• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

I am the worst driver....I get 9 mpg...help!

Share this page

You can also check to see if your choke is partially on.This would affect in town driving the most,and not affect hiway driving as much.Are your choke return springs on and working correctly? Choke cables properly adjusted?
 
wheelwright said:
You can also check to see if your choke is partially on.This would affect in town driving the most,and not affect hiway driving as much.Are your choke return springs on and working correctly? Choke cables properly adjusted?
Click to expand...

Thank you for your input.
That was one of the first things we checked.
I was hoping it was that, for it seemed like a simple fix.
Cheers,
Roger
 
Haven't weighed in, as I've been pondering.......

But.......

Seems like in my foggy memory banks there was an issue when folks swapped out spring in the slides......

Wrong tension seemed to do that, or the inverse.....

Have you looked at another with same carbs, cleaners off, and lifted the slides, seeing what pressure there was, and checking yours?

Just a random, unconnected thought process......
 
Ah HA!

The feeble old grey cells actually remembered something!
Found it!

He says that if the spring is too strong, the piston will not rise to its optimum position, thereby causing the engine to run too rich.

https://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601d.htm

The condition of two of those plugs shows too rich.
The rest of the engine seems to be compensating, and burning off the fuel.

There may be other issues, but I think I'd check them springs out.

Too thick of dashpot oil will affect operation in the rich sector, by inhibiting the pistion to caome up, effectively keeping the choke on.

Good write-up.
 
Question:
I re-read all 4 pages (quickly, but not quick enough....my eyes are now crossed), what version AH?

MKI is red spring, MKII is red or green, and a 6-port head is yellow spring.

Paper or oil element on the air filter?
 
TOC said:
Ah HA!

There may be other issues, but I think I'd check them springs out.

Too thick of dashpot oil will affect operation in the rich sector, by inhibiting the pistion to caome up, effectively keeping the choke on.
Click to expand...

1964 BJ8 phase two. HD8 carbs.

My springs are not colored anymore, maybe because they are old. How do I tell which ones they are?

Also, I noticed that the front dashpot hardly had any oil, but the rear was full. The oil I use is from Moss, SU dashpot oil. My bad for not checking the oil level.

What's your take on the front low and not the back?
What would cause that?

Could I be so lucky to just have the wrong springs? Plus finding out why the front dashpot oil was low?
I'm not out of the woods yet, but I'm hoping for something simple.

Cheers,
Roger
 
The way these carbs work is on vacuum.
If you read the links, you actually get "pulses" from the intake that can "pulse" the pistons.
One of the reasons the books tell you to have the valves adjusted and ignition right before you start futzing with them.

There was a write-up on using motor oil mixed with machine oil.
Too light an oil will get sucked into the engine and burned.
My presonal feeling is, you don't know where you are with the carbs, as they've been into countless times.
Either send them off and have them "done" right, or get the correct springs and see what happens.

No oil, the piston will "pop" first, leaning the mixture, and the other will try to take over.

I forget...does the HD8 have the button for checking mixture correctness?

The "drop test" is with oil.....should be stiff going up, and a specified time to drop.

But, wrong springs won't be revealed by any of this.
If you take the springs out and wipe them carefully with a rag, no colour is evident at all?

The problem I once had was some yay-who that mixed parts, thinking "all SU's are the same" and one was so different internally, I don't know how the car ran.

The springs are identified by tension, but then you'd have to find some way of testing that.

A) No vacuum leaks? (and see what idle vacuum actually is)
B) Valves adjusted?
C) Compression good and even?
D) Dashpot oil correct and full?
E) Pistons moving freely?
F) Float levels okay?
G) Springs
 
The springs are cheap and easy to change out. Why not just change them and see what happens. It may be that they are old and weak even if they are the correct ones. One may be weaker than the other do to engine heat. I thought you had your mechanic go over all that. Take charge and learn by doing. Its fun and helps you on the side of the road. Guys like Joe can put it straight and do the testing. Some monkeys polish/sand the piston or inside the housing and that will destroy the suction. Also there is a small hole in the angle tube at the top of the chamber that can get blocked. And do you have open holes in the black caps.
 
Roger,
My carbs were bought used. My car is a BT7 that would typically use 1 3/4" HD6 carbs. I bought the used 2" HD8s and just sent them to Joe to rebuild before I even tried them. So.... I have no "before" to compare to the "after."

Lin
 
tahoe healey said:
I thought you had your mechanic go over all that. Take charge and learn by doing. Its fun and helps you on the side of the road.
Click to expand...

Hi Richard, yes he has.
I would never re-do what my mechanic has tuned.
Some things I certainly will take charge and learn by doing. I did a nuts and bolts restoration (with a lot of help from the people here on this forum).

My British Mechanic, exclusive of Brit cars for over 40 years, knows more about my Healey then I ever will.

My Mechanic and David Nock have both given my car a full tune up. And that includes checking everything, timeing, drop test, vacuum leak, new spark plugs, wires, etc etc. They both say, the car is running great and the plugs show so.

They can't pin point it and say, just live with the poor mileage, it is what it is.

I just gave my car to my mechanic an hour ago. It will now be his daily driver back and forth to work. He will go through a full tank and tell me what he gets for mpg.

I'm thankful to all the people here who are offering suggestions. I'm relaying them to my mechanic.

Cheers,
Roger
 
Sticking or slow to rise carb pistons is what I was thinking also.
.. You could have the wrong/stronger springs installed than
standard on top of the carb pistons.
.. Are you using a thick oil such as 30w or 20w-50 motor
oil in your dampers ? If so, try using marvel mystery oil or
some other very low viscosity oil in your dampers.
.. Either your dampers or your pistons could be rubbing/sticking
during normal movement of the carb pistons.

Ed
 
Somehow I missed seeing the multiple pages of posts when I wrote the above post.
I see that you have moved on to plug gaps and notice that you are now just trying a .035 plug gap.
That is what I have been using with my petronix and matching 40kv coil along with Bosch platinum plugs.
It runs great and I get good mileage 21 MPG average last time I checked.
I use the trip odometer and head to a Shell station after the trip odometer exceeds 200 miles. It seems to take about 10 gallons to fill it back up.

Ed
 
I didn't notice for quite a while that I had two different dampers installed in my carbs. Who knows where the wrong part was installed ? Maybe the factory ran out of the right dampers one day. Could be the wrong springs in your carbs for the same reason or any previous mechanic may have thought he was helping the way the car ran by installing stronger springs.
Try new springs as others have suggested and change to a low viscosity damper oil to see if it makes a difference. Yes, the lower viscosity oil gets sucked out sooner than the thicker oil but it seems to work better at least in my car. I just check it more often and fill it back up as necessary. I am in there checking the oil and fluid levels anyway.
 
Ed,

I'm using the recommended SU oil, sold by MOSS.
Why are you using Bosch platinum plugs?
What wires are you using?
You average 21 mpg?
What do you get highway?
What do you get town?
 
Hi Roger,

Platinum plugs have lasted longer for me than regular plugs in any car that I have used them in. Bosch is a good quality plug but there are other platinum plugs that would probably be just
as good.

I am using Magnecor red wires. I really like them and they keep all of the noise and pops out of my music system. Their design limits current flow because the wire is wrapped like a long stretched spring inside of the insolation. This helps the spark plugs, rotor and cap last a lot longer than if straight metal core wires are used.

I think I am averaging around 20 - 21 MPG. I know it is better than that on trips and less if I drive around town and don't use highways. But I know it is better than 9 MPG around town.
The .035 gap may help your mileage some if you have a 40 KV coil.
As an experiment, take out the SU oil and put in some marvel mystery oil. Also, order a new set of springs for your carbs. Try one thing at a time so you will know if any one thing helps. Do you have matching dampers ?
Good luck in your quest !

Ed
 
Roger,

FYI I keep a small bottle of 20W-50 oil in the glove box and add a few drops to my HD8s at every fill-up or two. It makes people at the gas station think you know what you are doing.
 
I'm resurrecting this thread because I have some more info about coil voltage from a new source (ASE master automotive technician, former Sun Electric instructor and 15 years as a technical training instructor for a car company):

"I can tell you for a fact that if you give me 5 minutes with you, a car, and an ignition oscilloscope I could prove their statement 100% false.
Looking at an O-scope trace the only time the voltage will equal the total output of the coil is when the wire is open circuited.
Look at this waveform https://images.picoauto.com/copwaveform.png See the tall vertical line in the middle of the pattern? That is the spark line, in this case it is taking about 18KV to fire this plug. Assuming this is at idle, if we were to snap the throttle wide open the pressure in the cylinder would increase which acts as an insulator, and the spark line needs to rise by maybe 40% or so to let's say 25KV to fire the plug during that period of wide open throttle. So if the spark line is at 100% of coil output at idle, where does that extra 7 KV come from? This right there proves their statement false.
Next look at the almost horizontal line extending to the right of the spark line trending downwards slightly and ends with some oscillations. That is the firing line, and the more powerful the coil, the longer that line is. The less powerful the coil, the shorter that line is. In the illustration, that is a pretty healthy coil, the burn time is about 2 ms. This system can maybe throw 40-50KV if we were to open circuit a plug wire.
But electricity being lazy will only use the voltage needed to overcome the dominant gap in the system, which is usually the gap at the plug.
When I taught scope classes I used to have a plug wire that was cut in half and encased in a clear plastic tube, so I could slide the two ends closer and further apart. Using that it was very easy to see exactly what happened when the gap got bigger or smaller."

-- and --

"The easiest way I can think of to explain it is to think of wattage. 1 volt at 100 amps is 100 watts. 10 Volts at 10 Amps is 100 watts. 100 volts at 1 amp is also 100 watts. Coil output is similar, only instead of amperage think of time. The more voltage required to ionize the gap, the less time the the spark will last. There is a finite amount of power in each discharge of the coil. The more voltage that is used to create the spark, the shorter the time the spark will last (shorter spark line) The extra 10KV never really existed. It is just power that can be used to either ionize the gap, after that any excess power is used to maintain the spark line (keep the fire burning)
some modern ignition systems are capable of 80-90KV outputs, yet their spark lines are between 15-20 KV at idle. where did all the extra voltage go? It is turned into a very long burn time to fully burn the contents of the cylinder.
I agree with you that you can open the gap with a higher performance coil, but as rich as LBCs run, it probably isn't noticeable as it is on a modern car.

yes of course you may quote me and if there are questions, I would be happy to respond."


Not saying I understand this completely--yet--but this seems to confirm my initial belief that the spark voltage is determined by the gap, NOT the theoretical output of the coil.
 
Hello Roger,

just a thought in an different direction. I understand you have done a tune up , but how did you know your mechanic works well ?
I don´t know how your pertronix realise the centrifugal force controlling. Did you check the centrifugal force controller or vacuum input?

Forget the sparks as reason, they can only be an indicator for bad combustion.



But anyway you live in cheap petrol area... ;-)

Bye Michel-who has an original distributor
 
Bob_Spidell said:
"I can tell you for a fact that if you give me 5 minutes with you, a car, and an ignition oscilloscope I could prove their statement 100% false.
Click to expand...
Hi Bob,
Just curious which statement he is referring to?
 
Back
Top