Which carb?

[blooper]

Question, I have an pm blaster that just came out of the shop from having a new top end built. Engine has been bored over to largest size before having to go to new one, weisco piston. I have a 34 PJ carb on now with a CFM intake box with uni filter and outerwears. Bike has DMC complate exhaust. I am in southern California in the desert at just under 2k above sea level.

*Question is this too much carb for the bike? I have a OKO 30mm in the box that I used on an old blaster, should I switch to that carb or leave the 34mm on? Bike runs well and quick, just a little studded at about quarter throttle.

 

[joeak47]

Is there any porting done to the cylinder?
Was the head re-chambered for more compression?
Why did you bore it to 68mm when there is no power increase and all the over bore/repair potential is lost?
The stock Mikuni VM26 will work awesome with what you have.
It even works great with porting!
That PJ34 (34mm venturi) doesn't add any power, and will be harder to tune due to the reduced air velocity that flows through it compared to the 26mm venturi of the stock blaster carb.

 

[Larry's Shee]

What have you tried tuning wise?
What pilot, air screw turns, needle # and clip position?

Is it when transitioning through that point of when held steady?

 

[blooper]

This engine has been had top end done a few times. The reason it was bored all the way was not for improvement but because it needed to to have a new top end. This engine has porting done on the intake side. Just got it back the other day so have not checked to what jets are in it now.

 

[blooper]

It studders a little when you rip through the gears and its right about the 1/4 to 1/2 throttle.

 

[Larry's Shee]

So this carb has been on for a while and always had this stutter? Or is it a new problem ? Or all new set-up ?

Start with cleaning carb, setting float level, recording jet and needle numbers.
Search for other set ups similar for jetting specs.
You could try a bit of tuning, try richening air screw, if no better, try leaner from original. Only try 1/16- 1/8 of a turn at a time.
If no improvement, go back to original setting and try needle richer, then leaner. ALWAYS try richer before leaner when tuning.

Joe is probably right about carb size vs. modifications, and velocities are wacky at thàt opening/rpm and you may never get rid of the stutter . Minor porting and low end pipe doesn't support need for that big of a carb, jmho.
If you try the OKO, search for some jetting specs first.

BTW, have you ever done a plug chop to confirm main jet?
Have you done a leakdown test since the rebuild?

 

[blooper]

OK just to clarify, I do not need help jetting or cleaning the carb. This bike just came out of well known shop in the area. I am asking if I should leave the 34 PJ on the bike or will I get a better product from the 30mm OKO. The studder could come from the bike being jetted with an LRD pipe on and it was switched out to a DMC. Just trying see which carb would be better for my setup.

 

[joeak47]

I would leak down test it for peace of mind. I would plug chop it to make sure your safe from a lean condition, then go from there. That 34mm carb is huge for that set up in my opinion. Big carbs don't make power. If you happen to have a stock carb laying around and a stock factory intake boot, perhaps give that a shot too. You should be able to work out that stumble with some work.

You mentioned that you had the top end gone through multiple times... what issues were you having to have to rebuilt it so often? Any idea how many hours you had on a top end before having to have to do it again?

What two stroke oil and ratio are you running?

 

[blooper]

The Blaster I have now was a pieced together blaster from my other 3 bikes before I moved out west. Same bike as in my profile pic with different engine. The shop that rebuilt said it has been bored before do to the size and that they had to extend to the largest size. No idea why the engine had work previously, this was just an engine I had laying around. No stock parts and no stock carb. I think I will switch the carb to the 30mm OKO I have and see if it runs any better. The bike runs great besides the little studder right now and has top of power through the gears, alot more than the stock blaster I had.

 

[joeak47]

I would try the OKO carb seeing that you already have it. I just thought you may have a box of parts with a spare carb to experiment with.

 

[blooper]

Ya boxes of aftermarket parts for Blasters, I think the only stock thing I have is the stock carb boot and front shocks. I will play with the 30mm some.

 

[Larry's Shee]

OK just to clarify, I do not need help jetting or cleaning the carb. This bike just came out of well known shop in the area. I am asking if I should leave the 34 PJ on the bike or will I get a better product from the 30mm OKO. The studder could come from the bike being jetted with an LRD pipe on and it was switched out to a DMC. Just trying see which carb would be better for my setup.

Nice attitude dude !!
Well, if nothing else others may learn something.
Had you mentioned the porting and pipe change (without rejetting) would have made a difference.

You seemed determined to use the 34, hence reason to help sort it out, because it sounded so close.
A little experimentation on your part would have answered your own question.

 

[blooper]

OK switched the carb out and tightened everything down, stock intake to jug boot, 30mm OKO carb with 2 to 2.5 coupler on the CFM air box. I started with 50 pilot and 155 main along with the N80 needle that comes with carb on middle setting. Bike start right up and sounds fine but the idle hangs for a minute when you rev it. I looked at my settings compared to others with same up I found so that's why I started with those jets. Air/fuel is at 1.5 out but I adjusted and the hano time does not go away. Searching lead me to believe it is running lean. Any suggestions on next pilot to try or adjusting needle?

 

[jbuckster07]

Hanging Idle usually means air leak.

 

[blooper]

No leak on the engine after I conducted a leak down test, unless it is leaking from the coupler on intake side of the carb and the CFM intake box? Could this be the only reason or is it possible the pilot is too small or the needle?

 

[jbuckster07]

A leak would have to be AFTER the carb. But if it passed a leak test, not sure what would cause it. Every time I have had a hanging idle its had a leak.