need some info on a Jr. Magoo's motor

[accbr]

When I was putting my motor together I bolted the cylinder on there before I ported it to see what the compression was. I had a milled stock head that gave 180 with stock porting. I ported it with the Race Logic templates, and now the compression is around 165. This is on a 81mm bore and a 61x cylinder.

 

[douglee25]

Stock cranking compression on a triple is usually in the neighborhood of 120psi.

Doug

 

[addicted]

Yeah, 160 on a tripple is like 190 on a twin.

Spoke with paul saturday. Looks like I have good cylinders and pistons with a ******** crank:banghead: :banghead: :banghead:

 

[Mile9c1]

In that case it is definitely possible to run 93 octane on 160psi in a triple. But with incorrect dome shape, timing, jetting, etc. you could blow it up.

 

[waxhead]

i would think the higher compression has more to do with crankshaft flex than anything and trying to stop the back cylinder going to advanced due to it

 

[Waste Land]

Porting definitely affects what gas you have to run. I'm not sure exactly why it affects it, but I do know that I am able to run 93 octane with the 35cc domes (185psi) that are in my stock 701, but once I have porting done I won't be able to run 93 with those same domes anymore.

Porting LOWERS compression. When the exhuast port is raised that leaves more time and port for air to leave the cylinder before compression starts. So now you need to actually get smaller domes to make up for the compression you just lost.

 

[SuperJETT]

In that case it is definitely possible to run 93 octane on 160psi in a triple. But with incorrect dome shape, timing, jetting, etc. you could blow it up.

Actually, no, because a sitdown is going to see extended wot runs so you have to go even more conservative.

 

[Big Kahuna]

Porting LOWERS compression. When the exhuast port is raised that leaves more time and port for air to leave the cylinder before compression starts. So now you need to actually get smaller domes to make up for the compression you just lost.

Not always, many port jobs never touch the roof, just the sides, if that is the case then no change in port timing which will be no change in compression.

 

[Mile9c1]

Actually, no, because a sitdown is going to see extended wot runs so you have to go even more conservative.

Actually, yes, if it is set up correctly :biggthumpup:

 

[Matt_E]

Not always, many port jobs never touch the roof, just the sides, if that is the case then no change in port timing which will be no change in compression.

Not quite correct.
Most any porting that increases exhaust area will lower compression. Freestyle port jobs do not raise the roof, you are correct. However, they widen the port. In effect, that provides more area for the air to escape. The shape of the port also greatly affects compression. Example: Paul's FS port seems to make the exhaust more rectangular (stock shape is elliptical). It's a great way of getting more area without raising the roof in the center.
However, the outer edges of the roof is indeed getting raised, with absolutely no ill effect towards port timing.

Good practical example: Three of my port jobs never touched the roof. And yet, compression dropped 10-20 psi.

 

[addicted]

Actually, yes, if it is set up correctly :biggthumpup:

Exactly. I don't understand how some guys can claim to have a dialed in motor, but can't run it wide open without it grenading. If you can't keep it pinned, your carbs aren't set up right- But that's my opinion.:Banane01:

 

[shepster]

More to it than that im affraid, you have to look at the whole story, compression, timing, fuel, jetting, bore clearance, squish etc etc. To much of one and things soon turn bad and get expensive.

 

[addicted]

Sorry to dig this one up guys. . .

Paul,

I got the beast together and running. Not bad, but I think the jetting is way off. I already changed the lows to 115's and the n&s to 1.5. Bottom end is sluggish and loads up at idle real fast. It was worse with the low jets and needles that were in there when I got it(stock). Compression is 150, so I am running 93 octane with dominator at 50:1. Here is the piston wash after 13 gallons and cleaning the tops of the pistons. Any suggestions?

Thanks,
Mike

 

[douglee25]

Sorry to dig this one up guys. . .

Paul,

I got the beast together and running. Not bad, but I think the jetting is way off. I already changed the lows to 115's and the n&s to 1.5. Bottom end is sluggish and loads up at idle real fast. It was worse with the low jets and needles that were in there when I got it(stock). Compression is 150, so I am running 93 octane with dominator at 50:1. Here is the piston wash after 13 gallons and cleaning the tops of the pistons. Any suggestions?

Thanks,
Mike

I'd say the jetting looks pretty damn good if you're reading piston wash. Maybe you need to adjust the carbs?

Doug

 

[Jr.]

Sorry to dig this one up guys. . .

Paul,

I got the beast together and running. Not bad, but I think the jetting is way off. I already changed the lows to 115's and the n&s to 1.5. Bottom end is sluggish and loads up at idle real fast. It was worse with the low jets and needles that were in there when I got it(stock). Compression is 150, so I am running 93 octane with dominator at 50:1. Here is the piston wash after 13 gallons and cleaning the tops of the pistons. Any suggestions?

Thanks,
Mike

Hi Mike, Give me a call at the shop sometime today, I'll be happy to work you thru this.

ski ya, Paul