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    Re: Re-engine Buggy with a (modified) Predator 670
    #21
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    Glad your maiden run was a success.

    I wonder if the stock carb, petcock, and fuel line are big enough with the governor removed?
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    Re: Re-engine Buggy with a (modified) Predator 670
    #22
    Senior Member darwinpayne2000's Avatar
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    I noticed that Performance 670 sells a replacement fuel pump, so I checked online and it's a common problem for these engines to starve for fuel when running full-throttle. Apparently, they cheaped-out on the fuel pump. What a surprise.

    I have a spare pulse fuel pump that is better quality than the Harbor Freight model. Theoretically, that will fix my problem, but most people replaced the pulse pump with a low pressure electric pump. More consistent performance when running WOT. I'll go that route if I still have issues after the swap.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #23
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    In their defense, it was probably designed to supply enough fuel for governed RPM's. When you removed the governor you probably doubled the fuel requirement.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #24
    Senior Member darwinpayne2000's Avatar
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    I'm not sure how this topic came up, but I discovered that the Predator 670 alternator only puts out about 7 amps (84 watts). I was assuming that a big engine puts out big amps, but that's not always the case. That isn't enough amps to run my current headlamps, so I need to pick up a pair of LED bulbs, pronto. The LED bulbs will only draw about 18 watts total, vs the 110 watts used by a pair of standard headlamp bulbs. I rarely drive at night (over 40 eyes = no fun driving in the dark), but if I do, I would like as much light as possible. I might squeeze in a third LED headlamp to help out.

    I worked on several issues over the past few days. Since I haven't been driving the buggy for awhile, the annoying behaviors you get "used to" suddenly stood out. The front steering was a little sloppy from worn heim joints, so I replaced the outer pair. While I was at it, I also moved the steering points out to fix the less-than-Ackerman steering on the buggy. The Ackerman should be spot-on, but it isn't. After moving out the steering points, it's pretty close.

    My son also commented that the front shocks were a little too soft and they allowed the tires to float out instead of digging in when turning corners. So, the el cheapo Empi shocks have been replaced with a pair of Ryde Fx shocks that have been cooling their heels on the shelf.

    And finally . . . I replaced the pulse fuel pump. I had an older Walbro pump sitting in a box and Plan A was to install it on the Predator engine, but I had second thoughts and decided to try the Mikuni unit off of the Vanguard engine. Go with a known quantity, right? As I was cleaning up the Mikuni, I dropped it on the concrete floor and broke off one of the intakes. Rats!!! It's not like I could just glue it back on. Virtually nothing holds up to gasoline and road vibrations.

    Well, I'm nothing if not resourceful. I drilled out the hole and tapped it with threads. Then I grabbed a brass barb that I had laying around and threaded it to match. Voila!! Problem solved. However, after mounting the pump on the engine, it was apparent that the Walbro would be a better fit with the layout of the hoses, so I'm running the Walbro right now. We'll see how well it does out on the trails.

    Some pics showing the front-end layout, the Predator fuel pump and the Mikuni.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #25
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    At least the carb fix was easy.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #26
    Senior Member darwinpayne2000's Avatar
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    I was digging around in my parts bin the other day and I found my old Tiny Tach tachometer/hour meter. I used it in the past to measure the rpm's on my Polaris ATV's when I was adjusting the timing. I haven't needed it in a long time, but I thought it would be handy to use it with this new engine. It will keep track of the running-hours, plus show me the rpms as I scoot along the trails.

    With the tach and the new fuel pump installed, I took my buggy out for a spin yesterday afternoon. The tachometer worked great. It shows the engine running at 1500-1700 rpm as I cruise down the trails. That's much lower than I surmised. I have the same gear ratio that I used with the Vanguard engine, but it seems to take fewer rpms to move the buggy at a comfortable pace. I assume that its due to the higher torque output from the engine?

    I was less than satisfied with the fuel pump/carburetor, however. It seemed okay when I was cruising along, but I tried to tackle a steep hill and it coughed out just short of the top. If I stopped on a steep slope, the engine would also refuse to start. Part of the issue may be due to a low idle. I changed out the throttle spring and the new spring is doing it's job a little too well. The idle drops below 600 rpms and dies if I sit on idle too long. I just need to fine-tune it and spend more time in the seat to figure out where the issues really lie, but I'm concerned that this carb doesn't like steep slopes.

    I also brought along my Mikuni fuel pump as a backup. I switched it out with the Walbro, but the initial performance was worse. I couldn't go more than a few feet before the engine died. Maybe I didn't give the fuel pump enough time to sort itself out. In any case, as soon as it started to misbehave, my son basically said "can we be done now" and wanted to go back home. "You can sort it out in the garage", he said. Well, yes, but part of the reason for bringing the buggy out on the trails is to sort it out here, where I have a real-world environment. It was getting a little late, so . . . back home we went.

    To preclude any further fuel pump issues, I started looking at options. An electric fuel pump is a good option, but I'm hesitant to go that route. It sucks up some of the precious amps from the stator and the reviews on the cheap fuel pumps are a little disappointing. The most common complaint is "it stopped working after a couple of hours. That doesn't sound like fun, especially if you're out on the trails. Supposedly, the best low pressure fuel pump is the Facet Posi-flow. Good reviews on several sites, but Amazon wants $50-60 for it. Well if it works . . . maybe.

    Pulse fuel pumps are popular because they have a simple design and they are very reliable. The most recommended fuel pump is the Mikuni DF-44 (the rectangular one). There's hardly a bad word said about it -- anywhere. Reliable, rebuildable, yada, yada, yada. The only issue I found is that it's recommended for "20 horsepower and lower" engines. Not where I'm headed, although it must work with the Predator engine because that's the fuel pump sold by Performance 670. Instead, I ordered the Mikuni Dual-Round for $30. It supports dual carburetors if I decide to go that route and it will pump more way more fuel than I need to run this engine. It's supposed to be here sometime tomorrow, so that means more garage time.

    A couple of pictures of the Tiny Tach tachometer (it looks better in real-life) and the Mikuni fuel pump that I ordered.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #27
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    Are you sure it's not just a float level issue? Maybe see how high you can set it before it overflows or runs rich?
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    Re: Re-engine Buggy with a (modified) Predator 670
    #28
    Senior Member darwinpayne2000's Avatar
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    Under normal circumstances, if I disconnect the fuel line and run the engine on the fuel in the carburetor bowl, it will run for several minutes. I don't think it is a float level issue, but it wouldn't hurt to take a look. I wanted to measure the size of the venturis on the carb, so I can kill two birds with one stone by taking the carb off and checking it out.

    I've also noticed that it's a little hard to start, but that may also be a symptom of the idle setting.

    I have a replacement carb, but I wanted to get a good baseline for stock performance before I start in with major modifications.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #29
    Senior Member darwinpayne2000's Avatar
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    I was browsing for information on the Predator 670 and I came across some comments about hard starting and poor idle on the Predator. It caught my attention because I was having the same issues. The engine just didn't start and run as smoothly as the old Briggs engine. Most commenters stated that the engine was adjusted too lean in order to meet EPA emissions requirements.

    Luckily, one person figured out that the carburetor still has idle mixture screws. They're just hidden under caps (per the EPA, apparently). He popped off the caps, gave the screws an extra half-turn open and the starting/idle issues went away. He posted a picture and diagram showing where the screws are located, but you had to be registered to view the attachments. So, I registered. I still couldn't see the images because a sysadmin still needed to approve my account. I wanted to take the buggy out for a spin on Saturday and I didn't want to wait for some sysadmin, so I read through the description again. He said that he used a Dremel to cut a groove in the cap, then popped it off. After looking at a an exploded diagram of the carb, plus a little deduction, I figured out that the caps on the two "horns" on the top of the carb were the caps that needed to be removed. I used a cut-off wheel on my dremel to cut a small groove on the edge of the horn, then tapped in a small nail to pry off the caps. Voila!! There were the idle mixture screws!!

    Along with the idle mixture adjustment, I added the new Mikuni fuel pump. If you read up on the double-pumper, some people recommend that you run a return line back to the fuel tank to keep the pressure from getting too high for the carb float valve. Other people state that the pressure from the double-pumper isn't high enough to cause a problem with the float valve. So, what gives?

    I think I know the reason (as will others) A few years ago, I swapped out the Mikuni carburetor on my ATV. It was essentially the same carburetor as the old one, but I noticed that the carburetor would starve for fuel if I ran at WOT for very long. After digging into it, I noticed that the float valve on the new carburetor was labeled "2.5". The old carburetor valve seat was a "3.5". The difference? Gravity-fed vs fuel pump carburetors. The new carb was configured for a fuel pump and had a smaller diameter valve seat to control the fuel pressure. Conversely, if you add a fuel pump to an engine that was previously gravity-fed, you'll need to keep the fuel pressure low with a return line to avoid overwhelming the float valve. This won't be an issue with the Predator since it's already configured for a fuel pump.

    So, the double-pumper is mounted, per recommendations. It's a big pump, so there was no way it would go back onto the engine. Also, users recommend that the pump be mounted on the frame, not the engine, to avoid vibrations. It also needs to be mounted horizontal with the vacuum port on the bottom. That left very few places on my buggy for mounting it, so I put it in front of the gas tank. I ran one of the fuel lines to the carburetor and capped off the other line.

    Here a some pics showing the idle mixture screws and the fuel pump.
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    Re: Re-engine Buggy with a (modified) Predator 670
    #30
    Senior Member darwinpayne2000's Avatar
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    So, to continue the story . . .

    I took the buggy out Saturday to test out the new fuel pump and the idle jet settings. Changing the idle mixture was definitely an improvement. The engine now starts up easily and idles smoothly. Fuel delivery also seemed better, except . . .

    The engine would still stall out if I took a corner too sharp. That was the main symptom I saw on the last outing with the Walbro fuel pump. There was no way that this fuel pump was under-performing, so the only options were that the carburetor couldn't handle sharp turns or maybe the low oil sensor was the culprit.

    Fortunately, I had looked at a Youtube video earlier in the day where they showed how to disable the sensor (spoiler alert: disconnect the yellow wire). I started poking around in the wiring and figured out which yellow wire was THE yellow wire to the oil sensor. (Naturally, there was more than one.) I pulled the connector apart and clenched my teeth as I started the buggy. It fired right up and off I went down the trails.

    Disabling the sensor was the fix. No more stalling around the corners and the engine purred like a kitten, no matter how I drove it. I've since concluded that the Walbro (or the Mikuni) fuel pump actually fixed the fuel delivery issue, but the sensor kept cutting in and interrupting the fuel as I was running up/down steep slopes or taking sharp corners. (Thanks carburetor solenoid!!) It was a frustrating experience, but I'm glad I figured out the actual culprit and didn't chase any more phantoms.

    That still leaves the issue of oil starvation during cornering, etc. but that issue probably existed in my Briggs engine and, basically most other engines. The only fix I'm aware of is to run a dry sump and that's outside the scope (and budget) of this project.
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