Author Topic: Jetting recommendations (Read 2391 times)

Dimsi · « **on:** November 07, 2013, 10:12:51 pm »

So I am in the process of rebuilding the bottom end (first time) after a piston meltdown. I am back to 65mm stock bore from 67mm when I bought it. I have no Idea what the jetting should be considering there is some aftermarket parts:

Carb is Keihin PWK 39mm.
Airbox is open with a huge air filter inside (not sure what it is) connected to the carb via radiator hose wide enough to force onto the carb.
Boyesen reeds/cage
FMF fatty with fmf muffler (not sure which).

I live in Arizona so I ride in summer-like weather. The jets in the pwk right now are 180 and 45. Although, I did melt the piston at WOT so maybe the 180 was not enough..

Any tips on where to start from as far as jetting? Any advice/warnings since I will also be doing the break-in while trying to get the jetting right?

LT250RWV · « **Reply #1 on:** November 08, 2013, 06:00:11 am »

Wat year 250 you have 65mm piston in. 87 to 92 stock is 67mm 85 86 is 70mm I think.

Dimsi · « **Reply #2 on:** November 08, 2013, 07:22:00 am »

Thank you, my mistake; it is a 91 LT and it is at 67mm, don't know why I thought 65...

GrkGuy · « **Reply #3 on:** November 08, 2013, 08:32:34 am »

thats the same jets i run in my 250.
i think its a 68mm q pipe, 250r eleminator intake with k&n. v2 reeds. but that is for 70 deg plus.
and mine runs great i thought i might be a bit lean, but plug chops look good.
go bigger and jet down till you get it dialed in.

where is needle clip at.

not sure if it matters but my carb is a air striker bored out

Rainman56 · « **Reply #4 on:** November 08, 2013, 09:26:41 am »

What year 250?87 big intake or 88-92 small intake?

MD2smoker · « **Reply #5 on:** November 08, 2013, 09:48:07 am »

Make sure you do a leak down test on your motor after you get the top end together. Your melt down could have been from an air leak, not necessarily jetting.

Dimsi · « **Reply #6 on:** November 08, 2013, 09:59:59 am »

It's a 91 (small intake).

Needle clip is at the 2nd position from the bottom.

Definitely will do a leak down test before I attempt anything. Appreciate the advice, keep it coming!

Dimsi · « **Reply #7 on:** November 11, 2013, 10:30:48 am »

I'm starting to think the 39mm PWK might be a little too big for my new stock-size piston. What do you guys think?

GrkGuy · « **Reply #8 on:** November 11, 2013, 10:57:42 am »

i dont have any porting in mine, just a bigger piston, and that really does not make any difference that you can feel
i think my carb is 41mm. its just thses carbs once you find the right jets you will love it.

Dimsi · « **Reply #9 on:** November 11, 2013, 11:09:06 am »

I remember when I had the piston at 39mm, power was somewhat lacking on the low rpm range; hope it doesn't get worse now since it will be moving even less air through the wide carb...

Dimsi · « **Reply #10 on:** November 11, 2013, 11:23:13 am »

I should add that the cylinder has porting done to in; don't know the details though but the shop that resleeved it told me it was a pretty good job..

GrkGuy · « **Reply #11 on:** November 11, 2013, 11:41:12 am »

sounds like you need to get carb dialed in,
i guess you can get it back together and run through the steps.
ride bike at 1/4 throttle, does it want to rev up or run away on you, or does it stumble.
try it at half and 3/4 throttle. once you get jets where it hold its rpm, then do the plug chop.

pm me you phone number, i have a good text i can send you that may help you.

how does it run if you pull choke.

you may need to move needle clip down also, might be lean in a spot where you ride all the time

Dimsi · « **Reply #12 on:** November 11, 2013, 03:02:27 pm »

Well, before the piston melt down, to be honest I never used the choke. Living in the AZ heat, I guess I didn't really need it and I never thought of checking how it rides with it on. While riding, it was very responsive and overall smooth, except sometimes when accelerating in 2nd gear in low rpm i would get hesitation but not always. I also never got it to idle quite steadily, the rpms would either be low, or would start surging, no matter how much I played with the air screw.
I did have a problem with the carb overfilling and dripping when the engine was off so I assumed that this is one of the causes for the idleing issues. Tried cleaning it many times with not much result. Now when I had it off and cleaning it again i noticed that there is actually a slot cut in where the needle valve sits, so I guess it is meant to drip a tiny bit even when seated? I took a small drill bit and turned it in the seat by hand few times, hoping to remove any other build up/uneven parts of the seating surface. Another thing I found out is that my bowl gasket was not sealing good at all so I used some black rtv and closed it. Still need to try and see how that will hold up.

I also noticed that my reeds were sitting slightly open (about 1mm) and I wonder what could that affect besides starting issues (even though it started fairly easy most of the time, 2-3rd kick). I have replacement reeds on the way.

You are right, it will be a lengthy process, especially now that all of these changes are happening: smaller cyl. displacement, new reeds, less leaking (hopefully) carb, decarbonized pipe and silencer, and repacked silencer... I bet it will be a tedious process. I am still waiting on a replacement bearing to arrive so I can finally put together the bottom end and close it up, so that is kind of holding me behind.

BTW, what difference a 39PWK offer compared to a 36mm carb, considering the boyesen reed valve inlet is only 35mm? It doesn't make sense to me.

Sending you PM right now.

GrkGuy · « **Reply #13 on:** November 11, 2013, 03:47:41 pm »

you can take a Q tip and a cordless drill and clean the seat. put polish on your finger rub it into the q tip. you dont want any excess polish on it.
then put it in there and give it a few good revs on the drill. this will help clean the area so you have a good seal. and when you put seat valve back in put some of the oil for your gas on the rubber o-ring to help lube it up. but you need to get floats set right and sounds like you need new reeds. this will be your hesitation fix. i hold reed cage up to light if you see light coming through they are bad.

Dimsi · « **Reply #14 on:** November 11, 2013, 03:58:41 pm »

That q-tip idea is great, opening the carb back up! My reeds were so bad, I didn't need the light check, they were clearly "gaping".

GrkGuy · « **Reply #15 on:** November 11, 2013, 04:09:30 pm »

if you check some drug stores the have wooden q tips. thats what i use when i did mine. if your not not in a big hurry send me your addy and i will send you a hand full of them.

Dimsi · « **Reply #16 on:** November 11, 2013, 04:17:20 pm »

Didn't know these existed, i have a walgreens around the corner so I'll just pop in there and pick some up. Do you use any type of metal polish? Or does it need to be specifically for brass?

GrkGuy · « **Reply #17 on:** November 11, 2013, 04:49:33 pm »

i just used mothers chrome polish, but i am sure you can use brass polish. i just used what i had.

and lmk about the Q tips, i can have wife send them out tomorrow. for FREE.. lol its just gonna cost a stamp to send them.
i am not hurting for $.50 cents. lol my wife runs a compounding lab so she orders them in for me.
i have seen them wiht the baby stuff before too.

Dimsi · « **Reply #18 on:** November 11, 2013, 06:12:37 pm »

Great, I'll give it a go and see how it goes. I already picked some qtips earlier, but thank you for the kind offer! I will pick up some of that mother's polish, might try it on the pipe as well.

Dimsi · « **Reply #19 on:** December 12, 2013, 07:28:19 pm »

So I finally started the atv for the first time today after the rebuild and it's running way too rich. I have replaced the 39/40mm PWK with a smaller 36 PWK and I am trying to get the jetting right. I have already dropped the pilot 2 sizes to a #42 and after warming up and riding it at 0-1/4 throttle it is still rich (smokes a lot and plug comes out wet.)

Is it normal to be so rich at such small pilot size?? I expected to be around 45-50, instead I might have to go to 38! Anyone jetted a 36 PWK on these quad before?

Again, im at stock bore (67mm) with open airbox and fatty pipe.

Dezsled · « **Reply #20 on:** December 12, 2013, 08:07:31 pm »

I've never jetted an lt250 before, but before you go to a 38 turn the air screw out 1.5 turns from all the way in. My trx250r with a 39 pwk uses a 58 pilot dgg needle 4th clip but it's in so cal sea level

What needle is in the carb?

Chuckie25 · « **Reply #21 on:** December 12, 2013, 08:34:15 pm »

Quote

That's why I called Q was for some jetting advice.
Starting off with
50 pilot
162 main
1 1/2 turns out on the air screw
Clip in the middle position on the unknown needle.

But that's a quad vent PJ38 with an unknown needle on a ported to hades engine and PSI pipe

Dimsi · « **Reply #22 on:** December 12, 2013, 08:40:37 pm »

Forgot to mention that the above results were with the air screw at 3-5 turns out... and the carb has been thoroughly cleaned and the floats adjusted, so it's gotta be the jetting??

Chuckie25 · « **Reply #23 on:** December 12, 2013, 08:50:25 pm »

I'd say the jetting would have to be off.

When I was talking to Q about the jetting he told me that the Keihin are picky on jetting, and the 250's are picky on being jetted and when you put the two together they are even worse on being picky and he also went on to say that the air/fuel screw on the Keihin doesn't do much for tuning, you've gotta have the pilot as close to dead on as possible.

Again, I've not started messing with my PJ yet because cold weather, wanting to get my bike back together to go play,, that's just what Q told me while I was talking to him.

Side note: my bike, with the TM34 and jetting it's got now, will go piiigggg rich from just sitting and idling or being under 1/2 throttle for too long. I've gotta hammer on it to get it good and crisp again

Dimsi · « **Reply #24 on:** December 12, 2013, 10:55:51 pm »

I donno, it's my first bike and the first time I do jetting, so I can't really compare. Although, from different posts here and there I've been left with the impression that PWKs are supposed to be simple to do.

Regarding your TM34, I guess you could check the float and the valve on your carb to make sure fuel level sits where it's supposed to before throwing any money/time for jetting?

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Author Topic: Jetting recommendations (Read 2391 times)

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