GS750 Starting problem

[tomo]

Hi !, I`m a new member having only just found out about this site, which seems really good, so I`m hoping someone out there might be able to help !. I`ve been riding GS750`s for about twenty years here in sunny England, and have always been able to keep em running, but this time I`m stumped. The bike (1979 model with slide carbs) was starting and running fine every day until a month ago, when it just would not start. After much cranking, blowing in the breather tubes of the carbs (old trick which usually works well) it eventually caught and ran well once going. Next day, same problem, but only started with minimum choke, then would not keep going until full applied. Suspecting carb trouble, I stripped & cleaned them. Same problem still. Next replaced plugs, points and condensors with new, checked timing was cock on, but still refused to start. Swapped coils next for a new pair, no luck. Stripped carbs and cleaned again. No luck. Checked fuel getting to carbs which it is, also that good spark at all plugs which it is. A compression test showed low readings of 55,55,65 & 75psi. A drop of oil in each pot did`nt improve it I guess that points to valves. Bores must have some wear but she does`nt lose much oil or blow smoke. I don`t know whether such a big loss of compression can just be down to valve seating alone though. If anyone can advise I`d really appreciate it. I guess a top end strip is on the cards, but thought I`d ask first !. Thanks.

 

[BadBillyB]

Tomo, welcome to the GSR......Sounds to me like the cylinders were "washed down" with fuel. To restore the compression, crank the motor with the plugs out for at least 10 seconds. A drop of oil is not enough. Put a teaspoon of oil in each cylinder and let it sit for a couple minutes. Now see if your compression has come up to an acceptable level......If it does, you just need to find out why this happened. The most likely thing is inadequate spark. You can have spark at the plugs with them out of the cylinders but no spark when they are subjected to cylinder pressures.....It is fairly common for these older bikes to have low voltage at the coils B+ terminal as the voltage travels through many marginal connections and switches....Some members have used a relay to power up the coils for just this reason. The power to the coils will then just have one switch (the relay) to pass through.....Hope this helps you.....BadBillyB

 

[49er]

Hi Tomo
Welcome. Those compression figures are sad. Did you check them with the engine hot and the throttle wide open? Try checking your valve clearances. If you haven't checked them recently and they are all tight, it could affect your pressures considerably. If the clearances are within spec, it sounds like you may need a top end OH. How many miles on the old girl?
Cheers
Ian

 

[froldt]

Hi Tomo! Glad to see you here! I've got a GS750T and I'm having what sounds like the same problem you are. Best of luck to you! I can't wait to get my Gerty back up and running, soon as I figure out what's wrong with her.

 

[tomo]

Thanks for your advice guys. The bike has 89000 miles on the clock so I guess she is getting a bit tired. I did try wiring the coils direct to the battery (fully charged) but no luck. Unfortunately the compression test was done cold as she won`t even try to fire up. I will try again tomorrow with a bit more oil in each pot, as I did only put a few drops in so it probably was`nt enough. Thing that puzzles me is why she was starting and running fine one day then dead the next. If it is the valves I`d have thought she would have shown symptoms before like misfiring, which is why I tried checking out the fuel and electrics first. But the compression is certainly low, guess it should be over 100psi ?. Anyone know what range the figures should be for a healthy 750 ?. The funny thing is my other GS750 chop, has an engine in that was built from parts of three old scrappers and just thrown together. She goes a few months between runs but starts first time and has`nt missed a beat in twelve years !. Well, maybe I should`t have said that without touching wood !. Thanks again for taking time to reply. I`ll keep you posted as to how I get on.

 

[49er]

tomo
Even with the engine warm, your figures aren't likely to be around 100psi except for maybe #4.
With tight valves, if you suddenly had particularly cold weather, that could increase the valve to seat gap enough to make it difficult to start.
You don't always get backfiring from a non seating valve especially at rpm's above idle.
The Clymer manual quotes the normal compression range for the 850's from 128-171, with a variation max of 28psi and a service limit of 100psi. The 750 should be similar.
At 89,000 miles, she'll need a top end massage anyway.
Cheers
Ian