looking to increase bottom end

njkx

Member
Apr 13, 2004
209
1
the bike is a 05 YZ 250f. Lately is seems that there is a lack of bottom end power. It could be the performance of the bike or it could be that I have become more comfortable and more skilled with it over time. In any case I am looking to increase bottom end power. Everything is stock on the bike now except I went up 1 tooth on the rear sprocket (49).

Is an $800 investment in an aftermarket exhaust worth it? What are some other suggestions?
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
3
Are your valves properly adjusted and sealing properly? How about the piston and rings? Any loss of compression past a leaky valve, worn piston or loose rings will hurt bottom end power.

I would verify your compression, and make sure that the valve train and piston/rings are in good shape before spending money to 'upgrade' the exhaust.
 

njkx

Member
Apr 13, 2004
209
1
valves were checked and in spec....but maybe 10-15 hours on bike since checked...still starts 1st or 2nd kick if that means anything.
 
Jan 3, 2007
1,860
0
I think a Hot Cams stage one will increase the bottom end power for far less money then an exhaust system. You could try your hand at a JD jetting kit for more bottom end as well as a boyseen quick shot.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,820
16,889
Chicago
njkx said:
What are some other suggestions?

How often are you replacing the piston and rings?
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,820
16,889
Chicago
njkx said:
Ahhh, well...uuhhmm, never. At least I am honest.

approx 50 hours total on the bike.

That's where your bottom end power wandered off to. ;)
 

150rguy

I got fat bars!
Member
Dec 21, 2006
654
0
I might be wrong, but doesn't the smaller rear sprocket give it less bottom end and more top?
 

njkx

Member
Apr 13, 2004
209
1
thanks for all your help..gonna have a friend do the top end. Maybe a high-comp. piston. I have a feeling that will be all I need.
 

njkx

Member
Apr 13, 2004
209
1
Any suggestion on compression ratio for this bike? Also manufacturer? CP, JE and Wiseco seem to be the most popular.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,820
16,889
Chicago
All three companies make excellent pistons.

The Wiseco 4882M07700 is the piston I would recommend. It's a 12.5:1 compression ratio with 2 compression rings. I've put together a lot of YZ250Fs using this basic piston design and always have had excellent reliability and power.

Wiseco also has single compression ring race piston 4858M07700 that works really well but requires the rings and piston to be changed much more often.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,820
16,889
Chicago
njkx said:
Rich, isn't this the stock compression ratio? You don't think it makes sense to go with a higher ratio?

The actual compression ratio is rarely as high as the spec lists in a stock bike, but yes 12.5:1 is what Yamaha claims for this model.


These bikes run great at 13.5:1 compression and can still use premium pump gas without problems but you do have to contend with some additional heat. If you are running the bike on an MX track it's rarely an issue, if you are running it slow in the woods or in places where the airflow through the radiators can get restricted it can be a problem.

whenfoxforks-ruled said:
Rich, how much shorter life does the 14.8:1 have over the 12? And the 2 rings to 1? How much performance loss is there putting the ss valves in yamahas?

Fox, I've never seen a piston from Wiseco that is higher than 13.7:1 so I can't comment on reliability. Honestly I don't think you can get to a true 14.8 CR without a lot of other compromises in that engine so it might be a moot point.

The single compression ring Wiseco YZF pistons have service intervals similar to the CRFs. They start to lose power at 5-8 hours and I personally wouldn't run them for more than 20 hours in a RACE engine with a fast rider twisting the throttle. As you well know, fast riders wear stuff out in a hurry.

I've never seen a legitimate dyno run that I trust that showed a power loss running properly set up Kibblewhite stainless steel valves and their racing valve spring kit. I've ridden a number of bikes with Kibblewhite stainless parts and never felt a fall off of power either.
Kevin Staples did tons of dyno testing on YZ250Fs and found the Kibblewhite stainless steel setup was a bit more stable at high speeds than the OEM Titanium parts. The quality of the valve springs in the Kibblewhite kits compared to the cheapo OEM springs is likely the reason for this.

There are a lot of junk valve springs on the market that will surely kill power with stainless valves, and there are a lot of nit-wits assembling heads without the first bit of a clue. So my comments are specific to the Kibblewhite kits. I've installed over 100 of them myself and I think between Eric and I the number is probably close to 300.

Mismatched parts in a 13,500 valve train is just plain stupid but people do it all the time. Cheap springs are the usual culprit when problems arise.

Even the good parts will cause problems if they get installed by someone with no idea what they are doing. Setting up a 13,500 rpm valve train is fairly specialized work (assuming you want it to make power and be reliable), but people are convinced if they can get the parts in the bike and it runs, then all is well. :bang:

Unfortunately most of the guys who complain about stainless steel valves did the work themselves, and frankly should have spent the money to get the work done by a professional.

The sad reality of high speed four-strokes is some of the maintenance is no longer destined to be done at home in the garage.

Of course that won't stop people from doing it themselves and then getting on the internet to bitch about X companies bad parts when their do it yourself work causes issues.
 
Last edited:

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
I believe it was Eric who mentioned the 14.8, cause I found a 14, for the cr! I have no issues with the kibbles, but I have found out the importance of modifying the valve seats with them! I will never snivel about running race gas! It smells better, runs waaaaaay better, and I get to see Dave! It is just something does not sit right tossing ti valves in the garbage for ss! The biggest piston, kibbles and who to abuse for porting, Rich? I seen a few 08's alleging 13:1 out the door. So, they have to run race gas? The 13:1 in the 450 would not run on premium. Is there a magic number, or is it watch your plug? Going past lean to the land of detonation is not a lot of fun,,
 
Jun 22, 2007
31
0
replace your piston and rings, youll get the power back, and dont go using that 800 on an exhaust, as stated; get a high compression piston, your going to need to replace your piston VERY SOON anyways so its a great time to look into a high compression piston.
 

njkx

Member
Apr 13, 2004
209
1
OK...had a new piston & rings put in...13.5:1 ratio. Valves are in spec. valve train looks good....I'll try it out this weekend.
 

_JOE_

~SPONSOR~
May 10, 2007
4,697
3
Awesome, this thread needs to be read by anyone who wants to mod thier bike. Glad to hear she's back to normal.
 
Top Bottom