CB750 F backfire

On Oct 8, 9:54 am, "Ken Abrams" <harvest_t... DeleteThis @scum.suckers> wrote:
> "fweddybear" <fweddyb... DeleteThis @cox.net> wrote
>
>
>
> > The backfiring usually indicates that the bike is running lean, or
> there
> > is a slight leak somewhere in and around the intake....
>
> That is usually the case. Absent any other symptoms, I would agree.
> HOWEVER.........
> In this case, I don't think that applies. The non-firing cylinder is likely
> pumping raw gas into the exhaust which then ignites IN THE MUFFLER when it
> reaches critical mixture. This is not good for the engine in general and
> for the exhaust specifically. One of those explosions might just leave part
> of the exhaust system on the ground or flying through the air.
>
> You need to fix that dead cylinder; the "backfire" will then likely
> disappear.
> Might be as simple as a fouled (or shorted) plug. If you haven't already,
> they all probably should be changed. Next I'd look for a stuck or sunk
> float in that carb.
> Good luck!



The plug and wire are both new, so I presume I'm looking at pulling
the carb apart to check that float.

Is this something that someone like myself who is considered by the
general public to be a danger to myself and others within a 150'
radius can do or should I make the effort to find someone with a brain
to actually approach the project..?

Your help is greatly appreciated!
 >> Stay informed about: CB750 F backfire 

<tokenhire RemoveThis @yahoo.com> wrote in message
news:1191843495.608716.37060@19g2000hsx.googlegroups.com...
> Hey, gang!
>
> I have recently acqired a 1980 CB750F Supersport in moderately decent
> condition.
>
> I am frightening small children and Vietnam vets whenever I'm
> decelerating or maintaining speed at 3K to 6K RPM. It will backfire
> two shots at a time about .5 to 1 second apart from each other in
> varying intervals (the pairs of shots are more frequent in lower gears
> than higher).
>
> The backyard mechanic who made the bike roadworthy for me cleaned the
> carbs, but replaced nothing. He claims there is a cylinder that isn't
> firing correctly (he yanked the sparkplug wire off that cylinder with
> the engine idling and there was no appreciable change), and that there
> are "air mixture adjusters" that need to be tweaked but without a
> Clymers he doesn't know what the settings should be.
>
> The gas in the tank is fresh, there is a new inline fuel filter, the
> oil and filter have been changed, and the stock megaphone exhausts
> were replace with Aprilia racers (no model number on the exhaust
> pipes).
>
> Does anyone have any thoughts? I dread having to apply for a firearm
> permit in order to drive around town.
>
> Thanks much.
>
>
> Token

The backfiring usually indicates that the bike is running lean, or there
is a slight leak somewhere in and around the intake....you can check for
leaks by starting your bike and letting it idle...then spray some carb
cleaner around the carbs and intake to hear if the idle changes slightly...
if it does, then you have found your leak... if no change, then I would
consider adjusting your idle mixture screws...they are located at the bottom
of your carbs...they usually are sealed, but most bikes ive seen have
already had the seals off of them. You can start by making sure they are
turned out approx 1 1/2 turns. Take your bike out for a spin and then
remove each spark plug and see what color they are.....brown indicates
proper air/fuel mixture... white indicates a lean mixture and needs
adjustment (turning the screw 1/4 turn at a time)...black indicates rich and
also needs adjustment...if you get to the point of having the screws turned
out more than 2 1/2 turns, I would then change the idle or slow jet.

Hope this helps...

Fwed
 >> Stay informed about: CB750 F backfire 

"fweddybear" <fweddybear.DeleteThis@cox.net> wrote

> The backfiring usually indicates that the bike is running lean, or
there
> is a slight leak somewhere in and around the intake....

That is usually the case. Absent any other symptoms, I would agree.
HOWEVER.........
In this case, I don't think that applies. The non-firing cylinder is likely
pumping raw gas into the exhaust which then ignites IN THE MUFFLER when it
reaches critical mixture. This is not good for the engine in general and
for the exhaust specifically. One of those explosions might just leave part
of the exhaust system on the ground or flying through the air.

You need to fix that dead cylinder; the "backfire" will then likely
disappear.
Might be as simple as a fouled (or shorted) plug. If you haven't already,
they all probably should be changed. Next I'd look for a stuck or sunk
float in that carb.
Good luck!
 >> Stay informed about: CB750 F backfire 

"Ken Abrams" <harvest_this RemoveThis @scum.suckers> wrote in message
news:uHqOi.4684$oA2.1564@nlpi068.nbdc.sbc.com...
>
> "fweddybear" <fweddybear RemoveThis @cox.net> wrote
>
>> The backfiring usually indicates that the bike is running lean, or
> there
>> is a slight leak somewhere in and around the intake....
>
> That is usually the case. Absent any other symptoms, I would agree.
> HOWEVER.........
> In this case, I don't think that applies. The non-firing cylinder is
> likely
> pumping raw gas into the exhaust which then ignites IN THE MUFFLER when it
> reaches critical mixture. This is not good for the engine in general and
> for the exhaust specifically. One of those explosions might just leave
> part
> of the exhaust system on the ground or flying through the air.
>
> You need to fix that dead cylinder; the "backfire" will then likely
> disappear.
> Might be as simple as a fouled (or shorted) plug. If you haven't already,
> they all probably should be changed. Next I'd look for a stuck or sunk
> float in that carb.
> Good luck!

YIkes... I did read the text, but for some reason when I was typing, I
totally blanked on tthat paragraph about the problem cylinder....my
mistake....shows ya how skimming can get ya in trouble.... sorry guys..

Fwed
 >> Stay informed about: CB750 F backfire 

tokenhire.RemoveThis@yahoo.com wrote:

>I am frightening small children and Vietnam vets whenever I'm
>decelerating or maintaining speed at 3K to 6K RPM. It will backfire
>two shots at a time about .5 to 1 second apart from each other in
>varying intervals (the pairs of shots are more frequent in lower gears
>than higher).

If the backfires are snaps or sound like a dog with distemper coughing, the
mixture is too lean and the carbs need cleaning out. The exhaust smell will
be sooty, with stinky alkyde odors.

If the backfires sound like a shotgun going off, the mixture is too rich and
I recommend checking the float level for being too high. You might notice a
smell of raw gasoline out the exhaust pipe.
>
>The backyard mechanic who made the bike roadworthy for me cleaned the
>carbs, but replaced nothing. He claims there is a cylinder that isn't
>firing correctly (he yanked the sparkplug wire off that cylinder with
>the engine idling and there was no appreciable change),

Check your valve clearances and the cranking compression of all four
cylinders.

You should get no less that 140 psi from a warm engine after five compression
strokes with the throttle held open.

Most Japanese multicylinder motorcycles use a simplified ignition system that
fires two cylinders at the same time. One cylinder doesn't need the spark
because its exhaust valve is open. This results in rumbling and crackling
sounds coming from the exhaust under deceleration.

Your Honda has a mechanical ignition advancer under the left hand "points"
cover. It is spring-loaded to the full retarded position to enhance low end
torque and reduce backfiring on throttle roll off. Sometimes the advancer
will stick in the full advancer position because of corrosion or because a
spring broke.
A cylinder that doesn't fire will build up fuel in the exhaust system and the
waste spark from the ignition coil will light the mixture off.

>and that there are "air mixture adjusters" that need to be tweaked

The carburetors have fuel air mixture screws that are called "screw set A" on
the parts fiche. Anytime you see an o-ring included in "screw set A", you
turn the mixtures screws clockwise to lean up the mixture, and
counterclockwise to richen it.

>but without a Clymers he doesn't know what the settings should be.

Fire him. Any mechanic worth his pay should be able to adjust idle mixture
screws without a manual.

--
Message posted via MotorcycleKB.com
http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200710/1
 >> Stay informed about: CB750 F backfire 

<tokenhire.DeleteThis@yahoo.com> wrote

> Is this something that someone like myself who is considered by the
> general public to be a danger to myself and others within a 150'
> radius can do or should I make the effort to find someone with a brain
> to actually approach the project..?
>

No and yes, in that order.
 >> Stay informed about: CB750 F backfire 

On Oct 9, 9:56 am, "Ken Abrams" <harvest_t....RemoveThis@scum.suckers> wrote:
> <tokenh....RemoveThis@yahoo.com> wrote
>
> > Is this something that someone like myself who is considered by the
> > general public to be a danger to myself and others within a 150'
> > radius can do or should I make the effort to find someone with a brain
> > to actually approach the project..?
>
> No and yes, in that order.



Thanks for your help. Much appreciated.

The backfiring seems to have remedied itself in the wake of the choke
cable being stuck. It will not retreat completely (about a quarter
inch). However, now it stalls at intersections and runs pretty rough
regardless of rpm.

Think I can disengage the cable and live a reasonably fruitful life..?
 >> Stay informed about: CB750 F backfire 

On Oct 8, 12:24 pm, "Albrecht via MotorcycleKB.com" <u33665@uwe>
wrote:
> tokenh....DeleteThis@yahoo.com wrote:
> >I am frightening small children and Vietnam vets whenever I'm
> >decelerating or maintaining speed at 3K to 6K RPM. It will backfire
> >two shots at a time about .5 to 1 second apart from each other in
> >varying intervals (the pairs of shots are more frequent in lower gears
> >than higher).
>
> If the backfires are snaps or sound like a dog with distemper coughing, the
> mixture is too lean and the carbs need cleaning out. The exhaust smell will
> be sooty, with stinky alkyde odors.
>
> If the backfires sound like a shotgun going off, the mixture is too rich and
> I recommend checking the float level for being too high. You might notice a
> smell of raw gasoline out the exhaust pipe.
>
>
>
> >The backyard mechanic who made the bike roadworthy for me cleaned the
> >carbs, but replaced nothing. He claims there is a cylinder that isn't
> >firing correctly (he yanked the sparkplug wire off that cylinder with
> >the engine idling and there was no appreciable change),
>
> Check your valve clearances and the cranking compression of all four
> cylinders.
>
> You should get no less that 140 psi from a warm engine after five compression
> strokes with the throttle held open.
>
> Most Japanese multicylinder motorcycles use a simplified ignition system that
> fires two cylinders at the same time. One cylinder doesn't need the spark
> because its exhaust valve is open. This results in rumbling and crackling
> sounds coming from the exhaust under deceleration.
>
> Your Honda has a mechanical ignition advancer under the left hand "points"
> cover. It is spring-loaded to the full retarded position to enhance low end
> torque and reduce backfiring on throttle roll off. Sometimes the advancer
> will stick in the full advancer position because of corrosion or because a
> spring broke.
> A cylinder that doesn't fire will build up fuel in the exhaust system and the
> waste spark from the ignition coil will light the mixture off.
>
> >and that there are "air mixture adjusters" that need to be tweaked
>
> The carburetors have fuel air mixture screws that are called "screw set A" on
> the parts fiche. Anytime you see an o-ring included in "screw set A", you
> turn the mixtures screws clockwise to lean up the mixture, and
> counterclockwise to richen it.
>
> >but without a Clymers he doesn't know what the settings should be.
>
> Fire him. Any mechanic worth his pay should be able to adjust idle mixture
> screws without a manual.
>
> --
> Message posted via MotorcycleKB.comhttp://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200710/1



Not to worry. When weighing my options including the aforementioned
"mechanic" I'm forced to consider myself as the other option. I'm
considering disconnecting the choke cable. Recently it decided that
it didn't want to retreat completely and now she stalls at idle and
runs like a woman running to the restroom while passing shoe store
after shoe store. Never quite sure if it wants to rev up with the
thottle or hesitate. Think I can survive without the choke for a
bit..?
 >> Stay informed about: CB750 F backfire 

<tokenhire.RemoveThis@yahoo.com> wrote

> Think I can disengage the cable and live a reasonably fruitful life..?
>

Maybe but probably not.
See previous question and answer!

> should I make the effort to find someone with a brain
> to actually approach the project..?

Yes!
 >> Stay informed about: CB750 F backfire