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Adjusting HD8 Carbs

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On another thread about spark plugs we started a conversation about getting 400 rpm idle out of HD8 carbs. Because the HD8 carbs are suppose to run at idle with the throttle plates completely closed, as per the Bentley Manual, I find that i easily can achieve a 400 rpm idle IF I KEEP THE THROTTLE PLATES COMPLETELY CLOSED. That's the key. The HD8s were designed to idle on the amount of fuel/air that is by-passed around the closed throttle plates and restricted or regulated by the slow run valve. This is nothing more than a passageway around the throttle plates with a tapered needle point screw threaded into the passageway to regulate the amount passing thru. With the jet set to as close to the correct mixture as possible (using the jet adjusting screw) it is only necessary to turn in or out the slow run screw to achieve the desired idle speed. The manual states that the slow run screw is usually set at about 2 1/4 turns from bottom and if after tuning the carb one finds that the idle speed is too fast it is only necessary to turn in the slow run screw a bit to achieve the desired idle speed. (remember the throttle plates are completely closed). BUT IN REALITY THIS IS WHAT i HAVE ACTUALLY FOUND, in my case with my car I have found that I am not getting enough "flow" via the slow run screws alone. But I am getting enough "flow" to sustain about 400 rpms. But the drawback to this I think I am discovering is that I can't run the jet as rich as should be and am forced to keep it alittle lean. And this is the way I have been running it most of the summer. But about 3 weeks ago I decided I wanted to up the RPMs and also richen the mixture. To achieve the 400 rpm idle on the slow run screws alone, I had the slow run screws turned out to about 3 1/2 turns. Bringing them out more did not help the idle at all. But maybe increasing the richness of the jet would have yielded more rpm. I tried different jet and slow run settings and never got a good idle above 400 rpms. So I finally gave in and decided to crack the throttle plates just a tesch. So you realize this is going to allow more fuel/air around the plates and down the bore. So I set both the throttle plates just a smidgin of a bit open using the throttle shaft screws. Now I again fine tuned my idle by using the slow run screws. By this method I am compromising the system by allowing mixture to go around the plates and then also adjusting the slow run screws. With this method I can achieve any idle speed I want, and in this configuration with the slow run screws at about 3 turns out (a point near what they were before) the engine is idling at about 700 rpms. I will try to attach an exert from the Bentley Manual. But one more thing I would like to explain which might be relevant to your higher idle speed. Some years ago before I took my car apart for restoration, I was driving it regularly for 7 summers. I could never get the idle down much lower than about 900 rpms. The slow run screws pretty much did nothing, had no effect. So I turned the slow run screws all the way down tight. Pretty much taking that passage out of commission. Then I adjusted my HD8 carbs the same as the way you would adjust the earlier SUs. Set the jet height, balance the carbs and then set idle with the throttle shafts. But I couldn't understand why the slow run screws had no effect. When I took the carbs off during restoration I found that the diaphragm on the rear jet was very stiff and it never let the jet return to its most upward position there-by causing that carb to be rich all the time. This caused the whole adjustment of both carbs to be off and this is why I never could get the engine to idle below 900 rpms. With the jet being very rich, you needed more flow to compensate er go more rpms. Well I guess I have pretty much blown this one out of the water, maybe some of this diatribe will help. But my on going frustration is why I can't get the proper idle and responses with just the slow run screws and the throttle plates completely closed. The BOOK says so. Dave.
 

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Thanks Dave,

As mentioned prior, over the years I have rebuilt my carbs many times and have never attained an idle even close to 400 RPMs. Yes, I am running a little rich and although my car runs strong, 900 RPMs is as low as I have attained that will could be sustained at a summer traffic light stop. My conclusion to the reason for the high idle in a BJ8 has always been the HD8s in combination with the CAM and it was something you needed to live with. Although I find it difficult to accept the fact that the multiple times I have rebuilt my carbs I was installing a diaphragm that was too stiff or that I have been setting the carbs wrong for so many years, I must do so as I can't argue with your success and am eager to follow your approach.

How did you determine that the diaphragm was too stiff? What or where did you find the replacements you are presently using? What timing advance are you set to?

Thanks for posting and all the best,
Ray(64BJ8P1
 
IMO it's not the cam, as I'm running HD8s on a 29D engine and have the same idling problem of above 1000 rpm.

Yesterday when I was readjusting my richness for my sooty front plugs issue, at times the idle dropped into the mid-hundreds. Can't say if this was richer or leaner. I did not at any time change the large idle screw - my carbs are set up to idle with the butterflys closed.
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Derek,

My intent is to get idle closer to 700 RPMs that is even and consistent rather then the 1,000 RPM I now have while keeping the operational performance.

Ray (64BJ8P1)
 
RAC68 said:
Derek,

My intent is to get idle closer to 700 RPMs that is even and consistent rather then the 1,000 RPM I now have while keeping the operational performance.

Ray (64BJ8P1)
Click to expand...

:iagree: OR 1000 RPM on the car tach.

"N" All HD8 carbs.. set up to idle with the baffles closed is a Myth.
 
I agree that 400 rpms is too low. But I say that based on a different reasoning. I don't think you can achieve 400 rpms idle without the carbs jet being too lean. Even though the car ran well, it didn't have good low rpm grunt. But the interesting thing to me was that the idle was very stable at all weather conditions once the engine was up to proper operating temps. I have read some say in the past that you can't achieve a proper adjustment of HD8 carbs with the throttle plates completely closed. If that is a myth, and in my opinion the jury is still out, it is a myth that is perpetuated by the shop manual. See my previous attachment from the BOOK.
Having said that, while trying different things with the carbs, I continually tried to get more rpms with the throttle plates closed and bringing the slow run screw out more turns. If I left the jets alone at one position, I had at one point unscrewed the slow run screws as much as 8 full turns and never got more than about 500 rpms. At 8 turns I thought that was excessive and did not leave it that way. that's when I decided to crack the throttles a tesch. Note, the slow run screws are suppose to have a rubber 'O' ring around their shank. If that is not there you will suck air from outside the screw and sometimes hear a whistling. At this point I still believe it is possible to get a proper idle on the slow run screws with the throttle plates closed, but i believe it is in conjunction with the appropriate amount of jet openning (rich/lean adjustment).
Ray, from my memory about the stiffness of the diaphragm, it was just a physical comparison between a new one and the old one. But also when I noticed it, i deliberately put the old one back into the body of the disassembled carb to see where the jet set relative to the bridge by looking thru the carb body. With the old diaphragm the jet was noticeably below the bridge. Dave.
 
Timing of course could have alot to do with the issue. If my memory serves, i have set my timing at about 5/8th to 3/4th of an inch BTDC. My guess is that is somewhere between 10 and 15 degrees. I do not try to time my engines for maximum high RPM power. Dave.
 
Last edited:
I do not try to time my engines for maximum high RPM power. Dave.

I don't either but I do have a very good Dwell Meter & Snap-On aqdvace/RPM
Timing light.

OH yeah that shop manual just full of myths.--:highly_amused:
Check out the one tell you how to remove the steering column.:playful:
 
Dave,

Am I reading incorrectly or are you actually setting your timing After and not Before TDC?

Ray (64BJ8P1)
 
Dwell Meter, What's that ? Does that mean to repose or move slowly?? Or something to measure one's reposing or moving slowly. :friendly_wink:
The last time I used a timing light it bit me. Never Again, no, no, no.
 
Whoops, Sorry Ray, another one of them old timers brain infarction, Before Top Dead Center.
 
vette said:
Whoops, Sorry Ray, another one of them old timers brain infarction, Before Top Dead Center.
Click to expand...

Yep I knew IT!!!------:highly_amused:
 
Ray,
I always respect you advice and experience.
I have been fighting with my new HD8 (new not rebuilt) carbs for about a month on my BJ8
Not the usual things.
Idles great at around 800 and runs smooth through the speed and torque range.
Issue I am having is that the carbs will not settle after light throttle driving; happens about 70% of the time.
RPM stays about 300-400 RPM higher than regular idle.
Settles immediately with a blip of the throttle.
There is definitely clearance between the fast idle stops.
Have new linkages and return springs.
Have lubed the linkages and looked for a baddie, nothing seems to help. All seems smooth.
The U shaped links move together. and butterflies open together.
When it is in the fast mode if I push on the throttle I can detect about a 5 thou movement and speed settles.
I am beginning to suspect the intermediate shaft length is incorrect. At Idle the long linkage (over manifold) is vertical whereas the one on the interconnecting linkage one is about 2 deg over vertical. Are there any specs on this setup ?
Any ideas or thoughts welcome.
rg
 
volts101 said:
Ray,
I always respect you advice and experience.
I have been fighting with my new HD8 (new not rebuilt) carbs for about a month on my BJ8
Not the usual things.
Idles great at around 800 and runs smooth through the speed and torque range.
Issue I am having is that the carbs will not settle after light throttle driving; happens about 70% of the time.
RPM stays about 300-400 RPM higher than regular idle.
Settles immediately with a blip of the throttle.
There is definitely clearance between the fast idle stops.
Have new linkages and return springs.
Have lubed the linkages and looked for a baddie, nothing seems to help. All seems smooth.
The U shaped links move together. and butterflies open together.
When it is in the fast mode if I push on the throttle I can detect about a 5 thou movement and speed settles.
I am beginning to suspect the intermediate shaft length is incorrect. At Idle the long linkage (over manifold) is vertical whereas the one on the interconnecting linkage one is about 2 deg over vertical. Are there any specs on this setup ?
Any ideas or thoughts welcome.
rg
Click to expand...

This one goes back almost as far as the medieval one about the number of angels on the head of a pin or whether gold can be made from base metals.

Mine do the same exact thing unless the slow run screws are set so it almost dies.
 
Rg

That is a very common problem and you can't cure it by adjusting the carbs, which sound like they are working fine. I'm afraid the problem is with play and slack in the throttle linkages. Its a pain to eradicate especially as you can't easily access an important part of the linkage system.
 
Hi RG,

Over the years, adjusting my HD8s has never been that successful and every so often, although infrequent, I meet an owner with a BJ8 that is idling steady and even at around 600 or 700 RPMs. Jn almost every encounter, the owner of the car has little or no involvement or knowledge of how or why he gets such a good idle. A while back, Steve Byers wrote an article on adjusting the HD8s that had some good advice and also included linkage adjustments (www.healey6.com/Technical/HD8.pdf).

Additionally, check your chokes to make sure they are releasing and fully returned. This has been a constant issue on my car and was resolved by removing the single spring and "Y" wire that was hanging up when pulling returning both chokes and replacing with 2 lighter springs mounted on each carburetor choke cable between choke arm and cable support.

One last thought. A while back, when working with a friend on his car, he also experienced similar symptoms as you have describe. In that case, we found that only one carburetor was hanging up and it was due to a misalignment of the pin and its slight drag on the seat. Although he had not replaced the pins, the problem started shortly after the car came back from a spring tune-up at a local garage where they pulled the pistons when adjusting the carbs, were distracted, and exchanged the pistons by accident. I bring this up even though it is probably a remote possibility.

Good luck,
Ray(64BJ8P1)
 
RAC68 said:
Hi RG,

Over the years, adjusting my HD8s has never been that successful and every so often, although infrequent, I meet an owner with a BJ8 that is idling steady and even at around 600 or 700 RPMs. Jn almost every encounter, the owner of the car has little or no involvement or knowledge of how or why he gets such a good idle. A while back, Steve Byers wrote an article on adjusting the HD8s that had some good advice and also included linkage adjustments (www.healey6.com/Technical/HD8.pdf).

Additionally, check your chokes to make sure they are releasing and fully returned. This has been a constant issue on my car and was resolved by removing the single spring and "Y" wire that was hanging up when pulling returning both chokes and replacing with 2 lighter springs mounted on each carburetor choke cable between choke arm and cable support.

One last thought. A while back, when working with a friend on his car, he also experienced similar symptoms as you have describe. In that case, we found that only one carburetor was hanging up and it was due to a misalignment of the pin and its slight drag on the seat. Although he had not replaced the pins, the problem started shortly after the car came back from a spring tune-up at a local garage where they pulled the pistons when adjusting the carbs, were distracted, and exchanged the pistons by accident. I bring this up even though it is probably a remote possibility.

Good luck,
Ray(64BJ8P1)
Click to expand...

Thank you
Nothing like sitting down and writing out a message to focus your attention on the problem.
I think that yesterday I may have cracked it; but will wait a week or so before declaring total victory.
Was OK last night with about 10 traffic stops...
The interconnecting link between the carbs was floating (left to right) and I had set it like that thinking it would be better than having the clamps too close to the carb bodies. So when it floats the shaft of the interconnecting link link can be not fully into the carb body; in fact this hole seems be stepped and if the shaft is skewed to one side the other side shaft is not deep enough into the carb body and this increases the play significantly and causing the stops to be out of wack. Surprised that there are no bushes there, as even though there is limited movement over the years it must wear. Adjusting the clamp position on the shaft sorted it out.
Now of course have the problem of the slo running jet adjustment having little effect....
Roger
 
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