How to get compression back after porting

MURDER YAMAHA

MURDER YAMAHA

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So, I think I have things figured out, but, my compression will be about 105 - 107 PSI on all three holes.
That is using a stock head, with a single layer head gasket.
Without porting, that usually yields about 130 to 135 PSI on my tester.

So, my squish is at about .51, which I know is safe with the fuel we use.

My question is, how do I properly get my compression back up to the 130ish range without increasing the octane I need to run?

Since the porting lowered the compression, am I OK to deck the tops of the cylinders ?
But, then my squish will decrease.
 
I am no engine builder but I have seen the heads shaved and the chambers reshaped to bring the compressions back up but you will still need to have a good squish to prevent detonation. If you could get a hold of Don (MrViper) he would know better then most.
 
Trying to use your guys knowledge to figure this out.
Don is tough to get ahold of and is usually very busy.
I have already bothered him enough to last a lifetime Im sure. Lol
 
i dont mind the questions and am sorry i am not around like i use to be, life just changes sometimes.
 
I'm guessing you raised the exhaust port timing and is why you have lost compression? Shaving the head is the answer, if it gets extreme then you may have to alter the squish band.
 
I don't mean to hijack this thread, but feel as thought it goes with your original question...

Picked up a 2000 SXR 700 that was supposedly done up by Hauk. It has their early pipes (Simmons CPR), clutching, and VForce reeds.

So far I am only getting 103 PSI across the board. It looks as though the head gasket is stock. The seller mentioned it should've been ported, but I have no clue where to check to see if that was done.

Whats the stock compression spec? Sled runs great, but I would rather do a top end now if I need to instead of blowing it up. The head gasket appears to have all three layer remaining.

Thanks!
 
Brdmh44 said:
So far I am only getting 103 PSI across the board. It looks as though the head gasket is stock. The seller mentioned it should've been ported, but I have no clue where to check to see if that was done.

Whats the stock compression spec? Sled runs great, but I would rather do a top end now if I need to instead of blowing it up. The head gasket appears to have all three layer remaining.

Thanks!
Click to expand...

The only way to really know what's going on in your engine is to cc the heads and check the squish clearance to determine the compression ratio.

Compression numbers vary from gage to gage and the technique. What is important is that all three cylinders are within a couple percent of each other. Normally the compression gage should read approx 120-130 psi.[snap on gage]

If the engine is ported the heads are normally cut as well or the head gasket is thinned.

If you determine your gage is accurate and want to gain compression you can remove layers of head gasket. For each layer of head gasket you remove on the 700 SXs you gain approx 10lbs on the compression gage. If you do this you need to check the squish clearance! Don't go any tighter then .050" of clearance.

Any increase in compression will increase the importance of proper jetting and fuel quality....so maybe..."if it aint broke...don't fix it!"
 
SideshowBob,

Thanks for the info! I tested my other sled for comparison sake and got the same readings. I will buy another compression tester to verify readings. The "head gasket mod" isn't a big deal to me. However, if the top end is worn out I'd rather rebuild now than burn it down and cause more damage and expense. The sled runs great, starts with half a pull and doesn't feel down on power.

We all know that 2 strokes need at least 90 PSI to run good, but I figured stock would be around 120 like most others. In my mind if 120 is where it should be, and the engine does in fact only have 103 then its most likely due for a rebuild for both reliability and power.
 
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