Author Topic: Spark knock, WTF?  (Read 5509 times)

corvan

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Spark knock, WTF?
« on: May 19, 2008, 07:49:46 AM »
This bike was 78.5, but was slapping the piston and had hairline cracks in skirt, I happened to have a 81.50 jug laying around that needed boring, so I took my dremel and cleaned up the ports and sent her out to be bored to 82mm, first let me complain that the guy was told to bore it to .004 clearance so I could do a long easy break-in, this guy gives it back to me and I can slide a .005 feeler guage along with the piston down the bore, a .006 will go also but it was real tight. The ring gap is at .025 well I went ahead and put her together, start it for about 60 seconds and listened for any problems. Then I checked the compression, it was at 140, I used only a single layer of the head gasket, trying to get the squishband reduced, I want to get it at .045. will need to do some milling and shaping to get it. Anyway, after a few heat cycles on this thing, I go ride her, after she is warmed up good, she will spark knock some at low to mid throttle. I am worried about it detonating and puff kaboom! Should I just go ahead and put race gas in her?
1985 fl350, 2007 honda 450r, 2008 polaris RZR

hoodlum

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Re: Spark knock, WTF?
« Reply #1 on: May 19, 2008, 12:53:24 PM »
Quote from: "corvan"
This bike was 78.5, but was slapping the piston and had hairline cracks in skirt, I happened to have a 81.50 jug laying around that needed boring, so I took my dremel and cleaned up the ports and sent her out to be bored to 82mm, first let me complain that the guy was told to bore it to .004 clearance so I could do a long easy break-in, this guy gives it back to me and I can slide a .005 feeler guage along with the piston down the bore, a .006 will go also but it was real tight. The ring gap is at .025 well I went ahead and put her together, start it for about 60 seconds and listened for any problems. Then I checked the compression, it was at 140, I used only a single layer of the head gasket, trying to get the squishband reduced, I want to get it at .045. will need to do some milling and shaping to get it. Anyway, after a few heat cycles on this thing, I go ride her, after she is warmed up good, she will spark knock some at low to mid throttle. I am worried about it detonating and puff kaboom! Should I just go ahead and put race gas in her?




140 is not high enough to run race gas...How old was the fuel you were running in it? Are you sure it was detonating?Could you have been hearing clutch noise or something else?
I am at 175 on my compression, and have been able to get by with pump gas so far....
I would say you are probably lucky he didn't bore it to .004" over,especially if you are running a wiseco piston.......That's too tight.....006" or even .007" over will be fine as long as you let it warm up good before running...Service limit on a Honda piston clearance is .0069,and they don't expand near as much as a wiseco....
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corvan

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Spark knock, WTF?
« Reply #2 on: May 19, 2008, 01:57:48 PM »
Well it sounds like a ping, it sure did not make that sound before, as a matter of fact with the stock piston the engine was much quieter, it is a sharp tinny ping sound that happens when it is warmed up and you stab the throttle about halfway. It is the same sound the old 250r used to make, I had the head modified to set the squishband at .040 and compression at 175, was hoping that would be the answer here. I am sure that setting the squish clearance to .045 here would at least help performance. Any of you pro tuners, please chime in and let me know if I am going the wrong way. I am just an old 250r guy, and dont know much about these (yet)
1985 fl350, 2007 honda 450r, 2008 polaris RZR

Adnoh

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Spark knock, WTF?
« Reply #3 on: May 19, 2008, 04:19:12 PM »
Not much up on 350's but ssince your 82mm  I chime in. By chance were you driving around in the mid range maybe mid range lean. Clearence issues. The dome portion of the pistonis smaller in dia than the skirt area to compasate for swell as it get hot. Also you should notice that the piston is accual oval. What you ask. The dia ex port to intake  has more clearence than pin side the more out of round more you ca tighten up the pin side in piston the wall clearence. Thsi will build more power and reduce the slap effect that snaps off the piston skirt. If you have old piston mic it at 10mm from skirt than dome above top ring and than just above pin in an x and y position x being same direction of pin then y at 90 deg from x you will see what i mean. wiescos swell at a greater rate than stock honda and the wiesco,s will have a greater out of round measurment. the ring gap is the key. bore it to the end gap for the heat range your going to run more the heat greatter the gap not to exceed the max clearence. Please ask around about bridge relief. the big bore will reduce the amount of bridge area and it will also sweell furture into the piston. This is to say the amount of steel is less and as it heats up it bends sooner. This is another reason for greater clearence. An air cooled motor will swell at a greater rate due there no thermal offset ( coolent assorbing some of the heat) in the jug. Heres an example
Piston OD                  
            X = Intake/Exhaust                                    Y = pin
   10 mm from skirt         3.149435                                          0
   24mm from skirt         3.149430                                      3.149375
   67mm from skirt   Below bottom ring                                     3.149340                                                                      3.149295
   79mm from skirt   above top ring                                      3.149240      3.1492425

Adnoh

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Spark knock, WTF?
« Reply #4 on: May 19, 2008, 04:26:39 PM »
That did not come out right. I'll try  this.

Adnoh

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Spark knock, WTF?
« Reply #5 on: May 19, 2008, 05:11:14 PM »
Oops got it back wards "The dia ex port to intake has more clearence than pin side the more out of round more you ca tighten up the pin side in piston the wall clearence."  Should have said ex to intake has less clearence than the pin side. This is so the clearence beween skirt and cylinder wall can be as tight as possible so the skirt will not vibrate ( piston slap) and snap off. This will allow the piston to swell out and push out in the Y direction and not sieze in the X direction. Wisco make several pistons with differenc clearences. This is why a lot of early siezures accured before they got a handle on them. As long as the rings can hold back pressure  using a good mix ( syn or semi syn)the can handle a tighter clearence. This is to say the lubracating effect can be maintained between the cylinder and piston. Any blow by and good by lub and siezure or big end damage could acure. As far as squish band I take it you were talking about clearence. the squish area should be arould 40 to 50% of the bore with the angle bieng 5 to 7deg greater than the piston crown. In some case a dual angle of (pryamid) angle will increas power and burn rate which will keep timming in place  allowing an increas in compression and lowering compression ratio, to large of band and improper angle can lead to pre enightinion. Even at low compression this is caused by an increass in compression ratio. The ratio can be changed by increas in second angle or dome area in cc's This can be calculated by knowing cc on cylinder at a time in degrees when the ex port is closed and the cc of the head. Each head is diff so be carfull.

Adnoh

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Spark knock, WTF?
« Reply #6 on: May 19, 2008, 05:19:44 PM »
One final thought on compression ratios. the greater the compression ratios on that big of bore is refered to as flash horsepower which build a lot of heat and the down side is it usally causes you to wash the piston in fuel and negating any advantage you gained by incresaing head pressure and compression ratio. This ( high CR ) also is hard on bearings and rods. On the plus side you get the angle and area right it will be a major bennifit. I would defenty pick the brains of the 350 guys.

corvan

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Spark knock, WTF?
« Reply #7 on: May 20, 2008, 05:19:18 AM »
I rode some more last night, I still hear that noise just occassionally after warmup, I was bored so I took the head off and surfaced it on a piece of glass with sandpaper, took out the lip that was there that recessed the chamber so now the chamber rolls right into gasket surface. Put it back on, compression went up to 150, Cant say that it took away the little knock noise, is it possible that this is just a characteristic of a WISECO?
1985 fl350, 2007 honda 450r, 2008 polaris RZR

Moskito

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Spark knock, WTF?
« Reply #8 on: May 20, 2008, 06:58:03 AM »
How old is the Siezco piston you're running?  It may be what you're hearing.

Do you know what the clearance between the Siezco and your jug is?  If you put it to Seizco's specs (I believe .004 - .0045), you will seize it.  For some reason they just don't like to run in Pilots at the mfg's recommended specs.

Most people have best luck with the clearance upped by about .002".

Watch that piston - since you need to run with a slightly looser tolerance, they rock and the piston skirt is prone to collapsing (only a few thou, but they do, none the less) sooner than OEM or other cast pistons.  How much sooner?  Not sure, but it does happen.

After the collapse, the exhaust side of the skirt will crack, fall off and attack the exhaust port, rings and lower end bearings.  It's not pretty.

I'm not saying that the Siezco doesn't work, they're just a bit more finicky and require a higher level of babysitting over OEM.
Moskito - Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming \'WOW-What a Ride!\'

corvan

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Spark knock, WTF?
« Reply #9 on: May 20, 2008, 11:19:09 AM »
The piston was brand new in box, dont know the build date, but it was new. It has .005-.0055 clearance. I am gonna get a egt and stop worrying about it. Was hoping someone could tell me if my jetting is in the ballpark, 48 pilot, 180 main?
1985 fl350, 2007 honda 450r, 2008 polaris RZR

stumped

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Spark knock, WTF?
« Reply #10 on: May 20, 2008, 01:47:38 PM »
What size carb. are you running?

Sounds like a Keihin carb. to me

corvan

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Spark knock, WTF?
« Reply #11 on: May 20, 2008, 02:04:22 PM »
Brand new Keihin 38 airstriker
1985 fl350, 2007 honda 450r, 2008 polaris RZR

corvan

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Spark knock, WTF?
« Reply #12 on: May 21, 2008, 05:19:21 AM »
I changed to a 52 pilot jet instead of a 48, it was a little better.
1985 fl350, 2007 honda 450r, 2008 polaris RZR