Jetting my 78 GS1000

[Guest]

Looking for some advise on pilot and main jet sizes for my particular application. Motor is bone stock except for a VHR header. I run it with the stock air filter lid off, through a factory air filter. I ordered 20 pilots and 105 mains, thinking this would be real close. I am at sea level here in Fla. so keep that in mind. Carbs are freshly stripped and dipped with all new "O" rings, new 2.5 needle and seat, and new bowl gaskets. They are now sitting, fully assembled, meticulously bench synced, waiting for jets to arrive.....Anyone think these jet sizes are too rich ???.......Billy

[KEITH KRAUSE]

As always, trial and error but I'll give it a shot.
Stock pilot jet is #15...so I'd try an "additional" 1/2 to 3/4 turn out on the pilot fuel screws underneath. If that doesn't work, then I'd re-set the pilot fuel screws to where they were and try a 17.5 pilot jet. 20 is too much for just a lid removal and pipe.
I'd probably try 112.5 mains to start.
Your mods will require the jet needles to be raised. I'd try 1 position richer than stock on them. So place the jet needle e-clip in the 4th position from the top.
Set float levels to .94".
Removing the two floatbowl vent lines will help the bowls vent better and avoid fuel starvation. Leave the vent ports open. Don't try using shorter vent lines...doesn't work.
After disturbing the jet needles on your VM carbs, you MUST re-bench synch the slides, followed by a vacuum synch.
Get plug reads at 1/3 throttle for the jet needles...full throttle for the mains...and minimal throttle (4th gear cruise at about 35 mph) for the pilot circuit.
Do what the performance/plug reads say to do.

[BadBillyB]

Thanks for the input Keith. I have a brand new set of 110 mains, so I will try those and see how the plug chop goes...Following what you said, I will be trying the 15 pilot jets with a slight enrichment of the fuel mix screws. These have been adjusted before by the PO, so I am assuming your talking about 1 1/4 to 1 3/4 turns out correct??...... About the needle clip position, I was thinking of leaving it in the 3rd from the top (stock) for now, and getting a feel for how my low to mid throttle response is. The carbs are bench synched to the max, and I hate to do it all over again unless you feel pretty strong about moving the needles. Not trying to put pressure on you but I respect your experience and wondered what you would do if they were your carbs......It's a PITA to change the needle clip positions....Thanks...Billy

[KEITH KRAUSE]

I figure an additional 1/2 to 3/4 turn may work instead of a larger pilot jet. That would generally mean the screws would be somewhere between 1 1/4 and 1 3/4 including the additional richening as you said. If I tried to make my guess, I'd start at 1 1/4 to 1 1/2. If there's excessive decel' pop or slow warm up or slow to return idle (lean conditions), etc, I'd try the 17.5 then and tinker with the screws again. You may find the screws will not be needed to be richened much. Hard to say sometimes how the pilot circuit is effected. Hard to get accurate/consistant reads too. Near impossible without an actual vacuum synch.
Your 110 mains may work. Like I said, just guessing. I haven't jetted as much for air box mod's as I have pods, but I believe somewhere from 110 to 115 should work. That's why I decided on 112.5 trying to get it right the first time.
I just looked up some old jetting notes I kept for your model with no lid and RC 4-1 pipe (no baffle) the only mod's, and I first tried 120 mains and the bike fell on its face during any roll ons. The bike ran well with 115 mains. The owner didn't want a baffle and I can't really say how that effected the jetting on that bike, if at all.
I must have given him the other notes because I never throw this stuff away. I wish I could remember what we had to do for the pilot circuit but I don't. But I believe my above suggestions will work for the pilot circuit.
As for the jet needle, I assure you that removing the lid has quite an effect on air flow. I know that the jet needle is the most work, but there's simply no chance that the bike will have the correct mixture from just off idle to 3/4 throttle unless you raise the jet needles. Don't let a lean mixture fool you. Lean mixtures can make more power and you'll think the bike runs fine. Some people don't feel surging/hesitation as easily as others. Only if you have low compression, rich float levels or other compensating issue, could you see a tan/grayish color on the plugs at 1/3 throttle if you leave them in the factory (3rd) position.
With most pods/pipe combo's, the stock needles ALWAYS have to be raised to the 5th position and even then the stock needles frequently won't work and a jet kit must be used. With your lid off, I feel the stock needle will probably work and I doubt you would need to go to the 5th position, but the "4 1/2" position may be needed which would mean you do install the e-clip in the 5th position, but you add an approximately .023" jetting spacer directly on top the e-clip to achieve the 1/2 position richer adjustment. I still think you should try the 4th position first though. In all cases, BE SURE the stock plastic jet needle spacers are re-installed in their factory order. Thicker spacer (ring) on top the clip, thinner spacer (plate) below.
I know what work is involved and I'm trying to help you get it right the first try. Short cuts will just add time and trouble to your re-jet. If I guess wrong, then I'm learning all the time too.
Bench synching these carbs is very easy if you have decent eye sight. A vacuum synch must follow though to get ACCURATE plugs reads and to fairly judge performance.
Let me add that the ignition timing and valve clearances need to be within spec' to accurately vacuum synch. Also before vacuum synching and with the initial pilot fuel screw adjustments made first, you must adjust the side air screws using the highest rpm method. Set them initially to about 1 3/4 turns out, warm up the bike fully and adjust them as I've mentioned in many past posts. Then vacuum synch. I give a detailed bench synch post for your carbs somewhere here. I know I gave that info to Mark (Hoomgar) in his post asking for me to help him re-jet his 1000 (1000 re-jet/poor gas mileage/Keith Krause)...was the topic?. If you need that info, look it up or let me know and I'll try to find it. The synch, both fully closed and fully opened positions, must be done right as an important part of any re-jet. A good bench synch makes the vacuum synch go much quicker and easier.
Lotsa work, I know, but I'm just telling you what I would do if it was my bike, as you asked.
PS: I hate the new editing thing. The old editing didn't show you made edits unless someone already posted before you could edit your latest post. It makes you look kind of stupid, like you're changing your mind or something.
My edits are because I spelled things wrong and didn't want to leave them. Just so you know.

[BadBillyB]

Thanks a bunch Keith. Finished putting them together today, but have not fired it up yet. Changed the needle position to the 4th clip groove, went with the 15 pilots, 110 on the mains and set float height to 24mm w/o the gasket. The synch was way off afterwards, as you said it would be, so I did it again. Should be warm enough tomorrow afternoon to take it for a ride. Went with 1 1/2 on the fuel screws and 1 3/4 on the air screw.

The bike ran fairly good before except for some surging/sputtering at low rpm. Had a friend synch them with his carb sticks, but wasn't much better. It also popped and spit from the carb and the pipe when warming up with the choke.....I didn't find anything wrong with them after teardown except the bowl gaskets were deteriorated right aroung where the emulsion tube for the choke goes through it and down into the bowl. Never thought a bowl gasket could cause issues but I hope the new gaskets will make a difference......Thanks again Keith......Billy

[KEITH KRAUSE]

The surging and spitting you describe are an obvious lean condition. As you said, most noticable at lower rpm/smaller throttle openings, which always brings out any lean issues. Raising the needles will help, and hopefully, eliminate it and give the plugs a good burn.
Spitting out the carbs during warm up means it was pretty lean. Wish you'd mentioned that before. That doesn't mean that pilot fuel screw adjustments won't work with the stock 15 jets, but we'll just have to see. I may have leaned toward the 17.5 pilots knowing that. Changing the pilot jets is easy though if it must be done. Incorrect float levels and dirty carb passages can cause spitting too. Hopefully you have the correct length of pilot jet in there now. They do come in two lengths and I've seen some unusual changes inside of carbs in the past. Most likely, yours are the correct length.
Good luck on the testing and let me know how she runs. Be sure you did that vacuum synch very well, along with all other basics... clean carbs, valves, timing...Even the side air screw adjustments are important. I'll assume you adjusted them for highest rpm with a base setting at 1,000/1,100 rpm's and BEFORE the vacuum synch. I know your vacuum synch would already tell you if there's improvement, before the other testing. Some guys just pre-set them to a certain position and figure that's good enough, which isn't true. They must be tuned, finding the "sweet spot".
Jetting the VM carbs isn't that hard if you know any problems are truly because of the jetting and not something to do with other tuning issues or the correct order to do things. Mixing jetting and troubleshooting is a PITA.
I can tell you that not all re-jets go perfectly, not even with jet kits. Sometimes you'll have to accept minor annoyances, or "driveability" issues, as jet kits put it. For example, you may still have some minor spitting at warm up, but a larger pilot jet may eliminate that but cause a more annoying problem in return. It doesn't take much to create some minor spitting or decel' popping that just can't be jetted out because of jet needle taper, slide cut-away or just the design of the parts you're using. I've seen many jetting problems go away by trying a different pipe instead of different jets. Of course, most of us aren't going to change any parts and so we have to live with the best jetting we can come up with.

[BadBillyB]

Keith, all the spitting and popping happened only with the choke on....I am 99 percent sure that the float bowl gaskets cured that issue. The bike would not start before without using the choke. And you could not turn the choke off for about a minute or it would threaten to stall. After warm up the only issue was a bit of surging/sputtering at very low rpm (slow traffic). The only parts I replaced were the gaskets, the mains and the needle and seat. I fired it up today and it started instantly with no choke and idled right at 1K RPM. I blipped the throttle and it sounded and felt real nice. Let it warm slightly and the idle stayed right at 1K...Nice...Too bad it's raining and its going down to about 25 degrees here in Fla come Monday, so the test ride maybe a few weeks ahead. My fuel mix and air mix settings are pretty much the same as they were before. My final conclusion is that the FB gasket was the main problem and all the other changes are just going to make the bike run smoother and stronger.......Being a master tech (cars) for over 15 years, I made sure the compression, the ign timing, valve clearances, intake "O"rings, etc. were spot on before taking the carbs off. I think it just paid off...Will post test ride results when Mother Nature allows.....Thanks again ..Billy

[KEITH KRAUSE]

OK. Good luck on the testing. Be careful.

[BadBillyB]

For the most part, carbs work pretty well.....I cant seem to get rid of the sputtering right at 2 1/2 to 3 K RPM....It's not too bad but noticeable. As this is around 1/8th throttle, it's just about got to be an issue with the pilot circuit. Turning in the fuel screw to 3/4 turns out and the air mix screws to around 2 1/2 turns out made the symptom noticeably less...but still there. I am thinking either the float height is too high or I just need to sync them with my friends "carb styx"..........Pulling slightly up on the choke when it is sputtering seems to make it worse.....The carb bowls have fuel on and around them after sitting for a while and I think it may be coming from either the bowl gaskets (level too high) or from the fuel supply tubes that connect from carb to carb. The supply tubes seem to be obsolete so I hope it's not from them. I am planning on using fluorescent dye to pinpoint the source of the fuel leak, but that will require removing the carbs from the bike. The weather has been so nice here in Fla. , I think I will get some riding in first......Any thoughts on the float level. I set them W/O the gaskets in place like my Clymer book said to do..............Billy

[earlfor]

At 2 1/2 to 3k rpm, you would be in transition between pilots and needles.
I think you can solve your problem by resynching your initial vacuum levels to a value about 5 to 10% less than what they are now. Yeah, I know the #3 carb is the control carb and there is no adjustment for that one, but you can change its draw by tweaking the other carbs.

Earl

For the most part, carbs work pretty well.....I cant seem to get rid of the sputtering right at 2 1/2 to 3 K RPM....It's not too bad but noticeable. As this is around 1/8th throttle, it's just about got to be an issue with the pilot circuit. Turning in the fuel screw to 3/4 turns out and the air mix screws to around 2 1/2 turns out made the symptom noticeably less...but still there. I am thinking either the float height is too high or I just need to sync them with my friends "carb styx"..........Pulling slightly up on the choke when it is sputtering seems to make it worse.....The carb bowls have fuel on and around them after sitting for a while and I think it may be coming from either the bowl gaskets (level too high) or from the fuel supply tubes that connect from carb to carb. The supply tubes seem to be obsolete so I hope it's not from them. I am planning on using fluorescent dye to pinpoint the source of the fuel leak, but that will require removing the carbs from the bike. The weather has been so nice here in Fla. , I think I will get some riding in first......Any thoughts on the float level. I set them W/O the gaskets in place like my Clymer book said to do..............Billy

[KEITH KRAUSE]

A vacuum synch is a must when re-jetting. You can't judge performance or get accurate plug reads without it. If it STILL does it...
What you're describing sounds like a richness issue. The problem is actually happening right when the cut-away controls things. You can try tweaking the pilot fuel screws to a point, but this is generally a sign of the stock jet needle not working well. But vacuum synch first.
And by the way, you don't just turn the side air screws anywhere you want. They need to be set using the highest rpm method. If the rest of the bike is in tune, this method works well. Adjust these before the synch as I said earlier.
You shouldn't have a float related problem if you set them to .94" correctly.

[BadBillyB]

The problem is actually happening right when the cut-away controls things. You can try tweaking the pilot fuel screws to a point, but this is generally a sign of the stock jet needle not working well. But vacuum synch first.
And by the way, you don't just turn the side air screws anywhere you want. They need to be set using the highest rpm method.
You shouldn't have a float related problem if you set them to .94" correctly.

The sputter or flat spot is right at 1/8 throttle as indicated by my tape marks. As the slides are the correct 1.5 stock pieces, the problem has got to be in the pilot circuit. The air screws were approaching 3 to 3 1/2 turns out and the rpm's were still climbing, although not as smooth and obvious as you would like to see.....Today I pulled the air box off and idled the motor with open carbs. Using a propane bottle set at a very low rate, I went from carb throat to carb throat and got different results....Cyl #1 dropped rpm, cyl#2 picked up rpm, cyl#3 picked up rpm and cyl#4 dropped rpm....So 1 and 4 are rich and 2 and 3 are lean....Question is should I stagger the pilot fuel settings or the pilot air settings to make them even?? Or should I stagger both ??

Keep in mind the bike runs excellent except for idle to a little over 3K.......Billy

[BadBillyB]

I am fairly convinced that the extra fuel (that is causing 1 & 4 to be so rich)must be coming from the choke circuit. I tried plugging the holes that feed air to the choke circuit on the air filter side with no change. That does not mean, I dont think, that there is not fuel being drawn through the choke circuit fuel supply. The sure fire test would be to plug the choke delivery hole on the engine side of the slide and see if the fuel mix leans out. The only problem with this is, if whatever I use comes loose, the motor is going to eat it up. Maybe I could fashion a tapered plug out of a pencil eraser and run a piece of dental floss through it for a safety device just in case. The floss could just rest under the slide at idle. If I did this on one cylinder (like #1) and saw a definite lean out (using the propane and plug color methods), this would confirm if the extra fuel is coming from the choke circuit. It's the only thing that makes sense...Double checked my float settings today and they are right on the money. Re-tightening the float bowls fixed the fuel leak I had up to this point........I'm in pursuit of perfection and am not ready to settle for less....yet.......Billy

[80gs1000e]

Loving this thread, hopefully in a month or less I'll have my MAC 4 into 1 and Dynajet stage 3 kit w/ k&n pods. Gonna need tips like these.

(subscribed to thread)

[KEITH KRAUSE]

Just a quick reply, gotta get to work.
Opening the throttle by-passes the choke circuit. I don't think that's your problem. Have you vacuum synched the carbs? Bench synching isn't good enough. Will try to reply better later tonight.