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Carb Tuning

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An update to close out this thread. I am now sootless! Well my plugs are. Browsing around the archives I came across a post saying the plug gap should be increased by 5 thou if fitting electronic ignition. It actually says this in the Petronix FAQ website. The gaps on my NGK BP6ES are now set at .030. Problem solved.
The Colourtune still shows a yellow flame on tickover (700 Revs) but it turns Blue when I rev the engine. The plugs are now a dark chocolate colour with no soot. I'll leave it at that for now as the PO said that it likes to run a little rich. Lifting the piston pin increases the revs sightly, it's never done that before.

Thanks to all
AJ
 
AJ, Can you fill us in as to which process you choose to adjust the carbs. Thanks.
 
The illustrations and explanations in the Burlen instructions can be quite confusing. The type of HD8s fitted to Austin Healeys do not have 'throttle adjusting screws' and some people confuse these with the 'fast idle screws' but they are completely different things.
 
OK but remember I'm just following the book! For info this is a fast road engine, alloy head, fast cam, electronic ignition (not sure if it's Petronix).
Carbs rebuilt using a Burlen kit from Ahead4Healeys.co.uk. It says Jaguar on the box and I found out after we fitted them that the throttle spindles were too long and I had to do some 'adjusting' to make the linkage between the carbs fit.
I set the jets .075" below the bridge and screwed the Slow Running Valves (SRV) right down. I used a Unisync to balance the carbs using the throttle adjusting screws. It's still set up that way.
I tried that for a while but the plugs were sooty when I took them out. I played around with a Gunsen Gas Tester for a while but didn't get much useful info from it. I'm not sure it was calibrated right. When I've finished tinkering I'll try again.
So I bought a Colourtune. Using the mixture screws I found the point where the plug changes from yellow to blue on No 2 and 4 cyl's. When you get to the balance point I found there's about a quarter of a turn of the screw to get the flame to change colour. I found it very sensitive. I left it idling just on the yellow side as I've been told they like to run rich. I noted that reving the engine turns the flame blue.
I still got sooty plugs! After a lot of research I increased the plug gap from .025 to .03. This seems to have fixed it. No more sooty plugs.
BUT, this morning I thought I'd take a reference point for future use as I'd done a few miles yesterday. I measured the jet depression below the bridge. Front car = .088" OK, I deliberately set it rich. Back carb .039 Why's the back carb different! It should be too lean at that setting. No 4 plug was the same colour as Nos 1,2 & 3 (dark choc no soot). I didn't take No5 out yesterday but today it's the same colour as the others but shows signs of soot. Could that be the engine breather? I put the Colourtune in 4 not 5, does that account for it?
This pm I'm going to check again with the Colourtune in No 5 cyl. The good news is that the car starts on the button, it passes the 'pin lift' test, the plug colour is dark chocolate and it runs ok. If it ain't broke don't touch it?
AJ
 
Congratulations AJ. You are now an Expert carb tuner. Your results are equal to or above the norm and you know more about it than many. Now, as you say, 'if it ain't broke, don't fix it'.
 
Hello Derek, I agree with you that it can be quite confusing especially because there has been so much written and rewritten bought the HD style carbs. I have a booklet written by Donald Healey and Tommy Wisdom called The Austin Healey Guide. In it the section on adjusting HD style SU carburetors has text and pictures labeling that screw as the 'Throttle Adjusting Screw'. But I believe you are correct in calling this screw the 'fast idle screw' because it is designed to be used to set the 'automatic idle speed increase' when pulling the choke out.
 
Great news RJ. I'm glad you are getting it to run well, the only thing I would suggest is to aim for the jets to be roughly the same distance down from the bridge. Since you now have .088 and .039, I would spit the difference & set them both to about .060 and see how it runs, then fine tune from there. I run triple HD6s and usually they are all within .005-.010 of each other.
One thing that gets overlooked sometimes is that there is a balance port in the manifold between the front an rear sections and this means that both carbs affect all 6 cylinders to some degree, ie richen one carb and you richen all 6 cylinders. Most books I've read urge having as much sameness as possible in multi-carb applications - same air flow at idle and off idle, and same jet setting/fuel flow.
Now, go & enjoy your hard-earned success.
Dave
 
Another thing would be to remove the piston needles and verify they're both labelled the same on the shank. Verify both needles are correctly positioned with either the shoulder or the bottom of the groove even with the surface of the piston rod (depending on the design of the needles). This area is about 1/2" D and may be recessed slightly into the piston bottom.
 
I have owned my BJ8 since 1984 and learned long ago that the available tuning manuals for the HD8 carbs were inconsistent, confusing, and didn't produce the results that I thought were correct. They change nomenclature for a part from one part of the procedure to another, tell you how to adjust a single carb and then refer you to a part of the manual for dual carbs that refer to parts not on an HD8, among other deficiencies. Part of my career involved writing maintenance manuals for U.S. Navy aircraft and I think I do it pretty well. So, using the basics from the SU manual, workshop manual, Glenn's, Chilton, etc. I wrote my own procedures for tuning the carbs and they have worked well for me. I've shared them with several other people and have received no complaints.
The procedures are in a .docx file and that is invalid for an attachment here. Anyone who wants the procedures can request them to sbyers@ec.rr.com
 
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Hi All,

Steve's Tuning directions are both pragmatically progressive, complete, and understandable. However, I have found that the difficulties most of us have is the HD8 is that each Carb, in concert with their related engine, forms a separate personality of their own. I appreciate that they all seam to understand the language of SU, as instructed by Steve's document, but you have to provide the recognizable accent each HD8 needs to properly interpret the instructions and requests. Are the HD8s alive, sometimes I think yes.

I have found that following Steve's directions gets me organized and very close to the performance I am looking for. However, I have found that no HD8 on any 2 Healeys will either accept or respond to the same degree, or in the exact manor, so even not-so-fine personalized adjustments are necessary.

Thank you Steve, your directions have Helped Immensely in corralling and setting up my unruly pair.
Ray(64BJ8P1)
 
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This book is invaluable for those who would understand and tune their carbs.

For instance, it shows with pictures how to replace the shaft bushings in HD8 carbs.

screenshot.1300.jpg
 
Tentando superabimus! Put more simply, never give up. Sorry!

If you've been following the above you'll know I stalled out when I found that the back carb jet wasn't as far below the bridge as the front carb, about half as deep.. I've been struggling to understand this, maybe it's too lean now!

So what about this for a theory. You can see from the attached image that the brake servo hose is connected to the manifold very close to the rear carb. I'm wondering whether the vacuum at the back of the manifold draws fuel from the front and makes the front carb work harder than the back. I noticed this when trying to balance the carbs. The front one seems to need to be opened up a bit more.
Any thoughts from you more experienced guys would be appreciated.
AJ
IMG_3071.jpg
 
AJ
not more experienced but i too found the same thing on the rear carb of my tri-carb and do not have a brake servo. i had read this thread before trying the colortune plug. so when i started to lean my carbs out (i was running rich), i would adjust all three carbs the same each time i adjusted the first carb and second carb. trying to keep them balanced. but when i got to the back carb, i still had to adjust it in more.
i just pulled my rear breather and cleaned it out thinking maybe it might be clogged a little from oil coming from the vent hose over the years. did not get a lot out of it so i don't think that will help much.
 
Drone Dog said:
AJ
not more experienced but i too found the same thing on the rear carb of my tri-carb and do not have a brake servo. i had read this thread before trying the colortune plug. so when i started to lean my carbs out (i was running rich), i would adjust all three carbs the same each time i adjusted the first carb and second carb. trying to keep them balanced. but when i got to the back carb, i still had to adjust it in more.
i just pulled my rear breather and cleaned it out thinking maybe it might be clogged a little from oil coming from the vent hose over the years. did not get a lot out of it so i don't think that will help much.
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The Brake Booster has a check valve so I don’t think that is causing you Carb problems and if the check valve was malfunctioning then you’d have brake problems. To verify, cap off the hose connection on the Intake manifold to see if there’s any change. You won't have power brakes so be careful when driving. Have you sprayed ether around the intake area of the back Carb to see if the engine speed increases? Also spray at the Carb throttle bushings too.
 
I noticed this when trying to balance the carbs. The front one seems to need to be opened up a bit more.​
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If you mean the front carb is drawing more air, go back and re-balance - you want the same air flow both at idle and off idle for both carbs.
If you do have more air flow through the front carb, that may explain why the jet is lower in the front - more air needs more fuel.
As I said earlier, you want the jets down about the same amount so split the difference and set them about .060 and if it needs to be richer, richen both the same amount.
And again, there is a balance port inside the manifold that acts the same as the balance tubes on a tri-carb. This means that all carbs (2 or 3) affect all 6 cylinders to some degree - this is not a case of one carb feeding just 3 cylinders so sameness of all carb settings is essential.
Dave
 
I have been adjusting my HD carbs. I have adjusted the "Slow-running valve" screw to get the idle RPM correct. BUT,,,,,, what exactly/specifically does this screw do? How does it change RPM? I don't think it adjusts the throttle linkage, like the "Throttle-adjusting" screw does.
 
The screw controls the amount of air to its carb at idle since the throttle butterfly that normally controls the amount of air should be completely closed during adjustment of the screw.
 
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kodpkd said:
I have been adjusting my HD carbs. I have adjusted the "Slow-running valve" screw to get the idle RPM correct. BUT,,,,,, what exactly/specifically does this screw do? How does it change RPM? I don't think it adjusts the throttle linkage, like the "Throttle-adjusting" screw does.
Click to expand...
The "slow running valve" controls a bypass around the throttle plate (butterfly). On HD8s, the throttle butterflys should be completely closed at idle.
 
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