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Old 02-14-2010, 04:20 PM   #1
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Default funny sound in my engine

OK, i tune my engine right where i want it, then after awhile (i am guessing when it gets all the way up to temp) It will start sounding real strange off throttle (not necessarily idling). It's not a lean bog or a too rich bog. It doesn't necessarily sound like a 4 cycle idle either. It almost sounds like it wants to rev up but is stuttering at the same time. It is very hard to describe the sound.

But, it only does it off throttle. If i Wait a couple seconds it drops down to a normal idle. I guess when it loads up with fuel.

My first guess was it was too lean, so i richen it up on the low end some and it helped a little (about a quarter turn) If i richen it anymore it is to rich and dies. Talk about a narrow window.

Anybody want to take a stab at it??
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Old 02-14-2010, 04:56 PM   #2
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OK, i tune my engine right where i want it, then after awhile (i am guessing when it gets all the way up to temp) It will start sounding real strange off throttle (not necessarily idling). It's not a lean bog or a too rich bog. It doesn't necessarily sound like a 4 cycle idle either. It almost sounds like it wants to rev up but is stuttering at the same time. It is very hard to describe the sound.

But, it only does it off throttle. If i Wait a couple seconds it drops down to a normal idle. I guess when it loads up with fuel.

My first guess was it was too lean, so i richen it up on the low end some and it helped a little (about a quarter turn) If i richen it anymore it is to rich and dies. Talk about a narrow window.

Anybody want to take a stab at it??
to many variables ..sounds like a lean bottom end ....have you tried looking at the manual for the motor and taking it back to factory settings ? if not try that first to see if it is tuned wrong ...after that check all the usual suspects ...tank line airleaks fuel filter carb orings .....maybe the head is loose
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Old 02-14-2010, 04:59 PM   #3
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Check the clutch also, make sure the bearings are good and no shoes are hanging.
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Old 02-14-2010, 05:52 PM   #4
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One thing that threw a red flag up for me was you said you tune the engine then after it get's all the way up to temp. it changes . I would not tune an engine until it gets to full temp. Does it happen after a long straight ?
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Old 02-14-2010, 06:53 PM   #5
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I know exactly what you are hearing. There could be a few things happening. If you richened the lsn and it helped but went too far and it is still doing it, there are a couple things to try. The first thing I would do would be to drop down the heat range of the plug. If you are running hot or ultra hot, try a medium plug. You could also add a shim to the combustion chamber. That sound you are hearing is a result of preignition which means the charge in the chamber reaches flashpoint before it is supposed. In regular engines, the ignition system determines when the system fires where as these engines rely solely on heat and compression. If the ratio is too high and/or the plug is too hot, it causes detonation. You could also lower nitro content but not necessary. What I would do in order is:

Lower idle and richen both needles slightly. If problem persists, lower plug temp. If problem persists, add shim, if problem persists, check front bearing for leaks but 99% of the time, you can solve the issue by lowering idle and richening one or both needles. The plug and shim change should only happen if you can't solve it with the needle adjustments. These are the cheapest and easiest tests to perform in order. Just a tip, if you have richened the lsn to the point where it loads up in 3-4 seconds after it returns to a normal idle, the hsn is either too lean and/or the idle is too high. If you go up and down the board on the needles with no luck, try the plug first and then the shim. Let us know how you make out.
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Old 02-14-2010, 07:20 PM   #6
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when i went to check my plug when i came back from the track it was so tight i had to put channel locks on my plug wrench to get it off. But it has done it before and just ignored it because it really did not effect performance.

I do have some p4 plugs (which people recommend for the ninja anyway) and i also have some 20% nitro.

I never thought of predetonation. Interesting.

Now you said richen it up and lower the idle. I did that already and it helped a little but did not completely go away. matter of fact i richened it up more then i thought, as the plug was pretty wet when i took it out.

Will try the new plug.

You people are so helpful
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Old 02-14-2010, 07:23 PM   #7
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One thing that threw a red flag up for me was you said you tune the engine then after it get's all the way up to temp. it changes . I would not tune an engine until it gets to full temp. Does it happen after a long straight ?
doesn't really matter. I tune it after it comes up to temp.
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Old 02-14-2010, 07:27 PM   #8
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dreaux, this one is easy. What you are hearing is crackling fuel in the pipe on decel after a burst down the straight. This happens when an engine is in it's temp range and tuned only to temp, but is too lean on top and too rich on the bottom. When you run at WOT it runs out good as it leans and continues to lean up top at anything over half throttle. Then when you let off fuel enters the engine again from the rich low end, passes thru the engine into the pipe(as it always does during the power cycle) giving the crackling sound.

To set it straight, try a few hours richer up top and a few hours leaner on the bottom. This will most likely cause your idle to rise, requiring an adjustment down with the idle screw.
If it works well, try a couple more hours on both ends until the idle is steadier and the noise is gone.

Don't worry too much about this tuning mistake, I see it on 8 of 10 cars at the track.


Another method to see if your low end is in the ball park is to see how rapidly the temp lowers in the pits while idling. After about 3-4 minutes of warm up laps, bring it in, check the temp. If it is at say 230, watch the temp with your gun. It should lower a couple of degrees every few seconds, but not drop like a rock and not rise either of course!

This is just a baseline, most tuning is done by ear, with temp used for referencing changing conditions, like humidity and density only after you have a baseline temp. As long as you start a little rich and do not start lean, then don't push things too far, you never REALLY need a temp gun.


Good luck,
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Old 02-14-2010, 07:57 PM   #9
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won't be able to trouble shoot til friday. I will try richening up the top and lowering the bottome first.

both ideas sound great

but tuning should always be the first option.

to be honest it was kinda laging on the top end. Also, i never tune to temp anymore. You know the sound when it's up to temp.


and my ninja don't lag
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Old 02-14-2010, 09:42 PM   #10
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Before I ran the engine again I would take the time to remove the head and button and inspect the top of the piston along with the bottom of the button. Looking for small pits or demples in either is a direct sign of preignition like mark is talking about. I believe I remember a few months back you asking if it was alright to run the motor with only one shim, the answer is no and this is what will happen-the jx should have 1 copper and 1 aluminum shim for 30%.
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Old 02-15-2010, 06:21 PM   #11
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Lagging is possible when it is too rich, or too lean. The engine will pull harder and harder as it leans, then at some point fall down off the pipe again starving for fuel(and overheating) until you lift.

If they are overheated bad enough they will do that Baaaahhhh, Baaaaaahhhhhh, Baaaaaaahhhhhhhhhhhhh as it cools down and stops boiling the fuel(aka Vapor lock), then all of a sudden it will take off again to repeat the cycle.

If any of this has happened, I would definately look a the plug with a magnifying glass for traces of shiny metallic particles...aka, aluminum. It will look just like the metallic in paint. If there are any, pull the head to check the piston and button, etc. But in my experience the pinch will go away with high temps as well as possible pitting of the piston and button, so once this happens it's time for a repinch or piston and sleeve.

This usually happens when guys try to win drag races with their tune up, I was guilty of it myself back in the day(late 80's)...in reality races are won in the corners and when landing jumps, not down the straight.


Hope this may help
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Old 02-15-2010, 07:30 PM   #12
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Ok. Checked it, and there is no pitting. But, there is a large amount of carbon build up. This would normally be no big deal to me if i didn't clean the piston/button less then half a gallon ago.

This P/S has a little less then 2 gallons and all the pinch is gone at tdc. But it still has great compression.

Seems like it's been run a little lean but is ok.

Plug very gray but not distorted (that i can tell, i have bad eye sight)

No matter how much i try sometimes, no matter how much experience, i am just not that good of a tuner. But i still usually get a good tune.

And yes it is and has been shimmed properly.
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