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Old 07-11-2009, 09:18 AM   #1
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Default QUESTION FOR THE NITRO ENGINE EXPERTS !!

I have a Ninja JX.21 in a MBX6 ........this engine has been absolutley awsome and running flawless since day one with no leaking or flameouts what so ever .......I am 1/2 way through my 5th gallon of fuel and I have really tuned the motor 1 hr rich or lean since I've bought it for the first 4 gallons with no issues .....recently is has slowly started to become somewhat of a pain ......doesn't idle near as well ,flames out after a fuel stop " which it has never done once in 4 gallons " .....and overall just seem weird and fincky to tune on the bottom end .

So here is what I am getting at ....with the glow plug out ..cold .. there is no mechanical pinch what so ever but with the plug in it has a ton of compression .......the motor runs and has good power but it's ease of tuning manners have far deteriorated ..........I would not be unhappy if it were worn out at 4 1/2 gal. but I just don't want to struggle with a motor on a race day . Is this just how they start to act when they get worn out ???????

My friend has a 21-4 Nova in his car with 1/2 the compression mine does and no pinch at all that will run 45 minutes straight with no issues ..........I know all motors are different but I just need a little advice here on what to do because I don't trust this motor to finish a race anymore . Thanks guys !!!!
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Old 07-11-2009, 09:27 AM   #2
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Thats around what a friend of mine got out of his engine fuel wise/lifetime.

Otherwise, start checking all the usuals...

Air leaks, fresh glowplug, fuel and pressure lines for pinholes (or just replace 'em) blocked in-tank or external fuel filter.

I've also got someone running a +4 Nova as you mentioned and it too has nominal compression and pinch, but runs great. They've played with their metallurgy again...
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Old 07-11-2009, 09:30 AM   #3
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Originally Posted by dead1dave View Post
I have a Ninja JX.21 in a MBX6 ........this engine has been absolutley awsome and running flawless since day one with no leaking or flameouts what so ever .......I am 1/2 way through my 5th gallon of fuel and I have really tuned the motor 1 hr rich or lean since I've bought it for the first 4 gallons with no issues .....recently is has slowly started to become somewhat of a pain ......doesn't idle near as well ,flames out after a fuel stop " which it has never done once in 4 gallons " .....and overall just seem weird and fincky to tune on the bottom end .

So here is what I am getting at ....with the glow plug out ..cold .. there is no mechanical pinch what so ever but with the plug in it has a ton of compression .......the motor runs and has good power but it's ease of tuning manners have far deteriorated ..........I would not be unhappy if it were worn out at 4 1/2 gal. but I just don't want to struggle with a motor on a race day . Is this just how they start to act when they get worn out ???????

My friend has a 21-4 Nova in his car with 1/2 the compression mine does and no pinch at all that will run 45 minutes straight with no issues ..........I know all motors are different but I just need a little advice here on what to do because I don't trust this motor to finish a race anymore . Thanks guys !!!!
just my opinion ...the jx is o.s. based so it would probably be in the worn out category ....are you noticing any leakage around the front bearing ? that can be a possibility ...but other than that you may need a new piston sleeve and rod ....o.s. motors allthough easy to tune on race day ...dont have the longevity of say a nova rossi ....just my .02
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Old 07-11-2009, 09:46 AM   #4
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Check your case for pressure, check your connecting rod for stretch, check your pistons wrist pin, and the pin on your crankshaft, and check your bearings..i would think 1 or more of those can be replaced and give you a like-new engine
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Old 07-11-2009, 11:17 AM   #5
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Dave- Give it Chicken to pinch and replace the front bearing if you haven't done so already. Check rod for wear and let it rip. It sounds like the compression gets low when hot which would indicate a need for a pinch. It's worth it for you to pinch and replace the bearing. Engines are made from different grades of metals and the fact Chicken's Plus 4 has half the compression and runs good is the simple fact that the brass and chrome lining are different from the OS. The OS sleeves expand more than most under heat and pressure because of a different grade of brass and chrome and that is what you are experiencing when the engine comes to temp. I have had several Novas with over 5 gallons each and all have enough mechanical pinch to push the sleeve out of the block cold and oiled. I know you guys never preheat your engines before running them and that is the quickest way to send them to the grave. Some guys snicker at the engine heaters I have but when they last into the 10-12 gallon range with no problems and original pinch and it is the only engine I run all season. I am the one snickering at the guys who buy 3 engines in one summer just to make it by or the guy that has 6 engines and none of them run decent. It's an investment to have and use the heaters and I have a bunch in stock if you want any. Not trying to discourage you, just trying to point you in the right direction. See you at the next race buddy!
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Old 07-11-2009, 11:48 AM   #6
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Try another gallon of fuel or barrow some from another racer a tank full
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Old 07-11-2009, 12:03 PM   #7
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you said it has tons of compression left so its NOT worn out. i would go towards air leak. get new bearings, new fuel line, and reseal the motor, get a new glow plug while your at it, if you never changed the conrod i would at this point. 50-70 bucks and you should get many more gallons thru it
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Old 07-11-2009, 12:13 PM   #8
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Just because it has compression cold, does not mean it has compression hot and on the track.

Try some fresh fuel and a new plug first. If you still have problems, time for a full rebuild.
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Old 07-11-2009, 12:15 PM   #9
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Just because it has compression cold, does not mean it has compression hot and on the track.

Try some fresh fuel and a new plug first. If you still have problems, time for a full rebuild.
i missed that part, didnt see he checked it cold, thank you for clarifying that so he didnt get mistaken
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Old 07-11-2009, 12:44 PM   #10
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Did you change plug brands/temps?Is your fuel fresh and verified to run well in another car?Have you checked your head bolts, conrod, and front bearing AND FUEL TANK?Sounds like your lean on bottom or you have an air leak.Reseal the motor and carb.Take out one head shim.Install a new P3 plug and you should be good.Having compression when the engine is cold doesn't mean a whole lot.Check it when it's warmed up.Unless you have ran it very hot or seriously neglected your air filter you should have at least a couple more gallons left to go...Pulling one or two head shims will sometimes give tired engines new life because it increases compression.Hope It works out for ya. Good luck.
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Old 07-11-2009, 01:03 PM   #11
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motor has no leaks ........fuel is good , fuel line is good , tried a new p3 ........this motor obviously has less compression hot than cold but some motors like my buddy's 21-4 have less cold than mine hot ..LOL!! . It's not the tune either because I would not have been able to get 4 gallons through it without one single issue if I could not tune .

I can throw it on the track and it will run fine but it's not the motor it was for the first 4 gal. I can't have random flameouts or fuel stop issues .
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Old 07-11-2009, 01:15 PM   #12
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Unfortunately you are comparing your motor to one of the most durable ones on the planet. Nova's will run with holes in them
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Old 07-11-2009, 02:14 PM   #13
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He'll fix it for $22. Check bearings and rod. You'll want a rod at the least just for insurance. I also recommend rebuilding the carb and resealing the backplate and carb base.
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Old 07-11-2009, 03:00 PM   #14
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during fuel stops ?? im not trying to be weird here but do you blip the gas everytime refuel ?? or let me say constant throtle while they refueling it
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Old 07-11-2009, 04:14 PM   #15
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during fuel stops ?? im not trying to be weird here but do you blip the gas everytime refuel ?? or let me say constant throtle while they refueling it
idk if your tryin to say this or not...

but when refueling you dont wanna touch the throttle, you can cause air to get in the line and flame out,
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