"new" Mikuni fuel pump plumbing

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crkckr

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Apr 26, 2016
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Warrenton, MO
I've been having trouble with the Rotax 447 on my Firefly not wanting to idle below about 2400 rpm and I also have to directly inject fuel into the carb to get it started, which was not the case previously. So, I bought a new Mikuni from Spruce and it arrived... with 3 ports instead of the 2 on the old one. I've done a lot of searching on the 'net and have found nothing but confusion! I have seen the pump referred to as both a vacuum and impulse pump (on the same site!) and variously as one or the other on several other sites (I suppose it could be called both but most of the actual references I've seen call it a pulse pump). I am at a loss as to how to plumb the center (vertical) port... I've seen everything from plumbing it back to the tank, into the carb (although there's no place on the Bing I can see) and even seen it plugged up completely on one site. And while this pump is advertised at having a weep hole, it does not, at least in the place it's usually shown, a small dimple on the front of the pump case (I'm just guessing the 3rd port takes the place of the weep hole but I can find no obvious reference to that). Does anyone actually know with certainty what the 3rd port is for and how it's supposed to be plumbed? Also, I've seen several references to the orientation of the pump body - none of which agree with each other! Is it supposed to be with the 3rd port up or down, and the body vertical or horizontal? Or does it not make any difference?
TIA,
Cheers,
crkckr
 

Hot Wings

Grumpy Cynic
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Can't help with the pump plumbing, but it sounds like your problem is more likely a vacuum leak in the case?
 

bebedriver

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Sep 7, 2020
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You have a twin carb pump. If you are running a single carb, simply fit a short length of fuel pipe and plug the second outlet (I used a short bolt and pipe clamp, the thread makes for a good grip when clamped). It's a higher output pump. Hope this helps :pilot:
 

crkckr

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Joined
Apr 26, 2016
Messages
50
Location
Warrenton, MO
bebedriver, it is definitely not a twin carb pump, it is a single and is listed as such in the Spruce catalog. The twin carb pumps actually have 4 inlets/outlets! Here are the pictures of the pumps from the Spruce webpages. Mine is the square one on the right.

Hot Wings, that is one thing I haven't checked and is certainly a possibility. I'll have to check that out as soon as I get the new pump plumbed and get ready to run the engine again. Hopefully, the new pump will cure everything and I won't have to go looking for a case leak!
 

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bebedriver

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Sep 7, 2020
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Ah, having seen it I get that! However, centre (vertical) outlet must be for the pulse pipe?
 

Dana

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CT, USA
Your old pump must have had three connections too, otherwise one was broken off or it wasn't a pulse pump. The center connection is the pulse line which connects to the crankcase fitting, the side connections are in and out, the arrows indicate the direction of fuel flow.

The dual pump has one inlet, two outlets (one of which may be plugged), and the center pulse line connection.

If you didn't have a pulse line connected, the pump wouldn't pump. Where is the fuel tank located relative to the engine? Do you have a picture of the old pump?
 

David L. Downey

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Aug 7, 2019
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101
all you old 2-strokers: in getting the old Ultrastar back to flight status, I noticed that some references to the Mikuni single pump say that there is a 1/64" weep hole required adjacent to the pulse fitting. There is indeed a depression in mine but it i nto drilled. should I disassemble the pump and drill/deburr that hole?
Thanks!
 

Dana

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If the pump is located above the engine with the pulse connection pointing down, no weep hole is required. That's how I had it on my Ultrastar, mounted to a plate attached to the engine mount.
 

crkckr

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Apr 26, 2016
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Location
Warrenton, MO
As much as I hate to admit it, I am a dunce! Yes, the old pump has 3 lines, exactly as the new one. I honestly don't know what the devil I was thinking but it certainly wasn't "clearly!" My setup is standard Firefly, the tank is about 2' below the engine, the pump is attached to the frame about 6" below the engine, which is the length of the pulse line. Duh! I hate when I'm being a dummy...

Thing is, while the new pump says it has the weep hole, in fact it does not, it just has the dimple (I looked with an 8x glass to be sure, from the inside as well). I drilled the old one with the smallest bit I have, a #60 (.040), but the hole looks (is!) huge compared to one that is actually drilled at the factory, which is, according to CPS, 1/64th (.015). I'm going to have to look around for a bit that small to drill it myself. I have a suspicion that the hole I drilled might just be the problem, although there is nothing coming out of the hole. Once I replace the old pump, I may seal the hole with JB Weld on the old one and re-drill it with the 1/64th bit.

What is funny is that everything worked just fine before I did some winter work on the airplane, drilling the required weep hole, and including pickling the engine for the winter. THAT was a huge mistake, as everything seemed to go wrong once I tried to get back into flying status. The engine wouldn't idle normally and when pulled back to normal 2000 rpm idle would run rough and eventually would simply quit. I will order some 1/64th bits today but at the rate the weather is going (we had snow yesterday!) it might yet be a while before I can get the necessary painting done before taking the plane back to the airport!
Cheers,
crkckr
 

David L. Downey

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Aug 7, 2019
Messages
101
If the pump is located above the engine with the pulse connection pointing down, no weep hole is required. That's how I had it on my Ultrastar, mounted to a plate attached to the engine mount.
Thanks Dana. That is exactly when I discovered the disparity in the information. But if yours worked installed that way I'll do mine that way too.
 
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