59 283 230hp tech help

[Steve Bone]

I have a 59 283 230hp car that is rough at idle below 800rpm. The vacuum guage jumps from 14psi to 15psi at idle. I was told it was probably a bad valve. The car does not smoke at all and runs out perfect above 800rpm.

I ran both a compression and a leak down test on the car and even double checked it. The numbers are as follows

Cylinder#----------Compression------------Leak Down %
1-----------------------------140 ----------------------11%
2-----------------------------145 ----------------------12%
5-----------------------------145 ----------------------10%
7-----------------------------145 ----------------------13%
2-----------------------------110 ----------------------11%
4-----------------------------140 -----------------------9%
6-----------------------------145 ----------------------11%
8-----------------------------140 ----------------------10%

The # 2 cyclinder is down to 110 psi on compression and at 11% leak down. I can not understand why the compression is down with only 11% leak down.

The spark plug wear is very consistant over the 8 cyclinders with the exception of a little less wear on the number 2 plug.

I also did a wet test on cylinder # 2 and the compression remained the same. This tells me the the rings are not failing.

Any help on this would be greatly appreciated.

Regards,
Steve

[Joe Fisher]

Re: 59 283 230hp tech help

Possible weak valve spring, tight lifter adjustment or worn valve guide.

[Duke W.]

Re: 59 283 230hp tech help

I'm probably not going to make your day, Steve, but both the compression and leak down numbers tell me that your engine is getting old and tired, I don't have the compression spec for your engine, but it's probably about 160 and the shop manuals usually say max variation between cylinders of 20 psi.

Also, you've got double digit leak down numbers. A good engine should be in three to seven percent range.

It seems to have worn fairly evenly, but number seven looks the worst and is probably what's causing the idle instability and vacuum flutter. I think Clem Z says that number seven often sees the most wear due to some cooling issues.

Leakdown is usually done at TDC, but compression tests the cylinder sealing surface along the entire stroke. There's usually a correlation, but it's not always perfect. I think the old girl is just getting tired and needs an overhaul.

How many miles are on the clock?

Duke

[Dave Suesz]

59 283 230hp tech help

You should do a very careful valve adjustment and repeat the test. If the results are unchanged, you have a valve (or cam) problem. On the off chance it's the cam, you can check it on the car with a dial indicator and degree wheel. Most likely, though, you need head work.

[Steve Bone]

Re: 59 283 230hp tech help

Thanks guys, I have read the snap on manual very carefully regarding the Leak down test with their guage. It indicates that below 20% leakage on a cylinder is an idication of light wear.

I belive the engine has less then 25k miles on it since rebuild. It is bored 30 over. The heads were gone through but, the valves were not replaced. I am hoping that I have a bad intake valve. I would certainly prefer to rework the head and not touch the cam.

Regards,
Steve