[Ande780]

Recently just installed a new head gasket, cleaned carbs rebuilt water pump, and verified that all the valves are good on my dirt bike. After assembly, I started the bike up and it felt like it had less than usual compression when kicking it over. It did start third kick though. I let it idle and within a few seconds it started to over heat but this time there was not coolant puking out and it was actually smoking out the manifold and mid joints where the pipe connects to the muffler. I verified that they’re torqued to spec. I re-kicked it over and this time it didn’t over heat as much but it was still running hot and back firing when I had the throttle open about half way. I adjusted the mixture screw on the carb because it appeared lean and that seemed to help a little bit but it’s still hot. I don’t understand why because I just cleaned the carb and all the needles as well as installed a new main jet. A 168 was in there since I’ve owned it and it ran good every time before until I had a head gasket leak. I put a new 168 main jet in there and didn’t adjust anything else. I’m not sure after cleaning the carb and putting everything back to how it was when I took it apart that all of a sudden it runs worse. Could recent work and a clean carb require more adjustments? I believe from studying up it’s probably a jetting issue but I don’t have much experience with that. Maybe it’s possible that there’s still a leak in the exhaust somewhere and that’s why it’s back firing.
Any suggestions?

[JimDirt]

My guess would be Pilot jet , unsure why you replaced main jet (with same size) but not the Pilot , as that is what is most likely to be clogged , unless the inside of the bowl looked like a swamp and was green I would not see the need to replace the main jet unless doing it for altitude adjustment ... The pilot however is very small in the opening and a spec of sand can clog it easily , , so start with the Pilot ....... As far as muffler leak , yes a mid-pipe leak would cause popping thru the muffler , it is basically a air leak , so it acts like a lean condition ...... so make sure the composite seal is in place and intact if it is the stock muffler/pipe and that the clamp is tightened down so the pipe absolutely does not move at that joint when the muffler mount bolts are removed , as it should not twist or wobble

Overheating:
Since you replaced the head gasket , then that means you had to refill the coolant ... the systems in these bikes are a "closed system" , which means they can get a vair lock , which means there is air in the system and it acts like a barrier , preventing the coolant from circulating properly ... What you need to do is bleed the system by opening the bleed/drain at the water pump (the bottom screw that has the washer on it) and basically you need to get all the air out so the coolant can circulate , once that is done , then removing the radiator cap will not add any air into the system , so that is not a worry , the air is trapped internally in one of the passages in the engine , and it needs to be forced out ................ that should help with the overheating , but remember that running the engine on the stand for any longer than 1 minute or so will cause the bike to overheat , the system is cooled by air flowing thru the radiators , so if the bike is stationary , then there is no airflow , if you are going to run it for a short time and want to prevent most of the overheating , try placing a large box fan in front of the bike so the air is directed thru the radiators , that will help , but not prevent it from overheating while you are working on it ....

[Ande780]

Sorry I forgot to mention that when I cleaned the carb, I pulled the main jet and I guess the previous owner must’ve twisted off the threads because half of the threads were on the and I had to extract the remaining threads from the holder. I will replace the pilot too. I live just on the border of Canada and USA in Alberta and thought maybe a 168 is a little big.... I typically ride in 15-25 C weather. Also with bleeding the coolant, do I have to run it a bit as I remove some coolant or just do it on the stand? And I’ll verify again the exhaust system. And should I be worried if the bike is a lot easier to kick over now compared to before? Or is that just cause things have been replaced? I verified the timing was correct when assembling so I’m not sure what could’ve caused that... thanks for the quick reply

[Leardriver]

A 168 main jet near sea level in cooler temps is just fine. You're not using the main very much unless you are a ferocious rider.

Why did you replace the head gasket? Was it leaking? Did you correct the reason for the leak if so? I rarely have a head gasket act up, and only replace them at new piston time.

[Ande780]

I took it off to inspect it and found that the previous owner had already reused it and tried to goop it up and it just wore out and leaked. Initially I was going to replace it anyway to eliminate all possibilities of the overheating and I also replaced the water pump as the water seal was bad as well as cleaned the carbs and what not. Now it doesn’t leak coolant when overheating, just runs hot and backfires.

[Ande780]

I also did notice that when i unbolted the exhaust from the manifold that it was loose at the mid joint to the point where I could almost slip it out and I had that clamped down to spec... is there any remedies or techniques in making sure it’s fitted correctly or to seal the gap even more? According to a person diagram, there’s a gasket where the pipe goes onto the manifold as well as where it connect to the muffler. I may just order both those in and then re-secure it and see if that helps. There is a dent in the exhaust pipe due to a lay over from somebody else so it’s possible that maybe it bent the pipe and it isn’t sealing quite like it should.

[JimDirt]

The mid pipe gasket/seal needs to be tight/sealed , so yes replacement is the best bet as they do compress over time and just will not re-expand , they are basically made from Graphite and will get loose after several times pulling the muffler off , usually for repacking , etc.

Part # 1 and # 13 is what you need:

https://partsfinder.onlinemicrofiche.co ... %20MUFFLER

The only way a bent pipe would be a issue is if it is collapsed , and/or if it is bent , but it would have to take a pretty hard hit to bend enough to compromise the muffler seal , I would expect a hit to the header itself near the head would gause a header to crack but generally not leak unless the bolts were loose .... same goes for the muffler , aside from a direct hit to the joint , I would think routine maintenance would be the most likely cause of it not sealing , along with not tightening the clamp properly ....

[Ande780]

I will do the repairs and let you know how it goes. Would that effect the compression in anyway as it feels really weak? And when driving the coolant to get the air out, should I keep the bike idling or just drain it and top it up when it’s shut off?

[JimDirt]

What will make it feel like less compression is usually the Decompressor adjustment , if it is off even a little , it will affect how much compression you have when kicking , what it does is lower the compression by slightly holding open 1 exhaust valve while you are kicking the engine over , then once it starts and the centrifugal weight , keeps it from holding open the valve and your compression goes back to normal

Also when kicking , you will notice if you slowly push the kickstarter down a few times you will feel 1 spot that has more compression than the rest .... that is where you usually stop and then allow the kickstart to return , then give it a full kick from the top , as at that point you are on the compression stroke (remember this is a 4 stroke , so only 1 of them is compression)

You need to burp/bleed the system before riding , otherwise it will continue to overheat , also with the engine off , try tilting the bike from side to side , turn off the gas flow and lay it most of the way over , then back to level , then repeat laying it the other way , then up again , then crank the engine over without starting it to circulate the water , then tilt it again , and kick over again , then start the bike (make sure to turn the gas back on and also make sure the radiator cap is on when tilting and before starting) once the bike fires , loosen the bleed screw , it is basically like bleeding brakes , you just crack it till fluid comes out instead of fluid and air , see if air comes out (it will kind of spit air/coolant) , just like a garden hose does when you first turn it on till all the air is gone , then you get only water , once you are done then let the bike cool down for about 15 minutes , then pull the radiator cap and check the coolant level and top off if needed and you should be good ....... usually when putting the engine together , if you add the coolant very slowly and rock the bike from side to side , it will not get a air pocket , it usually only gets one from trying to fill it too fast when the system is empty from draining the system ....

[Ande780]

Thanks for the help. I will get everything in order and see how it goes.