Holley owners would kill for spark plugs like that!!
Your 270s cam is a variable duration cam, that can have lash from 0.015" to 0.030", & depending on where the lash is set, the adv duration will vary +/- 10* from 260 to 280*.
- Your vac seems low.
- Jet size has nothing to do with engine size. A big engine might use a smaller jet with the same carb that was on a smaller engine.
- Stalling; check that float level is 7/16".
- both rich & lean mixtures can cause surging.
- backfiring on decel is most likely valves too tight [ lash ]...or lobes/lifters wiping/wiped.
- first thing you need to do is get the idle timing correct. Re-adjust valves FIRST. Set idle to about 700 rpm & adjust mixture screws for reasonable idle quality [ not critical ]. Engine idling, in gear, loosen dist & turn dist slowly, starting from about 12* BTDC [ not critical ]. When highest rpm is reached, stop. Check timing. I am betting it will be between 20 -30*. Whatever it is, this is what THIS engine combination wants. Anything outside of this will result in LESS than optimal idle quality & tip in response.
- to get the desired timing, use an adjustable VA unit connected to manifold vacuum. you may have to make a stop for the VA unit. Eg: best idle was with 28*. 15* initial, make a stop so that the VA unit adds 13*. This where the carb adjustment starts: with idle timing, see the link.
I suggest you try this & report back.
Your 270s cam is a variable duration cam, that can have lash from 0.015" to 0.030", & depending on where the lash is set, the adv duration will vary +/- 10* from 260 to 280*.
- Your vac seems low.
- Jet size has nothing to do with engine size. A big engine might use a smaller jet with the same carb that was on a smaller engine.
- Stalling; check that float level is 7/16".
- both rich & lean mixtures can cause surging.
- backfiring on decel is most likely valves too tight [ lash ]...or lobes/lifters wiping/wiped.
- first thing you need to do is get the idle timing correct. Re-adjust valves FIRST. Set idle to about 700 rpm & adjust mixture screws for reasonable idle quality [ not critical ]. Engine idling, in gear, loosen dist & turn dist slowly, starting from about 12* BTDC [ not critical ]. When highest rpm is reached, stop. Check timing. I am betting it will be between 20 -30*. Whatever it is, this is what THIS engine combination wants. Anything outside of this will result in LESS than optimal idle quality & tip in response.
- to get the desired timing, use an adjustable VA unit connected to manifold vacuum. you may have to make a stop for the VA unit. Eg: best idle was with 28*. 15* initial, make a stop so that the VA unit adds 13*. This where the carb adjustment starts: with idle timing, see the link.
I suggest you try this & report back.
