Enough fuel flow to carbs?

[Shu]

Greetings GS500ers,

I have a question about fuel flow to the carbs.  I'm jetting for a pipe and K&N, and there is plenty of good info here at GStwin on that.  But first I wanted to check the flow to the bowls so I attached drain hoses from both taps to two bottles, and collected fuel for 90 seconds.  I did not use prime, instead I just provided vacuum to the fuelcock from another running bike so that I could simulate as close as possible the actual operating conditions without the bike running.   Both bottles filled evenly and each had 9 oz of gas for a total of 18 oz, or 12 oz a minute.

I realize that the drain set-up I used may have restricted flow, but this showed me that the carbs can get all the fuel they need up to a rate of at least 5-6 gallons/hour.  So I wonder, is this enough at WOT?

Thanks

[Adam R]

It might be enough, but it wasn't in my case (150mains, K&N pods and Vance and Hines exhaust) so I installed a Pingel petcock and now run a direct line from the tank to the carbs.  No problems either with overfilling, knock on wood.



Adam

[Shu]

Yeah, I'm running a stock fuel supply except I put a large inline filter between the fuel valve and the carb inlet, one that seems to have little if any restriction.  

Somewhere I read that you need about a half a pound of fuel per hour, per HP in a "hot" street engine - most likely talking about cars but the same might apply to bikes.  So for 40 hp that means 20 lbs of fuel per hour, about 3 gallons per hour.  If this is true my measured 5 gph should be plenty.  :dunno:

I have corrected the float/fuel level using the u-tube method, but I know that the level will drop if the supply can't keep up with the high load/rpm fuel demand, making main jet tuning impossible.  :x

[GRU]

i think that you didin't get enough fuel is because the engine was ideling...if you were to increase the rpm, the fuel pressure would increase

[Kerry]

3 gallons per hour?  Whoa!  

When I load my stock bike with 180 lbs of me+gear and 50 lbs of luggage I still use just over 1 gallon per hour going down the freeway doing 70-80 mph.

Would the combination of larger jets, higher-flow air filter/exhaust, and a wide-open throttle make that big of a difference?  If so, I'm impressed!

[Shu]

I may be confusing things by saying "gallons per hour" but that is what  I've seen as a common unit of flow, as on fuel pump specs.  But that three gallons an hour would not be of line for a stock GS500, IF it was going say 105 mph for that hour.  35 mpg is certainly not bad for anything going that fast.

No, what I mean is the fuel requirement at peak power, which is of course not used all the time.  But when called upon to do so, I want to be sure that my fuel supply can provide enough gas to keep the levels right in the float bowls.  My simple flow test, done with the engine off and with "donor vacuum" to simulate running conditions at the fuel valve, showed me that at least 5 gph gets to the carb bowls, and that sounds like more than enough to me, but I'm not sure.

[Shu]

GRU, you have me thinking about something I never considered before.  Does the fuel valve really increase the flow with engine RPM?  I always thought of the vacuum line controlled "ON" position as a safety thing, so the fuel flow would stop if the engine cut off in a spill, or as a safeguard against hydralock.  But  the strong pulsing vacuum could let more fuel through as the rpm climbs, is this what you mean?  This would make the fuel valve a flow regulator, not just a vacuum shut-off control.

[The Buddha]

I have jetted a number of bikes and never had fuel starvation issues. 150/40/1 washer/3 turns is well within stock petcock flow capacities. The fuel will flow out of the jets and into the venturi at speed much faster than it will drain out the drain spigot.
Cool.
Srinath.

[KevinC]

You actually could get less flow at bigger throttle openings. There is less carb vacuum with the throttle open than at idle, and since carb vacuum is what opens the vacuum petcock, it could close somewhat at WOT, restricting the fuel flow. This is a gravity flow system, so there is no fuel pump that will run faster at higher rpm or anything.

I don't suppose any of you have seen vacuum operated windshield wipers they used to use on cars in the '40s? Wipers went like hell when you were at idle, and slowed to a crawl when you opened the throttle - not exactly the best!

The stock petcock on my GS definitely did not flow enough fuel for sustained WOT, and that seems to be the case on quite a few GS500s.

[The Buddha]

Wowee KevinC... I did not know that. I can easily see how it wont be able to keep up a long WOT run. But I can drain a full tank in ~ 20 mins with  drain spigot off 1 carb. So that's about 4 gal in 20 mins, and with the 2 it will be 4 gal in 10 mins. OK OK not neccesarily linear but... say 12 mins max. Even at WOT (say 120mph) you'll manage ~22-24 mpg which is a gallon in 11-12 mins. You have 4 in 12. Of course The fuel flow will be fantastic with a full tank, and slow to a trickle when its low, and that makes for a very uneven rate of flow. Also you racers run only about as much as you need in the race, so you tend to not fill up to the top. I know this guy that cut a hole in his tank and inserted a coffee can sized metal container to make the fuel pressure higher with a lesser quantity of gas. To let him run less fuel and not starve for gas, and it already had a pingel in it. Of course he might be cutting it even closer than usual, cos when you devise one way of making it better we tend to over do it and end up right back where we started.
Cool.
Srinath.

[KevinC]

I suspect the spring is too strong in some of the vacuum petcocks, and when the vacuum decreases at WOT, they partially close.

I was having fuel flow problems even when it was completely stock and with a full tank. There is something wrong with a lot of the GS vacuum petcocks.

[dgyver]

My valve does not have the vacuum line to the carbs. The internals must have been gutted. No problem with fuel flow.

[Shu]

Looks like I'm not getting nearly as much flow as Seshadri when draining thru both carbs, only one gallon in ten minutes.  But from what we have said here that should be enough.  If my problem is getting less fuel as the vacuum drops at WOT, then maybe I can just operate the bike on PRIme? I will have to remember to set it back to "ON" before shutting off the bike. No different from having to remember to shut the Pingel off.