'81 CB750K DOHC Hesitating, Stumped!

[Nussbaumer]

Hello All, I've been searching this forum and gaining a lot of information, and for that I'm extremely grateful. I've never had much experience with motorcycles and this forum has given me the confidence to tackle the work myself. But my current problem has me stumped.

The bike hesitates and won't rev past 5500 or 6000 RPMs. It fires up great and idles nicely at about 1200 RPMs (according to the tach). Here is a list of things I've done to it.

-Removed and cleaned carbs (Pine Sol soak) and installed new gaskets and stock sized jets
-Checked for vacuum leaks (found none)
-Installed new OEM Air filter in stock air box
-Checked and corrected valve clearances
-Carb Sync with Motion Pro sync tool
-Adjusted Cam Chain Tensioner
-New fuel line and fuel filter
-New Spark plugs (NGK Iridium), Spark plug caps, and Wires
-Checked dynamic timing
      -When I checked the timing, it was good at idle. When I tried to do the advanced timing, it started spewing oil at a tremendous rate (it was about cup of oil or more on the floor). And it was spraying a mist of oil at the same time. So I didn't want to continue the dynamic testing trying to stare directly into source of the oil mist.

Also, there was quite a bit of baffling(?) sticking out of the exhaust. It looked like steel wool. So i grabbed it and pulled out a couple of fistfuls of it.

I''m not sure what the next step is to take and I would really appreciate a nudge in the right direction. If there is any more information i can give, let me know.

[shinyribs]

It really sounds like you have the makings of a good runner, considering all the work you've done to make it 'right'.

Does it always have a hesitation,or only at that upper rpm? Does pulling the choke closed have any affect on the hesitation: worse or better?

First thing that comes to mind is the accelerator pump. That can cause a hesitation if it's not working, but once you're in the upper rpms it's not really needed/will cause a hesitation.

Is your battery/charging system healthy? The ignition system on these bikes (especially the DOHC's) do not like low battery voltage at all. They will run horrible on a weak battery,especially at upper rpms.

Also, since you weren't able to check the spark advance dynamically, maybe you can check it manually. Just reach in there and see if you can twist it just to make sure it's not stuck or frozen. That can definitely cause the symptoms you described.

[Nussbaumer]

Thanks for the quick response. Yes it does only hesitate at higher RPMs. I pulled the choke in once and if I remember correctly it made it marginally worse.

I'm not sure how to check the accelerator pump.

I've been doing a lot of idling while troubleshooting lately,  so when I'm done working I put the battery on a tender. And I upgraded to a gel cell. So i think the battery should be good. As far as the charging system, I don't know how to check it.

I'll check the advancer manually tomorrow. Thanks for your help. Without this forum, I never would've bought this bike. And I'm glad I did. I've really been enjoying tinkering around with it, and I've been able to get a few good rides out of it as well.

Also forgot to mention. The idle screws are 3 turns out.

[Nussbaumer]

And my compression on all 4 were about 140 each.

[TOOLS1]

Did you mix up the jets when you replaced them? The pilots could be where the mains go, and vice-versa.
TOOLS

[Nussbaumer]

It's funny you should mention that, Tools. I did that the first time, but I caught it before remounting the carbs. I had to check the manual, but I'm sure it's correct now.

[Johnnylevi]

Did you check the charging system? If thats not working up to speed, then that could be why you can't rev past 6k. did you try adjusting you fuel/air mixture screws? Some stuff to look at.

[Nussbaumer]

I don't really know how to check the charging system.  I have a volt meter I just don't know what to look for.

On Mar 8, 2014 11:12 PM, "Johnnylevi [via Honda CB750'S]" <[hidden email]> wrote:

Did you check the charging system? If thats not working up to speed, then that could be why you can't rev past 6k. did you try adjusting you fuel/air mixture screws? Some stuff to look at.

1981 SS with Kerker 4-1 exhaust and extremely attractive rider.

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[Johnnylevi]

does it also measure ohms? You'd have to take the right side cover off and place your - and + on two rings on the charging rotor. Or I suppose you could check by placing the voltmeter on the batter terminals and revving the bike and see where the volts are. I believe its supposed to be 13+?... not 100% on that one. the rotor thing is pretty easy. The ohms should be ~3-4.

edited! +14v at 3000 rpm. Thats what mine read with a healthy charging system.
http://honda-cb750-s.456789.n3.nabble.com/1981-CB750-Super-Sport-td4025511i160.html

[Nussbaumer]

I'll try that tomorrow, thanks.

[Rodgil]

You mentioned the steel wool coming out your exhaust. I had a Yamaha XS650 that had the muffler stuffed with fiberglass wool to make it quiet. It ran ok enless you tried to go hard. I removed the wool and it ran much better! Make sure your exhaust is not blocked.

[Nussbaumer]

Could it also be that I took out too much and now there is not enough back pressure.

[TOOLS1]

Nussbaumer wrote

Could it also be that I took out too much and now there is not enough back pressure.

Highly unlikely.
TOOLS

[Nussbaumer]

I checked the continuity of the stator between the yellow wires and got around 0.13ohms the manual says I should get around 0.4 or 0.5 ohms. Is that a problem? Also, I prodded around in the rotor and didn't get any readings. What exactly am I supposed to touch my leads to in there?

Everything seems a little rusty. Here are some pics.

[Lucky 1]

Exact symptoms when the exhaust is plugged up.   You can double check that with a vacuum gauge.

[rich]

+1 on what Lucky1 said. Pull the mufflers and see what's going on in there.

Here's a post with some videos showing how to test the rotor - http://honda-cb750-s.456789.n3.nabble.com/Will-bad-ignitors-cause-charging-problems-tp1277377p3331341.html 

[Nussbaumer]

It turns out I have a bad rotor, I think. I touched my leads to the two copper rings on the face of the rotor and I got infinite resistance. I touched the leads together to make sure the meter worked and it does. So I guess it's time for a new rotor. Any tips on a good place to buy one.

I will also be checking out the exhaust to see what's going on there. Can I take out all of the steel wool or should I leave some in? I wouldn't mind getting a little more sound out of it.

[rich]

I'm just repeating what I've read here and other forums. I got lucky with a used rotor off ebay for around $60 bucks.

The best are rewound rotors from Tim Parrott in Alabama. Go to http://www.tpe-usa.com/. You may have to send him yours and have it rebuilt if he's out of cores.

Stay away from the rotors on ebay, made by electorsport or something similar. Ricks (http://ricksmotorsportelectrics.com/) aren't too bad and have a 1 year warranty.

The stock mufflers didn't use any steel wool and most aftermarkets I've seen just have baffles. Not sure what you've got there. But most times the DOHCs will run fine with a low restriction exhaust so I wouldn't be afraid to pull that crap out of there.

[Nussbaumer]

I found this on Amazon. It seems to be one of Rick's. On Rick's website they go for $175. I think I might get it unless someone had a bad experience with these.

[shinyribs]

Another twin cam with a dead rotor? 

You fellas have me wanting to replace mine just for good measure.

I'm not 100% on the correct way to test the rotor. But I know it's laid out in detail in the FSM. Sure would be nice to find a cheaper and easier fix than that. I hear those rotors can be quite tough to remove sometimes.