Low compression on one cylinder and miss

[newshelby]

Hi guys need your advice. Just bought a 68 Shelby gt500 with a 428 police interceptor engine that was rebuilt less than 1000 miles ago but about 18 years ago. Took the car into the shop and they could not get it to run right so they checked for compression and number two cylinder was low (only about 40 psi). The shop recommended taking off the heads and checking for a burnt exhaust valve. Attached is the build list that was done when the engine was rebuilt. Any ideas? Should I just have the whole top end rebuilt in fear that another Exhaust valve will blow? Maybe a cam issue? Your input is greatly appreciated. Want to do this right the first time. Thanks. See image of rebuild list.

[KR Convertible]

You could do a leak down test to verify the problem.  This is done by pressurizing the cylinder in question when the valves are supposed to be closed.  You might get lucky and find that the valves were not adjusted correctly and are not closing.  Good luck!

[gt350hr]

  +1  If a leakdown tester isn't available ,  it is also possible to pressurize the cylinder after removing the rocker assembly ( so all the valve are closed for sure). If you hear air escaping from the tail pipe , you have a bad exhaust valve . Or if it is coming from the carburetor , it's an intake valve ( doubtful) or if it is coming out of the breather in the valve cover , it's a piston or ring issue. Once "found" you can go in the right direction.

[1967 eight barrel]

This is what I used to keep the valves in place when I replaced my inner springs. You just need an air compressor and the correct fitting that works with your air hose. Remove the rocker shaft on the right bank, remove the plug and use the tool listed below.  If the valve isn't seated you'll hear it hissing through the intake.  That's the "cheap" way to do it at home. Anything Randy tells you can be taken at face value.

                                                                              -Keith

https://www.amazon.com/Lisle-19700-Valve-Holder/dp/B000COC7ZU/ref=asc_df_B000COC7ZU/?tag=hyprod-20&linkCode=df0&hvadid=241975700326&hvpos=1o16&hvnetw=g&hvrand=14808006321744823036&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9027279&hvtargid=pla-570366407029&psc=1

[gt350hr]

  Thanks Keith . I've been around awhile so I've had to "improvise". The method you and I describe is really no different than using a leakdown tester , it just doesn't tell you how "bad" the leak is since the gauge isn't being used.
     Randy

[KR Convertible]

I don't have a real leak down tester.  I use one of the adapters from my compression tester.  You just have to take the valve core out of it. You can add a gauge and an air valve if you want.  Pressurize it slowly because the air will push the piston down.  Listen for the air at the carb, the tailpipe and the oil fill cap to determine where the leak is.

[pbf777]

Hi guys need your advice. ................Took the car into the shop and they could not get it to run right so they checked for compression and number two cylinder was low (only about 40 psi). The shop recommended taking off the heads and checking for a burnt exhaust valve.

     The question I would have is, has any of the process as had been indicated by others above been executed?  If the diagnostic effort stopped with solely the "compression test", and post that singular observation you have been advised to begin disassembly, I would advise you to quickly extricate your vehicle from this shop, and go find another, or do it yourself.       

Just bought a 68 Shelby gt500 with a 428 police interceptor engine that was rebuilt less than 1000 miles ago but about 18 years ago. 

     Now, if I were to gander as to the issue (utilizing the all knowing crystal ball....... ), at 40 p.s.i. one would be lead to believe the the valve is not "stuck", but if the vehicle has been sitting without operating for a period of time (you didn't indicate clearly), then particularly the exhaust seats will tend to corrode and the rust does not provide a good seating surface, hence will demonstrate leakage.  If this were the possibility, then I would advise one to just drive the vehicle for a couple of weeks and then retest.  But this procedure is only to be attempted after ruling out other possibilities where this would be contraindicated.

Maybe a cam issue?

     The "cam issue" perhaps should have been answered as part and parcel of the original diagnostic inquiry, and the fact that you are inquiring here indicates that you are have been left attempting to diagnose what you contracted someone else to answer.  Perhaps this is just a failure in communication, or as some customers I have encountered state: "I just didn't MAKE them understand" (and I counter with: "the use of the baseball bat has been outlawed"....... ), but this also leads one to fear a perhaps unpleasant outcome on the horizon.       

Want to do this right the first time. 

     This "want", does not always mesh well with old cars, and monetary budgets, put I understand the point of view that having to do a task twice in order to get it right isn't economical either.  Me....... , being in the business, witnessing upon disassembly many so called "rebuilt" engines, and being the prima donna that I am, not accepting the "acceptable" or what is described as "the industry standard", that puppy's coming out-a-there, for a complete tear-down, inspection, and thorough rebuilding. But, also note, that's why my projects are never finished.       

     Scott.

     

[gt350hr]

    I thought you were more of a "primo don" than a prima donna.

[pbf777]

     And I don't, and you wouldn't to hear me, sing either!       

     Scott.

[newshelby]

Thanks to all for the helpful advice! Well... one thing led to another and the shop ended up pulling the top end for a reubuild. Any advice on what top-end rebuild kit/parts to use? The cylinders and pistons look good, but going to have the top end redone just to be sure everything's tight when putting it back together! Thank you!

[1967 eight barrel]

Did you determine if it was rings or valves?  I would hate to see you get the top end done and still have low compression on the same hole. Don't let them use cheap head gaskets. Felpro 1020's and Felpro 1247 S3.
Nothing worse than doing something multiple times.