YFZ Central banner

Slow cranking YFZ450 PLEASE HELP tried everything

24007 Views 33 Replies 9 Participants Last post by  Alex450r
Okay so here is the history from day 1 on the issue at hand... 2005 YFZ450 (big gun rev box, jetted carb, GYTR exhaust, no lid on air box)

- Few years back the stator and computer went bad simultaneously. The computer we determined went bad when the back wouldn't start up. (I believe this happened after hitting a large rock in the trail may have given it enough of a jolt). The stator went bad because the allen key bolts on the fly wheel sheared off hitting the stator and breaking apart the windings. The computer was replaced with a "Big Gun" rev box and the stator and allen keys were replaced with a OEM parts.

- September 2013, I had to install a tether style kill switch for a race. The following night (while upstate) I went to turn on my head lights and they did not work however my tail lights and brake lights worked fine. We tried to trouble shoot the problem but with limited tools and time I was forced to race with no lights. Quad ran great all weekend.

- October 2013, Traced the lighting issue back to the stator as the headlight are on there own circuit off the stator. This time the stator sheared off the engine cover, making the cover unusable as well.( this would mean the stator did not work for the whole race and weekend. So it ran strictly off the battery? and fired up fine every time, battery never felt weak) Under a tight deadline for the next trip I purchased a used case and stator off e-bay. (I know not to buy used electrical parts but the cover was too expensive new and the stator seamed to be good in clean condition).

- October 2013 (Day 1), I installed the cover and stator but did not have time to run the quad before loading it on the trailer.

- October 2013 (Day 2), Upstate I started the quad and still had no headlights, once again limited time and tools we decided to ride for the day with out the lights functioning. After riding for maybe 5 hours of trails, open roads, and jumps the quad died on me. This was after a good stretch of road in which the quad was almost at the top speed for maybe a minute or two. Thinking that the quad died due to the battery dieing as I hit the starter and got nothing we decided to swap battery with my Dads battery from his 2004 Raptor 660. The battery's were not the same size so we let my dads charge up mine for about 10min then stuck it back in mine. This gave it enough juice to start and get back to a road. From the road we towed it back to the trailer. We took the battery out of my quad and charged it on a trickle charger in the hotel room over night.

- October 2013 (Day 3), Took the newly charged battery and put it on the quad. the battery did not have the power to crank over the quad. We pop started it after pushing it around for a half hr. The quad ran like shit for about 2 min around the parking lot. While towing it back to the trailer i tried once more to pop start it and it just wanted to skid out the back tires. Frustrated we packed up and went home.

- November 2013, Tried to get the quad to run. Tried Pop starting it and the tires rolled freely but not enough to start. Pulled the spark plug and still cranked slowly. Tried jumping it off a 2012 F-150 and almost stalled the truck. (learned that this was a bad idea from Yamaha as it puts to much power to the quad). Bought a new OEM battery and still cranked over slowly. Gave the carb a shot of starter fluid and "BOOM" the motor locked up.

- November 2013, Pulled the motor out and determined the rod was seized to the crank. (thought this is why it was cranking so slowly) This was because of 1 or 2 things. Either the bad oiling system the 04-05 YFZ's are known for (which while I had it apart I did an 07 oil mod which added an oil nozzle to the top side of the crank and a higher volume oil pump) or when the stator blew apart a few pieces of coil were stuck to the oil pick up. I replaced the crank/rod and bearing with a stock stroke "Hot rods" crank, and new OEM piston rings. (no issues with the rebuild, everything went smooth and took our time, correct torque specs etc. followed the manual to the T) Left all the valves alone, never flipped the head over, simply unbolt and rebolt.

January 2014, Put the motor back in and fully reassembled. Went to start the quad and cranked over slowly. Pulled the spark plug and still kicked over slow. Disassembled enough to get the starter out. Disassembled and cleaned as per the owners manual. Brushes looked very worn but put it back on and gave it a go. Still had the same slow cranking issue. Bought a new aftermarket starter. Put that in and still cranking slow. Bypassed the starter solenoid and still cranked slow. Ran jumper cables (car size) from battery to starter and still cranks slow. Pulled the stator cover off and all the gears were put in the correct way.

All of this is without the spark plug in as well so I am not fighting any compression. Can spin the motor over by hand on the crank shaft and can spin the motor and trans freely by spinning the back tires. Starter spins freely when now on the motor. All gears seam to be mashing together nicely.

So the million dollar question is why is it cranking over so slow, so slow it cant fire up. (read a million threads and tried everything that I have read so far seams like I must be missing something but cant see what.)

Thank you for the anticipated help!!!
See less See more
1 - 20 of 34 Posts
Are the cams tight in the journals? maybe during disassembly one of the cap caps warped? are they torqued right? pull the valve cover and the timing chain off to see if you can spin both cams by hand while they are still torqued down. if the rod seized due to lack of oil maybe there was a lack of oil to the head as well.
Yes the cams spin freely and were torqued the correct Yamaha specs. (stock cams) Motor is believed to be timed correctly as per the manual but have yet the run the bike. I feel as thought there has to be something binding up in the starter gears but they don't appear to be worn in any odd fashion.
Does the motor spin freely by hand?
Yes motor spins free by hand. You can grab the magnet/flywheel with your hand and spin the motor over. Also if you put the quad in gear you can spin the motor over by spinning the back tires effortlessly.

Also while building the motor I was sure to lube all moving parts and spun everything as I was putting the motor together. Everything went together perfectly and spun nicely.
are all the ground wires making a good connection?
Yes the wires are all connected good and solid. Just to eliminate any wiring issues I ran jumper cables straight from the new and fully charged battery to the starter to bypass the solenoid and any possible wiring issues.
the next thing I would do is get a new starter
Starter is brand new as is the battery...
I think I might have just found the issue. The idler gear, the fist gear that the starter spins (the gear that can be removed by taking just the 2 screws off the cover) should the larger and smaller gear be locked together or move independently one eachother? Mine were locked together by a circlip being jammed. Unless they're supposed to move as one I think this is the issue. I will look further into it tomorrow after work.
Yamaha part # 5TG-15560-00-00 DAMPER ASSY

This is the gear I was talking about above. Should the larger and smaller gear oh this one unit be locked together or move independently of one another.
Yamaha part # 5TG-15560-00-00 DAMPER ASSY

Just received the new assy. This unit is not a 1 way gear. the 1 way is located on the flywheel which is good on my quad...

Please does anyone have any idea what could be my issue:angry I'm on my last straw.
Im at loss dude, I cant really think of anything else really but to go over every part of the starting system, and maybe the decompression pin on the cam
I have the spark plug out, so there is no compression.

Thank you for the help, hopefully someone else on the forum may have another idea.
Is the timing chain too tight?
Don't believe it is to tight, it has the stock tensioner which if my memory serves me right is just spring loaded. (anything that had a torque spec was torqued 100% correctly)

I will put the new gears in tomorrow and see if that somehow works, which I highly doubt because they appear to be the same as my old ones but I'm at a loss at this point.
just to rule that it is too tight either loosen the tensioner a bit or remove the cams, if it still tight then its gotta be something in the bottom end
if ur lights are now working, you can do a test to see why u have a slow crank. if not ur gonna need a multi meter with an amps probe. for the test u just turn the headlights on and crank the engine over. make sure the battery is at a fully charged 12.6 volts. if the lights go dim this means u have a high current draw and the problem could be from a tight engine or binding starter. if the lights stay bright, this means there is a low current draw and u most likely have high resistance in one of the cables either going to the starter solenoid or from the solenoid to the starter. to test the cables u can do a voltage drop test. use a volt meter on a low dc volt setting and hook one lead to battery positive and one to positive on the solenoid. crank the engine over and u should not be reading more than .5v. u should also go from the solenoid to positive on the starter. u can also test from the negative battery terminal to the case of the starter since that is the ground. lastly try the test across the 2 posts on the solenoid but for this one you shouldn't read more than .2v. hope this helps!
See less See more
Thank you for the input I will try this next. Sunday 2/9/14 I pulled the cams out and everything spun nicely. Valve clearance was within specs. Decompression valve is moving freely. Tried with new damper assy. and still same results. Put a car charger on the battery and while that was hooked up I tried and the quad spun over quickly as it should. Put an amp gauge on the battery without the charger and it went from 12.6amps to 4-8amps (jumping). This would mean the NEW battery was shot. Took it to Yamaha they ran tests and showed me that the battery is actually better than the other one they had on the shelf (13.6 and under load went to 10.2). I put my battery in an 04 Raptor 660 and it cranked that motor over just fine. Put the raptors battery in my yfz and still cranked slow.

Any further ideas????? Thank you for the help!
Headlights may or may not work at the point as the quad needs to be running in order for them to work. They have a lead off the stator not the battery to power them. Once the quad is running I will have to check to see whether or not they will work
1 - 20 of 34 Posts