Gk76a compression readings

[dsr91]

Hi people , does anyone know roughly what compression a gk76a non sp should have ? I can only find readings in manual for a 250 engine , basicly I put my bike up for sale and it must have know because riding home from work it struggled to pull away then started mis firing and got worse the futher I went , check plug and they were black ( bare in mind I have just rebuild carbs and cleaned plugs only 20 miles ago so I thought floats were stuck ) done compression test to check valves and results were a shock , sitting on bike looking forward readings are left to right , cylinder 1 - 130 psi .cylinder 2 - 30 psi . Cylinder 3 - 150 psi . Cylinder 4 - 120 psi . For cylindef 2 that is not a typing error it really is 30 psi

[Marsbar]

ouch 30 psi on number 2, reminds me of my first gsxr 400 engine, that was like 50psi, also on pot 2 for some reason. Compression shouldn't be more then 20psi difference from min-max if i remember correctly.
did you use sealing tape when you screwed in the compression tester?

Also did you balance your carbs when you rebuild them? as it makes night and day difference. I got a Carbtune kit from http://www.carbtune.co.uk/ really good price and allot better then the 4 dial version in my opinion.

Low compression can be caused my warn rings, witch unfortunately is about £200 i think. As you shouldn't really just replace the one pot as there will be a massive difference in pressures witch can put a strain on your crank ect.
This seller is extremly good value on his kit, but you will need to see if yours are standard or be bored.

http://www.ebay.co.uk/itm/190798782733? ... 1423.l2649

Or the problem might just be that your valve seats are coated in carbon witch was the problem with mine.



I would also be a wise idea if you do decide to re-lap the valves is to change the valve stem seals as i got told that it is a common fault for them to go hard and thus causing the engine to burn a fair bit of oil...witch my first engine also did.

I hope this information is helpful to you and you can get the bike back on the road before spring sets in.

[Maelstrom]

Compression test is usually taken at full throttle which can be problematic with CV carbs. In any case, sounds like a leakdown test is needed now to determine exactly where the compression went.
cheers

[dsr91]

Thanks for reply but no I borrowed a adapter from local garage for compression tester but now brought my own so will test with securing tape , also I've been told to do a wet compression test ( put small amount of oil down each hole ) apparently is a cheats way of checking rings but my local garage say it's valve seats ? Reason for that is if spark plugs are clean the bike pulls like a train but over fuels and mainly below 5rpm it tryed to die .as for sync tools I only got standard cheap ones of ebay ,(4 gauges mounted to a blue bit of metal )

[stripes1976]

There are two things here to check as a first port of call.
Firstly and easiest is valve clearances as the valves on the 76a stretch rather than wear so you can lose compression that way.
Secondly is change your rings. If your bike has had carb issues for any length of time you will have worn the rings as the excess fuel will have been washing the bores.

From your other readings of compression you are low on all cylinders anyway but 30psi my money is on rings.
If you need gaskets, stem seals and rings give michelle at robinsons foundry a call.
Word of warning if you are rebuilding then a few pointers. Make sure you change the bolts on the clutch basket for new as once tightened the will shear if you try and reuse as they are torqued beyond there tensile limit. Use the suzuki sealant on the engine cases although expensive it works. And make sure you take your time. These engines have a lot of little fitting quirks that if it feels tight or doesn't just drop in place stop and look why.