followup on a 79 honda 750 sputtering at 6000 rpm

Hi group...

Just a follow up on the sputtering problem I was having at 6000 rpm.....

although I adjusted the cdi unit to .020 on the gap, I also checked the
plugs and the number one and two cylinder plugs were white at the tips.....
3 and 4 were looking normal, so I turned out both idle mixture screws for
each cylinder about a 1/4 turn, making it a little richer and took it out
for a test ride. I found the sputtering went away leaving only a slight
hesitation if that, so in about 100 miles, i will check the plugs again and
follow the same proceedure on the lean cylinder(s).....once again, I wanna
thank krusty kritter for all the help and wealth of info in getting my bike
running in tip top shape for a bike of that age. It really has some great
pick up for a lil street bike.....I guess back in those days it was one of
the fastest.....

Fwed

 

OK, so you've helped the carburetion a bit by richening up the idle
mixture somewhat. It can be a *bad* mistake to try to cover up the
problem of a small main jet by richening up the idle mixture and
raising the jet needle...

The idle ports pass less and less fuel as the throttle is opened
further until they pass no fuel at all...

At some point before full throttle, the jet needle has pulled up so
high out of the needle jet, the main jet is the only source of fuel the
engine's getting...

An air cooled engine like your Honda wastes a lot of fuel just cooling
itself. The early 16 valve Honda engines were based on an older design
that has actually been around since about 1910, believe it or not. The
combustion chamber shape is inefficient, compared to later designs like
the Suzuki GS-series with the Twin Swirl combustion chambers. I'm not
saying this to make you feel bad about the motorbike you love, it's
introduction to how the engine will behave when it gets hot...

Those early Honda 16-valve engines take too long to burn the mixture,
so a lot more heat gets transferred to the pistons and to the cylinder
head than an engine with compact combustion chambers and flat-topped
pistons experiences...

You need some extra fuel to cool the combustion chambers if you're
going to run the engine hard and you're not going to use the stock
airbox...

And, you said that you're running separate air filters, no airbox and a
#105 main jet. That's why you need a larger main jet than stock. I'm a
bit concerned about your main jet size.

To find out if your main jet is big enough, you need to go out on an
empty road on a nice warm sunny day and run the engine up to redline
for a few miles at full throttle. I can understand it if you don't want
to do this because of law enforcement activity. But, to save your
engine from damage, I recommend
intalling the optimum size main jets...

To get the best, most even top end power (full throttle/after 7500
rpm), select the main jet that produces the highest top speed / pulls
hardest at high rpm.

If the engine pulls harder at high rpm when cold and less hard when
fully warmed up, the main jet is too large. Install a smaller main jet
and retest until you find the main jet that pulls the hardest at high
rpm when fully warmed up. This is supposed to be done first, before
trying to tune the jet needle and the idle mixture...

If the bike doesn't pull well at high rpm when cold and gets only
slightly better when fully warmed up, the main jet is too small.

You have to expect to run the engine hard for at least 10 to 15
minutes, getting up to redline at full throttle in order to properly
tune the midrange and low rpm carburetion,

Do not pay too much attention to the low-end mixture when you are
changing main jets, as I described above, nothing is coming through the
idle jets and the jet needle isn't affecting mixture at full throttle.
Running the engine for a few miles at wide open throttle should blow
any carbon off the plugs, except that you want to see that black ring
of carbon deep down inside the spark plug.

Even though you've helped the engine by tweaking the idle screws, you
still need to be using the main jets that produce the best power at
high rpm. You can adjust the idle mixture and the needles after you
get the main jet just right...

If you're not interested in getting the main jet just right, then
remember what I said about pinging causing little black specks on the
spark plug insulators at first, and extended pinging causing little
blobs of aluminum to melt off the piston tops and stick to the spark
plug...

Light pinging sounds like loose valves. Heavy pinging occurs when the
engine is hot and you're trying to accelerate in stop and go traffic.
Don't let lean mixture burn a hole in one of your pistons. Trust me. It
ain't purty...

 

although I adjusted the cdi unit to .020 on the gap, I also checked

this is great advise.... although I will not be riding the bike hard, I
wanted to make sure it wasn't hesitating or sputtering.... I'm not sure
where you got where I am running separate air filters and no airbox...that
musta come from someone else because I still have the stock set up as far as
that goes....i do have the 105's in there which is also stock....i believe
the main jets are correct as it seemed like the higher the rpm, the quicker
the bike wanted to accelerate when fully warmed up.....the only thing that
is different is the kerker exhaust... 4 into 1..... I will pull the plugs
again in 50 or 100 miles to see how they are burning....if cylinder 1 and
two are black, then i will lean it up a little... but if they are that tan
color, i know I adjusted them properly....I know they shouldn't be burning
white though....

stay tuned...

fwed