1979 GS750 black plugs - jet needle height?

[Coady]

Bike runs great, starts straight away, idles smoothly, pulls hard to redline. But plugs are still black and fluffy i.e. carbon fouled. And mileage is around 7.5-7.8 litres per 100km (32mpg), that's commuting through the city.

Just replaced valve stem seals to get rid of smokey morning startup which I thought may have fixed problem.

Have been running hotter B7ES plugs to date which were always dark and thought I'd now put in stock B8ES as I had no excuses, but they're even darker. I mainly commute, 1/2 hour each way so a lot of idling but battery voltage at idle is around 12.7 V.

Got a fat blue spark - checked idle mixture with colortune - bunsen blue with no orange flashes.

Now the thing is, I have the air screw an additional 1/4 - 1/2 turn out from highest idle / optimum colortune setting, to compensate for rich bogging at 2500 - 3000.

Could this be the jet needle too high?

1979 GS750E
Foam Uni filter in airbox
4 in 1 exhaust
Float height at 26mm.
Pilot jet #15
Fuel screw 1.25 turns
Air screw 2.00turns

Thanks

[alke46]

I gotta watch this thread as I have a similar condition on my 1000. My bike has an aftermarket exhaust which is the Mac 4 into 2 with slight turn out. The PO said it was installed by a shop but they did not do any re-jetting. I don't know if this is the cause of my problem or not.
I will be watching and waiting on the opinions of the forum.

[SqDancerLynn1]

I'm not the jetting expert but I think you have the fuel screw out too far stock is only 5/8 turn out also make sure you have 12 volts at the coil. Try 1 turn and work with the air screw. A good carb sync is a must. 32 mpg is not real bad but it should be close to 40. But you milage depends alot on how you ride The later CV carbs get better milage than the VM slide

[crapwacker]

Have you checked where to needle is set? And where do you run your throttle, mild, or wild?

[SqDancerLynn1]

if you want the specs check on page 5 under the technical forum sticky for carb settings The mixture screw they are talking about is what you refer to as the air screw

[Coady]

Have you checked where to needle is set? And where do you run your throttle, mild, or wild?

No I haven't and was wondering if that could be the reason for the fouling and worth a check.

[Coady]

I'm not the jetting expert but I think you have the fuel screw out too far stock is only 5/8 turn out also make sure you have 12 volts at the coil. Try 1 turn and work with the air screw. A good carb sync is a must. 32 mpg is not real bad but it should be close to 40. But you milage depends alot on how you ride The later CV carbs get better milage than the VM slide

When I got the bike the fuel screws were at 3/4 - 1 and air screws at 1 - Then it bogged down real bad at 2500-3000. Opening up the air screws to 1 3/4 fixed the bogging, but then I had to open up the fuel screws to get a good idle.

I ride between 3500-5000 with the odd spirited dash.

Thanks

[dlanes]

Black plugs mean it's running rich right? I changed my plugs and put those pods on now runs much better. It doesnt seem to bog anymore and checked the plugs and they looked good (they were tan not white).

[Coady]

OK voltage at coils is 12.30, battery 12.55 - 0.25V drop which is good.

Also tested coils. 5.4 / 5.5 ohms across each, and 22,000 / 30,000 between plug caps which I believe is also good.

I'll pull carbs and check needle next.

[Coady]

OK so I've pulled carbs and found my jet needles are 5DL36, which from searching appear to be stock for a GS1000 whereas a GS750 should have 5F21-3 needles.

They are currently in the middle position.

Mains are 102.5 and pilot 15.

Should I try them in the second position from top (leaner) or get stock needles?

[Coady]

Just found this which I am posting in carb spec thread from this website:

http://motorbikearchives.com/Bike-Tests/Road-and-Street/Suzuki-GS750EN-1979.html

The GS750B came with a # 100 main jet, 0-6 needle, 5F21-3 jet needle, and # 15 pilot jet. The GS750C (1978) and GS750N (1979) carburetors were changed to a #102.5 main jet, 0-4 needle, 5DL36-2 jet needle and #15 pilot jet.

What that all means is that the GS750N is leaner at the small throttle openings usually encountered in EPA emissions test modes. The dual-taper 5DL36-2 jet needle is initially leaner, secondarily richer to smooth the transition from idle to off idle, (or from small, steady-state carb openings at low speeds to gentle acceleration, as encountered in traffic). The float level was also lowered between the GS750B and GS750N, from 26mm to 24mm.


I'll go to these settings and work back from there.

[Coady]

So I cleaned my carbs, replacing o-rings and dropped the needle (leaner) to 2nd clip position. Bike ran way lean. Throttle raced and had to keep dialling in idle as it got warmer. Was hard to vacuum sync too.

Put needle back in middle position. Re-synced carbs and it goes great over 3000rpm. But it won't hold idle under this and I have to keep revving it at the lights etc or it dies. Blowing a bit of smoke too. Starts up real easy with choke but fouls the plugs.

Fuel and air screw settings don't make much difference

I accidently dicovered it holds idle fine when warm on side stand?

I also found the vacuum level to be less when I synced the carbs the second time, after I put the needle back to the original position; went from 24cmHg down to 15-17cmHg. When I synced them 6months ago they were at 24 ish.

Any suggestions much appreciated. Thanks.

[Big T]

You replaced which O rings?

In the carbs?

At the head?

It sound to me like you should go up one size on the pilot jet to regain idle and low end, and tweak your mixture screws from stock settings

Why don't you post us some pics of your plugs?

[Coady]

I replaced the carb O-rings.

Plugs are all filthy black and wet when not firing.

Ran pretty well with even idle up until I pulled carbs and cleaned them.

It's always run a rich til now even with stock pilot #15 - I opened the fuel screws to 1.5 turns and side air screws to 2 and black smoke started coming at the exhaust.

Weird thing is that its dying like its lean...

[crapwacker]

Could your air filter be over-oiled, causing lots of vacuum in the carbs?