How much fuel flow on a hotrod XS650?

gggGary

gggGary

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Got a thread stated on "madness", 750 hot cam, big pipes, way over carbed, VM38's! Those carbs have BIG 5/16"? inlets, bike had BIG outlet pingles which unfortunately leak like sieves and were butchered onto a special tank with other "issues". So how much fuel flow do I need? can "stock" dual petcocks supply a "big breather" like this?
I am not headed for the track, but it's hilly here and I enjoy a full on run through the gears.
Any opinions or :yikes: facts?
 
My 40MM mikuni on my Road Star feeds 1602CC through a single carb with a 2.1 float valve and 5/16" line, but they use a fuel pump. When removing the pump, we switch to the Grizzly 660's float valve, which is a 3.0 and will drown the motor if it sticks, even at W.O.T.
My EX500 had 1/4" piping, no fuel pump on the 36's, and it redlined at 11K.

I'd be willing to bet the carbs can more than do the job, but I might want to know what size float valve is in there, and if you can get a bigger one, just in case you encounter starvation, but I'm willing to bet it'll work well as it is, or at least that's what I'm hoping for ya!
Don't know if that helps much or not.
 
gggGary, here's some info that may help you get in the 'ballpark'.
A typical, well-running 4-stroke carbureted engine has a Brake Specific Fuel Consumption (BSFC) of about 1/2 lb of fuel per Horsepower per Hour. Gasoline weighs about 6 lbs per gallon. So, an engine that consumes 1 gallon per hour should produce about 12 horsepower. If you estimate engine horsepower at 60 hp, then fuel consumption will be around 5 gallons per hour (gph). Again, using rough numbers, 3600 secs in an hour, 4300 cc per gallon, 5 gph comes close to (( 5 * 4300 ) / 3600 ) or about 6 CC's per second. Measure your fuel system flow into a calibrated vessel over your choice of time, see how much you get and compare. (Adding each petcock flow, if you have 2, of course)
 
I will do that and report back. The 77 project has me kind of far back on the honey do list.
 
gggGary said:
can "stock" dual petcocks supply a "big breather" like this?
Click to expand...

Think of it this way - can you burn gas faster than it can run out of a 1/4" pipe? I think not! :D If you can't visualize it, pull the hose off your stock petcock and turn it on. Can you burn gas that fast?
 
can you burn gas faster than it can run out of a 1/4" pipe?"

Duh! Oh, you mean in a 650/750 motor.
LOL
I'll try a stock dual petcock tank the fuel line thing may be tricky.
 
Here's why I HAD to swap tanks.
madness 001.jpg

Note to self; PPO's JB Weld and epoxy is not an acceptable method to mount recessed fuel caps. :banghead:

OK you guys convinced me (sort of) I have a shiny inside 79 special tank with dual vacuum petcocks. All the holes / fittings on the vacuum petcocks are bigger than the earlier non vacuum ones. So a pair of those should flow well (enough fuel). So this will be the "mule" tank at least. And the fittings are large enough to seal to 5/16" fuel line with worm clamps.
While looking the petcocks over I answered a question I have thought/wondered about a few times. When a vacuum petcock is in the "prime" position it is pulling from the reserve port not the "on" port.
Bonus find today a 3/8" PEX brass 90 from the local plumbing isle (2 for $3) is a great match for 5/16" ID fuel line and the big fittings found on VM34, VM36, VM38 carbs.

madness 002.jpg madness 003.jpg
Helps with fuel line routing.

Also today The UNI filters were too long, hit the battery box and were "scrunched".
I cut some foam out of the sides at the rubber carb adapter end, removed the old glue from the rubber, cleaned all oil off rubber and foam with soap and water, then carb cleaner, and finally acetone. Glued foam to rubber with 3M "bondo" plastic bumper epoxy. VERY strong flexible bond. If I ever have any issues with that glue bond I'll report back.

Final note for today. Anytime you have the petcocks off remove ALL paint from the sealing area on the bottom of the tank. Razor blade paint off then sand the metal smooth. New o-ring seal = no leaks. I always find paint stuck to the O-ring so fuel can leak between the paint and the metal tank.
madness 004.jpg
Too many other things today to finish the petcocks and fuel it up, but hopefully tomorrow. Then I might be able to do a "real" road test of "madness" and find out if the carbs have half way close jetting.

Advance to the head of the class; what is "wrong" with those carbs?
 
I don't really know those carbs, but it does not look like anything is attached to the choke (or enrichener) plunger on the right one. Pardon my ignorance if that's not what that is!
 
Okay, I'll take a stab at it,
Pic#1 - Disconnected black hose, that's a left carb mounted on right side, missing enrichener lever, vacuum spigot pointing outward, air filter not fully seated, manifold clamp backwards (exposed nut, picky-picky).
Pic#2 - Concerned about that bend in the throttle cable at the top of the left carb, from this view angle looks like carbs may not be perfectly vertical, vacuum spigot pointing outward, can you manipulate the enrichener lever on that left carb?
 
ding ding ding, yeah basically two RH carbs. The biggest issue with that is the idle mixture and throttle stop are on the inside of the LH carb.......
No doubt the throttle cables are crowded. If I stay with a stock tank I will have to look into angled cable adjusters
48-8027.jpg

I want to see how these run but have a set of VM34's ready to bolt in.

Those are nut out T bolt type manifold clamps Mr. eagle eyes! :D
And yes they are not tightened in and fully hooked up yet.
May have to leave them at a slight angle so the cables (kinda) clear the petcocks.
 
That gas tank is really something. It's amazing the things people do. I think this was a case of the original owner/builder having more money than mechanical skills and common sense. Once he got the thing built (mostly by paying others to do it), he quickly found out he couldn't complete the fine tuning or probably even do the required maintenance to keep it running. It sat for 3 years and he unloaded it, lol. I think I can hear him singing right now "Welcome to my nightmare ...."
 

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