1979 GS 850 Help with Plug colour and idle please

[Guest]

Hi Everyone,

I would really appreciate your help on my ongoing 1979 GS850 issues. Sorry for the long note but think it’s all needed.

Here she is.


VM26SS carbs, K&N filter, Vance and Hines 4 into 1 exhaust
Dyna S ignition


Issues:
- Since I’ve had the bike, which I bought in Victoria to ride to Toronto (I eventually made it with AZR’s help), I’ve had the following problem.

Cylinder’s 3 and 4 plugs, especially 3 very sooty. One and two variously either seeming hot/lean or reasonably colored depending on the ride. Also I’ve had uneven idle, some popping on deceleration, lately what may be hanging idle at about 2K.
I had previously tried all kinds of fuel/air screw combinations with no success to get the plugs looking good.
No smoke from the pipes that I’ve noticed.

Carb internals are
- 115 Mains
- #15 Pilots
- Needle is in middle groove
- I’m assuming the float needle and seat are aftermarket, seemed to be in good shape with the springs holding the floats with no movement when upside down

What I’ve done recently
- Switched out the Pingle petcock with a brand new OEM Susuki vacume petcock. Checked this for proper functioning by sucking on the vacume and confirming good flow that stops when I stop. (The plug situation was pretty much identical with both petcocks)
- New green dyna coils, non suppression wires and NGK caps. Checked voltage at the coils and was only losing about .15 volts compared to the battery.
- Carbs disassembled and dipped in Pine Sol (I’m in Canada, no good stuff), all passages sprayed with carb cleaner and air, new o-rings from Mr. Barr.
- All floats at 24mm (middle of spec)
- Intake O-rings from Mr. Barr installed. Boots examined and looked and felt in good shape. No cracks, fairly supple.
- All clamps on tight
- Brand new B8ES plugs purchased often…
- Cam chain timing confirmed ok
- Ignition timing on Dyna S checked with Strobe at full advance and seems perfect for both 1-4 and 2-3
- Compression checked recently while warm, with all plugs out and throttle on full.. not great… all about 115 (#1 was a bit lower… 110 or so)
- Found out that the #2 and 3 plug caps were 10K ohm resistance and the 1 and 4 were the correct 5K ohms. Now all four are new 5K ohm NGK caps.
- Valve clearances adjusted and all between .03 and 09mm. none were tight previously. Two exhausts were a bit wide and needed one size thicker shim.

After all that, here is a picture of the plugs when I took her out. Brand new installed and then I went to work and back, about a 22km round trip. Most of the trip would have been on idle or very slight throttle(traffic). The idle fairly quickly went up to about 2K rpms after it was warm so there was a lot of time at stoplights at that idle.
Generally speaking after the new coils she started up easier, had smoother idle (even though it seemed to hand at 2K, it stayed there pretty solid mostly… better than before the new coils and plug caps and carb cleaning) and had better power. It almost seems like maybe the electric upgrades (coils and plug caps) may be making her fire on all 4 more reliably. But there’s still something really wrong with her high idle and this plug situation.

Number 1


Number 2


Number 3


Number 4



Ideas I have at the moment. I see a lot of talk about sealing the airbox. I'll post pictures of the box next. Notice the PO cut out holes in the side of the airbox. Not so sealed eh? But I think I’ve heard this is not uncommon? Also one side’s sealing rubber is cut badly and I'm sure is letting more air in air. Sooooo idea number 1 is maybe seal up those holes and replace /fix the rubber? And button up the airbox and see what might change? Question I have is… I’ve heard and seen that a lean condition can cause hanging idle. Is it possible that the high idle is a result of those holes in the airbox and that the high idle might make plugs 3 and 4 sooty and only 1 and 2 looking lean?


I don’t know why it’s just 3 and 4 mostly.

One thing I’ll do this weekend is get her warm and spray a bit of carb cleaner on the boots to see if there’s a leak there (even with the new o-rings)

Before I take her apart again and check valves and such (wondering if maybe they are burnt). What else do ya’ll think I can check/do? Airbox? Fuel/air screws? Other ideas? I’d love it if it was something I can adjust without digging into the top end in a big way.

I have not adjusted the air or fuel screws on the carbs since I cleaned them before this last ride. As I said adjustments to these before the carb dip and new coils/caps didn’t seem to solve the issue.
All 4 fuel screws at ¾ turns out
All 4 air screws at 1 and a half out.

Thanks a bunch.

[RobertGS]

Here's the questionable airbox work the PO did...

[chuck hahn]

The decell pop is from the 2 lean plugs most likely. Go to the side mixture screws on the two sooty ones and turn those screws out ( leaning the mixture in those cylinders ) 1/4 turn and turn the 2 that are ashen IN 1/4 turn ( Richening the mixture in those 2 ) The side mixture screws meter the air to fuel ratios. Turning them out will lean that cylinder and IN will richen a cylinder. Clean up the plugs..or better yet..get new ones and see what they look like then. I have one of these and its handy for adjusting mixtures and not needing a dozen sets of plugs...clean and proceed. Way cheaper than this at Harbor Freight.

https://www.pricefalls.com/product/a...ype={MatchType}

[RobertGS]

Thanks Chuck,
Easy enough to try. I"ll give it a go today.

[2BRacing]

Have you done a vacuum synch on the carbs? This will not affect the mixture, but gets them all working evenly together.

[RobertGS]

Hi yes both bench and vacuum synced.

[azr]

Easy enough to cover over those airbox holes with tape and cardboard to see if it makes a difference, but with the 4 into 1 and the K&N filter it would make sense why the holes are there.

[RobertGS]

Here's the report so far with adjusting the air mixture screws (remember fuel is at 3/4 out on all 4)

Went in 1/4 on 1 and 2, and 1/2 out on 3 and 4. - No difference on 3 and 4 still sooty.
Went another 1/4 in on 1 and 2, and 1/4 out on 3 and 4 - No difference on 3 and 4 still sooty.
Taped over left side airbox hole (side for 1 and 2 which were the lean ones, away from 3 and 4 the rich ones)- No difference 3 and 4 still sooty
Finally another 1/4 in on 1 and 2 and 1/4 out on 3 and 4 - No difference on 3 and 4 still sooty.

Through this 1 and 2 at least now don't look as lean. In any case they don't look rich.
Air mixture screws are now at
1 turn out on 1 and 2, 2.5 turns out on 3 and 4

So I'm thinking next I'll try turning in the fuel screws on 3 and 4 a bit at a time

[RobertGS]

I adjusted the fuel screws on 3 and 4 by turning them in 1/4 turn. They are now 1/2 out

It's raining here so instead of a ride I warmed it up and let it idle at 11OO rpms for 10 minutes or so (big fan on it of course).

And I'm reserving celebration, but it sure seems that 3 and 4 were no longer sooty.

So assuming the issue was the idle circuit the latest changes would seem to have made a difference.

Fingers crossed, I'll report back after a real ride.

[azr]

Remember, if you adjust the fuel you must adjust the air screws to match the changes you made. Fastest idle method can work well once you get the knack of it.