*THE HULL TRUTH is the world's largest FREE network for the discussion of Boating & Fishing. Whether you're researching a new boat, or are a seasoned Captain, you'll find The Hull Truth Boating & Fishing Message Forum contains a wealth of information from Boaters and Sportfishermen around the world.
Welcome to the updated THT!
If you are having trouble signing in, please email feedback@thehulltruth.com with your username and we will help you. We thank you for your patience as we help you access the new site!
Random Quote: The Truth, The Hull Truth, and nothing but the Truth
the motor wont give me full power i checked ignition system and compression is perfect. but when i took the intake off and run the engine i choked each card to see if it was getting fuel and all was getting fuel except one. i could of felt the negative and positive crankcase pressure when i choked all but execpt one. its the middle right carburetor looking from the front of the intake side, i check to see if the carb had fuel and the bowl full. so what could cause this pressure lost problem becuz the compression is good on all cylinders? i was thinking mayb reeds....................?
Possibly a reed, could also be a blown out fuel pump gasket. Or could be a cracked piston skirt. Just because you have compression on one side of the piston, doesn't meen your still getting it on the other side. I would start off by removing all of the carbs, and take a light and look in and inspect all of the reeds. Then while your there, turn the engine over and see how much the reeds pop open. Make sure they all open the same amount. If the dead cylinder isn't opening the reeds, then you most likely need to tear into the engine. Did this problem happen all of a sudden? Or gradually?
i wont know if the problem just happened or it gradually happened becuz im fixing the engine for some1 and they wont know when it happen so i wont know.
i took the reeds out and they're all good.....the i took the head and piston out and inspected the piston to see if the skirt was good..and its good now what....? im lost please help
You took the piston out by only taking the reeds out and the head off? I hope I read that wrong. Fuel pumps should have been checked first for blown diapraghms after the choke test.
The fuel pump can still pump gas with a bad diapraghm, awhile back yamaha had a white plastic like diapraghm that would crack and allow excess fuel directly into the cylinder through the vacuum pulse hole causing the engine to fllood on that particular cylinder. They actually went back to the black diapraghms and quit with the white ones.Take the fuelpumps loose from the block and pump the primer bulb up real hard(hose still hooke to carbs) and check for fuel leaking out of the center hole on the backside of the pumps(between the two bolt holes) Granted you can get to the connecting rods, I just have always used a alignment block when I install my caps with the block still in half. Granted the caps are snapped off during manufacturing creating an odd break which helps with realignment it just sound like a pain to do it through the intake.
what was the compression on that particular cylinder? What were the other cylinders compression? How does the cylinder wall look, any scoring, cross hatching? What did the piston look like, rings move freely, carbon buildup? I ask these questions because I have had a customer say compression was good and it ended up being to low. Just a note... I read back over your original post and when you feathered or choked the cylinder you would not have felt any positive pressure, just negative. Also how did the reeds look, did you see if they still have a good bit of spring to them? I have seen reeds actually stick in the shut position.
all the cylinders was 100 psi except 2. the #1 was 95psi and #3 was 90psi all the others was 100psi. no scoring in the cylinder i can still see cross hatching piston and rings are perfect. no carbon buildup . i meant to say i have no pressure and all when i put my hand over the carb. all the other cyls would try and suck and push my hand except that 1......?? what u think now?? please help
This is going to sound stupid but looking at the carbs which carb was not sucking? And looking at the pistons which one did you remove? I am going through manuals but I really am stumped without it being in front of me. If the reeds are good and compression is good and it is not sucking gas it has to be sucking air somewhere else. How did the intake seals look? Recirculation lines all good? Crankcas halves look to be sealed? If it was the lower cylinder it could be the lower crankshaft seal and vice versa for the upper cylinder? But it should still take the path of least resistance to get air and that would be through the carb, i'll keep looking.
yamaha 250 carb vx76 problems 
