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Beginner Questions about carbs, tickover, choke

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jjbunn

jjbunn

Jedi Knight
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Harold starts readily enough. I don't use any choke, figuring it's warm enough in SoCal. Once started, tickover is about 1500rpm. Manifold vacuum is 20psi, engine oil pressure is between 50 and 75psi. If I set it the tickover lower using the adjustment screws on the carbs, the engine tends to run roughly. Using more choke doesn't seem to make much difference.

Until the engine has been running awhile, pressing the gas pedal tends to make the engine splutter and die.

I have a feeling I need to adjust the carbs. The plugs are new, as are the plug wires. The distributor cap and points are in great condition, and I've cleaned them.

One doubt I have is that the plug gap I'm using is .035, whereas I have seen various numbers, e.g. .025 in the Bentley manual. Is 0.035 good?

I've also cleaned the pistons and needles in both carbs, and the float chambers.

(Harold used to have emissions equipment installed at some point in its life, but all that is connected now is the breather tube from the cylinder head.)

I have replaced the non-stock airfilters with new ones of my own design (pictured in a previous post). When tinkering/tuning the carbs, should this be done without the air filters in place?

In short, I'd welcome some advice on how to approach getting the engine to idle correctly (I think at 900 rpm), what areas to look at first, what literature to read, and so on.
 
Re: Beginner Questions about carbs, tickover, chok

What I've seen said many times elsewhere is that the carburetor adjustments are the last things to check.. What about the timing?

If you snap the throttle open it sputters and cuts out.. What if you slowly increase the throttle? Will the engine gradually climb, or will it also die? If the later, perhaps it's a fuel delivery problem.
 
Re: Beginner Questions about carbs, tickover, chok

I think I would adjust the timeing to stock, disconnect vacume advance first.

Then I would work on the carbs. Checking first fuel flow from the tank, then mixture, then balance, then idle. Then I would check mixture, then idle then balance. Round and round we go till we get it right.

When you hit the throtle it should scream back at you.
 
Re: Beginner Questions about carbs, tickover, chok

Carbs are HS4.
 
Re: Beginner Questions about carbs, tickover, chok

[ QUOTE ]
What I've seen said many times elsewhere is that the carburetor adjustments are the last things to check.. What about the timing?

If you snap the throttle open it sputters and cuts out.. What if you slowly increase the throttle? Will the engine gradually climb, or will it also die? If the later, perhaps it's a fuel delivery problem.

[/ QUOTE ]

It's very sensitive: if I press verrrry slowly and gently, it will climb. But a little too much and it dies. It's especially sensitive just after starting.
 
Re: Beginner Questions about carbs, tickover, chok

From description it's running "fat" (rich).

The carb(s) question Scott asks is for qualifying just how to adjust the mixture. It -should- be running between 500 and 800 RPM at tickover. Have you "jiggled" the throttle shafts with the engine off to see if there's any 'slop' in the bushings where the shafts go thru the carb bodies? If there's ANY room for the introduction of air past those shafts, they're worn and can cause what you've described.
 
Re: Beginner Questions about carbs, tickover, chok

One way to test to see if it's sucking in 'false air' around worn throttle bushings (a common problem in old SUs, I'm led to believe) is to spray starting fluid on the throttle shaft with the engine running. If you hear an appreciable change in engine idle, it's sucking in air where it shouldn't be.

Do those HS4 carbs have an adjustable brass lever in each float bowl which determines the fuel shut-off height? On my Healey's carbs I had to adjust the lever for the proper height.. Perhaps the carbs are starving because the float level is too low?

Or perhaps, as DrEntropy says, it's just overly rich. Here's a document I found very helpful when tuning dual SUs https://www.team.net/sol/tech/su-tune.html

A good book on the subject: https://thezstore.com/page/TZS/PROD/70-1053
(I found the book to be overkill for the simple stuff I wanted to do.)
 
Re: Beginner Questions about carbs, tickover, chok

You can test the shafts with about anything from ether to water in a squirt bottle, BTW. If it's suckin' air past the shafts it'll affect the idle immediately.
 
Re: Beginner Questions about carbs, tickover, chok

If she starts right up without any choke, and from other symptoms, I'd tend to agree with running rich.

The link from Dab (above) is a good one. You also might want to check out https://carburetor411.com/SU_Main.html. I quickly threw this together for my own use when I was starting to figure out how to keep the cars running.

One easy way to tell if she is too rich is if the tail pipe has black soot...
 
Re: Beginner Questions about carbs, tickover, chok

I always like to start by making sure the carbs are in sync and working from there....Get an SU tuning gauge or a hose ur can use to listen to the suction and then get the buggers balanced. Another thing to check is the choke cable(I am thinking dual strongbergs here, ala TR6...so this might not be true for SU's), if it's not releasing properly on one of the carbs(out of adjustment)it will cause similar symptoms.
 
Re: Beginner Questions about carbs, tickover, chok

Soot's a good indicator as well. But it's the "cold" throttle application causing it to "fluff" and die wot's giving me the over-rich "signals."

Julian: You've got engine oil in the dampers, yes? Sounds like the pistons may be "slamming" to the top: full rich on slight throttle app. You should feel a great deal of resistance if you try to manually push the pistons up from the front of the throats with yer finger.
 
Re: Beginner Questions about carbs, tickover, chok

All excellent advise but maybe too much. Back to basics. Mixture, balance, idle. Good stuff in the shop manual for just how to go about it. If that fails check for air leaks.
 
Re: Beginner Questions about carbs, tickover, chok

He's got a single cable for both units. All linkage and likely not a concern. The shaft-thru-body is more an issue.
 
Re: Beginner Questions about carbs, tickover, chok

Can you post a photo of the engine bay; I'm curious about where your smog remnants are routed/connected. A 73 should have a whole bunch of smog widgets (a charcoal canister, gulp valve, shutoff/dump solenoid, tappet chest vent tube, about 15ft of plumbing connecting it all).

I would adjust the valves and set the timing before fiddling with the carbs. Adjust the points too, assuming you do not have electronic ignition. If you don't already have a Unisyn, it is a worthwhile investment.
 
Re: Beginner Questions about carbs, tickover, chok

also what kind of "rough" idle are you getting when you lower the idle RPM? I've noticed with a "too rich" setting it tends to sound cyclic (my '76 can be used as a metronome when it's too rich), and when it's "too lean" my car sounds like it has a miss at random intervals.

However, like everyone else is guessing, I'd have to guess you're currently on the rich side since you can start without the choke. I don't live too far from Pasadena, and I choke every morning because it makes the car a LOT easier to start after sitting all night, especially since it's starting to cool off (like this AM).
 
Re: Beginner Questions about carbs, tickover, chok

Roger roger. /ubbthreads/images/graemlins/iagree.gif
 
If Harolds carburetters were adjusted correctly for idle speed he probably would not start without the choke when cold. It is evident that the sputter is caused by too lean a cold mixture which is the purpose for the choke in the first place. Get the factory manual. Optimise the ignition system (points, condenser, timing, cap, rotor etc...), then adjust the valves. Then and only then are you ready to adjust the mixture and synchronization of the carburetters as well as adjust the cold fast idle. ALWAYS remember: Ignition ,valves, and then carburetters, in that order. Otherwise you'll chase your tail forever, I've observed supposedly experienced mechanics who seem never to have learned this. The correct procedures for all these things are well covered in the factory manual or a Bentley reprint thereof. You can't go wrong with one of these. Have patience and good luck
 
Re: Beginner Questions about carbs, tickover, chok

[ QUOTE ]
Can you post a photo of the engine bay; I'm curious about where your smog remnants are routed/connected. A 73 should have a whole bunch of smog widgets (a charcoal canister, gulp valve, shutoff/dump solenoid, tappet chest vent tube, about 15ft of plumbing connecting it all).

I would adjust the valves and set the timing before fiddling with the carbs. Adjust the points too, assuming you do not have electronic ignition. If you don't already have a Unisyn, it is a worthwhile investment.

[/ QUOTE ]

Thanks muchly for the input so far.

Doc: there is 20W/50 oil in the dampers to the correct level. The pistons can be moved up with some effort by a finger, and plonk down rapidly with a nice chunky noise.

Nunyas: I don't like to adjust the idle downwards, because the engine starts to wobble about and sounds very uneven. Not cyclic, I don't think.

motorbill66: I have the Bentley and Haynes manuals. My problem is I don't know where to start!

Here's a couple of pics of the engine bay. I really need to take a new one because I've cleaned and replaced some bits, but these will do:

263436929_8840951846.jpg


Bigger version here: https://static.flickr.com/110/263436929_8840951846_o.jpg

Closeup of carbs and new filters:
303892763_6371c309e9.jpg
 
Re: Beginner Questions about carbs, tickover, chok

I wonder, I just wonder how much air is leaking by the throtle rods or gaskets? If it is too lean on the bottom end and is mostly right else where it must be leaning from someplace.
 
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