CB750 F backfire
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CB750 F backfire tokenhire 10-08-2007
`--> Re: CB750 F backfire Albrecht via Mo...10-08-2007
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Posted by on October 8, 2007, 7:38 am
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


Posted by fweddybear on October 8, 2007, 8:57 am
> 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



Posted by Ken Abrams on October 8, 2007, 9:54 am


> 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!



Posted by Ken Abrams on October 9, 2007, 9:56 am


> 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. ;-)



Posted by on October 8, 2007, 11:14 am
On Oct 8, 7:38 am, tokenh...@yahoo.com wrote:
> 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



I think I see the idle mixture screws. They come out from the bottom
of each carb. Mine have a fin on one side of each screw which looks
as though it's a stop as it hits a fixed aluminum tab coming off the
carb casing, so it will not quite make 360 degrees in total range of
motion.

I'm on my way to get cleaner and try that test as well.

I'll keep you posted on the results. You help is much apprecitated.


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