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Worked up 670 tuning

C
Feb 23, 2011
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I have a 670 bored to 720 it has 44mm Mikuni round slides, crank shop twin pipes and I'm running AV 100LL. My problem is no matter what I do with the jetting it falls on its face at about 6000-6500rpm. It spits and sputters and pops and bangs. I cant get my egts over 1000* and the pto side is way cooler on average through the rpm range, I expect a bit but sometimes its 200* or so. I am going to start trying electronics ie: plug gap, coil, cdi. But I was wondering if there was some 670 gurus on here with some experience that knew a couple tricks. I'm by no means motor illiterate, I just have never owned a 670 and thought if there was some special stuff I need to know (besides the crank is made of cardboard) that someone would want to fill me in.
Thanks.
 
C
Feb 23, 2011
38
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Sorry, could have been more thorough. Im not sure of CR but it tests at 155 psi, has porting work done, again not sure of specifics. I bought the sled sight unseen because it was a great deal for the chassis alone. Right now I'm tuning at 3200 feet, but ride to 7000' at times. It has 500 mains right now, have gone down to 450's with no real change in character. Sled had 480's when i got it. It idles around 1800rpm and gets on the pipe at 3000, runs strong until around 6000, but the egt's won't pass 1000*. Any other info you think would help, let me know. Thanks.
 
S
Dec 3, 2007
184
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columbus mt
Unless your squish is below .058 you shouldn't have to run av gas. The 110 LL is designed to run at high altitudes... Above 10000 feet and at low rpms. Try pump gas and 420-430 mains needle in the middle slot ans 55 pilots 1.5 turns. Watch your plugs and egts.

My 670 has 420-410 mains with jaws twins at 7000 alt. Porting and head. Carbon valve, short intake and so on.

Also try twisting your spark plug boots on tighter.

While your at it check the snycing of your carbs when your re-jetting.
 
C
Feb 23, 2011
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I'll try everything you said. It sure would be easier if I knew what exactly was done to the head and porting without doing a tear down. As for the fuel, I've tried premium, vp, different ratios and ratings of vp, and nothing helps. A friend of mine has an ultralight aircraft with a 583, he rarely goes over 6000' and says 100LL is hands down the best fuel he's used. Do you think it's possible the spark plug gap is too big? When I was running nitrous on my 700 I had to gap the plugs cuz it was "four stroking" at higher rpm's. ie: blowing out the spark at high squeeze, high rpm.
Thanks for the tips.
 
S
Dec 3, 2007
184
13
18
columbus mt
I used to fly for a living... Beech 1900's and avgas was - is easy to get. His 583 probably turns 6000 rpm at most to get reliability out of the engine.

That's why the 583 he's using is rated at 50 horse. My 583 is about 100 horse.
What ignition years are you using? X models with dpm? Yes gap plugs at 0. 018 and try that.

Good luck
 
C
Feb 23, 2011
38
3
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I'll try the settings you recommended, with premium pump gas, you probly no better than my pal. The motor is an X but no dpm. It's kind of an abortion, 2003 ZX with a 141" metric M10, dropped and rolled belt drive, Polaris tunnel skis and all kinds of random bits and bobs. I planned to use the good chassis parts on my 02 700 renegade, but I feel like this 670 has some potential. I'll try out everything you said tonight and let you know how I make out. I appreciate all your help
 
C
Feb 23, 2011
38
3
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Why no dpm?
Because that would only help with tuning, whoever built this sled did things as outside of the box as they could, just to be different I guess. I do have a twin dually dial-a-jet that I plan on installing, but of course I have to get it running top notch before I do that.
 
S
Dec 3, 2007
184
13
18
columbus mt
9's are fine buy the solid cap ones.

I don't think the aviation fuel is causing the issue just way over jetted as long as you have an airbox on it.

Sounds like soon as you get into the mainjet circut it is laying over.
Make some changes and let us know.
 
C
Feb 23, 2011
38
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Make sure your fuel pump is supplying enough fuel.
I have two 31L/Hr. pumps on it. Of course I run one at a time, Its getting fuel.

I'm pretty sure its not solely the jetting. I've been down to 450's and it does the same thing. I've rode rich before and it doesnt feel that way. I'm about to head to the house and try out everything you suggested, I'll let you know how I make out.

Thanks for everything.

P.S- Its got Uni pods
 
C
Feb 23, 2011
38
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Nope, no water. Just fire. The f@cking thing caught on fire. I'm thinking it was jetted a bit fat. But no sense jetting it now, I'm officially done messing with it. I'm going to find myself a wrecked ZX with a 800 and go that route.
Thanks again.
 
C
Feb 23, 2011
38
3
8
The carbs were probably so rich it was blowing excessive gas back into the socks. All it needed was a burp and boom. What are you doing with the engine?

Usually its 1/2 a squirt from the primer and one pull to get going, last night I gave it four pulls and it gave a real weak huff and the next thing I know I'm hand bombing snow on my pod. This is all after going down to 430/440 mains, 55 pilots, and 1.5 turns out. I'm thinking this motor is my arch nemesis, kind of inspires me to get 150 h.p out of it, but its also kind of discouraging
I'm not sure what to do with the engine, either keep it around and give it a complete tear down to learn what all the builder did, or if the price is right I'll just sell it.

I think a bellow or two are hooped.

I had the valves apart not too long ago and everything checked out fine.
The motor only has maybe 4 hrs on it since re-build.

I've had this sled since Jan. 2010 and its been a nightmare since. When I bought it I went on a lark because the seller wanted it gone now because he was moving in two days and couldnt take it. I figured it was my dream sled. I've been running an 02 700 renegade since new and have been more than happy with it. I had NOS on it for a season, took that off and took .0040" off the head and went that route instead, and have never had an issue besides a circlip working loose and a cracked reed. The seller of this told me it had a 700 worked up to a 740 by mongoose. I was amped, great motor in a M-10'd, dropped and rolled, lightweight 144". I couldnt be happier. and he only wanted $2500, so stupid is as stupid does, I had him drop it off at my house while I was in camp and my wife paid him.

When I got home I was like a kid on christmas morning, until I opened the hood and thought to myself, "since when did a 03 700 have rotary valves....motherf*&^er". Nonetheless I still took it out and went for a spin, was running poorly so I went to the lake to tune (about 10 km ride from my house). Tuned it for a bit then decided to head home. Water temp was getting up there so I decided to dip into the deeper stuff and spin my track to get some snow on the coolers, and guess what, thats the one and only time this sled has ever got on the pipe and did the damn thing. Right into a rock. Yup, 14 kms and a smashed bulk head. There not easy to come by anymore either. By the time I got a bulkhead, put it all back together the season was pretty much over, and that takes us to now...fire. awesome.

Thats the background of this thing, thought maybe you would enjoy hearing it. It makes it all a bit less frustrating to tell it in a humerous way.

Know anybody who wants to buy a 670? I'm up in northern B.C, Fort St. John.
 
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