Vacuum Reservoir for Deep-Throated Carbs?

[Wheelman-111]

Greetings:

Home from work Friday, Flash 3 has been running great. On a long downwind run at 60+, speedo needle trying to poke a hole in the housing. WOT for about a minute, it briefly lost power, a light chuffing backfire or two, then regained power within 5 seconds of releasing the throttle. Never could see if t clear float bowl contained fuel or not. Back home I checked plug and looked good, compression was fine. I recognized the behavior as identical to what Flash I did when it had a dodgy vacline connection.

So is the Gaping Maw of the 30mm OKO held open allowing manifold vacuum to drop below petcock-activating pressure? Anybody ever experience this issue?

If so, my solution is a vacuum reservoir modeled after a 1985 500SL door locking system. The locks worked off vacuum instead of electric in those years. Those wacky Germans!

Anyway on the scooter all that's needed is a one-way vacuum valve and a vacline Tee connection to a small bottle that will hold negative pressure applied to the petcock for some time even in the absence of manifold vacuum.

I note that there are several Euro scooter stores that offer "Boost Bottle" systems that seem basically similar to what I'm contemplating, ostensibly for performance-enhancement. Anybody have info on this?

Thanks Dawgs!

[Bear45-70]

For years Ford used a plain old juice can of about a quart size with a nipple soldered into it with a couple of check valves in the vacuum lines as a vacuum reservoir.

Now another thing to try would be to connect a vacuum pump to the vacuum nipple on the petcock and see what the minimum vacuum is to keep it open. The connect a vacuum gage to your intake and go run the thing and see what your minimum vacuum is at WOT. Then you will have your answer.

[Wheelman-111]

Greetings:

The Bear gives good advice! Mi-T-Vac at the ready to test both values.

[losthope]

My scooter kept shutting down and i couldnt figure out why. played with all kinds of jetting rich or lean it would randomly shut down. I thought... i wish i had a clear float bowl to see if im running the bowl dry. Instead i bought a clear inline filter so i could see if its running out of gas. And i havent had a problem since. Bike has yet to shutdown. My theory is that the filter is acting like a surge tank keeping a supply of fuel ready making up for low vacuume, or Dirty petcock conditions.

[Wheelman-111]

Greetings:

Good point about the filter keeping a bit of fuel in reserve. Unfortunately Flash's lustyappetite quickly consumes this few ml. of fuel, presuming this theory is correct. I'm searching for a suitable container to act as a reservoir. I might see if the frame itself is airtight, drill and tap a barb right into that!

[Clivester]

How about doing away with the manifold vacuum completely and just hook up a small low pressure electric vacuum pump to the petcock? Wire the pump up to something that's live when the ignition is on.

Of course, both the vacuum reservoir and pump idea may be slightly less safer than stock for a tipped bike - ie fuel woyuld not immediately shut off in an accident.

You could maybe even do away with the petcock by using a solenoid valve like this:

http://www.mccampingsupplies.com/index. ... i2nh269ha7

[turtle13]

You could maybe even do away with the petcock by using a solenoid valve like this:

i had thought about doing this in the past, however ruled against it if the battery goes bad...

[turtle13]

why not try adding some extra length to your fuel line. once the petcock opens it will let a larger amount of fuel flow down. leaving a little extra fuel on reserve for those special times when you need some extra to fill the bowl.

[Wheelman-111]

Greetings:

Thanks for all the great suggestions, Dawgs!

Turns out the problem was my second nipple, uh... Vacuum barb had managed to back out of skillfully-cut threads. I think it's stripped in fact. CT pot metal isn't the best recipient for a 4mm tap. Red LocTited it and snugged down, all good. Sure felt like vacuum was giving out at WOT, and it was!

Also found Mi-T-Vac is too crude for petcock evaluation; it flowed with the slightest squeeze of vacuum. The "0" on the scale is a wide band that goes to ~2" of mercury, and just that was enough for fuel to dribble. Any more and it flowed full-column. I can't imagine trying to read the thing at WOT, so I'll just trust that sealing the threads will do the trick.

Sadly my Noble Pump Plus Premix experiment came to a close this evening. Flash 3 seemed to like it, but prep for re-launching F4 means switching to 32:1 HP-2. I drained the 60 ounces remaining in the tank using Mi-T-Vac and added an ounce or so to the ounce that it already contained (65:1) then refilled from my 4-gallon tank of 32:1. Ran a bit differently, but close enough.

The pump is still connected - no plug available. I just ran a longer line back to the oil tank for a recirculating closed system. This way the pump stays lubed without dumping the oil on the ground. I plan to Tee the oil tank outlet to the fuel line for an extra quart of capacity once I re-install Flash 4.

[Bear45-70]

Wheelboy, did you try using the plastic reservoir in line between the petcock and the pump as a volume increaser to make the needle move less with each squeeze?.

[Wheelman-111]

Greetings:

Yessir, the reservoir was connected. The slightest squeeze moved the needle a width up, still inside the "Zero" box, and fuel was a-tricklin'. The flow stopped when I hit the trigger to break vacuum, so the petcock's fine. Just really dang sensitive to slightest vacuum. The drag of the vanturi and a well-oiled pod is surely enough. Ran WOT coming back from filling my tank - no chuffing. I think I'll forget about the reservoir.

BTW the frame tube has open ends pointing at the ground - no vacuum chamber option there without welding the ends shut.

[Bear45-70]

How about doing away with the manifold vacuum completely and just hook up a small low pressure electric vacuum pump to the petcock? Wire the pump up to something that's live when the ignition is on.

Of course, both the vacuum reservoir and pump idea may be slightly less safer than stock for a tipped bike - ie fuel woyuld not immediately shut off in an accident.

You could maybe even do away with the petcock by using a solenoid valve like this:

http://www.mccampingsupplies.com/index. ... i2nh269ha7

Yeah, but a simple mercury switch would fix the thing so it would shut off in an accident.

[maddog]

a 30mm carb to go 60+mph! you have the worst setup, nobody here uses that carb size, also vacuum boost bottles don't work

[Trafficjamz]

a 30mm carb to go 60+mph! you have the worst setup, nobody here uses that carb size, also vacuum boost bottles don't work

Why not?

[toboggan]

a 30mm carb to go 60+mph! you have the worst setup, nobody here uses that carb size, also vacuum boost bottles don't work

Why not?

Maddog just likes to state his opinion when most people don't care... just ignore him. lol