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New owner

  • Thread starter Thread starter Mikey82
  • Start date Start date
M

Mikey82

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Hi I,m new to the forum and from the Uk..Please be kind👍👍
I have a 1977 Gs750 had long term problem of firing on 3 cylinders and number 3 cylinder staying cold until after 5 mins of running time.
Compression test shows 100-105 psi in all 4 cylinders.
i found that some of the wire sheath had been worn away and missing in small sections of the wire from points to coils, so replaced the wires. Slight improvement in cylinder 3 but not much.
Next stop I checked the carb settings I loosened the locknut on top of the carbs (after winding out the throttle tick over screw) and screwed all the screws in until they stopped, and then tightened the locknuts I now have no.2,3 and 4 getting hot immediately and no.1 just warm.
Took bike for short run, very sluggish with a top speed of 70mph checked plugs and very black and sooty.
Going to tune carbs tomorrow, and wind out air screw ( presently unscrewed 1 1/2 turns from closed.
Any further tests you could suggest would be very useful.
New coils, plugs (ngk8es) points capacitor, carb kit. Battery,
Thankyou in advance.
 
What's the history of the bike? How long have you owned?

It's possible the carbs need a good, thorough cleaning. Also, you should check the valve clearances and adjust the clearance with new shims if the clearance is too small.

The work you described on the carbs sounds like an so so attempt at carb syncing. Typically, this is done after cleaning the carbs and preparing to reinstall on the bike. Once reinstalled, you should be a proper vacuum sync.

Read this guide to learn how to properly clean the carbs: http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf
 
#3 misfiring means your petcock is bad.
Test by removing the vacuum line at the petcock and plugging the line
Run the bike using the Prime setting and see if #3 doesn't start to fire.

You need to go back and properly sync your carbs
Have you adjusted the valves yet?
 
Thanks for the replies, I have been running the bike with a temp tank feeding the engine with no petcock,and vacuum pipe plugged, and as I have good equal compression i,m guessng that the valves and pistons are good.
Hoping to balance carbs and get improvements will let you know.
 
you can confirm that the problem isn't on the electrical end of things by swapping #2 and #3 plug wires. if the problem moves to #2, you've got a coil/wire/plug problem. i had an intermittent short in a plug wire once, showed good spark against the engine but failed when in place. if original, i'd replace the plug caps.

once you've got a nice spark at all four, adjust the valves. having good compression doesn't mean your valves have the correct clearances.

the recommended carb tutorial is excellent! find the float height spec somewhere and measure them at the correct spot.
really sounds like jet(s) or circuits are clogged, no amount of fiddling will correct that.
 
Guessing?

We use Ed's motto - to measure is to know

Adjust your valves, sync your carbs and see what you have then
 
All good advise Thankyou, I have balanced the carbs and now have all 4 cylinders firing.
Taking bike for a run it feels sluggish so even though all 4 are firing good, the tick over and pilot side of the carb is not yet right.
Next stop valves and maybe ultra clean of the carbs.
Will I need to get shims for the valves or could it just be adjustment?
 
Last edited:
Mikey82 said:
All good advise Thankyou, I have balanced the carbs and now have all 4 cylinders firing.
Taking bike for a run it feels sluggish so even though all 4 are firing good, the tick over and pilot side of the carb is not yet right.
Next stop valves and maybe ultra clean of the carbs.
Will I need to get shims for the valves or could it just be adjustment?
Click to expand...

Yes, you'll need to get shims. The thickness of the shim IS the adjustment.
 
just curious, all four get quickly and evenly hot on start-up?

my project needed three shims. local dealer was cheap enough at $5 ea, the shim tool tricky to use at first.
 
Well Thanks to the carb clean procedure posted on this thread I,ve completely dismantled cleaned, renewed o rings etc and took the bike for a spin.
A lot better, smoother running at low speed, opened her up and lots of power, even though carbs weren’t balanced.
Running rich with the air screws turned out 2 full turns on each carb.
Plugs were sooty, but def getting there.
Thankyou for you help..
Ordered a Morgan’s carb balancer, rather than borrow one, will then balance carbs and will report back.����
 
Remember that the air screw only controls the idle mixture.

Where did you set the fuel screws?
 
I set the fuel screw one turn open from the closed position if the bike is running rich after I,ve balanced the carbs should I look at changing these settings?.
 
Also, as others have stated, adjust your valves. The carb sync is no good without properly adjusted valves. Also, check the rubber boots and o rings from carb to head, and the airbox integrity and the rubber boots to the airbox. Boots are the only mildly pricey thing on the list, but when I did carb overhaul, valve adjustment, airbox overhaul, new boots and o rings, My '80 GS1000 went from aargh to wow.
 
Mikey82 said:
I set the fuel screw one turn open from the closed position if the bike is running rich after I,ve balanced the carbs should I look at changing these settings?.
Click to expand...

Again, those screws only affect your idle mix, so if you're running rich on other circuits, you need to make changes as needed

The standard setting for the fuel screw is 5/8 to 3/4 turn, so you're rich there.

Are you running the airbox and stock exhaust?
 
Last edited:
Big T said:
Again, those screws only affect your idle mix, so if you're running rich on other circuits, you need to make changes as needed

The standard setting for the fuel screw is 5/8 to 3/4 turn, so you're rich there.

Are you running the airbox and stock exhaust?
Click to expand...


Yes running airbox and stock exhaust, i did the compression test again this time the correct way (ie. remove all plugs and open throttle fully) I had 125psi on each cylinder.
i,m being told by people that if I have that sort of compression the piston rings and valves are good..����

New inlet and outlet carb rubbers and o rings
 
Rings and valves may be good, but the compression test doesn't tell you if your valves are properly adjusted. It only tells you that they probably don't leak when closed. As far as I know, the only way to tell if they are adjusted correctly is the hard way.
 
Appreciate your help and advise Thankyou, what plugs do you guys run your bikes on,I have been using NGK 8ES
Looking at all my options to cure the sooty plug syndrome..
I presently have a max speed of 80 mph and after a short run the plugs get rich...causing a judder response when blipping throttle, but once throttle is opened I have good power.
I know I,m getting closer👍👌💪
 
Afternoon. From experience with my '78 GS750 I have the fuel screws (the ones underneath the carbs closest to the engine when in situ) turned out 1/2 turn and the air mixture screws (those on the sides of my VM slide carbs) out between 2.1/2 and 3 turns. The plugs are a light tan colour, I can hit 85-90'sh without too many issues. I run a Marshall Deeptone exhaust and foam filters and have spent many a wonderful hour fettling the carbs getting them to work right (nearly there !).

The advise to check the shims is very good advice. After re-shimming mine I had to strip it all off as the valve guide on number 1 cylinder (the one that only ever got warm, not hot) was in at a slight angle.

Guide now replaced and she's nearly running properly but it's sometimes like herding cats getting her to run properly but every time I change something, the bike improves...?.then I shag it all up by diving in without engaging my brain.
 
B8ES is the correct plug for a stock or nearly stock setup. The plugs are most likely not the problem.
 
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