Author Topic: Float Height/Float Valve Struggles (Read 9562 times)

theofam · « **on:** April 10, 2011, 10:24:55 AM »

After a couple days and three carb dismounts, I've gotta figure out float height. Before I go into this, I've filled all the carb bowls to see if I've a cracked overflow tube, but they're all good. Also, my floats all seem to move easily without dragging or sticking.

These carbs are PD42Bs off a '78 CB750K, but I'd like your opinion if you've worked on the older carbs, too.

I can't tell from the manual's photo what angle to hold the carb when setting float height, because the photo doesn't match the text. The manual reads, "Remove the float chamber and set the float arm as shown in the Fig. 6-7 so that it just barely touches the valve and in this position, check the position of the float with the gauge set vertically (italics mine)." For the gauge to be set vertically, the carb needs to be upside down or right side up, unlike the manual's photo. So, do you point the main jet to 12 o'clock when setting float height? 3 o'clock? Maybe 1:35?!

Another question is how tight should the float valve fit in its seat? All of my float valves slide in and out of the seat easily. As a result, regardless of how the carb is oriented, the float arm just barely touches the valve. For instance, when the float is hanging with the carb upright, a portion of the float valve will slide out of the seat (maybe 1/5 its length). Here's a pic of the float valve with the carb as it would be on the bike. Note the float arm is touching the float valve.

If I orient the carb in a more sideways fashion as depicted in the manual's photo, because my float valves slide in their seats, the float arm just barely touches the valve at a much higher float setting than the 14.5mm required of these carbs.

I'm dreading the answer that my float valve seats are worn, because they weren't included in my Keyster carb rebuild kits, and I can't think of a place where I've seen them for sale. The PD42Bs have a press-in float valve seat unlike the 1970-1976 carbs that had threaded seats. So, I don't expect I can swap out the seats.

Do your float valves slide in the float valve seats like mine in the pics? Or, is it more of a press fit like a press-fit idle/pilot jet?

My weekend has ground to an abrupt halt, so I appreciate any help you can provide!

Kong · « **Reply #1 on:** April 10, 2011, 10:54:30 AM »

Some folks say that you set the float level with the carbs laying so that the float swing is vertical and down, so float is just hanging there just barely touching the end of the float valve. Others, and I am one of these, set the carb so that the float chamber's top is facing up and the float is on top with its weight resting on the float valve - we set the float measuring from the lip of the chamber top to the top surface of the float itself. Proponents of the first way say that the little spring on the end of the float valve's stem is compressed by the weight of the float if its done the second say. They may be right, and they may be wrong, but I know for sure that people do it successfully both ways.

theofam · « **Reply #2 on:** April 10, 2011, 11:20:26 AM »

Kong, thank you. That's helpful. I don't know if you can tell from my pics, but the float design changed on the PD42s from a cylindrical float to a flat bottomed float. I think the cylindrical floats are measured at their highest point on the circumference.

Regardless of which measurement method I'd choose, the float height is shorter near the float pin and float height increases out to the tip of the float. Any idea where I'd measure the 14.5mm for these floats? Maybe dead center?

mrrch · « **Reply #3 on:** April 10, 2011, 11:35:45 AM »

One way to check the actual fuel height is to run some clear hose from the bottom drain fitting and run it up beside the carb body, crack open the drain screw and the fuel will rise to the fuel level inside the carb. float needle can't be leaking or it won't read properly. I use a small popoff tester to check for leaking needles.

theofam · « **Reply #4 on:** April 10, 2011, 11:41:15 AM »

mrrch, I think I saw the clear tube trick in another thread. I'll do that if this round of float setting doesn't cure my problem. I'm not sure what you mean when you say a "small popoff tester" to check leaking needles. Can you explain a bit more or provide a pic? Thanks!

I'm still wondering about the float valve. Is it OK for it to slide inside the seat, or is it more of a press fit?

mrrch · « **Reply #5 on:** April 10, 2011, 11:48:15 AM »

http://www.jackssmallengines.com/service_tools5.cfm
You connect this to the fuel fitting on the carb when it is upside down and pump it a few times, needle should hold 4-6 psi approx before it pops open. If it does not hold any pressure the valve may be dirty or worn out.
If you have a Mityvac with the pressure feature it would work also.

bollingball · « **Reply #6 on:** April 10, 2011, 12:04:18 PM »

theofam: your float valve is working fine as far as movement it slides in and out as the float moves up and down I have the same carbs and measure half way on the float but I now use the clear tube less room for error I shoot for about 1mm below the gasket.
Ken

theofam · « **Reply #7 on:** April 10, 2011, 12:37:56 PM »

Ken, sounds like I'm buying a few feet of clear tube when the scooter shop opens Tuesday! Thanks for chiming in. How do you position your carbs when off the bike to measure float height?

I've torn them back apart to figure out why the accelerator pump isn't working. All the passages are clear with the exception of the check valve at the bottom of the pump. Soaking it now. The other check valve that feeds the carb passages had a bunch of poop in it! Could have been my issue - can't believe I didn't blow it out when I originally tore them down.

bollingball · « **Reply #8 on:** April 10, 2011, 12:55:23 PM »

You can use tubing from wallmart buy the fish stuff it won't be in the gas long enough to worry just don't use it on the bike.

I built a jig out of 2x4 lumber two uprights a board with the carb rack screw to it then screw that board to the uprights with one screw at each end so it will pivot then you can tilt the carbs to any angle.

HondanutRider · « **Reply #9 on:** April 10, 2011, 02:22:11 PM »

Hey guys, I'm pretty sure the valve (the vane "thingy" with the springy end) rides in a holder that has the seat for shutting off the gas flow at the bottom of its hole. The valve is free to slide up and down this hole so that when its pointed end is pressed into the seat, the gas flow is shut off. This happens when the float rises and presses it up. You want all that movement to be free, or else your carb bowls are likely going to do some pissing.

When you set the float level using a gauge or a measurement as described in the manuals, the valve should be just sitting on the seat at the bottom of its holder, and the float tang is just touching the spring-loaded end of the valve. If you hang the carb body right side up, then the measurement will be made as you raise the float tang to the valve until it just starts to compress the spring end of the valve, assuming the valve has seated. If you turn the carb up side down, then the valve should sit on its seat and the tang will rest on the spring end, maybe depressing it just a bit under its weight. A good way to confirm the correct float height, is to use the clear tubing as previously described, but by then you've buttoned-up and probably remounted the carb rack.

theofam · « **Reply #10 on:** April 10, 2011, 02:31:02 PM »

Hondanutrider, that's the best description I've read on how the float valve works and it now makes sense to me why it needs to slide inside it's seat. Thanks!

BB, I want to see your shop! Sounds like you fab up tools to help with various situations. Kind of frees up your hands for the actual work!

I've got both check valves free flowing. They were very gummed up still. I'll get the floats set, carbs reassembled mount and hope it all works. If not, I'll put some clear tubing on them to figure out how far off I am.

Thanks to each of you for helping me out with this!

bollingball · « **Reply #11 on:** April 10, 2011, 02:37:52 PM »

theo I do all my work on the jig turn carbs until level lock down screws so the carbs don't move. Just put bowls on no gaskets check with clear tube make adj. I use a liquid soap bottle full of gas with short hose to fill carbs With the carbs on a swivel it is easy. When you are happy then put the gaskets on.

madmtnmotors · « **Reply #12 on:** April 10, 2011, 04:18:29 PM »

Your float valve seats should be fine. The 78' 750 carbs use rubber-tipped float needles and don't wear the seats quite as badly as hard needles. I chased problems with the float valves on my 78' 750 until I replaced the float needles. I think the springs get weak or intermittently sticky but my carbs quit overflowing after replacing the float needles, always using the same procedure for setting the float height. I turn the carbs upside down with the bowls off and measure the high side (furthest from the needle) on my 78'. When the carbs tried initiating my "Ghost Rider" impression at stop lights by puddling fuel under the bike I decided to replace the float needles. Glad I did, no problems since. There is a source for the 78' float needles that comes with a bowl gasket. I think they called it a "Po' Boy" rebuild kit. I would have to do some research but Sirrius might have been the source. Good luck and bring your patience for carb tuning!

theofam · « **Reply #13 on:** April 10, 2011, 04:25:25 PM »

madmtnmotors, I'll stay suspect of the float needles. I just adjusted them upside down at the mid-point of the float. I hope it works! By the way, you did a great job with your ride report and IT. Thanks for kicking off the Rally. I'm trying like crazy to have Paddy running by May 3rd to join in on the fun here in Denver!

theofam · « **Reply #14 on:** April 11, 2011, 01:01:35 PM »

Paddy's carbs aren't leaking anymore! I suspect the real issue wasn't float height but instead poorly seated carb drain screws.

The bike is running pretty well above 2K RPM, but it stumbles/dies off idle. I'm running the stock 38 slow/pilot/idle jets and have the carbs' air screws turned out 1.5 turns. I took a video (17.5MB) and posted it so you Honda intelligentsia can help me troubleshoot!

CB750 troubleshooting

HondanutRider · « **Reply #15 on:** April 11, 2011, 02:42:40 PM »

Sounds pretty rough. You need to check and adjust the timing and valves, and then do a carb synch. That should smooth it out and make it run much better. If not, then there may be more problems.

bollingball · « **Reply #16 on:** April 11, 2011, 04:14:26 PM »

Theo Just reread your first post you said the carbs are off 750K8. Is that what your bike is? Also stock 42b has a #35 pilot jet not a #38. First I need to confirm the bike is a 750 K8. Did you confirm on the bench that the accelerator was pumping on all four carbs and that both mickey mouse ears were open? In the video your problems seems to be in your pilot circuit or pump.but could be float level. Have you done ALL of the other 3K tune up this needs to be done before fine tuning carbs. If the accelerator pump is not squirting out all four nozzles it will never run good.

Spanner 1 · « **Reply #17 on:** April 11, 2011, 08:16:35 PM »

Float height for the 42B carbs is 14.5mm. At a 14.5mm setting the bottom of the float should be just parallel
with the bowl gasket surface...... I think your gas level in the bowls is too low due to incorrect float level or slow fuel supply to the carbs. Here's why, the slow jet sits much higher in the carb and would be the first jet to be effected by low or slow fuel. In your video the bike revved o.k the first few tries then stumbled as the slow jet was now above the fuel level due to the main jet 'using up' the available gas. Gummed-up petcock will reduce flow and produce these symptoms even with good float level..... IMO. Test the flow by opening the 4 drain screws ( at the same time ) and turn on the gas. Collect the fuel in a clean container and you can return it to the tank afterwards. Let the fuel flow about 2 mins. to be sure of no restriction or vacuum, good luck

theofam · « **Reply #18 on:** April 11, 2011, 08:45:13 PM »

As background, I've done the 3K service, with the exception of condensors. So, cam chain tension set; new points, gapped and timed; valves set a Hondaman's recommended .003" intake/.004" exhaust; new NGK 5K OHM spark plug caps; new NGK D8EA plugs gapped at .025"; all electrical scrubbed with DeOxIt and then dialectric grease; carbs disassembled/reassembled with Keyster carb kits (didn't use the kits' slide needles based on negative forum feedback); bench sync using the flashlight method.

BB, I probably misspoke. I thought 38s were stock. They're likely 35s, but I can check. Yep, the bike is a '78 750K running the stock PD42B carbs.

Spanner, it sounded like a choir of angels to me! Just the fact that it was running and revving was music to my ears!

I think I'm suffering a "belt and suspenders" problem when it comes to fuel flow. I was skeptical about the tank (although it's apparently in great shape - check out the Paddy link in my autosig below for some pics when I used my shrink ray to get teeny and parachuted into the tank for some awesome pics) dropping stuff in the carbs, so I put in an inline fuel filter (yes, in addition to the in-tank fuel filter). So, tomorrow I'm going to pull the inline fuel filter and see if it flows better. I'm also going to pull the airbox to ensure I'm getting good shots from all four accelerator pump injectors. If not, I'll pull the carb assembly . . . AGAIN. . . and drill out the pump's mickey mouse ear holes. That would also give me a good opportunity to confirm idle jet size for BB!

By tomorrow afternoon, Paddy will be sounding like a well tuned Irish tenor!

Thanks for your help, fellas! Any other ideas are always welcome.

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Author Topic: Float Height/Float Valve Struggles (Read 9562 times)

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