Topic: CFM (cubic feet per minute) for the main jets.

[noahspop]

Hi I had to switch out an engine for an extra one that came with my bike. The engine seems to run fine. It just runs really rich, I mean really rich. The new engine is #CB750E-2516444 that came with a set of carbs # 7AIK.
The engine I broke a valve on is # CB750E-2610988 that has a set of carbs #41BWPD that I want to use on the bike. The other carbs don't fit. I would like to know what the CBFM is for each of these engines and carbs. Sorry if none of this really makes sense. I'm sure you can tell I'm fairly new at this.

A short background story:
The original engine (that was on the bike when I got it) ran just fine until the valve incident.
We took out the busted engine and put in the engine that came separate with the bike when I bought it. That engine sounds and runs great. It’s just that the carbs run to rich. They were synced properly, and torn apart multiple times to find the problem. What needs to be done now is to get smaller jets for the carbs to lean them out. That’s why We need to know the CFM to see what kind We can get.
When I say "We", I mean my good friend who is a trusted wrencher with plenty of experience in working on anything with nuts, bolts, and grease. He just asked me to do the research, but I couldn't find any threads on this site or google. If maybe someone can direct me in the right way. Anything would be helpful.

[mlinder]

You've listed the same engine as the new one, and broken one.

What is the new engine?
And I don't know what #AIK carbs are.

Let me see if we've got this right:

You installed a new engine, but want to use the old carbs from the engine you blew up, is this correct?
Need to know what we are working with. Finding out the cubic feet per minute the engine can take in, trying to guess how much the carbs can take in, guessing how much restriction is caused by the air filtration system (or lack thereof) isn't going to get you too far.
So, which engine are you running, and which carbs do you want to run, what main jets are currently in there, and what will you be running for air filtration?

[noahspop]

Sorry for any confusions.
I made the change to the engine #.
The carb # that you are unfamiliar with is the # my buddy read off to me.
I currently run and plan on running pods. They were with the bike when I bought it.
I have read the many pros and cons of pods vs. stock air filters on this site.
It's just that the bike ran fine with the pods before the engine change.

"You installed a new engine, but want to use the old carbs from the engine you blew up, is this correct?" - YES.

I'm not sure what size are already in there, but I was told that there is a possiblility of using a smaller size jet to lean it out. Currently I'm using the orginal carbs that where on the blown engine with the # 41BWPD

Thanks for your quick response.

[mlinder]

The carbs should work fine, if they worked on the other cb750 engine.
You sure the choke is open all the way? Any other changes? Still running the pods? Only change has been engine itself?

[noahspop]

We ran it with the pods on and off.
He took it out for 20 mile ride and it went through 2 gal of gas, he got the rpms up to about 8 1/2 in hopes that it just needed to get the excess of anything burned off. It was burning off extra gas.

Yes the engine, and clutch was the only change.

I feel like I'm in an episode of House.
The bike is a 1977 cb750 supersport. Now I'm not even sure that the engine I blew was the original engine.

Here's a pic.

[mlinder]

Nope, both your engines are 76 K's, I think.
With engines being pretty much identical, you've got something else going on if you are running everything else the same.
Same carbs, same filtration, same exhaust.

Again, make sure the choke is opening all the way, and make sure you have the right spark plugs.

[noahspop]

Yeah, I was thinking they were both K models when I was checking the engine numbers (CB750E-.....)

[noahspop]

Just trying to bump the thread, to get some more feedback.
thanks for your help mlinder. I just what others have to say about the situation.

The main question that I have is how to find the CFM for engine #CB750E-2516444
with a carburator # 41BWPD.
 
If this is even possible to find this out.

I need to get a smaller jet size to lean the carbs out.
Any and all suggestions are greatfully welcomed.

[mlinder]

It's not that simple, Noah.

The cb750, could, theoretically, fi there were NO restrictions for intake and exhaust, move 110 cubic feet of air per minute at 8500rpm. That's 27.5 cubic feet per minute per cylinder.
That wouldn't work very well, as you'd be using almost 14 gallons of gas per cylinder per minute

And how much do the carbs flow? That a tough question, as it relies heavily on the method of filtration, etc.

You need to find out why you are running rich.
Again, what plugs are you running, have you confirmed your choke mechanism is working correctly?
Nothing obstructing carb intake? Timing and valves adjusted properly?

[noahspop]

It never is simple, is it?

I did purchase the proper plugs, and gapped correctly.
Not sure on the choke setup. I will look into that too. I'll go over the valves and timing again too.

The bike was running with the pods on and off, still the same result.

Thanks for all your help Mark.

Scott. Noah is one of my sons.

[mlinder]

It never is simple, is it?

I did purchase the proper plugs, and gapped correctly.
Not sure on the choke setup. I will look into that too. I'll go over the valves and timing again too.

The bike was running with the pods on and off, still the same result.

Thanks for all your help Mark.

Scott. Noah is one of my sons.

Oh, sorry, Scott.
Didn't put the "Noah" and "s" then "pop" together.
I have brain damage.

Out of curiousity, just to say we made sure, WHICH plugs did you get?
And yes, check the the choke mechanism right away.

[noahspop]

This is where I got the plugs from.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=370167277968&viewitem=


They were the same brand as the ones that were in the blown enginge.
They seem to work for various years and models.

Again I will look into the choke senario.
Thanks,
Scott.

"I have brain damage."
From falling on your bike, too much fumes?

I have brain damage from falling off of my skateboard too many times.

[mlinder]

Which ones are they?

[noahspop]

Sorry, I have a bad habit of doing that.

This is where I got the plugs from.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=370167277968&viewitem=

[mlinder]

Yep, those are the correct plugs.
Next thing to do is check choke mechanism, and if you can, check your main jet size.

[mystic_1]

Wait, maybe I'm missing something, but, carb jets are sold by number according to their hole size, so 140 jets are larger than 120 jets and so forth.

Need smaller jets, buy smaller numbered jets.

mystic_1

[mlinder]

Wait, maybe I'm missing something, but, carb jets are sold by number according to their hole size, so 140 jets are larger than 120 jets and so forth.

Need smaller jets, buy smaller numbered jets.

mystic_1

He may not need smaller jets, though. That's the issue.

[noahspop]

Almost road worthy again.....