Topic: 750 carb overflow

[flybox1]

float pin.  did it get polished?
any corrosion here can hang-up a float.

[madmtnmotors]

Does your 78's carbs have the vent tube you attach a separate tube to or are they small holes cast into the body of the carbs?

The PD carbs have a vent tube with a brass nipple out the side that connects to the adjacent carb via a short piece of rubber tubing that ultimately vents out the #2 carb with a single rubber tube that clips to the top of the airbox, open to the atmosphere with the open end facing to the rear. Good call since that was not on my checklist. Blow compressed air through this vent opening in the bottom of each carb and make sure the air flows freely out of the vent tube on the #2 carb. The #2 carb has two brass nipples facing upwards, the smaller one is the vent, and the larger one is for fuel.

[750K]

Does your 78's carbs have the vent tube you attach a separate tube to or are they small holes cast into the body of the carbs?

The PD carbs have a vent tube with a brass nipple out the side that connects to the adjacent carb via a short piece of rubber tubing that ultimately vents out the #2 carb with a single rubber tube that clips to the top of the airbox, open to the atmosphere with the open end facing to the rear. Good call since that was not on my checklist. Blow compressed air through this vent opening in the bottom of each carb and make sure the air flows freely out of the vent tube on the #2 carb. The #2 carb has two brass nipples facing upwards, the smaller one is the vent, and the larger one is for fuel.

The set of pd carbs that came on my 77k don't have the brass vent nipple, the ones I swapped  from my 78k parts bike do have a vent nipple. The 77 carbs have a vent hole cast into the body of the carbs, it took me awhile to figure it out. On the 77 rack where the vent nipple should be is just a flat spot in the casting on the body. I had a couple stripped threads on the key hole tops I had re tapped to non metric threads add that to the venting issue and I was happy to start over with the 78 carb bodies with my 77's internals (adjustable needles etc), as well as having a spare parts rack.

If photobucket hadn't #$%* the bed with 3rd party hosting I'd post pics of the 77k carbs lack of vent nipple, if you message me an email I'll be happy to send you detailed pics.

[750K]

Your leak chase sounds like a mirror image of mine, what vent tube are you speaking of and what ended e up solving your issue

If yours have the vent nipple it will be near and look like the fuel nipple but be smaller, my set didn't have a nipple. Only small vent holes in the carb bodies themselves. It was super frustrating trying to figure out the overflow problem, I had a little bottle set up to gravity feed fuel to the rack attached to the stand. I guess there was nowhere for the atmospheric pressure to vent so it would cause the float bowls to fill and vent out the overflow tube leaking everywhere.

[750Moto]

I have been wondering what this was for, it was capped by the previous owner, I will Un-cap it and blow air through it and see if it solves anything

[Airborne 82nd]

Make yourself a wooden rack you can attach the rack of carbs to, attach it to a bench or put it in a vise. It's so much easier to work on carbs when not on the bike, especially doing the clear tube test. Does your 78's carbs have the vent tube you attach a separate tube to or are they small holes cast into the body of the carbs? I had overflow problems due to the cast holes likely being plugged, swapped carb bodies to one with a vent nipple and problem solved.

I'd set the floats, do the clear tube check and would have things hold levels. Drain carbs and recheck only to have one carb overflow erratically, drain re clean and adjust only to have the problem happen on a different carb # that was not overflowing before. Chasing it around and around was pretty frustrating. New jets, new gaskets, float and needles. Floats weren't hanging up anywhere, needlle seats and carb bodies were extra clean. Man it was a pain in the ass, make sure your vents are clean and working.

[flybox1]

That PO's a moron! 

capping the vent like that...smh 

Its best to plumb that right into the top of your airbox, but removing the red cap will be sufficient.

[madmtnmotors]

I have been wondering what this was for, it was capped by the previous owner, I will Un-cap it and blow air through it and see if it solves anything

Don't get too aggressive with the compressed air since you will be pressurizing the bowls by blowing air (and dirt?) into the bowls. The small overflow tubes (and possibly the fuel line if the float needles are open) are the only outlet for this pressure...

I still think you may have more than one issue.

[Stev-o]

Aftermarket carb parts are assumed to be Keyster....no bueno!
Does not state viton so prolly is not.


http://4into1.com/keyster-carburetor-float-bowl-gasket-float-needle-set-honda-cb550-cb750-1977-1978/

[750Moto]

Is there any way one of the carbs could be overflowing into the one next to it causing them both to leak

[madmtnmotors]

Aftermarket carb parts are assumed to be Keyster....no bueno!
Does not state viton so prolly is not.
http://4into1.com/keyster-carburetor-float-bowl-gasket-float-needle-set-honda-cb550-cb750-1977-1978/

I'm pretty sure that 4into1 does carry Viton rubber tipped float needles and bowl gaskets, but the description specifically says so, They do carry other PD float needles and bowl gaskets that do not specify "Viton". I am currently using 4into1 Viton float needles and bowl gaskets with no issues.


http://4into1.com/k-l-keihin-float-needle-rubber-viton-tip-honda-cm200t-cb400t-cx500-cb550-cb750/

[madmtnmotors]

I do not see where they carry Viton bowl gaskets. I must have sourced the bowl gaskets somewhere else (or they quit carrying them).

[750Moto]

I looked back, I bought mine via amazon and they say viton tips

[madmtnmotors]

I looked back, I bought mine via amazon and they say viton tips

That's good news. Now to eliminate the other possibilities...

[tlbranth]

I'm not familiar with the '78 but on my bike the persistent leaks were from the drain screws. It may LOOK like the fuel is coming from the overflow but be coming from the drains. I had a whale of a time finding these leaks and had to resurface the mating surfaces on one carb.

[madmtnmotors]

I'm not familiar with the '78 but on my bike the persistent leaks were from the drain screws. It may LOOK like the fuel is coming from the overflow but be coming from the drains. I had a whale of a time finding these leaks and had to resurface the mating surfaces on one carb.

Yet another item to add to the checklist, but the problem seems to be intermittent and the clear tube method is yielding varying results after a short ride. This is worth checking but my money is on float or float needle restriction/obstruction.

The devil is in the details!

[madmtnmotors]

If photobucket hadn't #$%* the bed with 3rd party hosting I'd post pics of the 77k carbs lack of vent nipple, if you message me an email I'll be happy to send you detailed pics.

You can attach photos to your post by using the "advanced" option. Below the text insertion box expand the "Attachments and other options" and attach photos. 

I use Google photos to host imbedded photo links:

[tlbranth]

I went to great lengths to get a decent clear tube setup and lost confidence in it. With help and suggestions I came up with what I think is a much better, and easier to do, float level setup.
Look at this post close to the bottom of the first page:
http://forums.sohc4.net/index.php?topic=166447.0

[750K]

If photobucket hadn't #$%* the bed with 3rd party hosting I'd post pics of the 77k carbs lack of vent nipple, if you message me an email I'll be happy to send you detailed pics.

You can attach photos to your post by using the "advanced" option. Below the text insertion box expand the "Attachments and other options" and attach photos. 

I use Google photos to host imbedded photo links:

Thanks for that, I will dig out my 77 pd41a's without the vent nipple and take a pic of them and see if I can post them.

[flybox1]

Is there any way one of the carbs could be overflowing into the one next to it causing them both to leak

No. but the fuel connection between carbs can be a leak source if its old/cracked.
Positive fuel is only coming out the drain tube, and not just dripping off of it?

[750Moto]

I removed the vent cap and it leaked, so I toot the carbs off and took the floats out, reset them to the factory 14.5mm (because I started testing my own measurements after lots of failure at stock spec). No leaks after sitting all night and a 15 minute ride this morning.

    Now my next question, what is proper temperature of the header pipe right out of the manifold after a ride. Mine are reading nearly 600 and I'm afraid I'm running the engine very lean and risking damage

[750K]

Nice, no leaks is great. The vent tube must have been the ticket!

[flybox1]

I removed the vent cap and it leaked, so I toot the carbs off and took the floats out, reset them to the factory 14.5mm (because I started testing my own measurements after lots of failure at stock spec). No leaks after sitting all night and a 15 minute ride this morning.

    Now my next question, what is proper temperature of the header pipe right out of the manifold after a ride. Mine are reading nearly 600 and I'm afraid I'm running the engine very lean and risking damage

Perform an idle plug chop with new plugs.  This will help you determine if it is lean, and make IMS adjustments.
Any idling should be done under a fan, btw.  Air cooled engines need air flow....

[750K]

So here's some pics of my old 77 pd41a carbs with out vent nipple or vent tubing, hopefully you can see the casting with the vent hole. Each body has a cast vent hole, I would guess they are early carbs and they switched to the later vent nippple and tubes. Also pics of my wooden rack, the rack attaches through the throttle shaft spring hole and sits in my bench vise. Easy to do the clear tube method or pull bowls and change jets, bench sync.