How to tell if a cam is worn?

[FreeBeeTony]

I am still not satisfied with the power in my '78 20' Grady with an IL6 165 Mercruiser. I know it's not going to be a speed boat.....but in order to achieve WOT of 3900RPM's I can only turn a 15" prop. I thought the engine would have been a bit stronger........I completely rebuilt it last year: bored out 0.030, new pistons/rings, bearngs, oil/water/fuel pump, new lifters, recond head/valves, new alt, rebuilt carb. The block is a '67. I used the existing crank and cam. Is it possible the cam is worn and I am loosing power due to this? Is there an aftermarket cam I could install and get more power? It runs very well.........would just like to get more power!!! w/o adding cylinders....

 

[KaGee]

Re: How to tell if a cam is worn?

I would suppose you would check it out of the engine with a micrometer. Measuring the lobe height.<br /><br />I've never had an engine, auto or otherwise that had a bad cam. Saw one, one time and it was obvious watching the valve train. When a cam goes bad, from what I've read, you begin to have ignition troubles. Usually a cylinder or two won't fire or fire intermittent. Worn lobes do not push the valves open enough to acheive proper intake and exhaust.<br /><br />I doubt you have a cam issue, but you can examine that possibility. My buddy has a 21ft Grady fishing cuddy of that vintage. He has a 470 Merc 4cyl whis is rated at 175HP. The fastest we can get that boat to run is 23-24 mph by GPS on a smooth lake. Tried different prop ratios, trim tabs, It's just a heavy boat and under powered if speed is what you want.

 

[FreeBeeTony]

Re: How to tell if a cam is worn?

Thanks KaGee, that's the kind of comparison I am looking for. I can do a little batter than 25 mph by gps trimmed at wot. Maybe this is the best I can expect........I already have cabin fever........and it's only Thanksgiving! Just put the bird in the oven........whilr looking at the boat in the backyard on the trailer!

 

[rodbolt]

Re: How to tell if a cam is worn?

hello<br /> normally cam lobes that wear enough to affect valvetrain action can be diagnosed by the amount of metal in the oil. a quick test is to look at the tip of the cam lobe. if its worn all the way across the face its most likely worn out. the face of the cam is not flat but is actualy angled slightly and the lifter foot is actually slightly convexed. this causes the lifter and pushrod to rotate while moving. when it quits rotating it will fail. the cam is cast iron that is flame or induction hardened. this is only .01" or so deep. if the cam was harder or the hardness deeper it would break.<br /> once the hard surface is worn through the lobe will round off rapidly resulting in little or no rocker arm motion.<br /> I doubt your problem is a worn cam. a worn cam lobe invariably leads to popping back through the carb.it can be traced quickly with a vacum gauge. it will look like a broken spring or a burned valve on the gauge.

 

[rodbolt]

Re: How to tell if a cam is worn?

hello<br /> and with over 1/8 of an inch about 1/2 inch wide of metal shavings I will bet you have mettalic oil. that metal went someplace. it did not just dissapear. if you inspect the rod and mains you will find out what is know as Embedability of a bearing. most the time when you have that much wear ,especially on a carbed motor, you will hear popping at WOT. EFI on the other hand will run fair with a wiped cam. the difference is in how the EFI and carbs move fuel from the gas tank to the cylinders and how the ECU on an efi motor can compensate rapidly for RPM and engine vacum changes. and the WOT thing is a major difference between us techs and auto wrenches. go on down to the chebby hause and talk to the tech about a high speed miss at 4600 but only after 30 mins.<br /> he will look at you rather dumb.<br /> while your looking at the cam look to see if the lobes are shiney all the way acrossed or only about half way or so

 

[Buttanic]

Re: How to tell if a cam is worn?

I have seen worn cams and the engine does not pop back. I noticed you said you changed lifters but you did not change the cam. Putting new lifters on a used cam is risky. Some times it works out but most times it will eventually ware down some or all the lobes. Right now I am changing the cam in a friend truck. He rebuilt the engine two years ago and put new lifters on a used cam. At least one lobe is worn from .302 lift to .150 lift. I didn't check them all but I expect to find others also worn.

 

[FreeBeeTony]

Re: How to tell if a cam is worn?

Hi tom, yes the distributor is the original, I installed a petronix kit. I know the timing is advancing but never measured how much. That might be something I should do in the spring.<br /><br />As fars a WOT, the spec calls for 3800-4200, so I'm figuring 3900 isn't too bad.

 

[tommays]

Re: How to tell if a cam is worn?

tony<br /><br />i think you should start back at the simpler things first like what should the WOT be 4400 ?<br /><br />no matter how heavy the boat the motor should hit the WOT your not getting anywere near your max power at 3900 rpm<br /><br />things like total timeing advance is this the orginal distributer when we repowered the 1977 correct craft this year we started with the old distributer and it caused all kinds of problems as it was as worn out as the old motor <br /><br />the old spring style advance just did not work right anymore and the orginal carb was a sorce of many probems also <br /><br />while these may not be your problem we found that when repowering everything is old not just the block<br /><br />the difference the updated ingition system alone was huge<br /><br />tommays

 

[tommays]

Re: How to tell if a cam is worn?

tony<br />i think if you can find the last 300 rpm it will make a big difference<br /><br /><br />tommays

 

[FreeBeeTony]

Re: How to tell if a cam is worn?

I don't want to find the last 300 rpm by changing the prop though............<br />I will try to measure the total advance in the spring. What should it be anyway?