Tuning my 250 I6

[creed1969]

Hi everyone, I like to avoid asking too many questions without first doing my best to figure things out, but lets just call this "utilizing your wisdom". I've encountered a couple problems in tuning my 250.

*1967 chev 250 I6, stock rebuild, stock cam, Rochester 1bbl, Hedman long tube headers, pertronix breakerless ignition on rebuilt dist, no emissions equipment

Here's a pic from last year for no reason other than to grab your attention haha. I know we all like pics.


A few weeks ago I was doing the initial setup of my rebuilt 250 (timing, idle mixture, etc). When I put the timing light on it (vacuum advance disconnected and plugged), I found it idled best at about 14 or 15 degrees BTDC. The timing was advanced above the timing tab. My Haynes book says the timing should be 4 degrees BTDC...but when I tried setting it to 4, the motor would hardly run. So I assumed my damper had slipped, and I just set the timing with the vacuum gauge (advance for max vacuum, and retard for 1"HG drop). Indicated somewhere around 15 BTDC.

The truck runs strong this way mostly...and I've put a few hundred miles on it. But when cruising at partial throttle, there is a constant popping/muffled backfire sound in the exhaust note. If I accelerate, it sounds smooth and normal, but cruising its got this annoying stutter, and decelerating it crackles and pops more than I think it should.

So last night I decided to take another try at setting the timing...I started by confirming my timing mark is correct using a piston stop and marking the pulley. I once again tried setting the timing to 4 degrees, and actually drove the truck a couple blocks...but it ran awfully. So I'm back at 15 degrees.

Tonight my plan is to adjust the rocker arms and check the lifters to make sure everything is working right...my vacuum gauge has a constant vibration spanning about 2"HG, and I'm starting to suspect my popping/backfiring is the result of a bad lifter or something.

So ultimately...why would my motor not run well at factory timing?
And, what may be the cause of my popping/backfiring at cruise? Ideas? I'll be thankful for any wisdom you all can impart on me

[trac209]

Could be balancer has slipped or timing light setting is off or faulty.
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[68gmsee]

I have an HEI on my 250 and I have the timing at 10º advanced. 10-14 is not too much.

Too much sputtering or popping out the exhaust when decelerating could be a vacuum leak or carb adjustment. Check the vacuum advance to make sure it's not leaking and check carefully around the intake and the base of the carb (I use a short section of garden hose as a stethoscope). Sometimes the middle gasket in the carb body may not be tight enough.

Other things to check:
Adjust the idle air screw for maximum vacuum with vacuum gauge.
Swap spark plugs with different brand (just in case).
I'm not familiar with the distributor you are using but if it requires the full 12vdc make sure you have rewired it and eliminated the resistive wire.