Quote:
Originally Posted by Megadan
Slight apples to oranges considering our engines are vastly different for several reasons, but a 42 pilot is pretty darn big. I run a 38 with a cam and port work with a full exhaust. But, that is again apples to oranges given our different engines and mods, so neither here nor there.
Also, as aggressive as the needle taper is on the PE carbs out of the box, being all the way in the bottom notch is impressive. I am in the second notch lol. Admittedly, my main jet is quite big compared to what you are running, so that plays some factor, but even most near stock engines I know of don't go below the middle notch.
My method for getting a carb figured out is to first start with the pilot jet and have the needle in a neutral position (3rd notch). The main reason why is to try and eliminate variables. I actually adjust the needle last.
I figure out the pilot jet by first sorting out the idle mixture. This can give a clue into the pilot jet being potentially big or small. The 2 turns out is promising, but it never hurts to verify. With the engine up to temp - having run at least 5+ minutes - I will check and set the mixture based on the idle drop method. Simply turning the air screw in 1/4 turn at a time to richen the mixture until the idle drops off. I then back off 1/4 turn to the last adjustment where a steady higher idle was present. If you want to go a bit leaner you could back off another 1/4 turn, and as long as you don't get any lean symptoms like a hanging idle, this is a good range to be in. Generally, I aim to have this adjustment more than 1 turn out and less than or equal to 2 turns out.
Once that is achieved you can actually test to see if the pilot jet itself is lean or rich by simply riding the bike in a steady off idle to 1/8th throttle position (trying to keep the needle out of it). I find puttering around my neighborhood in second or third gear on flat ground at 10-20mph is a good way to achieve this. If it hesitates and bucks a bit, it's like too lean. You can always hold this state for a fair amount of time and do a plug chop to read the plug if desired.
Once you have that sorted, we gloss over the needle and do the main jet. We do this, because like the pilot on the bottom end of the throttle position scale, the main jet is the single biggest factor on fueling past 3/4 throttle. WOT plug chops are the poor mans best way of reading main jet fueling state.
Once you know for sure the main is a good size, then you can determine and make needle adjustments. The reason why? The needles fueling curve is greatly influenced by the main jet. I run a 135 main jet on my bike, which is a good bit bigger than a bike without all of my engine work, so the part throttle fueling on my bike is a lot richer than another persons might be. That is why I have mine in the 2nd notch.
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I did pretty much exactly what you described here. Starting with the needle in the middle position, and riding the bike at different throttle positions to see if it's surging popping or giving any other weirdness. Then after riding I pull the plug and see what the plug looks like. If I go smaller on the pilot I get terrible bog off idle. Same if I go up one size on the pilot. Regardless of needle position. I get about half of a turn on the air screw toward the Richer side before the idle changes. And about half a turn leaner before the idle changes. Right where it's set the idle is perfectly smooth with no rich or lean idle. And that was letting it idle for about 20 minutes and then pulling the plug to verify. The needle position has me baffled. It should be in the number five slot as per what the book calls the number five that came with the carburetor. Meaning the needle is at its highest position toward the throttle cable. In this position I have just the slightest hesitation when idling and then cracking the throttle well riding the bike. If I drop it down even one position, that slight hesitation turns into a bog. The farther down I go the worse it gets. When I go up on the main jet I get choppy wide open throttle. When I go down on the main jet, I get the same feeling you get when the bike is running out of gas. It basically cuts out and won't go at all.
The part that has me incredibly stumped is the air filter. When my dad suggested putting a sock, just a normal sock that you wear on your feet, over the air filter I decided to give it a try since I have been trying to get this bike tuned for 12 straight hours. Putting the sock over the air filter did make an improvement. It greatly reduced that hesitation but not completely. And it caused an almost unnoticeable difference in my top end, reducing power. So for the time being I ordered two completely different air filters that have a slightly smaller filter media to see if I can cause just enough restriction to solve the problem without causing too much restriction hurting my top end. I've heard other people have had issues tuning a carburetor with a pod filter, but I didn't expect it to be this much of a nightmare.
The reason it takes so long on this bike is you have to remove the seat and gas tank to get to the carburetor. Both seat and gas tank have to be removed just to change the needle valve setting. The entire carburetor and intake manifold have to be removed to change the Jets.. absolutely no room to work on anything while it's attached to the bike. That being said I have taken this bike apart so many times yesterday, I can guarantee you that I could do it blindfolded.