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Big Block Talk! Everything about swapping a big block engine into your cart!



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Old 04-27-2021, 07:29 PM   #11
slonomo
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Default Re: Common 670 Solutions

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Originally Posted by smackley View Post
This is great info, and great timing for my own 670 project. I was racking my head around the idle issues. Thanks for taking the time to make this.

Also, what the heck is the cranks shaft bolt thread and size? Nothing in my random bolt bucket is threading in. I googled a bunch of verbiage that I thought would turn up results easily but I only saw info for predator 212cc 3/4 shafts.
I can't find any info on the crank bolt either. I do know it's a fine thread. I believe I used the same bolt from my Vanguard engines, I think they were either 3/8-24 or 7/16-20. I'm afraid I don't have the 670 anymore so I can't check it.
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Old 04-29-2021, 09:27 PM   #12
PharmGuy85
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Default Re: Common 670 Solutions

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Originally Posted by smackley View Post
This is great info, and great timing for my own 670 project. I was racking my head around the idle issues. Thanks for taking the time to make this.

Also, what the heck is the cranks shaft bolt thread and size? Nothing in my random bolt bucket is threading in. I googled a bunch of verbiage that I thought would turn up results easily but I only saw info for predator 212cc 3/4 shafts.
With Comet 780 clutch use 3⁄8 x 24tpi x 3” Clutch Bolt

With CVTech clutch use 3⁄8 x 24tpi x 4” Clutch Bolt

That's from the Vegas Carts installation manual.
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Old 06-18-2024, 12:05 PM   #13
dpalacio1
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Default Re: Common 670 Solutions

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The first issue I ran into was the idle air screw adjusters. First off they were hidden with caps, an EPA requirement so people don't mess with the air fuel ratio and cloud the earth with carbon. Whatever. All these do is make it impossible for the diy mechanic to adjust the carb, because we all know they don't stay adjusted correctly, ever!

Most of us already know that you can take the little metal caps off to expose the idle air screws. (see blue arrows)

All I did to remove mine was just drill a small hole through the cap, then use a finish nail to pry the cap out. They popped out super easy, they are just lightly pressed in there.

This will expose the low speed air/fuel adjuster screw. The screws on my carb were way off and it wouldn't barely idle.

Adjusting them is quite easy. I first gently bottomed out the adjuster, then backed each one out 1.5 turns as a baseline. Then I started and idled the engine at about 2200 rpm, let it sit and idle to warm up to full temp. Then I adjusted the screws until the engine sounded smooth, and the idle speed reached a peak, around 2500 rpm for me. When the idle speed started to slow down is the point where I went too far, so I backed up to the previous setting. Then I backed the idle speed screw down to get it to idle around 1000 rpm. I also used a temp laser gun to measure the exhaust manifold and head temp to double check the settings. More fuel = lower temps and less fuel = higher temps. The temp maybe had a 150 degree swing from one end of the spectrum to the other. Not super scientific, but it's just something else to look at, worked for me.

The only other way to get the air fuel ratio correct is to use a sensor, which I don't own, and neither do most people. But that's the only way to get it "spot on" by the numbers.

There are carb tuning videos galore on YouTube, I suggest watching and learning as much as you can before you attempt this.
When adjusting these screws, did you go in or out and did you do them si,ultaneously or 1 at a time?
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Old 06-20-2024, 10:28 AM   #14
slonomo
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Default Re: Common 670 Solutions

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Originally Posted by dpalacio1 View Post
When adjusting these screws, did you go in or out and did you do them si,ultaneously or 1 at a time?
Set your screws to zero, all the way in, GENTLY. If you screw them in to tight you will damage the needle.

Back the screw out about 1.5 turns as a baseline, sometimes it takes 2 full turns. That will get you started.

Adjust out (unscrew) to increase fuel ratio.

Adjust in to decrease fuel ratio.

I did one side at a time, but together, if that makes sense. So adjust one circuit, use the thermometer, take note of the temp, then match the other to it. You'll need to be a good listener and know what a properly adjusted engine sounds like. If you don't have that experience, you'll have to do your best. This Vtwin engine is capable of idling very smoothly with no shaking.

Keep in mind this is all done at idle. Try and set your initial idle to 1000 rpm and then gradually back down to 800 if it will go. This is the critical part of getting this engine to work with a cvt setup for gear changes.
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Old 12-20-2024, 12:30 AM   #15
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Default Re: Common 670 Solutions

I am having a bit of an issue with my 670. My idle RPM is about 4500. I have adjusted both the idle screw, as well as the two tapped screws with nothing making a difference. I cannot seem to get it to change idle RPM, except for when I hold the choke shut, which I can then get it to about 2250. Any suggestions? I did follow the Vegas Carts instructions for modifications, so keep that in mind.
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Old 12-21-2024, 11:58 AM   #16
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Default Re: Common 670 Solutions

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Originally Posted by jscola View Post
I am having a bit of an issue with my 670. My idle RPM is about 4500. I have adjusted both the idle screw, as well as the two tapped screws with nothing making a difference. I cannot seem to get it to change idle RPM, except for when I hold the choke shut, which I can then get it to about 2250. Any suggestions? I did follow the Vegas Carts instructions for modifications, so keep that in mind.
4500 rpm is maxed out on that engine. How are you measuring rpm?

It sounds like your throttle plate is open. Make sure the throttle plate is all the way closed, there is an ajustment screw on the linkage that will adjust that. At idle, the throttle plate needs to be 100% closed, there are small holes that allow "just enough" air to enter for idle. Then, adjust the idle air screws to factor setting, about 1.5 turns out as a base adjustment. Start it, warm it up, then adjust the idle air screws per my previous posts.

Post some pics of your setup, throttle linkage, etc.
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Old 02-03-2025, 01:22 PM   #17
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Default Re: Common 670 Solutions

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Originally Posted by slonomo View Post
4500 rpm is maxed out on that engine. How are you measuring rpm?

It sounds like your throttle plate is open. Make sure the throttle plate is all the way closed, there is an ajustment screw on the linkage that will adjust that. At idle, the throttle plate needs to be 100% closed, there are small holes that allow "just enough" air to enter for idle. Then, adjust the idle air screws to factor setting, about 1.5 turns out as a base adjustment. Start it, warm it up, then adjust the idle air screws per my previous posts.

Post some pics of your setup, throttle linkage, etc.
Sorry I just saw this reply. After this post I did some investigating. I was able to find out the the modification I made pulled the throttle all the way open (had the solid rod replacement pulling the wrong direction). It runs good now, with good control.
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Old 02-03-2025, 06:40 PM   #18
slonomo
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Default Re: Common 670 Solutions

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Originally Posted by jscola View Post
Sorry I just saw this reply. After this post I did some investigating. I was able to find out the the modification I made pulled the throttle all the way open (had the solid rod replacement pulling the wrong direction). It runs good now, with good control.
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