Stumble around 3000 RPM and 1/8th throttle

[opasnost]

My Bike is a 750es and its running great all over the rev range. But it stumbles at 3000rpm and an 1/8th throttle. Less or more throttle smooths it out. I notice that when the bike is cold the problem goes away and gets worse as it warms up indicating that its rich. But I also wonder that it might be something to do with ignition or timing since its right only in a specific RPM range.

This morning I added a washer in the needle jet to lean it out a little. Its tough to say if its helped yet maybe a little.

Any ideas?

[Tom R]

In that range I'd look at idle mixture screws and float level - a washer under the needle will raise it and result in a richer mixture.
Air bleeds clean?

[Tom R]

Does the advance mechanism move freely?

[Steve]

... - a washer under the needle will raise it and result in a richer mixture.

That depends on where he put the washer. If he put it over the clip, the needle will not be pushed as high, and it will be leaned out.
Adding a washer under the spring does nothing to change the jetting, it only increases preload on the spring.

Air bleeds clean?

What are "air bleeds"?

.

[opasnost]

I did put the washer over the clip to lean it out a bit...And I am also wondering what are air bleeds.

Im just adjusting my idle screws and it seems like 2 turns out gets me the highest idle....they were at 2.5.

On a side note, when adjusting for highest idle, do you turn all the screws in or out together or just one at a time to get the highest idle....also it seems to me like the outer cylinders are affected greatly by the idle mixture screws where as the middle two not so much or even at all...seems strange......

[opasnost]

Does the advance mechanism move freely?

How would I go about checking that?


So after turning in the screws a half turn it seems like the problem is not better or worse. I did a little more investigating and the adjustment screw for the carb which is in the middle on the left side has no effect on idle(is that the #2 carb?)...turning it all the way in or out did not change the idle speed.

I was paying more attention this time to the stumble at it comes on a little after 2000 RPM and continues to about 3000 then clears up.

[Tom R]

That depends on where he put the washer. If he put it over the clip, the needle will not be pushed as high, and it will be leaned out.
Adding a washer under the spring does nothing to change the jetting, it only increases preload on the spring.



What are "air bleeds"?

.

Doh! It's been a while since I moved the needle on my carbs - apologies and glad you caught it.
You might know them as air jets - regardless, they have to be clean.

[Tom R]

How would I go about checking that?

Underneath the timing plate, there's an advance mechanism that consists of weights and springs. It should be free of corrosion and move freely. Since it's pretty easy to check you probably should, but I have a feeling it's a carb related stumble - If they haven't been cleaned in a while, it might be time.

[opasnost]

Underneath the timing plate, there's an advance mechanism that consists of weights and springs. It should be free of corrosion and move freely. Since it's pretty easy to check you probably should, but I have a feeling it's a carb related stumble - If they haven't been cleaned in a while, it might be time.

I just cleaned them.....but my gut feeling is that the pilot circuit in the number 2 carb is causing the problem. Its the only carb that is not responding to idle screw adjustments. The bike jerks forward and back when it stumbles but in a consistent way so I think its that one carb. Also it goes away over 3000rpm maybe because thats getting into the needle jet range. I'm just gonna list off the things I should be looking for please let me know if I miss anything. Or should I approach this differently.

-check and clean pilot jet
-check float level
-air pilot jet
-clean pilot circut
-inspect pilot adjustment screw
-check for vacuum leaks

The last one checking for leaks. How do you do this? I read that you can spray carb cleaner on the rubber intake boots or wherever you suspect a leak and that should effect idle is that true? safe?

[Steve]

On a side note, when adjusting for highest idle, do you turn all the screws in or out together or just one at a time to get the highest idle....also it seems to me like the outer cylinders are affected greatly by the idle mixture screws where as the middle two not so much or even at all...seems strange......

Each carburetor is just a little bit different from ALL the other carburetors out there, so its ideal setting will be just a little bit different, too.

EXAMPLE: When I got my son's 650 going, it was running a bit rough, like it was idling on three cylinders. I start with the mixture screws out three turns, which is usually plenty rich to start the bike and start tuning. Adjusting the screws on carbs 1, 2 and 4 made them sound happier at about 2 to 2 1/4 turns, but #3 made no difference. TheCafeKid came over to lend a hand. He kept turning the mixture screw out farther, and it finally kicked in at about 4 1/2 turns out. After further fine-tuning, that carb is happiest with the screw out 4 turns, the other three carbs are happy at just over 2.

I did a little more investigating and the adjustment screw for the carb which is in the middle on the left side has no effect on idle(is that the #2 carb?)...turning it all the way in or out did not change the idle speed.

Yes, that is the #2 carb. If adjusting it makes no difference, it's possible that the tip is broken off and jammed in the carb. You don't need to remove the carbs to verify, but you will probably need to remove the tank for access. Remove the screw, examine the tip. It should go down to a very fine point. If it is blunt at all, the tip is broken, and you WILL have to remove the carbs to get that tip out. Don't forget to order a new mixture screw.


.

[opasnost]

Well I had it idling for a while adjusting the screws so I went and took a look at the plugs 1 and 2....I broke the #2 so i'm off to buy four more...but what I found was that #2 was sooty meaning running rich and #1 was just fine (I'll upload the pictures in a bit) also checked the adjustment screw and the tip is fine....so from that list

-check and clean pilot jet (dirt would make it lean)
-check float level (too high could be causing this)
-air pilot jet (blocked would cause rich....right?)
-clean pilot circuit (blockage would make lean)
-inspect pilot adjustment screw (checked seems fine)
-check for vacuum leaks (would cause lean)

so we got the float level and blocked air pilot jet right...before I delve into that any other suggestions???

I think I am going to try to screw the #2 adjuster all the way in and see if that helps and/or take off #2 air filter pod...but first new spark plugs

[opasnost]

Underneath the timing plate, there's an advance mechanism that consists of weights and springs. It should be free of corrosion and move freely. Since it's pretty easy to check you probably should, but I have a feeling it's a carb related stumble - If they haven't been cleaned in a while, it might be time.

Where is the timing plate? Is that the cover over the crankshaft on the right side?

[opasnost]

Underneath the timing plate, there's an advance mechanism that consists of weights and springs. It should be free of corrosion and move freely. Since it's pretty easy to check you probably should, but I have a feeling it's a carb related stumble - If they haven't been cleaned in a while, it might be time.

I can't seem to find these weights and springs where are they?

[opasnost]

I can't seem to find these weights and springs where are they?

Is it under this?

.....This is not from my bike mine is a 83 gs750es

[Steve]

On your '83, I think the advance is built into the ignitor, you have no mechanical advance that uses weights.

.