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Discussion Starter · #1 ·
Hi all. I bought an 07 YFZ 450 last year, it worked great for the most part, but it always was a bit hard to start (after sitting for a few days) and would die instantly when trying to ride it after that first start, but eventually after revving it slightly until it heats up for like 2 minutes then it would run fine (i don't know if this is normal or not, but i think not). Also it pops from the exhaust when i'm accelerating and let go of the throttle. It came with a GYTR pipe, which i guess is not the stock one.

Recently it started to lose coolant like crazy, and so i decided to do some maintenance by myself in various parts to fix this and try to fix the other issues. So this is what i did:

Bought a water pump seal kit and bearing, and replaced them. The seals were bad, but the bearing looked fine, but i also replaced it. The shaft had some little scoring, but i didn't change it since i didn't have a new one in hand.

Removed the carb, cleaned it (wasn't all that bad) and reassembled it (keeping the stock 155 main 42 pilot ) changing only the air mixture screw with a longer numbered one so i could adjust by hand.

(HERE I MESSED UP) Adjusted the valve clearance: intakes were 0.10mm and exhaust 0.15mm first and 0.20mm second -> to 0.20mm intakes and 0.30mm exhausts (i've lookup up the WRONG clearance table - my bad). Also, during the process i dropped the half shim from the cam inside the chain hole, and had to unbolt the lower part, disconnect the piston from it to reach the shim with a magnet (IT WAS A NIGHTMARE), but then i reassembled it aligning all the piston rings as per the manual spec, the correct timing and bolt Nm specs.

SO, after all that trouble reassembling everything i tried to run it, and my new issues started. It does not turn on without applying some throttle, and will not idle anymore. With the idle screw turned ALL IN it idles a bit, but barely and then shuts off. Also when running it was popping like crazy. So i was out of ideas and got to google/youtube for clues.

Then i realized i messed up with the valve clearance... I will get back to it and adjust correctly this week.
But my question is: will this affect IDLING? If not, what should i look for? Also, should i change the jetting from the 155 main / 42 pilot? I live in a hot area like 20~35 degrees Celsius most of the time, and 700 meters above sea level. I ran it with only premium fuel also (97 ron).

I'm out of ideas right now...
Sorry for the huge text, and appreciate any help!
 

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  1. Check the valve clearances are right.
  2. Check that the carb slide flap is not upside down. If it is upside down the bike will not idle at all.
  3. Put the fuel screw at 1 1/4 to 1 1/2
  4. Confirm that the accelerator pump is adjusted correctly. Basically, the correct adjustment is no gap between the arm or any play when you press the throttle/ the o ring or zip tie mod just makes this better but that can go wrong if anything moving rubs.
  5. Get the bike to idle. Even with the wrong jetting it should still idle it just won't accelerate well.
  6. Your jetting may be close but it's hard to tell. Personally, I would guess that 165-170 main with a pilot of 42-48 if you have no lid and this is a slip-on with no aftermarket intake.
 

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Discussion Starter · #3 ·
  1. Check the valve clearances are right.
  2. Check that the carb slide flap is not upside down. If it is upside down the bike will not idle at all.
  3. Put the fuel screw at 1 1/4 to 1 1/2
  4. Confirm that the accelerator pump is adjusted correctly. Basically, the correct adjustment is no gap between the arm or any play when you press the throttle/ the o ring or zip tie mod just makes this better but that can go wrong if anything moving rubs.
  5. Get the bike to idle. Even with the wrong jetting it should still idle it just won't accelerate well.
  6. Your jetting may be close but it's hard to tell. Personally, I would guess that 165-170 main with a pilot of 42-48 if you have no lid and this is a slip-on with no aftermarket intake.
Yeah, number 2 did it for me, it was upside down... :sneaky:
I have also adjusted the valve clearances again, to ~0.22mm exhaust and ~0.12mm intakes, which is in manual spec.

It idles and runs now, but the fuel screw i had to adjust it to around 3 ~ 3 1/2 turns for it to work reliably (1 1/2 didnt cut it), and i noticed the spark plug was dry and very clean when i removed it after riding a while.

Should i try another jetting like 46+165?
 

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Yeah, number 2 did it for me, it was upside down... :sneaky:
I have also adjusted the valve clearances again, to ~0.22mm exhaust and ~0.12mm intakes, which is in manual spec.

It idles and runs now, but the fuel screw i had to adjust it to around 3 ~ 3 1/2 turns for it to work reliably (1 1/2 didnt cut it), and i noticed the spark plug was dry and very clean when i removed it after riding a while.

Should i try another jetting like 46+165?
Anything more than 2 1/2 turns you need bigger jets and you also risk losing the fuel screw at more.
 

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Discussion Starter · #5 ·
Anything more than 2 1/2 turns you need bigger jets and you also risk losing the fuel screw at more.
Took it to track and trail yesterday. It is as before a bit hard to start, and if it dies by mistake while maneuvering in the trail (hot) it gets pretty hard to start.
But in the track, I did many laps in a row and it worked great overall. I didn't remove the air filter lid, mine is completely stock in this regard.
I will try a bigger jetting next to see if this helps...

Anyway, unfortunately, my coolant "leak" is not fixed. While idling or trail riding (very tight trail, with lots of stops and clutch action to go out of tight spots) I noticed that a small hose underneath the motor keeps dripping coolant, if I let it at neutral idling with some revving it drips even more at the point the back reservoir gets empty, BUT while riding at the track reasonably (at my skill level anyway) it does not decrease at all and seen to not drip by that same hose! What's up with that?!?
 

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You need to go back through the list that was in post #2.
Out of spec valves make hard start issues
Incorrect jetting will cause “hot” running engines.
Ensure carb is clean(pilot) and that there are no air leaks.


Sent from my iPhone using Tapatalk
 

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Discussion Starter · #7 ·
You need to go back through the list that was in post #2.
Out of spec valves make hard start issues
Incorrect jetting will cause “hot” running engines.
Ensure carb is clean(pilot) and that there are no air leaks.
The valve clearances are in spec now, as I said in post #3 (I double checked), carb is clean also.
Air leak is possible, I haven't checked it. The engine overheating light never turned on, the radiator fan turns on occasionally.

Just to clarify the symptoms:

"A bit hard to start" when cold -> I have to push the starter like 2 or at most 3 times (for like 1~3 seconds at most every push) for the first start, and keep the choke on for a bit, but after that if I try to ride it it bogs hard below 1/2 throttle and dies. A have to wait a little more time for it, and rev it above 1/2 throttle for it to launch at 1st gear and then ride for like 15 ~ 30 seconds in for it to become usable below 1/2 throttle (it has been like this since i got it last year, after all that i did in the previous posts - carb cleaning and valve adjustment this hasn't changed).

"Hard to start" when hot after dying by mistake (without hitting the off switch) -> I have to push the starter like 2~5 times (for like 2~5 seconds at most every push).

Anyways, I am most concerned about the coolant leaking as i said in post #5 now.
 

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The valve clearances are in spec now, as I said in post #3 (I double checked), carb is clean also.
Air leak is possible, I haven't checked it. The engine overheating light never turned on, the radiator fan turns on occasionally.

Just to clarify the symptoms:

"A bit hard to start" when cold -> I have to push the starter like 2 or at most 3 times (for like 1~3 seconds at most every push) for the first start, and keep the choke on for a bit, but after that if I try to ride it it bogs hard below 1/2 throttle and dies. A have to wait a little more time for it, and rev it above 1/2 throttle for it to launch at 1st gear and then ride for like 15 ~ 30 seconds in for it to become usable below 1/2 throttle (it has been like this since i got it last year, after all that i did in the previous posts - carb cleaning and valve adjustment this hasn't changed).

"Hard to start" when hot after dying by mistake (without hitting the off switch) -> I have to push the starter like 2~5 times (for like 2~5 seconds at most every push).

Anyways, I am most concerned about the coolant leaking as i said in post #5 now.
As far as leaking coolant, I would do the 2 seals and buy a new impeller shaft. it's easy to do and just make sure both seal cups face liquid so they will look like this ] [ not like this[ ]. If you put them in backward water and oil pass through the seals. I also say replace the impellor shaft because sometimes it still leaks due to the wear marks from the seals polishing the shaft.

Hard to start is almost always related to valve specs. Since you confirmed that that sounds good I would verify that the timing is not off at top dead center because a stretched chain can mimic ill-adjusted valves. I am sure your jetting is wrong and a bit off but the bog and hesitation can be due to the accelerator pump being set wrong and doing the zip tie or o-ring mod would stop that delay that a slow accel pump can have.
 

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Discussion Starter · #9 ·
As far as leaking coolant, I would do the 2 seals and buy a new impeller shaft. it's easy to do and just make sure both seal cups face liquid so they will look like this ] [ not like this[ ]. If you put them in backward water and oil pass through the seals. I also say replace the impellor shaft because sometimes it still leaks due to the wear marks from the seals polishing the shaft.

Hard to start is almost always related to valve specs. Since you confirmed that that sounds good I would verify that the timing is not off at top dead center because a stretched chain can mimic ill-adjusted valves. I am sure your jetting is wrong and a bit off but the bog and hesitation can be due to the accelerator pump being set wrong and doing the zip tie or o-ring mod would stop that delay that a slow accel pump can have.
I'm waiting for the new shaft to arrive, then I will open it again and check if the new seals I installed are facing correctly, and change the old shaft. HOPE this fixes it... But I remember I did install the seals exactly as they were before I took the old ones apart, I'm "hoping" they were already incorrect from the previous owner...

The timing is correct I guess, I have aligned the third and final "I" mark (there were two before, connected by a horizontal line) in the A.C magneto rotor with the stationary pointer in the magneto cover and adjusted both camshafts sprockets with their punch marks matching the cylinder head surface as shown in the service manual and rechecked the markings after I released the chain tensioner (also I'm not sure what you mean by "stretched chain", since it relies on that spring tensioner). I don't know if I could screw this up any other way ?!

As for jetting, I have only a 48 pilot and a 165 main laying around. Thinking of trying them together.

Also, I have the zip-tie mod already (it was even worse before). I did it a few months before I started this whole maintenance.
 
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