http://www.secondchancegarage.com/public/186.cfmwhen you first rev it the vac reading should rise then lower. the 10 inches is a little low, how is your timing? If the timing is not at 36 total it may read low, you can adjust your dizzy with the vac gauge to see if you can get more at idle. raise it some, and see if the motor likes it. then back it off 2 inches. Double check with a timing ioght to make sure you don't go over 36-38 total timing.
Can my PVC routing have anything to do with it? I have oil filler tube breather then a PVC routes from the road draft tube opening to the base plate of the center carb Posted traditionally using tin cans and a string.
No one has addressed the possibility of a restrictive paper overseas made air cleaner? A restriction above the throttle plates can cause higher readings off idle. Exhaust restrictions cause excessively low readings. RLFFRL
I know this might seem stupid. But Is all your runners connected to one plenum with two carbs on top? I tought you ran with 2 carbs with 2 plenums and 4runners pr. Plenum. And was trying to balance them to each other. My bad. Please insert a smiley with red ears.
I will remove the air cleaners and exhaust baffles this evening to see if there's any change. Posted traditionally using tin cans and a string.
The end carbs are dedicated carbs the have no idle circuit the power valves and air bleeds have been removed. I still have accelerator pumps in them and that's about it. The leak marginally. I sent one of the bases to ****ster and he sealed it up pretty well. I can feel a slight vacuum putting my hand over the carbs. I'm not sure if I can get them to seal any better. Posted traditionally using tin cans and a string.
No worries. I have a tri power with all the carbs connected to the same plenum. Thanks Posted traditionally using tin cans and a string.
Wouldn't you still need the high speed air bleeds intact on the secondary carbs? I know you would not need the low speed/idle air bleeds as they do not have those circuits, but do the other air bleeds still need to function?
Before you start tearing into the car too much you may try checking vacuum at the center carb where you have the PCV connected first. May be an odd effect of placement on the other port that's causing it to behave like a siphon tube on a sandblast nozzle or collector tube on a pan evac system. I think that's what volvobrynk was getting at in post #15.
Uhhhhhhhh...230/234 degrees [at 0.050] on a 110 degree LSA is a pretty lumpy cam for a little 327. I'd expect the engine to have low vacuum at idle if it was me.
I have a crane vacuum advance connected to full manifold vacuum. I adjusted the unit with the hex key and limiter plate to start at 13" hg and be out at 6" hg. Maybe I want it to come in at 15"? Posted traditionally using tin cans and a string.
Can I just unplug the PVC for the time being and tap into that port (plugging the other port obviously)? Posted traditionally using tin cans and a string.
It's a solid lifter so I figure that taking valve lash into consideration it's more like a 222/228 hydraulic cam. The engine is 0.060 overbored to 336 CI, I don't think this cam is too crazy. I have about 9.8 static compression and 8.3 dynamic. The quench is real tight at 0.035 and It doesn't seem to exhibit signs of pinging or detonation. The motor seems to like a lot of advance but I think in a little lean. This is all in a model a roadster with a close ratio muncie and 4.11 gears. Posted traditionally using tin cans and a string.
Your car wieght may explain you vacuum numbers. My pickup has pretty close to the same vacuum curve that your does. At idle it makes 12 inches of va***e at cruise it makes 18 getting the power circuit to kick in requires a fairly aggresive shift in throttle position if you are to light on the pedal it never causes a vacuum drop in fact vacuum goes up as it leans out. The truck is 2400lbs, it does not take much throtle blade to cruise at that weight. At what point do you think you secondaries start tipping in, somewhere around 60% I would image. You would really have to snap the throttle blades open fast to see a decrease in the vacuum on your setup I would think. And if you are doing any of this in netrual in the garage then vacuum readings will be even more abnormal looking. My Dads couple with a three duece shows close to the same pattern your car has and his car is in the 2600 to 2800lbs range.
I think you nailed my scenario exactly. It seems to run ok, I'm getting a stumble miss at wider throttle settings, but I think that fattening up the mixture will resolve some of this. I'm going to try lightening the power piston spring a little bit to help the power valve kick in a bit sooner. Thanks Posted traditionally using tin cans and a string.
Well i unplugged my PVC and took a reading with the vacuum gauge hooked to the center carb (manifold vacuum) and left my vacuum advance hooked up to the runner port. I got a really nice and steady 16" hg reading. Hitting the throttle it would quickly drop to 10" then immediately rise to 25". I don't think I'm going to mess with it any more. My question now is should I set my vacuum can to 17" ? Thanks for everyone's insight Posted traditionally using tin cans and a string.
Which vac advance can are you using that allows you to set the pressures at which it gives specific advance? I run an MSD dizzy that uses the Accel GM-style adjustable vac can, and all I can do is limit the maximum vacuum advance by turning the screw inside the can. Seems like you'd have to have some kind of spring adjustment to adjust the pressure where it first moves?
I have the crane can. It hast the Allen screw Thad adjusts the rate and the an adjustable limiting plate that limits the travel. I hooked it up to my mitt vac and ****ed around with it unto I got the settings I was looking for. At first I didn't use the limiting plate and it pulls way too much advance. Posted traditionally using tin cans and a string.
I would try to get the vacuum can to be advanced at idle vacuum. how many degrees is in the can? Usually 10 to 15 is what you would need. Also vacuum advance at idle will help with heating up (running hot) at idle.
Maybe, but from what I've read if you want to make an apples to apples comparison between the behavior of a hydraulic vs. solid cam, you knock about 8 degrees off of the duration numbers. Here's one of the sources of this info: http://speedtalk.com/forum/viewtopic.php?f=1&t=19384&start=0 http://www.carcraft.com/techarticles/ccrp_9812_secrets_of_camshaft_power/flat_tappets_camshaft.html Granted, this is from the net, but those figures seem to be the general concensus out there from other sources as well. From what I can tell, the cam is very street able and idles smoothly with the vacuum advance hooked up. With the mechanical advance only there a lot more lope, more erratic vacuum numbers and it seems to want a lo of initial timing.
The article by Duke Williams on the hamb about vac advance. You want the vac advance can pulled fully in at 2" below idle vacuum. He states adamant about manifold vacuum not ported. Do a search on hamb for distributor vacuum advance control units specs and facts for GM distributors. Ago
Thanks for the link Ago, that's excellent info. I recall having to ghave the vacuum advance be a couple inches different than the manifold Vacuum Looks like I got it backwards. So with the 16" manifold vacuum I have now and 13" in the can I think it's all good. My vac advance is hooked to the full time manifold vacuum source not ported.
Yes the engine seems to run great. I'm still tuning. I think I need to fatten up the mixture for part throttle transition, there's a bit of lean surge. My original question was why the vacuum gauge readings showed increased vacuum when applying throttle rather than reduced vacuum.
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