Topic: Need help getting it to run right

[guitarking135]

So I figure I would start here since y'all have a lot more experience with these bikes than I do. So what I got is a 78 cb750 that I've made into a bobber… I got it running but it doesn't run very well. Let's start with the set up

Stock engine-that I'm aware of.
4 Drag pipes
Stock carbs w/ 4 open filter elements

The problem is the bike will not run with out the choke pulled out all the way. When I push the choke in to start it sounds like it's sucking in ALOT of air. You can hear a whistling noise. I'm assuming that it's probably an air fuel mixture issue but I'm kind of at a loss as to where to start. Is there any test that I can do to try to isolate the problem. rich or lean?

[Tempast]

Well for one, you're getting WAY more air into the engine than stock, and with drag pipes and no backpressure WAY more air OUT of the engine.
The problem is most likely you not getting enough FUEL into the air going into the engine. Time to play with the jets. Stock size is 120 on the 750 I believe (not sure what year)  Try going up a couple sizes and see if it runs better.


Holding the choke makes it harder for the engine to breath, so you'll get more less air into it, and making the air/fuel ratio better.

[dusterdude]

Yea,you're way lean.richen that bad boy up a bit


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[Tempast]

Wow my grammar in that post. "more less air" GJ. What I meant was you'll get less air, and more fuel.

[harisuluv]

You are still on stock settings?  With that setup you would need to raise the needles and increase the main jet size.

[guitarking135]

I went up to a 120 jet from a 110 per the suggestion of a vintage Japanese bike mechanic. Let me go back. I went out to start it this morning and it wouldn't fire up. After putting in the `120 jets It acted like it wanted to start but just wouldn't go. Is it possible part of my problem could be in the Air/fuel screw?

[guitarking135]

Oh and the only way it would sputter and act like it wanted to start was with the throttle close to all the way open

[guitarking135]

You are still on stock settings?  With that setup you would need to raise the needles and increase the main jet size.

How do I go about raising the needles?

[flybox1]

if you have the correct PD carbs for that bike, you cannot change needle clip position , its fixed, so shimming the needle is the only way.
here's how...
http://forums.sohc4.net/index.php?topic=71245.0
Good PD carb info thread, as you have 2 other important systems on those carbs.  An accelerator pump, and a fast idle cam.
http://forums.sohc4.net/index.php?topic=48858.0

...and with your exhaust and intake,  you'll be very close to the setup listed in my sig    with maybe #130 main jets

[harisuluv]

Just making that main jet change shouldn't cause it to not even start.  The main jet should only affect 3/4 to WOT throttle range anyways.  You have other problems, most likely at least dirty idle/slow jets. 

[guitarking135]

I got it starting and can push the choke in a little. Might try the 130 jets and go from there. I think it wasn't getting fuel before. Let it sit and it fired right up. 

[guitarking135]

I'll go ahead and throw this out there too. I was playing around with it today i can get it to start but it runs really high idle then eventually comes down then dies. Almost like it's out of fuel. Sputters and puts. If I try giving it gas it dies but if I pull the choke out more it will come back to that really high idle. Do you think it might be an issue with getting fuel to the carb?

[lucky]

I went up to a 120 jet from a 110 per the suggestion of a vintage Japanese bike mechanic. Let me go back. I went out to start it this morning and it wouldn't fire up. After putting in the `120 jets It acted like it wanted to start but just wouldn't go. Is it possible part of my problem could be in the Air/fuel screw?

You cannot expect it to run right by just changing the main jet. That would have almost no effect at all.

[lucky]

I am going to tell you how to make it work.

First make sure the accelerator pump system is working. Both one way check balls have to work. ONLY the #2 carb has the accelerator pump. IT is the master carb.
 Check the spray nozzles with a bright LED pocket flashlight. With the KEY OFF,  make sure that when you turn the engine over that each nozzle squirts gas.

Make sure you measure the float height as given in the Clymers' manual.
Measure them like the book shows.
Make sure all of the float needles are new.
Remember they could be 40 year old parts. Toothpaste is not going to fix them.

Make sure to remove all 4 mixture screws and dig out any old O rings.
Put in 4 new O rings with the spring and washer on each mixture screw. You can just buy a set of 4 mixture screws and you get all new everything with them.

Put in #42 idle jets.  40's will not work.
Mixture screws all at 3/4 turns out.

Slide needle will need to be shimmed or replaced with 1977 or 1976 slide needles.
Needle clip one notch down from stock.
It will make your job easier.

Main jets #125.

Sync the carbs LAST.

[guitarking135]

Where can I get shims and idle jets at?