Diagnosing odd blowing (not sucking) on intake after breakdown...cam timing?

[Indydave]

(I have only moderate repairing skills and limited tools) I was riding my 1979 CB750F when it suddenly shut down with some rattling noise toward the front of the engine indicating what I thought was a broken timing chain. The engine would crank and seemed to have similar compression as before. It made a hint of trying to fire perhaps on only one of the four cylinders. I removed the valve cover to try to confirm if the chain broke and it did not, and seemed to have good tightness. Everything looked fine when I cranked it. However when I put my hand over the number one (left as I sit on it) cylinder, it did not feel like it was sucking and it actually was puffing air out fairly strongly. To me, this suggests that the cam timing is off badly and that suggests that the chain somehow slipped but did not break. Perhaps there was wear on the sprockets or a broken tooth. My question is, is there a way to confirm my suspicion without doing major tear down? Is there an easy (relatively) way to reset the timing? Could I loosen the tension and get the chain off to allow me to turn the sprockets so they can be set at the correct timing marks?

 

[Indydave]

Also, if the timing chain slipped while the engine was running would that automatically mean that a valve(s) was bent...and therefore the engine must be removed from the frame and the head removed in order to replace valves?

 

[BB74]

How lose is the timing chain? It would need to be very sloppy in order for it to slip a tooth.

The puffing air could be a bent valve or poor valve sealing on that cylinder.

Check for spark and fuel before you dig into taking stuff apart.

 

[Indydave]

Thanks BB74! The chain is quite snug. I did check the fuel and spark. There is spark on three cylinders but I could not see it on the 4th. Could it possibly be that the negative ground to the engine is corroded and that is what caused it to shut down suddenly? But that should not have caused any change to the timing and therefore the air should not be blowing (o casional puffs) out the intake. (Trying to not sound argumentative, just explaining).

I will check the negative ground and also perhaps connect a jumper cable from the negative battery terminal to the engine.

 

[BB74]

Cylinder 1 and 4 run off the left hand coil. check for loose wires at the front of the coil.

Mine were loose and caused a intermittent misfire when I first got my bike.

While you have the valve cover off check the intake valves shim clearance on cylinder #1 to see if the valve is getting stuck open.

 

[dirtdigger]

More than likely the valve clearance is tight causing valve to be held open.

 

[Indydave]

I am doubtful that valve clearance would have caused a sudden and complete shutdown. Also, my idea that the negative ground strap is corroded is probably refuted by the lights and starter working.

Good idea to check for loose coil wire. However #4 has spark.

 

[dirtdigger]

OK good luck

 

[Indydave]

Would anyone say that the blowing out of the intake is a definite indicator that the chain has slipped?

 

[BB74]

Not a definite indicator to me. It could even be a bit of timing overlap at top dead center at the end of the exhaust stroke that you are feeling puffing out of the intake port.

Put cylinder number 1 and 4 at top dead center and see if the punch marks on the intake sprocket align with the top surface of the cylinder head. This will tell you if the camshaft is properly aligned in the motor (intake camshaft timing is right) and the chain hasn't skipped a tooth.

 

[Indydave]

I understand what you are saying. I think a big reason that I am thinking the chain must have slipped is that the engine shut down instantly with no misfiring before it shut down. If indeed it slipped and the marks do not line up, is it possible to loosen the chain and slip it off the sprocket and then after repositioning the cam slip it back on or would I have to remove the cams to do that? Thanks to each of you for your suggestions.

 

[Indydave]

Also if you happen to know, what is the easiest way to check for the number one and number 4 cylinders to be at top dead center?

 

[BB74]

Check page 94 on the following link for the info on installing and aligning the camshafts.
cb750 cylinder head Manual

 

[Indydave]

I have confirmed there are no loose wires on the coils. I was able to locate the T 1.4 mark, and when I had the pointer aimed at it, the punch marks on the two sprockets were not perfectly lined up with the edges of the head. When I lined those up, I could no longer see the T 1.4 mark. I am going to assume that it is a problem caused by the chain slipping one of the teeth of the sprockets. I'm not sure I have the capability to remove the cam holders in order to reinstall the chain. I have found a shop that might do the work for about $360 and that seems like what I should do. But he is not giving any guarantees. Does anyone know for sure if you slip the chain, will that automatically bend a valve or two? Thanks in advance for replies.

 

[BB74]

Getting a shop to do it maybe your best bet. Or if you feel it something you can do yourself get a Clymer manual and take your time.

Either way you may want to spend a few dollars on new cam chains instead of trying to get the same old chains to work as they could be stretched to a point that they will skip a tooth again.

Good luck.

 

[Indydave]

New chain certainly sounds like a good idea but without removing the engine from the frame it seems to be almost impossible to install a new one. And I am not set up to be able to do that because it is not in a garage. That would require me to separate a link and feed the new chain around by connecting it to the old chain and then reconnecting the link. I some saw someone do a video on that and it seemed like a pretty tough job. I think I will take my chances using the old chain and hopefully the repair guy tightening the tensioners will solve the problem. I think that's what caused this because I have never tightened them before. I'll update when it's done.

I'm just wondering if I trashed the valves. Does that usually happen?

 

[BB74]

It could bend the valves but you can't really tell until you take it apart or do a compression test on it.

I'd try the following.

 

[Indydave]

UPDATE: I believe I have it timed correctly with the punch marks lined up with the edge of the head and it is at top dead center where the 1-4 mark is on the crankshaft. It has spark and fuel but it won't fire. I no longer have blowing out of the carburetor but I also don't have much sucking. I have to conclude that valves were damaged when the chain slipped. I hope to have a compression test done soon but in the meantime is there something I may be overlooking? Unfortunately I'm very near the point of sending it to the junkyard because it hardly seems worth it to pay someone to pull the engine remove the head and do a valve job and reinstall the engine. if someone lives near Indianapolis and wants to buy it for $750 I would probably take that. Thanks to everyone for their suggestions.

 

[BB74]

Sorry to hear things didn't get sorted out.

You could probably find another one for less then the cost of paying someone else to fix it. Maybe sell it to someone who can use it for parts.

 

[Indydave]

I know I have spark and today I sprayed starter fluid into the holes going into the head on the back side of the carburetor. And yet it still would not fire. Even if a valve or two is bent I believe it should have fired. So I have to believe that I don't have it timed properly. I saw the following in another thread and I think that it could solve the problem. Perhaps I set the cam timing marks to top dead center but it was on the exhaust stroke. Could someone confirm if this advice is correct?

"When the 1.4 S.F mark is on you are at TDC......................................BUT, WHICH ONE???

There are TWO, a 4 stroke has to rotate TWO complete revolutions (720 degrees total) to finish an entire cycle, why it's called a 4 stroke...........2 strokes up and 2 down..........intake, compression, power stroke and exhaust, in that order. Two TDCs happen then, one at the end of exhaust stroke and beginning of intake, and the other at end of compression and beginning of power.

Easiest way is to yank #1 plug and with finger over the hole turn the engine over until you feel the compression beginning to push your finger off, the next TDC (1.4 S.F) you roll to will be the correct one if you don't go too far. Engine MUST be turned counterclockwise on that side (left)."