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SU HIF4 fun and games

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RickB

RickB

Yoda
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Don't know how many of us with Spridgets try and use these HIF4 carbs but I've just today finally understood how the jet richening / leaning mechanism really works.
After studying many diagrams and cutaways I've finally figured it out.
Turning the screw clockwise richens, counterclockwise leans them.
Here's why:

There's a pivot in there and a spring (#24 & #25) and an L shaped lever and the screw (#21) lets that lever lean out toward the outside of the body or pushes it in toward the center. In toward the center lowers the jet while out toward the body raises the jet. Moving the screw anti-cw raises the jet and cw lowers it. Finally I "get it". BTW currently my car is running with the screws turned almost all the way in, due to an error on my part on just exactly which direction I should have been turning to achieve which results.

Diagram of the metering screw and associated parts:

carbparts.jpg

From this larger diagram of the whole carb:
carbpartsbig.gif

This cutaway drawing (on the right) was very helpful:
tech_su_01a.jpg
 
After work I raised the bonnet and leaned both carbs out by two full turns of the screws.
Here are the first noticeable effects:
1.
Lifting the pins previous to leaning caused the idle to rise immediately and stay higher than normal.
After leaning when I lift the pin on each carb the idle falls off a few hundred RPM. Still runs, just falls off.
2.
The idle increased by a few hundred rpm, and I can't adjust it down because the idle adjustment screws are not touching.
I'd think this indicates a vacuum leak, but I haven't been able to find one.

The car runs good and there's no visible soot coming out the exhaust like there was before.
I won't know about gas mileage for a while, I filled it up after adjusting the carbs so after I put maybe 50 miles on it I'll fill up again and see.
 
I've been told that #25 is a screw that adjusts startup enrichening and there's a temperature compensator on the L shaped piece.
I believe that's the bi-metal part that I've heard about.
I would assume that turning it cw would cause richening and ccw would cause leaning, since cw would seem to cause the jet to lower and ccw should cause it to rise.
Maybe some time soon I'll take another one apart and verify all this for myself.
 
Rick, if I understand you correctly I believe you have it backwords. In a SU lowering the jet richens the mixture and raising the opposite. Remember that as you lower, the taper of the needle allows more fuel in for a given throttle opening.

Kurt.
 
nomad said:
Rick, if I understand you correctly I believe you have it backwords. In a SU lowering the jet richens the mixture and raising the opposite. Remember that as you lower, the taper of the needle allows more fuel in for a given throttle opening.

Kurt.
Click to expand...

Kurt that's true, however this screw is mounted with the head at the bottom and turning it cw (tightening) actually (I believe) lowers the jet - so tightening (turning it clockwise) richens by pulling the bimetal piece downward.
There is a spring under the screw head between the bimetal piece and the screw head, tightening the screw compresses that spring.
I am not sure of all this, I feel the need to experiment.
In this image I have put a large black arrow where the startup richening screw is located and a smaller white arrow where the jet adjustment screw is located.

SU_hif4_adj.jpg
 
OK Rick. Guess you are right but I can't really tell how the mechanism all works and I have never had a HIF apart. Is the black arrowed screw acessable from the outside? Guess it would have to be but I can't see what would hold it in position while it moved the jet up and down so I guess you experiment or get hold of the MGB guys.

Kurt.
 
No you have to take the bottom cover off the float bowl to access that screw.
 
The first time I tuned an HIF I read the manual and tuned it, but like you I was scratching my head about how it works. So I went home and looked it up the same evening. Even though it is still a simple design, I still prefer the non-integrated float bowl design.
 
Trevor Jessie said:
The first time I tuned an HIF I read the manual and tuned it, but like you I was scratching my head about how it works. So I went home and looked it up the same evening. Even though it is still a simple design, I still prefer the non-integrated float bowl design.
Click to expand...

I too prefer the HS2, HS4, HS6 type SU's. If I were to put in an earlier heater box I could probably fit them (if they would fit my intake manifold).
I'm having trouble remembering at the moment - are the mounting areas the same between HIF and HS carbs? (guess I could go look this up....)
 
SUCCESS!!! :encouragement: :applause: :cool:

After putting 94 miles on it (back and forth this week the 13 miles from home to work several times) I filled up the tank and kept clicking the filler handle until it wouldn't hold any more.

3.100 gallons

Roughly 30.3 miles per gallon!
Not driving any more sanely than ever and the engine is running really well.
The rpm of the idle is up a bit from before but the idle is smooth and strong.
No acceleration problems and no pinging.
I'm very pleased with this result!

Can't wait for the next road trip, in 2 weeks we will be going on our club's Tulip Rallye - the annual rallye that takes place around Mt Vernon and Anacortes Wa. We should get some good gas mileage on the way up and the way back - I'll be sure to check it. Todays result makes me think that 35 - 40mpg is within reach.
 
Rick, my A15 Datsun engined midget with the single HS6 regularly beats 40MPG. I'll bet you can go a bit leaner.

Kurt.
 
Yeah, but a pair of HIF4's maybe pull a little more than a single even if it's the 2" HS6.
I'm pretty happy with the 30+ mpg and the amount of power I'm experiencing in the seat of my pants!
Ew - did that come out right?
 
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