Help with proper jetting

C

chuey_316

Member
Feb 9, 2010
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I just picked up a 98 blaster. It has a full FMF exhaust, and a cone style filter coming straight off of the carb. The air box is not being used.

The previous owner said that he just replaced the top end. I want to be sure that the previous damage wasn't caused by not rejetting after the piping and filter change. I haven't opened the carb yet to see what main is in there.

Can anybody tell me where to start as far as proper jetting? My elevation is 761 ft.
 
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direct on the carb filters are best left to wide open throttle only and drag machines.
you'll have better luck with the stock airbox or at least a 4-6" tube between the carb and filter, to allow the air to straighten out and flow over the jets better.

not taking the jetting headache a direct filter causes, I'd buy 270 thru 310 mains
start large and plug chop down
https://www.blasterforum.com/threads/how-to-plug-chop.53237/
 
Thanks. Since the the intake boot is missing, and the previous owner blocked off the opening to the air box, I'll probably run to the hardware store tomorrow and get some pvc to run from the carb to the box. Going to try to get out to the garage and pull the carb tonight to see what main is in it at the moment.
 
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Thanks for the link. I was checking some out on ebay as well. Any idea if the stock boot would give better performance than just a pvc tube? I'm on a non-existent budget right now, but if the pvc would hinder performance, then I'll go with the stock boot. Otherwise, I'd use pvc for now, then purchase the stock boot later.

I was able to go out and check the carb. It's got a 250 main right now. Also the oil injection is eliminated.
 
the pvc will help with what you have currently, minimal difference in performance between the 2
I wouldn't run it again with that 250 main, way too small for those mods
 
Thanks Awk. I found a guy in my area that has about 20 blasters and tons of parts. Got a boot from him, and grabbed some jets from the cycle shop.

Got a 290 in it now with no filter (have to order one). Seems to run ok, but after about 10 minutes it started to surge a bit. Sounded like when you hit a rev limiter, but it wasn't revving high though.
 
no air filter ?
don't run it like that or she's gonna blow, from too much air or dirt getting sucked in
just wait until you get a filter, then do the plug chops as I linked you to above
 
Ok, got a new filter in it, and went for a WOT run and it wouldn't get over 49mph, and sounded like it wasn't fully revving out. Took the lid off and it seemed to be a little better, but I was back home at that point so haven't been able to do another WOT run. I changed the jet from a 290 to a 270, and put the lid back on, and it feels a little better, but I only have room to get into 3rd gear.

I still need to put it back on the trailer, and head out to the country to do another run. Trying to get it as good a I can before doing some plug chops.

Does a 270 sound somewhat close for my altitude, and pipe? It's a fresh top end, and only has a 1/2 tank ran through it, so I'm a bit worried about running WOT while still trying to determine the jet size.
 
Just went out again. Same thing. Low end feels good, but it's not revving out on the top end. It's hard to explain, but it's kind of like a surge. I can feel it bucking a bit.

I'll see if I can get a video tomorrow.
 
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Also, not sure if it means anything, but the exhaust doesn't smell anything like any 2 cycle I've ever owned or been around. Not real sure what to compare the smell to, but nothing at all like a normal 2 stroke smell.

I figured maybe it was the oil that the previous owner used. But when I found out today that the petcock leaks if left on, and opened the tank to find it dry, I mixed a fresh can and it still has that awful smell.
 
Another problem I'm having that I'm sure is affecting the way it's running, is that the boot from the air box won't stay on the carb. I've tried everything, but it keeps pulling off the back of the carb. Clamps are good and tight, and no cracks that I can see on the boot. Only thing else I can think of is to get a rubber hose to fit over the carb and boot, and seal it that way.
 
chuey_316 said:
Also, not sure if it means anything, but the exhaust doesn't smell anything like any 2 cycle I've ever owned or been around. Not real sure what to compare the smell to, but nothing at all like a normal 2 stroke smell.

I figured maybe it was the oil that the previous owner used. But when I found out today that the petcock leaks if left on, and opened the tank to find it dry, I mixed a fresh can and it still has that awful smell.
Click to expand...

Stinky exhaust, especially with white smoke means leaking crank seal. Unless YOU know they've been changed, they are 12-29 years old. Consider it routine maintenance.

Clean outside of carb and inside of boot with brake cleaner. Consider something like 3bond to glue it. NO SILICONE !!
 
Thanks. I'll pull it out tomorrow to clean the carb and boot, and take a good look at the exhaust. Also did a compression test, and topped out at 150psi cold. Not sure if that's about normal, or high. I've searched for what the compression of a fresh top end should be, but only found conflicting #'s from different people, ranging from 127 to 160.

Any idea what octane I would want to run with my compression?? I don't want any pinging or detonation.
 
Octane is also relative to elevation/compression. For you a new stock engine should be@125 psi. Not 100% sure but seems I've seen 150-160 psi for pump premium.
 
Thanks. Reading through some of the threads now, and I'm seeing multiple post stating that to get 150 psi, the head would have to have .030 milled. I'll do some more reading tonight. I'm also going to do another compression test tomorrow with my other gauge just to rule out a bad test.
 
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Don't just mill it ! Re-chamber it. Either send it to Ken O'Connor, or watch youYouT video by AK-47 and do it yourself.
There's more to it than just increasing the compression, IF you want maximum power increase.
 
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Did another test with the other gauge. Topped out at 154. I'm not looking to do any head work. Just wondering if one of the previous owner's may have done some. 150+ psi seems a little high for a stock head from what I've read so far. That was a good vid on YouTube btw. Makes me wish I had a lathe.
 
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