350 chevy problem

[bobzak]

I have a Chevy rebuilt motor with 42 hours on it in my boat. Just picked the boat up over the winter from Hatteras. reconditioned the rest of the boat and put in the water. The boat ran fine the fist couple of times then had a fuel problem coming in one day. come to find out that my secondary float was stuck on my Holley carb and dumping fuel when it shouldn't. I removed the bowl and cleaned out and the float started working properly. i noticed a miss and backfire the last couple of times out then it really got bad. removed all plugs and found #8 and #5 had got really hot and they were slightly loose when removed. the electrode was actually metled. put all new plugs in and miss still there. did a compression check yesterday and found that all over 125 pounds except #8 and #5. they are at zero with no needle movement at all. removed the tester and turned over with finger in hole. no air pressure. do you think i have stuck rings, hole in pistons, or burnt valves. not sure if i should pull the boat back out of slip to remove motor or pull the heads in the slip.

 

[RogersJetboat454]

Re: 350 chevy problem

I have a Chevy rebuilt motor with 42 hours on it in my boat. Just picked the boat up over the winter from Hatteras. reconditioned the rest of the boat and put in the water. The boat ran fine the fist couple of times then had a fuel problem coming in one day. come to find out that my secondary float was stuck on my Holley carb and dumping fuel when it shouldn't. I removed the bowl and cleaned out and the float started working properly. i noticed a miss and backfire the last couple of times out then it really got bad. removed all plugs and found #8 and #5 had got really hot and they were slightly loose when removed. the electrode was actually metled. put all new plugs in and miss still there. did a compression check yesterday and found that all over 125 pounds except #8 and #5. they are at zero with no needle movement at all. removed the tester and turned over with finger in hole. no air pressure. do you think i have stuck rings, hole in pistons, or burnt valves. not sure if i should pull the boat back out of slip to remove motor or pull the heads in the slip.

Welcome to Iboats.

Sounds like your engine ran lean. When you pulled the spark plugs, did you happen to notice dull gray specks peppering the plugs? If you did that's melted bits of piston.

All of your above thoughts are plausible. Have you tried a compression test after adding oil to the problem cylinders? If you add oil to the cylinders and the compression jumps, you would most likely have a ring sealing problem. If it doesn't your rings and pistons could really be trashed, or you have a valve issue. A cylinder leak down test would also be handy at pin-pointing your problem. If you can get your hands on a bore scope, you could take a peek into your cylinders to see if you can spot the issue. You may also want to pull your valve covers, and see if you have any valves hung open, or other valve train damage.

Good luck!

 

[bobzak]

Re: 350 chevy problem

thanks, i do have a scope and will check that tonight. I also had to get a funnel today that would reach around the riser so i can get oil into the problem cyl's. one of the plugs just looked like it was fouled and the other had the electrode melted down. Motor only has 42 hours on it and it ran great the first 3 times out. it all started when the secondary float stuck. which really has me puzzled as the dumping of the fuel shouls not have caused this. you would think the fuel would have kept the cyl cool. I will update after test tonight. I really appreciate your help.

 

[RogersJetboat454]

Re: 350 chevy problem

thanks, i do have a scope and will check that tonight. I also had to get a funnel today that would reach around the riser so i can get oil into the problem cyl's. one of the plugs just looked like it was fouled and the other had the electrode melted down. Motor only has 42 hours on it and it ran great the first 3 times out. it all started when the secondary float stuck. which really has me puzzled as the dumping of the fuel shouls not have caused this. you would think the fuel would have kept the cyl cool. I will update after test tonight. I really appreciate your help.

Gotta wonder why the float stuck though. If there is trash in the fuel system maybe it hung up the float, and also restricted fuel flow. Other problem with a stuck float is if you have lots of fuel running into the cylinders, you are washing all the oil off the cylinder walls, and will burn up your rings. See what's going on with your scope.

 

[Bondo]

Re: 350 chevy problem

Sounds like your engine ran lean.

Ayuh,... I agree,... Has the fuel system been checked out,..??

There could be Issues, startin' with the tank vent, All the way to the Carb...

Welcome Aboard,.... Too Bad ya burnt up a brand new motor...


Btw,... What killed the previous motor,..??

 

[sswheeler]

Re: 350 chevy problem

If your compression guage isn't moving at all, there is abig hole somewhere. Piston or valves would be my guess. Stuck rings would still build some pressure. IMO, Holley carbs are great when they're set up and adjusted properly. The problem is those carbs have a LOT of adjustment in them. It's easy for them to act fine and be way off. Good Luck with the rebuild.

 

[krisnowicki]

Re: 350 chevy problem

Could be your valves are not closing

 

[odie241]

Re: 350 chevy problem

Before you go pulling the heads,pull the valve covers,and have someone crank it over and watch the rockers on those 2 cylinders to see if they are moving,if they arent,then you probably have bent valves,or maybe you got lucky enough to just have a couple bent pushrods.

 

[Richeskis]

Re: 350 chevy problem

I had the same problem with my 350, about 13 to 14 years ago. it was running great just before it started to back fire, right after a nice full speed run down river. First off you now have a blown power valve on your Holley unless you have a back fire mod on the Holley to prevent the power valve from being blown out. Just like "odie241" said, Pull your valve covers and you may find on both the cylinders that have no compression, the valves and stems have stretched out and the valve springs can not set the valves any more, you ran it too lean....just need to to a valve job..