1978 cb750 carb swap

[Lucky 1]

Thanks SHULTZ for posting your results. Sharing your experience.

You used a #77mm or a numbered 77 drill bit, drill on WHAT???  The idle jet? The main jet????
"Turned in the screw 1/2 a turn"   From what? From 1.5 turns out stock setting???

And you are running PODS right?

If you drilled the IDLE jet with a numbered drill size NO.77  that would be .018 thousandths.


Very unclear.

[shultz_1978]

Lucky
 I used a number 77 drill bit on the idle jet. On the screw I always count my turns out just in case there is something in the jet. Then I clean the screws and jet seats. When I took them out they where all about 5 turns to get them out so when I put them back in I went 5 1/2. I know in theory it should run rich.  I am not a pro or anything just trying this whole carb thing out.  You can see my setup in the picture from my previous post.

[Re-run]

No, I did not have to change jets at all. I ran it with no filter and did not have to change the jets. I retuned my idle mix and it ran just fine. I can haul ass easily into redline and pull a ton no problem. I currently run a K&N in the airbox, I run my plugs a very light tan and run regular fuel. Stock coils and plugs, only change is a dyna s.
I did tune without the bottom half of the box and in light wind it ran great. But, light wind in my area is only slightly more common than finding a 5 carat diamond laying on the road. High wind affected the air flow too much for smooth operation, an issue that happens to pods as well.

Part of my mantra with carbs is to clean them till you could drink from them, it may be that others may not do this. These carbs are more sensitive to being dirty, kinda like CV carbs are.

[shultz_1978]

First 35 deg. is a we bit chilly first thing in the morning. Bike seams to be running alot better. Maybe alittle lean. But it still runs better then it did. I love not having to play with the choke. Pulled the choke hit the button fire right up shut off the choke. IT sat for about a minute and off to give her a drink then to work. Not bad sofar.

[Re-run]

That actually sounds a bit rich really. When cold, you should have to use choke, or if you rode earlier that day but the engine is cool, choke should be required to start again. I generally use choke for a couple minutes. I use full choke to start and then back it off a little. idle for a short amount of time and then ride easy for a couple minutes and gradually decrease choke.
I would get your bike to full temp, ride for about 10 minutes at low rpm around town or something, then get it back home and let it cool and pull your plugs. Honestly, anything over a medium tan is technically on the rich side for this year of bike and I would look at shimming the needle as that fattens up the mix sooner on the transition from idle to high rpm.

[shultz_1978]

Thanks for the info Rerun I will have to take a ride after work. I guess I got to what I got to do ride. If I need shims I may have some looking to do. We do not have any bike shops close that seam to know what they are doing. They are the one that told me to get the 80 carbs. When I called and told htem they do not fit. Reply was I don't know what to said. NICE So I have a set of 80 carbs that I polished up a bit and can not use them. But hay they look good on the bench. lol

[shultz_1978]

I pulled the plugs and they are a light tan. I think I will run it like this for a few days and pick up some new plugs as these are the plugs I was running before I started messing with the carbs. Thanks again for all the input. Great site and happy with the members I have met. Everybody seems to try to help. Maybe we should start a bike shop. ok ok maybe not. lol

[Lucky 1]

You mean running a 1978  lot better with..... #45 idle jet and??? pods? Right?

You need to be real specific with the words and facts, with carb tuning posts.
Someone may copy them.

[Lucky 1]

OK....A K&N in the STOCK airbox. Not individual K&N pods.
So basically you are stock except for the K&N filter and exposed bottom half of the filter box.

So I would just call it stock really. Do you agree?

[shultz_1978]

Like I stated I am not a pro or anything just trying the whole carb thing out. I don't know what # idle jet I am running. What I do know is I used a #77 bit on it and my mix screw is ran in 5 1/2 turns. I can not tell you how many turns out from full set it is. The bike is running pods I do not know the brand... they where on the bike when I got it. The bike also has drag pipes and is punched out to a 805.

So number 77 bit to the idle jet

5 1/2 turns in on the screw

 It is running rich I can smell it. How rich I do not know. What shims to use I do not know that either.

I do not know that this will work on any other set up. But my bike is running better now then before I started this little project. I just need to guess on the shims to use and see what happens. I would feel bad if someone tried this to their carbs and just messed up their jets with a bad out come, I had nothing to lose. It is a fine line between better and worse. Only hope to stay on the better side.

[Re-run]

I run a k&n now. When I dropped the airbox bottom, there was no filter, so essentially, completely open.
no filter is less filter than pods. And while not straight pipes, I run aftermarket slip ons that are less restrictive than the stock pipes were.
With no filter and more open pipes, if these carbs were that unresponsive to changes, I would have had to rejet. I did not and had no trouble.

In a few months, I will be slightly boring, porting the head, and dropping in a hotter cam. I will use my stock carbs and I foresee no issues.

[shinyribs]

Reading this thread is tempting me to keep my keyholes I was wanting to trade for a set of roundtops. Thanks for all the real world info guys. I very much appreciate it.

[Lucky 1]

I am very interested in what you are doing SHULTZ.

I am sorry I did not take my bike on the road for a read test
 with that set up WITH the shims.
Maybe it would have been OK.

IF you do put shims under the needles notice that .012 pocket in the bottom of the slide.
Either put the shim OVER the pocket or in the pocket and measure you shims.
I had two .022 shims + the .012 pocket depth so it was about .056 thousandths.
Make sure this does not happen: See photo below.

Shim stuck.

Spring must center on the needle.

Needle crooked from stuck shim.

[shultz_1978]

I am pretty shore I will be shiming. As I do think it is running rich on the low end. But It runs out nice. I rode it last night and it never choke up on me or tried to cut out.

I do not know what or where to get the shims. I will probably have to have our awsome bike shop order them.   I have to say they really turned out nice.

[Lucky 1]

You CANNOT get the shims at a motorcycle shop. Forget that.
It is NOT something that you can order.

Trust me.

You will have to get them at a real hardware store.
Not one of those big box stores.


You are showing the Vacuum operated carbs but I thought
you were talking about shimming
the PD 1978 carbs.

[shultz_1978]

Those are the 80 carbs I was told to but and they do not fit. Guess I should have tried that before I polished them. I just thought they looked good setting on the bench.

[shultz_1978]

Lucky
 The shims you guys talk about looking at your picture are they small washers? I work for a supply house and we sell all kinds of washers. Of course I will have to get a mic to check the thickness. But will they work if I find the right ones. Any starter sizes to try. Thanks

[Lucky 1]

Yes...Shims and washers same thing.

Shims is just a fancy word for washers or spacers.

[Re-run]

Raising the needles will richen the mix sooner as it transitions from idle to mains.

[shultz_1978]

Thanks rerun but if it is rich on the low end. Do I need to lower the needle?