Author Topic: Compression Question, Kinda long (Read 3250 times)

SRwitt · « **on:** September 03, 2006, 05:08:55 PM »

Alright, I've been on here off and on for the last year asking questions about my bike. Since last summer I've been working on doing a top end on my bike a 91 GSF400. Everything is back together after finding my head gasket was shot, since the engine was already torn down I decided to measure my bores and re ring it. The bores all measured fine according to the manual so I re honed it, remeasured the bores (they were still within spec after honing) and got a new set of stock sized rings to put back in it. Now the bike is reassembled and my compression is low on all 4 cylinders but it is equal. I talked to a guy I work with and he said to redo the compression test but with oil in the cylinders. My batteries kinda dead but I got to test one cylinder, the compression jumped dramatically which would say it's a problem with the rings. Well when I reassembled the engine I applied an assembly lube to the cylinder walls,(meant to wash off by oil and heat) does anyone know if this could affect the compression readings? Or is it time for a new engine or order the Big Bore kit offered here and go for a full bore/hone.

FWIW before I added oil to the cylinders all of them tested between 70-80 PSI with oil in the one it jumped to well over 120 before my battery lost power.

Thief400 · « **Reply #1 on:** September 03, 2006, 08:25:52 PM »

You have to run the engine to BREAK in the new rings before you ever try checking the compression!!!!!! Some say up to 500 miles

gsxr400 racer · « **Reply #2 on:** September 03, 2006, 09:24:26 PM »

well i re rang my motor 3 times all times it relly never went above 145 all bores measured good and i check before after and during compression I just like to know and compare. but now i have 180-190 psi compression after i bored mine out . Not a sales pitch just sharing my experience.

Thief400 · « **Reply #3 on:** September 03, 2006, 11:04:16 PM »

2 reasons Jay, First there is nothing better than a nice straight, true, freshly bores and honed cylinder to provide a good seal, second your cramming a larger volume of air into the same size combustion chamber so the pressure is going to be higher.

SRwitt · « **Reply #4 on:** September 05, 2006, 01:19:09 AM »

Ok, I didnt know about breaking the engine in first, I had tried startingthe engine since the rebuild and it wouldnt start. I got pops and sputters but it would never catch and start running. From there I checked that I was getting fuel and spark. Then I thought to do a compression test. I got the battery charged yesterday and did all four cylinders with oil and they all tested at 145-170 psi. I think I may just need to try some new spark plugs.

gsxr400 racer · « **Reply #5 on:** September 05, 2006, 01:22:10 AM »

Quote from: Thief400

2 reasons Jay, First there is nothing better than a nice straight, true, freshly bores and honed cylinder to provide a good seal, second your cramming a larger volume of air into the same size combustion chamber so the pressure is going to be higher.

Okay ill buy that but then why does the manual list 140-145 as the lowest spec and 180-200 as the high?

Thief400 · « **Reply #6 on:** September 05, 2006, 01:05:32 PM »

Machine tolerances and carbon deposits Jay