Jetting my 78 GS1000

[BadBillyB]

Got frustrated as no adjustment I made to the fuel mix or air screw seemed to make a difference. So I put them back where I started from 1 1/2 fuel mix and 2 turns out on the air mix screw. Same results, bike runs OK but will start and idle fine, in cold weather w/o choking it and the plugs are black as can be, especially 1 and 4....Sputtering from just off idle and worse around 3K RPM.....Adjusting the air mix screw has very little effect on RPM, if any.....So, I decided to move the clip on the needle back to the middle position on 1 and 4 to see if there was any improvement. I did this w/o pulling the carbs for test purposes. When I fired it up my idle went from a normal 1K RPM to a little over 2K RPM......So I idled it back down to 1K and took it for a short ride. The sputtering is all but gone and the low end, up to 3K, feels crisper.......I have learned from working on cars w/carbs that if the throttle is opened up farther than normal to obtain the correct idle RPM (because of whatever problem exists), when you try and adjust the idle mixture, the motor does not respond very well to these adjustments.....I am hoping that after setting 2 and 3 to the middle clip position, my idle will go up again and I can back the slides off, allowing me to trim the idle properly with the air mix screws........So apparently some fuel is being drawn past the jet needle even at idle..........Still not given up as my bike almost purred on the way home from work today.......Billy

 

[KEITH KRAUSE]

Just a quick message as I'm on my way to work.
Compensation jetting, such as having the jet needles at different positions, never works. It may seem to make things improve, but it's only because you've adjusted something wrong or are dealing with worn/damaged parts.
Compensation jetting will drive you nuts.
I'll try to help after work tonight if I can. I'm also working Saturday so I don't have much time to help.
I can tell you that even the cheaper pods and pipes will require raising the jet needles from the factory (3rd) position. AT LEAST the 4th position is needed. If you have at least two plugs running black and the floats, synch, etc, are set correctly, then you have possible coil, compression issues. The motor, with those mods, should easily handle rasing the needles 1 position. Simply makes no sense otherwise.
Are the factory jet needle spacers in the correct order...thicker plastic spacer above the e-clip, thinner below?? O-rings inside in good shape, especially the o-ring around the needle jet/bleeder pipe?
1 and 4 run from one coil and you may have a weak coil too.

 

[BadBillyB]

Had to take a break from my carb issues as it was consuming too much of my time and I was not getting anywhere. Tried lowering the float levels and like Keith said, that wasn't it. The fact that it made an improvement when I lowered the needles back to stock position made me concerned whether the needle jets are possibly worn. The P.O. installed the small plates with the dimple in it...(the one that sits on top of the needle)...upside down, which flattened out the dimple. I have since replaced these with a set from some bone yard Kaw carbs.....dimple down of course. Was hoping Keith or someone could confirm the correct size needle jet for my 78 GS1000N and possibly help locate some new or good used ones........I think an 0-4 jet is the correct size. Is it possible for the needle jets to be worn or ditched out after 30K miles ???......Billy

 

[KEITH KRAUSE]

Stock needle jet is "O-2". Jet needle, "5DL36".
Always possible there could be wear, hard to say if it's contributing to any problem. I kind of doubt it, just 30K.
If the PO had trouble remembering correct re-assembly order of parts, I have to wonder what else may have been done? The thicker plastic spacer is still above the jet needle e-clip? The thinner spacer under the clip? Should be.
You have to take plug reads at specific throttle positions to know what each circuit is doing. Removing the air box lid will greatly increase flow and your pipe should improve flow too. So if the bike is running better with the e-clip in the factory position, something's wrong. It should be lean. Many people are fooled by a lean running bike. They think it feels OK. But the plugs will tell you the truth.

 

[BadBillyB]

Thanks Keith for keeping up with my slow progress. Weathers been too good for downtime and like I said, it runs OK....Plastic spacers are in the correct position. Was talking with a friend who was telling me that the Bandits have an issue with the needle jets having enough wear on them at 30K to cause problems. Also noticed, browsing the web, that other models may also be affected. I pulled one out this morning and it looked (to my 51 year old eyes) like it was maybe pitted from past corrosion. It is a series 258 0-2 needle jet and I immediately sent an e-mail off to motorcyclecarbs.com to see if they are available. Got a quick response and they said that they are still available from Mikuni Corp. and will cost like $14 each. So roughly $75 shipped. I figure that this is a fair price IF it fixes my problem. I totally agree with what you said about the fact that there must be something wrong if the bike runs better with the clip in the 3rd notch. I am pretty sure that it may even be better with it in the 2nd notch. But this is just covering up the real problem and would most likely create new issues at different throttle openings. Number 4 plug always comes out black as can be and my fuel mileage indicates excessive richness after a normal ride. Cyl #3 always has a noticeably cleaner (leaner) spark plug read, but is also a bit on the rich side. I am going to swap the needle jet between 3 and 4 and see if the plugs tell me I am onto something. I became a certified MC mechanic back in 1973 (Standard Technical Institute K.C.) but have never worked in a bike shop. I'm a master technician though and work on cars for a living. It's very humbling to have my butt kicked by these old carbs but I would like to see this through. I feel pretty certain that I have ruled out any ignition or engine mechanical problem. Cranking rhythm and compression are good. Valves are adjusted properly. I swapped the coils and wires weeks ago to rule these out. There is just too much fuel going into the motor at low throttle openings at cruise. Mid-range and top end are very strong and will be tuned after I find out why my plugs are all black........90 percent of my riding is cruising and my gas mileage must be awful......Billy

 

[KEITH KRAUSE]

OK. Good luck with it.
I'll be on vacation 'til next Tuesday. Just so you know if you ask for help.
All I can say for now is to check all the basics I always talk about. Electrical, etc. It seems your problems are carb related though and you just need to re-check things. If I could think of something to add, I would. But there's so many things that can cause richness problems. Funny that #4 is running richer than the others despite setting it up the same? Something's different.
Just don't try compensation jetting again, such as un-uniform jet needle adjustments. Compensation jetting never works.
Richness problems are commonly float level/float valve related, or clogged air jets or bleed pipe, side air screw not adjusted for highest rpm...
I'll check in when I get back to see if you need help. I'm sure others here can help too.

 

[Sandy]

I just read through your struggle with your jetting issues and one common denominator I see is a problem with 1 and 4. Sometimes stupid questions have to be asked and sometimes stupid questions turn out to be not so stupid. Anyway, is there a possibily you're trying to chase down a carb issue when you really should be chasing down an ignition issue such as a bad coil?

 

[BadBillyB]

Dont think it could be an ignition issue. In my last post, I mentioned that I had swapped the coils and wires to rule these out. I also had the problem before I took out the factory points and put my Dyna S in it. I did carefully inspect the ignition advance mechanism for proper function also. Also a week ago I decided to change the oil in my forks and when I tried to take the connector from the ignition switch apart, it crumbled, exposing 4 very corroded connectors. Now it has new connectors. Bike runs the same. Figure I saved myself a future headache down the road. Cant help but consider an ignition problem when 1 and 4 are black and 2 and 3 are noticeably lighter in color. Your advise is well received, but I feel like I have covered all the bases......Billy

 

[wiredgeorge]

First of all, setting the float height is more or less a waste of time on the VM26 series of carbs... The floats have been adjusted many times and typically folks will bend the floats rather than use the float tang so the original relationship of the float will be way off... I suggest you use the service fuel level method and do the adjustments wet by filling the float bowls with an external tank and measuring the actual amount of gas in the float bowl. If you try this method you will find that your float levels are very likely off even if their heights were adjusted. I would also recommend #17.5 pilots. Your problems sound like an off idle circuit issue that is likely tuneable via adjustments to the pilot fuel mixture circuit. I would set the air screws to about 1 1/4 turns out AFTER setting the fuel level (about 3.5mm below gasket line if memory serves) and the pilot screws at about 1.5 turns out from VERY VERY lightly seated. Once you have the bike idling smoothly, adjust the air screws at idle each in turn for maximum idle speed. Then do a road test with the pilot fuel mixture screws set at 1.5 turns using the #17.5 pilots. Use new orings on the pilot fuel mixture screws! 1.5 turns is close and you may need to close the screws just slightly to lean the mixture if the bike bogs some... work only in 1/4 turn increments and try in 1/4 turn first if you have a bog and then reset to 1.5 turns and then out 1/4 turn if the problem worsens when your tried to lean the mixture... You shouldn't need more than 1/4 turn either way from 1.5 turns...

As far as the leaking of the fuel pipes, this is another issue. I fix these pipes from leaking when I rebuild sets by recoating them with liquid nitrile rubber and wrapping them with teflon tape. The combo permanently stops leaks in this area. If you have leaks with the gaskets, double gasket. the other common cause for leaks is a cracked overflow tube. CLOSELY look at the brass pipe in the bowl and if it is cracked, replace the bowl OR use some JB Weld to patch the crack as JB Weld isn't affected by gas.

 

[BadBillyB]

Thanks Wiredgeorge, fuel leaks quit after retightening bowls. New gaskets are bedded in now. Float level on 1 and 4 have been set by using service method w/clear plastic tube. Fuel is even with the surface that the bowl screws contact. This might even be a hair low. Today I swapped the needle jets in 3 and 4 to see if the problem follows the jet. So

With....
The fuel mixture screws at 1 turn out
Air mix screw 2 1/2 turns out
15 pilot jet
110 mains
New O rings
Carbs synched
Needle clip in factory (3rd) position
New needle and seats
Choke plungers synched (metal yoke correctly oriented)

My Plugs look like this (cyl 3 on left cyl 4 on right)

 

[KEITH KRAUSE]

Back from my vacation. Not sure what's happening Billy.
Is that photo AFTER the needle jet swaps? Also, I assume these reads are after "general" riding around town? Throttle position matters in trying to narrow it down. Also, are you still having "richness" trouble with 1 and 4, or just 4? If both 1 and 4, and considering all four carbs are set up identically(?) I'd still have to wonder about the spark quality. I know you said you tried another coil but that doesn't prove anything, especially if it was just another used coil. Was it tested?
Have you tried swapping the 1 and 4 plug leads (and the coil terminals) to the coil for 2 and 3? That would prove any weak coil issue.
Also, the pilot fuel screws are not always set at the same point for best performance. They're the fine tuning of the pilot circuit and are commonly set at different points to allow for small differences in each cylinder. It's common to end up with a final setting of: Cyl 1: 1 turn out, Cyl 2: 1 1/4 turns out, Cy: 3: 3/4 turns out, Cyl 4: 1 turn out, or similar. Larger differences may show something's wrong though. I really don't think this is your main problem but wanted to mention it may be contributing to a problem. You don't just set all four of them uniformly. You'll find this out but you have to reach the fine tuning stage first.
Try the coil swap and let us know what specific throttle position these reads are at and if it's 1 and 4, or just 4. Also, the needle jet swap question.
If you still have a problem, it may help to come up with a list of things that can cause richness in one cylinder and we can eliminate things and find the problem.

 

[BadBillyB]

Back from my vacation. Not sure what's happening Billy.
Is that photo AFTER the needle jet swaps? Also, I assume these reads are after "general" riding around town? Throttle position matters in trying to narrow it down. Also, are you still having "richness" trouble with 1 and 4, or just 4? If both 1 and 4, and considering all four carbs are set up identically(?) I'd still have to wonder about the spark quality. I know you said you tried another coil but that doesn't prove anything, especially if it was just another used coil. Was it tested?
Have you tried swapping the 1 and 4 plug leads (and the coil terminals) to the coil for 2 and 3? That would prove any weak coil issue.
Also, the pilot fuel screws are not always set at the same point for best performance. They're the fine tuning of the pilot circuit and are commonly set at different points to allow for small differences in each cylinder. It's common to end up with a final setting of: Cyl 1: 1 turn out, Cyl 2: 1 1/4 turns out, Cy: 3: 3/4 turns out, Cyl 4: 1 turn out, or similar. Larger differences may show something's wrong though. I really don't think this is your main problem but wanted to mention it may be contributing to a problem. You don't just set all four of them uniformly. You'll find this out but you have to reach the fine tuning stage first.
Try the coil swap and let us know what specific throttle position these reads are at and if it's 1 and 4, or just 4. Also, the needle jet swap question.
If you still have a problem, it may help to come up with a list of things that can cause richness in one cylinder and we can eliminate things and find the problem.

Hey Keith, welcome back and thanks for sticking with me. Photo is before needle jet swap. Cyl 2 looks like cyl 3 (slightly rich). Cyl 1 is almost as black as cyl 4 but not quite.....I didn't use a test coil, I just swapped the plug wires and coil terminals from 1&4 and 2&3.....This rules out the coils and wires....I have already swapped out the jets and re-installed the carbs but unfortunately it got cold again here in North Fla and have not yet done the test ride/plug read. Tomorrow night I will ride it home from work and then back again the following morning. The verdict on the needle jets will be in on wed night. The sputtering/stuttering is at less than 1/8 throttle opening, so the sources of extra fuel are limited, in my mind, to the float level, pilot jet, choke circuit, jet needle, needle setting or the needle jet itself. Looking into the bore of my needle jets, they look like crap, all pitted looking and rough. There does not seem to be an obvious "ovaling" of the holes though. So, I guess we will see what the jet swap tells us before shelling out $70 bucks for 4 new ones.......BadBillyB

 

[KEITH KRAUSE]

OK. Sounds like those needle jets aren't helping things. Should be smooth.
Because it's easier to ask instead of searching back: these needle jets, besides being in poor condition, are the wrong size also, correct? Also, did you go through ALL the electrical connections? Dirty coil conections and plug caps can cause poor spark and a false rich burn. Just trying to be sure all the basics are done.

 

[BadBillyB]

OK. Sounds like those needle jets aren't helping things. Should be smooth.
Because it's easier to ask instead of searching back: these needle jets, besides being in poor condition, are the wrong size also, correct? Also, did you go through ALL the electrical connections? Dirty coil conections and plug caps can cause poor spark and a false rich burn. Just trying to be sure all the basics are done.

Well crapola, nothing I have tried has made any difference. The needle jets are the correct ones Keith, they are series 258 0-2 and swapping 3 and 4 made no difference. All 4 plugs are black after hours of mixed driving, mostly cruising.....I tried different set of stock size pilot jets in it just in case PO had drilled on them. No dice.....Maybe the carbs are fine and I am just missing something.. I rechecked ignition timing and verified that the advance mechanism is working correctly and giving the correct amount of advance....I do know that my timing chain is badly stretched as I locked down the side bolt on the tensioner and then removed it. I then marked how far the plunger was sticking out before releasing it. It was just about at the limit of the tensioners ability to remove slack from the chain. I am wondering if my valve timing is so retarded that it's messing with the bottom end. Especially the intake cam timing as it is a long run to the crank......What do you think ???.....BadBillyB

 

[KEITH KRAUSE]

Just saw your reply Billy. Late for work. Sorry. Will try to help later tonight if I can.

 

[KEITH KRAUSE]

If you think the cam chain needs to be replaced soon, you should do it, especially if you think it's giving you problems. Putting it off is asking for trouble if you try to squeeze every mile out of it. But if the tensioner is still doing its job and there's no significant chain slack noise, I doubt it causing any richness issues.
As for the performance, you said the bike's running fairly well, but the black plugs are an obvious sign it's rich. OK.
I've followed your progress and what you say you've done to fix the problem and it would be easy to say you've missed something or done something wrong, but to move ahead I have to assume you've done things right and haven't cut corners on any of the basics to tuning/jetting. That puts us at a point that doesn't make sense.
You've installed a freer flowing pipe and removed the air box lid. Both of these mod's would lean the stock jetting. You said the plugs were black from general riding around, which would be mostly the jet needle circuit. You said the jet needle e-clips are in the 3rd (factory) position. As far as you know, all the other jetting components are stock, correct float levels, "ballpark" pilot fuel/air screw adjustments, clean carbs, carbs synched, fresh o-rings/manifold o-rings, good spark/timing, compression was checked, etc. But you still have black plugs.
Just wondering. Didn't you have black plugs at only 1 and 4 earlier? But now it's all four? No adjustments since, just more miles of testing?
I see you're on line. Please bring me up to speed and I'll try to think of something to try.

 

[BadBillyB]

Well theres finally light at the end of the tunnel....I got my new needle jets (258 series 0-2) from motorcyclecarbs.com the other day and decided to just install them without making any other changes. I was hoping the bike would run great and it did not. It would not hardly run past idle. It was running OK before the change, but had a "burble" or flat spot at around 3K and the plugs were always black.....Then remembering what Keith said about the jet needle clip needing to be in at least the 4th groove from the top with my particular mods, I decided to try his advise. The bike runs better now than ever before. It is very close to being perfect and I am finally getting good results. The old needle jets were so bad that the bike ran better with the needle clip in the 2nd groove.....I think now that I will try 17.5 pilot jets and go down to a 100 main jet (from a 110) as I can feel it lay down a bit when it gets on the mains.....Seems to run the best with the fuel mix screws at around 1 1/4 turns out, but still experimenting with these....Thanks Keith, you were right, but it only works if you have good parts (jets).......BadBillyB

 

[Hoomgar]

How did I not see this thread until now?!?!? :-s :shock: :?

Looks like the experts got you covered so I wont muddy the water but if you need any additional thought and trouble shooting angles don't hesitate to let me know.

 

[BadBillyB]

How did I not see this thread until now?!?!? :-s :shock: :?

Looks like the experts got you covered so I wont muddy the water but if you need any additional thought and trouble shooting angles don't hesitate to let me know.

Thanks Hoomie.....I thinks its all downhill from here. Tuning carbs with bad parts in them is like tuning a motor with a weak cylinder. Lots of effort and little results....I'm not going nowhere, I dig this forum, so many great minds.......and great bikes......BadBillyB

 

[KEITH KRAUSE]

You say the bike is running very close to perfect but still needs some work.
If the "fuel mix screws" (actually pilot fuel screws) are at 1 1/4 and I assume your pilot circuit reads are lean (?), I'd try richening them a little (about 1/4 to 1/2 turn more ) and stick with the 15 pilot jet. Then re-set the side air screws for best idle at 1,000 rpm. Test, get plug reads at minimal throttle.
I don't know why the bike is "laying down" as you say at open throttle? With the air box lid off and that pipe, it should be able to handle a 110 to 115 main jet. All bikes can be different but going to a 100 main jet is not enough in my experience. I think something's wrong or missed but there's not much I can do from here.
Without reading back, what have you done to the carbs? If you list what you've done, I can try to list things that you may have missed.
I assume the carbs were vacuum synched well?
The inner o-rings are all replaced, especially the o-ring on the needle jet/bleed pipe?
The floats are set at .94 and the float needle valves HAVE GOOD SPRINGS? Not sagged?
The factory jet needle plastic spacers were re-installed in correct order? Thicker one above the e-clip/thinner under the clip?
Floatbowl vent lines removed so the bowls can breath easy?
No possible intake leaks?
Ignition timing, etc, all good?
Valve clearances set?
Compression was good?