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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member darwinpayne2000's Avatar
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    I went out camping this weekend, but I didn't take my buggy. Most of the trails near the campsite are limited to 50" and my buggy is 54". Plus, some of those trails have stupid tight turns that my buggy hates to negotiate on its own.

    However, before I left on the camp trip, the idle jets arrived from jetsrus. I excitedly opened the package and saw a bunch of . . . the wrong jets!!!

    For those of you following along at home, the Mikuni TMX35 has an "early" and a "late" design. They still sell both, for some reason. I have the early version, so I need the shorter idle jets. What I got was a #10 jet that was the correct type and a #12.5 that's intended for the late design. I really wanted to test with the #12.5, but that was not to be.

    I dropped in the #10, turned the air screw out one turn and fired it up. What I got was an unexpected result. The AFR number at idle didn't change. It's still reading about 11, which is what it was reading with the #15 jet. Turning the air screw in and out also had no effect.

    Carburetor theory would suggest that the carburetor is not running on the idle jet (0 to 1/8 throttle), but on the slide cutaway and the needle jet (1/8 to 1/4 throttle). However, I don't have the idle screw turned in very far and the throttle cable has plenty of slack. When I tested with the VM34 carb last week, changing the idle jet and the air screw settings had a noticeable effect, so I'm scratching my head on this one.

    Any ideas or suggestions?

    On the picture, the jet on the left is the correct jet and the longer jet on the right is for the "newer" carb.
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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member darwinpayne2000's Avatar
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    Back to basics!!

    I started over on dialing in the idle mixture. The Mikuni TMX35 came with a #20 idle jet. I turned the idle screw (not to be confused with the air screw) all the way out, so the slide was resting at the very bottom of the carburetor. I fired it up and discovered that if I turned the idle screw in very far, it would start running on the 1/8 throttle setting (cutaway and jet needle). I let it blubber along for a few minutes and decided that the #20 idle jet was too rich.

    I took it down to the #17.5 jet As soon as I started up the engine, the idle was running much higher and smoother, so it was starting to get within range, but two turns out on the air screw still didn't bring it into the goldilocks zone for air/fuel mixture. Still too rich.

    So, that took me back to the #15 idle jet. After running the engine with the idle screw barely turned in and the air screw out about 2 turns, it started idling smoothly. the AFR was landing in the 12.5 - 13.5 range, so it's still slightly rich. I'm betting that if I had the elusive #12.5 jet, I could get it dialed in with the air screw close to 1 turn out. That would be Nirvana.

    With the idle jet closer to optimal, I decided to take the buggy out to the Little Sahara sand dunes for some real-world testing.

    An observation. After I installed the Predator 507 camshaft from Performance670, I noticed a bump in performance, but it didn't have me saying "whoa!" when I hit the throttle. Now that I have the carburetor closer to optimal, I was saying "whoa!" a whole bunch of times out on the dunes. It pulls hard all the way from idle up through WOT. No flat spots.

    The only disappointing thing about the ride was the condition of the sand. I was riding at Sand Mountain and people had been riding the heck out it. Pure washboard. I couldn't make any good runs without getting shaken to death. I headed over to the Southeast corner of Sand Mountain and it was a little smoother. Not as rough because most of the tracks were running the same direction.

    The other issue was the bright sunlight. It was early afternoon and I couldn't read the AFR gauge very well. I knew this would be a problem and I had intended on making a shade to keep the direct light of the gauge, but I forgot about it as I was rushing along to get the buggy loaded and heading off to the sand dunes.

    Time for a McGuyver solution!! I tore some paper strips from a slice-o-pizza box in the pickup. I also had some aluminum tape in the glove box. I fashioned a shade that mostly worked. At least, I could read it from time-to-time and get some idea what the air/fuel ratio was at various throttle settings.

    . . . and the gauge indicated that I was still running a little rich. I dropped the main jet down from a #330 to a #320. That helped. It was still running rich, but I was okay with that for now. It was fairly warm today, about 88-90 degrees, and the engine is fan-cooled/oil-cooled. I was pushing it really hard, so the last thing I needed was the engine overheating and shutting down the fun.

    As I was loading the buggy, I noticed that I had a fair amount of oil leaking out of the PCV hose. I have it routed into a plastic bag with some material to absorb any oil, but it's way more than I was expecting. I need to understand why that's happening and also get a PCV canister to capture the oil.

    I also realized on the way home that I never once looked at the tachometer. I would be helpful to know if the CVT is keeping the engine at the optimal torque rpm. It feels like it, but . . . knowing is better than surmising.

    Here's a picture of my makeshift shade for the AFR gauge.
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    Last edited by darwinpayne2000; 09-13-2020 at 04:19 AM.
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    Re: Re-engine Buggy with a (modified) Predator 670
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    I don't remember if it's been said before or not, but the idealistic 14.7:1 A/F ratio is mathematically optimal but usually not in practice. In an engine, things don't usually burn 100% completely, so the normal air/fuel ratio that tends to make the most power is around 12-13 if I recall correctly, so you sound like you're in the right ballpark. Keep on keepin on
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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member darwinpayne2000's Avatar
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    According to the Internet (and they're never wrong ), the recommended setting for idle is in the 13-14.5 range. I assume because the engine would be running more efficiently at idle. For mid-range and WOT, the number is somewhere between 12-13.5. I'm pretty close. At WOT, running uphill, it was hitting in the 10-11 air/fuel range with the #330 jet. When I dropped it down, it was closer to 11-12. Mid-range was in the 12's most of the time. The seat of the pants driving also says it's pretty close.

    Of course, the numbers I'm quoting are for a car engine. An industrial engine built to lower (i.e. Chinese) specifications may be happier at the richer settings. We'll find out. I may have to tell my wife that I need to spend every weekend for the next two months out recreating on order to get the carb settings just right. Those honey-do projects might have to wait until next spring.
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    Re: Re-engine Buggy with a (modified) Predator 670
    Keeper of the Asylum K-fab's Avatar
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    Quote Originally Posted by darwinpayne2000 View Post
    According to the Internet (and they're never wrong ), the recommended setting for idle is in the 13-14.5 range. I assume because the engine would be running more efficiently at idle. For mid-range and WOT, the number is somewhere between 12-13.5. I'm pretty close. At WOT, running uphill, it was hitting in the 10-11 air/fuel range with the #330 jet. When I dropped it down, it was closer to 11-12. Mid-range was in the 12's most of the time. The seat of the pants driving also says it's pretty close.

    Of course, the numbers I'm quoting are for a car engine. An industrial engine built to lower (i.e. Chinese) specifications may be happier at the richer settings. We'll find out. I may have to tell my wife that I need to spend every weekend for the next two months out recreating on order to get the carb settings just right. Those honey-do projects might have to wait until next spring.
    LOL on that last paragraph.

    As I’m learning about tuning it jives with what you’re saying. I was recommended 13.6 as the base mapping with the PowerCommander auto tuner I’ve attached to the system. Then, from there fine tune - like you mentioned, leaner on idle, richer on top WFO.
    Sand is for fast cars

    Dirt is for fast drivers



    Yellow Dog Racing
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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member darwinpayne2000's Avatar
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    I went out camping up in the mountains this past weekend and I took my buggy this time. No surprise, it was running rich at 8000 feet.

    Two things I learned from this trip.

    If I were to do this again, I would get an AFR gauge with an LCD display. This red LED display is nearly impossible to read in the daylight, even with my new and improved shade around the gauge. Mornings and afternoons are okay if you're driving the right direction, but bright daylight doesn't work. The faint numbers complicate the dial-in process. Not only are you going stupid fast down the road as you try to read your WOT value, you're mainly focused on reading the faint numbers on the gauge and not really watching the road. I did take it for a ride after dark and the gauge numbers showed up really well! Which leads to my second observation.

    I was always told that the engine would go rich as you let off the accelerator. Well, I was seeing the opposite result. At WOT, it would be reading somewhere around 10-11 on the gauge, then go to 14-15 as I let off the gas. If that's the case, then that would explain why I was having so much trouble dialing in the carburetor using seat-of-the pants performance. When I would accelerate, then let off the gas and get a power boost, I kept thinking that I was running a main jet that was too lean and needed to go BIGGER. Instead, I should have been going smaller.

    A final note. I was also keeping an eye on the tachometer. It's a Tiny Tach clone that replaced my genuine Tiny Tach that died this past spring. Unknown to me, these units have a sealed battery that goes dead after a few years. Stupid, stupid design. In any case, the rpm value seemed low, maybe by half, at full throttle. I tried to press the buttons and adjust the settings. Totally incomprehensible. I looked up the "manual" for this device and it was just a product literature flyer. It didn't explain the 1P2r, 2P1r values at all. I finally found another device that uses the same firmware and they explained what the values mean. There's only two possible settings for this engine, depending on whether it uses a waste spark, or not. I need to play with it again and get it set correctly.
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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member jimmyg's Avatar
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    Your dialed in! Enjoy driving it already. That's an order! Lol
    All dressed up and waiting for the beer to arrive.
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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member darwinpayne2000's Avatar
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    Quote Originally Posted by jimmyg View Post
    Your dialed in! Enjoy driving it already. That's an order! Lol

    Yeah, sand dunes, here I come!!! I have . . . (glum expression) yard work this coming weekend, but I'm planning a trip to the dunes in about two weeks, then another trip in four weeks. I also want to ride around the Cricket Mountains before the snow flies. Based upon the temperatures lately, I'll have until January to make that trip.
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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member darwinpayne2000's Avatar
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    Smile
    So, I accomplished both goals for this fall. I went down to the Cricket Mountains a few weeks ago and I hit the Little Sahara sand dunes this past weekend. Last weekend was Fall Break for the kiddies (Thursday, Friday off), so a ton of people were there.

    First, the Cricket Mountains. Most of the trails are easy travel for an off-road vehicle, so it was one of those trips where you press down hard on the gas pedal and hang on for dear life!!! It was all fun and games until . . . the battery died.

    I stopped to check out a couple of flat rocks that looked like they might work as paving stones for a back yard project. Something told me to leave the engine running, but I didn't listen. When I went to start it . . . Nothing. Dead. This engine doesn't have a pull-start as a fall-back so I was stuck. Also, no cell signal, so I couldn't phone home, so-to-speak, and there was a big ridge that kept my walkie-talkie from working line-of-sight back to camp. Last time I was here at the Cricket Mountains, I ended up walking for an hour-and-a-half, uphill, to get a cell signal to call for help. Same story this time, but at least the ground was mostly flat. I called my wife, she brought me the battery booster (Note to self: Carry the Battery Booster in the toolbag) and gave me a ride back to the buggy. I fired it up and got the buggy back to camp.

    At home, I used a voltmeter to trace the problem and it turned out to be a bad circuit breaker between the charging circuit and the battery. The battery ran down because it wasn't getting charged. No idea how long that was a problem because the battery will last a long time before the charge is depleted. I replaced the circuit breaker with a glass fuse holder and everything was good again. To avoid the issue in the future, I installed a small voltmeter/ammeter combo gauge in the circuit so I can monitor what's going on.

    (An aside here. I've seen various claims about the amp rating on the stator for the Predator 670. Some sources claim that it's only 2 amps and can only charge the battery. A fairly reliable source stated that it's 7 amps. My ammeter confirmed that at full electrical load and running at 3,000 rpm, it puts out a steady 7 amps.)

    The trip to the sand dunes was fun x 2. I was worried at first because it hasn't rained in . . . forever and the sand was almost powdery. I had also thought about using a smaller set of tires on the back to gear it down a bit and get a little more climbing power, but I needn't have worried. The Predator engine with the new camshaft/carburetor setup handled any dune I cared to climb. It was a blast to run around on the dunes and hear that engine open up with that deep growl whenever it felt challenged. At one point, one of the other riders had trouble with their ATV and I was able to tow him across the dunes and back to camp without much problem.

    I know I'm just supposed to ride the buggy around and have fun, but it's also fun to look at the AFR readings and tweak the carb a bit. I dropped the main jet down to a #280. The jet needle clip is at the top of the needle and the mid-range is still running rich, mostly in the 10-11 range right now, so I ordered a leaner jet needle for the carb. I think it will run better overall if I can get the mid-range closer to 12-13. Plus, it will give me an excuse to head back to the dunes if I get the new jet needle and I have to "dial it in".

    A few technical notes:

    I finally got the tachometer to show the correct rpm's. The CVT clutch keeps the motor in the 2800-3200 rpm range when cruising down the trails, which puts the engine right where the torque curve starts to flatten out. It also showed it running at 4,200-4,400 rpm when I rapped-out the engine, which is below the 5,000 rpm limit recommended by Performance 670, so I'm well within the range.

    I tried two different ammeters on the buggy. The first was a car-style analog gauge that connects between the charging circuit and the battery. That shows the total amps in use by the buggy. The second ammeter is part of the voltmeter/ammeter combo unit (pictured). It is intended for hobbyists and is designed to show the current draw of a motor, etc. That ammeter is wired between the battery ground and the common ground for your electrical devices. It won't show you how many amps are being used to charge the battery, but that isn't a big concern for me right now. I mostly wanted to see the charging volts when the engine is running and get a heads-up if a fuse blows, etc and the engine stops charging.

    The AFR gauge/O2 sensor combo draws about 1.3 amps when the O2 sensor is heating and .78 amps when up-and-running.

    If I flip on the lights, the charging voltage drops from 14.1 to 13.1 volts. The pair of (Harbor Freight) lights pull about 3 amps. If you're looking to run a bunch of lights, you'll need the external alternator kit sold by Performance 670.

    The "instrument cluster" is due for a revamp. I keep adding on gauges and it's looking a little like Frankenstein.


    IMG_2415.jpg
    Last edited by darwinpayne2000; 10-19-2020 at 07:23 PM.
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    Re: Re-engine Buggy with a (modified) Predator 670
    Senior Member jimmyg's Avatar
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    I love it!

    High 10's is prity rich but still safe but high 11's low 12's would be better.
    All dressed up and waiting for the beer to arrive.
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