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Can’t get idle speed below 1300 RPM

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SteveHall64Healey

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Following an engine rebuild, I’m struggling to get my HD8 carbs set correctly. I’ve set the jet adjusting screw to the flush position, then set the slow run adjusting screw to the minimum setting to obtain smooth running at idle, and balanced flow across the carbs. The problem is that the idle speed now sits at 1300 RPM with the idle adjusting screws fully backed out.

Any advice?
 
SteveHall64Healey said:
Following an engine rebuild, I’m struggling to get my HD8 carbs set correctly. I’ve set the jet adjusting screw to the flush position, then set the slow run adjusting screw to the minimum setting to obtain smooth running at idle, and balanced flow across the carbs. The problem is that the idle speed now sits at 1300 RPM with the idle adjusting screws fully backed out.

Any advice?
Click to expand...
What are the Slow Run Valve screws set at? Start at 2 1/2 turns out then fine tune. Check Jet bridge adjustments. Start at .075. Check Tach accuracy you too.
 
I had the same problem until I rebuilt the carbs - IMO, it's caused by air leaking past the butterfly shaft bushes. Most important is to fit new plastic bushes around the butterfly shafts.

Rebuilding HD8s is easy. I also fitted new needles, jet diaphragms and slow-running screw washers.

I'd get some extra plastic bushes as they're easy to bung up on install.

This invaluable book has step-by-step instructions on the above:
screenshot.1290.jpg
 
The tach was just rebuilt and calibrated by Nisonger and it is audibly evident that idle speed is quite high.

Re the slow run valve screws, they are currently set at 2 - 1/2 turns out. Below that there is hesitation at low engine speed. Above that, idle speed increases further still.

Re the jet bridge, will try 0.075” and advise. Assuming current settting are lower than that, that should mean I can adjust to 0.075” then reduce 2-1/2 to a lower number of turns out and therefore reduce idle speed?

I am using the adjustment procedure posted by Steve Byers ....

thanks,
Steve
 
How's your (ignition) timing? That too has an affect on idle speed.

The jet bridge isn't so easily defined as a measurement number; a slight flat spot on one (1) of the needles will render that method useless. Besides, that has negligible effect on idle speed anyway, its purpose is mixture strength.

Are the throttle plates being held open by any of their adjustment screws? Or choke linkage not fully releasing?
 
steveg said:
I had the same problem until I rebuilt the carbs - IMO, it's caused by air leaking past the butterfly shaft bushes. Most important is to fit new plastic bushes around the butterfly shafts.

Rebuilding HD8s is easy. I also fitted new needles, jet diaphragms and slow-running screw washers.

I'd get some extra plastic bushes as they're easy to bung up on install.

This invaluable book has step-by-step instructions on the above:
View attachment 53529
Click to expand...
One of my favorite books!
 
Randy Forbes said:
How's your (ignition) timing? That too has an affect on idle speed.

The jet bridge isn't so easily defined as a measurement number; a slight flat spot on one (1) of the needles will render that method useless. Besides, that has negligible effect on idle speed anyway, its purpose is mixture strength.

Are the throttle plates being held open by any of their adjustment screws? Or choke linkage not fully releasing?
Click to expand...
True, but he may have more than one thing out of adjustment or worn out. I had a high idle on a 100 I owned several years ago and it was directly related to someone using shim stock to correct for worn out shaft bushings. We had a guy in our club who showed up at our monthly Tech Session idling over 2k and all of the adjustments were done by a mechanic who obvious didn’t know SU carbs. When he left his idle was normal. The Slow Runs were screwed all the way down/tight. More than a few adjustments were needed to get the idle and mixture strength correct. For some reason the HD8’s seem to idle better with the butterfly’s cracked a skosh in my experience, but too much opening is not good.
 
The idle adjusting screws you mention in your original post I think are the fast idle screw that are only active when the choke is pulled, when the choke is off they do nothing (some folks like to set idle with these but that isn't what they were designed for).
The idle speed is controlled by the slow run screws (with the butterflies closed fully) - they close off the port that carries air/fuel past/around the butterflies, so turning them in lowers the air flow and backing them out increases the air flow. As you turn them in to reduce idle speed you may need to adjust the jets to achieve the best idle. The 2 1/2 turn setting is only an initial starting point and you may end up quite different as you tune them.
Dave
 
Hi All,

I have had a similar problem since the car was new. Yes, in the beginning I did some things wrong but learned over the past 54 years and still idle at colse to 1K RPMs. During the time, I have increased compression by requesting (incorrectly) the head be shaved 0.060 instead of the 0.006 I should have had. However, this has proven a benifit and did add some additional umph. I use a minimum of 93 Octane fuel and higher; when economically available, to eliminate pre-ignition and also tend to set the carbs a bit rich. I dynamically balance the carbs and set both carburetors as close as possible with 2 Color Tune units (each fixed-attached to a carburetor). Last, I have a Crane electronic ignition and use an adjustable timing light to dynamically set the timing at 15 deg. BTDC. Again, with all this, I can't seem to get a smooth even idle under 1K RPMs but can briskly accelerate and the engine winds freely to 5K+.

Recently, a friend who had a similar problem suggested I try one more thing. It seems he brought his Healey to an experienced tuner who had achieved a smooth even idle on his car for the first time at close to 600 RPM. Curious, upon investigation, he found that the carbs differed from his setting in only one condition, the pin step was not even with the bottom of the piston but set about 1/10" deeper.

Since his HD8s are standard BJ8 carburetors and I am not sure what this change would do to idle performance. However, I will be trying to find out within the next few weeks.

Ray(64BJ8P1)
 
Bob,

No argument, although I do have an alternator. Isn't 600 RPM the idle speed mentioned within the workshop manual?

Ray(64BJ8P1)
 
RAC68 said:
Bob,

No argument, although I do have an alternator. Isn't 600 RPM the idle speed mentioned within the workshop manual?

Ray(64BJ8P1)
Click to expand...

Yeah. But, I like to be charging if I can. May not be relevant, but all the more modern cars I drive usually idle around 750, which is more-or-less what my BJ8 and BN2 like.
 
RAC68 said:
Hi All,

I have had a similar problem since the car was new. Yes, in the beginning I did some things wrong but learned over the past 54 years and still idle at colse to 1K RPMs. During the time, I have increased compression by requesting (incorrectly) the head be shaved 0.060 instead of the 0.006 I should have had. However, this has proven a benifit and did add some additional umph. I use a minimum of 93 Octane fuel and higher; when economically available, to eliminate pre-ignition and also tend to set the carbs a bit rich. I dynamically balance the carbs and set both carburetors as close as possible with 2 Color Tune units (each fixed-attached to a carburetor). Last, I have a Crane electronic ignition and use an adjustable timing light to dynamically set the timing at 15 deg. BTDC. Again, with all this, I can't seem to get a smooth even idle under 1K RPMs but can briskly accelerate and the engine winds freely to 5K+.

Recently, a friend who had a similar problem suggested I try one more thing. It seems he brought his Healey to an experienced tuner who had achieved a smooth even idle on his car for the first time at close to 600 RPM. Curious, upon investigation, he found that the carbs differed from his setting in only one condition, the pin step was not even with the bottom of the piston but set about 1/10" deeper.

Since his HD8s are standard BJ8 carburetors and I am not sure what this change would do to idle performance. However, I will be trying to find out within the next few weeks.

Ray(64BJ8P1)
Click to expand...
What is 1/10” in thousands? Explain what “pin step” is please.
 
Wouldn't be worried about not charging at idle.

Once I ran my car for 2 weeks and a number of local outings including with the driving lights on before it drained the battery. I'd forgotten to connect the ignition switch wire from the coil to the alternator. (Got a voltmeter now to keep an eye on the charging situation).
 
when i had a car that would not idle down and i had gone thru the other things like timing, float levels, etc, i usually start checking for a vacuum leak. get some carb cleaner and spray at the gaskets, the shafts, etc. easy thing to eliminate.

on my healey

the first issue i had on my rebuilt engine was that the heat speacers had a clear cellophane on one side, on two of the spacers. i could not see it at all when i put them in. when i took them back off, i noticed they had not stuck to the gasket. then i saw some wrinkles and pulled it off. put back together with some aviation sealant and i was fine.... well...

my second issue was my linkage getting caught up and not quite going back fully closed. it only takes a little bit on one carb to make it idle up. i had to move some linkage around to allow it to close. sometimes it still sticks just a little when i first start it.
 
steveg said:
Wouldn't be worried about not charging at idle.
Click to expand...

I drive thousands of miles from home, sometimes a hundred miles from help and often out of cellphone coverage. I worry about everything. Also, having a little higher idle gives me some margin when the idle drops at 10,000 feet.
 
Hi Patrick/Steve,

Patrick,
First, 1/10" = 0.1" or 2.54 mm. I mis-spoke about the needle step and really ment the to say the line on the needle that you set to the bottom of the piston.

Steve,
I agree that setting the needle a 1/10" deeper into the piston would be equivalent to dropping the valve 1/10" to increase richness. However, that would be at run and not at idle where the butterfly is closed and the HD idle is supposed to be solely dependent on the setting of the slow idle needle valve adjustment. Maybe there is a reason this works that I don't understand and the reason I qualified the level of my knowledge and what this action would do.

Ray(64BJ8P1)
 
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