77-79 GS (550) Jetting questions, poor performance due to carbs, +upgraded exhaust

[rrok220]

My girlfriend has a 1977 GS550 that we have been tinkering with for a while. It had a stuck valve at first and wasn't running on all cylinders until I started fooling with it. This is after she worked out numerous problems on her own to get it halfways roadworthy. I did some compression testing when I discovered only 90psi in #4, 110-120 in the other cylinders on three cranks with WOT. Wet with two oil jug caps of oil was in the 150-170 psi range. I thought there may be some wear on the rings and bores, but this sounded reasonable. Then I read in the manual that 100psi or less means rebuild the engine, but acceptable is 128-170psi. I have to go and retest these again by the manual instructions of "crank engine until the gauge needle ceases to rise."

The bike is rideable, but was falling on it's face on the highway with two riders at rpm's above 6000, and struggles to keep up with highway traffic, hills, headwinds, and won't go over 80. The bike sounded great accelerating from 4000-5500rpm,s but then would start to die out. The previous owner had "rebuilt the carbs," installed EMGO pods. We upgraded to electronic igniton and I increased the spark plug gap to the max that I read on here of around .030".
Due to one side of the exhaust having an increasing crack in the muffler case that eventually split, I chopped them off, welded on a 2" stub of pipe to the Y-pipes, fabbed up some custom L-brackets, and mounted a set of straight through EMGO "Dunstall-replica" straight through glasspacks. Sound is great up through mid-rpms! Now, however, I can hear the right side missing a lot, and sometimes just cutting out for a split second completely, and it seems worse at higher speeds.

The #3 and #4 exhaust headers were already discolored (side with giant crack in muffler). This cued me into the fact that they were probably running pretty lean especially with the free flowing right bank exhaust.

I haven't really looked into valve clearances and shimming, but it seems that the cam chain on these probably isn't the culprit (self adjusting?), and I am left believing that the carbs are just all messed up even after her friend cleaned them (without compressed air, and not understanding what "checking float height" meant (ouch!). I looked at them and saw cracks in the brass tubes that dip into the fuel bowl (where the needle jet goes into???), and one of the plugs in the end of one of the cracked brass tubes (is that the main jet?) totally missing on one, where her friend said "oh that's probably not too important."

We also ran the bike on the "PRI" petcock setting to see if that diaphragm was bad, but no change in riding performance, and also ran with choke slightly on to see if that pointed to lean mixture due to extra free flow on intake/exhaust.

Anyhow, I am trying to narrow things down, but I am pretty certain that the carbs are definitely a huge part of the problem, and who knows where they are set at as far as the notches on the slide/needle. Left bank side plugs were looking fairly fouled out.

I also noticed when I rev it with the timing light on a plug wire that I see an occasional miss in the timing light. Wondered if when custom cutting plug wires for the ignition upgrade, if something went wrong there?


Looking mainly for some guidance on these issues, just dug up all the stock carb specs, and need to know what we should change with the jetting and carb settings with the upgraded exhaust and air filters?
Also looking to talk my coworker into selling me his almost finished 78 GS750 bobber style project bike now that I'm so familiar with her 550!

Thanks a lot!

Chuck & Reda

[rrok220]

Also I wasn't looking to buy an entire jetting kit for $100, but hoping just to buy one set of jets and try them out, maybe a second set if the results left a little to be desired still.

EDIT: just looked on the Dynajet website and they don't even list a 77-79 kit for the models I scrolled through, and in 550, nothing at all. some of the models in the 550-800 range listed back to 80 or 83.

Here is what I have compiled from this forum, some of these fields with question marks might need to be labeled as N/A for the VM22SS carbs, and that may be why they were left blank in the spreadsheet .jpg file.

http://www.sudco.com/Carburetor01.html looks to be the best place I have be pointed at so far to get parts for these carbs


VM22SS GS550 1977 specs

ID 47110
idle RPM 1150
fuel level 4.0mm +/-1.0mm
float height 24.0mm +/-1.0mm
main jet #80 stock
main air jet 1.4mm stock
jet needle # 5DL35-3
needle jet # O-6
pilot jet #15
pilot air jet N/A
second pilot jet #50
throttle valve cutaway 1.5
throttle valve ????
bypass 1.0mm
pilot jet outlet 0.6mm
valve seat???
starter jet #????
pilot screw 1 (turn out???)
pilot air jet #??????
cable play ????mm
mixture screw setting 2 turns out

[tkent02]

Adjust the valve clearance and then retest the compression.
Jets, there's no kit, just take an educated guess and buy them individually from Z1.
Ditch the Emgos, they don't filter much nor do they flow much. All they do it create tuning problems.

[rrok220]

K&N pods instead? I think they are EMGO's, but not 100%. Previous owner's upgrade. The EMGO Dunstall style mufflers are straight through, but do have a perforated baffle that covers 100% of the inner core, but still perforated so that I can still see straight through them.

[rrok220]

Referring to a Vance and Hines instruction sheet for their 4 into 1 exhaust, and it recommended that for a GS550 (stock #80) to upsize to a #97 main jet. I was unsure of what years the recommendation was good for, as I am not sure if this would hold true for 77-79 VM22SS carbs the same as it would for 80+ CV(?) style carbs. Judging from another post on here regarding GS750's, it sounds like a move from 80 to 97 is just about right on the mains, and if I put K&N's on it, maybe go to a 100 or 102.5 mains? Pilots stay the same apparently, and you need to raise the needle a notch or two.

[rrok220]

Almost forgot that when her friend "cleaned" the carbs, I noticed that two of the needle jets had cracks running up and down them somehow... Now I look in the Sudco catalog, and the #257 is what they spec for the needle jet, but factory GS called for an o-6, and I see the o-6 in that catalog spec'd for a VM20, and referred to as a #239??? Maybe this is due for a different topic post. I see a VM22 listed as 257 N-8 Needle jet, but then a VM26 listed as a 182 N-8. I am not sure what this means, as I did not see the o-6 repeated as I did this duplicate of the N-8, nor do I know what the two mean. I think some of these VM22's are newer styles, as the spigot style diagram looks nothing like the one on our GS550.

see new topic on this

[Deano]

Hi, For main jets 102.5 would be about right and move the needles up one notch if i remember right.I have 105's in mine w/k&n's + Kerker but that was setup at sealevel when I used push the motor hard all the time so it tends to run a little rich now. You will need to get the parts or different carb body. I never took them out of mine so i'm no help with that. Also the points and valve adjustment need to be right or you'll have no luck setting up the carbs.

[almarconi]

The bike is rideable, but was falling on it's face on the highway with two riders at rpm's above 6000, and struggles to keep up with highway traffic, hills, headwinds, and won't go over 80.

Tall task IMHO for a 550 depending on how much you weigh. I weigh about 230lbs and a 550cc bike was minimal on the interstate. The 750 is adequate for the interstate with me on it but I don't think I would want to ride 2 up on it.

As far as your tuning problems, you'll need to replace the cracked parts in the carbs first. Really no sense fooling with the carbs until the valves are checked/adjusted. Once carbs are cleaned properly and valves adjusted then you can start tuning the bike. Use plug chops to confirm your jetting changes.

[Big T]

Almost forgot that when her friend "cleaned" the carbs, I noticed that two of the needle jets had cracks running up and down them somehow... Now I look in the Sudco catalog, and the #257 is what they spec for the needle jet, but factory GS called for an o-6, and I see the o-6 in that catalog spec'd for a VM20, and referred to as a #239??? Maybe this is due for a different topic post. I see a VM22 listed as 257 N-8 Needle jet, but then a VM26 listed as a 182 N-8. I am not sure what this means, as I did not see the o-6 repeated as I did this duplicate of the N-8, nor do I know what the two mean. I think some of these VM22's are newer styles, as the spigot style diagram looks nothing like the one on our GS550.

see new topic on this

You can call or email Sudco - they have folks who know everything about Mikunis

You may need to start shopping for a new set of carbs -try the For Sale section or Ebay

[rrok220]

I posted another thread, but then discovered after finally seeing an actual photo diagram rather than a line-drawing schematic, that the part on the bottom of the carb that I was looking at was not the removable needle jet, but rather the choke tube that appears pressed in (two of them were cracked), and one was missing the starter jet in the end. I started another thread on this. Might be time to look for a parts carb or two.
I also fear that with the amount of hacks who have had their wrenches on this bike, that the idle mixture screws on the bottom were probably tightened down and may have had the sharp tips broken off and possibly lodged into the carb like I read a thread on. Knowing the quality of work that was done to it, it wouldn't surprise me.

[rrok220]

Also discovered on a compression test that #2 cylinder has slightly low compression dry and wet (115 dry, where all the others were 132-146psi dry). I was getting 174-195 wet when I squirted a healthy amount of "Restore" (3 squirts from an old fashioned oil can) into the cylinders for a dual purpose wet test. Higher than when I had used just oil. #2 was still lower at about 162 psi with the 3 squirts of Restore. I am thinking that since the dry to wet increases were all about proportionate, that hopefully it was just a bad valve. Ideas? Anyone got a used head since ours has one exhaust bolt hole almost totally drilled out, and one with a heli-coil? (broken bolts from PO).