Topic: CB750 Carb Backfire / Spitting

[Boulevard_beck]

Hi everyone, I'm looking for a little guidance. I have a 78 CB750k that I'm in the middle of turning into a little cafe racer (http://forums.sohc4.net/index.php?topic=106175) and I'm running into some carburetor issues, i think.

I just got the bike running today. When I bought it a few months ago, it ran really rough. Since then, I've had the carbs rebuilt and installed new intake boots, and then I just let it sit while i did some other work on the bike. After a long session of trying to get it started with loads of carb cleaner, it finally caught and I let it run for a bit. Once it gets warm, it starts like a charm.

The issue I'm having now is that when the engine sits on idle, it seems like the carb is spitting. With the carbs exposed, I can see fuel being shot back out of the carb every few seconds. Plus, when I crank on the throttle, it really bogs down and can stall if I hold the throttle down. I have pods on for now to make it easier to work around down that, but I intend to put the airbox back on once everything is done.

Here's a video of the bike running: http://youtu.be/pDuKFc92s68

I did a search and found a comment stating "If the engine backfires through the carburetor, misses, cuts out or quits running, the main jet is lean. Increase the size of the main jet." Would this suggest that running the engine with the carbs exposed, or running it with pods is in fact causing the issue because there's too much air getting in?

Any help would be really appreciated. Thanks so much.

And, here's a pic...just because pictures are much more fun that loads of text.

[Dimitri13]

Yes. If you don't have bigger jets, running with pods, and especially without ANY intake on the carbs, you'll be lean.

Those seem like cheap universal ebay pods, which have no taper on the inside. Go back to the stock airbox, get it running perfectly, then see if you want to go with aftermarket intake.

[Boulevard_beck]

Thanks for the quick reply. I'll put the stock airbox back on tonight and see how it sounds. I'm amazed that what seems like a simple little modification (air pods) can have such a dramatic negative impact on the performance of my bike (backfire and dying on throttle).

[Dimitri13]

Yep. Not only will it lean it out, but it won't have the proper taper for the correct atomization of fuel (or something like that).

If you look at the airbox vs pods, you can see why it makes such a difference. With the airbox, it needs to go through a filter in a box with one opening, then through another box, which has the proper taper on the boots, then into the carbs. With pods, you only have to go through a filter, which has 360 degrees of intake.

[lucky]

Spitting and back firing, poor throttle response=too lean.

Yes..Matter of fact the intake area of the stock filter is 3.7 square inches!
So the airbox is limiting the intake to about 30%!

It is like a giant choke making the entire range of the carb richer.
Remove that air filter and box and the entire range is lean.

This set up will work.
stock air filter  and stock air box (sorry)
#42 idle jet.
#120 main JET.
needles non adjustable.
Mixture screw 3/4 turns open.


That will get you close.

FOR PODS.

If you insist on the pods then you will need to get some 1977 slide needles.
Put the stock needle next to the 1978 needle and lower the position on the 1977 needles TWO notches. You could try one notch also but I think it will take two based on my pervious experience. Install the 1977 needles.

Then you could move down on the idle jet to a #40 with 1-1/4 turns open.

Make sure your accelerator pump and nozzles work. all 4 of them.
See the video I posted on this forum.  Title- Accelerator pump video Excellent

[Flying J]

Or if you want your bike not only to work, as suggested above, but to run well you could go with the stock air box and the stock jetting.
http://www.motorcycleproject.com/motorcycle/text/specs.html

looks like a 110 and a 35.


Also you dont need to change out you needles if you want to run pods. You can buy shims made for that purpose
http://www.jetsrus.com/individual_parts/009_396_su.html
or here
http://carbjetkits.com/keihin-shims.html

In my experience with pods it requires farting with the carbs a bunch to find the right jetting. I could never get my 750 to run as well as i wanted so i changed back to stock.

[Boulevard_beck]

I studied quantum mechanics and astrophysics in university. I seriously don't think I have the knowledge to run pods and tune the jets correctly to work with this bike. I'll just leave it with a stock setup and paint a lightening bolt on the airbox or something.

[lucky]

You cannot run stock jets with the 4 into 1 exhaust.

You will find out.

[Boulevard_beck]

What about running stock jets with the stock airbox and some MAC 4-2's? I thought that the MAC exhaust systems were pretty close to stock - ie, not really performance systems with huge throughput.

[KC3]

Hey a fellow physicist!! I thought I was a lone wolf on this site. Well, let me tell ya, if you studied quantum mechanics, then you know linear algebra. You also know PDE's and probably have done quite a bit of complex analysis! If you have done even one of those things and actually were able to figure them out, then you should have no problem figuring this sucker out!

-I studied physics at Indiana University and am now an engineer and research staff for a neutron electric dipole measurement experiment and make electron spectrometers. I must say though, that much like quantum mechanics, I was very confused when learning about the air & fuel delivery systems of my cb550! I am, however, very happy that I decided to go with velocity stacks afterall! Even though I have put many hours of time into tuning since that decision.

Anyways, I would reccomend using either stock exhaust or rejetting as needed for the upgraded exhaust. Much like the upgraded air filter, the exhaust can have an "astronomical" effect on the performance of the engine. The backpressure, or lackthereof, can also be an issue. Just be sure to think about everything that will be affected with every change you decide to make. And remember that the engineers that designed the thing originally had already thought these things out.

Cheers-
KC

[OneWheelDrive]

I am, however, very happy that I decided to go with velocity stacks afterall! Even though I have put many hours of time into tuning since that decision.

Curious to see what jetting setup you went with to achieve success with the stacks. 

[Flying J]

There is a big difference between pods and velocity stacks. Go read the faq on carbs for a long explanation on that. But yes velocity stacks are very much doable. The stock airbox has velocity stacks in it.  They are effective in performance, that is why they run them on race bikes. The problem is they suck in all the dust and dirt that an air filter keeps out. If you want the compromise forum member tintop has built a velocity stack with an air filter. I haven't used mine yet but i hear it works great.

[lucky]

Hey a fellow physicist!! I thought I was a lone wolf on this site. Well, let me tell ya, if you studied quantum mechanics, then you know linear algebra. You also know PDE's and probably have done quite a bit of complex analysis! If you have done even one of those things and actually were able to figure them out, then you should have no problem figuring this sucker out!

-I studied physics at Indiana University and am now an engineer and research staff for a neutron electric dipole measurement experiment and make electron spectrometers. I must say though, that much like quantum mechanics, I was very confused when learning about the air & fuel delivery systems of my cb550! I am, however, very happy that I decided to go with velocity stacks afterall! Even though I have put many hours of time into tuning since that decision.

Anyways, I would reccomend using either stock exhaust or rejetting as needed for the upgraded exhaust. Much like the upgraded air filter, the exhaust can have an "astronomical" effect on the performance of the engine. The backpressure, or lackthereof, can also be an issue. Just be sure to think about everything that will be affected with every change you decide to make. And remember that the engineers that designed the thing originally had already thought these things out.

Cheers-
KC

[lucky]

Hey a fellow physicist!! I thought I was a lone wolf on this site. Well, let me tell ya, if you studied quantum mechanics, then you know linear algebra. You also know PDE's and probably have done quite a bit of complex analysis! If you have done even one of those things and actually were able to figure them out, then you should have no problem figuring this sucker out!

-I studied physics at Indiana University and am now an engineer and research staff for a neutron electric dipole measurement experiment and make electron spectrometers. I must say though, that much like quantum mechanics, I was very confused when learning about the air & fuel delivery systems of my cb550! I am, however, very happy that I decided to go with velocity stacks afterall! Even though I have put many hours of time into tuning since that decision.

Anyways, I would reccomend using either stock exhaust or rejetting as needed for the upgraded exhaust. Much like the upgraded air filter, the exhaust can have an "astronomical" effect on the performance of the engine. The backpressure, or lackthereof, can also be an issue. Just be sure to think about everything that will be affected with every change you decide to make. And remember that the engineers that designed the thing originally had already thought these things out.

Cheers-
KC

Yes...electron, diple etc.etc,interesting, but I had a good friend that was a physicist. He was wonderful to listen to, but after seeing him struggle for more than a year trying to put together has VW bug engine I helped him actually get it done and stop talking about it. Lol....lol..lol

[lucky]

There is a big difference between pods and velocity stacks. Go read the faq on carbs for a long explanation on that. But yes velocity stacks are very much doable. The stock airbox has velocity stacks in it.  They are effective in performance, that is why they run them on race bikes. The problem is they suck in all the dust and dirt that an air filter keeps out. If you want the compromise forum member tintop has built a velocity stack with an air filter. I haven't used mine yet but i hear it works great.

You are right.

The stock air box does have rubber velocity stacks BUT... it has to breath through a opening 3.7 sq. inches!!
So the entire air box is acting like a giant choke limiting intake air to about 30%!!

 But as far as jetting the pod and the velocity stack can be considered the same because if you have a screen on the velocity stack inside or on the outside it basically is about the same as far as the jetting. The screen in the velocity stack slows down the air also.

I spent a month working just on the jetting for stacks on my 1978 CB750 and came to the conclusion that the taper of the slide needle may need to be different. I got close but it was still rich.

I will give you the formula to get close though.
1978 CB750
Velocity stack with screen.
4 into whatever exhaust with some type of muffler of baffle.
idle jet #42mm.
Main jet #120mm.
Needle .040 shims on TOP of the pocket in the slide.
The pocket in the bottom of the slide is .012 thousandths deep.
Mixture screw one turn open.

If it is a 1977 try 2 clip positions richer.

IF you try that and it works then try a #40mm idle jet
and see if the throttle response is still good.

You will be close

If you put your mouth on the POD filter and breathe through it ,you will see that it offers almost NO restriction.

[lucky]

Forum member Brewsky is doing some very good research and testing on his motorcycle with some really good data.

He has the 1978 CB750 also.  We are just talking about these 1977-78 CB750's.

[Flying J]

There is a big difference between pods and velocity stacks. Go read the faq on carbs for a long explanation on that. But yes velocity stacks are very much doable. The stock airbox has velocity stacks in it.  They are effective in performance, that is why they run them on race bikes. The problem is they suck in all the dust and dirt that an air filter keeps out. If you want the compromise forum member tintop has built a velocity stack with an air filter. I haven't used mine yet but i hear it works great.

You are right.

The stock air box does have rubber velocity stacks BUT... it has to breath through a opening 3.7 sq. inches!! the openings of the carbs is 10.67 sq. inches.
So the entire air box is acting like a giant choke limiting intake air to about 30%!!

 But as far as jetting the pod and the velocity stack can be considered the same because if you have a screen on the velocity stack inside or on the outside it basically is about the same as far as the jetting. The screen in the velocity stack slows down the air also.

I spent a month working just on the jetting for stacks on my 1978 CB750 and came to the conclusion that the taper of the slide needle may need to be different. I got close but it was still rich.

I will give you the formula to get close though.
1978 CB750
Velocity stack with screen.
4 into whatever exhaust with some type of muffler of baffle.
idle jet #42mm.
Main jet #120mm.
Needle .040 shims on TOP of the pocket in the slide.
The pocket in the bottom of the slide is .012 thousandths deep.
Mixture screw one turn open.

If it is a 1977 try 2 clip positions richer.

IF you try that and it works then try a #40mm idle jet
and see if the throttle response is still good.

You will be close

If you put your mouth on the POD filter and breathe through it ,you will see that it offers almost NO restriction.

Oh so thats your scientific evidence for pods? You blew on it and it offered no Resistance?

The difference between pods and the stock box is the velocity stack that smooths the turbulent air going into the carb so that you get an even vacuum inside the throat.
Read this stuff. edit
http://forums.sohc4.net/index.php?topic=5410.msg741438#msg741438

Anyway, im not going to argue with the "suck on your pods" scientist. I, like many of you on here, am not a scientist. I read and do research to form my conclusion. i also like to consider my sources. There are quite a few guys on here with racing as credentials or are engineers or have a long history on this forum with good sound advice. Those are the peoples advice on motorcycle i look for. Lucky does not make that list for me.

[Boulevard_beck]

Thanks for everyone's great input. I'm very much a function over form person; I just want my bike to work, first and foremost. Second, there seems to be more than enough evidence to suggest to me that the safest best for me is to run the stock airbox over pods or stacks or "that guy's special stack filter thing".

Finding the secret formula of jets sizes and needle shims after taking apart my carbs repeatedly just doesn't sound like fun for me. I can appreciate perfectionists seeking out the perfect combination for their bike, their mods and riding style. However, I like to ride in the rain, I wash my bike every 3 months or so and I dont always keep up with the maintenance schedule. I just don't have the desire to tinker with my bike every other day to ensure that it's a high performance machine. Given my requirements, I think the stock airbox is safest.

[lucky]

I agree that for the 1977-1978 CB750 the best choice IS the stock air filter and air box because it incorporates velocity stacks and engine protection.
It just looks ugly.

If there was a good remedy and EXACT jetting specifications for pods or stacks for the 1977-1978 CB750 it would all ready be on the internet.
I doubt if you will ever find it. I have looked.
I have asked many and no one has the answer.

Also the dyno owners may have come up with a solution using their diagnostic tool but
so far not one person on this forum or any other forum has said they have the perfect solution for the 77-78 CB750 with pods OR stacks, EVEN IF YOU ARE WILLING YOU PAY.

Also some will say their bike runs good but later admit they have to baby the throttle. That is NOT a solution.  LUCKY

This is what I know works:

1978 CB750K
Stock air box and filter.
Aftermarket exhaust with internal baffle.
idle jet #42.
main jet #120.
Needle non adjustable -no shims.
mixture screw 3/4 turns out.

[KC3]

Let's try not to turn this into a 550 thread, but I am running my 76 cb550f with the 650 cam swap, velocity stacks with screens and outerwears in them. Im sporting 4 into 1 exhaust with a glass packed muffler that I made and running pd46a carbs with #115 mains and stock slow circuit with the stock needle position; no drilling of any jets or emulsion tubes...I'm also running a bit colder plug with the NGK D8EA.

@Lucky--you'll find that most theoretical physicists can barely tie their own shoes. Experimental physicists; however, are the complete opposite and (in my opinion) are just as mechanically inclined as most techs or engineers. I was an auto/diesel mechanic for many years before I went back to school for physics, so I don't count :-)

[Rigid]

Here in the backwoods, we just drill them jets out bigger and hold some throttle at the stoplight to keep the engine from quitting.  I have run with no pods or airbox, and with velo stacks, as well as the stock box.  It all depends on the situation.  I do like the lightning bolt painted on the stock box idea though. :-)

[lucky]

Let's try not to turn this into a 550 thread, but I am running my 76 cb550f with the 650 cam swap, velocity stacks with screens and outerwears in them. Im sporting 4 into 1 exhaust with a glass packed muffler that I made and running pd46a carbs with #115 mains and stock slow circuit with the stock needle position; no drilling of any jets or emulsion tubes...I'm also running a bit colder plug with the NGK D8EA.

@Lucky--you'll find that most theoretical physicists can barely tie their own shoes. Experimental physicists; however, are the complete opposite and (in my opinion) are just as mechanically inclined as most techs or engineers. I was an auto/diesel mechanic for many years before I went back to school for physics, so I don't count :-)

Your set up sounds right with that glass packed muffler. I can see why the stock idle jet would work.

I like R&D type people too because they will try anything as opposed to a more scientific type. R&D gets results. Worry about why later on.
 

[Rgconner]

I studied quantum mechanics and astrophysics in university. I seriously don't think I have the knowledge to run pods and tune the jets correctly to work with this bike. I'll just leave it with a stock setup and paint a lightening bolt on the airbox or something.

Yep, because carburetors are not orthogonal... It is like trying to solve for four bodies in a gravitational model!

Podding is a painstaking process to get very  little back, unless you like the "look".

But that is Fashion, not physics.

[KC3]

Let's try not to turn this into a 550 thread, but I am running my 76 cb550f with the 650 cam swap, velocity stacks with screens and outerwears in them. Im sporting 4 into 1 exhaust with a glass packed muffler that I made and running pd46a carbs with #115 mains and stock slow circuit with the stock needle position; no drilling of any jets or emulsion tubes...I'm also running a bit colder plug with the NGK D8EA.

@Lucky--you'll find that most theoretical physicists can barely tie their own shoes. Experimental physicists; however, are the complete opposite and (in my opinion) are just as mechanically inclined as most techs or engineers. I was an auto/diesel mechanic for many years before I went back to school for physics, so I don't count :-)

Your set up sounds right with that glass packed muffler. I can see why the stock idle jet would work.

I like R&D type people too because they will try anything as opposed to a more scientific type. R&D gets results. Worry about why later on.

Exactly. Theory is absolutely great when it is applied correctly!