03 cr250 head mod

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#1
after exhausting enough bolt on mods, i've decided i want to go the motor mod route. everybody always complains about hondas having lousy bottom end on the 250's. i've noticed they also have the lowest compression ratio at 8.5:1. my idea is to simply have the head turned down to raise the compression similar to a yamaha's at 9.5 to 1, and have the ports polished but not altered. will i see a substantial difference in the bottom end power, or will it be miniscule? the tuners i've spoken to are all pushing for a port job as well. i understand they can get bigger gains with additional porting, but i'd sooner take it one step at a time, and so would my wallet.
this bike runs fine on pump gas, which i also like. theoretically, raising compression requires higher octane levels, so i'm logically thinking i MAY need race gas now. but but but-
i have many friends running the higher compression yz250's on pump gas without incident, so how is that? is the octane requirement also dependent on the characteristics of the porting?
 
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#2
The increase in bottom end power will be very small if you can notice it at all. The port height must be lowered to get more bottom by turning down the base of the cylinder. Of course the head would then need to be modifed. The new CR motors just don't have the port layout for bottom without some major surgery. Add a couple of teeth to the rear sprocket to help the roll on and out of corner responce.
 

Faded

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#4
Originally posted by CHET
The port height must be lowered to get more bottom by turning down the base of the cylinder.
You can kill two birds with one stone and run a thinner base gasket (Cometic makes one). This will lower port height and increase compression. If it doesn't work out you can always reverse the change. Also you can add a couple of exhaust manifold gaskets to change the tuning length of the pipe to gain some bottom end. Something to think about... :think:
 

jmics19067

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#5
raising the compression might increase the power from low to mid at the possible expense of losing top end.

I don't believe port hieght will affect detonation too much since compresion is after the fact of the ports being closed by millingthe head. How the ports are shaped and the airflow of the intake in the combustion chamber might. the shape of the combustion chamber will.

the only benefit of polishing your ports is that it will look pretty in your hands whne you do a top end . I believe it might help to keep carbon from sticking in the exhaust port, otherwise basicly a wasteof time and effort.

If you want to experiment on increasing the bottom end and your cylinder and engine cases are in great shape, you can try removing the base gasket. Use a gasket sealer that will work for yamaha and kawasaki crankcase halves instead. If your squish clearance is still good on the head and you like the way the bike feels you should be able to safely cut off the crushed thickness of the base gasket off of the bottom of the cylinder. This will lower the ports and increase compression. All you would have to do is use two base gaskets to bring it back to stock spec.If you didnt like the way the bike feels or you run into other problems don't cut the cylinder and put the gasket back in .

I have used permatex#2 instead of a base gasket for probably about 20 hrs till I smoked a top end from other reasons.Didn't realize I damaged a radiator hose one ride and cooked it.
 

bclapham

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#6
i think them hondas have has issues with pinging without mods????

i messed with cylinder heads way too much and milling them down hasnt done me any favours other than making the bike run flat. and remember, your friends with YZ's are all running the Keihin carb too!;)

its all about port timing, and several tuners wont turn the base of a cylinder- from what ive learnt on here, thats quite often the most important aspect of a good port job- send it to Eric Gorr
 

Faded

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#7
Originally posted by bclapham
i think them hondas have has issues with pinging without mods????
B,

I finding (in my limited experience) that most of the pinging everyone complains about is due to poor jetting. I know you know how hard it is to jet the Mikuni... :moon:
 

bclapham

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#8
yep, thats what i meant- its hard enough to set up them bikes anyhow without looking for more trouble
 

bedell99

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#9
I know when EG did my jug he turned down the base and then modified the transfers and changed the squish to allow the use of race gas. My suggestions is the send out your jug, put a new top end in your bike and live happily ever after.

Erik
 

BEEF706

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#10
On my 02 I milled the head first, (keep in mind I am at altititude) this produced a small benefit in lower mid power, I didn,t notice a lack of over rev (not that much of a revver anyway) and I switched to a 50/50 race gas mix to avoid pinging(a preemptive strike on my part, I didn't try it without the gas). The best gain I got was from cylender mods, the base was lowered (I don't know the numbers but the same thickness as a stock base gasket) and the ports reshaped, the bike pulls MUCH harder through the low and mid ranges, I think it signs off a little sooner on top, but it may just be that it gets to that part of the range much quicker, the bike is still smooth for a 2-stroke, but makes power in a much more practical rev range (for me). With the stock cylinder and head mods I still went to a 49 tooth rear with the cylinder mods I went back to stock gearing and it still pulls better out of turns. Hope this helps.