new here.i have an 02 gsxr 1000.sold my second bike to a friend.88 cbr 600 about 6 months ago.he put 13000 miles on the bike in that time.bike was running great.now has 25000 miles on clock.lost compresion in 3&4.we pulled the motor changed rings cleaned heads new gaskets also changed o rings under cylender block sleeves.put motor back in .turns over good but back fires hard through carbs &sometimes exaust.we did set timing by the book.then pulled valve cover and double checked settings.any suggestions?thanks for any advise.could carbs be the problem?

 

Are you 100% positive you don't have the cams out 180º?

 

thats what i thought.pulled the valve cover off 2 times second guessing myself.ex. & intake are in the right place all timing marks in place with cam lobes facing away from center while looking from right hand side of the bike.

 

Did anybody play with the idle screw at all? Make sure the bike is getting fuel. I rebuilt my EX500 engine over a period of a couple weeks and had people in and out of the garage. I put it back together and had the same symptoms you are describing. I tore it apart and inspected the cams and I had them installed correctly. I found the idle screw was turned out all the way and the bike was not getting fuel. The incident was resolved by turning the screw a few times. I was pissed and relieved at the same time.

 

i did play with the idle screw after it wouldent start.i tryed spraying gas into carbs.still wont hit right.seams like its firing 180 out like u said.

 

Well, I'd start with a compression gauge and go from there. Check fuel, spark, air, compression. Did you install any rags or towels into the intake ports while the engine was pulled? Maybe you left a rag in there somewhere. (not trying to call you an idiot, I've seen it happen before)

 

no no rags.got fire & fuel.dont have a compression guage but spitting back through the carb hard. might have to get a comp. ga.

 

The compression test should give you a better understanding of where the issue is. You could also try a leakdown test. It is more involved but also more definite.

 

going play with it now & see what i can find out.

 

no luck.how do you do a pump down test?would it be in the book?