Topic: Cb750 K7 Jetting Recommendations

[KrautKoffin]

Picked up my first bike (77k7) a little while ago and just moved into a house with a garage.. So I tear into the bike that's been at a buddies for 2 months not running. Had a mobile mechanic rebuild the carbs and I went to check the jet sizes today and the carbs are full of oxidized aluminum. Oh well. I'm cleaning all that out now and wanted to get your opinions on jetting for APE pods and the MotoGPWerks pipe. Right now I have 110 mains and 35 slows. The bike has been running lean lately and has been a pain in the dick to start and idle. I think this is because of the clogged slows in 3 cylinders, but that'll be fixed today. Should I upset to compensate for the amount of air that the PO didn't compensate for? Get all of your pod rants out in another thread, I'll be sticking to pods.


Thanks,
Jordan

[martin99]

No anti pod rants from me.

On my 77F2 I have increased the main to 125, and raised the needles one notch. IMS screws one and a quarter turns out. Like you, I have 35 slows stock. I thought I had it running good, but I intend to up the mains to 130 over the weekend as after a recent sustained ride I think it's still a little lean from midrange to WOT. I don't intend to change the slows, it has a good idle and doesn't stumble on take off.

This is my third CB750 that I have jetted for pods. They are not all the same. My old F2 for example didn't need the needles raised whereas this one absolutely did. Make sure those carbs are super clean and that the accelerator pump is doing it's job. Make sure your ignition timing is absolutely on the money before you even think about jetting, and that there are no air leaks at the inlets as even the slightest leak will have you running around in circles. Keep an eye on the plugs to give an indication of too rich or too lean.

[lucky]

Picked up my first bike (77k7) a little while ago and just moved into a house with a garage.. So I tear into the bike that's been at a buddies for 2 months not running. Had a mobile mechanic rebuild the carbs and I went to check the jet sizes today and the carbs are full of oxidized aluminum. Oh well. I'm cleaning all that out now and wanted to get your opinions on jetting for APE pods and the MotoGPWerks pipe. Right now I have 110 mains and 35 slows. The bike has been running lean lately and has been a pain in the dick to start and idle. I think this is because of the clogged slows in 3 cylinders, but that'll be fixed today. Should I upset to compensate for the amount of air that the PO didn't compensate for? Get all of your pod rants out in another thread, I'll be sticking to pods.


Thanks,
Jordan

Dear Jordan,

I am very familiar with that carb and that set up.
This is my recommendation:

#120 main jets.
#42 idle/slow jets
Needle clip moved down one notch from the stock position.
One turn open on the mixture screw.

It will start right up.

Make sure the accelerator pump system is working and
make sure that "mechanic" did not throw away the small brass restrictors in the three accelerator pump hoses.
You will have to make sure the two steel one way check valves work properly.
The one in the bottom of the accelerator pump is prone to corrosion problems.
Then when the float bowls are full,engine off , air filters off. Look into the intakes at the brass spray jets and twist the throttle a few times and make sure you see all 4 nozzles squirt gas.
Use a small pocket LED light to see that.
Sync the carbs.
The 1977 had mixture needles that were adjustable. LUCKY

[KrautKoffin]

Thanks Lucky, these are PD42 carbs. Are the needles still adjustable?

[KrautKoffin]

And where can I get new slows?

[lucky]

Thanks Lucky, these are PD42 carbs. Are the needles still adjustable?

On the 1977 they were adjustable with the clip. 1978's were not.

[KrautKoffin]

Thanks Lucky, these are PD42 carbs. Are the needles still adjustable?

On the 1977 they were adjustable with the clip. 1978's were not.

These are PD42s..

[martin99]

PD42s do not have adjustable needles, so if necessary you will have to shim them. As long as you have the correct shims to hand, the job is no longer to do than if you had adjustable needles.

Lucky knows his stuff when it comes to pod tuning, as I said I have only tuned three CB750s (but plenty of others). I would still caution that you make sure everything else is in order too though (3K tune-up) if you want to get the best from your set-up.

[KrautKoffin]

PD42s do not have adjustable needles, so if necessary you will have to shim them. As long as you have the correct shims to hand, the job is no longer to do than if you had adjustable needles.

Lucky knows his stuff when it comes to pod tuning, as I said I have only tuned three CB750s (but plenty of others). I would still caution that you make sure everything else is in order too though (3K tune-up) if you want to get the best from your set-up.

Thanks, I will definitely check into everything before running the bike!

[lucky]

Best source for new slow jets is Sirius Consolidated .
Go to Ebay and look for the CB750 #42 (push in) idle jets and make sure the seller is Sirius Consolidated.

Or you could go to their website and order by phone.

[Black 750K8]

Sounds like you have a 77 with 78carbs.

[KrautKoffin]

Sounds like you have a 77 with 78carbs.

They are, my pd41s were in bad shape. Élan has them now!

[KrautKoffin]

Ordered my jets from Sirius, can anyone link me to the shims for the needles for the PD42 carbs?

[lucky]

Ordered my jets from Sirius, can anyone link me to the shims for the needles for the PD42 carbs?

Shims are just washers.
All washers are not the same thickness.
If you get the little brass washers they can vary by as much a .005 thousands.
Otherwise you will have to sand the washers so they all measure the same thickness.
Some places sell aluminum shims that fit inside the pocket in the bottom of the slide.

On those carbs use one shim .020 thousandths if it sits on TOP of the pocket in the bottom of the slide.

There is a pocket in the bottom of the slide that is .012 deep so with the .020 shim and the .012 deep pocket that is .032 thousandths or about 1/32 of an inch total.


IF the shim will fit inside that pocket in the bottom of the slide then you will need shims =to .032 +or - .005 thousandths.

You could just try to get some 1976 needles and use those, because they are the same taper and they have 5 grooves. Or find some 1977's.


The 77 and 78 carbs are the same but the needles were different.
Those are EPA needles that have only one groove. They did not want anybody adjusting anything. Even if it was too rich and polluting. Like high altitude places other than Colorado. That is why Honda came up with vacuum operated carbs in 1979.
The vacuum slide made operation in different altitudes possible.

Make sure this does not happen.
(photo coming)

Check out that float measuring coincidence. !977-78 ONLY.

[750K]

Ordered my jets from Sirius, can anyone link me to the shims for the needles for the PD42 carbs?

Sirius has the shim washers on their website also.

[lucky]

Thanks 750K.

 Do you happen to know if those shims sit in
 the bottom of the pocket of the slide, or what thickness they are?

[750K]

Not to sure lucky, I have the adjustable needles in my pd's so I've never checked the shims for fit. They have them listed as carb needle shims and for use as mixture screw washers, I have a pack I bought and have a spare set of pistons out my spare pd's. I can check next time I'm in the garage tinkering.

http://m.siriusconinc.com/products-detail.php?product_id=2188

[flybox1]

http://forums.sohc4.net/index.php?topic=71245.msg791509#msg791509

[KrautKoffin]

http://forums.sohc4.net/index.php?topic=71245.msg791509#msg791509

Thanks for the link! I just went to my local hardware store and they don't carry them. I'll keep looking around. To move up one position I need to use 2 washers, correct?

[martin99]

http://forums.sohc4.net/index.php?topic=103805.0

Take a look at reply #7 in this link, might be helpful if you can't find washers locally.

[flybox1]

http://forums.sohc4.net/index.php?topic=71245.msg791509#msg791509

Thanks for the link! I just went to my local hardware store and they don't carry them. I'll keep looking around. To move up one position I need to use 2 washers, correct?

YES.
you have GRAINGER in your area?

[lucky]

http://forums.sohc4.net/index.php?topic=71245.msg791509#msg791509

Thanks for the link! I just went to my local hardware store and they don't carry them. I'll keep looking around. To move up one position I need to use 2 washers, correct?

ONLY if that washer sits down inside  that recessed pocket in the bottom of the slide.
IF the washer sits on top of the pocket it will only need one washer.
First you have to measure the washer with a pair of calipers and find out how thick it is. Make sure they all match. The pocket is .012 thousands deep.

You want to raise the needle about .032-.035 thousandths.
About 1/32 of an inch.

[KrautKoffin]

Bump from the dead with updates!

Got my carbs back, swapped the slows to 42s, mains to 125s, and am shimming the needles now. Élan was nice enough to mail me back my old pd41s, can I use those needles with the adjustable needle positions in my pd42s or should I just shim them with the pd42 needles?

[flybox1]

I like the shims.  Its easy.  2 shims is about 1 clip position.  Allows for a 1/2 step if needed.

[KrautKoffin]

I used the adjustable needles for shats and giggles, I can always switch back later if they don't work out. The 42s are stamped 15A and the 41s are F2051E.

[lucky]

I hope you will report back to this same thread with your results and questions.

[KrautKoffin]

I hope you will report back to this same thread with your results and questions.

I thought I just did

I'll hopefully get a chance to start the bike Monday.