Documentation Of Proper Fuel Pressure For ST1100

[Uncle Phil]

I can find the flow rate but I cannot seem to find what the max pressure should be from the fuel pump.
I need documentation from Mother Honda, not from somebody's website.
Any place to look for that number?
Thanks.

 

[TerryS]

I can't see the number anywhere in the service manual, and I looked through a few other manuals for other Hondas with carbs/fuel pumps with the same result. Using some maths it does not take much pressure to move fuel from the bottom of the tank to the top of the carbs; if I allow 0.5m height and 0.8g/mL density for fuel then you only need 0.6psi or 0.04 Bar. The carbs certainly need no pressure to operate, just flow.

 

[Uncle Phil]

My problem is that a Quantum replacement fuel pump is putting out 6 PSI which I think is too much (don't need the 'choke') but they say they are 'OEM spec'. I've done the same search and can only find flow information not PSI.

 

[TerryS]

That pressure rate is only too high if the float valves are not able to shut off the pressure, in which case you would get fuel out of the overflows. Everything else in the carb is dependent solely on the fuel height determined by the floats. The Airtex that I used ran between 2 and 4 psi and did not cause any operational issues.

If you no longer need the enrichener circuit to start, there may be other issues like worn emulsion tubes, I had that on my RF900 and it meant that the gap between the needle and the tube was too big and it ran too rich all the time, giving poor economy and occasional fouled plugs. I was able to replace the emulsion tubes and it was a night and day difference on that bike. I'd suggest you hit up Adam Frymoyer on the other forum for some advice, he seems to be the 1100 carb guru and was really helpful to me.

 

[Uncle Phil]

Well, it was not a problem until I installed the Quantum pump - everything worked as it should with the stocker.
It just had intermittent failures.
So I would be surprised if the carbs went 'south' all of a sudden.
I need 'Honda Data' to give back to Quantum confirming what the proper pressure should be if I can find it.

 

[TerryS]

Fair enough, I hope you get to the bottom of the problem UP. Best of luck.

 

[TerryS]

There's some documented fuel pressure readings on this page:

http://www.st-riders.net/index.php?topic=1033.0

 

[Uncle Phil]

There's some documented fuel pressure readings on this page:
http://www.st-riders.net/index.php?topic=1033.0

How do you convert oz/in2 to PSI?
Divide by 16?
If so, their 'pressures' are way less than 2 PSI, closer to 1 PSI.
I've seen 2-5 PSI give numerous times so I'm still confused.

 

[TerryS]

Yeah, Oz/inches is a truly unique unit! Given 16 ounces to a pound I'd say you are correct with your numbers.

 

[dwalby]

FWIW, the Honda manual claims 640ml/minute to be the acceptable flow rate from the pump, which is much lower than the 500-700ml in 15 seconds that was reported for several random ST1100 pumps tested in the link provided earlier. This discrepancy was mentioned in the link article as well.

If we use 600ml/15 seconds as an average, that's 2.4LPM or 144 LPH, which seems to be in line with some pump specs I've looked at very briefly. Not sure what to make of the service manual number, it seems to be off by a factor of 4x from the empirically gathered data, so take the service manual figure with a grain of salt. It appears that the faulty pump still delivered 1000ml/minute, well in excess of the Honda manual spec, yet the pump was faulty (apparently a heat related failure mode where the pump would slow way down). The replacement pump delivered 550ml/15 sec, or 2200ml/minute.

 

[Andrew Shadow]

Hi Phil:

I can't help you with the Honda spec. but that article states that a new Honda fuel pump for a '94 ST1100 registered a pressure of 18.5 oz/in2. This converts to 1.16 PSI.

Another member had the same problem as you after he installed a Quantum fuel pump on his ST1100. The pump was putting out 9 PSI. The engine ran rich and the plugs would foul. He replaced the new Quantum pump with an Airtex pump and his problems went away. It sounds like the Quantum puts out to much pressure for the ST1100.

Thread is here- ST1100 Fuel Pressure.

 

[Uncle Phil]

I wonder where the oft quoted 2-5 PSI comes from?
The results from the reference article would imply 1-1.6 PSI maybe.
I've found an 'adjustable' inline pressure regulator that I may try just for grins.

 

[GGely]

Hi Uncle Phil, can you provide a link to the regulator?

Thanks!

 

[rwthomas1]

I'd be tempted to drill a small bleed hole as close to the pump as possible, start at a 1/16" and work up, until the pressure at the carbs is @2psi. Why 2psi? Arbitrary really, as long as the carbs are getting fuel at enough volume, there shouldn't be a problem. Unless the pump is designed to be "dead headed", the bleed hole should be easier on it than the regulator?

RT

 

[Uncle Phil]

Hi Uncle Phil, can you provide a link to the regulator? Thanks!

The thing is made in China (oh well) and I don't have a clue if it will work as advertised or not.
But here's the link -

Metal Adjustable Fuel Pressure Regulator for Carburetor Engines Universal W2H7 | eBay

This same 'device' is offered by lots of sellers at lots of different prices.
I figure I'd 'take the hit for the team' and see how it works.

 

[Uncle Phil]

I'd be tempted to drill a small bleed hole as close to the pump as possible, start at a 1/16" and work up, until the pressure at the carbs is @2psi. Why 2psi? Arbitrary really, as long as the carbs are getting fuel at enough volume, there shouldn't be a problem. Unless the pump is designed to be "dead headed", the bleed hole should be easier on it than the regulator?
RT

RT - That will be my next 'try' - the only problem is that blame submersible hose if pretty pricey if I mess it up.
Based on your knowledge, do you think 1/16" would be about right for a pump producing 6 PSI (as best I can measure it)?
If the regulator gadget works, then as the pump ages and pressure reduces, I could at least 'open' it up as this is an adjustable regulator.
I plan on mounting the regulator outside of the tank before the filter so I can fuss with it.
It may not be worth the postage it costs to get here but I figure I'll give it a shot.

 

[rwthomas1]

RT - That will be my next 'try' - the only problem is that blame submersible hose if pretty pricey if I mess it up.
Based on your knowledge, do you think 1/16" would be about right for a pump producing 6 PSI (as best I can measure it)?
If the regulator gadget works, then as the pump ages and pressure reduces, I could at least 'open' it up as this is an adjustable regulator.
I plan on mounting the regulator outside of the tank before the filter so I can fuss with it.
It may not be worth the postage it costs to get here but I figure I'll give it a shot.

Hahaha! No man, the size of the hole would be a WAG! Diameter hole is going to vary based on pump pressure, volume, length of hose, etc. Dunno if I'd punch a rubber hose, hard to get a clean hole in rubber that doesn't sorta self seal. I figured we were talking about the plastic flexible stuff I've seen on most in-tank pumps....

RT

 

[Uncle Phil]

Hahaha! No man, the size of the hole would be a WAG! Diameter hole is going to vary based on pump pressure, volume, length of hose, etc. Dunno if I'd punch a rubber hose, hard to get a clean hole in rubber that doesn't sorta self seal. I figured we were talking about the plastic flexible stuff I've seen on most in-tank pumps....
RT

Nope, I went with the 'real' Gates stuff - about $20 a foot for the proper spec.

 

[Uncle Phil]

Just got off the phone with Quantum - good conversation (at end of the thread) -

Article [11] - ST1100 - Fuel Pump Replacement | ST1100 Articles | Page 3 | ST-Owners.com (st-owners.com)

 

[Jim Van]

Uncle Phil :

I understand your concern. After I installed a low pressure auto fuel pump, I measured the flow and it was a heck of a lot higher than the Honda spec. If you do a calculation, you only need about 3 oz. flow per mile , if I recall my calculation properly. I was concerned about the pressure, too.

I removed the OEM filter, fitted a metal canister fuel filter in the tank, connected to the output line of the fuel pump, and drilled 3 small holes at the inlet side of the filter. There just happens to be 3 small dimples on the inlet end of the metal fuel filter - convenient. That reduced the output flow, but it was still above the OEM spec. I'm sure doing that also reduced the pressure.

I also removed the diaphragm fuel cut-off valve and replaced it with a mechanical spring-loaded one-way flow valve.

I never measured the output pressure, but I don't think I have a problem - the bike runs great and gets the same mileage, etc.