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Want to build a YZ144 engine

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I have a 2008 YZ125 that is used for Novice level motocross. I'd like to find another 125 engine and have it built to a 144. Are the 2005 and up engines interchangeable? What kit or machinist do you recommend? It appears that there are stroker kits out there and exclusively big bore kits as well. Just looking for any advice.

Thanks!

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From a budget perspective it is best to go bore only.

From a power perspective better results can be had with bore and stroke.

With proper port development the 144 as a bore only can pump out serious ponies, but the most important aspect is to make sure it actually turns faster laps - and that means it needs to come on the pipe early enough to have good corner speed (via early transition onto the gas)

Same engine from 05 to current.

The cases MUST be machined to fit larger piston - so either split the cases yourself (some dont I prefer to do it crank removed) or budget for that too. If you are goign to split cases - not a bad idea to replace the crank, main bearings, and seals anyways - dont want to ruin a new top end from a bad crank.

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I got a modded athena from Eric Gorr, He modded the ports,put his power valves in it and modded a stock head to run stock compression so there is no need for race fuel.Then I put a HGS "144" pipe on it,jetting was a breeze,If ya notice in posts where people seize them they were running aftermarket 125 pipes.Anyway I have been running this beast now for 2yrs trouble free.

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I got a modded athena from Eric Gorr, He modded the ports,put his power valves in it and modded a stock head to run stock compression so there is no need for race fuel.Then I put a HGS "144" pipe on it,jetting was a breeze,If ya notice in posts where people seize them they were running aftermarket 125 pipes.Anyway I have been running this beast now for 2yrs trouble free.

Thats good to hear, as we just bought a new Athena kit. Hoping to at least the preliminary install, then maybe have the motor ported and polished. Just wish there was a little more info on the net pertaining to the install. Maybe I am making it out to be harder than it really is. Looking forward to this project.

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Well did the install tonight. ( almost done. ) What would be the jetting specs for the athena 144 kit, stock pipe, 600-700ft elevation??? as always, thanks, Cman.

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I have run leaner jetting(main mostly) but stock is a good starting point and will be close. dirtrider and mxa built athena yz144 and they stuck with stock jetting or richer on main for some tracks so i would just try stock jetting first even though big bore should require leaner but even porting can throw that off.

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personally I would start off richer then work my way down what's a few fouled plugs compared to a top end siezure. My eric gorr 144 with high comp head I had to go richer on the main so your setup can determine your jetting needs.

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Thanks guys. I'll start off with stock jetting and read the plug to see how we are doing. Might go to the local Yamaha shop and pick up a couple larger mains just to have.

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i wouldnt run the fmf sst pipe on that bike. to much back presure! stock pipeis fine.

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personally I would start off richer then work my way down what's a few fouled plugs compared to a top end siezure. My eric gorr 144 with high comp head I had to go richer on the main so your setup can determine your jetting needs.

On a eric gorr maybe, but I have personal experience with the athena at 2200ft and went to a 370-380 and went 50hrs without a problem.So have no problem recommending stock jetting as a start point.

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On a eric gorr maybe, but I have personal experience with the athena at 2200ft and went to a 370-380 and went 50hrs without a problem.So have no problem recommending stock jetting as a start point.

You should stop pressing go leaner on the jetting spec's, I can't wait for your reply when they come back going hey dude you know that recommendation on jetting well guess what my bike locked up.

To the OP: Jetting is trail in error no one on here is gonna be able to tell ya the magic spec's better off safe then sorry.

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personally I would start off richer then work my way down what's a few fouled plugs compared to a top end siezure. My eric gorr 144 with high comp head I had to go richer on the main so your setup can determine your jetting needs.

it's always safer to start rich and work your way down. a spark plug is a lot cheaper than a new top end. my 125 started melting top ends after i installed fmf sst pipe and vforce reeds. i took me 4 pistons to get the jetting right because i started on a 410 stock main jet and worked my way up. i should have started with a 450 main and worked my way down and i could have saved a couple hundred bucks on pistons.

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