Got my 1978 CB750 out

[Lucky 1]

Got out on the highway today after waiting for my registration.

The bike set up:

Stock air box.
4 into 2 into 1 exhaust with Lossa megaphone/muffler.
idle jet #42mm (stock is #35)
main jet is #120mm ( stock is 110mm)
Needle non adjustable (no shims)
Mixture set at 1 turn.
Accelerator pump system working.

Out on the road it ran well and the plugs looked just
right when checked after the ride.

The idle kept climbing and so when I returned I open the
mixture screws from 3/4 turn open to 1 full turn open. Seems better.

I will take it out again tomorrow.

[shultz_1978]

That is a nice clean bike you got Lucky. Very nice well done.

[Lucky 1]

Thanks Shultz!

I just wanted others with the same set up to know what works.
I would really like to get some 1977 needles that are adjustable for the carbs.
I have this feeling that by richening up the needles one clip position would really help.

At one point I had velocity stacks with screens and 45mm idle jets and .045
worth of shims sitting on top of the pocket in the bottom of the slide and
the engine really screamed. It did not get a road test though.

[seestheday]

Thanks for the specs lucky.  I'm going to be moving to a 42 idle jet soon myself.  I have a 4-1 exhaust and I'm finding that I have to have the mixture screw way out otherwise it runs too lean.

[shinyribs]

How did your fairing do? Wind noise? Did it deflect good?Vibration?...all that jazz.

[Lucky 1]

The fairing directed the air too low and it was trying to lift my helmet at 60mph so I made an adjustment and tilted it up slightly. I will let you know the results on the next freeway ride. Should be better.
I am 5'10".

I took it for a short ride today and the mixture screw adjustment really helped.
Only 1/4 turn richer.

Oh...almost forgot. There was no noise from the fairing.

[Lucky 1]

Yes I have been through all of that.


Stock air filter/stock air box.
I tried #35 mm stock jet-will not work at all.
I tried #40 mm no good will not work, 1 turn out
I tried #42 works. 1 turn out. 3/4 was too lean.

By the way with these idle jet changes the results seemed very clear cut.
The #42 just works.
Also I tried it with two different light duty mufflers too. Almost the same exact result.

When i go to all the work of removing the carbs and changing jets
I really give it a complete test and re synch the carbs. I try different mixture settings.
I make sure that the combo will not work before moving on to another combo.

A large main jet will protect your bike from seizing up. It is cheap insurance.

[shinyribs]

Thanks for the info so far on the fairing.I'm still undecided on running one or not. We have cicadas here this year and I have whelps on my neck from those little buggers. A bug is just too darn big if you can spot it 100+ feet out at freeway speeds and can see it good enough to dodge it.

[Piute]

Those damn Cadeses ,Kill a couple for me ,there heaviest (lots every 13 yrs) I think ,When living in B-more riding around Pennsylvania Got hit in the mouth fatt lipp didn't tast good either ,yep kill lots fer me,you can use your bike to do it ,5-6 bugs on a full wind shield ya have to pull over and clean those nasty azz drunk flying fugly looking....

[Lucky 1]

I can see it now...A row of Cicada decals on the wind shield lower edge like a fighter plane.

[shinyribs]

HA! That would be funny!

[Piute]

Lucky 1 wrote

Got out on the highway today after waiting for my registration.

The bike set up:

Stock air box.
4 into 2 into 1 exhaust with Lossa megaphone/muffler.
idle jet #42mm (stock is #35)
main jet is #120mm ( stock is 110mm)
Needle non adjustable (no shims)
Mixture set at 1 turn.
Accelerator pump system working.

Out on the road it ran well and the plugs looked just
right when checked after the ride.

The idle kept climbing and so when I returned I open the
mixture screws from 3/4 turn open to 1 full turn open. Seems better.

I will take it out again tomorrow.

  Your not making it any easyer for me to wait on my tags(n such) Havn't hit the interstate myself,It's ? 30 mile's to work that way  Gotta wait  ..............

[seestheday]

Lucky 1 wrote

A large main jet will protect your bike from seizing up. It is cheap insurance.

Can you elaborate on this?  Genuinely curious.  I thought that a seized engine implied that the pistons are seized due to excessive heat/no oil/rust from sitting.  I just don't understand how a large main jet would help avoid that.

It really bugs me when I don't understand something.

[Lucky 1]

Pistons can seize in the cylinder bore for a number of reasons.

They can seize from neglect, water and rust.
They can seize from being run lean with too small of a main jet.
They can seize from the needle on the slide dropped down too far.
They can seize from a lack of oil.
They can seize from incorrect timing issues.
They can seize from any combination of incorrect jetting or lubrication issues.
A large main jet will make a rich mixture and keep the piston cool.

[rrgunslinger]

A motor running to lean Will melt the piston and cause it to seize.  Or just blow up. 

Harvey

On Jun 2, 2012 12:57 PM, "seestheday [via Honda CB750'S]" <[hidden email]> wrote:

Lucky 1 wrote

A large main jet will protect your bike from seizing up. It is cheap insurance.

Can you elaborate on this?  Genuinely curious.  I thought that a seized engine implied that the pistons are seized due to excessive heat/no oil/rust from sitting.  I just don't understand how a large main jet would help avoid that.

It really bugs me when I don't understand something.

1981 CB750K

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[shinyribs]

There is a point of diminishing returns,though.Yes,too lean can cause a piston to melt,or more common,just overheat and swell causing it to ''stick'' in the bore. But too much fuel can cause fuel wash.Fuel wash is when the motor consumes more fuel than it can handle,or more than the carbs can atomize,and the wet,raw fuel will strip the pistons and cylinder walls of oil and lubrication.Obviously that can seize one,too. Much easier to ruin a motor with a too lean mixture vs. too rich. Lean can create more heat which can help create horsepower ( energy is heat) IN SOME INSTANCES. Like we say at the track-''lean is mean ,but fat(rich)is happy!Best to err on the side of caution (rich) but it is possible to go too far.

[seestheday]

Very interesting.  Thanks guys.

Oh, I was mistaken about the 42 (I had actually researched this before and had already bought the jets prior to this conversation).

Stock on the DOHC is #68.  I moved up to #75 and changed the mixture screws from 4.5(!) turns out to 1.5 turns out.  I already had #105 mains.

While I was working on it I also did a cam chain tension adjustment, pulled the headers off because they still hadn't sealed properly and put them back on with lots of high temp gasket maker, and checked to make sure the carbs we still in sync (they were bang on).

Runs AMAZING now.  Fixed everything except for a very small amount of valve slap that I can hear if I really listen for it (I knew it was an issue and have shims on order).

Totally smooth acceleration up to 100 kph.  Popping on deceleration eliminated unless I really rev it in 1st and then let the revs drop substantially before shifting into 2nd.  

Irridium plugs that are in there are old and they were running for a while with the bike running lean so I won't be able to get a good read from them.  I'll be buying a new set soon.

[Lucky 1]

4.5 turns out on the mixture screws!!! WOW.

Sounds like that larger idle jet was certainly necessary!