. It will start OK and idle but as soon as you start to rev it it starts to misfire. If you hold the revs at about 3 - 4 thousand revs it runs totally crap but still seems to idle fine.

I have done the following

Cleaned the carb, checked the float height and checked the throttle plate is in the right way round

Sprayed starter fluid round the carb and intake to check for leaks

Checked the intake for rags or other blockages

Checked the exhaust for blockages

Changed the complete ignition coil from my other Quad which was running fine

Changed the CDI unit

Changed to the carb from the other quad

Checked the resistance figures on both carbs to check the throttle position sensor

Checked the stator resistance Both readings were fine

I have now changed the stator coils to a spare set I had and have fitted the wiring loom from the other quad

I have tried pulling the choke out while revving the engine, tried pulling in the hot start lever and letting the fuel run out. (the Hot Start is seating properly)

I am very very particular when setting the valve timing and clearances. I check for TDC by using a dial gauge through the plug hole rather than rely on the timing marks. The rocker cover will not come off with the engine fitted so checking the timing is an engine out job. So the timing is checked about 3 times before fitting the rocker cover. Besides the quad was running fine up to now, so unless the timing could have slipped I think the issue is not related to the timing.

The engine starts ok and will idle with no problems but it is only when it is revved that the misfire seems to kick in.

Where do I go next?

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Rossi 46

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No leaks in the exhaust. I even fitted a new sealing ring in the head to ensure a perfect seal to the front pipe. We use 98octane pump fuel that is readily available here in UK. Have never had any problems with it and besides the other quad is running fine on it. I will try some fresh fuel tomorrow.  It has just been suggested to me that there might be an issue with the de-compressor. Could this explain the problems I am having? Although its a 2007 engine we run the 2002 - 2006 de-compressor setup.

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Rossi 46

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If you still have the factory auto tensioner it might have jumped a tooth.. Check the flywheel and cam gear marks.

We use a manual cam chain tensioner. As explained above checking the timing marks means taking the engine out. Might still have to do that. Is it possible that the cam chain sprocket could have turned on the end of the cam shaft? Is there a way to check for this? Would timing issues such as this not have a major impact on starting the engine?

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Diggla117

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We use a manual cam chain tensioner. As explained above checking the timing marks means taking the engine out. Might still have to do that. Is it possible that the cam chain sprocket could have turned on the end of the cam shaft? Is there a way to check for this? Would timing issues such as this not have a major impact on starting the engine?

I know the cam gear can spin and ruin the camshaft. I've never had it happen though.
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Rossi 46

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Ok update. I should have mentioned that I have changed the plug. One of the first things you change really. When I changed the coil it was a complete unit, Coil Lead and Plugcap. The loom was also changed and in both cases the earth connection was connected to the coil mounting bolts. The engine/chassis are electrically connected through the several engine mountings so that should not be a problem. The parts I swopped were all from my other Quad which is running fine. I managed to lift the rocker cover enough to check the valve timing. I discovered the timing chain was a bit loose and it seems the manual cam chain tensioner had backed itself out a bit even though the locknut was still tight. I have tensioned the chain and checked the valve timing. (see Pics) Valve clearances were also checked and found to be fine.  I have now also changed the stator complete with a spare that I had. There was no sign of any metal particles either in the oil or the oil filter or the stator that was taken out. I am now taking all the parts off to get my other quad running for a race at the weekend. I might think of changing the cam itself just in case something might have happened to it.

2014-07-02 22.57.42.jpg

2014-07-02 22.57.22.jpg

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Diggla117

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Timing mark on the flywheel.. Might be off a tooth.. Supposed to look like this.. You have to get lower to see the gap that should be in between the two lines ImageUploadedByThumper Talk1404405079.274959.jpg

Your might be good but I'd check and make sure. Also the chain dosnt stretch so either you set it loose to begin with or something's up. Maybe the cam bearing is shot? You should of heard the chain slapping if it was too loose

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Rossi 46

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I see you swapped the stator with a spare. Do you know that its a working unit?? Otherwise id try the stator off your running bike.

The stator we used was a second hand one we had but there is no guarantee it is any good. I have now made a Peak Voltage Adaptor as described here https://www.thumpertalk.com/topic/1034259-build-your-own-peak-voltage-adapter/

With the engine running I have measured the following peak voltages as described in the manual.

Ignition Coil Max 6v     Should be a minimum of 100v according to the manual (how is the engine even running)

Exciter Coil Voltage 19v    Should be a minimum of 50V according to the manual

Pulse Coil Voltage 26v    Should be a minimum of 0.7v according to the manual (this reading seems high compared to the 0.7v minimum)

Exciter coil resistance is 17 Ohms  (normal)

Pulse coil resistance is 235 Ohms  (normal)

My Peak Voltage Adaptor Gives a peak voltage of 339v on my mains voltage of 245v so seems to be working alright.

Clearly there are some issues with my electrical system.

 

I have also now tried some fresh fuel but it has made no difference.

 

I dont' know about those. But I HATE the power those cams put out. Carry on.

What is wrong with the power of the stage 1 Hotcam? We use a Stage 2 cam in my sons quad as he is an aggressive rider and can make good use of the top end power. We use a Stage 1 cam in my daughters quad as she is somewhat less aggressive and hopefully benefits from the extra torque for lifting out of the corners.

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Rossi 46

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I have now tried 3 different stators, 3 different complete ignition coils, 3 different CDI units 2 new plugs and a different wiring loom, I have changed the fuel, checked the exhaust for blockages, cleaned the carb again, Tried going up a couple of sizes and down a couple of sizes on the main jet. I have now put in a brand new stage 1 Hotcam and checked the valve clearances. The timing has been checked, double checked, and triple checked and is spot on. The engine had new stainless inlet valves fitted with uprated springs a few months ago. The engine starts easily so I'm assuming there is no issues with the piston rings or valve seal. Although the electrical components I have used are not new it would be very odd for them all to suffer from the same fault. Its strange that the engine starts easily and will tick over perfectly but just won't rev without a misfire.  Is it possible that the flywheel could have lost some of its magnetism? It is a genuine Honda item. I just don't know where to go from here.

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