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Slow cranking YFZ450 PLEASE HELP tried everything

24006 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!!!
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sorry i forgot about the headlights not working until the wheeler is on but it does sound like u have a low current draw so do the tests for the cables like i said before and report back
IMO sounds like a timing issue. Check for correct timing and also make sure your valves are shimmed to spec. I use .005 intake and .009 exhaust hope this might help. I'm subscribed so I'll keep an eye for update
Just read a post yesterday where a guy had same problem and it was his oil pump .search it. It was interesting
I think the fact that he can turn the engine over smoothly by hand eliminates a timing or oil pump issue
I'm having same problem with my YfzR but mine will crank fast without the spark plug in it. I'm lost I've tried it all and it's stock compression fresh motor rebuild top to bottom.
Carb and exhaust was pulsating at both ends. Instead of sucking at the carb and blowing at the exhaust. Spun the crank one full revolution and put the cams back in. Quad is cranking and running smoothly now. Only starts with a 100% charged battery. Charging system and headlights are still not functioning correctly... This will be looked into further tomorrow morning.
exhaust valves are to spec. intake are out of spec in need of a shim kit. They will be ordered along with whatever maybe needed to get the headlights and charging system corrected.

Thank you all for the help so far!
Quad is now running and idling. Has intermittent starting issues still. Tracing the wiring harness and checking the TPS (although it has been disabled at the thumb throttle the continuity is not correct at the carb circuit). Reading the shop manual on the wiring harness. Will keep posting on the progress. Any input is greatly appreciated.
One of the Diodes was bad on the quad located behind the passenger head lamp near the key switch. I ordered both diodes that are on the quad and will change them both. I also ordered a high output "Trail Tech" stator & rectifier. Along with that I ordered "Hot Cams" valve shim kit to get the intake valves back to spec. Hopefully will have all the parts in sometime next week. Hopefully these last few things will get this 4 month project finished.
Not sure what the Diodes on the quad located behind the passenger head lamp near the key switch "power" but the quad runs great without them even in. I did replace the one that was bad and now have a spare for the other one. Once I installed the new stator and rectifier the quad runs great, charging well and headlights/tail lights are working properly. Still need to shim the valves but as far as I'm concerned I finally woke up from this nightmare. Thank you all for your input & I hope this is able to help someone else in the future!
Have you fixed this problem yet?? My 05 yfz450 has the same problem I got it with a gytr kicker installed and kicker harness cut up for just the basics, he cut the nutruel indicator wire, brake wire, clutch wires, a bunch of stuff. I ordered a stock wire harness hooked up everything I could and just had my quad at a shop that does electric work alot. They made me a start button for it bc mine was shot. I bought a cbr1000 battery like everyone says to do if u have slow cranking just today and the starter is new from last yr. my button cranks the engine slower then shit! I can kick the thing once and it starts right up. I want elec start to work bc I need nerfbars and mine have heel guards witch you can't have if u have a kicker..
Just noticed there was 2 pages lol. Glad you fixed your problem! I'll try buying new diodes, stator and rectifier for mine and see what happens, but mine had all that besides stator replaced before
Carb and exhaust was pulsating at both ends. Instead of sucking at the carb and blowing at the exhaust. Spun the crank one full revolution and put the cams back in. Quad is cranking and running smoothly now. Only starts with a 100% charged battery. Charging system and headlights are still not functioning correctly... This will be looked into further tomorrow morning.
Hello what you do to fix it i have the same problem
21 - 34 of 34 Posts