Topic: '78 CB750F Cafe Project Log (Minnesota)

[nick55555]

Nick,
When tunning the carbs (changing jets and adjusting needles) your best bet is to just remove them from the bike each time. I know in and out 50 times!! Make sure they seat well in the rubbers each time.
As far as adjusting the needles, here is what I remeber off the top of my head.
Remove the top cover of each carb.
Unscrew the slide arm, one screw on the shaft which crosses all four carbs.
From here you can lift the slide and disconnect the arm.
Pull the slide out from the carb body.
Down inside the slide there are two small screws which secure the arm to slide and hold the needle down, unscrew these.
Now the needle should come out for adjustment.

mireparo: Your clear description of how to do this worked like a charm! Now that I have gotten the needles out, I see that they are NON-ADJUSTABLE (see pictures below)!!! What the heck??? I am planning on doing pods and a Mac/Kerker 4 into 1 and I am pretty sure then engine is going to need more fuel throughout the range.

Should I get new needles? Sand them down? Any and all input will be appreciated!



They have "15A" stamped on them.

[TwoTired]

mireparo: Your clear description of how to do this worked like a charm! Now that I have gotten the needles out, I see that they are NON-ADJUSTABLE (see pictures below)!!! What the heck??? I am planning on doing pods and a Mac/Kerker 4 into 1 and I am pretty sure then engine is going to need more fuel throughout the range.

Should I get new needles? Sand them down? Any and all input will be appreciated!

It's not so much that you will need more fuel throughout the range, it's that the pods will reduce the vacuum in the carb throats that draw the fuel through the orifices and jet needle.

Most likely you will need to raise the needles.  Some have put washers under the E clip to raise the non adjustable needles you have.  However, if you can find needles from a 77 carb, they should have 5 grooves for the E clip.

Cheers,

[Pete944]

I'm also in Minnesota and looking for a project similar to yours.  My dad has a '71 cb500 that I love and I'd like a 750 cafe project.  Looks like you've got some good ideas for the bike.  I've been looking for a cheap 750 to build for about 9 months and haven't come across any under $1k.  If any of you local guys come across any, I'd appreciate a shout.

[Pete944]

And I'm in St. Paul, by the way. 

[nick55555]

Craigslist.org... found mine on there for $300.

Okay, I remember reading about the 'less vacuum with the pods' now and I also remember having a thought. What if one were to put some clear tape (only as an example) around the filter element of the pod, thereby reducing the surface area of the exposed element --> restricting the flow into the carbs --> increasing [from stock pods] the vacuum... anyone follow?

I am quite certain that some adjustments would still need to be made. But, I have heard several testimonials about these bikes running better with the stock airbox.

Unless I hear a convincing argument why I should not do this, I think I am going to give it a shot when I start putting her all together... and that is a good segue into a progress report:

Unneeded frame tabs have been ground off and sanded smooth, waiting on color samples from the powdercoater, should get the bike to him in a week so he can proceed to blast it and coat it. In the meantime I think I will put the carb back together (leave the needles alone for now) and tamper with the pods a bit. Prep calipers, master cylinders (?), forks, etc. etc. for painting, paint them, then rebuild them. Is that the proper way to do it? strip -> paint -> rebuild? Don't wanna scratch the paint, and don't wanna get paint where it don't belong.

Thanks again fellers,
-Nick

[mikedialect]

Craigslist.org... found mine on there for $300.

Okay, I remember reading about the 'less vacuum with the pods' now and I also remember having a thought. What if one were to put some clear tape (only as an example) around the filter element of the pod, thereby reducing the surface area of the exposed element --> restricting the flow into the carbs --> increasing [from stock pods] the vacuum... anyone follow?

I am quite certain that some adjustments would still need to be made. But, I have heard several testimonials about these bikes running better with the stock airbox.

Unless I hear a convincing argument why I should not do this, I think I am going to give it a shot when I start putting her all together... and that is a good segue into a progress report:

Unneeded frame tabs have been ground off and sanded smooth, waiting on color samples from the powdercoater, should get the bike to him in a week so he can proceed to blast it and coat it. In the meantime I think I will put the carb back together (leave the needles alone for now) and tamper with the pods a bit. Prep calipers, master cylinders (?), forks, etc. etc. for painting, paint them, then rebuild them. Is that the proper way to do it? strip -> paint -> rebuild? Don't wanna scratch the paint, and don't wanna get paint where it don't belong.

Thanks again fellers,
-Nick

let me know who you go through and what they cost- been looking for an affordable powder coater in state.

[Pete944]

I check Craigslist daily.  I guess I just haven't found the right deal yet, because most of the CB's seem to be well over $1k.

[mikedialect]

I check Craigslist daily.  I guess I just haven't found the right deal yet, because most of the CB's seem to be well over $1k.

http://www.carsoup.com/used/detail.asp?usedVehicleID=4199677&MakeID=31&maxYear=1978&searchID=2571700&vehicleTypeID=3&UVViewID=4&Page=1&DealerGroupID=

[Pete944]

Mike, thanks.  Hadn't thought to check carsoup.  I didn't realize they had any vintage stuff.  On another random note, do you happen to be a twin cities techno DJ?  And if that is the case, what are you doing posting at 130am when you should have been seeing Daniel Bell?

[mikedialect]

Mike, thanks.  Hadn't thought to check carsoup.  I didn't realize they had any vintage stuff.  On another random note, do you happen to be a twin cities techno DJ?  And if that is the case, what are you doing posting at 130am when you should have been seeing Daniel Bell?

I dj'd for a very long time in the city. I was at work and I've seen daniel bell 6 or 7 times- in detroit and here. I really don't go out any more because I've seen them all and it got stale after a while.

I have a couple cb450's and cb350f if you're interested in picking up something.

[mireparo]

mireparo: Your clear description of how to do this worked like a charm! Now that I have gotten the needles out, I see that they are NON-ADJUSTABLE (see pictures below)!!! What the heck??? I am planning on doing pods and a Mac/Kerker 4 into 1 and I am pretty sure then engine is going to need more fuel throughout the range.

Should I get new needles? Sand them down? Any and all input will be appreciated!

Nick,
Like TwoTired said, you'll probably need to raise the needles and washers work well. You'll also need significantly larger jets. You can drill them out but when you over drill them you can't go back. I ordered a few sets from cycledepot.com in the parts unlimited catalog. I'm not sure our bikes run the same carbs but I think I'm running 125 mains and the needles all the way up (equivilant to 3 washers the same thickness as the c-clip), however, I'm still not pleased with how it runs. I lost motivation in fall and won't mess with it till spring or summer again. I'm curious to hear what works well for you.

[nick55555]

mikedialect - I haven't brought them my frame yet (because I still haven't received the color samples) but Impact Powder Coating in Spooner, WI [(715) 635-2372 ] was recommended to me. They come down to the TC every or every other Friday to Beiswingers hardware at the intersection of 35W & Hwy 96 (which happens to be close to where I work). They have been very helpful so far, and the guy that recommended them to me does METICULOUS customization of his bikes... so it is probably good quality (and the cost is reasonable). I will post some pics and let you know when I have it done. ALSO, I have been looking for a CB450 or CB350 recently... let me know what you have!!!

pete944 - keep looking! The one I found was pretty far down the list, and the guy wasn't very motivated to sell it...

EVERYONE - No response to the 'decreasing the surface area/air flow through the pods' idea??? I know that someone on this forum has at least $0.02 on this! Even if I do try this, I will still probably raise the needles one or two washers... time shall tell.

[mireparo]

Nick,
First.., good luck with the carbs, my 2 cents has only confused me more.
Second.., let me know if you need some parts, I've still got all my spares around. I noticed your shocks are pretty rusted. I took my originals off which are shown on my bike earlier in this thread.
Just let me know what you need.
Micah

[TwoTired]

Fuel jet flow.

You'll notice that when the engine is not running, no fuel flows through the fuel jets.  This is because the carb throat pressure is exactly the same as the pressure on top of the fuel in the float chamber (vented to atmosphere).  When pressure is the same at both ends of the jet (carb throat to bowl supply), no fuel flows.

When the engine runs, the carb throat pressure drops due to two primary causes.  One is the suction from the intake stroke of the piston.  The other is the venturi effect at the air passage narrow point. (Increasing velocity, lowers pressure.)

The depth of the pressure drop therefore relates to both velocity and engine mechanical efficiency, as well as any restriction to the air supply presented to the carb throat entrance.

The pressure drop is what makes fuel flow through the carb fuel metering jets.  If you increase the pressure differential, more fuel flows through a given orifice size.  If you decrease the pressure differential, you need to use a larger jet orifice to obtain the same flow as the smaller jet with a larger pressure differential.

While the stock filter does not change the volume of air traveling through the carbs to engine cylinder, it DOES present a pressure drop to the carb throat due to the restrictive nature of the filter material disturbing the air flow.  (In fact, any tube will create a pressure drop even if open ended, as the walls present drag to air moving adjacent to it.)

If you shorten the runner (tube), or use a different filter media that presents less drag to the air movement, the carb throat pressure increases closer to that of the air pressing on the fuel in the carb bowl, and any given orifice size will flow less fuel volume.  If the engine volumetric efficiency has NOT changed, then the air fuel ratio then also changes.  (re-jet!!!)

Tape restriction of a pod filter.  In theory, a pod filter with inherent low restrictive qualities (lower than a stock airbox/filter) can be made more restrictive by closing off the inlet area.  However, a given restriction may be fine at idle speed and far too restrictive at air speeds and volume required at red line RPMs.

Which means; taping the filter may obviate the need to change slow jets, but may induce a required change to the main needle adjustment, and the main fuel jet to compensate for the higher restriction and higher throat pressure differential conditions when high volumes of air are required.   (re-jet!!!)

Filter media drag is non linear to airflow/volume increases.  Inlet area restriction is also non-linear, but the rates of change are different between the two....

I think. 

Cheers,

[KB02]

Everybody get that? 

I had to re-read a couple section twice myself, but I finally got it.

Your posts like that are one of thing that I love about you being around these boards, TT: technical, informative, and really helpful in figuing out what we're trying to do. 

[nick55555]

I second that... I guess I will REJET!!!

[nick55555]

OKAY!!! Finally, some more progress.

Since the last post I have:

- had the frame powder coated (Impact did a pretty good job, but I was under-quoted since the sandblaster charged about 200% more than anticipated)
- purchased an "All-Balls" tapered head bearing kit & one of Carpy's café seats
- cleaned up a bunch of misc parts & getting others ready to be sand blasted
- REJETTED (120)!!!
- put the carbs back together (with some extra parts... see below)
- failed to find small enough washers to increase the needle height. Does anyone have some '77 needles with the adjustable clip positions???

Here are some pictures to prove I've been working on it:

Pretty new coat. It was even allowed in the house for a while.


Pile up in the garage.

Just before I dropped the frame off at the powdercoater we noticed that the frame was split on both sides at the bottom. It looked like some water got in and froze. So, I knocked it back together and welded it up, which is why the wirefeed is out.

Carbs are back together with 120 mains (with the exception of having the needles adjusted). Got the ball bearing races off the lower clamp assy.


Ummmm.... these are some extra carb parts. They might be from the other carb I was playing with.... but I doubt it. Any ideas where these go?



So right now I am waiting to sand blast all of the parts that are going to to painted (which is nearly everything). I am waiting to do anything with the tank because I am pretty sure I am going to need to make some clip-on dents. Once [almost] everything is painted, I will put the front end together (with the tapered bearing) to see how it fits, plop the engine back in, update the swingarm bushings, and install the swingarm.

I am starting to get a bit wary of the whole rewiring deal, but then I am not very familiar with the wiring / diagrams.... I am sure I will be in the near future.


And now for my questions:

1) Does anyone have a line on some cb900f rear shocks? Any reviews on how they mount/perform on a CB750?

2) Where do those O-rings in the picture go???

3) Any pointers on how to go about rewiring? I am thinking of taking a wiring diagram, crossing off the stuff I don't/won't need, and seeing if what is left will work.

4) Tire recommendations (affordable yet good)?

5) Anyone in the Twin Cities have a sandblasting cabinet?

6) You guys are the experts! Anything I have overlooked or should be thinking about? Questions, comments, requests, complaints?

Thanks much!

[matt365]

I think the little o-rings are on the drain screws at the bottom of the float bowls.

Not sure about the larger one.

[mireparo]

Nick,
I've got those adjustable carb needles your looking for. I picked up a set of spare junker carbs because I needed a float bowl.
Let me know if you still need them.
Miach

[nick55555]

PM sent. 

[nick55555]

Okay, got the rings sorted out. The small ones were for the pilot jet screws (two per jet), and the larger one was between the float bowl and carb body on the #2 carb (where the accelerator pump is located).

Now that that is all sorted out... does anyone know if the front and rear brake rebuild kits for a '78 CB750F are interchangable? Anyone have a good source? I got a dual kit for my front brakes from partsnmore, but am still looking for a kit for the rear brake.

Also, does anyone know of a good place to get OEM Honda control cables (throttles and clutch)?

[moham]

Also, does anyone know of a good place to get OEM Honda control cables (throttles and clutch)?

http://www.discounthondaparts.com/fiche_select.asp

[bigsherm]

Come on, Nick!

     Let us see what you did with that yellow powercoated frame!

Sherm