WOT Bog above 4k

[johnny ro]

On the road today, finally.

It pulls fine to high rpm if I dial in the throttle rotation slowly. If I whack it wide open as I pass through middle rpms, it bogs, and recovers to accelerate hard if I back off a bit.

My SRX6 did this as delivered from factory, as the slides went too far into the bodies when throttle pulled all the way, so the 60 year old Yamaha guy told me, in a previous century. I miss that bike.

This 1989 GS500e powertrain is all unmodified OEM, except the recommended mild rejet for OEM setup. The float heights appear within the recommended range. The carbs are synched and idle fine. Valves within specs. New iridium plugs, clean looking older air filter. fresh gas.


Thoughts? Thanks.

John

[Meukowi]

had the same symptom, rejetting to smaller main jet did the work

[sledge]

On the face of it the slides are not responding quickly enough, but I would suspect a vacuum leak before going into the carbs,

Split or perished intake manifolds
Damaged or perished o-rings between the head and manifolds
Damaged or incorrectly fitted diaphragms
Leaks around the carb, ie the top cover.
The pipe leading to the fuel tap.

Some of those parts could be 30 years old, you need to ensure they are still serviceable.

[herennow]

had the same symptom, rejetting to smaller main jet did the work

My first thought also = too rich. The old story of a gs500 being lean from the factory just does not ring true to my experience.

What exactly was your rejet.?

[johnny ro]

what I read on the wiki.

Plus synch them.
Stock Air Filter   

Stock Exhaust
pilot 40
main 125
washers 1
turns 3
perfect
people 2

[johnny ro]

this bike shows 6,100 miles on the clock. a 1989, clocks are debatable. The title shows actual mileage, all along. With this and how the carbs look inside, they are probably not worn out. Some o-rings were, renewed now.

I get full power but must modulate the throttle on the way up. Does feel like a carb issue.

[Sporty]

On the road today, finally.

It pulls fine to high rpm if I dial in the throttle rotation slowly. If I whack it wide open as I pass through middle rpms, it bogs, and recovers to accelerate hard if I back off a bit.

My SRX6 did this as delivered from factory, as the slides went too far into the bodies when throttle pulled all the way, so the 60 year old Yamaha guy told me, in a previous century. I miss that bike.

This 1989 GS500e powertrain is all unmodified OEM, except the recommended mild rejet for OEM setup. The float heights appear within the recommended range. The carbs are synched and idle fine. Valves within specs. New iridium plugs, clean looking older air filter. fresh gas.


Thoughts? Thanks.

John

It seems to be a lean condition. If you back off and the engine picks up, that is classic example of lean. Rule out vacuum leaks first.

How about at full throttle? (WOT)
Does it slow down slightly if you back off, or does it pick up.  If it picks up, main jet is too small. If it slows down, main is likely fine.   

If it's ok WOT, but going lean in the midrange, try raising the needle.

[herennow]

Easy to test,
1. take out the air filter. Runs better? too rich (my bet 100% considering your symptom)
2. block 10% of the filter mouth area with duct tape. Runs better, you were too lean.

I went with a mild rejet and ended up with your exact problem and a month of f*&%king around going richer (all the folks online must be right, right..)

Penny eventually dropped and going back to stock main sorted it.

[johnny ro]

All good ideas. I doubt air leaks but most likely. Only changed the air screw, one washer in place, and the first het

Next weekend...try again on the GStoo.

The SR400 is open for its valve adjust (loose at 1,400 miles) and the nipple pulled out of the petcock when removing tank. Wait for expoxy to set and then work tomorrow. Reassemble mid week.

[mr72]

Stock Air Filter   

Stock Exhaust
pilot 40
main 125
washers 1
turns 3
perfect

If this is what you're running, you're almost definitely too rich.

40 pilot with three turns out will be way too rich at idle. WAY too rich. Unless you are way below sea level or you have a serious vacuum leak at idle.

I am now convinced that most of the forum experts who suggest these overrich settings will work correctly are overcorrecting for one or two things: they have major vacuum leaks resulting in the thing running lean on unmetered air, or they have set their idle speed way too high and are compensating for the extra air by pumping in too much fuel at idle. And usually you set the idle too high because of vacuum leaks making it lean at idle. So first fix all vacuum leaks, replace intake boot o rings and ensure intake boots are in good shape, make sure all other o-rings are replaced, then stock jetting works fine.

I have 125 jets, one washer, Yoshimura exhaust, Uni air filter, and header wrap, and chased a too-rich condition until recently.

My suspicion is you have leaking pilot needle o-rings and float level too high, in addition to having wide open pilot mixture setting and too much main jet. Stock exhaust and intake will work best with 40/122.5 and require about 1.5 to 2 turns out on the pilot mixture. IF you have any off-idle hesitation with this setting dialed in otherwise, then try one washer.

[johnny ro]

Thanks. I will read this again while opening up the carbs again. I do have new rings. Was some crumbled o-rings on this bike when I got it. So the consensus on the WIKI for OEM power plant non-EPA carb setting is based on a population of un-rebuilt carbs? Seems plausible.

[johnny ro]

Good advice here, thanks.

Opened her up today. The airbox boots are not fully tight when the clamps are screwed down tight. New boots on the way. Partzilla.

Will check float levels next, buy clean poly hoses online 2,3,4,5,6,7,8 mm. Usefull to have in the shop.

[user11235813]

When checking for air leaks don't forget the little rubber cap that seals off the right hand vacuum nozzle, it's held on with a tiny circlip.

[johnny ro]

found a leak at the left airbox boot. It was old and stiff and did not go back on right. New ones are on tight. One step done.