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crank ventilation effects on mixture

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pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, Nova Scotia, Canada   CAN
I'm assuming the car ran properly when it was new... so if it has the factory specified needles and springs and no air leaks the carbs should perform (more or less) properly.

Most people don't understand that the springs have a very large effect on the mixture. Weak springs give a weaker mixture and stronger springs give a richer mixture. Either way you need 2 the same. If they are not the correct "weight" then you need to get new ones.

You have to do a bit of homework and find out if you have the correct parts in your carbs if you want it to run properly. Same with the needles and emission/breather system.

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, California, USA   USA
This is something I need to research more. Out of curiosity, how do stiffer springs create a richer condition? I would think its the other way around but I'm new to these carbs.

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, Nova Scotia, Canada   CAN
Stiffer springs create a richer condition by increasing the vacuum over the jet. The position of the bottom of the piston dictates the venturi size and a stronger spring makes the venturi smaller at a given air flow. The airflow is controlled by the throttle valve. The vacuum over the jet is controlled by the "weight" air piston. Early SU's had no springs. They used pistons of different weights instead.

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, California, USA   USA
Thanks Adrian. Very clear explanation. I wonder if my springs are way too light and forcing me to drop the jets 3mm...

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, California, USA   USA
Also, tell me if I have this right. The oil viscosity dictates the rate of lift while the spring controls the height....

ice Avatar
ice Gold Member Larry Ice
Lawrenceville, GA, USA   USA
Peer, these carbs are very simple. There is very little that goes wrong with them. I think you have reached the phase where it's time to fish or cut bait. The post is getting so long I can't remember if you had your carbs built by someone who knew what they were doing. If not, by all means have them rebuilt professionally and have the individual who rebuilt them put them on the car and dial them in.
If you change change all the different parts/systems you will never get the engine to run properly and you're wasting all the time you could be driving. At times we all reach the end of the road with what we can do on our cars and then it's time to spend some money on them to achieve a good running car.



Iceman

Atlanta GA

60 AH MK1
62 AH MKII
67 Midget
71 Midget

ice Avatar
ice Gold Member Larry Ice
Lawrenceville, GA, USA   USA
Peer, if you want a car with decent highway cruising speed get a Miata. A Sprite or a Midget wasn't made for US roads and all the jerks that inhabit them. Cruise above 60 mph and your car's longevity is severely reduced.



Iceman

Atlanta GA

60 AH MK1
62 AH MKII
67 Midget
71 Midget

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pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, Nova Scotia, Canada   CAN
In reply to # 3657825 by Poundingsand Also, tell me if I have this right. The oil viscosity dictates the rate of lift while the spring controls the height....

Kind of. An SU is a constant depression carburetor. The spring sets the depression level over the jet. The height the piston reaches is governed by engine load by way of the vacuumed that is sent to the chamber above the piston.. Obviously if the spring was too strong it would not let the piston rise... but that is not its function. Note that early SU's had no springs.

There are volumes of miss information on the internet about damper oil. The oil is to dampen the rate at which the piston rises but only during sudden throttle openings. It has no effect on slow application of throttle. Damping the piston rise makes the mixture richer than it would be without damping. This is useful for sudden acceleration. Thick oil increases damping (and richness) and thin oil... does the opposite.

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.



Edited 1 time(s). Last edit at 2018-01-07 12:19 PM by pinkyponk.

matmire Avatar
matmire Matt Meyer
Studio City, California, USA   USA
If you want a very close substitute for genuine SU dashpot oil, but cheaper... 3-in-One “MOTOR” oil is about as close as you can get. That’s the Blue label version, NOT the “multi-purpose” 3-in-One that most hardware and auto parts stores carry. I found one at my neighborhood mom-and-pop hardware store. You can also get it on Amazon...

https://www.amazon.com/dp/B00065VGUC/ref=cm_sw_r_cp_api_xUNuAb940N646

It’s a straight sae 20 weight oil, same as SU oil. Typical motor oils are mult-weight, and so will behave differently at varying temperatures. You might want that in certain climates, but not in LA.

It also has the added benefit of coming in a container perfect for filling dashpots!

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, California, USA   USA
Larry, agreed. I think its time to farm it out. However, I am driving the car. Quite a bit with the warm spell here in LA! smiling smiley I am just trying to make the freeway ride more enjoyable. Its a blast on Mulholland, not so much the freeway back home...

I think I'm trying to get the car to do something it wasn't meant to, maybe. You say not over 60, but I see so many posts of people cruising at 80 for hours, or so it seems....I feel like maybe I am missing something.

Ill likely be calling a recommended mechanic tomorrow and see if he wants to tackle it, along with anything else he feels can use improvement. I don't mind spending the money, but wanted to tackle what I could. Any reason to hang out in the garage!

ice Avatar
ice Gold Member Larry Ice
Lawrenceville, GA, USA   USA
Hi Peer, wow, didn't know that many guys drive over 80mph. If yours can get to 80mph I think you have done a good job getting it running. If the car drives well enough to run it on the freeway just drive it and have fun. These are supposed to be fun driving little cars. I'm not sure which can be more enjoyable, driving them or working on them! Sometimes I think of them like a helicopter, fly it for an hour and spend ten working on it!



Iceman

Atlanta GA

60 AH MK1
62 AH MKII
67 Midget
71 Midget

refisk Avatar
refisk Rick Fisk
Frankenmuth, Michigan, USA   USA
80 mph is buzzing right along. I don't recall after 70+ posts - have you checked float levels?

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, California, USA   USA
Larry, agreed. I certainly enjoy working on mine. Unfortunately I don't get as much time as I would like.

Maybe not THAT many (and who knows with what mods) but I feel like it could be more freeway capable (and hopefully leak less oil in the process).
Rick, I have the plastic floats. From what I understand they are nonadjustable.



Edited 1 time(s). Last edit at 2018-01-07 06:07 PM by Poundingsand.

refisk Avatar
refisk Rick Fisk
Frankenmuth, Michigan, USA   USA
Peer,

Plastic floats are adjustable. You use a shim washer under the seat to raise or lower the float. To raise the float level use a thicker washer under the valve seat. And use a thinner washer or no washer to lower the float. smileys with beer

In reply to # 3658082 by Poundingsand Larry, agreed. I certainly enjoy working on mine. Unfortunately I don't get as much time as I would like.

Maybe not THAT many (and who knows with what mods) but I feel like it could be more freeway capable (and hopefully leak less oil in the process).
Rick, I have the plastic floats. From what I understand they are nonadjustable.

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, California, USA   USA
Thanks all, for the advice and information. I've decided to rebuild the carbs myself, since it's possible the seller overstated what they had done to them. I was going to farm it out, but decided "where's the fun in that"? I bought the car to have a project, and now I've got one!

I've ordered a rebuild kit, along with new springs, needles, etc. One question before I tackle this...aside from determining float fuel level by measuring the distance between the float top and the float bowl lid, I understand a more accurate reading can be made by measuring the fuel level in the jet. I've perused as many posts (from this forum and others) about this as I could, and seemed to find differing opinions on what the level should be.

The most popular seems to be to remove the piston and dashpot, drop the jet via the choke, and the levels should both be equal between the carbs, and just below the lip of the lowered jet. Does this sound about right?

Thanks!

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