Need Jetting Help, At Wit's End....78 GS750

[Ludeykrus]

Howdy all,

I've been trying to put my 78 750 back on the road for winter touring and I've never had it running right.

Long story short, I bought it a while back. The airbox was falling apart, and the PO had apparently pulled the baffles from the stock exhaust. I've had the carbs apart twice, and can't get them right.

Instead of spending a chunk of change on a full new airbox, I picked up a set of K&N-style oiled paper-element pods. Back when it ran decent on the stock airbox, it would 'hiccup' sometimes from around the carbs.

First, what jetting combination will get me close to decent? No DJ jet kit suggestions please. I I've played with the screws and the needles, and can't even get it to start easily, so I'd like a good start on initial settings so I can troubleshoot a bit easier.

Second, my far right carb is not giving fuel at idle. That cylinder is getting spark, but when I pull the plug wire while running the idle does not change. I do believe it's firing well under more throttle, so this suggests a blocked passage or jet. Where precisely should I clean and blow out to help solve this?

I've soaked all of the carbs in Chemdip already, sprayed out with carb cleaner, blown with compressed air, and installed new o-rings, so the fact that I'm still having such troubles is making me want to set this bike on fire and roll it off a cliff. It's given me more trouble than my old ironhead harley, so that's saying something!

Thanks in advance.

[msdosp1mp]

It sounds like one or more of your pilot fuel screws is broken off into the carb body.

[Tarbash 27]

Well, first off, you might want to go over the carbs and make sure everything is clean again if you have a dead cylinder. I would start with a #20 pilot jet, 115 main, and needle on 4th position from top to start with. Bench sync carbs. Adjust air screws to the highest idle. Make sure float height is adjusted properly.

Have you done a compression test lately? Make sure valves are adjusted properly.

The pilot jets and air screws will have an effect on idling.

[TheCafeKid]

I'm gonna disagree on the pilot jet size. #20 is way too big. 17.5 maybe but you should be able to get it running right with stock pilots. I think you're lean just a scoosh on 115 as I've found at least a 120 is required but then again I've seen people on here say 115 is good on theirs. I had 115s in mine for a while and it was too lean up top. There is no DJ kit for that bike so don't worry about that. MY suggestions:

stock pilot
somewhere between 120-125 main
needle clip in the second slot from the bottom of the needle
fuel screws 3/4 out
air screw 2 out
this is just a baseline. You HAVE to do plug chops to find the perfect combo. If you don't want to do that then get an airbox and go back to stock or suffer a poorly running bike. You simply can't guess your way to good tuning. Be patient only do one change at a time and write down what happens. Then go back and find the best combo.

[KEITH KRAUSE]

Sounds like you missed something while cleaning or possibly have a tip of a pilot fuel screw broken off in the carb throat. Sometimes you can feel the tip with your finger. If nothing, you can shoot carb cleaner up the hole and see if it squirts through. It should easily. If you feel the fuel is flowing at all other times then that's my best guess for the #4 idling problem.
If all above is OK, then it could be a vacuum problem that's at its most noticable at low rpm's.
You could test for flow to the bowls by draining number 3 and 4, and with the bike on the centerstand/level refill on prime for 10-15 seconds. No longer than that because you don't want a slow flowing carb to "catch up". Carefully drain again to see if the fuel amounts are the same. If #4 is significantly less then you have something blocking full flow and the carb is starving at more than just idle but it's just not that noticable.
Jetting is a bit of a question. Are you sure the paper filters are supposed to be oiled? This is just a stock exhaust without baffles?
You also say the bike has never been running right. That could mean many things need tuning. Sounds like you want to go straight at the carbs but you have to remember that basic inspection/tuning comes first, then carb tuning/jetting, not the other way around. Many here have made the same mistake out of lack of patience. This is a good recipe for frustration, which you already have.
Once at the carbs, they may have a bad history of neglect and worn or incorrect parts installed. Float levels need checking. Bench synch followed by vacuum tool synch. Pilot and side air screws may need tuning.
Your exhaust is not tuned now and the filters questionable. Hard to recommend jetting but you could start with 1 position richer on the jet needles. I don't know what to say for the mains...maybe 5 full sizes (25) up from stock? If things are flowing better than I think, maybe (30). Pilot jet should be no larger than 17.5 but you can first try to make the stock 15 work with richer pilot fuel screw adjustments. Set side air screws to 1 3/4 then tune using the highest rpm method. Vacuum synch is then part of tuning that shouldn't be skipped and would also show you if you have a vacuum problem at #4.

[boingk]

As other have said, make one change at a time.

Firstly, select your mainjet for best pull at high rpm. Analyse this once warm, obviously, as it won't give an accurate indication if cold. I'd suggest going at least 2 sizes up from stock.

After you've gotten the best top-end performance with your mainjet selection, adjust your needle position - think of it as fine-tuning the mainjet. This will give you better performance at mid-throttle.

Once you've gotten the above two right, you can fine tune any mixture screws. These should have been at the stock settings until now, although it can't hurt to adjust them if something is obviously wrong.

Cheers - boingk

[TheCafeKid]

The above poster brings up a good point once you're pretty close and trying to fine tune the mixes. Take note of how the bike runs when cold and when warm. This will go a long way in telling you which way you need to go with your tuning.

Running better when cold indicates the bike is a bit rich, because as the motor warms it naturally will richen up. If it runs better HOT as in whipped like
a rented mule, it's probably a tad lean. Again you need to take
note of how the bike runs in different throttle positions. Remember that RPMs have little to nothing to do with it other than a frame of reference. It helps to mark off your throttle with tape.

0-1/8 turn is the pilot circuit.
1/8-1/4 is what I'd consider your pilot/needle tradition point. This is where you're starting to come off the pilot circuit and into the needle circuit. This position (and the pilot) is generally where most of your even, steady cruise throttle (or "maintainence throttle") riding is done. It's also most often where people experience and notice jetting issues.
1/4-3/4 turn is your needle circuit. Surging can be felt but more often "bucking" is what people complain of. It's a sign that you're lean on the needle clip. To richen, the clip must be moved down on the needle to raise the needle higher in the needle jet, thereby allowing more fuel to pass sooner in the circuit. Sometimes to find the best spot a "shim" or a washer the size of the clip (or two...the clip spacing is usually about the size of the thickness of three washers) is used to essentially set it in between the stock clip settings. Many have found that richening the needle with a shim or two will smooth out some of the flat spots in the STOCK carb set up. More about that when you get closer to your goal.
3/4-WOT is your main jet. Though the very begining of this range is the needle to main transition and often a flat spot in between the needle and wot is noticed (roughly 6000 rpm in second gear usually is most noticeable)

Again plug chops done in these throttle positions will show you how the motor is responding to the jetting settings in these throttle openings. Hope this helps.