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View Full Version : Finally feel like I've got my carbs right



RickB
05-17-2013, 12:35 PM
After messing with them and messing with them I feel they are dialed in now.
3.5 turns out from all the way rich on each HIF4 and they are just right.
I couldn't believe I was getting so many miles on the tank of gas - it was just under 3/4 tank on the gauge at 126 miles.
So I filled it this morning. Took 3.6 gallons and no more.
That's 35 MPG with just normal driving, back and forth to Issaquah (about 30 miles round trip) all week and a couple trips to the grocery store.
The downside is I now have to connect the choke as it's a little cold blooded in the morning!

Trevor Jessie
05-17-2013, 01:09 PM
Congrats.

I've always been amused by people that brag that their SU equipped car starts right up without EVER having to use the choke.

Guest
05-17-2013, 01:32 PM
No matter what I Put on or what I do, 27 MAX avergae (got 31 before) with 22 average: DCOE, DGV, dual SU's...you name it.

bug_sixty
05-17-2013, 01:40 PM
Heavy foot? haha

RickB
05-17-2013, 02:06 PM
Yes Trevor, mine used to "start right up" no matter what.
Due to being very rich, like having the choke pulled.

I have a pretty heavy foot, there are some here who can testify to that.
These are 1.5 inch carbs and they are on a 1500 motor that's a Japanese copy of the A series British motor.
I've got about 9.5:1 compression and the engine really likes 92 octane pure gas.

The best I could get previously was 30 MPG around home, but I found the sweet spot and I still have plenty power just a bit cold on start up.

JPSmit
05-17-2013, 06:05 PM
No matter what I Put on or what I do, 27 MAX avergae (got 31 before) with 22 average: DCOE, DGV, dual SU's...you name it.

mine is about the same

Boink
05-17-2013, 06:20 PM
mine is about the same

Me too (1275 with dual HS2s).

nomad
05-17-2013, 11:10 PM
Just checked my BE with 1275 and 3.9 rear and mileage was 35.5. My Datsun engined Midget regularly runs at 39 to 40 with a single HS6

mrsprite
05-18-2013, 01:04 AM
Congrats! I had the carbs on my A15 Bugeye at the sweet spot once or twice, but then something else would go wrong and I'd have to fiddle with them again only to screw things up. I had it where I'd get 42MPG on the highway (hit 47 on one tank), but now I'm lucky to get 30MPG. In my defense I haven't really driven it much since the fender bender last February, so just driving it and adjusting things should make it better. I just moved down to Dallas and will be driving it down next week and plan on doing more driving this year. With the big altitude difference I will be adjusting the carbs anyway.....hopefully I can get them dialed in.

RickB
05-18-2013, 02:37 AM
Just checked my BE with 1275 and 3.9 rear and mileage was 35.5. My Datsun engined Midget regularly runs at 39 to 40 with a single HS6

Economy head?

nomad
05-18-2013, 08:32 AM
Yes, Rick, I forgot to mention that I do have the good Japonese head. H 89. I'm building another with the same head as you have.

Kurt.

RickB
05-18-2013, 01:40 PM
:fat: I sure like my throttle response and power range up to 7k... :fat:

nomad
05-18-2013, 02:10 PM
My HS6 may be restricting me on top end power. An HS6 should only be good for feeding about 80hp. Picked up a 40DCOE and manifold for a A datsun engine for in the future. Gas mileage will be gone! BTW we just got a $0.25 hike over night!!

Kurt.

Guest
05-18-2013, 09:06 PM
Picked up a 40DCOE and manifold for a A datsun engine for in the future. Gas mileage will be gone! BTW we just got a $0.25 hike over night!!

Kurt.

Made no difference with me.

owensdad74
05-22-2013, 04:50 PM
Where does the choke cable attach? I just got my single HIF 4 mounted on my 79 1500 and am not that familiar with it yet...


After messing with them and messing with them I feel they are dialed in now.
3.5 turns out from all the way rich on each HIF4 and they are just right.
I couldn't believe I was getting so many miles on the tank of gas - it was just under 3/4 tank on the gauge at 126 miles.
So I filled it this morning. Took 3.6 gallons and no more.
That's 35 MPG with just normal driving, back and forth to Issaquah (about 30 miles round trip) all week and a couple trips to the grocery store.
The downside is I now have to connect the choke as it's a little cold blooded in the morning!

RickB
05-23-2013, 10:31 AM
Where does the choke cable attach? I just got my single HIF 4 mounted on my 79 1500 and am not that familiar with it yet...

I was planning on attacking that this coming long weekend, but I have to admit to never connecting one on an HIF carb.
Maybe we will have to wander into the MGB portion of the forum and ask for help?

RickB
05-23-2013, 10:36 AM
I just posted the question over there, I'll relay what I find.

Trevor Jessie
05-23-2013, 10:41 AM
https://www.mgexp.com/phorum/read.php?1,2373310,2373495

RickB
05-23-2013, 10:50 AM
Wow - thanks Trevor!
That was just posted yesterday, great timing!

Flash_Harry
05-23-2013, 04:45 PM
Over on that post on the experience, I just added a scan of the HIF4 diagrams from the Haynes MGB manual. Couldn't attach it here - too big a file. Might help? But I could not find anything in two Haynes manuals on how to adjust the choke cable (or if it is necessary). I think someone told me that there should be a slight amount of slack, and that's how mine is now.
Harry

SaxMan
08-21-2013, 10:59 PM
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?

Boink
08-21-2013, 11:46 PM
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.

nomad
08-22-2013, 08:19 AM
Rick, nice to here you've got them working. 35mpg.....I get down in that range at 80mph!:highly_amused:


Kurt.

RickB
08-22-2013, 12:39 PM
Rick, nice to here you've got them working. 35mpg.....I get down in that range at 80mph!:highly_amused:


Kurt.

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:

nomad
08-22-2013, 10:35 PM
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:

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.

SaxMan
08-31-2013, 11:27 AM
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.

Billm
08-31-2013, 12:34 PM
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! ) :)
BillM

RickB
08-31-2013, 02:55 PM
Yeah, all those Corvette drivers didn't know what to think!

:wink-new:

nomad
09-01-2013, 08:54 PM
Yeah, all those Corvette drivers didn't know what to think!

:wink-new:

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
09-04-2013, 02:08 AM
Al lot of Hogs in there Black Hill's Did you cross hair Sturgis??

RickB
09-04-2013, 04:46 PM
Al lot of Hogs in there Black Hill's Did you cross hair Sturgis??

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...

nomad
09-05-2013, 08:28 AM
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.

SaxMan
10-04-2013, 08:34 PM
I'm starting to go in the right direction with the gas mileage. Last tank was about 23 - 24 mpg.

nomad
10-04-2013, 11:18 PM
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.

SaxMan
10-06-2013, 05:29 PM
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?

RickB
10-06-2013, 05:38 PM
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!

SaxMan
10-27-2013, 10:23 PM
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.

nomad
10-27-2013, 11:16 PM
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.

AN5Sprite
10-28-2013, 12:43 AM
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.

SaxMan
10-28-2013, 10:13 PM
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.

Bayless
10-29-2013, 10:34 AM
And if you remove the pots, be sure they go back on the same carb they came off of.

SaxMan
10-29-2013, 10:03 PM
Thanks, guys...again! I was able to get the carbs dialed in very nicely this evening. Baby Blue now purrs like a kitten when the engine is at idle and it is running smoothly through all speed ranges. We'll see if the gas mileage improves as well. If it does, great. If not, I'm going to chalk it up to the "there are worse problems to have" file.

Jim_Gruber
10-30-2013, 09:53 AM
When you fill up make sure you are using premium. These little engines were designed/built for when the quality of gasoline was substantially better than we can get today. Around town with your foot to the floor, don't tell me you aren't listening and enjoying that engine winding out 23-25 is acceptable but when you get on the Expressway, steady state, expect to get 30-35 or more. With Bugsy my '68 with a '66 1098 23-25 was about normal around town/going to the grocery 3 miles away and not giving the engine a complete chance to fully warm up.

SaxMan
10-30-2013, 10:54 PM
I do use premium, that was one thing the PO (longbridgehealey) advised me to do. Considering how small the tanks on these cars are, even with premium, it usually doesn't cost more than 20 - 22 bucks to fill up.

I took the car on the Westminster run for my daughter's speech today and it performed beautifully. The small of carbon was completely absent for the first time and based on the fuel gauge's movement on the 60 mile round trip, it appears to be doing better in that department, too. I probably won't be able to completely tell until the next tank, since I spent so much time idling the motor while adjusting the carbs this time around.

I'd say the carbs are about 90 - 95% perfect. However, given the variances in air pressure and temperature from day to day, I don't think I'll ever get them to 100%. I also don't think the benefit of that last 5 - 10% is going to be overly significant. Right now, the engine idles relatively smoothly just under 1000 rpm with a hint of the generator light coming on and runs through the speed range well. It was much quieter and smoother at 60 mph than it ever had been before.

nomad
10-31-2013, 10:59 PM
Great...good to here. Next you can try adjusting the mixture one flat at a time on both carbs so they stay matched. In for lean and down for richer. If it stumble's under acceleration you need a little richer. I set mine till they just give clean acceleration with a warmed up engine. In cold weather they don't always want to come up to operating temp and then I'll richen a flat to give clean acceleration. Mine are free enough that I can turn them with my finger's!

Kurt.

SaxMan
11-28-2013, 03:15 PM
Latest tank yielded just bit over 27 miles to the gallon. That sounds about right, considering I was throwing the car around a bit on this tank.