GX390 Build

ThunderKart79

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I think your 390 is a thing of beauty I actually have filled down all of my stock rods as far as the dipper. I have also filled in between the cooling fin on my block and head on my 390. To produce more tourqe your engine has to be more efficient and your 390 is going to be a tourqe monster. I will be taking my engine apart again so I can file my dipper like yours. Back in 2001 Yamaha had a sled called an SRX 700 it was a 700 triple and they are still one of the fastest NA snowmobiles ever produced. That was also the last year they produced it then they went to four strokes. To correct myself the SRX is one of the quickest sleds ever maid a good running one would get to 100 before you knew what happened the mid range was sick. I have friends back in Maine that still race the same ones they bought in 2001. But I have been away from Maine since 2005. If you get a chance I would love to hear your ideas about snowmobile improvements. I am also ordering some ceramic coating and bearings for my engine did you order the same size or two thin bearings for your side cover?
 

Mammoth

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I ended up ordering two individual bearings in ceramic for the side cover purchased through CBR Bearing.

I will pm you about the snowmobile. Once I make real progress on that project I will start a thread on it.
 

65ShelbyClone

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The new welder kept popping the breaker so I installed a step up transformer that took my 240 volt single phase service and bumped it up to 480 volts. This should allow the welder to run at full capacity without tripping the breaker and cut the power bill in half.

Volts x amps = watts.

115v x 200A is the same as 230v x 100A is the same as 460v x 50A, so current going from your wall into the transformer at 230v isn't going to be reduced nor will your power bill. Rather it will increase because the welder's demand won't change and the transformer will waste some wattage due to inefficiency. That Synchrowave 300 needs at least 50A at 460v and 100A at 230v, more if it doesn't have power factor correction.
 

Mammoth

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I see. The transformer still needs 100 amps to operate the welder at max 50 amps.

I had to find a solution for the size of the cable going to the welder. A 100 amp 240 volt cable is prety big with screw in connections.
By going through the transformer I can now plug it into a regular 50 amp outlet.

I did some nice heavy practice beads yesterday and it dident trip the breaker.
It's up and running just in time as the boat weld kit should be arriving soon.
 

Mammoth

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While turning it over on the dyno it seems like its having ignition issues. Can't quite figure it out.

The instructions say to set it at 18* before top, but it acts like its firing too early. I tried at 18, 9, 2, then went past top and tried 5, 10, 15, then 20.
It fired a few times in a row at 15 and thats the best it did.

It has great spark. Now matter how slow I turn the flywheel by hand it still sparks and always where I set it. Even with a timing light and using the dyno starter it sparks where its set.

Im not sure what to think of it. Got a little mystery on my hands.

Incountered another problem of rust in the bore. I noticed a squeak sound internally and after inspection realised the bore had accumulated surface rust and was causing some drag. Tore the whole engine down and did a light bore hone, but all the rest of the parts look ok. Now I have to reassembled everything.
 

Mammoth

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Got the engine all back together and it turns over alot smoother now. Still doesn't fire correctly, but I may have found an answer. I'm thinking the magnet on the flywheel I'm using to trigger the pickup is too long.
The instructions call for a 3/8" round magnet. I think the long bar magnet on the flywheel is confusing the control module.

I ordered a magnet to try and install on the flywheel. Fingers crossed.
 

NBGX270

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Got the engine all back together and it turns over alot smoother now. Still doesn't fire correctly, but I may have found an answer. I'm thinking the magnet on the flywheel I'm using to trigger the pickup is too long.
The instructions call for a 3/8" round magnet. I think the long bar magnet on the flywheel is confusing the control module.

I ordered a magnet to try and install on the flywheel. Fingers crossed
 

NBGX270

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There's a guy on Instagram that went the MSD route, he had the same issue and I believe you're hunch is correct. He contacted ARC and they built him a flywheel with the correct magnet. I don't know if I was supposed to mention what I have mentioned but I did LOL.
 

NBGX270

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Another battery idea that I caught from the same guy is a drill battery, that's what he used very small lightweight and compact! There's even a few companies out there that build billet battery mounts to accommodate them. I personally am using a battery very similar to yours from a company called anti-gravity, the baddest battery I've ever owned!
 

rockman96

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My understanding its not the longer stroke that makes more torque its the displacement being larger that makes more torque.

It would be the same if you took a short stroke engine and bored it out. The end result would be more torque.

Shorter strokes have been proven to yield higher rpm's.
My guess why you didn't see a difference between those two different stroke cranks would be due to this oiling systems design and how bad it is.

This is an awesome thread and project! A lot of good work here, it's sure nice to have your own resources.

I feel compelled to comment about strokers... A longer stroke DOES make more torque. It's all about leverage, pure and simple. When you increase stroke, you increase leverage, which increases torque. This is a constant, no exceptions. A bigger bore does not make more torque in and of itself. As a rule, stokers will make much better bottom end power but for a given bore size they will run out of air faster at higher rpm than a short stroke will... Therefore a shorter stroke motor will build more rpm.

Torque/horsepower curves for all motors will cross at around 5250 rpm. Torque=force=work, HP=how fast an engine can do this work. Simple math, HP=Torque x rpm

It also brings up things like piston speed (additional heat), increased rod angles, increased side loading of piston faces and cylinder walls, etc., which are all down sides of stroking. This being said, I prefer strokers for the lower range fun factor. As long as you don't go crazy with it, they can be very reliable and long running.
 

65ShelbyClone

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Got the engine all back together and it turns over alot smoother now. Still doesn't fire correctly, but I may have found an answer. I'm thinking the magnet on the flywheel I'm using to trigger the pickup is too long.
The instructions call for a 3/8" round magnet. I think the long bar magnet on the flywheel is confusing the control module.

I ordered a magnet to try and install on the flywheel. Fingers crossed.

If the bar magnet is longitudinally mounted so both ends pass the pickup, then both poles are passing as well and it's probably double-triggering and doing weird things. I've gone through the same problem trying to use factory flywheel magnets to trigger EFI.

The MSD setup you have is fairly....vintage. Back then it was common to use flying magnets to trigger a Hall-effect sensor. Nowadays it's much more common for the sensor to have an embedded magnet so the trigger just has to be ferrous metal.

I feel compelled to comment about strokers... A longer stroke DOES make more torque. It's all about leverage, pure and simple. When you increase stroke, you increase leverage, which increases torque. This is a constant, no exceptions.

This is an oversimplification and mechanical advantage is a small part of the overall equation. The video Mammoth posted above is a very good explanation of why.

A bigger bore does not make more torque in and of itself.

When it's increasing displacement, yes it does(it's not clear if that's what you mean). Force = pressure × area. Piston area increases so force on the crank throw increases. More force applied to a torque arm means more torque.

As a rule, stokers will make much better bottom end power

No, it's marginally better at best. Again, small part of the big picture.

but for a given bore size they will run out of air faster at higher rpm than a short stroke will... Therefore a shorter stroke motor will build more rpm.

All else being constant, how does one "run out of air" before the other?

Torque=force=work,

No, torque is force applied specifically about an axis of rotation. Work is force applied over distance.

Simple math, HP=Torque x rpm

No, horsepower = (torque × RPM) ÷ 5252.
 

rockman96

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This is an oversimplification and mechanical advantage is a small part of the overall equation. The video Mammoth posted above is a very good explanation of why.
Yes, its a simplification. But, mechanical advantage is not that small a contributor. Take an 80" Shovel or Evo, and stroke it to 88". Next, take that 88"motor and big bore it to 93". On the street, tell me which one made the biggest difference with all other variables remaining the same. I promise the stroker will be more impressive in that setting.
Granted, with these small clone engines the difference will not be as defined as the Harley is (which is where most of my experience is)... For stop light to stop light fun, the best thing to do IMO is to increase bore and stroke if you can, but if you can only do one then stroke it. Now, if you are turning high rpm's for extended periods on a track, you will definitely do better with increasing the bore.

When it's increasing displacement, yes it does(it's not clear if that's what you mean). Force = pressure × area. Piston area increases so force on the crank throw increases. More force applied to a torque arm means more torque.
I agree with you. But to take it another step, with increased mechanical leverage it will be more effortless for the engine to build torque.

All else being constant, how does one "run out of air" before the other?
Big bore usually means bigger elves, better breathing, and better VE.

No, horsepower = (torque × RPM) ÷ 5252.
You are correct sir.
 

65ShelbyClone

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But, mechanical advantage is not that small a contributor. Take an 80" Shovel or Evo, and stroke it to 88". Next, take that 88"motor and big bore it to 93". On the street, tell me which one made the biggest difference with all other variables remaining the same. I promise the stroker will be more impressive in that setting.
Torque is going to increase when displacement increases regardless of how the displacement is gained.

This is about two hypothetical engines with the same displacement and different bore/stroke configurations. The difference in how those two engines perform is very, very small. That's the point I'm making.
 

Mammoth

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To make a long story short bypassing all the failed ideas and get right to the point after weeks and dozens of attempts the engine finally fired up today.

I had installed a new magnet to where I thought it would be, but it wouldn't fire. Had an electrical engineer show up today with all his tool and several oscilloscope to do a battery of tests and came to the conclusion everything checks out.

After a day of trying to fire it at 18* before top center we decided to try it at 5* degrees sense we hadn't tried it there with the new magnet, and it actually fired and idled.

Have to make a few adjustments before the next run, but its finally running. It was a lot of work.

Here is the magnet holder. Had to make it out of aluminum not steel as I came to find out. Made an equal counterweight for the opposite side of the flywheel to keep it balanced.


It dropped quite a bit of weight off the flywheel.
 

Mammoth

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I have a coupler that will connect the 1" shaft to the transmission, and after that it will have a drive shaft to a propeller. Im going to try to increase the efficiency by not running a jet and go with a propeller built into a tunnel.
The tunnel is going to be more of an arch. After researching tunnel shape I drew up a basic design. It will be a bit of an experiment as I don't know exactly how it will will perform.
 

Mammoth

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Been consumed by work seven days a week for a month so I haven't had time for anything extra. I did take possession of the boat which took longer to receive than expected, but none the less it did arrive all in order.

After looking over the pieces and all the work put into the kit I'd say its a good price for whats there. There's a lot more parts than I was expecting.

This is a Minjet Inc. Avenger and its there second generation kit.

 

Budget GoKart

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question, i wanna build a budget engine and still be able to perform better that a bolt on engine, what simple things should i do, im talking like under 150$, i can tell you know your stuff.
 
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