Author Topic: 750 carb overflow (Read 3507 times)

750Moto · « **on:** August 09, 2017, 03:17:57 PM »

I have been fixing up a 1978 cb750, I rebuilt the engine and cleaned the carbs and have gotten it running pretty good. For the life of me I can not get the carburetors to stop leaking out the overflow nipples.

I have been trying to figure this out for weeks now. There is no crack in the brass overflow tube, the floats don't fill with fuel, the float needles are new, I can not figure this out. All carbs have leaked at one point or another, I have been focusing on adjusting the float height but am starting to think that is not the issue.

Does anyone know of anything I might be missing or something else that could be causing this leak. I'm desperate at this point and registered for the forum to try to solve this issue

ekpent · « **Reply #1 on:** August 09, 2017, 05:08:05 PM »

Are you sure its not leaking at a fuel tee or a split gas line or do you know for sure its from a carb overflowing,maybe from a float needle and seat not happy together.

750Moto · « **Reply #2 on:** August 09, 2017, 05:11:42 PM »

I'm positive it is coming from the overflow nipple on the bottom of the bowl. The float needles are new and the seats are cleaned with a Q-tip, as I understand the seats are not replaceable on the PD carbs

ekpent · « **Reply #3 on:** August 09, 2017, 05:19:26 PM »

Do your float needle have rubber tips or metal. Are they stock Honda OEM or aftermarket ?

750Moto · « **Reply #4 on:** August 09, 2017, 05:42:45 PM »

They are rubber tipped needles and are aftermarket, compared to the ones I took out they are identical

Gene · « **Reply #5 on:** August 09, 2017, 05:54:03 PM »

Quote from: 750Moto on August 09, 2017, 05:42:45 PM

They are rubber tipped needles and are aftermarket, compared to the ones I took out they are identical

Are they installed upside down? I'm not being a jerk, we've seen it happen.

Gene · « **Reply #6 on:** August 09, 2017, 05:54:57 PM »

Also - did you install new sleeves as well as needles?

madmtnmotors · « **Reply #7 on:** August 09, 2017, 06:08:37 PM »

Aftermarket rubber parts, like the rubber tips on the float needles in PD carbs, can be susceptible to ethanol based fuels. I had a set of aftermarket rubber tipped needles along with the bowl rubber gaskets (in the same kit) that turned to goo after just one week of exposure to ethanol based fuel. I am currently using aftermarket float needles and bowl gaskets with rubber tips made of Viton rubber.

Are you turning the carbs upside down to set the floats, or are you tilting them (usually about 45 degrees) so that the floats and needles are just barely closed? Once you think the floats are properly set, turn them right side up and sight down the length of the carb floats and see if they all have the same "droop". There are separate adjustment tangs for float height and float droop. If the floats drop too far they can potentially get in a bind.

Finally, run drain tubes from the bottom of the bowls away from the carbs and verify that the fuel is coming from the overflow tubes and not somewhere else. If you use clear tubing you can use it to verify your float height setting via the "clear tube method".

madmtnmotors · « **Reply #8 on:** August 09, 2017, 06:11:17 PM »

What measurement are you setting the floats at?

750Moto · « **Reply #9 on:** August 09, 2017, 06:23:49 PM »

Nothing is installed upside down I have worked on a good share of carbs and am positive they are installed correct. The rubber tip on the needles are not dissolved and they have been in for over a week now. I have set the floats countless times within the last 2 weeks using both the upside down and the 45 degree methods. I am setting them at 14.5mm.

When doing the clear hose test I am putting the bike level on center stand and measure it 3mm below the gasket surface. After I take it for a ride the float I just checked via the clear hose will be leaking and measure different then it did 10 minutes prior.

Stev-o · « **Reply #10 on:** August 09, 2017, 06:25:31 PM »

+1 float needles are suspect. Aftermarket carb kits are crap. ^^^Wilbur gave you great advice, he knows these carbs...

750Moto · « **Reply #11 on:** August 09, 2017, 06:26:44 PM »

Also I do have drain tubes that go to the ground because the first time it did I'd the hot engine and gas boiled the paint of my freshly painted engine.

I was under the impression the float needle seat is not replaceable on the 1977 and 1978 carbs?

madmtnmotors · « **Reply #12 on:** August 09, 2017, 09:04:52 PM »

Quote from: 750Moto on August 09, 2017, 06:26:44 PM

I was under the impression the float needle seat is not replaceable on the 1977 and 1978 carbs?

That is correct.

If your float level, as measured via the clear tube method, is constantly fluctuating, then I'm going to guess that the float levels that are fluctuating are binding somehow. As mentioned, if the floats drop too far (there is a maximum spec for this, likely in one of the factory manuals I've seen) this can cause binding. I usually just eyeball this setting to confirm that all of the floats have the same droop. Also, check the float pivot pin fitment through the tab on the float and make sure they do not fit too tightly and that the float can pivot freely on its pivot pin. Check for nicks or gouges on the insides of the posts that carry the floats to make sure the tab on the float isn't hanging up on the inside of the posts. Check the floats and make sure they are not twisted. This would cause one side of the float to ride higher than the other as well as exerting uneven pressure on the float needle. Are the floats oriented properly? When the carbs are upside down on the bench the flat side of the float should be facing up. The only other suspect would be that the springs inside the float needles themselves may be binding. Where did you get these needles from and what brand are they? Better quality needles may be required if everything else is ruled out. Do these needles have Viton tips?

I hate working on carbs because of all the potential minute details that can drive you nuts. I've gotten pretty good at it, but I still don't like it.

Some pictures of the carbs, upside down on the bench with the bowls off might help. Get some close ups too!

flybox1 · « **Reply #13 on:** August 10, 2017, 06:25:56 AM »

Once floats are set close to the same level as the pilot jet tops, perform a clear tube test of the fuel levels, and further adjust your floats to achieve a fuel level 3mm below the bowl gasket seam.

madmtnmotors · « **Reply #14 on:** August 10, 2017, 06:53:21 AM »

Quote from: flybox1 on August 10, 2017, 06:25:56 AM

Once floats are set close to the same level as the pilot jet tops, perform a clear tube test of the fuel levels, and further adjust your floats to achieve a fuel level 3mm below the bowl gasket seam.

He has done that but the float height measurements keep changing:

Quote from: 750Moto on August 09, 2017, 06:23:49 PM

When doing the clear hose test I am putting the bike level on center stand and measure it 3mm below the gasket surface. After I take it for a ride the float I just checked via the clear hose will be leaking and measure different then it did 10 minutes prior.

flybox1 · « **Reply #15 on:** August 10, 2017, 07:02:43 AM »

Those damn carb gremlins making adjustments after the bowls are put on.
I hate that!

Errors' in the process. Either the measurement, adjustment, or clear tube test is incorrect.

Is the float valve fully seated when taking the measurement?
These float valves have the wires surrounding the float tang. makes it a little tricky making measurements/adjustments.

madmtnmotors · « **Reply #16 on:** August 10, 2017, 07:08:31 AM »

Quote from: flybox1 on August 10, 2017, 07:02:43 AM

Those damn carb gremlins making adjustments after the bowls are put on.
I hate that!

I'm with ya there. Carb components and passages can be so small and persnickety that carful attention to the condition/appearance of every part must be made carefully. Then there's still no guarantee that you got it right.

One of these can help keep the gremlins at bay:

https://www.gremlin-bells.com/

Stev-o · « **Reply #17 on:** August 10, 2017, 07:12:10 AM »

Quote from: madmtnmotors on August 10, 2017, 07:08:31 AM

Quote from: flybox1 on August 10, 2017, 07:02:43 AM
Those damn carb gremlins making adjustments after the bowls are put on.
I hate that!

One of these can help keep the gremlins at bay:

https://www.gremlin-bells.com/

My buddy gave me one of these, no carb issues since!

750Moto · « **Reply #18 on:** August 10, 2017, 07:56:05 AM »

The float needles are from amazon by a guy with a website called 4into1. I am not sure of the material on the tip I just know it is rubbery. One thing that did concern me about the new needles when I got them is that the springs were softer than the old needles and when I installed them and put the carbs upside down on the bench, none of the springs were compressed the same amount, they all looked different.

flybox1 · « **Reply #19 on:** August 10, 2017, 07:59:00 AM »

4into1 is a good seller, and those are most likely viton tipped needles.
I dont think he carries ones that are not.
All aftermarket needles will have differences from OEM Keihin components...this is where the clear tube test comes in.
Float height should be set based on the test results, and not by the book value of 14.5mm.

madmtnmotors · « **Reply #20 on:** August 10, 2017, 08:09:29 AM »

To recap:

>if the floats drop too far (there is a maximum spec for this, likely in one of the factory manuals I've seen) this can cause binding. I usually just eyeball this setting to confirm that all of the floats have the same droop.
>Check the float pivot pin fitment through the tab on the float and make sure they do not fit too tightly and that the float can pivot freely on its pivot pin.
>Check for nicks or gouges on the insides of the posts that carry the floats to make sure the tab on the float isn't hanging up on the inside of the posts.
>Check the floats and make sure they are not twisted. This would cause one side of the float to ride higher than the other as well as exerting uneven pressure on the float needle.
>Are the floats oriented properly? When the carbs are upside down on the bench the flat side of the float should be facing up.
>The only other suspect would be that the springs inside the float needles themselves may be binding.
>~~Where did you get these needles from and what brand are they?~~ Better quality needles may be required if everything else is ruled out.
>Can you verify that these needles have Viton tips?
>Post pictures of the carbs on the bench upside down with the bowls off. Get a wide shot along with close ups. Some eagle eyes here can spot some of the finest details from good pictures.

750Moto · « **Reply #21 on:** August 10, 2017, 08:42:23 AM »

I will run through your checklist next time I take the rack off of the bike either tonight or tomorrow and will take some pics in the process

750K · « **Reply #22 on:** August 10, 2017, 09:04:48 AM »

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.

750Moto · « **Reply #23 on:** August 10, 2017, 09:09:00 AM »

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

750Moto · « **Reply #24 on:** August 10, 2017, 09:12:10 AM »

I have been working on the carbs off the bike, every time I think I get it fixed and put them back on it will either leak immediately or not leak at all even if I leave the gas on for an hour. If it does not leak at all I take it for a ride up the street and back and what do you know it starts leaking again.

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Author Topic: 750 carb overflow (Read 3507 times)