I rebuilt the stock Mikuni pump about 300 miles ago and found a lot of junk in there presumably from the 10% ethanol in my local fuel [thumbsdown]
The bike had been running fine until about a week ago when it felt like it was dropping a cylinder. I was testing some electrical and left my manual petcock open in between tests. I moved the bike and noticed fuel leaking out of the exhaust ??? The header was full of fuel.
So thinking it was a needle/seat issue, I rebuilt the carbs...three times [bang] I pulled the float covers while the carbs were on the bike so I could manually actuate the floats to see if they were in fact not stopping the fuel. They do and while I was holding them up, fuel starts running out of the vertical exhaust header :P this leads me to believe that the pump is letting fuel past the inlet from the vertical intake where it gets it's pulse.
Sooooo, do I buy a new Mikuni pump or go the electric route? I checked out the JC Whitney link in the cheap alternatives thread to the Mr. Gaskets but am unsure which actual one is being referred to.
Has anyone used an electric with the FCR's and had problems "overpowering" the needle/seats with the pressure they put out?
Any more options I am missing?
Thanks in advance for reading all this and offering any insight.
Chances are it's the vacuum petcock underneath the hinge bracket as opposed to the pump. The vacuum source is from one of the intake manifolds...horizontal I think.
You can replace it with any 5/16" in/out manual valve.
How does your bike run with the 41's? Most choose 39's for a 750.
Quote from: ducpainter on September 17, 2012, 12:29:44 PM
Chances are it's the vacuum petcock underneath the hinge bracket as opposed to the pump. The vacuum source is from one of the intake manifolds...horizontal I think.
You can replace it with any 5/16" in/out manual valve.
How does your bike run with the 41's? Most choose 39's for a 750.
I replaced it already with a manual one and the fuel flow does in fact stop when I close it.
I don't have any experience with 39's on a 750 but was encouraged by a members build (eonan) and was given Chris Kelly's OK that the 41's would work just as well if not better on the 750 especially if I started to mess around with hi-comps and the like.
To be honest, I have not experienced the general wheelie filled acceleration most comment about in spite of being geared (14/44 IIRC) and having it on a dyno once already to try to sort out a big lag when shifting into second. I just did a valve adjust/belt change/fuel system overhaul and was hoping to do more tuning but the clutch now seems to be starting to slip in addition to this latest issue [bang]
I am hopeful but there has been a lot of times when I have considered selling the 41's to buy 39's :-\
The pump does not stop fuel flow. It has no valves
Sounds like one of your float needles is leaking by and not stopping the effects of gravity.
You should fix it and shut your fuel off every time.
Koko64 (IIRC) reports having issues with his floats from minor amounts of dirt.
I don't doubt that 41's would be OK at WOT. I was wondering how driveability was.
Quote from: ducpainter on September 17, 2012, 02:01:59 PM
The pump does not stop fuel flow. It has no valves
Does this mean that any time I do not manually shut off the fuel, the static pressure will push fuel out of the pumps vacuum line and into the vertical cylinder? This would make me re think returning to or adding back the vacuum shutoff as I have already filled the header and crankcase with fuel (oil only had a few hundred miles on it to boot >:()
QuoteSounds like one of your float needles is leaking by and not stopping the effects of gravity.
I held the floats up manually while mounted on the bike and the fuel stopped flowing, I also blew into the inlet when the carbs were of the bike and no air seemed to pass when they were resting seated ???
QuoteYou should fix it and shut your fuel off every time.
Koko64 (IIRC) reports having issues with his floats from minor amounts of dirt.
My tank is treated with POR 15, I have new lines and filter, and I rebuilt/cleaned the pump and carbs. I don't know what else to do other than start throwing money at the problem in the form of a new pump and needle/seats :-[
QuoteI don't doubt that 41's would be OK at WOT. I was wondering how driveability was.
In what respect? I feel as though it should be more responsive and the bog in second is uninspiring. I had thoughts that a lighter flywheel would help but I really don't know...
Quote from: LP on September 17, 2012, 05:26:05 PM
Does this mean that any time I do not manually shut off the fuel, the static pressure will push fuel out of the pumps vacuum line and into the vertical cylinder? This would make me re think returning to or adding back the vacuum shutoff as I have already filled the header and crankcase with fuel (oil only had a few hundred miles on it to boot >:()Only if the needles were leaking
I held the floats up manually while mounted on the bike and the fuel stopped flowing, I also blew into the inlet when the carbs were of the bike and no air seemed to pass when they were resting seated ???
My tank is treated with POR 15, I have new lines and filter, and I rebuilt/cleaned the pump and carbs. I don't know what else to do other than start throwing money at the problem in the form of a new pump and needle/seats :-[new float valves might be in order.
In what respect? I feel as though it should be more responsive and the bog in second is uninspiring. I had thoughts that a lighter flywheel would help but I really don't know...how is part throttle response? A light flywheel helps every Duc.
A few things.
Your FCRs could be fine and still occasionally flood due to a little dirt or "waxy" fuels. You need a manual tap.
The flat spot can be caused by improper jetting. It could be an incorrect slow jet , incorrect slow air screw setting, needle clip position, accelerator pump requiring attention, even main jet size.
Some boggy performance can be caused by "moderate" flooding at speed and only resulting in fuel pissing on the ground at idle or low speeds.
Also, too large a carb for the bikes needs and state of tune can cause flat spots in the bottom end and midrange. The bigger the carb the more you have to "ride" the carb.
What mods to your motor?
Quote from: koko64 on September 17, 2012, 06:39:24 PM
A few things.
Your FCRs could be fine and still occasionally flood due to a little dirt or "waxy" fuels. You need a manual tap.
I have a manual tap
QuoteThe flat spot can be caused by improper jetting. It could be an incorrect slow jet , incorrect slow air screw setting, needle clip position, accelerator pump requiring attention, even main jet size.
Some boggy performance can be caused by "moderate" flooding at speed and only resulting in fuel pissing on the ground at idle or low speeds.
I haven't had any fuel leakage up until now and it it NOT coming from the carbs, it is coming via the pump vacuum inlet AFAIK. I am hoping and praying I can get the bike in much better shape by continuing jetting changes (I have already dropped to a 152 pilot and I think I am now at a 160 main as per the advice of my one and only dyno session). I haven't messed with the slow air screw much after the jetting changes and have not changed the clip
QuoteAlso, too large a carb for the bikes needs and state of tune can cause flat spots in the bottom end and midrange. The bigger the carb the more you have to "ride" the carb.
Can you please elaborate on what "ride" the carb means? Is it judicious application of throttle?
QuoteWhat mods to your motor?
Only mods are the 41's with pods and FBF slip ons
ducpainter: part throttle response seems ok albeit a bit slow especially if I go WOT
back to your original problem ...
1) manual petcock works fine when you use it (you mentioned forgetting to shut it off at least once)
2) you're convinced that the carb float valve is working right (OK, let's accept that for the moment)
3) when you forgot to shutoff the petcock, you think fuel made it through the pump vacuum line to the intake runner>cylinder>crankcase (assume you are right for now)
4) you rebuilt the pump recently (!?!)
OK, the only way that can happen is if the diaphragm in the fuel pump is torn/punctured. That can happen with age. That's the only route from the fuel-side to the vacuum-side in the pump, and obviously is not acceptable.
Pull the pump, gently blow air (use your mouth - not any compressed air source) into the vacuum line port .... do you get any air out of the fuel inlet/outlet? If yes, the diaphragm is shot (yeah, I read about the rebuild ... maybe that's when it happened, maybe the new diaphragm was bad, who knows). Early stages of diaphragm failure could have caused the doggy running with the vacuum leak/fuel crossover and low flow to the carbs.
The Mikuni DF-44 (http://www.mikuni.com/c-fuel_pumps.html) sold by Chris at CA Cycleworks and a number of other sources is a 14lph rebuildable replacement suitable for all of the carby Monsters. Much cheaper than OEM and smaller, too. Shop carefully and you'll have a new fuel pump for $30 - put it on and see if you are right about your carbs and petcock being OK.
STILL, if you forget to shut off the petcock, fuel may make its way to the carb>cylinder>crankcase with the least bit of crud in the float valve/seat - as stated by Ducpainter, the pump is not a valve.
Changing to an electric pump is a really unnecessary complication - the vacuum pumps work better than fine with very low maintenance.
Have Fun!
Quote from: LP on September 17, 2012, 05:26:05 PM
Does this mean that any time I do not manually shut off the fuel, the static pressure will push fuel out of the pumps vacuum line and into the vertical cylinder?
It shouldn't do - the vacuum side of the ump diaphragm should be dry. If you've got fuel in the vacuum line, something's wrong inside the pump.
big
Quote from: LP on September 17, 2012, 12:14:56 PM
So thinking it was a needle/seat issue, I rebuilt the carbs...three times [bang] I pulled the float covers while the carbs were on the bike so I could manually actuate the floats to see if they were in fact not stopping the fuel. They do and while I was holding them up, fuel starts running out of the vertical exhaust header :P this leads me to believe that the pump is letting fuel past the inlet from the vertical intake where it gets it's pulse.
Any more options I am missing?
Here's another possibility .... you can get the float to stop flow by liftng it manually ... fine, that doesn't mean it will shutoff when floating on fuel in the bowl. Have you meticulously set the float level correctly with the carbs at the correct angle as they sit in the bike?
By ridin' the carb I mean learning how the carb wants the throttle applied to get the best response i.e., snapping it open Vs gradually feeding it in, etc.
If you used a Winderosa rebuild kit they are known to fail.
First step: Remove vacuum line and see if it is wet with fuel.
Second step: With a hand vacuum pump apply vacuum to the fuel pump. Maybe use some clear tube so you can see what comes out. If gasoline comes out, the diaphragm is bad. Oh, no need for more than 15" vacuum.
The listen to what Koko is telling you about the 41s on a 750.
I pulled the vacuum line and when I opened the petcock fuel came out.
I pulled the pump, took it apart, and found the "screen" to be frayed and torn in two spots along the edge. I am assuming this is the problem and hope to rebuild it and get everything back together later today. I will report back.
Thank you all for your responses...now about those carbs :-\
That was it. Bike runs again :)
[thumbsup]
Interesting to see how she runs and if she needs any tuning.
Let us know.