Help jetting for a big bore bore


New Member
Apr 8, 2024
Hello , I have an 04 blaster that I’m having issues with jetting. currently it has the stock main, which is a 230 and it has a big bore fmf, fatty pipe and silencer and a cold air intake. My elavation is right around 300-400 ft above sea level. It does start right now but only with me holding the throttle and it doesn’t want to idle without the throttle and it bogs when I rev it. Hope that some of you can suggest where I should start with the main or the pilot, thanks.
While I am not intimately familiar with Mikuni jet sizing as I am with Keihin, I can say with confidence you should start with 4-5 steps richer on the main and about 2-3 steps richer on the pilot. You'll need to source a small assortment of genuine jets to dial it in. Jets R Us and PJ Motorsports are good sources. Don't buy chinesium jets. There's no way of knowing if they are sized correctly to what is indicated by the numbers.

Also you say you have a big bore. Are you talking about a stock cylinder with an oversized piston or an actual big bore cylinder?

Do not run the engine any more until you get it jetted fatter. From what you describe it sounds to be suffering a significant lean condition and can lead to engine damage if you continue to run it this way
thanks for the information but how much is one step about, like an increment of 5, 10 or 20? Also it’s the original cylinder with an oversized piston.
No problem.

As to jet size increments, as I said above I'm not real familiar with Mikunis jet sizing. Check PJ Motorsports or Jets R Us for jet size increments. You'll need to know what your carb model is to find the correct jets for it.

So a bored over cylinder and piston, not an actual big bore, this will require very little if any jetting adjustments compared to an actual big bore. Your exhaust system and air box lid being absent are the main culprits of your need to re-jet. This established, I'd still recommend getting a range of jets going appx 5 steps richer mains and appx 3 steps richer pilots. You'll never go wrong having more jets on hand. I have accumulated pretty close to the full range of Keihin mains and pilots for PWK carbs and it has come in handy on plenty of occasions
Hey I have some questions about head modification and potential power gains. Sorry if I kind of steal this thread.

I was looking at some blaster modifications for engine performance and am making a list of things I want to do. I currently am running a fully stock motor with the exception of a fmf fatty and powercore. I am going to be getting some work done on my cylinder soon because it is running extremely low compression (75 psi). So while the engine is out I wanted to do some things. I know for sure I will be doing an oil injection block off and removing the air box lid. I was also considering sending my cylinder to ken o Connor for some porting work. With that I would also like to have my head modified, but I am kind of in the fence between having ken do it or ordering a tach out performance head. I’m not sure which would give more power but not destroy my wallet. One more thing I have thought about is a stroker crank, probably a +3mm. Would this have any significant horsepower gain? And are there any other mods that you guys would recommend? Thanks in advance.
I trust Kens work. He rebuilt the crank for my daughters Blaster. Price was very reasonable and turn-around was fast. He knows his stuff.

A stroker crank will always add a good bump in power. Make sure and freshen up your bottom end of you are going that route. Crank bearings and seals, counterbalancer bearings. You'll be splitting the case to replace the crank, do those while in there. With the stroker crank, you will likely need to run a cylinder spacer if staying with the stock cylinder. I know 100% that running a stroker in a 250r requires a cylinder spacer, not 100% sure with the Blaster but I'd say very likely you will. At the very least, port timing will be off with the longer stroke which I would want corrected.

Also, if you do a stroker and other mods, upgrade your carb to at least a GENUINE Keihin PWK28. Amazon/eBay Chinesium knock-offs are hit and miss. Not to mention the fact you are buying a product from people stealing/copying the design of the real McCoy. Support those who support us. It's well worth it in the long run. Buy a legit carb
Thanks for the info, I am always up for suggestions. I think that is the way I will go. Later on I might buy a better top end pipe like toomey or fat bastard. I drive gravel roads and play riding with the occasional light trail so I think they fit my kind of riding, and I just like how they slap the powerband. :)
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