Fuel Starvation with Late Non-Vacuum Petcock?

Thanks for the pics and affirmation Gary; when I saw the size of the outlets I installed a second one (they provide two each of straight and 90 degree). however, there could also be a restriction through the petcock too. I dunno for sure.
It'll be at least until Thursday that I'll be able to do a flow test. Like I think I may have said, I've got a Pingel from my GSXR which is on the mend from a mishap at Willow, and that doesn't have any problems keeping the 4 36mm Flatslides fed.

Thanks TwomanyXS1Bs for the additional data on testing. Between you and Fred I'm to get a pretty comprehensive body of data.
 
Yes the intake tubes on those petcocks are also tiny. It's kinda common on the chincom petcocks, bought one for a royale star looked at it a few times and threw it in a parts bin, bought factory overhaul parts for the original instead. (what I do on XS650's also) I think they just take one design and change the baseplate to fit whatever, probably original design was a 50cc. If you don't change the petcock I'd try reserve for sure.
I have only ever had one Pingel, it came on a project bike and leaked like a sieve, so I'm biased, might even have been on madness, LOL.
 
gggGary said:
Wow those are tiny yeah I'd be worried won't flow enough. Maybe with two it'd be ok.
Pic of stock vacuum vs Mikes.
View attachment 118531 View attachment 118532
Click to expand...
gggGary said:
Yes the intake tubes on those petcocks are also tiny. It's kinda common on the chincom petcocks, bought one for a royale star looked at it a few times and threw it in a parts bin, bought factory overhaul parts for the original instead. (what I do on XS650's also) I think they just take one design and change the baseplate to fit whatever, probably original design was a 50cc. If you don't change the petcock I'd try reserve for sure.
I have only ever had one Pingel, it came on a project bike and leaked like a sieve, so I'm biased, might even have been on madness, LOL.
Click to expand...

You read my mind; I was planning on mentioning the tiny intake pipe as was walking back from my yearly job review. Chances are your suspicion is on the money regarding the one-size-is-good-enough design of the import petcocks.

Sorry to hear about your Pingel; I've got one on each of my Harley customs - one has been on there for ~15 years and they haven't let me down yet. Although the 15 year old one has had a lot of avgas going through it/sitting on it, so that may have something to do with its longevity. That stuff is magical in terms of storage for keeping fuel systems happy.
 
TwoManyXS1Bs said:
Some rough/raw fuel consumption trivia.
The typical 4-stroke has a BSFC of about 1/2 pound per horsepower per hour.
BSFC = Brake Specific Fuel Consumption
At about 6 lbs per gallon, that translates into:
1 GPH fuel consumption produces 12 horsepower.
1 GPH is roughly 1cc per second.
60 MPH at 60 MPG is 1 GPH, producing 12 Horsepower.
12 HP uses 1 GPH, 1cc/sec
24 HP uses 2 GPH, 2cc/sec
36 HP uses 3 GPH, 3cc/sec
48 HP uses 4 GPH, 4cc/sec
60 HP uses 5 GPH, 5cc/sec
72 HP uses 6 GPH, 6cc/sec

When racing flat-out on an XS650/750, use the higher 5-6 GPH, 5-6cc/sec consumption figures to determine minimum fuel flow requirements, plus maybe a 100% safety factor, or about 12cc/sec as your minimum fuel flow target.

At 12cc/sec, a 473cc pint should be drained in at least 40 seconds.


The scenario you describe in post#1 reminds me of another syndrome, "imminent seizure", due to heat. An easy way to check for this is to notice if engine braking increases during the scenario. Increased cooling to the cylinders doesn't always help, since it's the pistons that need to shrink. Notice the increased piston/cylinder clearances called out in XS650/750 race setups.

From page 11-7 of the August 1973 factory racing tips manual:
View attachment 118537
Click to expand...

I forgot to mention regarding the imminent seizure, It's possible that may have been happening, but when it would start carrying on I'd back off the throttle in hopes of letting the fuel level "catch up" and if it didn't actually go faster, it did run smoother after a bit. Unfortunately it also meant I'd also be more in the way of the modern litre bikes. Even when I'd pull into the pit with it carrying on all the way to the pit-in, it wouldn't labor to idle or putt back to the truck. My presumption is that I doubt I had some seizing going on.
 
TwoManyXS1Bs said:
Some rough/raw fuel consumption trivia.
The typical 4-stroke has a BSFC of about 1/2 pound per horsepower per hour.
BSFC = Brake Specific Fuel Consumption
At about 6 lbs per gallon, that translates into:
1 GPH fuel consumption produces 12 horsepower.
1 GPH is roughly 1cc per second.
60 MPH at 60 MPG is 1 GPH, producing 12 Horsepower.
12 HP uses 1 GPH, 1cc/sec
24 HP uses 2 GPH, 2cc/sec
36 HP uses 3 GPH, 3cc/sec
48 HP uses 4 GPH, 4cc/sec
60 HP uses 5 GPH, 5cc/sec
72 HP uses 6 GPH, 6cc/sec

When racing flat-out on an XS650/750, use the higher 5-6 GPH, 5-6cc/sec consumption figures to determine minimum fuel flow requirements, plus maybe a 100% safety factor, or about 12cc/sec as your minimum fuel flow target.

At 12cc/sec, a 473cc pint should be drained in at least 40 seconds.
Click to expand...

OK, so I just did a quickee flow test using the criteria above using a Ratio Rite and my cell phone's stopwatch. I also only used the hose for the LH carb because...I'm too lazy to investigate the difference between two hose flow and one. My guess is that one hose will flow faster the two though since the fuel still flows through the same petcock.

At 40 seconds I had 1/2 a pint. At 73 seconds I had a full pint. This was done in the "on" position, so the fuel saw the restriction of the smallish standpipe for that position.

Even if I went with the 48hp constant with a 100% safety factor, I'm barely more than keeping up at 60 seconds per pint.

I did try switching to reserve on the track and still had the issues I'm trying to figure out FWIW.

I have some brass on order and perhaps I'll look into alternative petcocks.

Thanks again for all the help guys!
 
No xs650 is going to burn fuel faster than a working xs650 petcock can pass it. How long would it take for the tank to drain onto the ground? Can't you ride a lot longer than that w/o refueling?
 
xjwmx said:
No xs650 is going to burn fuel faster than a working xs650 petcock can pass it. How long would it take for the tank to drain onto the ground? Can't you ride a lot longer than that w/o refueling?
Click to expand...
You are right, on the street, he's racing. Running that hard fuel usage, might be 2, 3 times what street use would be. It's pretty hard to run a motor near rated horsepower for more than a few seconds on the street.The track he was racing on, Buttonwillow, is a tight demanding 3 mile course. The petcocks are WAY at the back on a stock 650 tank he may be (probably is) unporting the standpipe during braking. Little google out there but 14-16 MPG from top modern roadrace bikes?

Damned old man stories of minor relevance; I tried to remember what kind of fuel mileage we got MC road racing but it was WAY too long ago. Best I could come up with was; starting with new, we had to change front brake pads to make it through a 6 hour endurance race. Even in hard street use you'd get hundreds of hours out of a set of pads. Couldn't figure out why my left wrist got so sore til we did the math and found at Blackhawk Farms Raceway we were shifting gears 800 times an hour. Buddy and I took turns riding, crewing LOL, swapping duties a tank at a time at a time?
 
TwoManyXS1Bs said:
Okay, now do the same test, measuring fuel delivery rate out of the bowl drains. This will let you know if the float valves are restrictive...
Click to expand...

Yes, good idea. As I sit and smoke out in the garage with the cat, I contemplate what else I can try. The cat is no help on that but I appreciate your affirmation.
I've been a little distracted though; I picked up another track bike and I'm trying to get it prepped for the next outing.

xjwmx said:
No xs650 is going to burn fuel faster than a working xs650 petcock can pass it. How long would it take for the tank to drain onto the ground? Can't you ride a lot longer than that w/o refueling?
Click to expand...

I get it. This is just my hypothesis for now to sort this out. It's showing a problem with demand over time - it may be a small time, but the problem doesn't show itself on the street, just on the track where It's typically WFO or brakes

gggGary said:
You are right, on the street, he's racing. Running that hard fuel usage, might be 2, 3 times what street use would be. It's pretty hard to run a motor near rated horsepower for more than a few seconds on the street.The track he was racing on, Buttonwillow, is a tight demanding 3 mile course. The petcocks are WAY at the back on a stock 650 tank he may be (probably is) unporting the standpipe during braking. Little google out there but 14-16 MPG from top modern roadrace bikes?

Damned old man stories of minor relevance; I tried to remember what kind of fuel mileage we got MC road racing but it was WAY too long ago. Best I could come up with was; starting with new, we had to change front brake pads to make it through a 6 hour endurance race. Even in hard street use you'd get hundreds of hours out of a set of pads. Couldn't figure out why my left wrist got so sore til we did the math and found at Blackhawk Farms Raceway we were shifting gears 800 times an hour. Buddy and I took turns riding, crewing LOL, swapping duties a tank at a time at a time?
Click to expand...

I measured the outlet holes and they're only .150" which looks like a pretty good restriction when the hose and carb inlets are around .300"
The 100% safety factor mentioned earlier doesn't seem excessive when one considers the bowl is constantly getting drained, yet the bowl's volume needs to be filled/maintained at a particular level.

The first paragraph in Gary's reply is what made this rabbit hole worth exploring.

Like I said, I'm a little distracted, but still want to try and get this sorted for the next outing. I gotta try and dig up the stock tap and measure that outlet.

Thanks for the continued interest!
 
If this helps, when the bowls get empty enough, takes maybe 2-3 min. at around 3000 rpm, it starts lean popping and running on one cylinder. This is from petcock turned off. Seems to me if you had starvation while riding it would eventually shut off; if wouldn't find a low fuel equilibrium and stay there. You could pass a longer clear tube out where you can see it to see whether it stays filled with gas. Maybe with some coloring; dash of red transmission fluid...
 
xjwmx said:
If this helps, when the bowls get empty enough, takes maybe 2-3 min. at around 3000 rpm, it starts lean popping and running on one cylinder. This is from petcock turned off. Seems to me if you had starvation while riding it would eventually shut off; if wouldn't find a low fuel equilibrium and stay there. You could pass a longer clear tube out where you can see it to see whether it stays filled with gas. Maybe with some coloring; dash of red transmission fluid...
Click to expand...
Not a bad idea and easy enough to throw together. I'm sure I can find a place to do this test w/out bothering anybody. In regard to shutting off, though, and this is just speculation, it's possible things could be leaning out (and yes, it does act lean under the circumstances I've described) with the fuel low enough that the pilot circuit is just sucking air.
FWIW, I looked at my plugs and they're a nice cocoa brown. However, this was after getting off the track and putting back to the truck. Haven't done a throttle chop and plug reading.
Also, I measured my stock petcock outlet and it's right around .195", but I still had the stock CVs on it with that petcock, and had not yet had it on the track.

I've gotten some brass so I'll be fiddling this weekend. Something to be aware of, and I'm *not* trying to do a public shaming here, on Mike's website the description for some of the jets say "sold in pairs" but don't say "Pkg of 2" in the Qty column. Some do. If you happen to order a jet that doesn't explicitly say "Pkg of 2", you'll only get one jet even if they say they're sold in pairs...just a public service announcement.

I still have yet to contact them about it so take this at face value, everybody. They do get the stuff out fast and I got a handwritten "Thanks" on the bill.
 
FWIW, I got an email back from Mike's. They're sending me another jet.
I just wanted to give them public kudos since I mentioned the issue a few posts back.
 
molochnik said:
... As I sit and smoke out in the garage with the cat, I contemplate what else I can try. The cat is no help ...
Click to expand...

Yeah, but they can be good listeners...
SmokingCat02.gif
 
If I were you, I'd stick exclusively with genuine Mikuni jets, not Mike's knock-offs. From one batch to the next, there's no telling if they will be sized the same or flow the same. That's happened to me several times with other stuff from them. I did use a few of their jets many years ago and they were fine, but since they raised their prices I can get the real ones for pretty much the same money. I find the best deals are the 4 packs from one of the big vendors like Dennis Kirk or Bike Bandit. Genuine Mikuni jets can be had for as little as $2.50 to $3 each that way.

Comparing the real jets to the knock-offs, I noticed something else. The tapered bore leading into the metering orifice differs in angle and depth between the two. I've got to think that has an effect on how they flow fuel. Since you're racing, I think that might be important to you.
 
5twins said:
If I were you, I'd stick exclusively with genuine Mikuni jets, not Mike's knock-offs. From one batch to the next, there's no telling if they will be sized the same or flow the same. That's happened to me several times with other stuff from them. I did use a few of their jets many years ago and they were fine, but since they raised their prices I can get the real ones for pretty much the same money. I find the best deals are the 4 packs from one of the big vendors like Dennis Kirk or Bike Bandit. Genuine Mikuni jets can be had for as little as $2.50 to $3 each that way.

Comparing the real jets to the knock-offs, I noticed something else. The tapered bore leading into the metering orifice differs in angle and depth between the two. I've got to think that has an effect on how they flow fuel. Since you're racing, I think that might be important to you.
Click to expand...

Your advice is well taken. I'll take a close look at them. The taper leading in and the outlet into the needle jet certainly will make a difference. I had an RD200 about 20 years ago that had Hitachi carbs. Air filters weren't available anymore and I couldn't find appropriate jets. Since it was given to me, I had my boss port it, ran open carbs, and jetted it by soldering the mains shut and re-drilling them. He explained how all the features of a jet have a purpose, even though the solder and drill method was his ol' school suggestion.

xjwmx said:
Or just mock you.
View attachment 119001
Click to expand...
That's how my mom's great dane looks at me when I speak to her (the dog) in Russian.
 

Similar threads

R
Fuel taps, vacuum rebuild kits or swap to non vacuum...?
2
Replies
28
Views
2K
Astrolite
Astrolite
mikeh36
Single petcock and vm34 carbs
Replies
6
Views
894
Team Junk
Team Junk
Noidea
Vm float height guage and testing
Replies
2
Views
322
ckahleer
C
P
Semi-generic carb theory question
Replies
8
Views
801
Pat D
P
Djfknwoo
Xs650 backfires at 2k-3500 rpm on left side
2 3 4
Replies
71
Views
7K
Mike Melvin
Mike Melvin
Back
Top