Somethin's wrong with new cylinders

[okbeast]

I'm bout tired of this fuckin build, this thing is about to be on ebay. Put the motor back together last Saturday and got about an hour of break-in on it. Checked compression with a (what I thought) was stock head milled .040. It was at 130/130, I thought that was kinda low but I figured the rings hadn't seated yet so it'd go up. Rode Sunday at the river, pretty much like normal, didn't pussy foot it but didn't hot rod it or anything just raced like normal. Checked compression last night, still 130/130...shit.

 

I then pulled the head off to see if it was really milled, and it's milled around .024. Ok, figured the compression with that should at least be around 140 or possibly higher.

 

Had another stock head that isn't milled so I threw that on just to see and I got around 110/110.

 

Have the rings not seated yet, or is it somethin else?

[dajogejr]

Ok...a few things.

What elevation are you at?

 

Are you kicking the bike until it no longer raises the needle...not giving up after 10 kicks?

Using a good quality compression gauge?

 

Did you measure piston to wall clearance? What about Ring end gap?

Did you check the cc's on the domes?

Lastly...and my personal favorite.

Did you check the squish?

 

If you're at or near sea level....a .040 milled head on a fresh top end, 130PSI is low. I'm thinking it should be in the 150ish range, give or take.

[okbeast]

Ok...a few things.

What elevation are you at?

 

Are you kicking the bike until it no longer raises the needle...not giving up after 10 kicks?

Using a good quality compression gauge?

 

Did you measure piston to wall clearance? What about Ring end gap?

Did you check the cc's on the domes?

Lastly...and my personal favorite.

Did you check the squish?

 

If you're at or near sea level....a .040 milled head on a fresh top end, 130PSI is low. I'm thinking it should be in the 150ish range, give or take.

 

Elevation is around 1200 ft. I kicked the hell out of it then had buddy kick it to make sure I wasn't bein a pussy. Compression guage is good we checked his bike and it reads around 160 with 20cc domes. I did check the piston to wall clearance on the piston that wouldn't fit through after I had to grind some out for it to fit (yeah I know weird, who knows but that's how this project's gone), but I talked to the guy who did the porting/bore and he did it according to the pistons in hand. Ring end gap was good. The domes in the .024 milled head roughly cc'd out to around 20.5-21ish (did a rough test). And a big 'ol negative on checking the squish. I thought maybe i got a big base gasket or somethin, not sure.

[dajogejr]

Um...please explain grinding a piston to get it to fit. That's a new one on me.

Also...you need to check squish...before you go any further.

.040 to .050 is really optimal. And make sure you check it correctly, parallel with the wrist pin so the piston doesn't "rock" back and forth.

[okbeast]

Well, from the beginning: Had the cylinder in a vise to put the studs in, lightly of course, held by the cast flashing at the bottom. It slippped and broke a piece off. Now I'm pretty sure cast doesn't bend, but for some reason on the cylinder on the bottom I wasn't able to get the piston through. It was rubbing some down on the bottom and I could see where it was on the piston. I honed it some and that helped but it was still rubbing, so I started slowly grinding some material away down by the flashing where it was rubbing till the piston fit, then sanded smooth. It is below where the rings seal off, so it shouldn't affect anything besides piston-wall clearance IMO.

 

I'll check the squish tonight, does it matter which of the heads I use, will that change anything?

[bigboybanshee]

Yes, it definitely matters which head you use to check the squish.

 

Are the cylinders ported? If they are, the porting can cause a decrease in compression. :shrug:

[okbeast]

Yes they are ported, that's what I had them off for. But shit it wouldn't affect it 20 psi would it?

[dajogejr]

Either head should have about the same squish...however, smaller cc domes will effect compression.

If the squish is WAY off, that can effect compression as well, like .060 or .080 or so...too much.

[blowit]

Yes they are ported, that's what I had them off for. But shit it wouldn't affect it 20 psi would it?

 

 

Sounds like a raised exhaust port to me!! Verify the squish but yes, a raised port can drop you 20lbs.

 

 

Brandon

[dajogejr]

I've seen full on drag ports drop 10 lbs...20 seems a bit high.

 

I would ask the guy who ported it how high he raised your exhaust

[okbeast]

Sent the builder a msg and I'll check the squish tonight.

 

I might add that with the .024milled head and the compression at 130 this thing ran great, it pulled very hard and I was impressed with how it ran for the little tuning I did.

[alcoholbanshee]

We really need to know the squish........you also need to ask the builder what base gasket you should be running.

[okbeast]

K he said the exhaust is 27.5 mm down from the deck. That don't mean shit to me tho

 

Any chance the rings didn't seat properly or something like that?

Edited August 8, 2007 by okbeast

[alcoholbanshee]

K he said the exhaust is 27.5 mm down from the deck. That don't mean shit to me tho

 

Any chance the rings didn't seat properly or something like that?

Doubtful....Measure your squish. And what base gasket do you have? If you have a thick base gasket and to much squish, there is your quick fix!!!!!!!!

[okbeast]

I've got the gaskets that come in the Rocky Mountain Cometic kit. Used the base and head gasket.