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big block hesitation

Hey, what's up, everyone? Been a while since I posted a topic here, but I finally finished rebuilding my engine and, of course, now I am running into problems tuning the thing. It is a 400 big block Chrysler .030" over with Keith black 9:1 hypereutectic pistons, Summit hydraulic cam and lifter kit with 282 degrees intake and 292 degrees exhaust advertised duration with .465" intake and .488" exhaust lift and a 114 degree lobe separation angle, stock crank and rods, rebuilt 452 heads w/ 88cc chambers and a freshly rebuilt Holley 4160 style model 3310 dual feed 750 cfm carburetor. The carb has a Quick Fuel Technologies externally adjustable vaccuum diaphragm (really cool!!)and a new power valve. The cam is also advanced 4 crankshaft degrees. Initial timing is 15 degrees before TDC. It runs very well, but hesitates when the throttle is yanked open. The floats are spot on, the idle mix seems to be about right. I don't recall exactly what size jets were in the carb when I tore it apart, but I believe they were like #76 or similar. It has a Mopar performance electronic ignition distributor with the stock weights and springs (for now). My first thought was carburetion, but I adjusted the secondaries throughout their entire opening range and adjusted the accelerator pump cam and have seen no difference as of yet. Anyone have any ideas?

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sackattack84
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Where did you plug the vaccum line in??? it should be on the outlet on the front pass side of the holley (ported Vacuum) not the one under the front off the base plate. (manifold vacuum)

15 initial sounds like a lot?? Total with vacuum unhooked and plugged is the way to tune it. Set total to 36 or 38 You may also want a advance spring kit add one at a time. Could be you need to tune the accelerator pump system on the front of the carb?? dodgem I like to plug vacuum advance till everything is sorted out then if you must try to work the vacuum in???? Try It If it causes pinging it must be tuned with a 3/32 allan wrench in through the vacuum port.
Reply to
dodge-him

The vaccuum advance on the distributor is capped off for the moment, as are the vaccuum line in on the baseplate (manifold vaccuum) and on the passenger side (ported vaccuum) of the carb. I have an advance spring kit, but have not installed it yet. I did atempt to adjust the accelerator pump cam, but it did not seem to have any effect. Maybe I just need to play with it a little more?

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sackattack84

It may need bigger squiters in the front. what is the total advance?? may need lighter advance springs. You say cam is advanced 4 deg you mean you degreed it and it cam out at 110 ATDC intake max lift (intake centerline) ???? or you advanved it another 4 from manufactures centerline 109 to 105???? You checked and rechecked timing marks?? (not nesesary if you actually degreed the cam) dodgem

Reply to
dodge-him

It's a vacuum secondarie carb right??? and you have tried a multitude of advance springs or has someone put a manul opening kit on it but with no provisions for accelerator pumps on the back two barrels?? Dodgem

Reply to
dodge-him

The power valve can be critical too it should be one to two numbers above idle in gear vacuum but optimum advance must be dialed in first. i would buy and install lighter advance springs. then set total to 35 to 37 if ping occures back total down. or advance it up till it pings and then retard it till ping gpes away if you have no way of setting total timing. Then get it idling in gear and check vacuum if its 7.5 to 8.5 go with a 6.5 valve if it is 6.5 to 7.5 go with a 5.5 Dodgem

Reply to
dodge-him

Thank you for your advice. Yes, it is a vaccuum secondary carb. Instead of the stock secondary diaphragm housing, however, I installed a billet secondary housing from quick fuel technology which allows you to adjust spring tension by turning a screw on the outside of the housing rather than having to actually change the spring. I will definitely recheck total timing and power valve size and let you know what I find. Thanks!

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sackattack84

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