Am I missing something ?

[tomo]

Thanks to everyone who answered my previous postings about my GS750 starting problems. Before I go tearing into the engine, just thought I`d ask whether anyone can think of something I`ve overlooked. To re-cap, bike was being used every day, starting fine & running well. Then one day, was a pig to start but ran fine. Over next few days the same thing, until after a two day lay up, would`nt start at all. I stripped the carbs & cleaned them, checked fuel getting to the carbs ok. Gapped & fitted new plugs, fitted new points & gapped, fitted new condensors, checked static timing. Fitted new coils & HT leads. I did`nt do all this at once, just as I worked through possible causes. I did a compression test, got low readings. Re-tested with oil in pots with little improvement. Checked valve clearances which seem ok. I am thinking I`ll need to take the head off to re-grind the valves, probably get the block re-bored & new pistons too, but I have a nagging feeling that it might be something really simple, gut feeling towards ignition, even though there is a spark at the plugs. I know the engine has high mileage & undoubtedly the compression is low, but the problem really appeared overnight, and I`d have thought if it was down to compression it would have shown up in poor running/performance but it did`nt. My other 750 has even lower compression but starts & runs really well too !. So, any further suggestions or advice would be much appreciated before I vanish to the garage weilding a large hammer !!.

[Dink]

Checked out the charging system?? Battery voltage?Voltage at the coils? These bikes really feel the difference between 12volts and 11.9 volts.

Dink

[bradleymaynar]

Replace the battery. Sounds like you may be over-thinking this. I have similar problems with my bike if it sits for an extended period without a tender connected. The problem seems to suit a bad, or under charged, battery. If that's not it, progress from there.

BTW, hey Dink, how're you and Jean?

Brad bt

[robertbarr]

Have you tried ether yet?

If you have compression, spark & fuel, it pretty much has to fire. Perhaps ether combined with a freshly charged battery and a bump start (or kickstart, if yours is a pre-80 bike).

I made this suggestion earlier to another person in the same situation -- try shutting off the fuel temporarily and starting it just on the ether. Repeat this a few times. Then turn the fuel on and see what happens.

I agree (as most would) that low compression alone wouldn't cause any sudden change in performance. If that were the case, most of my vehicles from earlier years wouldn't have gotten anywhere...

[BadBillyB]

Tomo, I still suspect that you have washed down the rings with gasoline. The engine must be mechanically sound before its going to start. 50 or 60 lbs. of compression is not going to cut it. Rings and valve seats do not go bad overnight. A malfunctioning petcock or an ignition issue (fouled plugs) can allow so much fuel into the motor that the rings lose their sealing ability. I would never tear down a motor that has poor or very low compression without finding out why beforehand....If the rings are washed down with fuel, you will want to determine this using the following steps.....
Remove the gas tank and drain the float bowls (eliminates making it worse)
Remove the spark plugs and crank motor over a few seconds (clears them)
Put 3 or 4 ounces of motor oil in each plug hole and let sit for 1/2 hour
Spin motor over (WITH PLUGS OUT) for a few seconds to blow out excess oil
Re-check your compression

Also check your oil for fuel contamination. Level will usually be higher than normal if there is fuel in crankcase. If unsure, drain it before doing above.....BadBillyB

[bradleymaynar]

Thanks BBB, hadn't even considered problem with the fuel delivery system. Your definitely going to lose compression values if fuel is getting into the oil. You may want to consider an oil and filter change. Fuel in the oil will "water down" the oil in the pan, and cause serious problems down the road.

Brad bt