EvilBanshee Posted September 7, 2011 Report Share Posted September 7, 2011 I could be wrong but with 22cc domes for a 421 is suppose to be ran on race fuel. To be on the safe side, if I was you I would buy 24cc domes & run the pump gas that your getting from your local gas station. From the looks of it, it looks like your detonation from low octane. If your going to buy new domes try the 24cc domes. I dont think that your crank is shot from the looks of the piston & domes. You could pull the cylinder off & inspect the crank for piece of mind. Good Luck Im not really sure why is would make a difference it being a stock 350 with 22cc domes or a 500cc with 22cc domes or ( equal compression) Compression and timing need higher octaine. Does matter how small or big your engine is. ( Just like a chevy 350CI or a 427ci big block.) If their both 9.5:1 engines they will run on 89 octain just fine. Quote Link to comment Share on other sites More sharing options...
252wheelieking Posted September 7, 2011 Report Share Posted September 7, 2011 ^^^I can't fully explain it as I just learned this from reading on here. Im sure somebody with more knowledge about this will chime in & explain it to you. I think it was gotta-go fast that explain this down to the Tee... Quote Link to comment Share on other sites More sharing options...
Strm Trpr Posted September 7, 2011 Report Share Posted September 7, 2011 Im not really sure why is would make a difference it being a stock 350 with 22cc domes or a 500cc with 22cc domes or ( equal compression) Compression and timing need higher octaine. Does matter how small or big your engine is. ( Just like a chevy 350CI or a 427ci big block.) If their both 9.5:1 engines they will run on 89 octain just fine. Wrap your head around this: Compression Ratio = (Total Cylinder Volume) / (Combustion Chamber Volume) A 350cc engine cut in half is 175cc's. One cylinder is going to ingest 175cc's + one 22cc dome worth of air/fuel = 197cc's. So, (197 / 22) = ~9:1 compression ratio So, now take a 500cc motor. Long story short, (250 + 22) / 22 = ~12.4:1 compression ratio. Little different, isn't it? Quote Link to comment Share on other sites More sharing options...
No Limit Powersports Posted September 7, 2011 Report Share Posted September 7, 2011 22cc dome is not the compression ratio just the volume of the head. A larger bore or longer stroke with the same volume of dome will yield a higher compression. Also a higher exhaust port will lower the compression ratio as the piston has less time to build compression. Quote Link to comment Share on other sites More sharing options...
No Limit Powersports Posted September 7, 2011 Report Share Posted September 7, 2011 Your math is a little off in this application because we are talking 2 strokes. We can't start building compression until our exhaust port is closed by the piston. Quote Link to comment Share on other sites More sharing options...
No Limit Powersports Posted September 7, 2011 Report Share Posted September 7, 2011 On the question at hand that is detonation without a doubt. Hard to tell by the picture but the domes appear to be flat domes with domed pistons that will promote detonation. A more favorable dome design might be beneficial to you. Quote Link to comment Share on other sites More sharing options...
camatv Posted September 7, 2011 Report Share Posted September 7, 2011 if its deto its the worst i have ever seen.. looks like crank is letting go or something else in the bottom end or dirt, sand, concrete etc.. from the pis it looks like more than a 22cc dome!! if you can CC that sucka and see what it really is.. the compression may have taken out the crank or some part of it.. Quote Link to comment Share on other sites More sharing options...
Strm Trpr Posted September 7, 2011 Report Share Posted September 7, 2011 Your math is a little off in this application because we are talking 2 strokes. We can't start building compression until our exhaust port is closed by the piston. I was trying to illustrate static compression ratio with everything else being equal except displacement. Wouldn't the math be correct if both the 350cc motor and the 500cc motor had identical exhaust port timings? Quote Link to comment Share on other sites More sharing options...
LS3 Machine Posted September 7, 2011 Report Share Posted September 7, 2011 In Portugal we have 95 and 98 octanes. Europe Octane ratings are different than US. 98 in Europe is like 91 in US. Trust me Ive lived in both. Quote Link to comment Share on other sites More sharing options...
RadarRacing Posted September 7, 2011 Report Share Posted September 7, 2011 I'm also for detonation. I have seen detonation worse than that a few times and it can really hammer the lower rod bearing. Check that bearing for sure. It sounds like your mainjet is plenty big for the carb size and a cub with those pipes unless you have a very odd needle, you might tell us what needle it has and pilot jet size should be in the 48-55 range. Check your squish clearance before you put it back together, or even check it with those domes and see what clearance you have. If your cyl was supposed to use for example a .022" (0.55mm) base gasket with those domes and you used a .012"(0.27mm gasket that can change your squish chearance from a safe clearance of say .040-.044" (1-1.25mm) to maybe .030" (0.75mm) which can cause detonation VERY easily. Make sure you use the correct base gasket, piston , and dome. Andy (M/M) is very good with cubs and can get you the correct domes, pistons, and gasket thickness to make that work correctly. Hope this helps, Radar Quote Link to comment Share on other sites More sharing options...
Sands Posted September 7, 2011 Author Report Share Posted September 7, 2011 I dont think both sides are damaged? I Think hes just talking about one side. I know what lean looks like. Judging by those plugs she is FAAAATT. The plugs you see in the domes are new plugs. They never ran. Quote Link to comment Share on other sites More sharing options...
Sands Posted September 7, 2011 Author Report Share Posted September 7, 2011 I could be wrong but with 22cc domes for a 421 is suppose to be ran on race fuel. To be on the safe side, if I was you I would buy 24cc domes & run the pump gas that your getting from your local gas station. From the looks of it, it looks like your detonation from low octane. If your going to buy new domes try the 24cc domes. I dont think that your crank is shot from the looks of the piston & domes. You could pull the cylinder off & inspect the crank for piece of mind. Good Luck I already pull of the cub and there is no sign of damage on crank, but I will inspect better later. I think today I will have a time to do that. Quote Link to comment Share on other sites More sharing options...
Sands Posted September 7, 2011 Author Report Share Posted September 7, 2011 On the question at hand that is detonation without a doubt. Hard to tell by the picture but the domes appear to be flat domes with domed pistons that will promote detonation. A more favorable dome design might be beneficial to you. Those domes are 22cc cub domes and I am using Wiseco 573M06800. The carbs are Keihin PWK39 with pilot jet 55 and main jet 178-180 ( I am not sure until open it ). The base gasket has 0.012" of thickness. It has more 5 degrees timing. Quote Link to comment Share on other sites More sharing options...
Sands Posted September 7, 2011 Author Report Share Posted September 7, 2011 Europe Octane ratings are different than US. 98 in Europe is like 91 in US. Trust me Ive lived in both. It shouldn't be... I think that they couldn't sell 98 octans if the fuel doesn't have it. Quote Link to comment Share on other sites More sharing options...
Sands Posted September 7, 2011 Author Report Share Posted September 7, 2011 I'm also for detonation. I have seen detonation worse than that a few times and it can really hammer the lower rod bearing. Check that bearing for sure. It sounds like your mainjet is plenty big for the carb size and a cub with those pipes unless you have a very odd needle, you might tell us what needle it has and pilot jet size should be in the 48-55 range. Check your squish clearance before you put it back together, or even check it with those domes and see what clearance you have. If your cyl was supposed to use for example a .022" (0.55mm) base gasket with those domes and you used a .012"(0.27mm gasket that can change your squish chearance from a safe clearance of say .040-.044" (1-1.25mm) to maybe .030" (0.75mm) which can cause detonation VERY easily. Make sure you use the correct base gasket, piston , and dome. Andy (M/M) is very good with cubs and can get you the correct domes, pistons, and gasket thickness to make that work correctly. Hope this helps, Radar Thanks for your reply! I already pull of the cub and right now I am not sure about squish it had because the base gasket is in the trash! about thickness. It is better use 0.022" or 0.012" base gasket? Quote Link to comment Share on other sites More sharing options...
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