Other What does "loading up" feel like when you are tuning carbs?

McDog

McDog

Other Administrator
Staff member
Location
South Florida
The last frontier of learning to do stuff on my ski is carb tuning. I just have never been good at it and I always end up with a buddy who does it for me. I know if it's right but can't really get a feel for too rich or too lean.

I'm trying to learn and understand it. I have read the mikuni manual. I know what lean hesitation feels like(I think) but the term "loading up" eludes me.

I need to tune my xs771 with bj44's. I think it's too lean because it gets better when I turn out the low speed adjuster. I have them turned out to the max. I think I need bigger jets or maybe to decrease popoff after reading the manual.

I had also screwed up and put the fuel filter on the return line thinking it was the in line. So basically the pressure built up in the carb because the fuel was stopped by the filter. I figured it out real quick when it stopped running and fuel was leaking out the "in" connector on the carb from around the fuel line from the pressure. I checked the jets to make sure they were clean afterwards but now I think I need to check the check valves too. It's always something.
 
Flash-FX

Flash-FX

No Square..No Round..FX-1
Location
South Jersey Shore
McDog said:
The last frontier of learning to do stuff on my ski is carb tuning. I just have never been good at it and I always end up with a buddy who does it for me. I know if it's right but can't really get a feel for too rich or too lean.
Where's your "tuner" buddy? Loading up on the low end? A tell tale sign is really wet plugs and lots of black "goo" dripping out of the exhaust. A real lean condition will make the engine stall (run out of fuel) when you grab a handfull of throttle from idle.
I'm trying to learn and understand it. I have read the mikuni manual. I know what lean hesitation feels like(I think) but the term "loading up" eludes me.

I need to tune my xs771 with bj44's. I think it's too lean because it gets better when I turn out the low speed adjuster. I have them turned out to the max. I think I need bigger jets or maybe to decrease popoff after reading the manual.
You are on the right track by starting with the idle. If the engine needs more fuel down low, and you can "fake" out the circuit by opening the low speed adjuster, check your current pop-off and low speed jets. Try a larger L/S jet and maybe lower the pop. How does everyone else jet their BJ 44's on a 771? The 771 can make some heat in the cyl's, so you don't want a lean mid to top end=toast.
I had also screwed up and put the fuel filter on the return line thinking it was the in line. So basically the pressure built up in the carb because the fuel was stopped by the filter. I figured it out real quick when it stopped running and fuel was leaking out the "in" connector on the carb from around the fuel line from the pressure. I checked the jets to make sure they were clean afterwards but now I think I need to check the check valves too. It's always something.
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The fuel filter would have to be all plugged up to do that on the return line with no clamps and loose hoses. Sounds like fuel tank pressure buildup from heat and a tight fuel cap. Try to use at least zip ties on those fuel line hoses. Years ago, I toasted a perfect 760 crank because the pulse line popped off the rear cylinder carb,(no clamps) when I rode it back in from the surf on 1 cyl. :crying:
 
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Dustin Mustangs

Dustin Mustangs

uʍop ǝpıs dn
Location
Holland, MI
Do your self a favor and call jetworks. Not only will Art be able to verify you have the proper jetting but he will also talk you blue in the face about tuning it in. Blackjacks are somewhat unique when it comes to jetting, esp pop off.
 
Waternut

Waternut

Customizing addict
Location
Macon, GA
I believe Mcdog had the carbs rebuilt and jetted by Art for this engine from the start. I believe he told me 130 pilots and 110 mains.

I tried to help tune these carbs yesterday and this thing has me stumped. If I idle for a bit and hammer the throttle, it sputters for a half second or so and then take off like a rocket. From my experience, that tells me the carbs are loading up and it's too rich. However, when we turned the adjuster screws in a little (lean them out) to verify my assumptions of being rich, it would die when I went to hammer the throttle off idle. Then I'd have to reprime the carbs to start it again. That tells me it's drastically lean. Regardless of what it does off idle though, everything runs like poo until you get it on the main jet.
 
McDog

McDog

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Staff member
Location
South Florida
Flash-FX said:
The fuel filter would have to be all plugged up to do that on the return line with no clamps and loose hoses. Sounds like fuel tank pressure buildup from heat and a tight fuel cap. Try to use at least zip ties on those fuel line hoses. Years ago, I toasted a perfect 760 crank because the pulse line popped off the rear cylinder carb,(no clamps) when I rode it back in from the surf on 1 cyl. :crying:
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It was really tight oem line with big, tight zip ties. Could be true about the tank pressurizing though.

I plan to talk to Art but I'm not calling him on a holiday weekend to bother him even though I could. I did email him. I put this on here just to grab some extra opinions in the meantime. I like to use the x as a sounding board as I work through issues.

Harrison, do you mean a stronger spring when you say go up one size? It seems like I would want to lower popoff to get more fuel but I could have my brain wrapped around it backwards.
 
Big Kahuna

Big Kahuna

Administrator
Location
Tuscaloosa, AL
McDog said:
It was really tight oem line with big, tight zip ties. Could be true about the tank pressurizing though.

I plan to talk to Art but I'm not calling him on a holiday weekend to bother him even though I could. I did email him. I put this on here just to grab some extra opinions in the meantime. I like to use the x as a sounding board as I work through issues.

Harrison, do you mean a stronger spring when you say go up one size? It seems like I would want to lower popoff to get more fuel but I could have my brain wrapped around it backwards.
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if he you had a weak spring, this would allow extra fuel to be dumping in at lower RPM's, once you clear this out your jetting is in its range and you make good power. by going to a stronger spring, it does not allow this buildup of additional fuel to be dumped in. I am by no means an expert, I struggle with tuning as well. I understand principal but not great with telling what needs to be done by "Feel" or "seat of the pants" tuning.
 
Dustin Mustangs

Dustin Mustangs

uʍop ǝpıs dn
Location
Holland, MI
Have you done the standard diagnostic testing to ensure the engine is in proper working order? Have you verified the pop off is where it is supposed to be?? I am surprised Art would set them up so far off with out there be something else playing into this.

The easiest way for me to tell if I am lean or loading is time. Run the engine at it's worst throttle position. Does it act different going from that throttle position to wot depending on how long you leave it there? If yes, loading. If no, lean. From what you describe I would say your pop off is too low, which is a common mistake when setting up bjs.
 
McDog

McDog

Other Administrator
Staff member
Location
South Florida
It ended up being problems with the fuel lines. I'm going back to Art's original recommended springs. He is still the man.
 
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