Mystery Hole

[e3steve]

Wilf, I was browsing The Forum, looking for a photo; I came across this helpful post which may be of interest to you:

http://www.honda305.com/forums/viewtopic.php?t=4618

Good luck!

Steve

[LOUD MOUSE]

Your bike being a 1966 issue (CB72?) will/may/should have a sort of square cutout at the front left of center in the steel plate which attaches to the top case under the carbs.
That is where HONDA installed the tube that year. (1963 and later)
It is/was long enough to go under the steel plate mount bracket to the hole under the larger hole where the stater/starter wires pass through.
The 1961/1962 HAWK/SUPER HAWK and 1962 CL72 had that vent tube (early CB72/77 engines used a vent at the right rear of the engine top case which vented through a flapper valve near the chain sprocket) follow the frame tube above the engine and was long enough to exit between the rear of the engine and frame same as the battery vent tube,
There is/was a special sort of clip held to the frame tube with a wire harness band at a location sort of inline with the carb air intake tubes which holds the vent tube to the frame tube. ..................lm

Hi LM,

The engine number is 1007539, on the wiring loom is 1966 printed.
It's sold here in 1967, so it's licensed as a 1967.

Why do you need the engine number ?

It doesn't have chrome fenders, nor the flipped up seat, but has the powerjet carbs. It does have the dutch / german special air filter cases and turning lights (winkers) standard. It' a original dutch model, not imported.

If you need more info, just ask,

jensen

[LOUD MOUSE]

Cross winds at the carb vents while riding?
Strange. .........lm

Even though there is not supposed to be tubes on the carb vents I found from experience that fitting them stopped the engine cutting out in certain crosswinds. I used to ride on a regular trip and between some hills in certain winds the bike cut out suggesting fuel starvation on one side, so two short bits of plastic hose from the carbs to under the cover above the gearbox solved the problem. The wind needed to come from nearly right angles to the bike.
Leon. long time rider

[LOUD MOUSE]

I have the PDF and would like to know the index listing.

I found that data and looks to me like a tube from the carb bowl over the rear of the engine case and between the frame.
If it went to That Hole how does it get from inside the CLAMS ?. ......lm

Well, I tied my shoelaces together in anticipation (e3steve's caution), and after spending much of today scouring all the photos I could find on Bill Silver's cd, I will again suggest that the hole is/was for carb overflow tubing.

I didn't think my battery vent tube suggestion was all that much out of line. Under normal circumstances, the battery should only vent water vapor or hydrogen and oxygen gas, no acid to speak of. And I don't really know where I would place a battery vent tube if it did contain acid--anywhere other than a couple of feet behind the rear axle seems bad to me.

In searching the "Shop Manual 250-305 1973" file on the Bill Silver cd I did find 5 photos that may shed some light on the mystery. Figures 8, 10, 12, 13 and 15 on pages 46 through 49 appear to me to show a bowl overflow tube running from the carb through this hole and down into the chain case. It seems odd that the end of this tube would spill fuel right close to one or both of the oil seals, but that's what I get from the photos (I think the figures depict a C72). And if you're spilling fuel from the bowl(s), you've got other problems anyway.

I am aware of the discussion regarding bowl overflow tubes, but I do believe that's what the brass spiggots were intended for. I base this on my experience with a 1968 CT90 carb which has exactly the same internal and external fitments on the bowl, and an expressly designed channel in the skid plate to accommodate the tube. I also see that the CB77 carbs have an atmospheric vent for the bowl, although it is quite tiny and easily obstructed by dust etc. which may have reduced its reliability and, ultimately, lead to its abandonment? I have yet to run across any pic's of a CB77 showing carb overflow tubes going down that hole. In any case, these are my current guesses...and footwear.

Thanks to all for your continuing interest.

Wilf

[jensen]

LM,

Yes, I checked, (as usual) you're right about the cutout, I'll change it in the winter to the original situation. And yes, it's a CB72, you're correct.

jensen

[jensen]

leonvjames,

I have had the same problem, I and I found out why.

If wind is blowing around a open pipe there will be an lower pressure (vacuum)in the pipe (in this case the end of the overflow hose is the open pipe).

When this low pressure is somewhere attached with the air above the petrol in the chamber, and the low pressure is the same or lower then the pressure in the carburetor venturi, the engine won't be able suck petrol out of the yet, so it will stall. (a combination of the law of Pascal and Bernoulli)

I'm happy to give a full explanation and/or calculation, but I think you can imagine what happens.

Because of this reason I have the overflow hoses ended not directly in the wind but on a place where this low pressure is more difficult to build up.

Jensen

[Wilf]

Steve...thanks for the caution on the corrosive battery fumes. I was unaware of anything other than hydrogen gas to watch out for when charging. I will run the vent tube as per your description. And thanks for the link re: advancer springs. I ran across that a day or two after I posted (don't know how I missed it) and it seems to be exactly the situation I have. New springs are on the way because even if I was able to bend the hooks back I don't know how worn the springs are.

LM...the pdf index listing is: Disassembly and Assembly / 1. Engine (C72-77) / A. Engine Replacement. The Figures I was looking at are 8, 10, 12, 13, 15 and 16.

Thanks all,

Wilf