The REAL reason...CRF Valve problems

[FFRacing]

RE: CRF450 Valve(?) problems
Posted: 06.05.2003 17:28

the problem has been around for along time the first time i had to deal with it was with the little honda engines atc 90s parents would get theses 3 wheelers for there kids and they would run them in low gear with the throttle pegged to the bar. these little engines were capable of over 11000 rpm what would happen is when they were shut off and let to cool they would not restart because of lack of compresion the intake valve would loose its clearance and then be held open when the motor cools. all most all engines gain clearance when hot.

after several adjustments it was impossible to make it work and the engine would have to have the valve replaced this always happend to the heavy intake valve and not to the hot running ex valve. mass vs force the problem was not with the valve metalurgy but inadequate spring force for the cam lobe profile and the rpm that it was being run at for the valve train to last the rocker arm or the bucket has to stay in contact with the cam lobe so that the valve could be set down lightly on the seat spring pressure does not cause seat and valve wear, again because a proper cam lobe is designed to slow the valve down at the end of its event so that it will be let down on its seat . when an engine is over reved and the spring is not adequate the valve leaves the lobe and bounces on the seat causing a peening action depending on which material is harder one has to go away. in the case of the honda its the valve most companys make there valve softer that the seat so that it is not brittle and will not break there has been other makes that i have seen this happen to kaw 250 mojave the 900 nija even a honda 600 ridden by Fritz Kling .1 weekend of 13000 rpm use did in the intake valves i replaced the intake valves with stock units and placed a cb 900 spring base that wAS .020 inch thicker under the stock springs and finnished the season with no problems. in the case of the honda crf 450 with its titanium valves it is capable of reving way past what the stock springs are capable of controling. by replacing the titanium valves with steel valves you can prolong the problem but it is only a bandaid fix the harder valve will last longer but it is heavy and the valve is still bouncing and could lead to breakage or bending the best cure is to use a better spring package with more force.

Ron RHC

Edited and populated post by Madman393

[works86yz250]

ive heard of this valve seat banging and valve sreaching problem before on the crf. and ya thought honda would be smarter that that....... a yzf has never had that problem. just shows whos REALLY beter in the end. for us off road desert riders, we all know the most importantnt thing iin a bike is realiability, rite? i mean, we gota get home. so while all you proud 'crf' riders are stranded with a broke bike after it cools, us yzf/wrf riders will be stuck towin you home, but its fun to roost the guy ur towin when its a honda so be my guest!

[cowboyona426]

Thanks NorCal, that is interesting. I wonder why YZF's don't seem to have this problem? Stiffer springs perhaps? Could it have anything to do with the DOHC/SOHC difference?

[Woody_393]

It is my opinion that he is Correct.

on TT this was talked about quite a bit. It seemed that if you started losing clearance that if you replaced your springs with aftermarket springs, the problem should go away.

I have a buddy that checked his '02 CRF the other day (at about 50 hours) and his were all within spec, but a little on the loose side. Odd eh?

I have yet to check mine, but I will soon.

I'm a little confused about what he means in the first paragraph.

[Rockey]

Im with Woody.
This is what I thought the problem was, aside from an overly aggressive cam profile setting the valve down too hard onto the valve seat. The spring makes more sense though.
Most people dont set their bikes on TDC when they shut them off. This is important because at TDC the valve spings are not compressed. You shouldnt leave your bike sitting for a week with the springs compressed, it weakens them. This could contribute to the problem.
The CRF has aggressive cam timing, and the valve needs to follow the cam lobe and seat softly into the valve seat. When the spring is worn, and the engine is spinning 9 grand, the valve is being "thrown" down by the cam lobe, and the valve jams right into the seat because it is "floating" and not following the cam and being set down into the cam lightly.
The SS valves will help, but it really isnt the answer. It needs stiffer springs. You see these cases everywhere, bike has 60 hours on it, and the intakes go. Ok, he replaces them. 20 hours later they are toast again. Ok, now the owner is pissed. He used OEM Honda valves, and were installed correctly. WTF? Well, the springs are still stock. Same case scenario. The valve is eventually toasted because the springs are still weak. Once the spring is toast, its only a matter of time for the valve to wear out. Thats why the new valves only lasted 20hrs when the first set lasted 60. In retrospect, if the spring set were replaced at 40 hours on the original set of valves, the first set of valves probably would have been fine.
Can anyone find out if the 03 CRF has stiffer valve springs than the 02?

[FFRacing]

Rockey, if you ever go to a DT race, stroll thru the pits. The first thing you will see just about every rider do when he gets off the track and back to the pits, is "back" the motor off. It's habit. How many riders leave their bikes strapped down compressing the forks for a week? Not many I bet.
Ron really knows his stuff, and I tried to convey his message MANY times over at the other forum, but just got chastised for not knowing what I was talking about. Oh well, TW

[Woody_393]

Originally posted by FFRacing11x
Rockey, if you ever go to a DT race, stroll thru the pits. The first thing you will see just about every rider do when he gets off the track and back to the pits, is "back" the motor off. It's habit.

What is this backing the motor off bit. I don't do this, but think I probably should. Is that just putting the motor so it rests at TDC?

And Rockey, thanks for the translation. I was confused and now I'm not.

[MotoMan]

I would have to agree with everyone on stiffer springs being the answer.

Given all the facts here this sounds logical to me.

I am interested to know about the "Backing off" procedure as well, I think Woody is right?

[Rockey]

Yep, thats exactly what it is. At TDC no valve springs are compressed, and thats what you want.

[bates393]

Are you at TDC when you push the kickstarter down until you feel resistance? Is this the best way to get to TDC? I will do this for sure from now on. Let me know if that is the right procedure? Definately good info.: :

[FFRacing]

"Backing it off" comes from keeping the bike in gear and rolling it backwards to TDC. This also aids in the starting of the DTers as until recently, with the advent of the "production" DTXers, they did not have kickstarters. By backing it off accomplished 2 things. First it kept the valves closed on a hot motor(taking any load off the valve springs and also keeping pressure on the valve seats, which in a Rotax, were prone to coming loose when hot).
Second, it allowed a full revolution of the motor when bump starting the bike before it hit the compression stroke.

[Florida 393]

This one is being copied to the four stroke KB forum tonight. Good job guys. Tryce you rock!

Bill

[motox_45]

I still dont get it? Put it in gear and back it up till when? How do i know it is TDC?

[FFRacing]

Try it...you will know. It won't go back any farther.

[motox_45]

Ok i just tried the "backing it up thing" and couldnt tell, but i kicked it over real slow. I am assuming it is when it is the hardest to kick. right after it gets real easy. I should leave it when it is real hard. Correct? Which would mean the pisten is all the way to the top and making compression. And thats why it is hard.

(Oh look i've gone cross-eyed, lol)

So leave it when it is the hardest?

[FFRacing]

You got it! TW

[Woody_393]

FFR, thank you very much. This has been very helpful. I hadn't even heard of doing this. Learn something new every day.

Dang ATM gets better and better every day.

Thanks again guys, I agree, this needs to be in the KB for sure.

One other question. I don't know that it matters, but would the backing up method vs the kick starter method be better or does it matter? I'm guessing it doesn't matter, but want to make sure.

[FFRacing]

Woody, as long as the bike has a kickstarter, using it to get to TDC is just fine. The reason we :back" it off with the DTers is 1) the don't all have a kickstarter and 2) it stops the motor just "after TDC" so it is easier to bump start. Either way you are accomplishing the same goal...taking the pressure off the springs. TW

[motox_45]

FFR, you are a wise man.

[MotoMan]

Thanks FFR, this is something that I never would have thought about.

Good stuff!