YZ400F 99' how tight is too tight spinning the intake cam by hand


6 replies to this topic
  • kcuf

Posted October 23, 2012 - 12:23 AM

#1

Hey guys, i've been collecting a few bits to fix up a non running YZ400F i picked up pretty cheap, looks as though the chain had stretched so much that the valve timing was out enough for intake valves to lightly kiss the piston, i've had the head serviced and all sorted out, but upon re-installing the cam's to do the valve clearances, the exhaust cam is fine, can push it around with the tip of your finger no problems, but the intake cam, you really need to get a grip with at least 2 fingers to get it moving, and it just feels generally tight, i was still able to measure clearances but i figured i should see if i should replace the cam/cap combo, now would be the time if that is advisable,

anything i should try? i am mainly concluding that the cam is slightly bent? or that the cap is damaged from the piston/valve kiss. or do i simply slather it in oil and try again?

thanks
Dan

  • grayracer513

Posted October 23, 2012 - 05:44 AM

#2

It's not unusual for the cams, intake in particular, to feel a bit snug when their cold. If you measure the cam for straightness with a dial, and you also measure the cam clearance with Plasti-Gage, and all that's OK, then you should be good. Clearance will increase as the head expands as it heats up.

As long as the cap and cam don't show any damage, it should be fine, but you can, if you like, polish the cam journals by hand: Tape up the ball bearing to shield it, and clamp the sprocket gently in the protected jaws of a vice. Then take a long 1/4" wide strip of very fine Emery cloth (600 grit or so), wind it all the way around the camshaft and pull it back and forth around the shaft as evenly as possible. Clean up and recheck the clearance occasionally. It's a slow process, and obviously not extreme high precision, but it works fine when all you want is to take off a half thousandth or so.

Also, be EXTREMELY careful seating and retorquing the caps. Seat them completely before tightening, but use only finger pressure and a light tap or two to do it. DON'T pull the caps into place with the bolts. I personally recommend using 75 inch pounds (6.25 ft/lb) as the torque value instead of the 86 inch pounds (7.2 ft/lb) called for in the manual. It's always worked for me, and 86 just feels tighter than necessary. Either way, torque them in 3 or 4 even steps in the prescribed order, and don't exceed 86 inch pounds under any circumstance.

  • kcuf

Posted October 25, 2012 - 01:29 AM

#3

It's not unusual for the cams, intake in particular, to feel a bit snug when their cold. If you measure the cam for straightness with a dial, and you also measure the cam clearance with Plasti-Gage, and all that's OK, then you should be good. Clearance will increase as the head expands as it heats up.

As long as the cap and cam don't show any damage, it should be fine, but you can, if you like, polish the cam journals by hand: Tape up the ball bearing to shield it, and clamp the sprocket gently in the protected jaws of a vice. Then take a long 1/4" wide strip of very fine Emery cloth (600 grit or so), wind it all the way around the camshaft and pull it back and forth around the shaft as evenly as possible. Clean up and recheck the clearance occasionally. It's a slow process, and obviously not extreme high precision, but it works fine when all you want is to take off a half thousandth or so.

Also, be EXTREMELY careful seating and retorquing the caps. Seat them completely before tightening, but use only finger pressure and a light tap or two to do it. DON'T pull the caps into place with the bolts. I personally recommend using 75 inch pounds (6.25 ft/lb) as the torque value instead of the 86 inch pounds (7.2 ft/lb) called for in the manual. It's always worked for me, and 86 just feels tighter than necessary. Either way, torque them in 3 or 4 even steps in the prescribed order, and don't exceed 86 inch pounds under any circumstance.


thanks for the reply, and all of the detail, it was much easier to turn with your torque specs so i just went with it, got the bike running this afternoon, took it for its first bit of a ride around, with the choke off you can only give it a little bit of throttle to get moving, if you leave the choke on you can give it a better rev, seems to start fine, even after its warmed up, had to use the choke and the hot start/air bypass button to make it easy though..

the previous owner stuffed a few threads at some point though, the oil drain plug in the bottom is stripped, and the lower hex head bolt on the oil filter cover is stripped also!!, you can get them to that "kind of" tight spot so i did that just for a quick ride up and down the paddock, will look into fixing them tomorrow, anyone got any idea on what thread they are, guess i'll just have to get a few helicoil kits.

so pretty well tomorrow i'll pull the carb and give it a complete dissasembly and clean, and re-assemble, hopefully i find something blocked that is causing it to die when you give it full throttle.

thanks again

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  • kcuf

Posted October 26, 2012 - 03:20 AM

#4

well ive got the full throttle business sorted, seems to go quite well which is good
was plenty of cleaning to do in the carb, everything had laquer/green on it, after all that got it back together and its starting a touch easier now, still wont idle unless the choke is on though, once you give it a rev you can turn the choke off, but as soon as you let off the throttle too long it just dies unless you turn the choke back on

ive played with the pilot jet in all kinds of positions but as soon as you try to let it idle, it dies,

now ive been reading around the other threads, and i did note one in particular you had posted in GR and for the life of me i cannot remember removing a O ring on the top of the pilot jet, and i can for sure say that i did not install one when i put it back in, so unless it was stuck up there and i didnt notice, i dont think its there, (unless it managed to stay and didnt dislodge with air/carb cleaner) plan to check that one before i pull the carb for the 3rd time to have a look.

  • High_Boost

Posted October 26, 2012 - 06:01 AM

#5

its been a while since Ive been in the carb, but I dont think there is an oring under the pilot jet..
there is definately a little tiny one that goes around the fuel screw that can get stuck up in the hole.. you might check to see that that is in there correctly

  • grayracer513

Posted October 26, 2012 - 06:16 AM

#6

Don't confuse the pilot SCREW with the pilot JET. The pilot jet is in the carb just forward of the main jet, up it it's own little well. That's most likely where the trouble is. The jet orifice is very small (on a number 45, the orifice is 0.45mm, or about .017"). Any little dried film in the orifice can easily make the jet effectively half as big as it should be.

Read:

http://www.thumperta...726#post8021726

  • kcuf

Posted October 29, 2012 - 12:08 AM

#7

thanks for that, got it all sorted now, took the carb back off, soaked the pilot jet again, got some copper wire and gave it a good see-saw clean, sprayed it some more, and gave it a blow dry with air, also got one of those adjustable pilot screws at the bike shop today turns out i had the o ring on the screw, but was missing the washer, easiest way to get the washer was get the adjustable screw.

anyway, put the carb back on, started 2nd kick with the choke on, turned the choke off, and it kept idling perfectly, let it warm up, turned it off, started first kick 4 or 5 times in a row with no choke, and idled perfect, so very happy with that

biggest dissapointment was that i broke the old cruddy chain when i went for a quick ride yesterday, picked up a 2nd hand good condition chain from a local shop today, worked on the carb first, took a few links out of the chain to get it on (went 2nd hand so i can decide on weather i'll stick with stock gearing, or change it when i order some newies) and the master link they sold me with it was the wrong one!!
will go sort that out tomorrow, also found a leak in the left radiator up where one of the tubes connects to the top tank, if you push on it it sprays water, so i'll go find someone to braze/solder that up for me tomorrow too.





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