Bogging in Mid-Top Throttle Under Load

[Riding Again]

The title is the problem I'm seeing when the bike is on the road. On the center stand, in neutral, it runs uninhibited.

Current Conditions:

• Stock carbs (32 CV);
• stock airbox (sealed with silicone around all edges and end covers as well as good foam on the filter;
• stock foam filter;
• stock pipes;
• mixture screws 1-3/4 turns out

What has been done:

• New O-rings (boots);
• New carb O-rings;
• new boots (both airbox and intake);
• carbs cleaned (dipped for 24h 3-years ago, ultrasonically for this build) following the guides;
• float levels verified;
• diaphragms inspected (no rips, tears, or holes identified);
• new plugs;
• new plug caps;
• new fuse box [can't believe the difference this made!];
• used K&N filter oil on the filter;
• carbs vacuumed synched

What hasn't been done:

• Plug chops

Where do I go from here? I'm thinking jetting.

Here are the original spec's

GS850 (80) the specs are for the CV's

idle.................... 1050 +/- 100 r/min
carb................... mik BS32SS
id no................... 45120
bore size ............ 32 (1.26)
float height ......... 22.4 +/- 1.0 (0.88 +/- 0.04)
fuel level ............ 5.0 +/- 0.5 (0.20" +/-0.02")
VALVE SEAT........ 2.0
STARTER JET....... 32.5
PILOT SCREW ..... 3.5
PILOT AIR JET ..... #200
pilot jet............... #42.5
jet needle........... 5D57-3 (the -3 3rd notch)
needle jet .......... X - 6
pilot outlet........... 0.7
BY PASS............. 1.0, 0.8, 0.8
MAIN AIR JET...... 1.7
main jet............. #115

(Thanks first timer)


and here is what I have

Needle 5D50 [non-adjustable]
Pilot Air Jet 180
Pilot Jet 40
Main Jet 115
Needle Jet 318EX-5
[Not certain on the rest of the spec's]

Doing some reading at http://www.iwt.com.au/mikunicarb.htm and the forums here, I believe the problem lies in the needle jet/needle circuit and main jet circuit due to the throttle position.

My thoughts for the jetting stems from the following charts (I added my elevation and typical riding temperature ranges).

from http://www.iwt.com.au/mikunicarb.htm

I figured a correction factor of jetting to be 0.93 and now to cross to this chart

from http://www.iwt.com.au/mikunicarb.htm


I should be running the following(?):

Needle ?? (Adjustable or direction to adjust)
Pilot Air Jet 167
Pilot Jet 37.5
Main Jet 107
Needle Jet same

and the best way to get these -- or am I overthinking this?

 

[MisterCinders]

Do a chop when it bogs to see what the plugs say. Bogging usually means rich, but the plugs will tell for sure. You might also try a ghetto chop by riding it until the bog happens and then adding some choke to see if it gets better or worse. Better = lean; worse = rich.

Once you figure out what direction to go, the needle is your friend at mid-throttle. Move the clip up to lean the mix and vice-versa.

If you can't adjust the needle for lack of clips, perhaps there is a way to shim it slightly? Otherwise, you may need to change needles.

Finally, those CV carbs use the main jet at all settings IIRC, so others with more CV knowledge may have a solution based on changing that jet size. v0v

Good luck

 

[AJ]

I don't think you're over-thinking the problem. Your good research will come in handy as you solve your issue.

Agree with Mr.C. that bogging like that usually means too rich.

But first... Was the bike ever running right before with this exact setup? That is, with the current carb settings (jet sizes, needle setting, and mixture setting) + air filter(s) + pipes, was it running ok? If the answer is yes, then don't go down the rabbit hole of changing those settings. If they were correct before, they are probably correct enough for now.

Several things come to mind:

(1) Over-oiled foam air filter. Blot as much oil out of the filter using rags or paper towels. If that doesn't help, remove the filter as a temporary measure to test if this is causing the problem. (BTW, is a foam filter stock? I though Zuk always used paper filters)

(2) Float setting too high. I know you checked it. Move it to the highest setting of the tolerance (biggest distance measured inverted) to drop the fuel level in the bowl. The pilot jet is fairly sensitive to fuel level in the CV carbs.

(3) Rubber plug in the pilot jet hole. Your carbs may not have this, but some CV carbs feed fuel to the pilot jet through a hole between it and the main jet. If that rubber plug is missing or in poor condition, the pilot jet can draw fuel directly from the bowl, and it will draw too much, causing it to be rich.

If you haven't seen this link before, this one is pretty clear:
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
BUT, one more time for emphasis, if your bike ran well previously with the current settings (jets, needles, mixture screw) in the current setup (air cleaner and pipes), then that setup is probably ok and you need to look elsewhere.

Hope this helps.

 

[Syncro]

had that same problem with my gs850g, traded before I checked it out, but my motor was begining to leak oil from god know where.s

 

[Riding Again]

Agree with Mr.C. that bogging like that usually means too rich.

But first... Was the bike ever running right before with this exact setup? That is, with the current carb settings (jet sizes, needle setting, and mixture setting) + air filter(s) + pipes, was it running ok? If the answer is yes, then don't go down the rabbit hole of changing those settings. If they were correct before, they are probably correct enough for now.

AJ-that is what's had me perplexed-is yes, it ran well before. I was thinking that I solved potential leaks when I replaced the boots and then this popped up. [Hold that thought....] Read on below

Several things come to mind:

(1) Over-oiled foam air filter. Blot as much oil out of the filter using rags or paper towels. If that doesn't help, remove the filter as a temporary measure to test if this is causing the problem. (BTW, is a foam filter stock? I though Zuk always used paper filters)

Don't know, but it ran well with the foam. Maybe next year I'll upgrade to K&N paper.

(2) Float setting too high. I know you checked it. Move it to the highest setting of the tolerance (biggest distance measured inverted) to drop the fuel level in the bowl. The pilot jet is fairly sensitive to fuel level in the CV carbs.

(3) Rubber plug in the pilot jet hole. Your carbs may not have this, but some CV carbs feed fuel to the pilot jet through a hole between it and the main jet. If that rubber plug is missing or in poor condition, the pilot jet can draw fuel directly from the bowl, and it will draw too much, causing it to be rich.

Still a possibility, but all plugs in and accounted for.

If you haven't seen this link before, this one is pretty clear:
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

That's what clinched it for me to start looking into the jetting.

BUT, one more time for emphasis, if your bike ran well previously with the current settings (jets, needles, mixture screw) in the current setup (air cleaner and pipes), then that setup is probably ok and you need to look elsewhere.

Ding ding ding---I think this is the answer! Look elsewhere

 

[Riding Again]

I stopped off at a local bike shop where a long ago acquaintance works as a motorcycle mechanic (once owned a suzuki shop as well) to talk with him. I should NOT need to rejet due to the altitude and that I should keep looking.

For the record, I shimmed the valves as well this go-a-round also. He suggested checking the compression and so I did. The results are not stellar, but they are consistent at 1: 107; 2: 106; 3: 110; 4: 107. [absolute air pressure is typically 12.5 psi at this altitude so I think it's reasonable].

But WHOA, what did I find when I pulled the plugs to do the compression check.

As a group

#1

#2

#3

#4

To entice others to look at this, I'll start a new thread "read the tea leaves (?) [plugs]