NF Question for inbourd 140 merc techs

[vinnimon]

Well this has happened once again,Dang.Camgear has failed for the second time.It is a composite fibre gear.As I was told, its to save the valve train just in case.

Just to give you the ins and outs,everything has been done,you name it,its been done.

Heres a list

New crank bearings,crank turned as well

New cam shaft and bearings

2 pistons heads,wrist pins and connecting rods replaced

New rings,head was plained as well

All valves ground,seated and seals replaced

Cam and crank gear replaced

New lifters and push rods

New starter,coil,ign wires

2 new batteies

mech fuel pump tested, excellent

new fuel filters and high end fuel lines

Shall I go on?

My problem is that this composite cam gear fails at high rpm under load,arfter a while.

Mind you that the oil pressure is fine,I watch for it constantlly.At idle 35 psi,full out i think its 60 plus.

Anyway, I know its a fail safe cam gear for the valve train.Is there something Im missing for this to gear to fail once again?

Any suggestions would be appreciated

Keep note that some of the work has been proffesionally done.

Thanks in advance V BTW 1985 honda 7.5 4 stroke kicker is awesome.

[Gerritt]

just a shot in the dark here... but have you considered you're prop? perhaps you are getting to much RPM.. or you have something completely out of whack.. what year in the motor? as ell as hours on it?

 

My initial thought Vince...

 

 

G

[DanD]

I ran into this a few times when the Iron Duke was popular in GMs; especially the Fieros. People use to think they had a race car and spun them up pretty high rpm wise.

There was an issue with the harmonic balancers causing or not controlling unwanted torsional twist of the crankshaft. This twisting and flexing back (washing machine action) would pound on the cam gears teeth, causing them to strip out at high rpm. This being a marine application; make sure the proper balancer or if available an up-dated balancer is installed and that it’s not an automotive one; I believe the cranks are balanced and configured differently for marine. I have seen people put on an automotive balancer to gain the pulley configuration to drive accessories like power steering.

The camshaft’s end play is also very important; to much and the cam and gear will walk in and out causing a sawing action against the fiber cam gear and steel crank gear. With the cam, cam thrust plate and gear installed there should be .0015” to no more then .0050” max; measured between thrust plate and gear flange. To tight can also cause the cam to bind again damaging the gear.

The next thing is the poor oiling that the gears get; the only oil they originally received is from whatever leaks out of the cam and crankshaft’s end bearings. To address this GM brought out a TSB saying to drill a .040” (1mm) hole in the oil galley end plug behind the cam gear. This will allow main line oil pressure to spray against the gear. Do not dill this hole any larger then the .040”; bigger isn’t better in this case. You’ll loose too much oil pressure through it and possibly starving something else. Maybe when the block was rebuilt and the galley plugs removed to clean the galley they didn’t drill the new plug?

Also always replace the gears in matched sets; if your supplier only offers them with individual part numbers they are old stock don’t use them.

Now mind you all of this is automotive but it probably also applies to marine the basic blocks are the same.

One last thing; make sure you pull the oil pan off and clean out all of the chewed up fiber gear that will be in there and in the oil pump pick-up tube.

 

Dan.

[The JAY]

Can't help with the diagnosis, but I can attest to the reliability of the kicker!!!

[vinnimon]

Thanks guys!

Another question: After I fix it,should I go with a 21 pitch prop?I have a 17 on it now.

Will it reduce the rpms at higher speeds and possibly prevent this from happening again?

Like I said everything has been done,some professionally as well.

Ive done a few total engine rebuilds and never an issue,just this one.

 

Before I forget, A big thanks to thejay for his help. And the day wasnt a waste, a few fish were caught as well.Couple sheep head,one gobie and a bunch of kitties.Biggest was 4 lbs or so.And a nice day to be out as well.