So I am aware that the valves lash is not adjustable on a stock 350 with the original shaft mounted rockers, however, what should this lash be? I am trying to diagnose a rough idle and bouncing vacuum needle and want to see if from any of the head work done to my motor might have resulted in the push rods now being slightly too long causing the valves to be tight. I would like to remove my valve covers and just confirm the lash and see if its worth getting adjustable rockers. Thanks in advance
That's always a tough one. You mentioned having some cylinder head work done, so if you could elaborate a bit? That could help. "Lash" doesn't really fit descriptively for a hydraulic lifter application, maybe a better term is "lifter preload" Things that can affect preload with cylinder heads could be: Removing material from the deck side of the heads Valve stem tip height (maybe due to excessively sunk valves after a valve job, or excessively cutting the stems). There's other stuff too besides the heads to affect this, like having the block's deck cut, wrong lifters (chevy lifters have the cup a bit deeper), an aftermarket camshaft with a taller or shorter base circle, what have you. The last thing we like to hear about, but happens, is a worn camshaft or failed lifer(s) And lastly, I've seen lifter preload all across to board, big and small...there just has to be some, even if it 0.010". Devon
So I just had the engine rebuilt last year. I bought the heads from a buddy and they had been previously refinished with larger valves, I believe they had been decked, intake port and polish. I really dont know how much they have been decked tho. I am running a TA 284 cam, lifters are from a comp cams buick 350 timing set. And the pushrods were checked for length by the engine builder, I never heard anything other than that, but this bouncing and low vacuum at idle in gear has me chasing ghosts. Everyone tells me something different. One person swears its the carb ( stock rebuilt q jet) another swears its the valve train. Thats why I figured if there was an adjustment I can check in the valves thats a pretty easy thing to check.
Sounds like a vacuum leak to me. I recently had to fix another one on my '67, brake booster grommet was the culprit, before that was throttle shaft leakage. Easy to check and fix.
Everything in the vacuum system should be almost brand new, not to say I couldnt have a defective part ( i.e. New booster that bleeds vacuum). Ive already sprayed carb cleaner around the carb base gasket, maybe I'll try unhooking all the vacuum lines that arent needed to start the car and plug the ports then try it again
That's a really good idea. You could even go so far as to plug PCV for the test. If it does get a lot smoother during that, you might be seeking intake manifold gasket leaks afterwards. Devon
If you pull the valve covers, do a visual check on the heights of all the valve tips or put a level on them to make sure they are all the same. If they are not, or one is higher than the others you may need an adjustable or custom length push rod there. A compression test will also show lower compression on a few if that's the case and it's bleeding off compression by opening too soon.
Updates: Pulled any non essential vacuum lines yesterday: no change in vacuum pressure Did a compression test last night, range in compression from 141-148 pounds of vacuum so I don't think my issue is there either. (All spark plugs looked good, maybe a touch on the leans side if anything. The only thing I can think is a carbuerator adjustment, I so have a stock rebuilt q jet carb on this mild cammed engine. I'm taking the car to carbs r us in Thomaston ct today (same guy who rebuilt it in the first place) he's going to try to work his magic, hopefully he can atleast identify the issue if he can't get her purring right.
Retarded ignition timing can cause what you are describing. Most cams love a lot of initial timing, up around 18-20* depending on the size of the cam. Of course you need to reduce the amount of mechanical advance in the distributor when you run more initial timing. You can temporarily advance your idle timing to see if it helps, just don't drive the car as you'll probably have too much total advance at higher engine speeds. As far as lifter pre load goes, adjustable push rods are a cheaper alternative to expensive roller rockers. You can turn the motor to TDC on each cylinder and adjust both valves of that cylinder. You can even use one adjustable push rod to determine what length push rod you need. As mentioned, if your valve tip heights are all over the place, than you need to use 16 adjustable push rods.
Put #1 at TDC (zero on the timing mark) and set half the lifters. ONE revolution of the crank will put #6 at TDC and set the others. I'd run it until warm and plenty of oil throughout valve train first. A simple "barely spin with some resistance" (nice oil film feel) should be set for zero lash. Any looser or tighter will mean wrong push rod length. Another 1/2 turn down to load hydraulics. Too bad Buicks arent adjustable. Push rod length is the only option there. If they become TIGHT, youll need shorter rods. Loose and youll need longer. If loose at the valve, say .005", youll need to do the math at .005" X 1.5 or 1.6 depending on rocker ratio. If questionable, I'd go adjustable across the board. Done. This is the correct order for each revolution. FWIW, if its a Dorf, all bets are off. This is for the 18436572 orders only! ws Get #1 to Top Dead Center Exh.- 1 3 4 -8 Int.- 1- 2 5 - 7 Get #6 to Top Dead Center (this is also #1 TDC on the exhaust stroke) Exh.- 2 5 6 7 Int.- 3 4- 6 - 8 Adjust until pushrod will not spin between fingers and then add ¼ turn and tighten poly lock. You will only turn the engine 2 times. TDC #1 and TDC #6
Can you define what you mean by a bouncing vacuum needle? A TA284 cam in a 350 is not going to idle smooth. Its definitely going to have a lope or a rougher idle than factory. And the needle on the vacuum gauge is not going to steady. What is the actual vacuum reading?
X2 A bouncing vacuum needle indicates a leaky valve. A varying vacuum reading is typical of a non stock cam. Does the engine idle with a da da da da da sound? That's indicative of a leaky intake or exhaust valve Or is it rumpita rumpita rumpita? Have you done a leak down test?
It does run with more of a rumpita rumpita, if I can I will take a video of it this weekend, the thing rocks quite a bit while running. I have not yet done a leakdown test, only compression. but I can put that on the list of things to try this weekend.
A stock rebuilt Q-jet can use some mods to the idle circuit to make it easier to idle a cammed engine. If your carb guy can't figure it out there are 2 guys here that custom calibrate carburetors, and they can. Q-jets were calibrated for the engines they came on. That no longer applies with the cam change.
