muggsyjack

During the past weekend we had power drop off at the top of each gear and on burnout (5700 to 5800 RPM). As my son states, it feels like misfire but no real banging or poping sounds. looking at carburator, all four intake throats have black buildup.

The drop is not the rev limiter as it still pulls some before he shifts. The rev limiter hits the wall and does not increase.

I have never used a hydraulic cam above 5000 RPM previously. Of course it was many years ago, and things do change. The cam came from Summit with lifters. .488 lift and 235 duration at .050. The engine is a 91 305 shortblock with H.O. 305 heads w/o swirl ports. Cranking comp. was 220 PSI. I am also using 1.6 roller rockers. The valve springs are the H-11 tool steel from Crane. One of the old heads is still here and a strong push opens the valve. The current heads require me to brace myself to push open. I know it is not scientific but it works. The current springs are much stronger.

Is it possible that the lifters are pumping up at high RPM? Is it more likely that the valve springs are to weak. All my vale float experience is with motorcycles. When you float valves, they usually backfire pretty violently.

Any comments? The only change on the car was going from 10-30 to 15-50 synthetic motor oil. I would not guess that the springs would weaken after sitting idle?

AlkyIROC

I'd check the heads to make sure they can take that much lift first. With the 1.6 rockers you're now getting .520 lift. What are the specs for the springs? Are they set up for that much lift? Are the heads set up for thet much lift?

If the valve guides haven't been machined down it's possible that the bottom of the spring retainer is hitting on the top of the guide because of the extra lift. The other possibility is you're getting coil bind.

Coil bind is hard to check with hydraulic lifters. You need a pair of solid lifters then rotate the engine until the valve is fully open. You should be able to fit a .050 feeler gauge between the coils. You can't check it with hydraulic lifters because the lifter will bleed down.

Hydraulic lifters don't usually have a pump up problem until over 6500 rpm.

Valve float is when the valve is closing. The springs close the valve as it comes off the ramp. If the springs aren't strong enough the valve will bounce on the seat as it comes down. This reduces cylinder pressures and reduces performance.

Personally I think you need some better head work. You can try installing some 1.5 rockers to see if that helps.

muggsyjack

Head supplier said .600 lift. No guide clearance trouble. No coil bind. Granted the engine was not running, but when valve is fully compressed I read .460" of lift compared to base. There is plenty of clearance in coils for more than another .120" of lift.

The funny thing is, this did not happen until I went to heavier oil. I am running a high perf pump and getting 50 PSI at idle.

I am thinking of backing off the preload as much as possible w/engine running and test.

muggsyjack

I also forgot to mention that none of the pushrods were bent either.

mod313

Sounds like not enough spring seat pressure. Also, be sure you only "zero" lash the valves. Doing that will allow your hyd lifter motor to zip all the way to 7k without issues as long as spring pressure is good.

AlkyIROC

Hydraulic lifters need .030 preload to work properly. You adjust the rockers until there is zero lash. You then turn the adjuster so that the pushrod goes down .030" into the lifter. That's usually 1/2 turn on the rocker nut.

mod313

Not necessarily the case Stephen. We do this with class motors that run hydraulic lifters, both flat tappet and roller. Zero lashing allows us to zing these motors in access of 7k easily. Not doing it will keep the motor revving lower.

muggsyjack

I zero lashed valves and did burnout at house. According to my driver, the 5800 RPM drop off is gone.

The real test will be Friday night.