gs700 runs like poop

[Anonymous]

so here's the lowdown. Took apart topend to replace head gasket that was leaking, upon rebuild, new head gasket, new head cover gasket, checked all valve clearences, new rubber gromet things for headcover bolts followed manual for all assembly details. swapped exhaust off a donor bike doesn't run, checked compression and it was within specs. put carbs back on with no filter, ran motor to see there were no leaks. ran ok but with slight hesitation in midrange rpm. put on new pod filters. drove bike. got some serious hesitation in low and mid range, assumed rejetting was the obvious fix as I had not done so yet. plugs were very light in color and based on the info that came with filters moved up two sizes in jets. now she barely runs at all. no power at low end appears to break free at something like 7k but not positive about that.

first Q: is did I miss a vacum line? off the tank there are 4 lines, main fuel goes to intake side of carb, small line with spring surround goes to exhaust side of carbs, long line is breather goes to rear of bike, fourth line is ? can't remember where it came from.

second Q: why do 3 of 4 pilot jets have a rubber plug blocking the tube they are in. is this correct?

third Q: did I re-jet incorrectly. I was at 122.5 with stock exhaust. went up two sizes to 125.5 i think. not sure what the new exhaust is, it looks like oe but doesn't have suzuki stamp on it. it was a black chrome color until i painted it.(swapped it off a donor bike doesn't run)

Help

 

[Anonymous]

second Q: why do 3 of 4 pilot jets have a rubber plug blocking the tube they are in. is this correct?

All 4 should have that little plug. I can't imagine it would run well at all without them in place. That plug forces the fuel to be drawn through the main jet and into the pilot circuit at idle. Without the plug, I'm not sure if that carb would draw more fuel or not enough, but it can't be a good thing.

third Q: did I re-jet incorrectly. I was at 122.5 with stock exhaust. went up two sizes to 125.5 i think.

Mikuni jets go in 2.5 increments, so you only went one step, not two. Sort the pilot circuit first, then your needles, then work on the main jets. You can run around town without needing the mains at all, so fix the bottom circuits first. Once it idles and runs smoothly at small throttle openings (<1/2 throttle), then you can worry about your mains. If you are worried about the mains being too lean, take them out and sort the pilot and needle with no mains in place. It will bog as it comes onto the mains, but at least it will not be lean.


Mark

 

[Anonymous]

i did plug the fourth tube but it had no benefit. I reasoned that there would be no logical reason the #3 carb would not have the plug but others would.

Must not have been clear in my description, the bike idles fine... when applying throttle it bogs. up till about 7k

and now that you mention it It does have the 127 jets so it is two steps up
I forgot the increments were 2.5

 

[dig dug mx6]

did it have pod filters before? or are those new after the rebuild.

A guess here Could it be possible that with pods and a stock exhaust you have too much in flow and not enough out?? I don't know just a guess

 

[Anonymous]

Must not have been clear in my description, the bike idles fine... when applying throttle it bogs. up till about 7k

That would be the needle circuit, then. It is more helpful if you use throttle openings than rpm, as the throttle position is what controls which circuit you are on:

idle-1/8 throttle is pilot
1/8-3/4 throttle is needle
3/4+ is main jets

There is some overlap between these of course, but these are the general areas.

Your plug description is lean, so I would try raising the needle one notch first. If the needle is not adjustable you either need to buy some shims or a jet kit. Radio Shack has a small washer pack that is a good source for shims. Try shimming it 0.040" to start, that is about equivalent to one notch on my needles.

Where are your mixture screws set at? Check that and set them to 2-2.5 turns out as a starting point depending on where they are now.

Before test riding it again, take a piece of tape and stick it to your throttle housing, then mark positions for 1/8, 1/4, 3/8, 1/2 throttle, etc. This makes it much easier to see how much throttle you have on and which circuit you are running on.

What I did was to go out on a quiet country road and get into 3rd or 4th, then run at 1/8 throttle until it stabilized, then I would try the choke to see how it responded. If it bogged, then I was good to slightly rich. If it ran better, then I was lean. I did this for a number of throttle positions to see how it was on the various circuits. Then I used the results to adjust accordingly. If you are stuck on a particular problem (and I was, on a stutter at WOT around 6000rpm), try going in one direction. If the problem improved, then you went the right way. If it got worse, you went the wrong direction... By being methodical and sorting one circuit at a time and only changing ONE setting at a time you can get it sorted with a bit of effort. If in doubt on a change, I will always try going richer first, as it is the safer direction, especially when trying to sort the main jets.


Mark

 

[oldschoolGS]

Must not have been clear in my description, the bike idles fine... when applying throttle it bogs. up till about 7k

That would be the needle circuit, then. It is more helpful if you use throttle openings than rpm, as the throttle position is what controls which circuit you are on:

idle-1/8 throttle is pilot
1/8-3/4 throttle is needle
3/4+ is main jets


Mark

excellent post,

I would not bother with trying to shim the stock needles, just buy a jet kit. Aftermarket needles are tapered in addition to being adjustable and will give you the best performance thru midrange. Good Luck, Ed.

1983 GS750ED
2005 GSF1200SZ

 

[Anonymous]

thanks for the help gents, as usual you are all very helpfull. not sure when I will get to do the work to see if I can solve the problem but thanks again for the head start.