How to diagnose cam shaft?

[thelawdoc]

Gentlemen, I have a stock '66 C10 250 w/2 spd Powerglide & Rochester B. The odometer says 65k. Truck was starting and running nice until about a month ago when she began to start with great difficulty and suffered frequent stalling at red lights. I rebuilt the carb, changed plugs and wires, changed point & condenser, changed both fuel filters, verified good fuel pressure, new battery, fresh gas and good spark with wires correct for firing order of 1-5-3-6-2-4. The other day I finished up this tune-up by doing the timing and adjusting carb. I take her for a test drive, she starts up and runs great for about one mile. Then stalls. She restarts easily and runs nice back to my house with a couple stalls. I check the timing again. The rotor is off by 180 degrees! And I could not restart her. I check distributor gear -it is like new. No marks. I look down into the distributor hole and everything looks fine in there too. The belt is fine. I pull harmonic balancer and take off timing gear cover and the gears look pretty new, though my harmonic balancer does not look very good (the rubber is separating from it). Mechanic/buddy says my harmonic balancer is shot and cam shaft must be worn out. But I am not confident about the cam shaft being bad. The only other clue I have concerns the carb needle: when I close it all the way in during adjustment of carb, the engine does not shut down as I believe it ought to. Also, I did not do timing with vac adv plugged. Any ideas? Could it be the cam shaft (I hope not)?

[awbrown]

sounds like the timing slipped ... what carb do you have ?? normally turning the idle mixture screw in and having it continue to run means there is an internal leak supplying fuel ,welch plugs leak often.. I have seen people use a high volume pump on 6 cyl. and it will rip the gear off the cam , make sure the distributor is still turning when you crank it .. the only other thing would be the roll pin ....you can tell if it needs a cam if any valve is not moving when the engine is cranked..

[MikeN]

The engine wouldn't run at all with the rotor out 180 degrees. Wouldn't start, wouldn't run, nothing. And there's no way it can "jump" from correctly phased to being out 180.

If your carb has a stuck needle & seat, or a float that has a pinhole and fills with fuel/sinks, you'll get flooding which will cause the symptoms you describe. Inoperable choke can also cause the problems you are having.

Bad points, condenser, or coil can also cause the symptoms you describe.

I seriously doubt your cam is a problem. You're chasing your tail there. Concentrate on the carb...I think you'll find that this is choke or needle & seat related. Your choke may not be opening properly once the engine has warmed up.

[thelawdoc]

Originally Posted by awbrown

sounds like the timing slipped ... what carb do you have ?? normally turning the idle mixture screw in and having it continue to run means there is an internal leak supplying fuel ,welch plugs leak often.. I have seen people use a high volume pump on 6 cyl. and it will rip the gear off the cam , make sure the distributor is still turning when you crank it .. the only other thing would be the roll pin ....you can tell if it needs a cam if any valve is not moving when the engine is cranked..

Awbrown, Thank you for responding. Yeah, I figure there is internal leak supplying fuel, but I do not see how that might put me in the situation I have. I do not know what a 'welch plug' is. My fuel pump is the stock fuel pump and seems to be working just fine. I will check today the movement of the dizzy while cranking. The roll pin is good. Right now, I have the timing gear cover off and we are trying to decide what to do about the cam shaft.

[thelawdoc]

Originally Posted by MikeN

The engine wouldn't run at all with the rotor out 180 degrees. Wouldn't start, wouldn't run, nothing. And there's no way it can "jump" from correctly phased to being out 180.

If your carb has a stuck needle & seat, or a float that has a pinhole and fills with fuel/sinks, you'll get flooding which will cause the symptoms you describe. Inoperable choke can also cause the problems you are having.

Bad points, condenser, or coil can also cause the symptoms you describe.

I seriously doubt your cam is a problem. You're chasing your tail there. Concentrate on the carb...I think you'll find that this is choke or needle & seat related. Your choke may not be opening properly once the engine has warmed up.

MikeN, I cannot figure what put the rotor out 180 except for a broken gear somewhere. I am pretty confident that the carb needle and seat are good, but that should not make for the situation I have, right? The carb is rebuilt with new, good parts. I rebuilt it myself. The manual choke works fine. Point/condenser are new. If it is not the cam, what would put the rotor off by 180?

[1971 GMC SHORTY]

I Had This Happen on my 250 Years Ago Timming Gears Are A Fiber Material Then When lead Gas Change oner I Burn Valve Seats Had to Puul Apart 2 Times In less Then A Month
Pull Cap Crank Engie Look At Dist Rotor Smooth or if it's Jump

[1971 GMC SHORTY]

Had This Happen on my 250 Years Ago Timming Gears Are A Fiber Material Then When lead Gas Change oner I Burn Valve Seats Had to Puul Apart 2 Times In less Then A Month
Pull Cap Crank Engie Look At Dist Rotor Smooth or if it's Jumping

[thelawdoc]

1971GMC Shorty, Thank you for responding. Yes, I intend on cranking the engine and having a look at the rotor movement. I will also check to see whether cam shaft moves the valves. I am very hesitant to start pulling the cam shaft- for some reason I feel that the cam shaft is not the cause here. My timing gears are new, metal. My mechanic says he can swap the engine for another I6/250 cheaper than replacing my cam shaft. I started out doing timing on my perfect truck and I end up now talking about swapping out the engine. Talk about thickening plots.

[geezer#99]

You've got thickening plots because you immediately jumped to conclusions.
You need to go back to basics.
After you check your rotor is turning then first you need to prove your timing marks on your balancer are right. Here's a link on that.
http://www.crankshaftcoalition.com/w...op_dead_center
Then put #1 on tdc compression. Line up your rotor pointing at #1 on the cap and rotate the dizzy ccw 1/2 inch and tighten it down.
Make sure you have fuel in the carb. Stroke the throttle and you should see fuel squirted inside.
It should start like that.
Once it's running and warmed up, set your idle speed and adjust your mixture screw for best idle.
Then plug the vac line to the dizzy vac pot and set your timing at 10 degrees. Hook the vac line back up and adjust your idle and mix screw again.
Do you have a pic of your engine bay you can post?

[thelawdoc]

Geezer#99, At the moment I have radiator out because I wanted to inspect the timing gears. Within the hour we will check the cam shaft action on the valves and the movement of the rotor. But yesterday, when I had completed the tune-up, the rotor was on No.1 and timing was set. I drove one mile and the rotor went off 180 deg. As I mentioned, I did neglect to plug vac.adv. when I set the timing, but I do not think that could put the rotor off 180. I figured only a broken tooth can put a rotor off 180. Am I wrong? I must be, because I cannot find any broken or damaged teeth anywhere. The cam was my last suspect. I will post photo of engine bay later today. I am suspecting that my harmonic balancer is bad and maybe that is putting the timing out of whack.

[geezer#99]

A little light bulb just went on in my little brain!! LOL!!
If you don't have the dizzy locked down with the bolt and clamp correctly, then the dizzy will rotate cw until it can't move anymore. The centrifugal rotation of the dizzy causes the housing to rotate.
I had that happen on my motor. Couldn't figure out why my timing kept changing.
Burn me off a pic of the clamp on your dizzy.

[thelawdoc]

Geezer#99, well I thought I tightened up the dizzy clamp properly, as well the cap screws, but I will be double checking everything of course. Right now, I have to order a new balancer because mine seems to be no good (the elastic ring looks bad). I don't want to play with a cam shaft until I have all other suspects ruled out.

[geezer#99]

I thought I'd tightened mine too.
Mine was the wire type with a clamp and bolt.

[thelawdoc]

UPDATE: Ok, so I have a happy ending and a lesson learned today. Firstly, the timing gears on my truck, while new and work fine, have dot-markers on them for some other engine. I discovered this when I brought the No. 1 piston to tdc and the dots did not meet up. Apparently, the p.o. did not think to fill in the inapplicable dots and drill in new ones when he used these gears. So that drove me crazy for a while until I decided to check the piston. Lesson learned: don't trust the dots! Secondly, a bad harmonic balancer will, indeed, throw off proper timing. I never gave this part much thought, but my balancer went for 50 years and 65k miles, so I think I got my money's worth out of it. Thirdly, when attaching spark plug wires, make sure you go clockwise on the dizzy for the firing order {duh}. At some point yesterday when I went down my checklist, I put the wires back on counting counter clockwise on the dizzy cap. Yeah, dumb. It was really cold out and my mind was not working. So, all in all, the day ends happily. Thank you gentlemen for the usual, generous help.

[geezer#99]

It's good you got some of it figured out.
But you didn't figure out the dots.
When they line up right opposite each other then you're on #6 tdc. To get to #1 tdc you rotate the crank once. It's the same on a six as it is on a v8.

[thelawdoc]

Geezer#99, Hmmm. I did not know that. I figured the p.o. used timing gears from an other engine. I will do as you suggest tomorrow and check it out. Thanks!

[old cruiser]

Originally Posted by geezer#99

It's good you got some of it figured out.
But you didn't figure out the dots.
When they line up right opposite each other then you're on #6 tdc. To get to #1 tdc you rotate the crank once. It's the same on a six as it is on a v8.

This doesn't sound right to me . When the dots are virtually touching each other #1 should be on tdc , ready to fire in this position your distributor should be set so the rotor is pointing to #1 . Timing is then set via marks on balancer and tab on timing case.

[geezer#99]

Originally Posted by old cruiser

This doesn't sound right to me . When the dots are virtually touching each other #1 should be on tdc , ready to fire in this position your distributor should be set so the rotor is pointing to #1 . Timing is then set via marks on balancer and tab on timing case.

Do you mean on a six or on a v8?

[old cruiser]

Originally Posted by geezer#99

Do you mean on a six or on a v8?

On a 6 .
V8 has dots to line up with marks on the chain

[thelawdoc]

Geezer#99, old cruiser, Why do I even need to look at the dots if I manually verify No.1 cylinder at tdc? I will have a look and see what, if anything, the service manual says about this.

[rideblue00]

hmmm, suscribed

[MikeN]

If it was running fine at some point, and the gears are fine, they are obviously phased correctly. Don't worry about it.

[thelawdoc]

MikeN, that is exactly my thinking. My truck was running smooooooth for many months after I acquired it. I do not know much about the dots on the gears. But I know tdc is tdc no matter what any dots say, lol.

[chevy_mike]

Make sure there is a woodruff key in the cam, holding the gear in place. If this is missing, it could allow the gear to spin on the end of the cam. This is not like a V8 where the gear bolts to the cam. See in my pic from my 250.

[thelawdoc]

Chevy Mike, Yep, I got the keys in place. And the gears are new. That is how I know either my p.o. used gears dotted for a different engine, or, as said above by Geezer#99, that the dots indicate tdc for No.6. In any event, no matter now for me. Truck fires up nice and runs nice.