Vacuum Diaphram assembly

[FANNING507]

I have overflow problems with #2 carb. I will bend the tang to put more pressure on the needle. Does anyone have a diagram of how the vaccum diaphram is to be assembled? I replaced the flexible diaphram, but fuel is leaking past the control. I think that it was assembled wrong before I got to it, and I replaced it as I found it. There isn't many pieces, but the repair manuel doesn't show the assembly. Kim

[AlanB]

Hi Fanning,

Welcome to the forum!

Many members have reported that some of the replacement diaphragms have a too-short centre spigot, and don't shut off.

If you have a Special (with the seperate vac unit... or "Octopus") then it might be best to just bypass it until you solve the problem.

Just run a single line from the rear outlet on each fuel cock direct to the carb inlets. Block the front (prime) outlets by linking 'em with a length of fuel line, and find an extra cover to block the nipple on the carb boot for the vac line.

Only downside is that it is wise to manually shut off the fuel while parked...... but you need to do that anyway if it isn't shutting off!

AlanB

[fredintoon]

Hi Kim & welcome,
your profile says your bike is an '81 Midnight Special. It don't say where you live so I don't know if you can more easily find find a cricketer or a baseball player. Whichever, he can throw your octopus valve further than you can and that's about all they are fit for. Your float valve leak is an independent problem. A thorough cleaning may be all the thing needs but the sure fix is to thoroughly clean the carb and then fit a new needle and seat. Once the carb seals properly plug the gas taps "prime" connections and the vacuum line taps on the intake boots. Run a single gas line with an inline filter in it from the leftside gas tap to the rightside fuel tee and vice-versa, turn both taps to "on" & then "reserve" to run and just like dad had to do, remember to turn them to "off" when you stop.

[dbeardslee]

Kim - A lot of assembly questions can be answered by the Yamaha fiche . There are all kinds of exploded diagrams there. Just click on 'Parts Catalog' and input the particulars on your machine. Then click on the part group you are interested in and a diagram as well as a numbered parts list will appear.

[AlanB]

Hi Fred,

Originally posted by fredintoon
I don't know if you can more easily find find a cricketer or a baseball player. Whichever, he can throw your octopus valve further than you

D@mn!..... I have just spat half a cup of coffee into my keyboard........ that is about the funniest thing I have read on here for a long while.

I feel equally strongly about the octy; (I even converted my in-tank vac cocks to non-vac).... I was just being nice to the new guy.

AlanB

[FANNING507]

Vacuum Diaphram (Octopus)

Really appreciate the help. I had a rebuild kit delivered for the fuel petcocks and it had the diaphram included. The Octopus has the two outside covers, with a separating middle spacer. The old diaphram was threaded thru the spacer so a membrane was on each side of this spacer. There appears to be a vent line on the spacer. Are the membranes supposed to be together instead of separated, (if so, downside of the vented side, or on the vented side)? I consulted with the OEM parts site, and the octopus is not diagramed except as a completed unit.
I'm puzzled by the engineering of this unit. I know what it's supposed to do, but not sure how it gets there.
Just a Mid West American trying to figure out the Japanese!
I appreciate all the input. I would prefer to keep the bike in restorable condition.

[AlanB]

Hi Kim,

The two membranes do go on opposite sides. the centre boss with the o-ring on it should fit into the fuel inlet to shut off the fuel at rest; it is pushed over by a light spring.

When the engine fires, the vacuum from the inlet is passed to the other side of the diaphragm, pulls it over against the spring, and lets the fuel flow......... unit IS plugged into the carb boot, isn't it??

As I said earlier, many of the replacement diaphragms have too short a spigot, and can NEVER shut off the fuel, so check it against the original.

As I also said, I would ondo the single screw which holds the unit to the carb bank and bypass it until you can get it working properly..... It will not take the bike into an "unrestorable" condition.

AlanB

[TopCatGr58]

Hey there Fanning,

I remember there being a post about the Octy, and that there was a vent on one of the plastic pieces that makes it, and IF that vent were in the wrong place, wrong side, etc., that it wouldn't work either. It may take a while, but doing a search in the FORUM for "octopus diaphragm" will probably reveal several pages of threads, but hopefully you'll find the one with that info, I think it even had photos....IF they are still linked from whoever posted it!?

IF you are talking about keeping it as CLOSE TO STOCK for a TRUE RESTORED CONDITION, then I can understand about trying to fix it, but unless you are going to sell it to a museum, these bikes as great as they are, are not in any high dollar range of value even for PURE STOCK conditions!

Good Luck!
T.C.

[fredintoon]

Hi Kim,
perhaps I mis-spoke. If you want to keep the bike 100% restorable by all means do not have a professional athlete throw the octopus away. Instead, carefully save it against the day that a wealthy enthusiast wants to give you big money for the bike so he can restore it to factory spec and put it in a museum. Meanwhile, go with what works.

[FANNING507]

Vacuum Diaphram

I really appreciate the responses. I followed your wisdom and reassembled according to your instructions. I will need to remount the carbs to see if it works; ---another day at least. Thank you for the advice. I also built a carb holder to test the fuel levels in the bowls last nite. Still need some time to work it out. Thank you for all the help. Kim

[FANNING507]

Vacuum Diaphram

Everything went back together fine. All four carbs show the same fuel level, and I guessed right the first time on how much to bend the tang down to put more pressure on the needle on the seat. Started up the first attempt. Diaphram seems to be working fine. One new problem. The starter button only seems to activate the starter relay periodically now. Worked fine until after it was started, then only seems to work when cooled down, or after a period of rest? Any quick tests? Kim

[SFerinTEXAS]

Fuel level

Someone correct me if I am wrong. Bending the float tang only changes the fuel level, it does not change the pressure applied to the seat.

[FANNING507]

Float level discussion

You are right. It was leaking fuel by not pushing the needle far enough into the seat to shut the fuel off. I used a poor choice of words. Still, I am pleased that problem is fixed. Showed my ignorance--Hey I'm still learning. Kim

[Mississippi]

Nice work Kim.

[TopCatGr58]

Re: Vacuum Diaphram

Originally posted by FANNING507
One new problem. The starter button only seems to activate the starter relay periodically now. Worked fine until after it was started, then only seems to work when cooled down, or after a period of rest? Any quick tests? Kim

Recommend you take the right handlebar switch assembly off and check and clean the button/spring and contacts and make sure the wire is still soldered to the contact pad! The wire is the ground that completes the circuit for the starter relay, and if it's not getting good ground, then it won't activate the relay properly or intermittently!
T.C.