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sbc 400 problems

4K views 38 replies 7 participants last post by  techinspector1 
#1 ·
hi guys new to the forum and need some info, i have a 400 sbc in my boat and at the river last weekend i lost some rpm its started running rough and limped it back to the dock,i changed the plugs the next morning , one plug was fried and all the other looked perfect, im not getting any smoke out the exhaust, there is no water in the oil, im getting spark to all plugs, i feel its the #5 cylinder. i do get a little oil coming through one of the bolts on the intake and when its running the side that the #5 cylinder is on has smokey oil coming through the breather and the other side dosnt. now what would be the proper steps to find the problem, i know im gonna do a compression test but what would be some other things to look for and do?
 
#3 ·
I would pull the intake manifold and inspect the gaskets for being pinched all the way around each and every port. If the seal is imperfect on any of the ports, oily vapor from the crankcase can be drawn into that cylinder on the intake stroke. This can lean out that cylinder, while the rest of them are running happily with the correct mixture.

The correct fix for this problem is to determine why the gap exists and fix it by machining the parts to fit each other (heads, manifold). The heads and intake must be parallel in both "X" and "Y" axes. This is why I am constantly carping on these fellows who are doing a rebuilt to begin with getting the main bearing bore straight and parallel, then cutting the block decks in a mill while the block is registered on the main saddle. If the block is higher on one or more corners than the other corners, the heads will sit askew on the block and the intake manifold has little chance of being parallel with the head ports.

When putting a motor together, I recommend using two sets of intake gaskets. Install a set just as you would if you were buttoning up the motor for the last time. Let it sit overnight so that the gaskets take a good set, then the following day, pull the intake and inspect the gaskets for being pinched all the way around each and every port between the heads and the intake. If all the ports on the gaskets are pinched shut all the way around each and every port, then put the other set of intake gaskets on and re-seal the motor, thanking your lucky stars that you have parallelism between the heads and intake sealing surfaces. If the ports are not pinched all the way around each and every port, then you have some machining in your future.

That's my best shot from long distance, not inspecting the motor first hand.
 
#7 ·
If your compression is really zero, or even close to it the leak is pretty significant so that air is going one of three places - crankcase, intake, or exhaust.

Pull your PCV valve out of the valve cover to see if there is a lot of pressure coming out of the crankcase (piston or rings).

Pull the valve cover and check the valves for any indication that one or more of them are not coming all the way back up (closed). Probably will have extra loose push rod depending on the original setting.
 
#8 ·
had my wife crank the motor by using the starter and all the cylinders will blow my finger off the sparkplug hole except #5 i just feel a little pressure and thats it,,, im scared its the rings but whats funny is the plugs were dry and it doesnt blow smoke out the exhaust, so more testing tomorrow
 
#18 ·
Ok guys got the heads off, the cylinders look fine. I flipped the heads over and filled the valve area with water and the 0 compression one drained out the intake valve real quick, and the cylinder with 110 drained as well just a little slower, the others were fine
 
#19 ·
Pull those two intake valves and look at the seat in the head and the seating face of the valves. It could be a chunk of carbon or it could be something distorted the valve seating areas. If all that looks good look at the valve stem where it goes into the guide for any bend or burr that maybe preventing the valve from closing against the seat.
 
#21 ·
Now for the tough question, what bent the valve? They typically do not bend on their own so most likely the engine injested something that got caught between the valve face and the seat and the valve spring bent the valve. But the obvious question is what was it and where did it go? Hate to see you put it all back together and later find there is a screw lodged in the top land area of the piston or some such "ah sh..." event.
 
#24 ·
Need an opinion guys.... My funds are low due to the economy so I'm trying to figure out the best options to get the motor back together so my kids can have some fun for the summer, here are some options

#1 just replace the bad valves and run the boat and do the major fix in the winter
#2 get some good 350 heads, drill the steam holes and run them, I can find them pretty cheap on Craigslist
#3 try and somehow get the money to have mine redone that will be around 400-450
 
#26 ·
I would go with option #1 - just replace the two intake valves that are bad/suspect, replace the head gasket, and put some time on it before you decide if it needs anything else.

The other cylinder compression number seemed OK, I gather the engine was running good prior to the episode.

No saying that a major engine overhaul would hurt anything but before you sink a lot of money into it I would want to know more.
 
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