1979 honda 750 sputtering at 6000rpm

Hi All..

Am having a problem with my 750 when I run thru the gears at 6000 rpm.
It has a tendency to sputter and was wondering if it was in need of bigger
jets, or if I need to look at the gap for the igniton spark....the bike has
a 4 into one kerker exhaust with 105 jets. The carbs were cleaned, synched,
and adjusted properly with new fuel filter and tank just kreemed, so I know
carbs are not an issue. Plugs are fine too as well as wires. Any opinions,
suggestions??

thanks in advance...

Fwed

 

Are you sure? If the bike's been standing for a long time, then simple
carb cleaner may not do the job.

 

The carbs were cleaned, synched,

Yes... positive.... they were completely rebuilt last year (by a repair
shop) and it ran good.... over the winter, i started it every 2 weeks or so
and early this season i went to take it out and it ran lousy.... i took it
back to the repair guy and he found little rust particles in the fuel
filter....so he then gave the tank a rekreeming (after the acetone and acid
bath) and recleaned the carbs.....the bike wants to lurch forward (and does
when it doesnt cough) so i am thinking the jets, but then i also remember
way back when, this happens when points are set too close....but this has a
dual spark advance system... i think checking the gap for each ignition coil
would help a little...just not sure if its that or jets...

Fwed

 

Gee, if it didn't do it last year and you didn't change jets over the
winter, I don't think it is jets. They don't shrink or grow on their
own. Sounds like a bit of crap in a small passage still. As point gap
changes, so does timing. The two are very closely interlinked. Look at
that too.

--
Gopher 33 28 19N 112 01 49W
'77 CB750K '78 CB750K
'00 ZG1000 '96 Ducati 900SS
**********pull 'mychain' to reply***********
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For the rest of my life I would continue to say precisely the wrong
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natural gift for it" W. F. Temple)

 

I beieve those bikes have fairly weak ignitions. My 1981 would sputter
if I didn't religiously change the plugs often.

 

You could try re-gapping your spark plugs to 0.025 inches, instead of
0.028 inches. That really helps more at very high RPM, like 10K...

Are those CV carbs or slide valve carbs you've got? A torn diaphragm is
a CV carb would show up at higher RPM, while the engine would idle
OK...

When you roll off the throttle, does the exhaust system go fartPOP! or
piffle-piffle-SNAP! ?

That's plugged up idle jets, or the idle mixture screws are set
wrong...

What gear are you mainly finding the "sputtering" in. If you have lots
of power running up into the midrange, and then it flattens out around
5K and there's actually a decrease in the power until you get up above
7K and then it runs OK up to the redline, that's the dreaded
interaction between the 4-into-1 exhaust sytem and the carburetors. The
exhaust systems pushes a pressure wave backwards through the carbs
about 5K and the engine can't inhale fresh fuel air mixture, because
the air is going backwards through the carburetor...

The carb doesn't know which way the air is flowing though, it sucks gas
out of the float bowl and when the air starts flowing the right way, it
sucks up even more fuel. Now the engine gets a double shot of gas and
it's too rich...

#110 or #115 mainjets are probably the biggest you can use to get a
rich enough mixture to run for long periods of time at 10,000 RPM, full
throttle, but there is no way to re-jet the carbs to get rid of the
dread flat spot around 5K to 7K...

I talked to the techs at Dynojet about the flat spot. They said that I
could just accelerate rapidly through the flat spot RPM band in the
lower gears and avoid riding in the flat spot zone. Fine, on a race
track I could do that, but I was riding on twisty canyon roads a lot...

Problem was, I couldn't just accelerate through the falt spots in those
slow curves. I had to stay in 2nd gear and buzz the engine, I couldn't
ride in the flat spot in 3rd gear...

The Dynojet guys, who'd taken my hard earned cash for their "tuning
solution" that just didn't *work* admitted that there was no way I
could ever tune the flat spot out of the original equipment carbs. They
said that other racers were installing the larger bore carbs off the
next year model on their GSXR's. The larger bore reduces intake
velocity and vacuum and the engine doesn't get as big a double shot of
gas in the flat spot RPM band...

4-into-2-into-1 exhausts systems are one solution to the flat spot.
Exhaust throttling valves such as Yamaha's ExUp or Suzuki's SET close
up at low RPM and do some throttling in the 5K to 7K RPM range to break
up the unwanted
postive pressure wave from the exhaust system...

Then there is the problem that you've removed the air box and installed
separate air filters. The Japanese figured out how to acoustically tune
those plastic airboxes to assist the engine in breathing as it passes
through the dreaded flat spot. Racers removed the airboxes and dynoed
their engines and found they had less mid range torque without the
airbox, so they re-installed the air boxes to get a flatter torque
curve...

The airbox is a resonant flask, just like a bottle with a long neck. If
you blow across the neck of an empty beer bottle, you'll hear an
organlike note as the air inside the bottle resonates. If the bottle is
half full, it will resonate at a higher frequency. That's like making
the airbox smaller in volume...

Bottles with shorter necks or wider mouths will resonate at a different
frequency. I don't think anybody has ever blown across the mouth of a
Mickey Big Mouth bottle and made it resonate at all. That's about the
situation you have if you take the lid off an airbox. Lid off the
airbox is about the same as installing K&N separate filters...

The length and diameter of the airbox's intake snorkel is *critical*...

I removed the rubber snorkel completely from my GS-1100's airbox,
thinking it would breathe better through a larger hole. I was wrong, it
needed the length and diameter of that snorkel just like the bottle
needs a neck in order to resonate at a frequency that would help the
engine's torque curve...

At about 6000 RPM in the higher gears, the engine would begin to lean
surge and stagger. The exhaust sound was waaah! uh! waah! uh! waaah!
uh! until I rolled off the throttle and reduced engine RPM. I
reinstalled the rubber snorkel and the surging was gone...

 

Am having a problem with my 750 when I run thru the gears at 6000

I have them gapped at .26, which is between the recommended .24 and
..28.... it does the waaaaaa uhhh waaaaaaaaaa uhhh... thing.... and pops
while decelerating...... in the meantime, I will check the gap for the
ignition coils, and mention this to the mechanic at the repair place....

thanks for all the advise all!!

Fwed

 

place....

Well, it does sound like your carburetion is lean in both the idle and
midrange circuits. If I was you, I would richen up the idle mixture
about 1/2 to 1 full turn, and ride the bike to see if that helps, and
if that didn't cure the problem, I would raise lower the needle clip
one notch or if there's no clip put about two half millimeter thick 3
millimeter inside diameter washers underneath the top of the needles to
richen up the mid range...

The engine would probably run better with the stock airbox and a single
K&N replacement air filter inside the box with the lid on the box, all
snorkels in place and no holes in the box...

I wouldn't run out and buy a bunch of main jets without understanding
how the sizing of round jets works. Google for "kaybearjr@aol +round
jet" to see how the orifice size increases exponentially...

And, as I said previously, no amount of re-jetting will tune out that
bad interaction between the exhaust system and the carbs...

The only real reason to install main jets that are around 4 sizes
larger than stock is because the engine is running really lean and
overheating when you hold the throttle wide open for long periods of
time, like when you're riding 130 mph on a long straight road...

Main jets get really expensive when you're guessing about what you
need, and
shade tree tuners usually wind up with a whole box full of useless jets
that are too big or too small for the climate and altitude they usually
ride in...

 

I have them gapped at .26, which is between the recommended .24

Since I posted last, I poked around at the cdi unit.... one side was
gapped at .20 and the other was gapped at .32..... so what I did was gap
them both at the .32...... then took it out for a ride and found that it
helped tremendously..... now I get a slight sputtering at 7000 rpm or above
and just briefly.... this is probably "normal", but was wondering what the
correct gap is for the cdi unit? This is a 79 honda 750 Ltd with (not that
this matters but) a 4 into one kerker exhaust....doesnt seem to pop on
deceleration much anymore .. just a little ....

Fwed

 

I would think that 0.020 (20 thousandths of an inch) would be more
realistic than what you claim to have set your pulser coil gap at...

..32 is more than 5/16ths of an inch, and your pulser coils would fire
the spark incredibly retarded, if they fired at all...

I know that you meant 0.032 inches. If setting the pulser coil there
helped a little, your ignition may be too far advanced or stuck partly
or fully advanced...

Your whole problem might be the mechanical ignition advancer...

I looked at the Left Crankcase Cover Spark Advancer fiche at
www.partsfiche.com. It shows that you apparently have a mechanical
spark advancer, item #3 on the parts list.

If you hook a timing light up to your engine and look past the spinning
timing boss, your light should light up that mechanical spark advancer,
and it should be advancing the spark from about 10 degrees at idle to
about 45 degrees or more BTDC when you rev the engine up...

(Actual engine timing specs may be on a sticker under the seat or on
the inside of a side cover...)

You should see two little flyweights expanding out as you rev the
engine and
contracting back as the engine idles down. If they don't move freely,
they may be stuck or a spring might be broken. If you can squirt some
penetrating oil at that thing, it will probably free it up so it work
smoother. A bolt with the timing boss on it attaches to the spark
advancer, and when the spark advancer does its thing, it rotates that
bolt and the advancer in whatever direction ignition advance should
be...

 

Since I posted last, I poked around at the cdi unit.... one side

Yes. i did mean 32 thousands of an inch... i can say it right, but i
cant type it correctly...I will check the sark advancer as well.... and make
sure it functions correctly... after all that, i will also pull the plugs to
see what they look like and determine if I should richen up the idle mixture
screw...

Thanks again Krusty... youve been very helpful (as always) and i can't thank
you enuff... next time you are in the northeast (Ct) I'll buy you a
drink....

fwed

 

Look for spark plugs that have rough looking bone white porcelin tips.
Use a lighted magnifying glass to look deep into the spark plug. You
should at least see a black carbon ring way down inside the plug. If
you don't have that, after you've run the engine full throttle in high
gear and done a plug chop, you need a bigger main jet...

Small black specks like pepper burned onto the spark plug nose indicate
pinging, ignition advanced too far, or a spark plug with the wrong heat
range (too hot). Little bits of carbon come off the top of the piston
and burn onto the plug when the engine pings...

If you look down the spark plug hole with a piston at TDC, you might
notice a bare aluminum spot on top of the piston. That means the piston
is really getting hot right under the spark plug...

The next step is when the piston starts melting and throwing little
bits of melted aluminum onto the spark plug. I have seen pistons melt,
burn holes in themselves, seize and break connecting rods. If you see a
tiny bright aluminum ball on a spark plug, your jetting or timing is
way off...

 

after all that, i will also pull the plugs to

Ok... will check the plugs that way.. i have recommended plugs in
already, so im sure the manufacturer wouldn't steer you wrong..(hmmmmm..
come to think of it... hehehe)

Some great info here... will keep that in mind when checking
things....thanks again!!

fwed

 

Honda is usually correct about spark plug heat range. Many years ago, I
had a Honda with 2 sets of plugs, 1 set for around town and 1 set for
long highway trips. I left the hot, around town plugs in the engine and
set out to ride it across the USA. I didn't get far before burning a
hole in one piston...

But modern large displacement bikes like yours don't need any special
spark plug selection---as long as they remain stock, with the original
air box and carburetor jets and exhaust system. With your modified
machine you need to get the main jets and idle mixture set right in
order to keep the engine running cool enough so the tips of the spark
plugs don't start glowing cherry red and ignite the mixture early,
before they actually fire a spark...

I had one manual recommend a really cold spark plug for my Jaguar. It
seemed that they believed I was going to race it at Le Mans, so they
recommended a Champion N3 plug. But I was driving around town, not on
the race track and my spark plugs would carbon up and I had to keep
cleaning them all the time. That got old fast...

Then one day, I remembered a set of Hitachi spark plugs that I'd
removed from a Jaguar cylinder head that I'd bought in a salvage yard.
I decided to try those Hitachis, and the engine ran so much better I
went down to the auto parts store to find an equivalent Champion plug.
The parts guy sold me a set of Champion N5's and asked what kind of car
I was putting them in. So I told him and he looked up the recommended
plug and said I should use Champion N5's. So I asked him, "What about
the N-3's recommended in the manual?" He said, "Oh, those are cold
spark plugs for high speed racing, like Daytona."

And that's how I learned about spark plug heat ranges---the hard way...