cam and distributor gears are in good shape? Your idle speed should come mostly from initial timing and the idle mixture, the idle speed screws should only be used for the last little fine adjustment. But if ypur idle mix screws doesn't have any effect like I recall you saying them there is another issue at play As far as the LSA goes, I've had a Poston 105a cam installed at 103° LSA that had a rock steady idle and timing and have a 107 window rattler now with rock steady timing and idle, pull the distributor and look at the distributor gear, maybe the chain has stretched or,.. Have pulled the covers and measured valve lift?
LSA is the angle between the center lines of the intake lobe and exhaust lobe. Smaller LSA's and longer durations increases the chance of overlap, when the exhaust and intake valves are open at the same time. More overlap decreases engine vacuum and contaminates the intake charge at lower RPM (idle). All of that contributes to the rougher idle. The rougher idle is one of the drawbacks of larger cams with more overlap. Normally, running more initial advance makes the idle more stable, but it doesn't eliminate it. If the springs are too light, the weights flutter at idle causing the timing to jump around. Put some stock heavy springs in and see if it helps. If it doesn't, check the distributor for shaft play, both side to side and up and down. There are shims that will take up too much up and down free play. You'll have to remove the distributor and gear to install them. I forget what the free play should be, but I think it is something like .015.
Sounds like closing off some of the secondary throttle opening and drilling a couple holes in the primaries might be a good direction to explore. I'm not actually there with everything in my hands, so I'm not really recommending drilling anything yet If more timing roughens idle, then back it off a ways. You should be OK making minor speed adjustments from the primary. I don't think you have a wild enough combo to necessarily need the sec's open at idle. The front mixture screws should have more control then. It's tough for us to guess, so a good way to learn is play with both avenues of thought until you either see something work or run down the "duh" thing that was missed.
More overlap 'potentially' decreases vacuum. It depends on how the specs compare to the cylinders being fed (and the induction's characteristics). You can make some extreme cams idle a LOT smoother than you would think possible.
The dist. has about 5,000mi. on it, the cam gear about 40,000mi. The vacuum reading fluctuates rapidly between 10-12, the best I can get with the ps front and rear and ds rear mixture screws 1 turn out to somewhat the smoothest idle, the ds front mixture screw bottoms out before it reaches it's best adjustment. The vacuum reading has always fluctuated rapidly with the V6, 350 with both the Q-jet and Holley, 1st rebuild and 2nd. I've tried 2 different vacuum gauges with the same results, maybe i'll try a 3rd. Cam chain and gears are new from TA, the crank moves about 2deg. before the dist. does, there doesn't seem to be any side to side play in the dist. but you can move the shaft up and down some. I'll keep swapping springs around and see if I can stop the flutter and then see where the initial and total are at. Here is the cam card. I gave all the info to JW and he had this cam custom ground for my car, the heads are bone stock, zero deck 10 to 1 comp. (once the heads come out I will then go to a roller cam).
The idle was smoother with the 1st rebuild and with the V6, but the vacuum flucuation has always been there and now it has a real raspy idle
Your cam card didn't show up for me, but I can almost guarantee that 10-12" is way too low for it. Still thinking that backing off timing would see that number climb. Many are using much more cam for the cubes and seeing better vacuum. 255* @ .050" can produce 10" of idle vacuum on 350" with a long stroke. Jittery needle on 2 gauges is not a gauge or mixture issue. (hopefully it isn't something like the gauge simply resting on the engine? I set it on a rag) Your going to have to put some time studying Google for vacuum readings and look for a basic mechanical issue. The up and down motion the distributor has IS the end play being referred to. Often times, leaky valve seats give the needle the jitters and surprisingly...I've seen more than one engine come out of a specific shop having the same problem due to poor machining procedures. On that one, the needle drops more severely as the throttles are opened, until the engine speed recovers. Every guess will border the obscure the further this goes... I don't mean for this to be frustrating to you but it would take maybe 15 minutes for a pro to figure out what's going on here.
Got a new vacuum gauge and now it reads a slow steady sweep between 10-11. Googled vacuum readings and it says that this is normal with the problem being just idle mixture or timing. The needle is a whole lot steadier than before. It also stated that you will loose 1 inch vacuum for every 1000ft elev., I live at 5800ft elev. And I let JW know what elev. I lived at so I had enough vacuum for power brakes, and I do. I will start with getting the timing to quit jumping around first and then go from there. Off idle this thing has killer torque and if it had any steeper gears than the 3:23's I'd have to take off real easy. Thanks, Keith
Sounds like you are using a dampened gauge now. Are you suggesting the other gauges are faulty? If your timing is jumping around, your vacuum will fluctuate a bit anyways. If it isn't, the gauge is hiding plenty.
I was going to say that 17-18" would be normal for a 9:1 version of this, so even with altitude there's still room for improvement (tuning).
223/235, 110 LSA. I wouldn't expect more than 15-16" of vacuum from that cam. Sounds like he is about where he should be for nearly 6000'.
You'll lose 1.5-2" with a Holley compared to a good Qjet there too, depending on what exactly. Definitely better info for people to work with, now knowing more of the circumstances, such as altitude. I still don't think everything is solved by the replacement of the gauge. It is nice to knock down one thing at a time while tuning.
The first 2 vacuum gauges I was using were off of hand vacuum bleeders for bleeding brakes, the gauge I now have is a actual vacuum/fuel pressure gauge
Definitely a more dampened gauge then Use it for tuning and if needing to discern a more difficult problem such as valve sealing, etc., then use the more sensitive ones. IF that scenario is happening, you'll find the carb won't fully respond to tuning changes. I see that one quite a bit from the local budget machine shop VJ's. Big name, low price. Performance cams hide sooo much I suppose the true test is to put the 2 gauges on a smooth running modern car. You don't have to agree with me, just leaving the info for educational purposes. I'd rather end up wrong half the time and have people solve their problem without it getting so costly.
I think the most accurate vacuum gauge I have ever seen is the Autometer ones. The commonly available gauges in Parts stores seem to disagree a lot.
The industrial brands are more consistent and purposefully built, such as Magnehelic and Dwyer (among others). The number itself is less relevant than the needle's movement or behavior in diagnosis. I don't use parts stores ones unless they are at someone else's place Anyways...the point of this tangent is that there's plenty of room for tuning and tons of valid aspects have been mentioned. People that have done dozens to hundreds of performance cammed carb tunings have dealt with plenty of puzzles. The only person with the ability to truly see what's happening is right in front of the vehicle, and is now armed with better information. It's just as frustrating to have the OP reply with "I called so and so and they said this..." or something else simple that negates what many in the thread see as an area of improvement. Put the sensitive gauges back on and quickly open the throttles heavy and see if the needle flicks a few times severely before the rpm catches up and raises the reading. The "in between" and transitional throttle play usually speaks volumes. Rule that out then keep playing with things.
