Predator 670 dual 28mm Mikuni: Jet sizes?

strantor · 10-29-2022, 07:41 PM

I put a couple of these knockoff "Maikuni" 28mm carburetors on my 670 and I'm trying to figure out what jets to put in it. Anyone have a clue?

slonomo · 10-29-2022, 11:19 PM

What size jets are in them now? What makes you think they need different jets?

strantor · 10-30-2022, 02:28 AM

Quote:

Originally Posted by slonomo

What makes you think they need different jets?

Because my luck is broken so I wouldn't expect anything to just work for me right out of the box. Also there are some subtle clues like the fact that the factory carb had 2 different sized main jets and that the performance670 website sells a dual 28mm mikuni kit yet the Amazon listing for the carbs I bought says 28mm is for a 75-125cc engine.

The Amazon listing is very generalized and uninformative and seems to tailored to an audience of scooter and dirt bike tuners whose engines probably rev 10k+ and are likely 2-stroke, so I'm not surprised that their "compatibility chart" doesn't translate to my application, but I think these should still work with proper jetting, and it seemed prudent (to me) to assume they wouldn't have proper jetting for my application out of the box.

Quote:

Originally Posted by slonomo

What size jets are in them now?

Low speed jets:
Carbs shipped with #38 installed and #35 in the box. With the 38s I couldn't get it to idle quite right. I assumed it was too lean because the Amazon listing left me with the impression that these carbs might be too small. So I took the 35s and drilled them out one size bigger than the 38s, and installed them. Then it idled even worse. So I assumed I had mistaken a lean condition for a rich condition. Having already drilled out my smaller jets, I ordered two 40-pc kits of jets. Right now I have a 35 in the right side and a 32 in the left side, and the idle is even worse.

I am at the point now that I am ready to publicly confess I have no clue what I'm doing and everything I try makes it worse. The best result I got was right out of the box and your question "What makes you think they need different jets?" seems uncannily insightful. So what is it you know that makes you ask that?

Main jets:
I haven't gotten around to tuning the main jets yet, but the carbs shipped with #120 installed and #110 & #115 in the box. Thanks to the jet kits I now have everything from #100 to #170. The factory carb had a #108 and a #105 so my plan was to start off with those numbers and probably go a bit larger.

If the factory carb had low speed jets I would copy those too, but it doesn't seem to have any. There are two #45 jets (black plastic jets, top of carb, covered by screw & washer) but I think those are mid-range jets.

strantor · 10-30-2022, 07:54 PM

I don't need help with jets for the time being.
I found the cause of my troubles. I'm not as big of an idiot with carb tuning as I thought I was. I'm just a big idiot in general. After hours and hours of attempting to tune, I found that the engine was running on only one cylinder. I had fuel, I had spark, and I had compression, but the carb wasn't sucking any air. Even at WOT, put my hand over it, no vacuum whatsoever. Broken rocker and bent pushrod. Intake valve wasn't opening.

slonomo · 10-30-2022, 11:51 PM

Well that would do it.

With those carbs, make sure the actual venturi measures 28mm and not the opening. I've seen where Amazon or Ebay carbs from China will be advertised as 28mm carbs, but the venturi actually measures much less. The 28mm often is the measurement of the inlet side of the carb, but not the venturi. Verify that. If it's 28mm you should be ok.

Also, I've used China carbs before without much luck. I have disassembled them and found metal shavings and dirt inside, so might want to take them apart and inspect. I've also found missing parts in some and damaged main jet needles or pilot jet needles from being mishandled. Just be on the lookout.

slonomo · 10-30-2022, 11:52 PM

And....you're not an idiot....you found the problem. That makes you a mechanic

strantor · 10-31-2022, 05:29 AM

Quote:

Originally Posted by slonomo

Well that would do it.

With those carbs, make sure the actual venturi measures 28mm and not the opening. I've seen where Amazon or Ebay carbs from China will be advertised as 28mm carbs, but the venturi actually measures much less. The 28mm often is the measurement of the inlet side of the carb, but not the venturi. Verify that. If it's 28mm you should be ok.

Also, I've used China carbs before without much luck. I have disassembled them and found metal shavings and dirt inside, so might want to take them apart and inspect. I've also found missing parts in some and damaged main jet needles or pilot jet needles from being mishandled. Just be on the lookout.

Yep they had trash in them, I cleaned them out before installing. And one of them had a damaged needle (not the one in the carb but the spare).

strantor · 10-31-2022, 05:30 AM

Quote:

Originally Posted by slonomo

And....you're not an idiot....you found the problem. That makes you a mechanic

Thanks. I just realized, I might never have found this if I didn't put dual carbs on.

strantor · 11-04-2022, 12:58 PM

Ok I got a got a new head off ebay and put on the engine. two reasons:
1. My head had small cracks in it.
2. I couldn't find stock replacement rockers anywhere and this head had rockers already on it.

Now I'm waiting on pushrods from Performance670.

I was told by someone in the predator 670 owners group on Facebook that a single Mikuni setup would use a #10 to a #15 idle/pilot jet. That seems unfathomably small to me, but it is backed up by some data posted here and jives with my experience that even with the smallest pilot jets in my kit (#32) it was still super rich at idle.

Since this is dual carbs, by my math on the cross-sectional area, two #7 jets would be equivalent to one #10 jet, and two #10 jets would be equivalent to one #15 jet.

Jets of size 7 and 10 do not exist for this carb, or maybe for any carb. I thought to fill in a jet with solder and then drill it out but that would require drill bits of size 105 or 101 respectively. Which also don't seem to exist.

So I have another idea I'm going to try once I get the rods, but until then I can't do anything but sit around and ponder it, so I thought I would ask for predictions. Here's the plan, and reasoning behind it:

A (unicorn) #7 jet would (in theory) have a diameter of .07mm and a cross sectional area of 0.003848451 mm^2
A #48 jet has a diameter of 0.48mm and a cross sectional area of 0.180955737 mm^2
26AWG wire has a diameter of 0.47498mm and a cross sectional area of 0.177190538 mm^2
A #48 jet with a piece of 26AWG wire shoved through it, would leave (0.180955737 mm^2 - 0.177190538 mm^2 =) 0.003765199 mm^2 worth of area for fuel to pass through (which would be equivalent to a #6.92 jet)

A (unicorn) #10 jet would (in theory) have a diameter of .10mm and a cross sectional area of 0.007853982 mm^2
A #50 jet has a diameter of 0.50mm and a cross sectional area of 0.196349541 mm^2
25AWG wire has a diameter of 0.48768mm and a cross sectional area of 0.186792645 mm^2
A #50 jet with a piece of 25AWG wire shoved through it, would leave (0.196349541 mm^2 - 0.186792645 mm^2 =) 0.009556896 mm^2 worth of area for fuel to pass through (which would be equivalent to a #11.03 jet)

So what do you say? Is cramming small wires into jets a good way to meter fuel? Is this retarded or is an old trick that I independently reinvented?

If you're not convinced of the existence of 25AWG wire, I assure you it does exist and even if it didn't, I could make some by stretching 24AWG wire until its diameter shrunk to what I need. In fact this experiment will likely involve a lot of stretching/shrinking of wire to get just the right diameter.

I can solder the wire at either end to the outside of the jet so it doesn't go flying down the engine's throat.

slonomo · 11-05-2022, 08:48 PM

Here's my 2 cents on jets:

1. Use only factory jets, unmodified
2. Drilling jets is not recommended, atomization will not be as good
3. Soldering and drilling jets is not recommended for the same reason as #2
4. Never use anything inside a jet like wire, bad idea

Carbs are very precise fuel metering devices that use vacuum pressure in a venturi to atomize fuel. Any alteration in the shape of the jet will cause a negative change in the fuel atomization. What you can't see is microscopic burrs and scratches that create turbulence and ultimately takes away from the correct flow of the fuel through the orifice.

Now I'm not saying it won't run doing what you're proposing, lord knows people do stuff like this all the time and engines run. But running is different than running correctly. I recommend using the correct jets for the application. If they aren't available, then you have to do what you have to do. But you won't find a "correct procedure" for resizing jets.....because it's not a "correct procedure".

10-29-2022, 07:41 PM	#1
strantor Not Yet Wild Join Date: Jun 2020 Posts: 15	Predator 670 dual 28mm Mikuni: Jet sizes? I put a couple of these knockoff "Maikuni" 28mm carburetors on my 670 and I'm trying to figure out what jets to put in it. Anyone have a clue?
strantor Not Yet Wild Join Date: Jun 2020 Posts: 15

10-30-2022, 07:54 PM	#4
strantor Not Yet Wild Join Date: Jun 2020 Posts: 15	Re: Predator 670 dual 28mm Mikuni: Jet sizes? I don't need help with jets for the time being. I found the cause of my troubles. I'm not as big of an idiot with carb tuning as I thought I was. I'm just a big idiot in general. After hours and hours of attempting to tune, I found that the engine was running on only one cylinder. I had fuel, I had spark, and I had compression, but the carb wasn't sucking any air. Even at WOT, put my hand over it, no vacuum whatsoever. Broken rocker and bent pushrod. Intake valve wasn't opening.
strantor Not Yet Wild Join Date: Jun 2020 Posts: 15

11-04-2022, 12:58 PM	#9
strantor Not Yet Wild Join Date: Jun 2020 Posts: 15	Re: Predator 670 dual 28mm Mikuni: Jet sizes? Ok I got a got a new head off ebay and put on the engine. two reasons: 1. My head had small cracks in it. 2. I couldn't find stock replacement rockers anywhere and this head had rockers already on it. Now I'm waiting on pushrods from Performance670. I was told by someone in the predator 670 owners group on Facebook that a single Mikuni setup would use a #10 to a #15 idle/pilot jet. That seems unfathomably small to me, but it is backed up by some data posted here and jives with my experience that even with the smallest pilot jets in my kit (#32) it was still super rich at idle. Since this is dual carbs, by my math on the cross-sectional area, two #7 jets would be equivalent to one #10 jet, and two #10 jets would be equivalent to one #15 jet. Jets of size 7 and 10 do not exist for this carb, or maybe for any carb. I thought to fill in a jet with solder and then drill it out but that would require drill bits of size 105 or 101 respectively. Which also don't seem to exist. So I have another idea I'm going to try once I get the rods, but until then I can't do anything but sit around and ponder it, so I thought I would ask for predictions. Here's the plan, and reasoning behind it: A (unicorn) #7 jet would (in theory) have a diameter of .07mm and a cross sectional area of 0.003848451 mm^2 A #48 jet has a diameter of 0.48mm and a cross sectional area of 0.180955737 mm^2 26AWG wire has a diameter of 0.47498mm and a cross sectional area of 0.177190538 mm^2 A #48 jet with a piece of 26AWG wire shoved through it, would leave (0.180955737 mm^2 - 0.177190538 mm^2 =) 0.003765199 mm^2 worth of area for fuel to pass through (which would be equivalent to a #6.92 jet) A (unicorn) #10 jet would (in theory) have a diameter of .10mm and a cross sectional area of 0.007853982 mm^2 A #50 jet has a diameter of 0.50mm and a cross sectional area of 0.196349541 mm^2 25AWG wire has a diameter of 0.48768mm and a cross sectional area of 0.186792645 mm^2 A #50 jet with a piece of 25AWG wire shoved through it, would leave (0.196349541 mm^2 - 0.186792645 mm^2 =) 0.009556896 mm^2 worth of area for fuel to pass through (which would be equivalent to a #11.03 jet) So what do you say? Is cramming small wires into jets a good way to meter fuel? Is this retarded or is an old trick that I independently reinvented? If you're not convinced of the existence of 25AWG wire, I assure you it does exist and even if it didn't, I could make some by stretching 24AWG wire until its diameter shrunk to what I need. In fact this experiment will likely involve a lot of stretching/shrinking of wire to get just the right diameter. I can solder the wire at either end to the outside of the jet so it doesn't go flying down the engine's throat.
strantor Not Yet Wild Join Date: Jun 2020 Posts: 15

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10-29-2022, 11:19 PM	#2
slonomo Searching for The Way Join Date: Oct 2009 Location: Medina, Ohio (NEOHIO) Posts: 11,442	Re: Predator 670 dual 28mm Mikuni: Jet sizes? What size jets are in them now? What makes you think they need different jets?

10-30-2022, 11:51 PM	#5
slonomo Searching for The Way Join Date: Oct 2009 Location: Medina, Ohio (NEOHIO) Posts: 11,442	Re: Predator 670 dual 28mm Mikuni: Jet sizes? Well that would do it. With those carbs, make sure the actual venturi measures 28mm and not the opening. I've seen where Amazon or Ebay carbs from China will be advertised as 28mm carbs, but the venturi actually measures much less. The 28mm often is the measurement of the inlet side of the carb, but not the venturi. Verify that. If it's 28mm you should be ok. Also, I've used China carbs before without much luck. I have disassembled them and found metal shavings and dirt inside, so might want to take them apart and inspect. I've also found missing parts in some and damaged main jet needles or pilot jet needles from being mishandled. Just be on the lookout.

10-30-2022, 11:52 PM	#6
slonomo Searching for The Way Join Date: Oct 2009 Location: Medina, Ohio (NEOHIO) Posts: 11,442	Re: Predator 670 dual 28mm Mikuni: Jet sizes? And....you're not an idiot....you found the problem. That makes you a mechanic

11-05-2022, 08:48 PM	#10
slonomo Searching for The Way Join Date: Oct 2009 Location: Medina, Ohio (NEOHIO) Posts: 11,442	Re: Predator 670 dual 28mm Mikuni: Jet sizes? Here's my 2 cents on jets: 1. Use only factory jets, unmodified 2. Drilling jets is not recommended, atomization will not be as good 3. Soldering and drilling jets is not recommended for the same reason as #2 4. Never use anything inside a jet like wire, bad idea Carbs are very precise fuel metering devices that use vacuum pressure in a venturi to atomize fuel. Any alteration in the shape of the jet will cause a negative change in the fuel atomization. What you can't see is microscopic burrs and scratches that create turbulence and ultimately takes away from the correct flow of the fuel through the orifice. Now I'm not saying it won't run doing what you're proposing, lord knows people do stuff like this all the time and engines run. But running is different than running correctly. I recommend using the correct jets for the application. If they aren't available, then you have to do what you have to do. But you won't find a "correct procedure" for resizing jets.....because it's not a "correct procedure".