421 cub jetting, on 28pwk's

[sprinklerman]

Eh. We run SB CPI in the dunes all day without heat issues on a 421. Several guys run them on 421 cubs here without issues as well. I dont know the parameters of their head designs though.

[apeik]

Ya, I'm going out this weekend to do more testing, hopefully I can figure something out. I'll have domes made if it could possibly be a problem. The first set of custom domes was a complete failure, I'm sure people remember that thread.

[sprinklerman]

Ya, I'm going out this weekend to do more testing, hopefully I can figure something out. I'll have domes made if it could possibly be a problem. The first set of custom domes was a complete failure, I'm sure people remember that thread.

Thats where I'd bet the problem might be. Keep us posted.

[WINDYCITYJOHN400]

Eh. We run SB CPI in the dunes all day without heat issues on a 421. Several guys run them on 421 cubs here without issues as well. I dont know the parameters of their head designs though.

Ok, well then I would agree with the dome/detonation as the likely suspect. With timing turned down and monster jets....it's gotta be a mechanical issue.

[sprinklerman]

Ok, well then I would agree with the dome/detonation as the likely suspect. With timing turned down and monster jets....it's gotta be a mechanical issue.

Yea..something is funky.

Did you check and make sure the silencer core isn't collapsed or stopped up?

[WINDYCITYJOHN400]

With all the jetting attempts you have tried, I'd say this is starting to sound like a Trinity kit. LOL

 

Gotta be the domes. Squish may be in the 50's...but if the shape has convergence....your going to get bad detonation.

[apeik]

There's nothing plumbed wrong, all lines are clear, silencers and pipes are clear, just went back on yesterday.  Im on my second set of domes, these look, by eye, about the same as any pump gas cub dome.  In case anyone missed it ill post it again, they are the bottom domes, old one's that were weird were on top.

[apeik]

if i cant figure it out this weekend, ill have some different domes made of course.  

[possum]

With all the jetting attempts you have tried, I'd say this is starting to sound like a Trinity kit. LOL

 

Gotta be the domes. Squish may be in the 50's...but if the shape has convergence....your going to get bad detonation.

 

I agree. the squish band angle dont look correct. maybe its just the pic but i would double check the angle to the piston.

[WINDYCITYJOHN400]

I agree. the squish band angle dont look correct. maybe its just the pic but i would double check the angle to the piston.

Yep. Just going off the picture....squish looks almost flat. = Convergence = Major detonation every time.

[apeik]

They're actually more angle than a 573.piston, I put a piston in the dome before I put them in. I'm pretty sure theyre 11 degrees, these domes were run in another pump gas cub before mine I think.

[apeik]

These domes were out of a good running cub on pump gas, never overheated. Just went back and read the text.

[WINDYCITYJOHN400]

They're actually more angle than a 573.piston, I put a piston in the dome before I put them in. I'm pretty sure theyre 11 degrees, these domes were run in another pump gas cub before mine I think.

 

Get out the clay or solder and know for sure……then you can move on to the next diagnosis.  If the squish is tighter towards the piston center on the squish band than on the outer edge…then you have found your issue.

[apeik]

It's definetly wider at the end of squish band, I have the solder still, ill measure it tomorrow. Think it was closer to .072 or more at inside of squish band

[WINDYCITYJOHN400]

It's definetly wider at the end of squish band, I have the solder still, ill measure it tomorrow. Think it was closer to .072 or more at inside of squish band

 

Ok…..on to the next best guess. LOL