Weird Exhaust heat

[MikeHawk7039]

I'm new so if I post in the wrong place sorry.
At night I realized when I'm cruising 55 on the gas going up hills or on the gas hard for a while my exhaust starts slowly getting red hot but only at the back of the "expansion" part and starts to creep halfway up it. When I look down it's not hot at the header close to the motor. I thought it had to be lean but I'm running 40 pilot with a 120 main, I had a 125 main and thought maybe it was too rich
I have a hawk 250 with Nibbi 28fl carb, needle at middle notch, nibbi filter, and ebay exhaust. I've check the valves too and set at . 05mm. I can't figure it out??

[XLsior]

Seems like the catalytic converter is doing some converting and forming hot spots. This could be a location of more/less flow obstruction.

Might have been fouled under a rich condition and then burning off buildup under a lean condition.

Check the spark plug to get a glance at the general lean/rich condition.

[MikeHawk7039]

It's an aftermarket catless eBay exhaust, the plug was dark like it was rich. I can only assume it's rich and it's just burning off right there? Maybe I should try a 115 main? It runs great though, I know normally red exhaust means it's lean

[XLsior]

if there is no catalytic element installed in the chamber then perhaps the empty expansion section is creating a turbulent hot spot...

Are you sure the 125 was rich...?

Think my 28fl carb on stock cylinder with head port/no cat was 38 or 40pilot/130main mid clip...

[Megadan]

My experience with tuning the PE28FL having moved from a PZ30B, was that the pilot jet size remains the same (38 for mine with a ported head, standard bore, cam, exhaust) the idle mixture adjustment was actually very similar too, even though one is and air screw and the other is fuel - approximately 1.5 to 1.75 turns out - and the main jet needed to go up 1 full step (5 numerically).

Others have reported similar, using a 120 main jet on a bike with just basic bolt ons type upgrades, with some people at sea level jumping to a 125, but also stating it was a bit rich. A PZ30 with those same mods will almost always be around a 115 main.

Even with the big bore cylinder my pilot jet and idle mixture really haven't changed.

I have no doubt the main jet is too rich at a 125, especially given the description and the scenario. It is also worth keeping in mind that the PE carbs have a more aggressive needle taper than the PZ carburetors, so the mid throttle mixture can also be tricky to get right, often just slightly too rich in the middle slot. I have had great luck with moving the clip up one slot to the second from the top to lean it out.

EDIT: Just thought of this too. What is your sprocket combination? If you are on a stock 15/50 sprocket setup, cruising at 55 means you're RPM's are getting up around 7500-8000. High RPM, wide open throttle, for a sustained pull, will absolutely start heating up the head pipe like that too.

[Thumper]

It's an aftermarket catless eBay exhaust, the plug was dark like it was rich. I can only assume it's rich and it's just burning off right there? Maybe I should try a 115 main? It runs great though, I know normally red exhaust means it's lean

Did you poke a coat hanger through that bulb before you installed it to check for a honeycomb? It sure looks like a cat bulb...

But, an open resonator chamber is supposed to reduce back pressure at the head pipe, which would be great if that's what it is.

[MikeHawk7039]

My sprockets are 17 and 45 or 46 whichever sounds right, it's turning 6500 at 55, I tried the 115 and it was definitely to lean so I tried a 130 (the only thing I haven't tried) and it barely gets hot after a big hill. It keeps pulling fine with a 130 and doesn't bog. That's gotta be to rich though? There's no engine work done so I don't see why it would want that much fuel? I'm in Arkansas at about 600-800 elevation

[MikeHawk7039]

I've sprayed carb cleaner around the carb to make sure there's not an air leak, it idles great too. I'm trying to think of anything that could be wrong because a 130 theoretically has to be to big. Its only the "cat" part that gets hot. I made sure it doesn't have one before I put it on. It's the same ebay exhaust I've seen everybody else put on

[XLsior]

130 was about right for me on a standard piston.

[Bill Hilly]

Maybe the chamber /bulb is just way thinner walled than the head pipe.

[Megadan]

The 130 may very well be the right jet for your bike then. No two engines and situations are the same. I was running a 145 on my PE28FL with my cam and port work. The only real way to know would be to do a WOT plug chop and read the plug, or install a wideband.

[Megadan]

Did you poke a coat hanger through that bulb before you installed it to check for a honeycomb? It sure looks like a cat bulb...

But, an open resonator chamber is supposed to reduce back pressure at the head pipe, which would be great if that's what it is.

It's not a cat on the cheap pipes. I have had 2 of those systems, and it's supposed to be a "power bulge" used to manipulate resonant wave tuning on the head pipe by creating an anode at a given distance from the exhaust valve in order to aid scavenging at a certain RPM point.

[MikeHawk7039]

Thanks for the help, after riding it some more the 130 seemed to do the job. I had searched the forums for hours and seen no one else had that problem so I thought something had to be wrong. Bike runs great and exhaust doesn't heat up. I just thought it was odd it was just the bulge heating up