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1978 crusader 270

jaybirdboat

New member
First off I am not a trained mechanic but I try to save money and work on stuff myself. I did an upper head job two years ago and now the engine is running rough. Changed plugs , but not wires nor distributor. The marine mechanic thinks a valve maybe sticking but I do not trust most mechanics. They rebuilt carb thinking that was it. Why would they think a valve first without changing common sense items first?

advice would be much appreciated.
 
rebuilding or changing parts without a thorough diagnostic approach = get another mechanic......

I'd start by asking when the last complete tune up was done? unless modified, a 78 will have points and they are high maintenance items...
in addition, its almost 40 years old....so it may be wise to assess the state of the engine - and the suggested vacuum gauge testing would be an excellent start.
 
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Another simple test would be to pull one ignition wire at a time to see if the idle is effected. If idle quality is unaffected by pulling one particular ignition wire the cylinder may be dead due to a stuck valve or an ignition problem. Next step would be a compression check on the suspect cylinder. Vacuum test would also be in order. No substitute for effective troubleshooting !!
 
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................ I did an upper head job two years ago and now the engine is running rough. Changed plugs , but not wires nor distributor. The marine mechanic thinks a valve maybe sticking but I do not trust most mechanics. They rebuilt carb thinking that was it. Why would they think a valve first without changing common sense items first?
This is typically what occurs when proper diagnostics are not performed.
Even if a good educated "guess" turns out to be correct, it is not science, whereas proper "diagnostics" is more in step with science!

rebuilding or changing parts without a thorough diagnostic approach = get another mechanic......
Yes, I agree.
Also, if proper diagnostics had been performed, you'd not be throwing money at work scope that did not correct the problem.


advice would be much appreciated.
Try doing these tests:

Cylinder pressure test (aka compression test). The highest cylinder pressure should be within 10% of the lowest cylinder pressure!
All spark plug will be removed.
Ignition system will be disabled (remove power source from ignition coil to prevent damage).
Each cylinder being tested will be cycled at least 3 Xs during each reading. (pressure will be accumulative.... this gives us a more accurate reading)

Cylinder leak-down test. This test will help determine which cylinder is low, and hopefully where the leak is occuring (piston rings vs vavles, etc.)

Vacuum test. Adjust the low speed circuits to obtain the highest idle speed. Idle stop may need to be re-adjusted.

See where the ignition advance is at idle speed. (aka BASE advance or Initial advance)

Q: How were the cam followers adjusted after the cylinder head work was done? If one or two were adjusted too tightly, you may have a burned valve or two by now!

Q: If still using a contact point ignition system, how are you adjusting the dwell angle?


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Great points. The engine was converted to electronic egnition several years ago. I am going to pass this on to the new mechanic that I have hired.
 
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