DTSFab.com (Desert, Trail and Sand)
Machinery, Trailers, Toyhaulers and Motorhomes => Machinery Builds => Topic started by: Nutz4sand on April 22, 2009, 09:27:48 PM
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My last rebuild on a Polaris Scrambler 400 2 smoke did a similair thing to the piston. It was jetted a tad rich. Had a pipe and reeds. And compression below 150 PSI.
I swapped a sled for a Polaris 400cc Two smoke Trailblazer and it ran out great. The compression was about 128 PSI. The machine was stone stock. I had it out in the Dunes at Silver Lake just getting around on it for fun. About a week ago I was out riding it and I thought I heard it detonate/detonating at high throttle. I stopped and looked it over but it seemed fine.
I was riding it around more and it was running out well then I went at the big hill and halfway up it started to detonate for sure. I backed off and spun around and mildy rode it back to my parked truck. It had noticeably less power.
Got it home and finnaly pulled the jug today and found this.
Weird on a stock machine. The carb is clean inside and no plugged ports. Main jet is a 230 which is what the manual calls for with my altitude and temps I was riding it in.
I am starting to think the octane ratings on the pump are bogus as all hell. 87 Octane is OK to use (according to the factory manual) but I had 92 in it.
So whatcha think?
The cylinder looks great as does the head. Nothing I can see that would have caused a hot spot to start detonation. But detonation sure got me.
Yah think the head looks like it was lean? The Plug is actually darker than the pic makes it look.
I pressure tested the crank in positive and negative pressure before I pulled it apart and it passed that with flying colors.
Never had these issues with my Banshees. But that was years ago.
The only commmon thing between these two Polarises I have (Besides being the same engine from the factory. On stock. One mildly modded) Is the gas. They each have different oil in them. Yet are scorching pistons the same way. Hmmm ???
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It's obviously the driver's fault. ;D
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It was running the carb out of fuel under a load. That's a lean burn on a piston if I've ever seen one.
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It was running the carb out of fuel under a load. That's a lean burn on a piston if I've ever seen one.
The needle and seat are not plugged and when I drained the tank to remove it the fuel flowed quite fast. I will have to look at this more though. How it may flow fuel fine at times and not at others?
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it wasn't detonation.det causes breakage not melting.
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it wasn't detonation.det causes breakage not melting.
I have heard that mild detonation can make cylinder temps go up and melt pistons? Is that not true? (Serioulsy asking)
I heard it knocking and it backed off the noise as I backed off the gas.
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Nt that I'm aware of. PREIGNITION that many mistake for detonation will tho.
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Nt that I'm aware of. PREIGNITION that many mistake for detonation will tho.
I always thought they were related and nearly the same. If not the same.
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nope. Detonation usually is due to the flame front starting at the plug normally and then the process erupts into an explosion instead of a steady flame front propagation resultin g in very high mechanical stresses but not a lot more heat.. Similar to diesel operation. Preignition is due to a hot spot or too advanced timing or lean F/A and will localize the heat in an area such as you see burned. Detonation would have likely have broken the top ring and likely a ring land instead of melting.
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I took a piston exactly like that to the guy that runs the crank shop a while back and he said straight off "looks like detonation to me". He then asked about compression ratio, jetting etc.
Looks to me like it may be bad fuel if the other things that usually cause lean condition etc can be ruled out.
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I'm with Fabr on this one.
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I took a piston exactly like that to the guy that runs the crank shop a while back and he said straight off "looks like detonation to me". He then asked about compression ratio, jetting etc.
Looks to me like it may be bad fuel if the other things that usually cause lean condition etc can be ruled out.
Looks like he doesn't know what detonation is. ;D ;D
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Detonation, BTW, is much less likely with a hemi combustion chamber. Detonation usually is more prevalent in a wedge combustion chamber.
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That is definately not a det. killed piston...
When they 1st started adding alcohol to all the gas We had alot of those, not burnt all the way thru but a bad spot on the exhaust side, running down to the ring...causing the drop in power...a couple of factors come into play here, the fuel is most likely the cause...when they add the alcohol, it takes 1.4X the fuel to keep the stroker happy, thus making it a tad lean on top...Not all 2 smoke oils are compatible with "oxygenated fuels"...In some cases I've seen this shit is up to 40% alcohol...It runs great at the lower rpms and throttle area's...But on top when things tend to get lean it leans way out and you get a melted edge,
Coulda just been a fluke... When was the last time the motor was gone thru ?
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I believe i have yet to see a 2 stroke popo "4 wheeler" that hasnt burnt at least one piston in its life...
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When a 2 smoke is running it's very best is when it will happen too! They LIKE being lean-for a bit anyway. ;D
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That is definately not a det. killed piston...
When they 1st started adding alcohol to all the gas We had alot of those, not burnt all the way thru but a bad spot on the exhaust side, running down to the ring...causing the drop in power...a couple of factors come into play here, the fuel is most likely the cause...when they add the alcohol, it takes 1.4X the fuel to keep the stroker happy, thus making it a tad lean on top...Not all 2 smoke oils are compatible with "oxygenated fuels"...In some cases I've seen this shit is up to 40% alcohol...It runs great at the lower rpms and throttle area's...But on top when things tend to get lean it leans way out and you get a melted edge,
Coulda just been a fluke... When was the last time the motor was gone thru ?
I could not tell you when this motor was gone thru last Spec. I got it used with little history. It LOOKS like I may be the first inside it. I think its the stock piston. Its a cast unit for sure.
Problem is my other machine (Scrambler 400) which started to loose a crank bearing (Polaris 400cc two smokes are famous for loosing the flywheel side bearing. Its due to how they HAVE to be put together it hurts the single bearing there. It gets stressed being pushed as the cases are assembled. Even freezing the bearing and heating the case (The clutch side has TWO bearings thank goodness)) That machine is doing the same thing to its piston. I can JUST see it starting to work the piston when I peek in the exhaust. I plan to pop the head back off and clean it up (when) if I can cure this problem. But I completely went thru that motor not that long ago.
Your talking about the alcohol content is something I have questioned before and told its not an issue. But apparently it is.
But I think Spec nailed my issue and what I suspected. I will be seeking much larger jets and or a better fuel source.
I am wondering if the fuel in my area is not extra cheesy. I buy 91 or 92 octane for my toys (93 when its at the station I am at) but My Honda Pilot tossed a couple pistons the same way. It was way built with way too high compression (170 to 200 PLUS PSI depending on which head and jug I ran. Long story. I got the parts reworked to 165 PSI max.) and I thought it was that high compression that screwed those pistons. I did better on $$$$ race gas but again I thought it was to much compression for the motor. As others told me it was not the gas I am getting today.
I actually was starting to think that the oils I used were not liking the gas and breaking down even. I tried a different brand that was alcohol compatible and it was a "little" better. But not much.
So anyone know of a fuel company that still makes "normal" GAS. Not alcohgas?
Thanx for all the input! Bill
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I run yamalube, are you premixing or oil injecting?
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Both machines are oil injected. The Scrambler runs Yamalube injection oil.
I forgot what the guy I got the Trailblazer from ran but I emailed him. Its a nice light green color but that does not tell me much about it.
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Nutz, these guys have NO idea what they are talking about. The issue is immediately obvious and Im surprised some of these guys have missed it. It is nothing to do with detonating or lean runs etc.
Mate Im amazed. I thought you would have picked it up easily. Im a bit disapointed in you over this......
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oops, almost forgot...
Your oil ring is missing.......
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Artie,are you SURE that YOU aren't missing a ring or 2? LOL!!!!! ;D ;D ;D ;D ;D ;D
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I think atie's been smokin'
OUTA THE BILLY BONG
b:1 ff: :slp jj: :m
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take it from someone who has melted more pistons than Wiseco, you are running lean. Alcohol in your fuel can make you run leaner/hotter, try looking for unoxygenated fuel, and consider the possibility you have a bad crank seal/carb boot. Also keep in mind that 2 strokes usually run the leanest/hottest in the upper midrange of the throttle, not necessarily W.O.T. Main jets will only do so much in the midrange so try adjusting your needles higher
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+1
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I think atie's been smokin'
OUTA THE BILLY BONG
3:24
http://www.youtube.com/watch?v=Pyp0yNIjWmY (http://www.youtube.com/watch?v=Pyp0yNIjWmY)
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Artie,are you SURE that YOU aren't missing a ring or 2? LOL!!!!! ;D ;D ;D ;D ;D ;D
That might explain one or two things that have been puzzling my therapist..... :-[
With regards to the above video clip... dont worry if it means jack shxt to you guys...definatley Aussie humour... still cracks me up..... you have to know the place names for it to 'do it' for ya....
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hi
running lean could be bad fuel or exhaust leak near cylinder
just my 2 cents
cheers
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hi
running lean could be bad fuel or exhaust leak near cylinder
just my 2 cents
cheers
I may just be misunderstanding but what are you meaning by an exhaust leak near cylinder causing it to be lean?
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It can suck fresh air into the cylinder on the downstroke and cause a lean condition due to the extra free oxygen in the fresh air compared to exhaust gas. On a four stroke using an O2 sensor it is IMPERATIVE that there be no exhaust leaks since a wideband O2 sensor is sensitive enough to "see" the extra O2 and will erroneously indicate the AF ratio. An engine ,2 stroke or four, will also "see" that extra O2 and run lean.
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I'am going with seals. Check your balancer oil levle. seal betweeen cylinders. To late for air test. 92 cot on an 87 oct motor will heat things up via burn rate. Still going with seals thu.
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Forgot to ask what plug was that. right one listed for stock or one step hot leaving to much heat in cylinder with to high of octane.
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I'am going with seals. Check your balancer oil levle. seal betweeen cylinders. To late for air test. 92 cot on an 87 oct motor will heat things up via burn rate. Still going with seals thu.
Oil level in balancer is fine. Its only a one popper (single cylinder so no seals between cylinders) I also did a positive and negative air pressure test on the motor before I pulled it apart (See my very first post) and will do another before I try to fire it up. (I always do on a two smokes.)
Explain why you think 92 octane will burn hotter than 87? (The only arguement I have ever heard on this is higher octane burns slower and due to this its still burning more as it leaves the cylinder. I question this as this is a bascally stock motor with moderate revs. Higher octane burns fine in racing engines turing MUCH faster speeds so I am somewhat questioning THAT explanation to me.) But I would like to hear your thoughts on this.
Champion 8902 is the plug.
As also noted the other machine I have (Polaris scrambler 400 (Same motor) With a NGKbr8es plug. NEW seals throughout. Fresh rebuild. Started to do the same thing. The only thing the motors had in common was the fuel being used.
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Crank seaks would make it way lean at idle and it would have a lean bog anytime on the throttle...it went south running hard and pulling...It most likely was fuel or just it's time...Nutz try the NGK equivilant plug...IMO they are a truer plug in terms of consistancy than the champions
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Thats the funny part Spec. The 2wd trailblazer had this champion plug. The Scrambler had a NGK in it. Both machines still did bit of the same noise at top end. The trailblazer just happened to have it affect it during a hard hill climb. Thus the piston damage.
I am pulling the jug off the Scrambler to check it as well.
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The Scrambler still has greta compression as I never was on it hard once it started to make the noise.
Both machines have the same motor but each motor is at different levels of mods.
I jsut find it funny that both seemed fine. Then one day they both started to detonate on the top end some.
The only common thing was fuel. Thats why I suspected it. I think I got a batch of crummy gas to be honest.
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I don't know if it applies in this case. I read that on a 2 stroke running alcohol you should not let off the throttle suddenly, it cuts off the cooling effect of the alcohol. I know the gas pumps here say there is at least 10% alcohol in the gas. I know alcohol adds octane. I wonder if they are just adding alcohol to get the 93 actane. Also, on a sprint car,it will melt the piston if it goes lean at all. I think alcohol in the gas is the problem. Tim
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Aside from $$$ pure race gas I need to find a gas station that simply sells gas that is still gas and just gas.
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Aside from $$$ pure race gas I need to find a gas station that simply sells gas that is still gas and just gas.
Good luck on that...
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One trick the six day riders used to use when they ran wide open for a long time was to hit the kill button with the throttle open. Fuel flowed to cool engine without ignition. Tim
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nutz, you are right race gas will not run hotter in your motor it will actually run cooler. It's a common misconception that race gas is more volatile...actually the opposite is true it burns slower than lower octane fuel which is why you need it in higher compression motors.....less chance of predetonation. You can try running a splash of race in with your regular fuel to boost the octane a bit but I think that would be a band-aid, not really fixing your problem.
You may have got a bad batch of fuel but I still think you have a lean condition probably caused by a bad crank seal. I would spend the extra few hours and change your seals while you have the motor apart, cheap insurance.
The only place I know of up here in Canada to get non oxygenated non alcohol 92 octane at the pump is esso marked marine gas. It's all I run in my sleds. If I'm going on a trip I bring enough so I don't run out. Maybe you can get it in your area? Make sure you look for the marked 92, the clear 91 has alc in it
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I guess I didn't win. Oh well just a guess. So the higher octane fule burn slower and this has no effect on the engne. I sure thought it would run hotter due to the slower burn and eginte the squish band gases that are used to cool the edge of the piston. I looked at it like retard in timming. If the squish gases are burned up the squish is ineffective. The trapped gases mixing with the egnited gase mix and help keep heat off of edge of crown as the exhaust gas dumpes into exhaust port. If the compressed mix in the squish is gone the edge of the piston on the exhaust side will melt away by lack of cooling. Why this does not happen on the intake side is because the intake ports open and cool that edge off and the intake charge is being pulled across the crown into the exhaust port for stuffing and building up on its compressen stroke once the intake and exhaust ports close. If the timing is changed and the ignition is late via burn rate the piston is to far down on the power stroke relesing the squish gases and mixing with the combustion chamber gases and making the squish ineffective. Brief but asking, wrong?
Plus is useing high octane on a low compressin motor good. I say no. I bet the four stroke guys who burn up those valves dont thnk so either. Using v-2 corrected you can use high cranking pressure to a point with a low CR and get away with pump gas.
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Yah I had some squish issues on my Honda Pilot thanx to that doofus who used to work at PCP. Dunno if he still does.
Needs to be banned from touching anything more than a plastic push car.
Polarises are NOTORIOUS for HUGE squish bands.
If you ever pop a stock Polaris two smoke apart the squish band is FRIGGING beyond huge compared to what most think they should be.
Yet most fo them run like this for years without a burp.
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Ya,
The PO's squish band is a touchy animal too...Mill off too much and it's just a boat anchor...1st couple I did ended badly...AHHH How I miss hi-school and no internet to edjimacate me :-*
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Lots of good info been put up here.
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Bummer about you PCP work. Hate to see that happen to anyone. The hugh squish may be a result of the clearence. Just a thought. If one was to tighten it up a little it would be more effective and less fuel sensitive. This of course if the clearnce is too great. It would be nice to now in degrees when the exhaust port closes and the deck clearences and head cc numbers to see what CR you have. Here's another thought. If one was ( if clearence allows)to tighten it up a little you could add a second angle based on the piston crown taper and increase the chamber cc for better power stroke and offset the rise in Cr or do this in and take 2 mm off top of exhaust port. This should yield a major increase in preformance with out hurting the motor. Rember just some thought for discussion on how to keep that edge on the piston.
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Heres a diff head #1 and #2 being the previous one both cut to 180 lbs with different cc's and clearences. This is the one I use on The wifes new pump gas motor. The diff in Cr is as follows before port modifaction #1 7.62 and #2 6.84 after port modifaction #1 7.40 and #2 6.64. #1 head is used on the wife new pump gas motor 92 oct.
I'am not saying do it just sharing info. Last pic is of the plug chop after jetting
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Monkeying with the squish band is something that should be left to a very qualified person. There are about 3 people I would trust to do this. Milling the heads is never a good idea if you taking off enough to get into the squish band. I have always taken it off the top of the cylinder. I have also never liked offset chambers.
Also, alcohol in the fuel while making it slightly leaner will not increase the EGT's as ALGOL burns much cooler than gas. Adding some alcohol cured overheating problems in My KX500 when I used it for a hill shooter. Only problem is finding oil that will mix with alcohol. Bel-ray used to make GK1, not sure what is out there now.
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I still reckon its missing an oil ring........
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No, I don't think running race gas in a low comp motor is a good idea. I've never heard of a 2 stroke burning down as a cause of burning higher octane fuel, but it does cause both a loss in performance and in wallet thickness
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Honestly I feel the problem is running lean. The thoughts about the squish band are valid tho.
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Adnoh has made really good points...
Especially about milling the block and not the heads...Getting into the squishband is bad...They make alot of different inserts to get away from milling off any thing...so If it's not what you expected you don't have to toss the parts in the recycle bin
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Here is one to compare to his style of head.
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Well the eggs on my face a good bit here. Heres a first for me.
As custom I always do a pressure and vacuum BEFORE I tear a two stroke down. Then after I button it up to check my work.
This one held 6 inches of vaccum for more then three minutes before I tore it down. It also held 6psi dropping only 1.5 psi in four minutes. (I think it was still warm/hot a bit when I did the test from a putt around the yard before I stopped it. That might have made it read good readings???)
After bolting the new top end on it I could not get it to hold neither vacuum nor pressure worth a dam. The gasket on the exhaust manifold was leaking and after removing it and replacing it and working it for a bit I finnally got the exhaust flange sealed with a different brand gasket.
But it still was leaking pressure and vacuum.
Being it passed the before teardown I just planned on a top end. I never removed the clutch cover just doing a top end.
There is a vent line on the flywheel side (for the counterbalancer oil reservoir) and I put it in a water container and it did not draw nor bubble. So that told me that side seal was holding inside its area. Soapy water all over the motor I could see no bubbles at any joints.
Sooooo I borrowed a clutch puller and popped the clutch off and the seal on the clutch side was blowing lil bubbles! :P (You can't see the clutch side seal with the drive clutch on.)
The cranks shaft could also be wiggled up and down JUST the slightest...
Sooooo I yanked the motor and gutted it and got all new bearings and seals coming and the whole motor will be "fresh" before she sees fire.
(Side note I am also upping the jetting just for sakes sake. I can drop it if need be after break in.)
I gotta admit I have never had a motor pass the "before teardown" vacuum and pressure test to find this later. Still thinking it passed as OK cause it was warm from driving it around a bit before I tested the motor the first time for vaccum and pressure. From here on out they will ALL be cold tests. Maybe even a cold test then a warm one If I can.
The bearings are out. The three main bearings on the crank all have a tiny bit of slop. (Scramblers have two big bearings on the clutch side and one on the other. Then a larger diameter but skinnier bearing past the counter balance area to support the flywheel. That bearing is solid but its getting replaced too.
More as events unfold.
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Well, while you have it apart you can fit an oil ring.......
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I am glad to see you found the problem.
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Well, while you have it apart you can fit an oil ring.......
I gotta ask,WHY??? ;D
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Artie thinks it needs Oil rings...
Wants it to be a 4 stroke....
Just a bit of a dill ;D
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That's strange that it pulled well with a crank seal out...My experience has been crank seals run way lean and won't get up to R's with out choking the piss out of the motor...But I guess that is the difference between the twins and tripples I run VS. the 1 lungers :-\
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Glad to see you found the colpurt. It was fun guessing thu.
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My Bro in laws oddy would only pull when it reved high, the seal would open up...
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I gotta ask,WHY??? ;D
and valves and a camshaft...... oh, and drop the oil in the fuel..... ;D
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and valves and a camshaft...... oh, and drop the oil in the fuel..... ;D
Put Mrs. artie on the line......
You need a good slap ;D