Timing the cams...did I use the WRONG TDC???

Indydave

CB750 Enthusiast
Messages
33
Reaction score
0
Points
6
Location
Indianapolis
My 1979 750f dohc slipped the timing chain and I believe I have it correctly timed but it won't fire even though I know I have spark and I have sprayed starting fluid directly into the cylinder while cranking it. I saw the following advice in another thread and I would like someone to help me understand if I could have it timed to the exhaust stroke and not the compression stroke. Does this advice seem 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).
 

BB74

CB750 Enthusiast
Messages
40
Reaction score
6
Points
8
Location
Winnipeg, Canada
Camshaft lobe positioning will tell you if it's on the compression stroke. Turn it over by hand with the valve cover off and watch how the valves open and close on that cylinder.

But I think these motors use a wasted spark system that fires on both the compression and exhaust stroke. So the cylinder should fire every time it his top dead center.
 

Indydave

CB750 Enthusiast
Messages
33
Reaction score
0
Points
6
Location
Indianapolis
That's been what I have been assuming. The #1 (left side while seated) cam lobs both point toward the spark plug and the punch marks are perfectly aligned with the top of the head...when the TDC 1-4 is aligned with index mark on crankshaft.

Even when I pull the carbs and spray starter fluid directly into the intakes while cranking, nothing fires. I have checked and saw spark. I was able to get a compression tester gauge from AutoZone, but the size of threads is wrong so tomorrow I will need to get some kind of an adapter.
 

Indydave

CB750 Enthusiast
Messages
33
Reaction score
0
Points
6
Location
Indianapolis
NOW it appears to have no spark at all, which could be a good thing if that is all the problem is. I have my doubts that it would be the coils, so I am wondering if perhaps when I was turning the crankshaft using a socket on the side where the timing mark is, if perhaps I bumped the electronic pickups (pulse generators?)

Does anyone have experience with this?
 

Indydave

CB750 Enthusiast
Messages
33
Reaction score
0
Points
6
Location
Indianapolis
I am beginning to wonder if it ever slipped the timing chain at all. When I looked at it it appeared to be one or two teeth off but now it seems to be perfectly timed and it won't fire. Previously when I checked for spark I could see it on three cylinders but I didn't consider that being the main problem. Now I can't see a spark on any of the four cylinders. Maybe the coils were still producing a little bit of spark but not enough to fire. I wonder if possibly both coils went bad and that's why it shut down. Do both coils go bad at the same time? I replaced both of them about 8 years ago which would be only about 5,000 MI. I will try to check for voltage at the front of the coils tomorrow. If I have voltage there is there a test I can do on the coils using a standard electrical test meter? Is there a certain amount of ohms I should see? THANK YOU in advance.
 

Indydave

CB750 Enthusiast
Messages
33
Reaction score
0
Points
6
Location
Indianapolis
UPDATE: I just finished doing a compression test and it pretty much confirms that the valves got bent when the chain slipped. I got 100 PSI on number four and zero on 1-3. I repeated the test and got zero again (with plugs out and throttle open. It has good spark now.

So would anyone want to buy it for $600? Any other 79 with 31,000 miles would be ready for a valve job anyway.
 
Top