Flat Spot in Carb. , Dead Cylinder, Fuel Pump?

[Guest]

It is spring in the northland so I uncovered the 'wagon. The engine is a 340. The carb. has been rebuilt. The motor idles excellent, has no noticeable ticks/knocks, and leaks no oil. Good oil pressure. However, last year #2 cylinder was "weak" and is now only blowing 25lbs. HHmmm? Off idle, the carb. must be "feathered" to getter go'in, and there is definate bog-down, then she takes off. What gives? Bad lifter or sticky Valve in #2 Cylinder? Accelerator pump in carb.? *Should I junk this engine and install a Chevy 350? (Ha! I'm so funny!)

[Guest]

My guess would be a bad head gasket. Any bubbles in the radiator or signs of losing water?

[The Old Guy]

It sounds to me that you have two problems. You have a dead cylinder which if the engine has miles on it ,is probably a burned valve.The second problem is more than likely an accelerator pump that is not doing the job. With the air cleaner off, look in the top of the carb and push the throttle linkage . If there is not a good squirt of fuel then you pump is not functioning.

[Guest]

First do a "wet" compression check. warm engine, remove #2 plug, install gauge, test and record reading, remove gauge squrt about 2 table spoons of engine oil in the cylinder,crank for 3 seconds, install gauge and test. If the number jumps 25# or more you have bad or stuck rings. If not, remove the valve cover check for broken rocker arms, broken valve spring, bent push rod,or stuck valve. turn engine by hand watch each valve open and close. use a dial indicator or other good measureing prosses to be sure both #2 valves are opening and closeing the same as other cylinders. start the engine and watch valve train work. A held open intake valve will cause many of the problems you state. As the cylinder comes up on compression the pressure is pushed back into the intake, this screws up the flow to the other cylinders. A burned or stuck open exhaust valve will just shot the gases out the tail pipe. It has less efect on engine driveablity than a missing spark plug wire because in that case the engine still must compress the cylinder with no power in return. 340's almost never lose head gaskets. head gasket failure shows up in several ways. two adjacent cylinders with low compression, water in cylinder, compression gases in water. etc JIM

[Guest]

Thanks for the feedback. The engine cools fine and there has been no noticeable loss of coolant, so I would rule out the head gasket as a problem. The Carb. gives a good dose of fuel, so that seems to work. I am more of a "shade-tree mechanic" so I don't have the necessary tools for an internal diagnosis. However, upon taking off the valve cover and watching the valve train, it appears that the intake (?) valve on #2 cyl. has minimal movement. Also of note, the exhaust (smell) seems to be "rich" (as in incomplete combustion). Apparently, this gives me a good excuse to take the motor out and have stainless valves (and a mild cam) installed. JimmieB.....What would you recommend for this engine? I am not sure of it's mileage, and prior to my getting it, it had been sitting in the desert for 16 years. Thanks for the help!

[Guest]

Go to the search at the top of this page put .050 in the field and it will get you to some of my postings. Then go to the general board and put in 340 vs 350 good stuff. if you have more questions post them JIM