I'd guess they are surface cracking defects from the casting process. The threaded hole was likely part of the casting unless you see mill marks or other machining evidence.
https://www.empcasting.com/how-to-deal-with-the-surface-defects-of-aluminum-die-casting.html

Well I don't see any parting line in the threads and it seems like it would be a whole lot more work to make a threaded sand core than to just drill and tap it afterwards.

I just bought a new head off ebay. It comes with the rocker I need and haven't been able to find.

 

I'm more concerned about the black gunk on the valve stems which indicates to me a very poor valve stem seal if any. The black muck on the heads is too much fuel to fully burn leaving carbon behind instead of making COO plus a bit of oil getting past the rings. One of the old problems with VW horizontal 4s being Hot Rodded was head temps getting too high and warping.

 

It looks like the Valve seized in the guide,
and that's what broke the Rocker.
Maybe bad overheating caused it.

The cracks are normal garbage Chinese-Castings, nothing unusual.
-------------------------------------------------------------------------------------------------
Run some of this stuff in the Gas at all times to prevent Carbon-build-up .........

A "Poly-Ether-Amine" based Fuel-Additive
works the best at cleaning Fuel-Injectors in Direct-Injection-Engines.
This is a so-called Fuel "Detergent", that is also supposed to
clean Combustion-Chamber-Deposits.

"Poly-Ether-Amine" is also known as polyethylene terephthalate,
which is also known as PET, or, Polyethylene-Glycol.

Products containing PEA .........

Stabil Marine Fuel-Izer
Stabil Pea Fuel Additive

Lucas-Oil Pea Fuel Additive

STP Pea Fuel Additive

Rislone Pea Fuel Additive

Royal Purple Pea Fuel Additive

Star Brite Pea Fuel Additive

Chevron-Techron works well too,
but I don't know what it contains .........
Content = "Trade-Secret".
.
.
.

 

RE:""running your Engine to ~5000-RPM (..) they WILL BREAK""

Generally dont agree! The heavy and large long-piston Diesels it is 100% true, but still most of personal car sized engines allow at least 7000 rpm, but so small engines as 49 cm3 mopeds allow with an ease about 9000-10 000 while the same engine boosting before the race it allows even 22 000 rpm yet for hardly limited time while race is going on, after what all the bearings are demolished and must be shifted. So, rpm are dependant on fuel ratio, cylinder ventilation factor (like windows opening, glideness/roughness, vorticles), compression, ignition system function, actual rpm. and, particularly fuel flame speed (detonation factor) and even (for two-cycle engines) of resonant exhaust period adjustments. And, of course, position of load on the curve.

 

Ok I got a got a new head off ebay and put on the engine.

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.

 

"" Is cramming small wires into jets a good way to meter fuel? ""

Yes ......... BUT ........

Soft-Copper Wires will eventually wear thin from Engine-Vibrations,
and, if it can be avoided, arbitrarily changing Jet-sizes is generally not a good plan.
You don't have 2 Oxygen-Sensors in your Exhaust-Pipes, so you're just guessing at best.

You can't tune a Carburetor strictly by Numbers, it doesn't work that way.
The requirements of the Engine vary substantially,
depending on ...... EVERY DETAIL.
Your Engine is not the same as the "other-guy's" Engine, guaranteed.

The Intake-Manifold-Design has a tremendous influence on how the Carburetor works.
You have no Intake-Plenum, and very-short-Runners,
this causes massive Spikes in the Air-Flow-Velocity through the Carburetor,
causing it to deliver more Fuel than might be expected.

In this case, the best way to "Lean-Out" the Mixture is with a Controlled-Air-Leak after the Carburetor.

You might be able to accomplish this with an Aquarium-Air-Regulator-Needle-Valve
plumbed into the Intake-Runners after the Carburetor.
Get the expensive "real" Brass-Needle-Valves, not Plastic.

Get creative, there are no Rules.

You could also put a threaded hole in the Intake-Manifold that will accept a Screw-In-Brass-Air-Jet,
or even a 1/8" NPT Brass-Pipe-Plug, which can be drilled with a set of "Number-Drills" until
You get the Mixture you're looking for.
This last idea makes for a very clean, and repeatable modification, and will never change by accident.

You might be surprised at how large the orifice needs to be.

Welding-Torch-Tip-Cleaner-Files work great for small increases in Air-Flow.
( They work for Fuel-Jets too )

This modification has the Bonus of making most Carburetors more responsive just above Idle.
The Throttle-Slides will need to be closed further than usual to compensate for
the increase in Idling-Speed caused by the new Air-Bleeds.
.
.
.

 

"" Is cramming small wires into jets a good way to meter fuel? ""

Yes ......... BUT ........

Soft-Copper Wires will eventually wear thin from Engine-Vibrations,
and, if it can be avoided, arbitrarily changing Jet-sizes is generally not a good plan.
You don't have 2 Oxygen-Sensors in your Exhaust-Pipes, so you're just guessing at best.

You can't tune a Carburetor strictly by Numbers, it doesn't work that way.
The requirements of the Engine vary substantially,
depending on ...... EVERY DETAIL.
Your Engine is not the same as the "other-guy's" Engine, guaranteed.

The Intake-Manifold-Design has a tremendous influence on how the Carburetor works.
You have no Intake-Plenum, and very-short-Runners,
this causes massive Spikes in the Air-Flow-Velocity through the Carburetor,
causing it to deliver more Fuel than might be expected.

In this case, the best way to "Lean-Out" the Mixture is with a Controlled-Air-Leak after the Carburetor.

You might be able to accomplish this with an Aquarium-Air-Regulator-Needle-Valve
plumbed into the Intake-Runners after the Carburetor.
Get the expensive "real" Brass-Needle-Valves, not Plastic.

Get creative, there are no Rules.

You could also put a threaded hole in the Intake-Manifold that will accept a Screw-In-Brass-Air-Jet,
or even a 1/8" NPT Brass-Pipe-Plug, which can be drilled with a set of "Number-Drills" until
You get the Mixture you're looking for.
This last idea makes for a very clean, and repeatable modification, and will never change by accident.

You might be surprised at how large the orifice needs to be.

Welding-Torch-Tip-Cleaner-Files work great for small increases in Air-Flow.
( They work for Fuel-Jets too )

This modification has the Bonus of making most Carburetors more responsive just above Idle.
The Throttle-Slides will need to be closed further than usual to compensate for
the increase in Idling-Speed caused by the new Air-Bleeds.
.
.
.

Coincidentally I have these handy-dandy 1/4" NPT ports that I just recently installed in my intakes for the purpose of balancing the carbs and maybe connecting them as @shortbus Recommended. I already have and could install "speed control valves" for pneumatic cylinders (just needle valves).

 

Carb jets (at least in most carbs) don't go just by the hole diameter. Many jets may have the same size hole and yet have a different number stamped on them. The countersink on the inside, the depth of the chamfer on the front side are just two things that have a bearing on the number stamped on it. The number is stamped on them after a fluid measurement test is done. I can foresee someone disagreeing with me on that.

The Mikuni and other slide carbs are called Variable Venturi carbs. Unlike a throttle blade carb their venturi is not machined into the carb bore. By restricting the slide/ throttle opening in the Mikuni type you can make the mixture richer or leaner. That at least back in the day is how it was done on that type carb, not by changing jets. I can also hear the disagreement on that one.

Did you see the dual carb manifolds they sell at that link I gave earlier? https://www.performance670.com/product-detail/predator-dual-carb-stand-offs- Compare the length of them to you manifolds. They have come up with what works for these carbs and these engines. I'd copy that as closely as I could. They have already done the research and development to find what works and works best.

 

Carb jets (at least in most carbs) don't go just by the hole diameter. Many jets may have the same size hole and yet have a different number stamped on them. The countersink on the inside, the depth of the chamfer on the front side are just two things that have a bearing on the number stamped on it. The number is stamped on them after a fluid measurement test is done. I can foresee someone disagreeing with me on that.

The Mikuni and other slide carbs are called Variable Venturi carbs. Unlike a throttle blade carb their venturi is not machined into the carb bore. By restricting the slide/ throttle opening in the Mikuni type you can make the mixture richer or leaner. That at least back in the day is how it was done on that type carb, not by changing jets. I can also hear the disagreement on that one.

Well this sounds like something I just need to tinker with and find something that works.

Did you see the dual carb manifolds they sell at that link I gave earlier? https://www.performance670.com/product-detail/predator-dual-carb-stand-offs- Compare the length of them to you manifolds. They have come up with what works for these carbs and these engines. I'd copy that as closely as I could. They have already done the research and development to find what works and works best.

Thanks, Yes I've seen them and I know others use them and they work well. But I don't have room for them. The engine is almost touching the gas tank and these standoffs would hit the tank with no carbs on them.



Adding the carbs onto the standoffs and then air filters onto the carbs, I would need to move the engine about a foot over. If it were a live axle cart that would be no problem, just move the sprocket; but this cart has a diff, so the engine is where it has to be to line up, and the gas tank is where it has to be because it's molded to be where it is. That's why I made the manifolds that take an immediate 90 degree turn going forward and aft; plenty of room to grow in those directions.

 

I added some seals onto the rods going through the tops of the carbs.



I was planning to do something of a dust cover to keep trash from falling in there but I saw a video which indicated the rubber boot on these carbs which normally couples the throttle cable to the top of the carb does more than just keep out dust. They said that this boot being missing or cracked can make your engine run poorly. So apparently it's an actual seal. So I sealed it. Now it my rotating linkage no longer rotates with the ease of a heavy butterfly valve. It has some stiction to it. So I guess I accelerated your prediction @shortbus.

The problem now, since the throttle is coupled (through the governor) via a spring, as you slowly press the accelerator nothing happens until the spring force overcomes the stiction and it jumps open to half throttle.

Going back to the original topic of this thread, I am playing with the idea of going full drive-by-wire. A throttle position potentiometer on the accelerator and a servo to rotate the throttle linkage. That would get rid of the springs and their slip-stick action and would also give me the option I've been dreaming of: different driving modes. It could be set to limit throttle not only based on engine RPM but also on ground speed. This buggy has a CVT so engine RPM doesn't necessarily translate to ground speed. I could have a "kiddie mode" where the kids can't kill themselves unless they can find a way to do it under 15mph, and I am free to kill myself at whatever speed I deem necessary to get the job done. But it would introduce a host unforeseeable (and seeable) potential failure modes. I could always put a big red mushroom-head E-stop kill switch on the dash though, in addition to a pull-type kill switch on a lanyard.

Or, maybe this air leak will be beneficial along the lines of what @LowQCab mentioned about letting some air bypass into the intake to lean it up. If I just put dust covers instead of seals, and jet the carb to account for it, maybe it will be fine. I don't know what I want to do just yet. Need to think about it some more.

 

I think you'll find the length of those standoff's were to overcome the problems inherent with Mikuni's. too close to the intake valve may be part of the overrich mixture. These Mikuni and other slide, variable venturi carbs kind of went out of fashion because of the inherent problems. To be truthful I didn't know they were still being made and sold.

People, me included, that grew up tuning blade throttle carbs had a hard time adapting to Mikuni's. The normal things didn't work.

Can I ask a question about your carbs? Is there a metering rod attached to the slide going into the main jet? The last Mikuni I worked with had that. On the order of a Carter carbs metering rod. The rod got thinner as the slide opened up letting more fuel to pass. The rod where it went into the slide had a C-clip on it to adjust the position in the jet to change the fuel mixture.

 

I think you'll find the length of those standoff's were to overcome the problems inherent with Mikuni's. too close to the intake valve may be part of the overrich mixture. These Mikuni and other slide, variable venturi carbs kind of went out of fashion because of the inherent problems. To be truthful I didn't know they were still being made and sold.

People, me included, that grew up tuning blade throttle carbs had a hard time adapting to Mikuni's. The normal things didn't work.

Can I ask a question about your carbs? Is there a metering rod attached to the slide going into the main jet? The last Mikuni I worked with had that. On the order of a Carter carbs metering rod. The rod got thinner as the slide opened up letting more fuel to pass. The rod where it went into the slide had a C-clip on it to adjust the position in the jet to change the fuel mixture.

These Mikunis (genuine and knockoff alike) are all the rage with today's small engine performance modification crowd. I didn't realize they had a rocky past. I must assume (hope?) improvements have been made over the years and they're not still the turds you knew them to be. Or maybe they're only popular because they're cheap? I thought I was upgrading and I'll cling to that hope until it's been thoroughly beaten out of me by first hand experience.

I believe that my intakes are just as long, if not longer than the standoffs that you linked to.

Yes there is a needle/rod that works just like you described with the clip and all.

 

Yes there is a needle/rod that works just like you described with the clip and all.

Then maybe this will help you - http://www.iwt.com.au/mikunicarb.htm That kind of info wasn't around back in the day. No body I was around ever knew about the different angles on the slides. There's a lot to read on his site about small engine performance and some parts he sells.

Lot of Youtube stuff too if you Google Mikuni metering rod.