Advice on building a 250f mod engine

Step 1: Call Twisted

Step 2: Pay lots of $$$

Step 3: get 5 more hp that you rarely use 

Or just buy a Kawasaki.

Get a 350!!

I thought you didnt like the Kawi, heavier(rear tire?) etc.

FWIW unless you’re down to sink $8k plus into just the motor you’re not going to feel much of a crazy difference. The mods you listed may be good, but porting the head is just the tip of the iceberg in a “built” motor. 

Pick up a new yz250f and send it to TD along with 10k. A shit load of race gas and a gym membership couldn't hurt either. Give 'em hell!

Every pro 250F racer would swap their fully built race engine for a stock KTM350 engine if it were an option.

Talk to a few reputable engine builders and they will be straight forward with expectations and options.  More specifically, inqire with BakerSpeed and TMR at least.   

Same power curve as a yz250f but 10hp more lol

Race Motors are fun... Fun Factor is High, Expense is High, Maintenance Costs are Higher, Reliability is lower. Exhaust, ECU, Good Fuel....Start There.You will be shocked at the gains.I would reach out to a motor builder that is a "Honda" guy and knows the ins and outs of the brand. Typically, they will have various levels (expense) of build packages. 

High compression piston, higher compression valves (not dished) vp mr12 fuel, polish transmission remap ecu on dyno at all throttle positions and rpm ranges.

I never had a fully built 250F but still have/had fairly decently modded ones. Right now have a '21 YZ250F w/ a TD Dual-injector, exhaust, Powerflow etc. To me, that was the best way to get a nice chunk of power, without tearing into the motor and really having to worry about maintainence intervals and reliability issues. I know on the YZFs, once you start building for power, then you essentially need to plan for a Carillo rod and probably a complete clutch etc. So as a vet rider / occasional racer, it wasn't worth it to go any further. And the power it makes has been outstanding, with a chassis that does exactly what I want. I have also gone with GYTR kits, as they too will give you a nice boost but without really taking away reliability. So depending on your 250F you are going to build, I'd maybe just do a Vortex and whatever hot mods there are for that bike, before going too crazy. And ask about how they hold up once built etc. I really only have done YZFs, so I can't say for other models. 

I have owned/ridden 350s and they are just not for me. At first I thought it was, but then there were too many things about it that I just don't like. I may be in the minority there. I think each gear makes the bike feel totally different (even after playing with different combos.) I don't like the (lack of) recovery of that bike either. I had practice days were it was an absolute rocket. Then I would go race it and once I couldn't take the lines I wanted, made a mistake or had someone force me out of the throttle, it felt like I would lose so much time. At least the modded YZ250Fs seem to get right back into the power and the gearing is so much nicer (although way slower up top.) So for me, I'd either do a nice (but not crazy) 250F or just get a 450. But this current YZF that I have is like the perfect bike for me, which is part of why I haven't gotten a new one. And even when that time comes, I will probably just hang onto it, just to have, and get a 450.

Here you go...Mild porting.... build it yourself. 

https://www.athena.eu/en-us/big-bore-cylinder-kit-85-mm-290-cc-to-incre…

Back when I was riding 250f's this is what I did:

Exhaust, made no noticable difference.

Porting and Cams, could definitely feel it.

290cc big bore, some will tell you that you'll loose over rev or high rpm power, but this isn't what I experienced. It had more power everywhere in the rpm range, cams probably helped with this. I ended up running 14/50 gearing to help smooth the power out. 

My next mod was going to be a stroker crank to get to 315cc, but then I bought a 450, detuned it a bit by removing timing, and sold the 290f 7 months later because it wasn't getting ridden...

Oh, we read the post and are actually trying to help you. Maybe you have not figured it out yet but it's a very bad idea.

450

I’ve never had a fully built motor but my last bike was a gasgas 250f with a 2nd fuel injector and a tuned ecu. For me that was the sweet spot of engine gains without sacrificing reliability/needing to rebuild the thing every 5 hours

I don’t care what 250f brand you got,  leave motor stock, put a pipe, ecu and power flow kit on a bike and it will be night and day difference. shooot, You can buy 2 bikes for the price of a good mod motor these days.   

Suspension, then motor. Be prepared for a lot more maintenance when you build a full race motor. Shorter service intervals. Especially on a modded 250. 

I completely understand your interest. I would be asking this question?
Am I a local pro that is capable of winning races? 
If you are not having your engine built like this will slow down your average lap times. The maintenance will be extremely expensive and often to avoid a catastrophic failure. 90% of the riders would be way better off doing suspension. 
When I was racing back in the early 80s I was a local pro that could not win a local race. I was faster on a lightly ported 125 than I was on my full race engine. I ended up racing my full race engine just for the starts. It was easier to get a great start and get passed than it was to get a mid pack start and pass my way forward. 
What I just said is not easy on the ego but it is a reality. 
In saying all this if you do build a full race engine do not skimp on parts. When a full race 4 stroke decides to shit it’s self it gets real expensive! 

1st question is how much do you want to spend? How much for the year?

 How much more power can you handle?

Full blown new HD crank, treated transmission, porting, cams, clutch, exhaust, ECU injectors, air box and more you are knocking on $10,000 - $20,000 

The more HP you go the more possible weak links that will surface and break because they can not handle the additional HP.

Have same bike, also have the money now 😀

Talked to a guy that a have Suter clutch on CRF250R.
Really big difference !! So you use the power the that the motor already have

XPR box and cam chain adjuster.
Electric waterpump
Powerflow

Call Phoenix Honda and ask if they can sell the airbox to the public. 
See the thread on Vital

 

Why can't anybody just answer the question? 

He already has his mind made up and said he's willing to spend. 

I think some headwork, with a vortex ecu tuned for race gas and an exhuast is your safest bet without sacrificing durability. Just gets the most out of the stock components. 

Different bike but in a AJ Catanzaro video, he rides a 24 yz250f with the TD airbox with ECU. T4 fuel with a full exhaust. Said the power was unbelievable without tearing into the engine.

I have owned a 290 yzf before, crazy bottom end .  I could get great starts and loved tight corners .  Top end did fall off compared to a stock 250g.  I currently ride a yz250 f with hi compression piston, gytr head, cams , valves  and gytr silencer .  I can tell a difference over stock.  I did kill the crank and rod after 4 years .  Bought used cases and a wiseco crank and have gotten a couple more years out of it.  I have ridden a friend 24 Ktm 350 with kit suspension and my old yzf didn’t seem that far off.

"If anyone has any input or suggestions please let me know!"

I guess he wasn't really looking or didn't like what he heard.

First questions no one is asking is 

Age and honest speed, how much do you ride yearly and what type of riding do you do. 

2 routes.

First full mod and 2nd mini mod with a big bore.

You can build a full twisted national ready motor that's got a 10 hour interval but if your not fast enough to actually stress the motor longevity will not be an issue. For reference we built a buddys 2020 crf250r full national holeshot getter build. We did everything you could do including a dual injector. For a vet A/B rider he was getting 30 hours out of a set of rings. Everything else was in spec. Longevity only becomes an issue when your running local A or the B guys that are faster then A and banging them off the Rev limiter.

That said my opinion is if you build a full mod 250 motor do it in this order and stop when you get to the desired amount of money to spend.

Air box mods, vortex, hi comp piston, a dual injection and 50/50 t4 and pump fuel. 

This is the limit to a stock crank, rod and clutch. If your basket is in good shape I strongly recommend a rekluses troque drive clutch pack. It will help keep up with the power you have added. From here on you'll need a beefier rod. Have crank works rebuild the crank with a pro rod and plain bearing set up they have. If you go big bore make sure you send the piston with so they can balance it to the piston.

 From there I'm porting the head, then cams then flat face valves and springs(flat face valves bump compression up a touch) exhaust likley a good slip on to save some as most headers and pipes are pretty good now. Have the trans tumbled not rem or cryo. Those are  good but not worth the money for the avg riders. This is where you should stop for the most part. Anything beyond this has minimal return for for massive money, like most throttle bodies unless swapped due to the builders recommendations on the build package he has. 

If your bike doesn't need to stay a 250 get the big bore. Best bang for your buck. This bikes like them. The heads and exhaust have good flow for the extra cc's. It's also my recommendation if it doesn't need to stay a 250.

CW 270 kit, deck cylinder  to match stock compression, head matched to the piston, vortex, dual injection, 50/50race gas, rekluse clutch pack crank and pro rod balanced to piston and a slip on. This will give you more then you proably need. It will fill the power out from top to bottom. It's also a bit cheaper. 

If you have any questions feel free to reach out. 

It’s not cheap but you can have a bad ass engine with ti exhaust for just over $5.2k for a CRF250R. I have XPR Mods for 22 CRF250R below. 

- XPR Piston and CAM with Spring kit 

- XPR electronic water pump 

- XPR Porting. I bumped up compression but didn’t max it out.

- Yoshimura exhaust TI

- Vortex ECU mapped for Pro 6 

- Sometimes the webpage offers 10% off 

https://xprmotorsportsparts.com/collections/all

Boom