With a cam, with a duration at about 282, (I can't find my card) and lift at 480, what type of vac*** should I see at idle? I'm trying to tune it, and with initial timing at 10 BTC it starts right up and idles as smooth as I expect it to with the cam. But when trying to tune the carb with a vacuum guage I have a vac reading of about 8-9 Hg. I can get a higher reading by advancing the dist, but then it gets way too much advance and is hard to start. If my valves were not adjsuted correcly would this be symptom?
What determines the vacuum level is mostly the overlap in the cam. If you have a narrow lobe separation angle and overlap, it will not make as much vacuum. I do think you could be a little higher, but it will not be like a stocker. If your valves were not adjusted correctly it would either make lots of noise form excess clearance or really run like **** if the valves are not fully closing, I doubt that is your problem. You should be able to run a bit more advance, just schecking that 10 degrees is without the vac advance hooked up right? Seems most higher performance engines like around 12-14 initial static, then the vac advance adds more.
I pulled the advance hose from the dist and pugged it, I did not plug the vac advance at the dist, should I have? I get a small click on a few cly, esp at start up, but on the p*** side as well after it warms up but you have to lean next to the valve cover to hear it. No poping or backfiring at all, I'll try advancing it more, at what point will starting become a problem with too much advance?
No need to plug the vacuum advance canister, just unplug from the vacuum source like you did. The click on start up is just the hydraulic lifters filling up after draining down. As long as it goes away quickly it is no big concern. You can set your valves on a Chevy easily with near perfect adjustment by doing it running. Get an old steel valve cover and cut the top loose or fold it up so it is on the outside. Put this in place of one side valve cover, after the engine is warmed up. With the open top valve cover in place start it up and then loosen each rocker until it clicks (audibly, you will know when it does). Then tighten just until it stops clicking. This is "zero lash". Then crank the nut down an additional 1/2 turn. The engine may stumble and want to die, it can help to temporarily increase idle speed. Do all 8 on one side and then switch the open valve cover to the other side and repeat. You now know for certain that your valves are all adjusted equally. The open top valve cover is just to contain the oil splash while you adjust the valves with engine running. You can adjust the valves without it as long as you like getting oil all over the place. As to your question on how much is too much advance for starting? It depends on how good your starter is! To a point, a higher torque starter will crank better than a std one. Also depends on your battery cables and good connections, grounds included. FWIW, my 38 Chevy's 454 has a bigger cam than you do and I only get like 6-7 inches vacuum, there are somethings that are built into the cam profile and you just have to live with, as my original reply stated.
Do you have the full cam card from the cam - if so post up specs. Sounds a little bit low to me but not awfully bad - I would have expected around 10-12"... You really want the total advance to be limited to around 32* = static + centrifigual. The vacuum should be near 0" at WOT so your vacuum advance should'nt be in play.... If you're not getting enough advance at idle with the vacuum on then get an adjustable advance unit and crank back the centrifigual total timing a little bit to compensate and not go above the total timing.
OK 1. i found my cam card, it is a crane H-228 480 lift, adv duration is 284 2. I re timed it with 14 BTC and while timing it the idle speed was about 700rpm so I raised it to 1000rpm, and when I did the vacuum reading went to a steady 10Hg. This was all done with the vac advance line disconnected and plugged. Seems to start fine, would it be hard to start when hot?
The Jalopy Journal
