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    Re: Re-engine Buggy with a (modified) Predator 670
    #51
    Senior Member darwinpayne2000's Avatar
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    Yeah, the more the merrier.

    Here's the background on my engine and my build-up process. The engine is a "small block", usually sold as a 14 -18 hp engine. I bought it cheap, relatively speaking, because it was a closeout model. Briggs has made some changes in their Vanguard engines over the years and mine had the older-style rocker arms. Another story for another time.

    When I got the engine, I ran it stock on my kids 2-seater go kart. The go kart came with a 10hp Tecumseh engine, so the 14hp was a nice upgrade. They lost interest in the go karts when we bought ATV's, but I thought they were fun to drive, so I moved the engine over to the single-seat go kart and started rebuilding the frame into a full-suspension minibuggy. (I have a description of my buggy in the "Show off Your Rides Here" but the pictures went "poof" thanks to Photobucket. I need to repost those pictures).

    I started chasing more horsepower by removing the governor. That helped a bit, but then I connected up with Al Hodge at Performance V-twins and I bought his rev kit. Another slight improvement. Next, I looked at the carburetor. The venturi on that 14hp carb was about the size of my finger. I ordered a carburetor for the 18hp engine and that had a very noticeable improvement.

    I also ordered a performance camshaft and, later, high compression pistons from Performance V-twins. Better, but nothing dramatic, so I revisited the carburetor setup. I built a custom intake manifold and installed a 28mm Mikuni. Yeah, it was better, but . . .. I also looked at affordablegokarts.com for some guidance and they sold a generic 32mm Mikuni for their racing engines. I gave that a try and it also helped. However, the big performance boost was switching to a 34mm Mikuni off my old Polaris Cyclone. It made a night-and-day difference in the performance. Later, I talked to the owner of affordablegokarts.com and they had started using the 34mm Mikuni flat-slide carb on their engines and they were ecstatic with the results. I stuck with the 34mm Mikuni round-slide because I already owned it and because I was realizing that the ultimate fix was more displacement.

    So, I spent a lot of money on the Vanguard, I doubled the horsepower, and it's a great trail-riding machine, but the performance on the sand dunes is lacking. I don't have that "war power" setting like the old fighter planes had to give me the extra "oomph" when I really need it. So, I'm hoping that the bigger displacement and the proper carburetor on Predator 670 will give me what I'm looking for. If needed, I can always throw in a camshaft for a little extra boost.

    Now back to the original question. Dual carbs was always an option on my old Vanguard and also on the Predator. The main issue is cost and synchronizing two carburetors. I already have a 34mm Mikuni and a 39mm Keihin. I wanted to try them first before I start spending a bunch of money.
    Last edited by darwinpayne2000; 09-20-2019 at 03:57 PM.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #52
    Senior Member darwinpayne2000's Avatar
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    So, I had to sit out the early morning while the rain poured down. Just wonderful.

    The rain tapered off by late morning so I dashed out, installed the carburetor and took the buggy for a quick drive. My impression? . . . Wow.

    I haven't tuned the carburetor yet, but that Keihin carburetor really puts the stock carburetor to shame. The acceleration was quick and smooth, even better than my old Vanguard. My plan is to take the buggy out tomorrow, brave the mud puddles (again) and dial in the carb. I think the jetting is pretty close. I played with the idle jet and moved it up from a 30 to a 35 and now if feels about right.

    The only issue I see right now is the air filter. The location of the carburetor puts it right into the muffler, so I had to make an adapter to point it up and away. Since the noise from the muffler is already an issue, I think the next task is to move the muffler towards the back of the engine.

    Here are a couple of pics of the carb and air filter.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #53
    Senior Member jimmyg's Avatar
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    All that heat off the muffler won't help also. Otherwise it sounds like your a happy camper.
    All dressed up and waiting for the beer to arrive.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #54
    Senior Member darwinpayne2000's Avatar
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    I took it out yesterday and did a formal test. Driving around the neighborhood works for a quick test, but you can't really open it up when you're surrounded by homes and small kids.

    The main jet was off by quite a bit. The jet was a 170 and when I floored it, the top-end basically died. Way too lean. Unfortunately, the largest jet I had was a 175. (Time to order some more jets.) I put in the 175 and that helped quite a bit, but I think I need to go at least another jet size larger. When I ran it at 3/4 throttle and backed-off, you could feel an acceleration bump.

    The idle jet is also off. When I was testing on Friday, I started with a #50, which is usually considered the stock jet on a PWK 39, but it was way too rich. It would hardly run. I gradually backed it down to a #35, thinking that seemed about right, but on Saturday, I idled it back to camp on 1/8 throttle for a fair distance and it burbled the whole time. When I checked the plug, it was black and sooty. I took it back down to a #30 then turned the air screw out two turns. That made a noticeable difference. Acceleration from idle to half-throttle was smoother and quicker, but it still feels like 1/8 throttle has a burble in it. The fix for that is a slide with a larger cutaway, but it's $70 for a new slide. Ouch!!

    I need to test it further, but I think I need at least a #7 slide to smooth out the idle circuit.

    Despite the carburetor woes, it still ran better than the stock carburetor. Acceleration was quicker and it felt like it had more power. I really want to see what it will do at WOT, but I need to wait for the new jets to arrive.
    Last edited by darwinpayne2000; 09-22-2019 at 11:47 AM.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #55
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    Going back a couple posts on here and saw you ran a tachometer but it was only reading like 1700rpm or so. To me, that sounds like it was reading only off one one cylinder instead of taking into account both cylinders. Just basing off that most CVTs dont stat to engage until about that much or higher. What CVT are you running?
    Last edited by JagerDanger; 09-23-2019 at 01:54 AM.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #56
    Senior Member darwinpayne2000's Avatar
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    Yeah, my settings on the tachometer were off. When I took it out on the next run, I realized that the engine sound and the tach reading didn't match. It can also be a little confusing because the Honda V-Twin has a "waste spark", so it was unclear which setting I should be using. After I played with it, I finally got the tach set up correctly. If I recall, it was running around 2800 rpm when I was cruising down the trails. I need to take a fresh look now that I have the new carburetor installed.

    The tachometer is actually quite handy. When I was working on the idle jet, I could watch the tach and see how changing the air screw would change the rpm of the engine. It wasn't always obvious just by listening to the engine.

    As far as the clutch, I'm running the Comet Duster drive/driven clutches with a black spring. There are three light, three medium and three heavy pucks in the clutch. The light pucks are there mostly to add surface grip to the clutch face. I started out with the pink spring, but that was too light once I started adding horsepower. I tried the blue spring a few years ago and it was too stiff. The black spring still seems to be the right fit at the moment, but I might reconsider the blue spring if I keep going with the engine mods.
    Last edited by darwinpayne2000; 09-23-2019 at 08:00 PM.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #57
    Senior Member darwinpayne2000's Avatar
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    The new slide with the #7 cutaway arrived last week and I finally got a chance to take the buggy out this past weekend to the sand dunes and give it a good workout.

    I was running a #30 pilot jet with the #6 slide, but I bumped it up to a #35 with the #7 slide to see how it would behave. I also bumped up the main jet to a #185.

    I took it out on the trails to dial-in the carburetor before tackling the sand dunes. The pilot jet was just right at idle, but it was still a little too rich on 1/8 throttle. I dropped it down to a #31 pilot jet and that fixed the 1/8 throttle, but it seemed a little lean on idle. It had a tendency to run at a high idle unless I bumped the throttle. I adjusted the air screw down to 1/4 turn and that helped quite a bit. I want to try the #32 jet and see if that fixes it.

    The main jet was the opposite problem. I thought a #185 might be too rich, but it's still running lean at wide-open-throttle (WOT). It's not as bad as it was, but you can feel the power drop when you get past 3/4 throttle. I installed the biggest jet I had, a #188, and that helped, but I think it needs to be closer to a #200 to get the carb dialed in.

    Riding on the dunes was also a big improvement with the modified Predator engine. The small and medium-sized dunes were no problem, but the bigger dunes are still a challenge. There's one specific dune-face that I use as a test bed for dialing in a carburetor. You drive a loop up the face of the dune, then back down. You can really feel it if the tuning on your engine is off. I was able to reach the top of the dune, but the engine was at the limit. Part of the problem was the lean main jet, but it could still use some extra horsepower to make dune riding enjoyable.

    The next steps are:

    1) Relocate/redesign the exhaust away from the air intake. That should also help with the noise from the forward exhaust.

    2) Mill the head by .040 and perform a minor port job on the ports. According to Performance 670, that should add an extra 4-5 horsepower.

    3) Swap out the camshaft for even more horsepower.

    I will probably work on the exhaust next, but the mill/port job and the camshaft will need to wait until this winter when I have some down-time (and some more money). For the milling/port job, there's a local machine shop that will mill the heads for less than what Performance 670 wants. The port job on the heads is also something I can tackle myself.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #58
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    How about boring the block out 0.020" for a bit more power? Might as well get an extra 2-5hp.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #59
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    I'm still waiting for the donuts video, or at least a little drifting..lol
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    Re: Re-engine Buggy with a (modified) Predator 670
    #60
    Senior Member darwinpayne2000's Avatar
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    I'm trying to work within a "budget" on this build, so I probably won't go all-out. At least, not initially. My original intent was to remove the governor, install a rev kit and upgrade the carburetor. I thought that would give me the biggest bang for the buck and I think that was correct. It's running really good and when I'm cruising down the trails, I'm barely stepping on the accelerator.

    The only reason/justification for putting more money into the engine is for better performance out on the sand dunes. After I bought the Polaris 400 and 500 ATV's a few years ago, the buggy performance was anemic by comparison, so I basically stopped riding it on the dunes. Now, the Predator 670 performance is pretty good, but it's still a little short of what I want.

    Looking at the options, I can gain 4-5 horsepower each by milling/porting the heads or by swapping out the camshaft. I can save some money by having the heads milled locally, rather than shipping them off to Performance 670. Likewise a mild port job is within my skillset. A mild port job won't make a huge difference, but I might as well clean up the intake/exhaust ports and smooth out the short-side radius on the heads while I have the opportunity.

    The camshaft is not so straightforward. Performance 670 offers three different camshafts for different needs. The 501 camshaft is basically for a stock engine and keeps the rpm's in the same range (3000-5000 rpm for an ungoverned engine). Three hundred bucks and you're done. The 505 camshaft is probably intended for tractor pulls, etc and is out-of-the-running as a camshaft option. That leaves the 507 camshaft. It moves the torque power curve into the 4000-6000 rpm range and max'es out the engine at 7000 rpm. What's not to like, right? Well, the 507 cam will really need a beefier valve train with steel push rods, heavier springs, more durable rocker arms, etc. That pushes up the cost considerably for the 507 camshaft solution.

    The third option, as JagerDanger pointed out, is to bore out the engine .020. I'm keeping that option in my hip pocket for now. After I put some miles on this engine and it's ready for a refresh, I can bore it out and pick up some extra horsepower as a bonus.

    Currenty, I'm leaning toward the 501 camshaft and the rev kit. My Polaris 500 (four-stroke) ATV is fairly low-revving, but it keeps up with those whiny 400's without any problem.

    As far a spinning some donuts, etc, when I'm out at the sand dunes again, I'll grab some video of the buggy in action.

    A final note: When I'm just cruising along unimproved trails, the engine is running around 3700 rpm. One of the guys following me on his ATV said that I was averaging about 35 mph. If I'm on an improved dirt or gravel road, I take it up to about 4400-4700 rpm. I don't have the exact mph, but it's probably around 50 mph. If I put it to the floor, the engine maxes-out around 5200 rpm, but I don't have the rev kit installed yet. I hardly need it. Once I get up around 4700 rpm, it's going as fast as I care to go on gravel roads. The front-end shocks are a little "springy", so the buggy tends to feel skittery if you go much faster.
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