Too much compression

[elgoyoloco]

I just put another hole in a piston for the second time in a row after a full engine rebuild. Stock cylinder .5mm over wiseco pistons, noss cool head with 21cc domes, +4 degrees advanced timing, 370 mains (I went big on the mains after the first piston melt down). Setup should have been fine for pump gas. So I put the head back on and checked compression on the "good" cylinder, 180 psi. How is this possible with 21cc domes? I'm goind to measure the dome cc's I'm thinking they got marked wrong, which has cost me 4 pistons so far. Lesson learned, always check compression after rebuild. Any other ideas what could give me that much compression?

[Lowflyin]

I usually see peole running 24cc domes on pump

 

 

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[BUILDER]

What is your elevation? 370 mains is massive for your setup and 21cc domes should be good for pump gas. Sounds to me like you have a problem going on somewhere else.

[elgoyoloco]

Im at sea level, mostly glamis. I was at 330 mains and it ran perfectly for 7 long and abusive seasons. Rebuilt the topend to freshen it up (.5mm over), but also added the cool head with 21cc domes and advanced the timing +4. It took a couple rides to put a hole in the first piston, then split cases to flush crank of debris and reassembled and uped the jetting. Well I got one break in trip then the next trip same thing, same cylinder (left). I split the cases everytime and replace crank seals to ensure no leaks. The issue is definately compression, but why so high is what is weird.

[T_Shee]

Wow that's pretty wild. Have u done leak down test? Air leak wouldn't explain your high compression but you should b fouling plugs and pig rich with those main jets if there's no air leaks

 

[elgoyoloco]

Okay so I measured the domes. If I plug the spark plug hole and fill the dome I'm at 18-19cc, hard to be exact with residual oil in measuring beaker. The domes are clearly stamped 21cc. I'm beginning the think pro design cost me a bunch of wasted money. Now should I order 21's again or be safer with 22's? Or am

I measuring wrong?

[bansheefromhell]

I'd call them up and tell them to pay you for the pistons you fried

[elgoyoloco]

Yeah I have a feeling they'd tell me I should've checked compression before running it.

[elgoyoloco]

Well I remeasured seems like 10 more times. I can't get an accurate reading. I reordered a set of 21's and I'll check the difference. It's the only thing that makes sense for the compression readings I'm getting. Piston dome shape looks similar to stock. Btw, I'm at 0.044" squish if anyone knows where I should be at with stock height cylinders, crank, wiseco pistons and pro design domes? This may also be a really dumb question because it seems like it to me but your not supposed to use head gaskets with cool heads right?

[bansheefromhell]

Well I remeasured seems like 10 more times. I can't get an accurate reading. I reordered a set of 21's and I'll check the difference. It's the only thing that makes sense for the compression readings I'm getting. Piston dome shape looks similar to stock. Btw, I'm at 0.044" squish if anyone knows where I should be at with stock height cylinders, crank, wiseco pistons and pro design domes? This may also be a really dumb question because it seems like it to me but your not supposed to use head gaskets with cool heads right?

 

never heard of anyone doing that, I use o-rings

[AKheathen]

yah, 21cc should not be a problem. you are stock stroke, right? at 370, you should be getting away with 90oct, or less. are you sure on the timing? don't have a timing key, and plate? local fuel screwing you? i actually had that happen to me this summer. supposed to be 90, but my tahoe kicked into low octane, and had to cut my ridding off when it felt real lean. pissed, especially, since i fueld a few miles from the cheveron main supply tanks that feed the state. i blame it on the fact that nobody wanted to pay the extra 20cents, given the prices, even though it makes the fuel actually cheaper per mile in over 90% of vehicles. no gasket on a cool-head. what are your average readings? get some plexi and grease-seal it to the head with a hole in th center. put a plug in and measure the olume. if that is what you are doing, then your domes should be at 21cc. shit, you should be getting away with 20cc on 93. also, re-check for air-leak sources. plugged jet, cracked carb boot/intake, etc...... 370 is quite off-the-chart.

[elgoyoloco]

Okay so another full engine rebuild, new pistons, new domes (measured old and new, both the same 21cc) and reset timing to +0 degrees. Checked compression, showing 165psi. Put in a head gasket and it dropped to around 158ish, I was okay with that. Checked squish, all okay. Ran a 50/50 mix of premium and race gas just to be sure. Broke it in and began jetting process. New plugs and 340 mains, right side plug looks good, left side completely white. Kept bumping up jetting until I reached a ridiculous 400 main jet. Right side is beginning to foul, left side completely white still. Parked it to no do any further damage. Removed pipes and carbs, went to home depot and built a leak down tester using pvc piping, rubber plugs and a cooling system pressure tester. Engine holds 6 psi for more than 15 minutes, even checked it under vacuum, all okay. No air leaks. WTF, looks like the only thing left is a bad left carb. Any other ideas? Also looking at 28mm or 33mm pwks, any suggestions?

[oldskool]

IDK man,maybe check your o-rings on air screw.

[do-work-son]

You got your choke tube in and your slides in the correct carbs ?

[EvilBanshee]

you got a air leak somewhere dude