what's that screw for anyway??? carburetor - idle screw

[Blaze]

I just thought of this because of the fuel/oil ratio discussion in another topic:

I should have noted that I run a 25:1 mixture. Since you just got your motor, I'm sure your mixture is pretty close to that anyway, so it doesn't matter for this conversation, but if somebody else reads this and they are running a different fuel/oil ratio, it could make a difference.

 

[pianoman8t8]

Good call, blaze, i never woulda thought of that. Also, i messed with the carb alot over the weekend. It came setup on the 2nd groove from the top, so I decided to mess around with it a little. I put it on the 3rd groove, and it had more bottom end, more power, but top end suffered. I also tried the first groove on the top, and it ran like crap. So i still have it back on the 2nd. I also tried the 4th just for the he** of it, and it ran like crap also. I have a good mind to get that upgrade one from the Grubee place.
Thanx guys for all the input, i apreciate it!
-pianoman

 

[mrsaxman99]

*

psychological self-fullfilling prophecy

...when i first got mine, i also thought it was for mixture, a closer look before install and i would've known better.

funny thing, tho:

when i thought it was a mixture screw, it behaved like one. then i learned it was for adjusting idle, now that's what it does

maybe i've got the magic carb....

 

[likemyspoiler]

I think there are 2 different carbs being talked about. The stock carb which has only one screw for idle adjustment & premium carb that has idle & mixture screw side by side.

 

[gone_fishin]

yup, that's right, but check the date of this topic. back then, we were dealing with the stock carby and only hearing vague rumors of something better. there was a bit, heck there was a LOT, of "mixture/idle screw" confusion when MBc first got busy. you had to be there but it pretty funny, and it's what inspired this thread

 

[Scottm]

I love reading these old threads. Good job likemyspoiler in all your research.

 

[Hollywood]

I have done alot of R&D on carbs and they can be a pain to get them to idle right.I have seen two types of carbs in engine kits.The old style what has one adjustment screw and a bigger(longer)carb with two adjustment (one air screw and one fuel screw) screws (to use this type carb you will need a lot of frame room to get it to fit).The first thing i will start with is air filter and air filter housing.What i have found by testing is that the filters that come in the kits are not much good.They do not let you get a good air flow.What i have found to work the best are filters you can make out of Scotch Brite.The Scotch Brite comes in four colors green,red,gray and white ( they can be had at any good hardware store or body shop supple store)I think the green and red give you the best air flow.You may want to use the gray or white ( the white is the finest ) if you live in a area where their is a lot of dust and dirt in the air.I put a light coat of oil on them to trap the dust.As for the air filter housings i seen and tested there are four types.Most kits come with a black plastic housing or a steel one with a white plastic cover.The black plastic housing gets its best air flow if you cut or grided off the four plastic flutes as short as you can get them.The steel housing with the white plastic cover works good by just changing out the filter with a Scotch Brite one.There is a all steel housing after market one you can get(i get mine from Dennis Foster).I have found this one to work the best for air flow of the ones i've tested when you change out the filter with the Scotch Brite filter.The new style carb works great with the K&T type filter that comes on it.This carb also works the best of the ones i have tested if you can get it to fit in your frame. Well my finger is getting sore(i am a one finger typer)Next time i will reply with what i have found out by my testing on the jet and screw adjustment.

 

[Sockmonkey]

I think it would be good to oil a Scotch Bright pad if U use it for a filter, as this will help it to trap more dust. I used to do this with my RC airplane engines.

As for the idle screw adjusting the mixture... What if by turning it out 5 turns it loosens it up enough to allow air to be sucked by it at high speeds thus leaning out the mix???

 

[Hollywood]

Don't turn your screw out five turns or you are going to loose it.If you want to play with the fuel/air mixture move the e-clip on the jet.The sweet spot for fuel/air is between 3 to 4 turns out to get a good idle spot.

 

[Jim H]

Lost my idle screw the first week I ran my 80... replaced it with a wood screw until I got a new one... ran like carp though. There was a young fella on this forum a while back who swore that his idle screw increased hp, torque, low end, top end, midrange and everything... I miss his wild stories...