Topic: New problem running my 71-750

[awiertalla]

Hey guys,

I have a 1971 cb750, it's been chopped up and ran great until recently. It appears to be running on 2 or 3 cylinders. I would be riding and it would suddenly bog way down and stay that way for a minute or two, then pop back up to normal. It has progressively gotten worse, to the point where I can't ride it at all. I originally thought it could have been a sticky float but I swapped my stock carbs with a brand new pair of mikuni 2-4 and the problem is still there. I checked psi, and I have 125 across all 4, I have a dyna ignition (haven't touched it). I pulled the valve cover, checked my tappets and noticed what looks like my cam chain "Jumping", tightened the stock chain tension and it got a little better. I'm wondering if it could be bad valve guides, or if I need to lap the valves? I also have black smoke sometimes out the exhaust (making me think the valve seals are bad). Any ideas or help is appreciated.

[ekpent]

 Black exhaust smoke usually indicates rich running or poor burning. Blue is oil and white is steam.

[awiertalla]

This is all great, I forgot to mention that when I checked the psi, I pulled and cleaned all of the ngk iridium plugs then checked for spark on each cylinder (held the plug to ground and turned the motor over). Would this fulfill all those checks? How would I tell if the dyna igntion was at fault?

It could be a number of things-

-It could be a voltage problem at the coils where they intermittently drop out, pick up, drop out.
- It could be the Dyna ignition itself. A few years back, Dyna had a bad batch of units that would drop out one pair of cylinders after they got hot, intermittently.

- It could be bad plug caps.
- It could be bad plugs.
- It could be a poor charging system creating some of the symptoms above.
- It could be, just as you described, a cam chain going haywire.

What I would do is perform a complete, comprehensive service of the bike as described in the manual under 3,000 Mile Service. Check the valves, cam chain, plugs, caps, timing. Check the fuel levels via the Clear Tube method (especially since you installed new Mikunis. I would definitely pull, inspect and probably replace the plugs. I would check the plug caps and coils to be sure they are operating properly.

This should all take you less than a weekend day, or 1-2 evenings. If your do all those things, I suspect you'll ascertain the very likely cause of your problem, or better still, resolve it.

[awiertalla]

In addition to what's been suggested, you can check if the tank is venting properly.
Simple to check, pop the cap open very slightly, and note if it affects drivability.

The cap I have definitely vents, when I overfill it sprays up out of the vent hole haha.

[awiertalla]

This is all great, I forgot to mention that when I checked the psi, I pulled and cleaned all of the ngk iridium plugs then checked for spark on each cylinder (held the plug to ground and turned the motor over). Would this fulfill all those checks? How would I tell if the dyna igntion was at fault?

Not "truly" a conclusive test. You're approximating the quality of the spark without any instrument other than your own eyeballometer.

Testing the Dyna is tricky. Because the unit tends to fail only when hot, you'd need to ride the motorcycle, then when the symptom occurs, pull over and grab a meter and commence measuring voltage at the coil. There is a static means of doing it, hidden somewhere on the inter web...

Black smoke from your pipe says fuel. But because you've changed your carbs around, is that black smoke before, after or even after the carb change? You definitely need to verify the fuel level via the Clear Tube method (see below).

When you pulled your plugs, what did they look like? When your bike begins to stumble and sputter, hop off and carefully test each header's temperature. Are they all equally sizzling hot? (Hint: use water or saliva) If not, what pipe(s) are cooler? This can point you in a direction too.

I wouldn't place much stock in the clogged vent hole being responsible for all the symptoms you're experiencing.

You really do need to do a comprehensive and complete tune up per the 3k Mile Service. It will tell you heaps. Especially given your prior statement about the cam chain.

If the dyna goes bad at high temps, then it makes me think it is not the problem. Simply because this symptom occurs even when the bike is cold, feel free to tell me this is wrong and I need to check it

The black smoke occurred well before the new carbs, but was much worse with the old carbs. The bike was running very rich (long story), but the last time I rode the bike my buddies said I had a huge plume of smoke come out after leaving a stop light (they said a diesel truck worth). With the new carbs it is significantly less but still a little bit while revving the engine.

When I pulled the plugs, they were black. I assumed it was because I was running so rich.

I'm trying not to ride the bike until I figure out the issue, but I will rev it and try the water technique (really smart) and see what I come up with. Ill have to unwrap the headers

I think the next time I come back on I will have completed the 3k tune up so I can provide you guys with the findings. I really miss riding this bike and want to get it back on road, your help is really appreciated

 

[Jerry Rxman Griffin aka MuthaF'er]

What's your standing battery voltage and voltages at every 1000 rpms?

[PeWe]

Try std NGK D8EA plugs. My engine did not like irridium plugs.
If it happens despite new NGK D8EA plugs:
Verify which cylinders that will not work. If it is either 1,4 or 2,3 that stop working check the ignition. Could be bad connection in the wiring from point to the coil.
My bike could get that problem for many years ago. Not fun when it suddenly happened on a night driving in Germany or Denmark and still 1000-750km left home on a trip from Italy via Austria earlier.

[my name is nobody]

What does the air filter look like? Is it stock one or aftermarket, and is there the possibility that it is too restrictive for whatever reason? Do all tests you have done so far have the same air cleaner in common? Any particular plug look worse than the others, or all the same? New carbs may be jetted too rich, old ones may be malfunctioning due to worn or mis adjusted jet needles or incorrect float levels. Plugs correct heat range?
Tune up first and foremost.

[awiertalla]

I just wanted to let you know I solved the problem, and you were spot on with the diagnosis. I let the bike sit for 2 years before deciding to get back to it. When I first posted, I ended up removing the head, lapping the valves and replacing the valve seals. Put it back together and tried it, and got absolutely nothing. I thought I might have installed my cam incorrectly, and left it at that. 2 years later, I checked the cam, and it was perfect. Tested to see if I had spark, and got nothing. I tested the coils, and they were fine. Tested the Dyna ignition and it failed, but not the unit itself. The p.o. installed the Dyna and ran the wires along the bottom of the frame, right next to the exhaust, and they ended up getting melted. I wasn’t able to see this though, until I pulled the old ignition out of the bike, then I found the melted wires. A relatively simple fix, that took 2 years haha.

It could be a number of things-

-It could be a voltage problem at the coils where they intermittently drop out, pick up, drop out.
- It could be the Dyna ignition itself. A few years back, Dyna had a bad batch of units that would drop out one pair of cylinders after they got hot, intermittently.

- It could be bad plug caps.
- It could be bad plugs.
- It could be a poor charging system creating some of the symptoms above.
- It could be, just as you described, a cam chain going haywire.

What I would do is perform a complete, comprehensive service of the bike as described in the manual under 3,000 Mile Service. Check the valves, cam chain, plugs, caps, timing. Check the fuel levels via the Clear Tube method (especially since you installed new Mikunis. I would definitely pull, inspect and probably replace the plugs. I would check the plug caps and coils to be sure they are operating properly.

This should all take you less than a weekend day, or 1-2 evenings. If your do all those things, I suspect you'll ascertain the very likely cause of your problem, or better still, resolve it.