Any possibility of "bad" carb causing overheating?

[johnny248]

I know this sounds crazy, but is anything that can be "bad" on a carb that would cause it to run hot?

I know that the first thing everyone will say is either its jetted too lean, or there is an air leak.

Here is the thing....

This engine was running hot from the initial build. After pressure testing the engine, I found that the base gasket was leaking around the area where the cases mate. There was a slight difference in height between the two cases. So, took it down all the way to splitting the case. Re-sealed the case, glass sanded the case deck area to give it a completely flat mating surface so I wasn't depending on the gasket to compensate. Back together, new gaskets, new crankshaft seals, etc. Pressure tested again and it help pressure for a couple of hours without any signs of leaking. Still runs hot.

Then, I found that the carb adapter boot in the intake would leak if you pushed or pulled on the carb. Glass sanded ever mating surface of the manifold, including the surface on the case where the manifold bolts on. Still running hot.

Since then, I've installed a different manifold, again with everything sanded. I've installed a different cylinder kit, replaced crank shaft seals a second time. Removed and resealed the oil pump plug, and added some Honda bond. Can not find any source of an air leak ANYWHERE on this thing. Still runs hot. Have run all the way up to a 130 mj, still runs hot.

The ONLY thing that has not been replaced is the Polini 24mm carb.

I don't know what could possibly be wrong with the carb, but is there ANY remote possibility that this carb is not doing something, or doing something is shouldn't be?

I'm trying to find another carb to try out on this thing, but I really am at my wits end trying to make this thing rideable.

Looking at the jetting chart, it seems like needle taper takes over from a little past 1/4ish throttle all the way to WOT. Is it possible that something as simple as not having the correct taper could throw this off?

Any thing I could POSSIBLY be over looking, or missing, or something crazy that could be causing issues? When it doesn't have a huge jet in it, it actually runs really well, minus the temp problem.

As this bike currently sits, it has the following:

Polini Corsa BBK
Malossi MHR Crank
C16 Pipe
Stage6 Intake Manifold
Polini 24mm Carb , incl needle on center section (seems to be sputtery if I lower the clip) , 42pj, every MJ from 100-130
Polini Pod filter
Polini Variator with 5.6g rollers
Polini 3g Clutch

[johnny248]

It is pretty crazy. I'm still messing around with it a little bit. I just got back in from a "test" ride, and I have it running too rich, however I did have some better results this time. I'm not going to jump to conclusions and say it is good now, but I did have a much better run today than yesterday. I used some copper rtv on the exhaust slip joint which I knew was leaking a little bit, but I didn't think it was enough that could cause any problems. I'm going to downjet a size tomorrow and see what happens.

BTW, I found out two things about this pipe. First off, I ground off part of the motor mount/swing arm suspeinsion thing that is on the bike side. I hit a mega pot hole and that d*** thing put a dent in my new pipe. Really * me off.

Also, one of the welds leaks a little bit on the part that bolts to the cylinder. I can take a picture. I was going to smear a little copper rtv over that weld.

[johnny248]

I didn't realize how much that pivot link moves. I don't think that weld is a huge deal. I just put some copper rtv on it, and ill let it dry overnight and see what happens s.

On a side note, so far I love your pipe! It looks tough, sounds great, and so far seems to work really well. It has great pull. Haven't run it wot yet.

[1man8scoots]

What does the plug look like.....
Temp gauge is nice but plug chops are what matters....
I've said it 20 times and all people seem to rely on is a temp gauge....
I run both a stage 6 egt and a tto or vapor and I still do plug chops...
Also try to clean your carb and check the float level. Use compressed air after you use caustic carb cleaner or ultrasonic bath it.

[Wheelman-111]

Greetings:

If there's a crack in the carb body itself, it might produce an air leak that'll overheat with any jet. But a more-likely possibility is a loose barb in the manifold, or (in my case) a missing cap on a manifold barb. I was pulling my hair out when I noticed the lack of a cap on an open fitting. The other thing to know is your static compression. If it's high and you run pump gas - even Premium - there's almost no way to keep the temps down.

[Wheelman-111]

Greetings:

Polini builds its stuff to high standards indeed, but no telling if some P.O. dropped the thing on the concrete floor. Still thinking an air leak in an improbable place is the most likely explanation. It takes a shockingly small amount of air to foul the best jetting plan.

[johnny248]

I went for about a 20 mile ride today, with one step smaller main jet than last night. With normal riding between 0 - 45-50ish (Im guessing), the temps seemed to be better than before. They were generally in the 280-310 range. I did get a stretch of higher speed with slight incline and reached about 370. I am going to assume that it would get hotter than that if I let it.

That being said, it is a better result than I have had so far. It still feels a little bit too rich from the sputter spots at different throttle opening. I will drop it down again tomorrow and see whats happens.

I have another carb lined up that I should have Thursday or Friday that I am going to try out to see if anything changes. It is not a Polini carb, but it is still a 24mm carb. I'm not sure if it uses a different needle or not, but I am still trying to learn more about how needle taper seems to have more effect throughout the throttle range than clip position. Wondering if a different taper needle would be better.

I will see what happens with this other carb. If I continue to have the same problem then I guess I will pull the shrouds, variator, fly wheel, etc and have another go at pressure testing this engine. After that, I feel like I'm out of ideas other than to give it to someone else to figure out.

[johnny248]

Your right wheelman. I have yet to ever see a cracked carb body. Thats a stretch. The bike has always ran hot, its most likely something else. Johnny im Happy to send you a loaner test oko. Id obviously replace the polini if it was problematic, its worth a shot, but im really doubtful thats the issue. Their standards are unlike any company i have ever dealt with

I appreciate that, but let me see what happens with this other carb I have lined up. I know it is probably doubtful that this is a carb issue, but I just don't have any more ideas since it ran hot from the first time I built it, and I have gone over it numerous times, and EVERYTHING has been changed since them except for the carb, and it still is having the same problem.

I guess there is also the possibility that I just completely suck building and setting up engines.

[johnny248]

What does the plug look like.....
Temp gauge is nice but plug chops are what matters....
I've said it 20 times and all people seem to rely on is a temp gauge....
I run both a stage 6 egt and a tto or vapor and I still do plug chops...
Also try to clean your carb and check the float level. Use compressed air after you use caustic carb cleaner or ultrasonic bath it.

On checking the plug, do I want to run this thing wide open to the point where it is getting really hot, then kill it and pull the plug?

Would the plug still read rich if it infact was rich, but there was an air leak somewhere in the system?

Going to the temp guage (which you don't want me too lol ) If this engine was running with an ideal a/f mixture, what would a reasonable WOT temp actually be? Assuming it is still ideal a/f mixture, would it maintain that wot temperature basically forever if run wot in the same condition for extended distance?

One other quick thing, I have a factory cooling fan, but I should have removed the intake grate on the shroud, or any of that foam inside of it?

[DeanP]

Just to give you an example of a temp, I was just in the mountains here and we climbed a big hill. It was about a 2500ft climb for about 4-5 miles. I was wide open going 40ish and my temp reached 388f. Once the hill ended, the temp dropped back to a normal 325ish.

[johnny248]

Yeah. I'm definitely not going to be able to ride 4-5 miles wide open

[1man8scoots]

A properly tuned engine should be able to hold wot for a long time. There is no set limit or restriction. If the temps do not stay down there is an air leak or compression is too high for the octane. I can hold my 24mm carbed corsa for a long time, like g9 said I can get on the highway and ride. I also live near and ride in the mountains which is way harder on the engine to maintain speeds due to less o2. I ride in the rocky mountains where it climbs 4-6k in elevation to top out over 11k feet. With out coming near to 388. I average less than 375 by time we hit the summits.
I also know how to tune for my riding style. I tune for 75-100% throttle. Ya it's boggy down low but I don't worry because my plugs are perfect. Tune with plug chops not the d*** gauges. If you dont know how to do a plug chop, do a little research. If your plug is covered in oil and fuel you're too rich and you have an air leak still. It's a 2 stroke, even hill billys can build them right.
You said you live near cruising, can you hold his wot with out over heating? I know I can on all of my bikes.

[johnny248]

I swapped out the carb. I did try some 110 octane before and it didn't seem to help. I'm kind of stuck. so.....

If there is anyone in the Metro Detroit area who is a pro and feels like being kind enough to help and take a look, I'm accepting all offers lol

Until something happens, I guess this thing is getting parked.

[johnny248]

If the engine did have an air leak, would the plug read lean?

And, lets just say for sake of the matter, if I was able to get this temps under control with like a 134 or 136 main, what would that mean? Can you ever compensate for an air leak with fuel? or is it always going run hot?

[1man8scoots]

You would still have a lean plug if you had an air leak and was to upjet to compensate. But you will have a lot of residue on the plug. That's why you should do a plug chop on a brand new plug to see if it's an air leak or you're just lean. Sometimes too rich and an air leak and a plug that is oil fouled can look similar minus the oil fouled would be black and the air leak plug would look yellowish to light tan.
If you've pressure tested or leak down it and it passed you have a compression issue or clogged up pipe or not enough air flow, or bad cdi (never been the cases but could happen), or plug is too hot in number, or something is causing a strain on the engine (rider weight, brakes dragging, too tall of gearing), or using too little of oil, or the scooter gods have forsaken that scooter.