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Jetting my 78 GS1000

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    #16
    As keith said, you must vacuum synch the cabs and ACCURATELY. You cannot adjust squat on a carb bank if they are not in synch.

    Earl
    All the robots copy robots.

    Komorebi-The light filtering through the trees.

    You are free to choose, but you are not free from the consequences of your choices.

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      #17
      OK. Got a little time to add some thoughts.
      As I've said before, short cuts just cause frustration. The carbs must be vacuum synched, preferably so the difference from the highest to lowest level is about 1/2". The side air screws must be set using the highest rpm method, using 1,000 rpm's as a base point and THEN the carbs are vacuum synched. Start this adjustment with the side air screws out 1 3/4 turns. Valve clearance adjustments and correct ignition timing is also required before the synch.
      On the VM carbs, it's best to adjust any higher vacuum level(s) you initially see, DOWN to meet the lower level(s), then fine tune. Torque the slide adjuster screw holder nuts to 3.5ft/lb.
      I doubt your problem is choke related. You should have a small amount of slack after the plungers are fully seated. Easy to see. Slowly lifting up the choke knob or pressing the choke lever should show the plunger brackets moving just a bit before the plungers start to rise. The brackets are fixed but are slightly adjustable if you see a problem in their uniformity. The brackets also must be installed "right side up" if they were ever removed from the shaft. Look at the shape of the little bump that meets the plunger. The slightly flatter side (yes, there's a difference) should point upward. You can see how reversing this bracket can cause the plungers to be out of synch and possibly stop a plunger from completely sealing. Sometimes the brackets are out of synch because the shaft set screws are cocked a bit. Look at yours, just a possibility.
      If your plungers have hardened rubber parts, then complete sealing will be impossible.
      I mentioned the cut-away only because it's the transition from pilot circuit to jet needle. Jet kits mention the slide must sometimes be changed to get good results, but only RARELY. This wouldn't be the case with your bike. However, as I said earlier, the stock jet needles are notorious for causing problems when the intake is increased significantly. They almost never work well with pods, but sometimes do with just air box mods, such as removing a lid. But only sometimes. I'm a big believer in the jet kits for the 1000. Their more tapered jet needles just work better. You may yet be able to tune well with the stock needles, but maybe not. Don't get into compensation jetting by trying to make another jetting circuit change the mixture of another.
      It's very easy to have a problem that you can't find. Even taking the jet needles out to adjust their clips can cause a problem. How? The needles have a spring to keep them under tension and so they don't rotate with vibration. A dimpled plate that goes over the entire assembly forces the needle to cock against one side of the needle jet. That's because the dimpled plate can't sit flat, so the needle can't point straight down. This set up stops rotation/vibration, but also causes one side of the jet needle to wear and also the needle jet. You can then see how removing the needle and re-installing it will change how the jet needle and needle jet "fit". The exposed wear areas will change how mixture flows. You may not like the results, and not knowing, start blaming other parts for the problem you have.
      Simple pilot fuel screw (underneath) adjustments will tell you if they are the answer. Keep a record of every adjustment. Then try them at minimum/maximum extremes, then fine tune. If the bike ran well before with them 3/4 turn out, then you KNOW the minimum setting, after lid removal, will be no less than 1 full turn out, to a maximum of about 1 1/2 turns out. Take a plug read for the pilot circuit, or judge by symptoms such as slow warm up, hanging idle rpm's, etc. Nothing special about your bike, it should run well with the screws set somewhere in that range. That is, if everything else has been set correctly.
      As for your side air screws being adjusted at 3 1/2 turns or close to, that's incorrect and shows a problem. The VM pilot air screw only regulates air. Turning it out increases air flow. On every correctly jetted/tuned VM equipped bike I've jetted, these screws end up generally 1 1/4 to maybe 2 turns out. You still have the stock 15 pilot jet and the lid removed. You're bound to be lean if anything and turning the screws out to 3 plus turns is just getting leaner. The pilot fuel screw factors in here too but has a limited range of effect. If the pilot fuel screw are about 1 1/4 to 1 1/2 turns out, the side air screws should be about 1 3/4 turns out. I told you to adjust them for highest rpm but if the rpm's keep rising at this point, you must be lean because of a problem somewhere/possibly intake leaks. Backing them out that far, especially if they have older o-rings around them, can cause leaks. No other explanation for the air screws giving those results.
      For just lid removal, a simple increase of about 1/2 turn on the pilot fuel screws should work and then the air screws set as I've said. If the pilot fuel screws don't give good results by the time you have them out to 2 turns, then you would need to try a 17.5 pilot jet and return the air and fuel screws back to something close to a stock bike and start over.
      I can't stress enough how important a vacuum synch is. Too much difference in vacuum between cylinders will cause mixture differences. You can't get plug reads because you don't know which one to use as a base point. You can't judge performance either. Many members don't synch, or try to get by on a bench synch because they don't have the tool. Vacuum synching is a necessary part of the re-jet.
      Hope this helps. I'll be busy this weekend but will try to check in if you still need help.
      And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
      Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

      Comment


        #18
        By the way, I suggest synching at about 3,000 rpm. Some have there own rpm preference. Synching at higher rpm's builds heat fast. Use two large fans.
        Synching near normal idle speeds is a waste of time, to me.
        Also, remove the float bowls vent lines, if you forgot to. Leave the ports open.
        And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
        Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

        Comment


          #19
          Thanks, Earl and Keith for prompt reply's.........I borrowed my friends Motion Pro set-up Saturday and found my bench synch to be very close. Made only a couple minor adjustments. Attempting to block the chokes fuel supply circuit did not give conclusive results. My focus is once again on the float height. It does not seem right to me that the float bowls leak fuel from the new gaskets. Using fluorescent dye in the fuel confirmed this.They never leak from the overflow tube. Needles and seats are new. I have transparent (clear) bowls on my Lectrons and you can clearly see the fuel is around 3/4 of the way to the top of the bowl. Forgetting float height setting, what level should the fuel be at in the bowl ?????.......Billy

          Comment


            #20
            Factory range for your carbs is .90 to .98" I set them at .94" in most cases.
            I have found the bowl gaskets will leak even if new. Has nothing to do with float level. I always replace the stock bowl screws with SS Allens and even then can get a little "varnishing" showing after a year or so, despite being able to snug them down much better. You'll find the gaskets give a bit and tightening them up a couple times after initial tightening may help. Just don't over-tighten. The bowls on the VM carbs can go in slightly cocked if you're not careful. You have to get a feel for it. You have to put them up and kind of jiggle them slightly and you'll feel them seat a little better. Hard to describe really.
            I have many times used a gasket sealer to assist sealing. "Permatex-Form a gasket sealant 2" is what I use. It's non-hardening, stays pliable. Obviously, just a very thin ribbon is all that you want. Don't want to get extra floating around after tightening. Works for me.
            I wish I could check the bike myself for this 1/8 throttle problem you have. Kind of hard to guess over the web. Should be easy to test and narrow it down to at least a rich or lean symptom. Does it get worse as the bike fully warms up, or better? It's still possible the jet needles just won't co-operate, but it seems to me you still have some tweaking you can try. There is quite a bit overlap effect at lower throttle positions between the pilot, cut-away, and jet needle.
            Have you done minimal, 1/3, and full throttle plug reads yet?
            And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
            Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

            Comment


              #21
              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

              Comment


                #22
                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.
                And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
                Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

                Comment


                  #23
                  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

                  Comment


                    #24
                    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.
                    And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
                    Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

                    Comment


                      #25
                      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

                      Comment


                        #26
                        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.
                        And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
                        Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

                        Comment


                          #27
                          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?
                          '84 GS750EF (Oct 2015 BOM) '79 GS1000N (June 2007 BOM) My Flickr site http://www.flickr.com/photos/soates50/

                          Comment


                            #28
                            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

                            Comment


                              #29
                              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.

                              Comment


                                #30
                                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)

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