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Running Rough & Out of Ideas

  • Thread starter Thread starter Roland
  • Start date Start date
R

Roland

Guest
1978 GS750E

I just recently completed a Valve Shim job. Prior to starting the shim job the bike ran very well (idled at 1500, no hesitation and very easy to make 80+mph, very little smoke on startup, very easy start with full choke followed by full off choke and down the road in 45 secs)

So I completed replacing the shims and got the valves within spec. and now it acts as though the carbs are out of sync. It won't idle below 2k, stalls at the light, boggs down on the smallest hill, boggs down really bad if I try to Throttle hard.

-Shim Clearances between .04 to .09
-Carbs are Precisely synced.
-Compression well within spec 115-140.
-Plugs are burning clean unless I idle too long in the garage.
-I adjust the Air Screw near 2.5 turns for best running unfortunately I do not get a clean blue flame using my Color Tune. Looking at the Air Screws they look in perfect condition.

I am open for positive suggestions on what might be going on.
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Running Rough & Out of Ideas

No I haven't checked them, although I do have a brand new unopened set of points.
 
.04 to .09 mm's I hope :D

How off were the valves before changing the shims? If the bikes condition changed right after the valve job logic would dictate that's where the problem lies. Have you tried/checked the carb sync, if the valve adjustment changed the flow then your carbs will now be out of sync.
 
When using your colortune, you need to be adjusting the fuel screws, not the air screws.

As stated, if the valves were way off, you need to re-synch.
 
Regarding the fuel screws, how far out are they?
 
Running Rough & Out of Ideas

I did do the carb sync and the compressions tests after I finished with the shims.

Air Screws = fuel/air mixture screws each are approximately 2.5-2.75 turns out from full tight.

Yes, .04-.09mm was the range.
Ex.....Orig......Final.....Gap
1.......2.50.....2.57......04mm
2.......2.60.....2.63......07mm
3.......2.60.....2.70......04mm
4.......2.65.....2.63......08mm

In.....Orig......Final.....Gap
1......2.55.....2.02......04mm
2......2.70.....2.75......04mm
3......2.80.....2.90......04mm
4......2.70.....2.70......04mm

Compression
Cyl....Cold.....Hot
1......136......140
2......117......122 (Prob. higher as I couldn't get wrench on fitting)
3......115......115 (Prob. higher as I couldn't get wrench on fitting)
4......120......130

CIMG4652.JPG
 
Last edited:
Fuel screws should be about 7/8-1 turn out and the air screws about 1.75. Tweak in small amounts from there.
 
Roland said:
I did do the carb sync and the compressions tests after I finished with the shims.

Air Screws = fuel/air mixture screws each are approximately 2.5-2.75 turns out from full tight.

Yes, .04-.09mm was the range.
Ex.....Orig......Final.....Gap
1.......2.50.....2.57......04mm
2.......2.60.....2.63......07mm
3.......2.60.....2.70......04mm
4.......2.65.....2.63......08mm

In.....Orig......Final.....Gap
1......2.55.....2.02......04mm
2......2.70.....2.75......04mm
3......2.80.....2.90......04mm
4......2.70.....2.70......04mm

Compression
Cyl....Cold.....Hot
1......136......140
2......117......122 (Prob. higher as I couldn't get wrench on fitting)
3......115......115 (Prob. higher as I couldn't get wrench on fitting)
4......120......130

CIMG4652.JPG
Click to expand...

The point the guys are getting at is that your carbs have two screws you adjust.

Usually the air screws are adjusted via the rpm method where you adjust each one until the idle revs the highest, then you bring it back down to correct idle.

Since you have a colortune, you adjust each cylinder/carb buy adjusting the fuel screw "under" your carbs.

I suggest checking the points as well because they can crap out on you and cause all sorts of odd running problems. This happened to me. Put on your new points/condensors, gap and then set your timing.

Rick
 
Nessism said:
Fuel screws should be about 7/8-1 turn out and the air screws about 1.75. Tweak in small amounts from there.
Click to expand...

Sorry to jump in here but could I ask what you recommend for a base setting for the air and fuel screws on a 78 GS1000?
Thank you!
 
Last edited:
Set the side air screws at 1 3/4 turns. Set the pilot fuel screws at 1 turn out from lightly seated. Don't seat the fuel screws too tight or you might break off the sharp tip.
 
Running Rough & Out of Ideas

I never knew about that fuel screw that you guys are describing. I just went and looked and I think I see the one are describing. It is at the bottom of the carb and is not attached to the float bowl. I have never adjusted those screws before.
 
Go careful here!
If you bottom those screws they will stick in the hole, the tips break off very easily, they are very long thin points.

Find a very short screwdriver that fits tight in the slot, see if they turn freely. It's not easy to get the inner two even if you do know how the screws work. Most are at least a little bit stuck from age unless they have been taken out already, some still have a spot of paint on the threads from the manufacturer's attempt at making us not be able adjust the carbs.

If they are it's probably best to pull the carbs so you can see what you are doing, put the carbs upside down so the screws are right side up. Get them free and turning easily, only then is it safe to try to bottom the screws lightly so you can count the turns out. Once you break off the tip in it's tiny port you have a much bigger project on your hands.
 
renobruce said:
Set the side air screws at 1 3/4 turns. Set the pilot fuel screws at 1 turn out from lightly seated. Don't seat the fuel screws too tight or you might break off the sharp tip.
Click to expand...


Thank you, I am not too far off of that I guess I need a little fine tuning.
 
While doing the basics (valve clearances, et al) did you also thoroughly clean the carbs (take off, dip carb bodies & jets & bowls & blow compressed air through all the holes)? For you to get a proper baseline, the carbs should be very clean.

Rick
 
blizzard said:
Thank you, I am not too far off of that I guess I need a little fine tuning.
Click to expand...

With the bike running, set the side air screws at highest idle. Then do some plug chops riding the bike at minimum throttle and adjust the fuel screws accordingly. turning the fuel screws out richens the mixture. If your bike is not ready to ride, you can run it stationary at minimum throttle (say 4,000 rpm) for a couple of minutes, then ready your plugs. If you do it that way, have a fan running on your engine so you don't overheat it.
 
Not sure if this helps, but in your first post you said it ran good and idled at 1500. Isn't that a high idle shouldn't it have been more like 1000-1100?
I would think something wasn't right to begin with if it idled at 1500. Just thinking, Good luck
 
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