carb question By mcgyvre (Read 244 times)

[mcgyvre]

Hey gang! I have been having a lot of fun scooting so far this spring and I owe most of it to y'all here at the forum. If not for the library, I'd be on the sidelines reading about your adventures. As usual, my 2008 Benzhou needs nearly constant attention but it's a labor of love. I bought it 10 weeks ago and have done much to it and with it. It had 5800 miles when I took it over and now has 7600. I was born to scoot!

Today's question has to do with tuning my carb. It's a 4 stroke 50cc 1p39QMB. I have torn it down, cleaned it out, and made sure to pay attention to Alley's tips on things to watch for. While following Alley's tuning procedure, I found it very difficult to note any difference in the RPM's while adjusting the air-fuel mixture. Although not 100% positive, I found a spot where I believe the revs are at their highest point. Once richening by 1/8 turn, I turned it in all the way, counting the turns. I returned it to my determined position, redid the procedure to verify my settings and came away quite satisfied with the results. It's running much smoother and am quite pleased. One note, in order to get a decent curb idle speed, I have to screw the idle adjust screw in almost all the way. Here's the rub. As I found it difficult to really hear much difference in revs, I decided to remove the adjustment screw entirely. When I did, the revs went way up and the engine sounded VERY smooth. Having been satisfied with my earlier efforts, I decided not to pursue any adjustments or experiments with the screw removed, so here I am, checking with you gurus of go. I don't know enough about the motion lotion apparatus to risk messing with it. Why would it run so smoothly with the a-f adjustment screw removed? Is this a sign that something is astray? Any tips, hints, suggestions, or insights would be appreciated. The only other thing I can think of to say about the setup is that the screw is factory, not a removed plug setup. If y'all need more info to help, lemme know. I'll tell ya all I know, just don't know what you may need to know! LOL!

[Bashan]

You are getting a boatload of air into the system with the screw out. It's abnormal for it to run like that. OK, before you took the A/F screw out, what did the plug look like? Was it black, brown, bare?

[Alleyoop]

Here is a Diagram of how the carb works, as you can see you allowed more fuel to be sucked out by taking the screw out PLUS more air.
Alleyoop

[mcgyvre]

There was no plug at all or evidence that one was there. I get the huge volume of air and it seemed odd to me as well, which is why I posted. Gonna review Alley's diagram and see if I can comprehend what's going on here. Additionally, I realize that properly sealed and functioning airbox, air passageways, intake and exhaust all affect these settings. A quick primer on airbox leaks effects on a=f mixture adjustment would be appreciated. Seems to me if I have leakage before the carb, I am getting more air than spec'd and would lead me to adjust the mixture to compensate. My mind says to adjust it to be leaner to compensate. That's my logic, anyway. How off is my thinking?

[Bashan]

I meant your sparkplug. That will tell us how the mixture was running. Rich

[Alleyoop]

May 11, 2014 18:24:40 GMT -6 mcgyvre said:

There was no plug at all or evidence that one was there. I get the huge volume of air and it seemed odd to me as well, which is why I posted. Gonna review Alley's diagram and see if I can comprehend what's going on here. Additionally, I realize that properly sealed and functioning airbox, air passageways, intake and exhaust all affect these settings. A quick primer on airbox leaks effects on a=f mixture adjustment would be appreciated. Seems to me if I have leakage before the carb, I am getting more air than spec'd and would lead me to adjust the mixture to compensate. My mind says to adjust it to be leaner to compensate. That's my logic, anyway. How off is my thinking?

If you have air leaks PERIOD that means like you said getting more air so what that makes someone do is ADJUST the Fuel Ratio Mixture RICHER to balance the fuel with the more air coming in. Otherwise it will run lean, so the first thing you want to do is make sure it is a sealed unit no extra air being sucked in from other than the air filter.
Alleyoop

[mcgyvre]

Rich, the spark plug had a thin film of black, soot-like residue on it that wiped off easily with a dry cloth.

Love the diagram! That helped a lot. Thanks, Alley. Spent some time today sealing the airbox. As happens anytime more than one thing is changed, there's no way to know which worked, which didn't, or which helped more than the other. All I know is she's running real strong after carb tuning and airbox sealing. Again, thanks to Alley for the tuning procedure.

Just to let ya all know that I earned my handle, my airbox sealing was accomplished using nothing more than a box of small screws, a tube of Super Glue seal all adhesive (not regular Super Glue) and an old frisbee. The same materials were also used to refashion a number of broken or damaged plastic panel tabs. The edges make good, stout screw holders and can also be cut into the flat for a sturdy 90 degree tab. The frisbee I used also had a small contour on the flat that worked well for securing slightly uneven pieces. Small velcro tabs were used in the front to secure the front panel since there is no side force to pull it off. Makes getting to the R/R, horn, and turn signal lamps quick and easy. Finally had a chance to get a scoot pic up in place of my squirrel.

[Alleyoop]

Good job Mcgyvere, where there is a will there is a way I always say
Alleyoop

[Bashan]

Black is too rich even if it wiped off. I miss the rodent. Rich