Technical engine tuning advice

I just finished the 500 mile break in after breaking in the cam on my tri-powered 327. the engine seems to run pretty well overall however there is a surging / bucking that I experience around half throttle - right before the outboard carbs open. It almost sounds like the engine is cutting in and out. This seems to happen when cruising at higher RPMS (I have 4.11s out back). During acceleration or varied throttle conditions the engine is well behaved.

The end carbs are set up properly; thick throttle blades, no idle circuits or power valves etc. I have done my best to eliminate vacuum leaks from the whole system overall. Ignition is vacuum advance with 36 degrees total and all in by 3000 rpm. The ignition components are all new. The spark plugs seem clean and whitish-tan, no signs of fouling etc. I have iron heads around 9.8 static compression and a solid lifter cam 230/236 - .488/.501. The engine runs almost too cool (160) so I've got a 195 thermostat on the way.

I've heard about lean surge, could this be it? suggestions about how to go about taking care of this is appreciated.

thanks

 

Do you have any idea what the spark advance curve is? Like you, I'm suspecting lean surge, or possibly a bit too much spark advance at that point.
Do you have vacuum advance on dist? If so, is it connected to manifold vacuum or ported vacuum? All of these are things you can play with to alleviate the surge. A dial back timing light, or a timing tape on the balancer used with a regular timing light are both good methods of checking spark advance, assuming you checked and made sure timing tab is set for true TDC on balancer. Good luck!

 

Dirty old man,

Vacuum advance Hooked to full manifold vacuum. 36 degrees all in by 3000 rpm. About 18 initial advance. New aftermarket dampened with degrees marks, tdc verified with a piston stop. AC R45 plugs. I have and adjustable vac canister. Maybe it's pulling in too fast?


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Easy way to find out is to disconnect and plug vac. advance and try again. Could be the problem as man. vac. is gonna be pretty high at steady throttle opening at higher rpm still on that one 2 bbl.
Then if this cures the problem, try ported vac (if such is available on your carbs), and/or play with the adjustment on the diaphragm.
If backing off advance doesn't cure it, then most likely it's a lean surge.

 

Thanks. I will try this. I thought vacuum dropped to near zero when the throttle opens?


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It sounds to me like a tip in problem, like maybe when you are transitioning you are getting too much air until you get the blades open far enough to let the secondary carbs work. Maybe make your accelerator pumps act a little faster, or change the RMP range that your secondaries start to hit.

 

Do id just the acc. pump by bending the rod? Good advice thanks


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