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Finally feel like I've got my carbs right

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I resurrected this thread to find out where everyone stood on Spridget gas mileage. The former owner of mine said he was getting around 30 mpg. Magazines at the times these cars were built seem to be closer to the 35 mpg range. I really can't tell exact numbers until I fill the car again (I just got it this past week) but I don't think I'm getting 30 mpg. My gut is that the engine is running rich. There's an awful lot of carbon on the tailpipe (not oil) and I can even smell the carbon driving around. The car tends to run cool, which I've heard is unusual for the 1275, as many run hot. This would also lead me to believe that the carbs are mixed too rich as a leaner mix should cause the engine to run hotter. It fires up no problem without any choke, granted the temps haven't been below the mid 70s since I bought the car.

I'm a complete newbie at this, but are my instincts on, or am I way out in left field?
 
With my 1275 with dual HS2s, I don't think I get over 25 (sometimes as low as maybe 20), but I tend to push it. My former 1275 with a single HIF carb in a Mini got closer to 30. I think a 948cc might get well over 30... again, driven peacefully.
 
Rick, nice to here you've got them working. 35mpg.....I get down in that range at 80mph!:highly_amused:


Kurt.
 
nomad said:
Rick, nice to here you've got them working. 35mpg.....I get down in that range at 80mph!:highly_amused:


Kurt.
Click to expand...

HS2's and small round ported head correct?
I have the HIF4's on a peanut head, so I'm feeling good about 35MPG!
Oh and I don't exactly drive it like an old lady... :devilgrin:
 
RickB said:
HS2's and small round ported head correct?
I have the HIF4's on a peanut head, so I'm feeling good about 35MPG!
Oh and I don't exactly drive it like an old lady... :devilgrin:
Click to expand...

No, No, No....on my A15 datsun engined midget. Single HS6.:eagerness:

[Actually with 2 200lb guys a couple of month's ago my check was 33...I fibbed.]:glee:

Kurt.
 
My first measured tank with the Sprite turned in a rather disappointing 20 mpg. 6 gallons, 120 miles. I'm still leaning towards the carbs being mixed too rich, the idle speed set too high, and now beginning to consider the possibility that I may have a dragging brake. I'll have to jack the car up and see what's going on to see if the dragging brake theory holds water.
 
Do remember that the way we drive these toys affects the gas mileage tremendously!
In town if I am feeling playful I get about 25 MPH but on the freeway on long trips (like Seattle to Wisconsin and back) at a steady 65 MPH we were getting 35 to 40 MPG! In a burst of fun going into (???) Wyoming we were racing and my mileage dropped to about 20 MPG for that tankful (Thanks RickB! ) :smile:
BillM
 
Yeah, all those Corvette drivers didn't know what to think!

:wink-new:
 
RickB said:
Yeah, all those Corvette drivers didn't know what to think!

:wink-new:
Click to expand...

Well if I had known you guys would encounter the annual corvette rally in the Black Hills I would have drove along just to show them what a real sports car is.:eagerness:

Kurt.
 
bug_sixty said:
Al lot of Hogs in there Black Hill's Did you cross hair Sturgis??
Click to expand...

We saw some, but the bulk of them either were before or after we drove through twice.
The Vette people were funniest, whole lotta credit cards goin' on...
 
Bike week used to start on the second Tuesday of August but believe it is now the first Monday. Anyway first part of August is a good time to avoid the Hill's! Unless you don't mind sleeping in your car!
Raced bikes out there in my youth....now no one even know's there are races.

Kurt.
 
I'm starting to go in the right direction with the gas mileage. Last tank was about 23 - 24 mpg.
 
Jets should be set so that you always require a little choke when starting cold. No reason that a 1275 shouldn't get, at least, in the low 30's at 2 lane speed's.

Kurt.
 
We had a 90 degree day today, and I had set the carbs when the temps were in the 60s. The result was that I did need to add a little bit of choke today to keep the motor running from cold. The idle speed was down around 750 - 800 rpm when warm, but didn't stall. I'm guessing the warm temps and lower barometric pressure leaned out the mix?
 
I find it funny that a little temperature and air pressure difference can make a noticeable difference in the way our engines run - the SU carb is such a wonder!
 
After another subpar tank of gas mileage-wise, I went to adjust the carbs again. This time, I discovered how far the engine could be leaned out before it would stall and then began increasing the richness of the mixture until the engine wouldn't stall. I seem to be having issues with the front carb. When I lift the carb piston, engine speed is not increasing. I adjusted the jet so it was all the way in (i.e. full rich) and it still had no effect when I raised the piston. I'm thinking that maybe the needle is now off center? I'm getting a decently smooth idle, but when I accelerate, I start getting some misfires and rough running in the 2500 - 3000 rpm range. I checked the throttle adjusting bolts and they are fairly well synchronized...both carb pistons are moving simultaneously when I pull on the throttle cable.
 
If the jet is screwed all the way in so that the jet is even with the bridge in the carb you are all the way lean. From there 12 flats of the adjusting screw underneath will get you in the ball park. Off center needle will only give you binding in the jet so that the piston won't drop all the way down. Quite obvious. Try 12 flats down on both with both throttle butterfly's opening at the same time. If there is an issue then I would suspect problem's with the fuel float needle and seat and fuel level.

Kurt.
 
Kurt's making good points, I would add that since your in there messing around, you might as well remove the dashpots/pistons and look for excessive wear on the needles and also look at the damper springs. Are they the same length and how long are they? With the dashpots off you can also set the jets accurately with a vernier caliper, then make adjustments on the road via "flats".

And... Take the needles out and see 1. if they match and 2. what ones you have. They could be "standard" or someone could have swapped them for a richer/leaner profile.

Also, removing the dashpots might seem like a big deal if you've never done it, but it's not. You're not going to disturb anything that you can't easily fix.
 
Got it. I'll try the 12-flat adjustment first. I did discover that my bicycle hub cone wrench makes the perfect tool for turning the mixture adjustment screw...yet another one of my "bicycle specific" tools that have found their way over to working on the Sprite.
 
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