Big bores 250cc

[bajatrailrider]

Terry Has ready our all bore 250 kits we bought 3 of them. So now the fun of what cam, what comp. How much to grind down dome of piston for no ping on pump gas. Mikes bike will be around 270cc. Since I have two 230s one is the stroker 254cc other is stock 230. For now I may just make the stocker bore only 250cc. Let the $$$$$$$   I mean work begin.

[bajatrailrider]

So ADN  SteveThe  Is using the EO big bore 250cc I think it is 11.1 or 11.5 Also web 89a cam ported head stock valve size. He has no ping on pump fuel.I was just wondering What cam combo to run pump fuel. It would be nice to have 11.1 comp. With 89a cam do you need stiffer valve springs , new short valve guilds. I already know Mike would only use steel guilds.

[SteveThe Snakes]

The 89a cam is a drop in cam. Heavier valve springs are needed but no valve guides. 

[adnohguy]

If your asking me Baja, I would never use an 89a no matter how much compression, porting, valve size, carb size, exhaust brand or header diameter, bore size or stoke length. There are many cams available now that will make an 89a boring and Ho-hum.

It (89a) was designed for an 05 & older 150f up to 175cc only. It was a compromised design with consideration for overlap, lobe centers and lift in a STOCK HEAD. (stock heads won't flow any more lift or support any more overlap or tighter lobe centers than an 89a)

The design of 15 year old 89a will hold your 247cc and above engines back from there true potential.

IMO:

A really good cam for 247cc and up is a custom grind from Terry based on Mike Coe's "MC-1" (Mega Cam #1)

Terry can take that cam and tighten the lobe centers (like mine) (for mid range punch), or leave it alone, or widen them more for a super wide power band that builds more and more power, very Linear and progressively from idle to 8K rpm. No surges, no "on the cam" no lag or waiting for things to start happening. Power is always there, no matter the rpm. (Easier to set oversized valve clearances on overlap also)

Ask Terry about different options and let him know what characteristics your looking for.

All versions of this cam have the same exhaust cam lobe as the 89a and again, Torque from just above idle till 8K rpm.

A stock Size intake valve will limit higher rpm pull/power with anything over 262cc (69x 70) so a 270 will need a 1mm oversized intake valve and some really good port work for best results.

This cam can run the same springs as an 89a, can use stock exhaust valve and guide but needs a .025 shorter (minimum) intake guide under the valve spring because the cam has more intake lift than an 89a.

If you must use an Iron guide, you could whittle on an OEM guide, but Terry's Bronze guides are absolutely great quality and totally reliable.

Be sure to "degree" your cam properly. (Every little improvement helps with smiles.)

2d advance for even more low rpm torque as well as it holds its tune longer as the cam chain wears, or straight up for more mid to 8K pull.

Do get a Tokyo Mods manual chain adjuster.

I understand that your limited on fuel quality, but these engine ABSOLUTELY LOVE Compression.

I have used up to 13.5-1 with excellent results.

The VERY LEAST Compression that I use is 12.5-1

Try to run at least 11.5-1 if you can stand it.

You have three choices if you have detonation.

(I use use 100 California street legal Sunoco)

Stock 230f rev box has the most mild, least aggressive advance curve. (For your fuel quality at Low altitudes)

The stock OEM 2005-older 150f box has a middle aggressive advance curve ( I use this box below 4,000 elevation) you may want this box above 4K elevation with your available fuel?

The "Pro Com" has the most aggressive advance curve. (I use this box above 4K elevation)

May be good for your fuel quality above 6K elevation ?

If you still have detonation then try 11-1 [emoji53]

Remember you have 15 lbs of rotating mass with a stock crankshaft and flywheel. 13oz is very easy to remove from the flywheel. With a stock crank even more can be removed. (Nothing, easily, can be removed from the crank with a stroker)

Even larger diameter "stepped" header than your running now will give your 270 more pull, from the bottom to the top, that is noticeable using the "seat of the pants" test method.

Good luck, this new engine, depending on how you build it has the potential to REALLY make your smiles HUGE without loosing all of the favorite characteristics of a 230f.

Enjoy.

[bajatrailrider]

Thanks for info  In past engines (not 230s or Terry's valve guilds)  The bronze valve guilds did not give us the long life we like. So if you can make stock guild shorter that's a plus. Is the EO piston 11.1 or 11.5. That is what Steve uses with no ping on pump gas. Its same old story in Baja on Fuel.We cant deal with race fuel or carry Octane booster. Like yesterdays ride a long one. Started with Mexican Hi test, Then at half tank fuel. We all got half a gallon reg sold out of milk jugs to get back. On Mikes 230 Build 6mm stroke/Terry's 250cc piston.We would like the right cam to start with as to run pump fuel at 11.01 or 11.5. We do not want to take motor back apart to cut dome down again.On my 250cc kit from Terry I may not take my 254cc stroker back apart. As its running strong without problems. Im thinking at this time my small wheel 230 put the bore only 250 kit in it. I already have the pwk 28mm carb not installed.Im thinking Terrys 2.5 cam would go with that build. Another yoush Header/muffler I love those pipes.Port the head maybe big intake valve.