1966. 289 compresśion is dropping

Malibujoel · Nov 3, 2018

Hi All.
I have a 66’ mustang with a 289. The motor was built in the 80’s and put around 10,000 miles on motor before pulling motor out of car as it got in a accident. Motor has been sitting in garage since the 80’s till now and reinstalled into another 66’. I replaced carb with a new Edelbrock AVS2 and replaced distributor with a HEI distributor. Replace valve seals and other seals as well. Car hard to start. Idles at 1200 rpm. When put in gear idle drops to 380-420 rpm. Took compression and range from 105-115 psi. Checked valve adjustment again and push rods at TDC are loose again. So readjust valves and have 60psi in one cylnder now and push rod is loose again. Also performed vacuum test. At idle 16.5-18 with vacuum needle fluctuation and when car is put in gear vacuum drops to 15. Is it possible lifters have failed after sitting in motor since the 80’s not running till now? Or?.
Any input will be appreciated.
Thanks

WORTH · Nov 4, 2018

DO you have good oil pressure? are the lifters noisey when it's running? You may have over tightened the rockers. I'd back it off, and readjust them running.

Eric

Malibujoel · Nov 4, 2018

I have good oil pressure. Lifters are not noisy. Has roller rockers so it sprays oil everywhere when adjusting valves while running. Not sure why compression will change on same cylinders at times from 90 psi to 115 psi. That’s why I was thinking lifters failing as it’s the only component inline that is hydraulic. But I never seen this before. But with motor sitting for 30 years plus anything is possible. I removed intake manifold and removed 2 lifters from the cylnder that had the largest drop in psi. They slid out 2/3 of the way and needed plyers to get them out of block as they got stuck at that point.

Malibujoel · Nov 4, 2018

I just took a look at top of valve stems of #2 cylnder were I removed the rockers to get out the lifters. The top of the exhaust valve is mushroomed on a angle. Measured from keepers to top of valve. 0.151”-0.105”. The intake measures from keepers 0.177” all the way around as it’s not mushroomed. I took a visual of the other valves that still have springs on them and there are a few more that are mushroomed at top of valve. Wonder what causes this? I looked at roller rockers and they look and roll ok.

WORTH · Nov 6, 2018

To start with you can't take your compression readings as gospel. There is no way of telling how accurate and consistent your gauge is. If your valves are mushroomed I'd say it had an oiling problem at some point in it's life. If it were mine I would pull the heads and do a valve job, and replace any questionable parts, and do the lifters if you question them. Oh and I usually idle it as slow as possible and cover the rockers with shop rags while I'm doing the valve adjust. Then spend an hour cleaning up the mess when I'm done. You can't adjust hydraulic lifters while its not running, you have no idea if they are inflated or not.

Malibujoel · Nov 7, 2018

WORTH said:
To start with you can't take your compression readings as gospel. There is no way of telling how accurate and consistent your gauge is. If your valves are mushroomed I'd say it had an oiling problem at some point in it's life. If it were mine I would pull the heads and do a valve job, and replace any questionable parts, and do the lifters if you question them. Oh and I usually idle it as slow as possible and cover the rockers with shop rags while I'm doing the valve adjust. Then spend an hour cleaning up the mess when I'm done. You can't adjust hydraulic lifters while its not running, you have no idea if they are inflated or not.[/QUOTE

Thank you Worth.
I did pull the heads off on the weekend and brought them to the machine shop. They found 8 rocker studs damaged. This motor has all full roller rockers and no guiset plates. So good chance rockers eventually dug into the studs. Most of the rockers had some damage to the roller tips that were burned or scored. I removed all lifters and the all still look great and so do the cam lobes. So it looks like I lucked out on the cam. Rebuilding heads now with new seats, valves and new springs and 16 new roller rockers with just rollers on the tips. Will post once heads are back in on motor for results.

WORTH · Nov 8, 2018

While it's open, and since it has signs of oiling issues in the past I would replace the lifters anyway. No way to tell how they will preform until it's on the road.

Eric

Malibujoel · Nov 8, 2018

WORTH said:
While it's open, and since it has signs of oiling issues in the past I would replace the lifters anyway. No way to tell how they will preform until it's on the road.

Eric

Will new lifters not harm the old cam lobes?

Malibujoel · Nov 9, 2018

Got heads back from machine shop. Replace 8 rocker studs. Some were starting to notch (see pic)and some were badly notched. Replace damage studs and repaired valve stems that were mushroomed at the top and replaced all springs. Going to install heads this weekend and retest.

1966. 289 compresśion is dropping

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