questions for the master mechanics...
- Thread starter deacon
- Start date
Deacon,
Comfortableshoes, Egor, 5-7Heaven, and some others here have some good thoughts. You suspicions about an air leak somewhere are probably correct based on what I've read in this thread.
The bit about having used the motor without the shroud is a red flag...it tends to warp part of a cylinder wall. (I've seen this happen on so many VW's where those bottom baffles were forgotten when a motor is overhauled. They go just above the pushrod tubes and deflect the torrent of air from the fan shroud to make it follow the other 90 degrees of the bottom sides of the cylinders. Often when you hone an old set of VW cylinders and see an area on one side of a cylinder wall that the hone isn't touching.....it's cause that cylinder may have came off a motor that was put back together where the baffle(s) was forgotten. It's bad for a 4 stroke VW cylinder but a disaster for a 2 stroke cylinder)
My first Yamaha was a small thumper 90cc. I could ride it a bit and it would foul out and not start. After it cooled down it would run again for a bit. Fortunately a buddy of mine had a spare cylinder off his that he had bored out and fitted with an oversize piston. It was ready to go. We pulled the cylinder off mine and installed the new piston/rings, bled the oil pump and got it up and running. We cleaned up the old piston and jug and used feeler gages to check the cylinder clearance an it was way over. (what was happening was the intake mixture in the crankcase was spewing up past the rings as the piston went down on the power stroke spoiling the direction of the intake charge often wetting the plug and fowling it and losing compression. Riding up a hill when losing rpm often brought this on. )
When the guys at the shop bore the cylinder to fit it to the first oversize they have to hone it and check the wall clearance. If it looks good to go...they then take a small die grinder and edge all the ports to raidus them out from the cylinder wall into the port. (so the rings don't somehow catch on a sharp edge and break)
The thoughts on the compression check is definately a good indication. Yes, do always hold the throttle wide open when checking it. The "pro" hot rodders often do a "Leak Down" check where they pressurize the cylinder and watch to see how well it holds the air pressure with the piston at different positions (above the ports with a 2 stroke) It would be up to you to determine if it would be worth having the motor bored to an oversize and fitting it with a new piston and rings.
I'm just wondering if the ring(s) is broken. But you've checked out all the other areas of air leaks...and the cylinder sounds more and more like the culprit. 2 stroke motors are really vunerable to air leaks.
I find more older weed whackers and chainsaws etc which have cracks where the tubing comes thru the tank going to the carb. I've found filters breaking off the line in the tank and then the unfiltered fuel stopps up the carb. Lastly...those carbs diaphrams......can be a real source of the problem. I have a McCullough Super Windstream 4 Vacuum Blower that would do what your motor is doing. It would run for awhile and just throw craps. Once I finally found a diaphram for it ran great with full power again too.
One guy near me who has a lawn mower shop has a syrenge with tubing he fits to the fuel intake side of a carb when he suspects the diaphram has problems. He has an assistant load the syrenge with fuel and he pulls the engine thru to get it started and the assistant applies gentle pressure to the syrenge so the fuel bypasses the diaphram circut of the carb to deliver fuel pressure. If it continues running this way and delivers full power he's trouble shot that problem.
So that's my experience & take on your problem. Good luck !
Comfortableshoes, Egor, 5-7Heaven, and some others here have some good thoughts. You suspicions about an air leak somewhere are probably correct based on what I've read in this thread.
The bit about having used the motor without the shroud is a red flag...it tends to warp part of a cylinder wall. (I've seen this happen on so many VW's where those bottom baffles were forgotten when a motor is overhauled. They go just above the pushrod tubes and deflect the torrent of air from the fan shroud to make it follow the other 90 degrees of the bottom sides of the cylinders. Often when you hone an old set of VW cylinders and see an area on one side of a cylinder wall that the hone isn't touching.....it's cause that cylinder may have came off a motor that was put back together where the baffle(s) was forgotten. It's bad for a 4 stroke VW cylinder but a disaster for a 2 stroke cylinder)
My first Yamaha was a small thumper 90cc. I could ride it a bit and it would foul out and not start. After it cooled down it would run again for a bit. Fortunately a buddy of mine had a spare cylinder off his that he had bored out and fitted with an oversize piston. It was ready to go. We pulled the cylinder off mine and installed the new piston/rings, bled the oil pump and got it up and running. We cleaned up the old piston and jug and used feeler gages to check the cylinder clearance an it was way over. (what was happening was the intake mixture in the crankcase was spewing up past the rings as the piston went down on the power stroke spoiling the direction of the intake charge often wetting the plug and fowling it and losing compression. Riding up a hill when losing rpm often brought this on. )
When the guys at the shop bore the cylinder to fit it to the first oversize they have to hone it and check the wall clearance. If it looks good to go...they then take a small die grinder and edge all the ports to raidus them out from the cylinder wall into the port. (so the rings don't somehow catch on a sharp edge and break)
The thoughts on the compression check is definately a good indication. Yes, do always hold the throttle wide open when checking it. The "pro" hot rodders often do a "Leak Down" check where they pressurize the cylinder and watch to see how well it holds the air pressure with the piston at different positions (above the ports with a 2 stroke) It would be up to you to determine if it would be worth having the motor bored to an oversize and fitting it with a new piston and rings.
I'm just wondering if the ring(s) is broken. But you've checked out all the other areas of air leaks...and the cylinder sounds more and more like the culprit. 2 stroke motors are really vunerable to air leaks.
I find more older weed whackers and chainsaws etc which have cracks where the tubing comes thru the tank going to the carb. I've found filters breaking off the line in the tank and then the unfiltered fuel stopps up the carb. Lastly...those carbs diaphrams......can be a real source of the problem. I have a McCullough Super Windstream 4 Vacuum Blower that would do what your motor is doing. It would run for awhile and just throw craps. Once I finally found a diaphram for it ran great with full power again too.
One guy near me who has a lawn mower shop has a syrenge with tubing he fits to the fuel intake side of a carb when he suspects the diaphram has problems. He has an assistant load the syrenge with fuel and he pulls the engine thru to get it started and the assistant applies gentle pressure to the syrenge so the fuel bypasses the diaphram circut of the carb to deliver fuel pressure. If it continues running this way and delivers full power he's trouble shot that problem.
So that's my experience & take on your problem. Good luck !
Last edited:
To be honest I have put all these engines on hold. I have two that are sick maybe and one that has a welded on sprocket.
Your take kind of matches mine. The leak of compression may be the hint that there is a real problem. I am going to mess with it some more one day when I'm not so worn out with it. It would be interesting to know what the problem really is.
Your take kind of matches mine. The leak of compression may be the hint that there is a real problem. I am going to mess with it some more one day when I'm not so worn out with it. It would be interesting to know what the problem really is.
okay so your saying that it isnt pulling gas in well it might need a carb clean out or a carby kit and another fact is the primer bulb could be screwed try replacing that
