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Old 06-17-2013, 12:28 PM
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sbc 400 problems

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?

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Old 06-17-2013, 01:09 PM
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leak down test,
valve springs,
cam, after doing standard ignition,fuel flow,compression etc
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Old 06-17-2013, 01:13 PM
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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.
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Old 06-17-2013, 04:44 PM
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Well till I pick up my friends compression tester I pulled the plugs and spun the motor with my fingers over the spark plug holes just to see if I can feel the air coming out and #4 #5 I can't feel anything blowing out.. How could that happen?
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Old 06-17-2013, 06:11 PM
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Quote:
Originally Posted by fordmugg View Post
Well till I pick up my friends compression tester I pulled the plugs and spun the motor with my fingers over the spark plug holes just to see if I can feel the air coming out and #4 #5 I can't feel anything blowing out.. How could that happen?
Blown head gasket, bent/burned valves, hole in piston, rocker arms off, camshaft lobe flat.
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Old 06-17-2013, 07:42 PM
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So before I tear it down whats the best way to test if its the rings or valves with a compression tester
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Old 06-17-2013, 08:09 PM
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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.
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Old 06-17-2013, 09:18 PM
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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
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Old 06-17-2013, 09:30 PM
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It could be that the intake valve rocker adjustment backed off and the push rod is not opening the intake valve. No intake coming in means no compression or air blowing out.
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Old 06-18-2013, 07:01 PM
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ok guys did a compression test, this is what i got

#1 170
#2 185
#3 175
#4 110
#5 0
#6 170
#7 155
#8 190

the #5 and #4 cylinder i squirted some oil in and #4 didnt change and # 5 went to 10

so whats your opinion, rings?
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Old 06-18-2013, 08:10 PM
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Given the small change with oil in the cylinder the rings are less suspect than the valves. Pull the rocker cover and let us know what you find.
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Old 06-18-2013, 08:37 PM
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I have pulled the valve covers and everything looks fine, they move just fine, I even adjusted the ones on the #5 cylinder to see if it made a change and nothing, so I'm thinking I should just pull the head and take a look inside
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Old 06-18-2013, 08:56 PM
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Sounds like your only real choice at this point. It could be the head gasket, piston, or still be valve related but it should be a lot clearer when you get the head off and can look inside.
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Old 06-18-2013, 09:04 PM
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Well I'll get it off tomorrow and keep you guys posted
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Old 06-18-2013, 10:12 PM
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Is it pressurizing the coolant system??.
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