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    Re: Re-engine Buggy with a (modified) Predator 670
    #71
    Keeper of the Asylum K-fab's Avatar
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    So glad to see that I'm not the only one that combats sound issues.
    Quiet is SO much better - and keeps the 3606 hertz tone (my tinnitus damage point) I hear 24/7 to a minimum.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #72
    Senior Member darwinpayne2000's Avatar
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    How I spent (part) of my weekend.

    Now that the muffler has been mostly addressed, it is time to swap out the camshaft and valve springs for "more power". After looking at the camshaft options on the Performance 670 website, I went with the Predator 507 cam and the stiffer springs that use the existing keepers, etc. The camshaft should allow for more open-ended performance down the road and the valve springs can be upgraded at a later date if I want to increase the rpm's.

    The first step was pulling off the valve covers, then removing the rocker arms and the push rods. Easy peasy. The second part, removing the camshaft, was also easy. You pull off the side cover, align the timing marks and remove the camshaft . . . then listen to the little parts that ride on the cam lobe fall out and go everywhere. I don't know the name for the part, but they're the equivalent of valve lifters in a car engine, except these look like an inverted roofing nail and must be inserted from inside the engine. I wasn't sure how I would make them stay in place if the camshaft was missing, but I just wiped the oil off and stuck them back up in the holes and they stayed put.

    The hardest part, really was swapping out the valve springs. I have a valve spring compressor, but it's made for larger valve springs and those smaller springs kept trying to pop out. I finally put a washer on the top and that helped, but it made it hard to insert the keepers. Not much room to maneuver the keepers into place. I finally just grabbed some scrap flat steel and made a lever to push down on the spring while I dropped the keepers into place.

    I also bought the "kit" for eliminating the gasket on the engine side cover. I assumed that the gasket would get destroyed when I opened up the case. No spare gaskets laying around and I have no idea how to find a replacement gasket. Also, I'm used to using MotoSeal to seal up the crankcase on my Polaris 2-stokes, so it just made sense to me.

    I put everything back together, cranked over the engine and . . . nothing. Boy, that worried me. I kept thinking that I screwed up the alignment of the timing marks, but I gave it a shot of starter fluid and it fired right up.

    Now, I have to start over on tuning the carburetor. The new camshaft has changed the dynamics between the engine and the carb. I had to turn in the idle screw a few turns to make it run. Also, it wasn't really a rip-snorter when I took it for a quick spin around the block. I was expecting something more profound and instead, the engine ran worse than before. I thought about it and decided that the carb was running too rich. I dropped down the idle and main jet and took it for another quick spin. Much better performance.

    Which leads to another thought about the "right" carburetor for the engine. I'm using the Keihin pwk 39 because it's what I had on hand. As I dialed in the carburetor last fall, I had to keep going larger and larger on the main jet. Normally, you run a pwk 39 with a 160 main jet. Of course, that's in a 2-stroke application and 4-strokes tend to suck more air. I was up to a 200 main jet and looking to go larger. With the new camshaft, the engine ran much better after dropping it down to a 185. I also have the carburetor off a Scrambler 500 and, as luck would have it, a Mikuni TMX35. The Mikuni is intended for my Polaris 400, but it's also the same carburetor they sell on the Performance 670 website. Maybe I should test it out with the Predator engine and see how it performs. The rule-of-thumb is that a smaller carb will have better throttle response. Maybe a smaller carb is what this engine wants.

    Here are a few pictures from the camshaft swap-out. Check out the picture that shows the difference between the old and new cam lobes. The new lobes are about a millimeter taller and a much longer duration.

    IMG_2368.JPGIMG_2369.JPGIMG_2371.JPGIMG_2374.JPG
    Last edited by darwinpayne2000; 02-17-2020 at 03:12 PM.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #73
    Keeper of the Asylum K-fab's Avatar
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    Interesting cam lobe profile, with the flat section on it. (Older one)

    Smaller carb - better throttle response, usually better low end but tend to choke out at higher revs. How much WFO do you run? The smaller carb may actually be a better choice if you’re not on the go pedal constantly.

    You've the equipment to play around now.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #74
    Senior Member darwinpayne2000's Avatar
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    Just a quick note. I played around with the Keihin carb this afternoon. I bumped up the idle jet to a #40 and dropped down the main jet to a #155. Much better idle and throttle response. That cam has definitely changed the performance profile of the engine. I'm also feeling better about the Keihin carb, but I still want to try the Mikuni TMX35. Unfortunately, the weather has been hanging around 35 degrees as the high these past few days and I don't think the manual recommends that temperature for carb tuning. I can still get it in the ballpark before I take it out in a couple of weeks and do some fine tuning. Maybe test all three carbs in the garage, then take them out for a final showdown.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #75
    Keeper of the Asylum K-fab's Avatar
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    Test in garage - uh.... Keep the doors open!
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    Re: Re-engine Buggy with a (modified) Predator 670
    #76
    Senior Member darwinpayne2000's Avatar
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    Well, "test in the garage" is a little misleading. Yes, the garage door is wide open and . . . freezing! I've been playing with the carburetor and exhaust tubing in the garage, but the garage door is always open because I'm constantly starting it up, checking the idle, etc. I also drive it around the neighborhood but I keep that to a minimum. I don't want to be "that neighbor" and, also, it's really cold driving around in the buggy. Just a quick loop around the block and that's enough. After two hours outside fussing with the buggy, I'm ready to go back inside and warm up.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #77
    Senior Member jimmyg's Avatar
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    Other than the cold weather sounds like your having a ball.
    All dressed up and waiting for the beer to arrive.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #78
    Keeper of the Asylum K-fab's Avatar
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    I do NOT miss being cold.  Props to ya for doing it when it's not nice out side (not nice is anything under 65F/18C)

    I wonder what the temp changes will do to jetting?
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    Re: Re-engine Buggy with a (modified) Predator 670
    #79
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    Temps can mess with jetting a fair bit in my experience, when I was younger I rebuilt an 08 Honda CRF150R, peppy little bike. But I did it in the garage during winter in upper Michigan, and that bike was so cold blooded that I needed to turn the heat on in the garage and heat up the cylinder with a torch just to get it to sputter, but when it finally got warmer outside it ran like a champ.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #80
    Senior Member darwinpayne2000's Avatar
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    Yeah, I've seen similar behavior. When I bought my first ATV's they were a pair of Polaris 250 2-strokes. One was an older Trail Boss and the other was a brand new Trailblazer. On some days, the Trailblazer seemed to run better. Other days, the Trail Boss had the edge. It just depended on how hot or cold it was. I finally figured out how to play with the jetting and I got both dialed in a little better so they ran good between 65 and 85 degrees.

    Speaking of carburetors, I played around with the Mikuni TMX35 and the Polaris Scrambler carburetors on the buggy. Overall, they ran about the same as the Keihin, but I didn't have the same set of main jets for those carbs. I couldn't really fine tune them, plus it was only about 42 degrees for the high on Saturday. I need to order a jet pack for each carburetor and take the buggy out for some real testing, but I'm thinking that I'll end up keeping the Keihin. I didn't see any performance out of the two carburetors that made me think "whoa, this is the real deal!", compared to the Keihin.

    Also, since we're talking about carburetors. If you're tempted to pick up a cheap Chinese clone of the Mikuni or some other carburetor, I would recommend against it. Most of my experience has been with the tiny carbs that come on the kids 50 and 90 cc ATV's and I've fussed with the Chinese carburetors to no avail. I finally shelled out the bucks to get the original, branded replacement and it ran like a champ. I also watched a Youtube video where the presenter showed the differences between a knock-off Mikuni carburetor and a genuine Mikuni. Externally, they looked the same but they took short-cuts with the clone and he could never get it to run as good as the genuine thing.
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