SU HS6 Tuning Questions

[LanceLyon]

Hi Everyone,

I'm trying to uderstand, if that's the word for it, the tuning concepts for the HS6 carburators. The basic concept for these carburators defies my traditional understanding of how a carburator works!

Here's where I am at: Engine is timed and running at a water temp about 160 degrees. Idle is set at about 1000 RPM, kinda rough, and from the odor of the exhaust, running rich. I follow the recommendations of the shop manual, and begin with the jets backed off two full turns, or 12 flats, from being seated all the way. The suction "hiss" through the two carburators has been equalled by adjusting the idle screw.

I begin to adjust the jets further out equally as well as further toward the seat, and find that the engine seems to run the smoothest, fastest, and with the best odor when the jets are both seated all the way back down. My instinct tells me that it should be too lean at this adjustment. The car starts cold in this setting without additional choke, but does not like to start hot.

Does anybody have any insights or experience to help me? Thanks!

 

[Dave Russell]

LL,
Here is a good explanation of the SU. It is for the Hitachi-SU but they are similar;
https://www.teglerizer.com/sucarbs/techtip6.htm
Also, this is pretty good;
https://heine.nusa.co.za/Library/SU%20Carburettors.htm

"I begin to adjust the jets further out equally as well as further toward the seat, and find that the engine seems to run the smoothest, fastest, and with the best odor when the jets are both seated all the way back down."

This is the first time that I have heard of "tuning by smell". Seriously, what you describe could be caused by worn parts or float levels too high or even wrong needles or jets.
D

[ 02-09-2004: Message edited by: Dave Russell ]</p>

 

[Randy Forbes]

You didn't mention, but are the needles and jets new? They both wear out, especially if they're not aligned properly.

The wear will allow more fuel to pass, hence the easy cold starting.

 

[aerog]

In addition to the wear you should also verify that the enrichener (ie: choke control) isn't sticking or isn't out of adjustment. When you pull the choke control you're just changing the idle speed and enrichening the mixture by moving the jets. If the "choke" cable cable/mechanism is allowing the jets to stay "enriched" slightly you end up compensating by adjusting the jets toward the lean side - something that should be relatively easy to eliminate as a problem by disconnecting the cable.

Check that the needles are installed in the pistons properly too. Too far into the piston and you'll have to compensate by moving the jets more toward "lean", too far out of the piston and you compensate toward "rich". In theory the shoulder of the needle is flush with the groove in the bottom of the piston.

The float needles/seats can also be a source of problems. If they're dirty or worn you can have a carb that tends to be rich.

Finally, when you read up on the theory behind this style carb you'll find that they're really pretty simple and logical. The way I look at it is, the carb simply reacts to the volume of air behing drawn through it by the engine. The greater the volume of air the higher the piston is lifted up (due to the air), when the piston is drawn up it opens the needle valve allowing more fuel into the venturi of the carb thus creating a balanced fuel/air mixture. Since the carb relies on metering the volume of air it's important to set the valves and timing before messing with the carbs (just like the SU book says) so you're starting with an air-pump (the engine) that is to spec - but it's also easy to see that the other parts that influence needle position (improper installation, dirty pistons, air leaks) can alter the whole action of the carb. Conversely if the air-side of the carb is right-on but you have fuel leakage (worn jets, seats, needles, improperly adjusted floats, etc) then you can't compensate the problems with adjustment.

An excellent reference on most all SU carbs, covering the theory, rebuild and tuning instructions is the "SU Carb Tuning Tips and Techniques" by G R Wade, Brooklands Books (Moss sells it I think).

[ 02-09-2004: Message edited by: aerog ]</p>

 

[LanceLyon]

I rebuilt both carbs using SU Master Rebuild kits, but no new needles or jets were included. Jets are centered and nothing hangs up. The engine has about 70K miles on it, and I have no idea if the needles and jets are original or not.

I took my first test-drive tonight of the car since getting it running, with a couple of very excited teenage daughters! About ten minutes after parking it we could not get it started again; not even a cough!

I'm downloading the info from Dave's links as I write this. Thanks Dave. I'll let you know if I get it figured out. Randy's thought about worn needles makes sense when you think about it. How often should needles and jets get replaced?


Just got aerog's post. The choke cables were relaxed as I set the jets, and I'm pretty sure that the needles in the pistons seated with the brass shoulder even with the aluminum base. Also, I did check the float levels and blew out the needle valves just before I installed an in-line fuel filter a couple of days ago.

Still perplexed, but happy to have all of your ideas and support!
[ 02-09-2004: Message edited by: LyonLance ]

[ 02-09-2004: Message edited by: LyonLance ]</p>

 

[Dave Russell]

"I'm downloading the info from Dave's links as I write this. Thanks Dave. I'll let you know if I get it figured out. Randy's thought about worn needles makes sense when you think about it. How often should needles and jets get replaced?"

"Also, I did check the float levels and blew out the needle valves just before I installed an in-line fuel filter a couple of days ago."
====================================
LL,
It's hard to say how often the needles & jets should be replaced. It takes such a small amount of wear (even hard to measure) to screw things up that if you have any doubts, replace them.

Even very minor float valve leaking will cause a flooding & hard start problem. As a final check, after the car has been sitting as you describe, remove the piston covers & pistons, look down into the jets & see what the actual fuel levels are. They should be about 1/8"below the jet bridge surface. If the fuel levels are higher than this or even with the bridge or maybe even overflowing a bit, you have found the problem. It takes a very small amount of float needle leakage to raise the level to overflow at the bridge after sitting for a while. (The fuel input pressure to the carbs does not go to zero when the pump is shut off.)
D

 

[LanceLyon]

Although I started this thread about 3-weeks ago, I have not found an answer to the situation that makes a difference.

Two additional questions at this time:

1. Remember, this car sat idle in a barn [for 25 years]in New hampshire before it came my way. When I first got the car I sprayed [mass quantities of] WD-40 in all the spark plug holes and then rebuilt the starter and various other electrical componants. When first I turned the engine over it was slow to go, then increased in speed as the (presumably) rust was scraped off of the cylender walls. After starting and running the engine for perhaps 1 hour, I changed the oil, which smelled of fuel. I have to think that the rings suffered damage durring the ordeal, and that I am getting "blow by". QUESTION 1: If worn rings is the culprit, why does it start first time every time when cold with no additional choke, but will not start at all when hot? Seems to me that worn rings would cause it to run lean, not rich.

2. After reading through the information provided in the various links that were suggested in the above posts it is apparent that a number of jet needles could be used. QUESTION 2: Does anyone have a learned opinion on which jet would work best in the Southern California climate, specifically in the Santa Barbara Area? Moss offers a Standard, Lean, and Rich option, but the literature suggests that there are closer to 250+ jets available from SU.

[ 02-29-2004: Message edited by: LyonLance ]</p>

 

[piman]

Hello Lyon,
I would not worry about the needles just at the moment. When the engine is at rest, there is little or no difference in the needle dimension whatever optional needle is fitted.
I re read your posts and it sounds as if the carburettors are set too rich as it does not need choke to start from cold.
If you screwed the jets downwards from the original 12 flats off full up then you are making the mixture richer. I would reset the jets to the original basic setting and re check for flooding.
If you have an electric fuel pump, disconnect that when you stop after a run. Then try a start, if it fires up then your problem is definitely flooding.
You mention that you are concerned about the rings, take the oil filler cap off when the engine is running and if there is an excessive amount of fumes coming out then you may have a problem but I don't think that is why you can't do a hot start.

Alec

[ 03-01-2004: Message edited by: piman ]</p>

 

[Johnny]

I agree with all the previous posts, however, you may need to start over,(adjust to 12 flats) then verify you don't have any air leaks via the intake manifold (check underneath, there should be tubes leading down the engine). While checking underneath, is there fuel being dumped on the ground?

 

[LanceLyon]

Alec & Johnny,

The seats for the jets are adjusted all of the way up and seated against the stops. This is lean, isn't it?

I'll check the oil filler cap suggestion and report back. Regarding the manifold leak, I have listened around the manifolds with a tube stuck in my ear and can hear no "hissing" noises that would suggest a leak. The two small tubes in the manifold have leaked fuel in the past, but that was due to a clogged needle valve that was remedied with cleaning and a fuel filter. I suppose that if the fuel pump is pushing fuel past the floats after the car is stopped that that might explain it. I'll have to look into the excess pressure issue somehow.

Thanks for your thoughts!

 

[tahoe healey]

Possibly over-looked but are the needles set too high in the pistons? Starting rich and going richer when the pistons rise. Have you played with the pistion lifters to check the richness at idle?

 

[LanceLyon]

Mission Success!! Tahoe Healy's last post made me think about a post earlier in this thread from Aerog regarding the needle location in the piston.

I foolishly thought that the needles, when fully inserted, would be at the proper position. I reset the needles as Aerog suggested and behold!

My thanks to all of you for your help. I'm embarassed for not checking the needles as suggested, and for needlesly exercising you all!

 

[Dave Russell]

Aerog & Tahoe,

Good call, I'm impressed. It never even occurred to me that this could happen. And I have even experimented with the needle position. The carbs can easily run out of adjustment range, about three turns, if the needles are moved very far.

I retract my original diagnosis & add another item to my possible problem list. Always something to learn.

In Lyon's case the carbs could have been running on their .0970 step instead of the proper .0990. A needle height change of .050" would result in an idle mixture fuel increase of around 150%.A big problem at idle but not so much at larger throttle settings.
D

[ 03-01-2004: Message edited by: Dave Russell ]</p>