Help me get this as close as possible

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  #1  
Old 10-04-06, 04:22 PM
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Smile Help me get this as close as possible

Have a 77 GMC Sierra SWB truck with inline 250 6 cylinder 1 BBL(head has been changed to standard versus integrated) Turbo 350 trans and 2.76 rear end from a Suburban. The previous owner bored the block to .060 and installed an aftermarket cam. I don't have any idea as to the specs of the cam and am unable to set timing according to specs. What is the best method to get the timing close because this is no longer a stock motor. I am not racing I just want to get ignition timing, curb and fast idle rpm, carburetion all in sync. I don't depend on this daily but I enjoy the comfort and utility it affords.
Additionally is it necessary to double nut the back valve on this head if all other efforts have failed.
Thanks for your time, patience, and consideration.
Steve
 
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  #2  
Old 10-04-06, 07:13 PM
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Steve:

Why can't you set the engine to the OEM specs? You probably won't maximize the performance of the engine by doing so, but it doesn't sound like that is your objective.

If you are wanting to max the performance with a layman's technique you can start with the OEM specs and at least 90 octane fuel in the tank. Then take it out to a long grade when you have some time. Accelerate up the grade and listen for any pinging. If you don't have any advance the timing slightly then up the grade again. When you start getting a little ping, back off the timing a couple of degrees and check for ping and spark knock. If it's ok leave it there.

Keep in mind you have maxxed out the timing, if you have any problems from this (hard starting or pinging) retard the timing slightly to take care of it.

If you have a high lift/long duration cam you may not get the smoothest idle. If the cam is too radical you should hope the valve springs were changed.

Regarding the back valve on the engine, I'm unclear as to the problem you're having. Are you saying the valve lash adjustment loosens? It really shouldn't be a problem holding the set with the locknut. If it is and double nutting is the only solution, I would do it. I've honestly never heard of the problem before.

If I'm on the wrong page, post back and let us know.

Hope this helps,

Bob
 
  #3  
Old 10-04-06, 09:03 PM
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Reply to Bob

I have retarded the timing as you stated with success and don't have smooth idle as desired. I'm pleased with the timing since have done as you stated. A mechanic told me it sounded like a high lift cam;therefore, is it necessary to make other adjustments to the carb? Lean or Rich? Rochester 1 BBL carbs are challenging. Is there a trade secret needed to avoid the inherent frustration?
As for the valves, I used new lockoloid nuts but still have a loose sound on the back valve. It is a new head with new parts.

The 250-6 with integrated head calls for 12 degrees BTDC. I changed to a standard head which was last used on 74' models
requiring timing be set at 8 degrees BTDC.

Thanks for your help and suggestions.
Steve
 
  #4  
Old 10-05-06, 05:10 AM
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A high lift cam isn't a good one to get a smooth idle. You may have to accept a compromise.

There's something going on with the back valve. You could have a bad lifter or a sticky valve. The adjustment lock on a rocker is pretty reliable - it shouldn't be loosening.

If you have the limiter caps on the idle mixture screws you may have to pop those off and try a variation in your idle mixture. I'm afraid that cam will set its own limits on what you can accomplish.

Hope this helps,

Bob
 
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