Topic: I'M AT THE END OF MY ROPE WITH A DEAD CYLINDER ISSUE

[paddy paul]

Here is the  problem, Dead # 3 on my 75 CB 750, decent compression 120psi ,adjusted the valves,good fuel delivery,spark to the plug,still nothing, run the bike 1,2,4 hot firing/working.. # 3 is cold and dead,checked the slow/main jets for obstruction,nothing,any ideas or suggestions would be greatly appreciated because I have just about had with this bike, thanks

[scottly]

Is #3 carb getting fuel in the bowl? Have you checked the level with the "clear tube method"? What does the spark plug look like?

[paddy paul]

yes plenty of fuel, and the spark is nice and fat and blue, , I also have set the points and timing, it's madding

[Rio_CB750]

Could you have inadvertently switched the spark plug wires?  Is it firing on the exhaust stroke so it won't register as hot?  Carefully check the wires to the cylinders, this happened on an old Chevy I had, which is why I bring it up, it had 6 cylinders though, anyway, something to try...

[paddy paul]

no, 2-3 and 1-4 are where they belong

[scottly]

Is the problem ALL the time, or only at idle; does it smooth out at road speeds under load? Also, what does the PLUG look like (not the spark)? A little history of the problem might help.

[david 750f]

Have you tried switching the 2&3 plug leads?

If the issue follows the plug leads it is most likely electrical. If it doesn't follow the plug leads then it is probably carb related.

[paddy paul]

 No I haven't I will go out and try that right now, thanks

[paddy paul]

every time you start it it only runs on 1,2 and 4, just bought the bike last month, the po said it had been sitting for about 3 years, the carbs where stuck shut and full of gum/varnish, so I put a set on that I had on the shelf for my other 750 project..

[Rio_CB750]

You might want to check the plugs again, not the order this time, but the angle of the boot.  You can pull the plug, hold it at angle and it fires and is blue hot.  BUT, put the plug back in, put the boot on, but the angle of the boot or stress on the wire may cause a failed connection.

Twist off the boot, maybe even trim the lead to get good copper, put some dialectric grease on, screw it back in, put some grease on the plug, check for spark, then put the whole thing back in the head and see if it fires.

[paddy paul]

switched the wires, and # 2 fires with the #3 wire but # 3 cylinder is still dead, so I don't think it's a spark/ wire issue, I will  recheck fuel delivery and level,and then maybe pull the carbs this weekend...

[Hush]

You don't say if #3 EVER worked so I'm guessing you are just putting the bike back on the road.
Can you manually lift the slide on #3 ?
I'm no expert on the 750 carbs but the older ones had seperate cables didn't they?
Can you pull that cable up and see if #3 chimes in?
I'm wondering if the needle valve has dropped down onto the main jet, can happen if the wee clips come out.
This would cause only #3 to not fire.
Just whistling in the dark PP but hey you might get lucky as you said they had been sitting in your shed for a while.

[Don R]

Pull the valve adjust caps and observe the rocker arms while the engine turns over. When they are not up on the cam, check for lash also. Maybe you could find a compression tester that would fit the honda spark plug thread size. Many people stick their finger over the plug hole and turn it over to check compression. Be very careful if you do this.
I've been at the end of my rope a few times, the guys here always helped.

[ChortlingSandcaster]

COUPLE IDEAS THERE.

1) You can buy a cheap spark tester at Auto Zone, or Manny-Moe-Jack, or Grand Auto,
or Kragen, or your local auto shop.  One of them will sell this sanity restorer.

It is clipped on to the spark plug wire.

You start the bike.

There is a light on this little spark checker tool that lites up when the coil 'sends the spark down.'  Now you're not checking the spark in an odd way -- you get to check for spark *with the plug and wire installed and the engine running.*    So if it's a marginal plug wire that gives a phony read when the lead wire's twisted up while the plug is removed -- this tool checks for spark *with everything installed.*



2)  I spent 3-4 days on a Kaw 400 triple on this same problem.  I had pulled the carbs for cleaning.  Put them back on.  I installed new plugs.  I trickle-charged the battery.   I flushed out the gas tank.  I cleaned the tank's petcock.  I bought 3 new rubber gas lines and attached them to the 3 nipples on that petcock.  I installed inline fuel filters, one per carb.

So, with a cold exhaust pipe on #1 (cylinder), here's what I did:


1) "Okay mebbe carb #1 is not getting fuel, after all -- I flushed the tank and petcock."
    - pulled the gas line off nipple for cylinder #1.  Turn gas on at the petcock.  PLENTY of fuel
      coming out of the nipple for cylinder #1.

2) "Okay the new fuel filter has a manufacturing mistake and is blocked, I bet carb #1 is not
      getting gas through my new inline filter."
      - pulled carb #1's fuel hose, turned gas on, NOPE plenty of fuel getting to carb #1.

3) "Well its probably the carb work I just did."
    - pull the carbs back off, nope, nothing sticking, float level correct, carbs are good.
    - still have cold #1 pipe

4) "Okay, mebbe when I pulled the carbs and re-installed them, opened a crack in #1's intake
      manifold and it's running too lean, too much air/not enough gas so no ignition.  let's fire up
      the bike and spray some WD40 at the intake manifold to cylinder #1."
      (Do this one OUTSIDE, if theres an intake leak at the intake manifold, the WD40 will enter
       the leak and burn mighty in the cylinder).
      - no intake leak, so 'running too lean,  too much air' wasnt the problem

5) "Okay check the battery.  These things don't run right with a poor charge."
     - battery fine, between 13-14 volts when rev the bike.

6) "Okay, check the coils."
     - fine.

7) "Well she's got three brand new NGK spark plugs.  I JUST KNOW IT AINT THEM NEW PLUGS.
      So I guess I have a timing problem or rotor/stator problem or.....various nightmares.  Well
      lets start down that path."

THE #1 PLUG WAS BAD.  A BRAND NEW NGK PLUG WAS BAD.  I knew the coil was good, I had just checked it.  But when I removed #1's brand new NGK plug and laid it against head -- NO DURNED SPARK.

Next time it will be the FIRST frigging thing I check.  At age 50, around motors since age 16, that is the first time that ever happened.  BRAND NEW DANGED PLUG.

Felt like an idjit.

[ChortlingSandcaster]

I saw that you did a valve adjustment.  I was going to suggest 'maybe the valve timing/opening is off.'

Right now I'm working on a kz900 that sat for long enough that the valves are sticking a bit in the valve guides, ie. there's a little corrosion on the valve stems from the bike sitting so long.

If the valve timing is wrong, and/or the valve are sticking in the guides, that could explain why no ignition (valve sticking in the guides and not opening/closing smoothly or to the full extent).   And I think you DID say that the bike sat for a long time.  Maybe you have a 'valve stem corrosion' problem too? 

Let me say that a different way.  If the reason you did the valve adjustment was "the valve clearances were too big" that *could* mean the valve is not closing all the way due to corrosion on the valve stem.

I found just this condition on this '76 Kaw 900 I'm working on.  I checked valve clearances as standard for me before getting an old bike running again.  THE GAPS WERE TOO BIG.  So I thought "Ok, I'll do a valve adjustment and reduce the valve clearances to bring them in spec."

WRONGO.  I caught myself on that one.  Valve clearances don't get BIGGER over time -- they get smaller, because the valve seat wears out and the valve stem moves UPWARD in the head, which does NOT make the valve clearance increase -- just the opposite.  So...why were the valve clearances too *big* ?? BECAUSE THE VALVE STEMS were corroded.  The valves weren't closing all the way.  So the valve stems at rest were too low in the head.   THAT MEANS -- the valves were not closing and no way am I getting a proper 4-cycle operation in that cylinder.  Pull the head, valve job time.

One easy thing to check, you'll need a shop manual for it, is -- check the cams positioning against the timing on the crankshaft.  The manual should explain how to do it.  I suspect it will be fine since 3 of 4 cylinders are firing. 

Another question.  Does this bike have stock points?  Or did someone put in an electronic ignition?  If the latter, you should check it.  If the timing's way out, your 'check for spark' with the plug laying on the cylinder could show spark but doesn't tell you if the TIMING of that spark is correct.

The other suggestion about doing a visual on the valves is important because you just adjusted them.

[750goes]

simple test

put another plug onto the end of the lead for #3 cylinder and ground it out - leaving the one in the head still there.

kick it over and watch for spark on the #3 plug

if you have spark - put in a new plug .......

[scottly]

Here is the  problem, Dead # 3 on my 75 CB 750, decent compression 120psi

Obviously, if the bike has 120psi, valves are not the issue. If the fat blue spark was from the center electrode of the plug to the ground strap, with the plug body grounded against the head, it rules out ignition. That leaves the carb.     

[BLUE71TURBO]

Here is the  problem, Dead # 3 on my 75 CB 750, decent compression 120psi

Obviously, if the bike has 120psi, valves are not the issue. If the fat blue spark was from the center electrode of the plug to the ground strap, with the plug body grounded against the head, it rules out ignition. That leaves the carb.     

 My feelings exactly. Just because you have fuel in the bowl doesn't mean it's getting through all of the passages in the carburetor.

[Hasenkopf]

if it were a small block chevy, I would suspect a worn cam lobe......

[ChortlingSandcaster]

Here is the  problem, Dead # 3 on my 75 CB 750, decent compression 120psi

Obviously, if the bike has 120psi, valves are not the issue. If the fat blue spark was from the center electrode of the plug to the ground strap, with the plug body grounded against the head, it rules out ignition. That leaves the carb.    

Did he check compression before or after the valve adjustment?
Besides, just because valves close enough to get 120psi tells you nothing about the timing.
Count the cam chain links using the procedure in the manual.

What if the gauge is inaccurate.

Its just that, the valve adjustment was just done -- WHY was an adjustment needed?  

You should also verify the cam timing using the procedure in the manual, re-check the compression if you first checked it *before* the valve adjustment and not afterward.

We still need to hear:  Is the reason for the valve adjustment that:
- the valve clearances were too BIG
- or the valve clearances were too SMALL

that will tell a lot.  Because a long-time sitting bike will get some surface corrosion on the valve stems.  This can make the valve clearances too big if the surface corrosion on the valve stem fails to allow the valve to close all the way, or to intermittently stick.  Then, you might adjust the valves to close the gap.  And now the valve contacts the piston crown....?

It could even be a broken valve.

We need to hear what the pre-valve-adjustment valve clearances were.

[Hush]

Simple cheap way to rule out the carb, pull #3 and eye drop some gas in there, replace plug and fire it up.
If it fires on all 4 the problem is fuel supply....always worked on my two strokers.

[paddy paul]

I just bought the compression tester Thursday, brand new, I have 115-120 on all cylinders, brand new plugs, when I pull off the lead and the plug the plug is firing, I ran the motor with the caps off to watch the rockers and they are operating, I think it has to be a fuel problem, I will pull the carbs tonight and look at them... the valves where adjusted because they where to big, way out of spec on all the cylinders,

[Duke McDukiedook]

Did you wire/compressed air clean out all orifices especially on #3?
Are you sure the float is working on #3? You checked with tube for fuel level?

I want to make sure you exhausted those steps because it sounds like it is the carb.

[ChortlingSandcaster]

every time you start it it only runs on 1,2 and 4, just bought the bike last month, the po said it had been sitting for about 3 years, the carbs where stuck shut and full of gum/varnish, so I put a set on that I had on the shelf for my other 750 project..

the valves where adjusted because they where to big, way out of spec on all the cylinders,

Wow dude.  Take a look at this.  You said the valve clearances were all too big!

Just think about how the valve is sitting -- as it opens/closes for all those miles, and the valve seats get worn -- ask yourself this:

- 'as the valve seats wear out, will the tip of the valve stem rise UPWARD?
   or DOWNWARD?'

The correct answer:  as the valve seat wears out, since the valve spring pushes the valve UPWARD to close it -- with less metal on the worn-out valve seat the tip of valve stem RISES UP HIGHER IN THE HEAD.  Not lower.  That *reduces* the valve clearance.  But you found that
the valve clearances "were to big, way out of spec on all the cylinders"

I realize it seems like the carb might be the answer.  But you have exactly the problem I have on an overhead cam bike -- valve clearances that grew over time.   Doesn't make sense to me.   And the bike sat for 3 years, plenty of time for some surface corrosion on the valve stems. 

The valve clearances get smaller over time as the tip of the valve stem rises up higher in the head due to the valve seat wearing out.

This may be a peripheral issue, but worth looking into.

[paddy paul]

 If the valves were not seating, I wouldn't have the compression that I do, before all this started with # 3. I was having trouble with the float sticking on #4, that's when I pulled off the carbs and and cleaned them and adjusted the floats, put the carbs back on, the #4 float problem  was taken care of, but the #3 no fire problem started, I'm going to pull the carbs tonight and recheck all the passages and orifices clean and reinstall tomorrow...