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Old 12-25-2021, 01:56 AM   #1
reeseman44
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Default Modded 670 jetting help

Have a heavily modified 670. Looking for jetting advice.
Performance 670 billet pistons, rods, heavy valve springs, ported n polished heads, 507? Cam (if I remember correctly) billet flywheel, Performance 670 mikuni 34mm flat slide carb kit with uni air filter. Deleted governor, low pressure fuel pump sef to 4.5 psi

Florida, pretty much sea level.

When im on the throttle around 4500 rpm, it cuts out and won't go higher. Not sure what jets in it now, whatever it came with.
Also revs very high, Adjusting the idle screw is futile, as it either revs at 1500+ or not at all.
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Old 12-28-2021, 08:28 PM   #2
slonomo
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Default Re: Modded 670 jetting help

Open it up and pull the jets and record them. Go to www.jetsrus.com and order some larger sizes, start there. I always rejet one size, install new plugs, take it for a ride, then remove the plugs check for soot. Google "plug reading".

Your build is pretty extreme and I'm betting you need more carb, with that setup I'm thinking 42mm. Just a guess.
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Old 12-28-2021, 09:13 PM   #3
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Default Re: Modded 670 jetting help

My 1000 cc heavily modded Harley Sportster Ironhead which pulls 7k is using a 38 mm Mikuni. IMHO I don't think you need a larger carb. Just the one you have jetted properly.
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Old 12-29-2021, 08:23 AM   #4
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Default Re: Modded 670 jetting help

Performance 670 sells dyno built engines with 40 some HP. Those have the 34 mm flat slide mikuni on them. I would think if a bigger carb worked better they would use it.
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Old 12-29-2021, 09:47 PM   #5
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Default Re: Modded 670 jetting help

Thanks for your opinion Ray. I just have one question for you.... How many Predator 670's or Vanguard Vtwins have you worked on?

Yamaha uses a 42mm CV style mikuni on a 660cc single cylinder engine in a 4x4 quad that revs to 7k and has 40 plus horsepower and does wheelies....guess they don't know what you know about carb sizing.

There are many other engine builders that use the 670 and have used larger carbs.....just sayin. Performance 670 isn't the only one doing this you know. The kits with 34mm carb are marketed toward the novice engine builder and are made for drivability, not maximum power.

Giving a Harley 1000 comparison isn't nearly the same type of engine nor is it the same type of vehicle with the same driving conditions. And are you saying Harley guys don't use bigger carbs? Ha that's funny. My neighbor across the street has 3 Hogs and I know he's using a 45mm carb on one of them with an S&S engine.

To the OP....Yes, as I said before in an earlier post, rejet the carb you have but do it methodically. You can likely get the 34 dialed in nicely though and have really good power. If you have increased the port sizes and displacement, you likely need more cfm to reach high rpm power levels. But if you're looking for max power, you're not going to get it from a 34mm carb. Think of it as a small block ford....stock a 500 cfm carb is plenty. Add an intake, big valve heads, cam etc....you'll need a 650 cfm.....same principle.

Tuning a Mikuni is fairly technical in procedure, but not difficult. I have found a few good videos on YouTube that really help explain the process and I've found them super helpful. If you haven't seen any yet I'd be glad to reference a couple. Let me know.
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Old 12-29-2021, 10:40 PM   #6
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Default Re: Modded 670 jetting help

Your neighbors HD's are likely to be 1200-1700 cc's. The larger HD's of course can work well with the carbs in the 46mm size range. In the case of the large single cylinder again it's all at once volume changes the whole set of carb parameters. The large S&S is 124 ci or little over 2000 ccs
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Old 12-30-2021, 06:03 AM   #7
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Default Re: Modded 670 jetting help

slonomo and raydhd, I warned you about this 9 months ago, enjoy your time off
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Old 12-31-2021, 10:16 AM   #8
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Default Re: Modded 670 jetting help

Not sure what I did
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Old 12-31-2021, 01:03 PM   #9
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Default Re: Modded 670 jetting help

Quote:
Originally Posted by slonomo View Post
Not sure what I did
You never are. You got some time to think about it
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Old 01-01-2022, 09:40 PM   #10
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Default Re: Modded 670 jetting help

These two really just need to meet up, slug it out, get it over with, then have a beer and hug and be done with the bickering
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