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Jetting suggestions?

  • Thread starter Thread starter pasteurized
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pasteurized

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Currently, my '77 GS750 has a Vance & Hines 4-1 and emgo pods. I am running 125 mains, 17.5 pilots, and I know I moved the e clip (to the 2nd from top position?). It is really boggy from idle -> 3k rpm once it is warm. Once totally warm, I have to be very deliberate with the throttle and clutch from a stop because it wants to die if you aren't careful. The fuel pilot screws are roughly 2 turns out each and the air screws are about 1.5-2, but I could never tell any rpm difference between turning them either way.

Does anyone have any jetting suggestions for this setup? It would be a LOT more fun to ride without the bog down low. Makes for riding two-up a nightmare at stop signs.

I am considering locating a stock airbox and using that, does anyone have suggestions for a '750 with stock airbox/filter and a 4-1?
 
I'll assume the bike is ready for jetting. If you read any of my posts I always say the jetting comes last. So that means good compression, valve clearances good, no intake leaks, clean carbs, carbs bench synched followed by a vacuum synch, floats set, ignition timing/spark/points good, fresh plugs, clean filters, good fuel flow, good carb/tank venting.....if all are good, then you know any problems are most likely the jetting. Any of the above things not checked can cause problems and make jetting a PITA.
First thing, BE SURE the two floatbowl vent lines are removed and leave the ports open.
I like to start jetting with a plug read/performance check but I only have your above description to go by. The problem you mention is a bog off idle.
The most glaring thing I see in your post is you have the jet needle e-clips at the second position from the top. I've never had a VM equipped 750 run correctly with the jet needles set that lean. Two positions richer will probably be right, possibly three. I've often seen the stock jet needles not work at all and then it's time for a stage 3 jet kit. Only problem I have making an exact suggestion is that I've seen contradictions on what is the factory e-clip position on '77 models. Some say the second position from the top, some say the third. The jet needle positioning is the most work and it's nice to get it right the first time if possible. So I'm saying if the factory position is number two, then place the e-clips in the fourth position from the top. If the factory position is number three, then place the clips at the bottom position. If you don't know, then I'd say place them in the fourth position, which will be two positions richer than they are now. Be sure you place any factory plastic jet needles spacers in the correct order during re-assembly.
After adjusting them, you must bench synch the slides as best as you can, then follow that with a vacuum tool synch. Accurate plug reads/performance checks depend on a good carb synch. I have a detailed/easy to follow post about bench synching the VM carbs at this site somewhere if you want to search. Sorry I forget where it is. I think it's at Hoomgars thread about jetting his 1000?
If the carbs need cleaning, I suggest you do it now and replace all inner o-rings with Robert Barrs o-ring kit. Also be sure the manifold o-rings are good at this time. More work I know, but it saves time in the long run.
Test the jet needles at 1/3 throttle and chop off and read.
Your main jet choice sounds good to me. Full throttle test will tell. If not right, you're certainly close according to my past experience.
As for the pilot circuit, it's hard to say where you're at without plug reads and specific performance notes at idle to low speed cruising. Past experience says a 17.5 pilot jet may be fine but you shouldn't need the pilot fuel screws (underneath) out two turns to further richen the pilot circuit. Stock pilot fuel screw setting is closer to 1 turn out in combo with a stock 15 pilot jet. Many times, especially with Emgo pods, the pilot circuit runs good with the stock jet and just richer (additional 1/2 to 1 turn on the screws). I usually try to adjust the screws out to about 2 1/2 turns before I give up and go with larger pilot jets. If I go with a 17.5, I generally get good results by setting the screws back to something close to 1 turn out. The screws simply fine tune/assist the pilot jet. They have quite an effect and I always give them a chance first.
I can't be sure, but I wonder if you went with a larger pilot because of your off-idle bog? The jet needle on VM's overlaps/kicks in much sooner than many think and a lean jet needle will often be mistaken for a pilot circuit problem.
After setting the pilot fuel screws, set the side air screws using the highest rpm method. If you get little or no change while attempting to achieve highest rpm, that suggests a pilot circuit problem (dirty carbs, incorrect pilot jet size, intake leak) or you're adjusting at too high an rpm. Always set at 1,000 rpm's or very close to that.
I get pilot circuit reads after minimal throttle position/cruising around 4th/5th gear at about 40 mph level or slightly uphill. Pilot reads are hard to get accurately and minor adjustments to the pilot fuel screws can take several miles to show a change in plug colors. It takes patience sometimes. If the bike starts up well and warms up in a fair amount of time, doesn't "spit" out the pods at lower speeds, and decel' pop is acceptable, then good performance in those areas usually means the plug reads will be decent too.
Hope this helps. Your 750 will run noticably quicker with the pods/pipe if jetted correctly. Expect to lose about 3 mpg if ridden the same as before. If you're not that concerned with a little more power and/or this re-jet is difficult for whatever reason, maybe you should consider a stock intake and exhaust. You'll still have to be sure all the other basic maintenance items are done if you want a good running bike though.
I don't have much trouble re-jetting the VM carbs if the rest of the bike is tuned right. But it can still take some patience and possibly taking things apart a few times. Each bike can be a little different.
 
Just re-read the last part of your post. I thought it said you may want to go back to stock intake AND exhaust, so I made a comment about that at the end of my post.
If you keep the pipe only, then you need to install the air box, with lid I assume, first. Then install the stock 15 pilot jets. 17.5 is simply too large for a pipe only. Try the pilot fuel screws at 1 1/4 turn out for starters. Set air screws as always.
Your STOCK jet needle e-clip positions are the question here, again. A pipe only mod sometimes doesn't require the jet needles to be richened but only 1/3 throttle tests/reads will say for sure. Whatever the factory position is, will POSSIBLY work. Test and get reads first. If the pipe really flows better than the stocker, then you WILL run a little leaner than stock with the needles at the factory position. If plug reads say a little lean, I bet a 1/2 position richer e-clip adjustment will be right. A 1/2 position richer is achieved by using an approx' .022" thick jetting spacer. If your stock e-clip position is number two position from the top, then you would place the e-clip at the third position from the top AND place the jetting spacer directly on top the clip. This is in effect, position "2 1/2". If stock is number three, then you'd go to position four and place the jetting spacer directly on top which would be position 3 1/2. Re-install any plastic spacers as factory installed.
Remember you MUST re-bench/vacuum synch any time the jet needles are moved or disturbed.
 
Dang. After all that yakkin' above, I forgot about a main jet suggestion for the pipe only.
That's a little difficult to get right the first time and I'm off to work in a second, but I'd allow 3 or 4 steps up from stock. So that's a 7.5 to 10 increase. Just a guess you know.:smile:
 
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