Lean idle cause?

[JJinSC]

Considering clean fuel system, carbs, and settings to factory, what could cause a lean idle situation? This 787 cranks easy, idles perfect and runs perfect all day long. It just has a problem transitioning from idle. Accelerator pump seems to work fine. If I apply throttle fast it will quit. If I apply throttle slow it will stumble ( on rare occasion it will transition fine). If I stop applying throttle and hold where it stumbles it will either clear and go or continue to stumble. If it continues to stumble I can massage the throttle (slightly back and forth) and it will clear and go. If I blip the choke, during the stumble it will immediately stumble worse for a split second then clear and go. I bought this machine new (97 XP) and it has always done this. Even after 1 rebuild, 1 carb cleaning (fuel line replacement) and 1 carb rebuild with OEM kit. I cannot seem to get this hesitation out. Please share ideas.


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[Bnelson]

Is the brass restrictor in the fuel line from the accelerator pump?

 

[JJinSC]

I am not sure. This is one area I am confused about. The small fuel line from the accelerator pump that splits to two lines for the carbs had something inline that is not in any parts diagram. It looks like a check valve. Therefore, other than check to make sure they were in good shape and clear passage I have not messed with those lines. ( did not replace). I as told this was a mod, but again, I bought this ski new in 97 and it came from the factory with this item inline.


Where would I find this restrictor within the fuel system and what is the purpose of it?


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[Bnelson]

That check valve you are talking is most likely the restrictor... It's just there to stop the accelerator pump from shooting in a huge shot of fuel off of the idle transition.... Maybe post a picture of this check valve?

 

[JJinSC]

Here is a picture of it.

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[Bnelson]

Yessir that's just a check valve..... I'm talking about an inline restrictor that's actually inside of the fuel line coming from the acc pump... Should be a small brass piece that looks like a carb jet

 

[JJinSC]

Being as I only removed the other ends from the carbs when cleaning, they should still be in there providing they were not missing from factory, correct?


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[Bnelson]

Yessir, if your confident the lines are factory and haven't been messed with or replaced then it should still be in there

 

[JJinSC]

Also, any ideas why this check valve would be there when it is not in any diagram and since there is actually check valves in this circuit on each carb? No one else seems to have this inline check valve, however, mine came this way from the factory.


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[Sportster-2001-951C-Stock]

If I apply throttle fast it will quit. If I apply throttle slow it will stumble ( on rare occasion it will transition fine). If I stop applying throttle and hold where it stumbles it will either clear and go or continue to stumble. If it continues to stumble I can massage the throttle (slightly back and forth) and it will clear and go. If I blip the choke, during the stumble it will immediately stumble worse for a split second then clear and go. Sent from my iPhone using Tapatalk

"Clear and go" implies it's too rich, "hesitates, falls on it's face and often quits" implies it's too lean.

Depending on which is the case, there a different direction to correct the issue.

"If I blip the choke, during the stumble it will immediately stumble worse"

This makes me think it's too rich off idle, and if it's not too rich at idle(idle mixture screws don't need adjusting) then I'd be tempted to disable the accelerator pump or perhaps drop one size on the low speed jet.

At this point I doubt a smaller jet is the solution unless it also does the off idle rich 4-stroking burble as well, and you didn't mention it does. So I'm unsure what exactly is going on.

 

[JJinSC]

I'll try and post up a video of what it is doing a little later.


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[Sportster-2001-951C-Stock]

That green check valve is pretty crazy looking. Depending on if it's spring loaded it might actually serve a purpose if it were on the other end of the fuel line at the tank (as an USCG required anti-siphon valve).

 

[Sportster-2001-951C-Stock]

I'd prefer just the audio, if possible.

 

[JJinSC]

At this point I doubt a smaller jet is the solution unless it also does the off idle rich 4-stroking burble as well, and you didn't mention it does. So I'm unsure what exactly is going on.

Can you elaborate on what this 4-stroke burble is like? I have read a description on here elsewhere but cannot quite get. When mine stumbles it is more like a woooooooom sound (bog).


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[JJinSC]

That green check valve is pretty crazy looking. Depending on if it's spring loaded it might actually serve a purpose if it were on the other end of the fuel line at the tank (as an USCG required anti-siphon valve).

It is right ( like 1 inch) from the a-pump.


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[mikidymac]

Have you verified your pop off pressures, are the needle and seat holding pressure with the diaphragms installed and are the butterflies synced?

 

[Sportster-2001-951C-Stock]

What happens if you give it sudden throttle, does it take off like the banshee it's supposed to be or does it take a while to clear out excess fuel, or possibly it immediately quits b/c there is not enuff fuel?

 

[Sportster-2001-951C-Stock]

It is right ( like 1 inch) from the a-pump.


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Oh, it's in the a-pump circuit? I thought it was in the main fuel line.

I'd have to pinch the a-pump line or block it off to disable the a-pump aif it's dumping too much fuel in your intakes that would explain the symptom you describe (assuming you're describing too rich).

Could be (no idea why!) those carbs aren't calibrated for an a-pump.

Short of doing these external tests, pop-off pressure and the basics of calibration need to be verified, such as knowing which size jets are installed. I'm kinda thinking along the lines of a calibration mismatch of some sort between the carbs and a-pump, too much fuel bieng dumped into the intakes for whatever reason.

The reason is, earlier carbs were calibrated richer and did not have a-pumps, so the wrong mismatch will leave your mixture too rich.

I'll come back to describing in more detail the 4-stroking subject in a bit, short hand description is it's a miss every other compression stroke due to too much fuel.

 

[JJinSC]

What happens if you give it sudden throttle, does it take off like the banshee it's supposed to be or does it take a while to clear out excess fuel, or possibly it immediately quits b/c there is not enuff fuel?

If I peg the throttle it will die. If I let off quick enough it returns to idle.


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[JJinSC]

Here is a video with audio of what it is doing. [video=youtube_share;VMOfJsQ42cM]http://youtu.be/VMOfJsQ42cM[/video]

 

[Coastiejoe]

In reading your post, it seems you have the original fuel lines. If so, I think you have the Grey Tempo lines. If you do, they need replaced. The ethanol in the fuel breaks the Grey Tempo lines down and create a green goo on the inside of them. This goo blocks (restricts) the fuel/oil delivery and causes many issues. The two biggest are poor performance and a lean condition. Lean is a BAD thing as it will destroy your engine. You will need to replace or do a MAJOR cleaning of your fuel selector, the carbs and the filters inside he carbs.

 

[1983]

video is private.....

Here is a video with audio of what it is doing. [video=youtube_share;VMOfJsQ42cM]http://youtu.be/VMOfJsQ42cM[/video]

 

[JJinSC]

In reading your post, it seems you have the original fuel lines. If so, I think you have the Grey Tempo lines. If you do, they need replaced. The ethanol in the fuel breaks the Grey Tempo lines down and create a green goo on the inside of them. This goo blocks (restricts) the fuel/oil delivery and causes many issues. The two biggest are poor performance and a lean condition. Lean is a BAD thing as it will destroy your engine. You will need to replace or do a MAJOR cleaning of your fuel selector, the carbs and the filters inside he carbs.

Grey tempo lines replaced years ago, please re-read my very first line that says "considering...". The reason for stating that is so that you will not lead me down lines of troubleshooting I have already been. Thanks.


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[JJinSC]

video is private.....

Sorry, I fixed that. I swear I have public marked but youtube is funny.


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[Sportster-2001-951C-Stock]

If I peg the throttle it will die. If I let off quick enough it returns to idle.


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I think you mean the engine will stop running completely, and if this is the case then it's my opinion that happens b/c there is not enough fuel to sustain combustion.

If there was too much fuel it would not stop running but it might sputter a bit before catching and then taking off normally.

So based on this we should concentrate on the condition of the carbs being dirty internally and restricting fuel flow. ie: Your engine is running out of fuel and is running lean.

A few suggestions:

* Replace original Tempo fuel lines, which corrode/disintegrate and contaminate your carburetors internally.
* Remove, disassemble and thoroughly clean the carburetors internally, they have Tempo Trash and years of fuel gum inside of them plugging the internal passages.
* Confirm the pop-off is repeatable and occurs at the pressure as per the shop manual specifications for your model year engine.
* While apart, confirm the high and low sped jets are the correct orifice size.
* I would recommend any carburetor rebuild should include a replacement needle and seat, these parts (specifically the needle tip) will be worn out over a period of 15~20 seasons use.
* Probably a good idea to replace the fuel selector valve, the o-rings are probably dry-rotted and unreliable, may allow air to leak inward as the fuel pump draws fuel from the tank.

It's entirely possible your green check valve in the a-pump circuit must be there yet is undocumented, especially if this was one of the early models when the a-pump was first introduced. You may recalibrate the original carbs to the earlier spec that didn't use the a-pump by installing a larger low speed pilot jet, etc. if you choose to eliminate the a-pump for some reason, such as if the internal diaphragm is damaged or other a-pump parts are broken and no longer available.

To test the a-pump you can remove the air box and watch inside the carb bore while actuating the throttle, it should squirt fuel into both of the carburetor throttle bores (not a small drizzle-drip but a healthy squirt).