1978 cb750 carb swap

[shultz_1978]

Ya it ran good the last time out. Just rich on the idle.

I did find out that the mix screws were 3 turns out so I put them at 1 turn.

I am still learning all of this carb stuff. But yes it is running better then it was. Before I started messing with it I had to choke the crap out of it to keep it running till it warmed up. Now I have it to were I just use the choke to get it started. Let it set for about a minute then go. No spitting or popping like it was.
It just seems to be a little rich at idle.

[Lucky 1]

Now I am going to say one more thing that could be an issue.

If there is no combination of jets and needle shims that will seem to work,

It could be that all of the increase of fuel from a much larger idle jet and shims on the needle,
X4, that maybe the fuel line from the stock gas tank which is 1/4-5/16 is not large enough
and the float bowls are getting low on gas.
That #2 carb getting low on fuel could be a big issue
if the accelerator pump system is not working all the time.

But lets wait and see how Shultz' bike runs after he gets it back together.

[Re-run]

If you still run into rich issues at idle, replacing the idle jet may be required. It is not so much the size, it is the loss of venturi action. See the hole in the jet acts as a venturi and when you drilled it, that action no longer occurs. It may or may not be an issue though, only time will tell.

[shultz_1978]

It is back together It is running 1/4 line. I check the accel. pump is working. Thanks for the how to check on that one Lucky.

Question on the mix screw. Which way is to lean and rich the mix.  They were at 3 full turns out, which seemed a little far to me. But I ran them all in then turned out 1 turn and the bike seems to not be running as good as it was.

The bike seems to be stumbling if I just jump on it. It seems ok if you just roll in to it.

You might be right Rerun. I may have to start over. Maybe I should have started with a little smaller bit?
But hay I can get the carbs off in about 10 minutes now. LOL

It take time and it is still raining. I just take it a round the block 4-5 minutes. Not long at all.

[Re-run]

The stumble is a bit hard to get rid of unless you crack it from fully closed and it blasts the carbs with fuel. I guess it depends on a person idea of "jumping" on it though.

As for the fuel adjustment, out is richer and in is leaner.

[shultz_1978]

Thanks Rerun

 That is what I thought. I really don't think I will need to get on it that hard. So the stumble I think is not going to be a problem. I was just setting in the drive and hammered on the gas and noticed it.

Thanks Hope the weather dries up so I can get it out for a real ride.

[Re-run]

If it helps, it seems the stumble is during the transition from the idle jets to the mains. The needle effects this area. This is one area I wish honda had not caved to the epa and kept the 77 needles. Get a micrometer and find a batch of washer that fit and mic them to find a group that are the same thickness. See then if you can tune out the stumble. I only get it if I REALLY crank open the throttle.

[Lucky 1]

The mixture screws richen when backed out.

With the #45mm idle jet and the .031 shims on the needles you may
only need to turn the mixture screws out about 3/4 or 1/2 turn.

Take one mixture screw and turn it in or out and carefully watch your tachometer.
When you get the highest reading on the tach the mixture screw is about right.
You can add 1/8 just to make sure it is not lean. Then do all of the others the same.

[Lucky 1]

The mixture screws richen when backed out.

What size mainjet?

With the #45mm idle jet and the .031 shims on the needles you may
only need to turn the mixture screws out about 3/4 or 1/2 turn.

Take one mixture screw and turn it in or out and carefully watch your tachometer.
When you get the highest reading on the tach the mixture screw is about right.
 Then do all of the others the same.

MAKE SURE THE PODS ARE ON and the engine is warmed up when adjusting the mixture.
The pods MUST be on to tell what is going on. They make a difference.

[shultz_1978]

The main is a 15a what ever that means stock?

It maybe a few days till I get back out. We have high in the low 40s and wet.

[Re-run]

Ah so you have the weather I had last week. Not overly warm here yet this week. 70 today and tomorrow and then tanks back to the 50s. I wish the lows stayed above 40 at least but no such luck.

[shultz_1978]

I can deal with the 40s It is the wet that I don't care for. But its all good I just get out the other play toy. hehehe

[Lucky 1]

That 15A is printed on the needle? Not the main jet. The main jet will have 3 numbers ,no letters.
The needle goes up and down through the main jet.

I have not seen any meanings to those numbers so far.
I have see Honda engines with the same year ,same carb etc.,
and the needles had different numbers.

If any forum members know anything about those numbers please tell us.
BTW Shultz I went from a #45mm idle jet to a #42mm idle jet and it ran worse.
But I had .044 thousandths shims on the needles not .031 thousandths.

[Lucky]

I found out today that the total intake area of the stock air box on this 1978 is less than 39% of the total area of the intake.
I thought the intake total area was 12.56, but it is less.
It is 10.67 square inches.
The intake is actually about 30% of the total area of the carb intake area.
I thought it was 39% but it is actually 30%!

These stock carbs are really choked down when you add that in with that paper restrictive filter.
But you know when you choke an engine it gets a richer mixture.

[Lucky]

I found out today that the total intake area of the stock air box on this 1978 is less than 39% of the total area of the intake.
I thought the intake total area was 12.56, but it is less.
It is 10.67 square inches.
The intake is actually about 30% of the total area of the carb intake area.
I thought it was 39% but it is actually 30%!

These stock carbs are really choked down when you add that in with that paper restrictive filter.
But you know when you choke an engine it gets a richer mixture.

[shultz_1978]

I had a bike shop tell me the best thing to do was chrome tape half of my air filters off. It did not change anything. Any more I think bike shops are getting to be like most car dealers. A JOKE !!!! I have payed out about $250.00 to two different shops for the bike to run the same as it did when I dropped it off. That will learn me LOL

 In time I will find a setting that makes me happy with the way it runs. I do not race or anything. So as long as it runs smooth with out all the choking to keep it running with out popping that is fine with me. It was running good after I drilled out the idle jet. Just running rich so I check the mix screws and they were out 3 to 3-1/2 turns. I have turned them in to about 1-1/2.

I have not had a chance to get it out and ride (weather) blaaa. Hope to get out in the next few days and see what it runs like.

[Lucky 1]

Today I got a surprise.
I had put the carbs for my 1978 back on with stock#35 idle jets,120 mains
 and mixture screws set at 1 turn open.

I put on the STOCK air box and after it was warmed up  it would bog and die on quick acceleration.

Still lean,
I guess that 4 into 2 into 1 exhaust even with a baffle still needs more gas.

Now I have to decide it I will put the #45mm idle jets in or the #42 idle jets.

I sure am getting fast at removing the carbs.LOL...lol

[shultz_1978]

LOL I two am pretty good at removing my carbs.

I wish the weather was a little nicer so I could go for a ride. Maybe tomorrow.

Good luck on you carbs. I will post my findings when I get a chance to ride.

[Lucky 1]

It is raining on and off here too.

I just finished changing idle jets and the carbs and filter is back on the bike.

I went with the #42mm's and I got excellent results.

It is interesting that I have not had to put any shims on the needle with the stock filter.
 But the idle is still affected by the performance exhaust.  You cannot run stock idle jets.

I just had to come upstairs to my computer and get a 9V battery for my RPM gauge to set my mixture screws.
They are one turn open now,but I think I can turn them down a bit with this #42 idle jet.


1978 CB750

So to sum up:  
Mains 120s.
Idle jets #42s
NO shims on the needles
Turn mixture screws IN about 1/8 from 1 full turn out.

Stock air filter in a stock air box.
Still need a road test. Should be fine.

Engine accelerates good. Even with a quick turn of the throttle.
Still need a sync job.

I'm Happy!   Sound good.

[shultz_1978]

Nice hope it work for you. I think its going to be fair here in the morning. If so I will ride the to work and see what happens. I think I may pick up a set of carbs.

The ones I have had some work done on them. Someone drilled out the srew hole and put in bigger srews. It also has a bowl that was broke. I can not polish them and I think it would look better with the bowls polished. Maybe by then I will have a understanding of how to set everything. Maybe