Flat spot/Bogging while cruising...

[anthony8488]

I have an 82 CB750SC, carpi 4-1 exhaust, the rest is all stock.

I am having an issue when cruising (have had it happen at constant speeds of 40-70 mph) that after several miles it feels like a couple of the cylinders drop/aren't firing. It very audible in the exhaust when it happens. During the flat spot/bogging throttle response is about half as responsive and power is about half as well. Also, when it happens if I don't blip the gas the bike will idle down and die, if it stalls it has always started with the same issue, if i blip the gas enough times it eventually goes away with no consistent time that it will happen again.

I recently changed plugs and wires, both coils checked good on the meter, plug ends are like a blonde coffee/caramel color. I also removed the inline fuel filter to trouble shoot, thinking maybe it was starving the motor some.

Starts right up cold or hot, idle is very steady, drives and accelerates great (outside of the issue).

I have seen some issues with hesitation and flat spots(ignition points/jets), but they were having noticeable issues before ever reaching cruising.

Any ideas on this one?

[TOOLS1]

I have two ideas. First, the gas cap vent might be plugged. Try opening the gas cap next time it does this and see if it starts running better. The second idea is that the electronic ignition is going bad.
TOOLS

[anthony8488]

Thanks for the reply, I ordered a new fuel cap, the one on the bike was gouged and dinged up from the previous owner anyhow. So new cap arrives tomorrow and will try to take the bike out and test it. 

If that doesn't fix it you mentioned the electronic ignition. Outside of checking the points is there another way to test if the ignition is failing?

On Saturday, July 30, 2016, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

I have two ideas. First, the gas cap vent might be plugged. Try opening the gas cap next time it does this and see if it starts running better. The second idea is that the electronic ignition is going bad.
TOOLS

Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."

1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/Flat-spot-Bogging-while-cruising-tp4061073p4061074.html

To unsubscribe from Flat spot/Bogging while cruising..., click here.
NAML

[TOOLS1]

You do not have points on an 82 model, you have an electronic ignition. Sometimes these ignitions go bad, and will only work when cold. When they heat up they quit working. It's well known on the forum that I do not trust electronic ignitions. However there some tests that can be performed. You will find detailed instructions in the Factory service manual which can be downloaded from the manuals thread at the top of the home page.
TOOLS

[anthony8488]

So new fuel cap(Genuine Honda OEM) came in yesterday. Swapped it out and drove today. I made
it about 15miles and then had a a couple seconds of the flat spots. Turned
around and it happened two more times in the 15 miles back. The longest was
maybe 3 seconds, had just enough time to put the key in the cap and it
stopped before I could remove the cap.

A definite reduction in the issue. But now there are little hiccups of it
happening. Does it still seem likely the ignition could be the issue?

Thanks,
Anthony

On Tuesday, August 2, 2016, TOOLS1 [via Honda CB750'S] <
[e-mail]> wrote:

> You do not have points on an 82 model, you have an electronic ignition.
> Sometimes these ignitions go bad, and will only work when cold. When they
> heat up they quit working. It's well known on the forum that I do not trust
> electronic ignitions. However there some tests that can be performed. You
> will find detailed instructions in the Factory service manual which can be
> downloaded from the manuals thread at the top of the home page.
> TOOLS
> Life is not about the number of breaths, you take, but the moments that
> take your breath away.
> I don't have an anger problem. I have an idiot problem. Hank Hill
> Never confuse education for intelligence.
> Happiness is a belt fed weapon.
> I just can't imagine what could go wrong.
> No fire? No explosions? So whats the point of your story?
> Those who are too smart to engage in politics are punished by being
> governed by those who are dumber. ~Plato
> It couldn't be done, but the darn fool didn't know it, and did it anyway.
> We all got problems. Ksharp
> I like vintage bikes because they take me away from the clutter of
> technology that I work with everyday and back to a simpler time of
> mechanical elegance and simplicity.. "ninadm"
> Darkwing Duck: The worst part of public transportation is the Public.
> "That is awesome shit there" Re-Run
> "Fear nothing, attack everything" Eric Berry
> " Oh, you read that on the internet? Clearly it IS a massive problem. Of
> course it CAN’t be normal operation."
>
> 1976 CB 750-A X 2
> 1977 CB 750-A X 4
> 1977 CB 750-K
> 1976 CB 750 F
> 1981 CB 750
> 1966 Kawasaki SG 250
> 1981 KZ 750 LTD
> 1973 CB 350
> 1979 CM 185 Twinstar
> 1982 Honda XL 80
> South of Eden (Kansas City MO)
>
>
> ------------------------------
> If you reply to this email, your message will be added to the discussion
> below:
> http://honda-cb750-s.456789.n3.nabble.com/Flat-spot-
> Bogging-while-cruising-tp4061073p4061090.html
> To unsubscribe from Flat spot/Bogging while cruising..., click here
> <http://honda-cb750-s.456789.n3.nabble.com/template/NamlServlet.jtp?macro=unsubscribe_by_code&node=4061073&code=YW50aG9ueTg0ODhAZ21haWwuY29tfDQwNjEwNzN8MTczMDUyNjI5NA==>
> .
> NAML
> <http://honda-cb750-s.456789.n3.nabble.com/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml>
>

[Re-run]

When it happens, I assume throttle is steady? Usually, when that has happened to me, it was a fuel issue. Either a small bit of crap made it through the filter and plugged a jet for a couple seconds, or the mix is off on a cylinder or 2. If conditions allow it, I would cruise and as soon as it happens, I would pull the clutch and hit the kill switch. This should hopefully capture on the plugs what might be happening.
Let things cool a bit so you don't burn yourself and then pull the plugs. The plugs may show an indication of what is happening.

I had something like this once, and the mix was too rich and the plug was getting loaded up. It would basically spit out some fuel and fire again for a bit.

[anthony8488]

You would be assuming correct, steady throttle maintaining cruising at speed. I've never checked the plugs like you suggested, so I'll have to give that a try and see. 

Did you also mess with your carbs or just readjust your mixture and all was ok again?

On Tuesday, August 9, 2016, Re-run [via Honda CB750'S] <[hidden email]> wrote:

When it happens, I assume throttle is steady? Usually, when that has happened to me, it was a fuel issue. Either a small bit of crap made it through the filter and plugged a jet for a couple seconds, or the mix is off on a cylinder or 2. If conditions allow it, I would cruise and as soon as it happens, I would pull the clutch and hit the kill switch. This should hopefully capture on the plugs what might be happening.
Let things cool a bit so you don't burn yourself and then pull the plugs. The plugs may show an indication of what is happening.

I had something like this once, and the mix was too rich and the plug was getting loaded up. It would basically spit out some fuel and fire again for a bit.

The ride IS the adventure. The destination is just to get gas!

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/Flat-spot-Bogging-while-cruising-tp4061073p4061127.html

To unsubscribe from Flat spot/Bogging while cruising..., click here.
NAML

[Re-run]

Mine was a combo of the fuel screws being slightly too rich and a too-restrictive air filter. Filter was clean, just was too restrictive. It was an emgo filter.

[anthony8488]

Ordered a K&N filter should arrive today or tomorrow. Old filter was dirty, and no brand on it to know if it too was a emgo. Once the filter comes in will take it out and see. Plugs look good where the fuel screws are currently. Will watch to see if the filter changes that. 

On Tuesday, August 9, 2016, Re-run [via Honda CB750'S] <[hidden email]> wrote:

Mine was a combo of the fuel screws being slightly too rich and a too-restrictive air filter. Filter was clean, just was too restrictive. It was an emgo filter.

The ride IS the adventure. The destination is just to get gas!

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/Flat-spot-Bogging-while-cruising-tp4061073p4061131.html

To unsubscribe from Flat spot/Bogging while cruising..., click here.
NAML

[Re-run]

Filter might change it slightly, but when I dropped a k&n into my 78, it worked much better than the emgo did.

[anthony8488]

So put the new K&N air filter in, turn the fuel on and it down pours, turn fuel off and when I look under the carbs my #2 fuel mixture screw is now gone. The other three are still in sturdy, not sure how I lost just the one. They are turned out 3-3/4 turns, which should be fine for a DOHC, I thought.

I'm trying to source the part, are all the fuel mixture screws the same across the DOHCs? I will need to buy a carb kit since the spring is gone as well, seeing rebuild kits for DOHC models but not stating nighthawk(cb750sc), i'd only be buying the kit for the fuel mixture screw. Thanks.

[Re-run]

That sucks, but I think as long as the model of carb is the same, the mix screw will be the same. Just make sure you have the screw, spring, oring and washer. They all work together.

[TOOLS1]

Honda should have those parts available. Check partzilla.com
TOOLS

[anthony8488]

Staying in this post as I am still having the same issue.

I sourced the mixture screw and swapped that in, found some 95 Honda coils and swapped to those incase my original factory coils were bad under load.

With the new coils the bike feels smoother but I am still having the same issue where it will randomly drop cylinders. Plugs are new, wires are new(copper core), plug caps are new (NGK replacements). It starts almost instantly and will idle all day, just intermittently loosing cylinders while riding.

 Sometimes it drops them for a few seconds to a min, today was by far the worst where it took 6 miles for all the cylinders to fire back up.

Could this be an issue with the CDIs? Carbs have been cleaned numerous times over the last year following every guide on here and using guitar wire to scrap passages, I feel like it's electrical just not sure where to start.

Also, since I have a CB750SC, when I thought it could be ignition I reached out to Dynatec, they said because the center on my ignition is a tear shape and nota circle they don't have a replacement. Anyone have any luck fitting a new ignition on a SC(nighthawk)

[crestview750f]

Just a thought: have you drained the float bowls lately?

[anthony8488]

I haven't drained them on the bike but the Carbs were off the bike last month and I had everything drained then took it out for a ride had the issue come up so got the bike back and then let it idle till it stalled with the fuel shut off. Ordered the plug wires then had a chance to swap those yesterday. Turned fuel on let the carbs fill and then a couple twist of the throttle the bike turned right over idle was good, good response blipping the throttle.

Took it out today got about 4 miles down the road and then felt the cylinders drop, took it about 6 more miles to get home on back roads and then as I turned into my neighborhood they all came back. I turned fuel off and let it stall out again incase I need to pull the carbs off again.

I need to find some meter readings on all the ignition components and start checking them but searching today saw some people reference the CDIs which made me want to ask about them.

[motogrady]

Mine did the same thing when I first got it.
Ignored it until it flat out quit one time, almost got run over by an 18 wheeler when stranded on the side of the road.

Cruising at the speeds you mentioned, the heat from the motor was washing back over the carbs, and  rubber fuel lines, which were kinda old.  
The rubber  fuel lines got hot, then soft, and were sucking into themselves.

Noticed it right after I almost got creamed by that truck.
Caught them real hot, real soft, and collapsed into themselves when I went off and pulled the tank on the side of the road.

As the hoses cooled, and vacuum in the line decreased, I watched them expand and return to a normal, round size.

That's why I could never catch it at home, in the garage, or on short runs down the road.
You need the constant steady heat, which goes away when you stop for a bit or park it for the night.

Next time you get it stalling, pull off, reach under the tank and see if the fuel lines are any kind I'd squishy.

[Rodgil]

Go back to Tools reply to you 3 August 2016. Get hold of a multi-meter and test the igniter coils. They are on the LH end of the crankshaft. The red connector where you test it is behind the LH side cover alongside the battery. The test is described in the factory manual you can download from this website and only takes a few minutes to do the test.  You don't have to dismantle anything else to do the test. They will definitely give you the symptoms you describe.

[Maz]

Do you have a fuel filter inline? If so, is it dirty and restricting the fuel flow so that at higher speeds, the motor is using fuel faster than the float bowls can fill up? Just a thought as I had exactly the same symptoms on my FJ1200 and found that the inline filter was full of cr*p.
Maz

[motogrady]

Maz is correct, even the pet clock has a real fine screen.
I ripped the one in the petcock out, felt it may have been adding to the vacuum increase in the fuel lines,
helping to collapse them when warm and under heavy load.

Lowes sells a pretty good clear plastic inline fuel filter in the law mower dept.  under 10 bucks,
and you can see at a glance what's happening in the fuel tank, garbage wise.