Rocker Arm Adjustment

[Janusz]

I recently removed my valve covers and found a chipped Crane roller rocker arm. I believe it was due to a clearance problem with the valve cover (came that way). Ordered a new arm and replaced the damaged unit and readjusted the remaining rocker arms following the Comp Cams manual:

Turn the engine until the exhaust pushrod just begins to move upward to open the exhaust valve. Adjust the intake rocker by spinning the pushrod until you feel resistance and then turn the adjusting nut 1/2 more turn.

Turn the engine until the intake pushrod comes all the way up and almost all the way down. Adjust the exhaust rocker the same way including the additional 1/2 turn.

The engine started easy and ran smooth before ....... now it struggles and stumbles, wanting to shut down. I found most of the pushrods spun freely in the above adjusting position prior to making any of the adjustments, so now I'm wondering if the preload is set too high, and if so, is there a better way of measuring it.

Engine is a 351W Sportsman block, with SVO aluminum "High-Flow" heads. Rocker arms are Crane adjustable arms and cam is a Comp Cam 35-430-8. Cam spec's are:

Valve lift .560 int / .560 exh
Tappet lift 280 int / 280 exh
Valve timing 34 BTDC open int / 66 ABDC close int
74 BBDC open exh / 26 ATDC close exh

Looking for any suggestions.

John

[HighPlainsDrifter]

Hi,
You followed the correct procedure. The valves are adjusted too tight if your engine runs rough. What happens is the lifters bleed down and loose their oil pressure, this makes it seem like the valves are adjusted too loose as the pushrods will turn. An easier way to adjust the valves is to pull out all the spark plugs. Turn the engine over by hand with a socket and wrench on the front crank bolt. Have someone stick their finger over the cylinder you want to adjust[follow firing order] When air rushes out and stops,that cylinder is at top and on the firing stroke, = both valves closed. NOW you can adjust both intake and exhaust valves. loosen off the rocker adjusting nut until there is a noticible clearance on the pushrod. Slowly tighten it until there is no up and down clearance. Now give it 1/2 turn more to set preload and lock nut.
Perry.

[Janusz]

Perry,

Thanks for the reply back and suggestions. I'll give your technique a try. My gut feel is that I was unable to tell at what point the roller was touching the valve and was therefore over-loading the spring. I'll drop you an e-mail back after I get a chance to pull the covers and re-adjust the rockers.

John

[Janusz]

Perry,

Quick update ..... pulled the valve covers and readjusted all of the rocker arms using a feeler gauge to determine when the rollers were touching (versus spinning the pushrods). Covers went back on and she turned right over and purred like a kitten (albeit a relatively large kitten with a raspy throat). Thanks again for the feedback.

John

[trularin]

I agree with most of Perry's explanation, however, you may want to adjust to the top of travel and leave it there. I do not add 1/2 turn after the lifter is adjusted. This gets rid of top end float if the lifter pumps up due to higher oil pressure.

This is a common cheby trick. Install truck lifters and adjust them at the top.

It is even better if the engine is running and the lifters are fresh.

Good luck.

Just my $0.02

[HighPlainsDrifter]

Glad it worked John,
It's great to get a positive reply.
Perry.

[Janusz]

Perry,

I was meaning to ask, when searching through some old posts, I saw your post around your undercar exhaust. Did you use a solid connection like a clamp to a welded fitting to tie back to your side pipes?

John

[robbury]

OK. I have a 1968 302 w/351 heads, that's balanced and blueprinted, heads ported, decked, polished. I 'little bit of a cam', and 'roller rockers', I'm told by the seller, who had it built. Fresh motor.

First I adjust the valves using the method that John described, and I wind up w/40 - 90 lbs compression in all cylinders; not very good. Reading on, I see Perry's method, and it strikes me as easier to identify TDC on all the cylinders, so I readjust the valves Perry's way. Now I have 90 - 120 lbs compression across all eight cylinders; still not very good. Beginning to sense a pattern developing, I adjust the valves on number one cylinder one last time by Perry's method EXCEPT -- instead of continuing to tighten the valves 1/2 turn (preload) after zero clearance, I BACK OFF 1/2 turn. Check compression again: 210 lbs.

After doing all eight cylinders the same way, compression is 180 - 220 all the way around, and my motor runs like the proverbial violated simean. I mean, it's noisy, but it tachs to 6300 faster than you can say '6,300'. I'm not complaining here, but does anyone know why my motor runs best with less than zero preload on the valves?

[HighPlainsDrifter]

Hi,
John, my side pipes are hung at front and midway points by short brackets to the body/frame. The 3" undercar exhaust is hung to the frame by thick fibre exhaust hangerclamp and exits via a 90 degree bend to the sidepipe exit. The back of the sidepipe has an opening cut to give clearance for the 3" bend.
To answer the valve adjustment question-loosen up to get more compression;
Your valves were adjusted way too tight, you probably have hi rev anti pump up lifters. These lifters can be run at zero tolerance, or only 1/8 turn preload.
Perry.

[Hotfingrs]

robbury......sounds like it's a mechanical cam...not hydraulic