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BN1 with H6 carbs : what setting ?

matt78

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Hello.

Two H6 carbs have been fitted on my BN1 by the previous owner (I don't think that camshaft nor pistons have been modified the same way as a 100M).

Incidentally, last week I noticed that the accel. linkage fitting was wrong, the angle between the link and the lever actuating the throttles preventing them to open fully (brilliant job done when replacing carbs seals by famous and very expensive Healey "specialists" here, boasting of racing @ Le Mans Classic. What a pity...).
Having solved this issue - and obviously got a better performance - even if the car seems to operate correctly (starting, idling, ride), I decided to check the whole tuning (just in case !), including sync, mixture...

So I have a question regarding mixture check : following the SU instructions, I push the small pins to slightly move the carbs pistons up, and always get decreasing rpm.
This should indicate a too weak mixture, BUT : working on the front carb (didn't try with the rear one), I already unscrewed the jet adjusting nut by 3 3/4 turns, and the engine keeps stalling when the piston is lifted...

As there seems to be not so many threads left on the jet bearing for further enrichment, I'm afraid that I missed something : any idea ?
Another hypothesis would be that the previous owner threw the carbs without thinking about their tuning, and the needle and / or the jet don't match the right setting for this engine. If so, what should be the right needle/jet needle combination ? I guess it should be similar to a 100M one, but couldn't find the information.

Thank you in advance for your help.

Matt
 
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steveg

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SU authority Des Hammill's initial settings, checked with tail of a dial caliper:

screenshot.2187.jpg


I see 2 full turns is the usual tuneup starting spec.

screenshot.2188.jpg
 

Dandare

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It might be an idea to check the position of the needle in the piston ie 'shoulder flush with the base of the piston'. All of the distance below bridge guides are based on the needle being in the correct position.

I would venture that almost all of the needles are the same size at the thick end (where it idles) so unlikely that this is the current tuning problem even is the needle is the wrong overall profile.

Danny
 
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matt78

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Thank you for your replies.
Yes, the needle shoulder is flush with the piston as described, and I'll check this evening the initial setting as mentionned, but let me remind you that the engine starts easiy, idles and runs correctly - so, why touch it will you ask ? Well, having found and solved the linkage issue, and then discovered the odd reaction of the jet setting nut action, I feel very suspicious about any other hidden surprise...

Thanks also for the needle data for 100M, but what about the jet ? I found out that on H carbs there are 3 # existing : 9, 1 and 125. Which one should be used ?
Because I fully agree with your comment about the limited impact of the needle on idling, but should the jet ID be inadequate vs engine displacement for example (reminder : I don't know where these carbs came from...), it may prove difficult to get the correct enrichment within the nut adjustment range : don't you think so ?

Matt
 

steveg

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According to the Burlen Catalogue, all the H types use the same jet bearing. However, the .125" needles won't fit in the .1" hole in your piston. Assume a .09" needle would be very loose and off-center.

Again according to Burlen, the H4 uses the "9" jet (AUC8182) and the H6 uses the "1" jet (AUC8183).
Refer to the needle shank diameter or the hole diameter in your pistons.

According to Burlen, it would be possible to have a 9 jet in the same jet bearing, which evidently works for both sizes. But you'd still be left with a too-loose 9 needle in the piston. The correct 1-size needle wouldn't allow the piston to properly seat as it would hang up part way into the 9 jet.

You need to adjust the one carb to have a more correct jet drop, but you probably won't be satisfied until you make sure both your needles and jets are correct, which you could do by removing the domes and pistons. If one jet was a 9, it would look visually different from the other 1 jet.
 
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matt78

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Here's a small update after two additional static checks :

- needle fitted on the front carb : #OA7, so std 100M. I didn't take the time to have a look to the rear carb, but logically - if this word applies to my engine - it should be the same.
- distance between jet's top and bridge : 1.4 mm on the rear carb, but 3.8 mm on the front one :cool-new:
So if the rear jet's position is almost nominal, the front one is way out, while still being at a too weak level.
I wasn't able to read the # figure stamped on the part : to check it, is it possible to take them apart from the carbs without having to re-center the needle and so on ? I wish to avoid dismantling too many things uselessly...

I understand that each carb size implies the use of a unique jet ID, so that my issue would be a needle affair. If the jets are really PHYSICALLY not interchangeable, so with different outer dimensions like length and diameter, then my hypothesis of somebody having replaced the original H6 0.1" jets by H4 0.09" ones for example is wrong, and I need to find another explanation.
Sorry for bothering you with my questions :eek:

Matt
 

red57

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Matt,

In your first post you said is that you were adjusting the front carb only - this will never give good results. Both carbs need to be set the same (or, at least very close), if it doesn't run right with them set the same, there is something wrong. Because there is a balance port inside the manifold that allows both carbs to affect all cylinders, it is possible to have one carb very rich and one carb very lean and still seem to be running fine - but it is not right.

Second thing, be very careful in lifting the piston that you don't go too much - 1/32" is only .031" and the pin is capable of lifting the piston 1/8" or more. If the jets are adjusted right and you lift the piston the 1/32" called for in the book, it should speed up slightly - but if you lift higher than 1/32" it will slow back down and stumble a bit.

Third, you need to check float settings in both - again, they must be the same. If your front float is set too low, you may need the jet a bit lower to compensate.

Steve is right that you can't have the wrong jets. I suggest you check that the rear carb has the same needle (probably is but check to be sure).

Also, check that the carbs are balanced - you need to have the same air flow thru both carbs (actually should be done first, then proceed with jet fine tuning). If you don't have a unisyn type device, at least listen to the hiss thru a tube stuck in your ear with the other end held in front of each carb throat.

Again, all elements of both carbs need to be set the same.

Usually, when I have had issues the best thing is to go back to the basics and set everything the same to the initial settings that Steve posted and then start fine-tuning over again.

Dave
 

steveg

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No bother at all, Matt. The forum helps keep the brain engaged during enforced idleness.

PS - If you're interested, this book is a great general resource for all the types of SU carbs. (Says 'unavailable' below because I'm accessing the UK site from the US):

screenshot.2189.jpg
 
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matt78

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Sorry Steve, I hadn't seen your last message when I replied yesterday. OK, now that's clear for me : for ONE carb diam. there is ONE range of jets and ONE specific range of needles. I thought they could be mixed, hence my insistence...
I already read this SU blue book, yes, that's why I started to fettle (or try to !) with the carbs, including testing the richness at idle, and subsequently to get rather puzzled !

Dave, thank you for your wise comments.
To make it clear, I didn't try at all to tune the whole system by adjusting the front carb only : I simply tried to experiment on the first carb the "piston lifting test" to see how it worked, and then stopped after having noticed its apparently "never rich enough" effect... But I wasn't careful enough with the lifting height, that's right, and will try again following your recommendation.

I'm fully aware that the two carbs MUST be tuned with similar settings, and again will go through all the whole process described in the book, line after line - which I already began to do before encountering this oddity.
I'll keep you posted.

Many thanks, and keep safe.

Matt
 
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matt78

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Hello.

Going deeper and deeper, here is the small update of the day :

- I checked the two needles which are OA7 --> OK
- I set the front and the rear jets at 1.25 mm as required --> OK
- I started the engine without changing anything else (one more time, it was for testing purpose only, not tuning): no problem, but with the choke off, it ran roughly at idle, seeming too weak, which wasn't really a surprise... so what else ?

Well George, an air leak, maybe ?
I started using my favorite brake cleaner spray to check everywhere, and bingo : rpm raised when applied to the protruding part of the front carb body where the choke cam plate pivot is fitted.
Having had a better look, it appears to have been "customized" in the past, as if somebody had glued it... Wouldn't have imagined that there could be a crack there !

I didn't find another air leak : at least, I got an explanation for the "never rich enough" behaviour of the front carb jet setting...

But now I'm not sure of the next step :
- buy a new body, but it's expensive and I need to have it reamed to accept a new throttle shaft : there's nobody around me with the right tools,
- have the current body repaired - brazing ? Again, nobody to help...
I hope - but can't be sure - that there's no other issue with this carb, or its rear partner !

Do you have any additional advice ?
Thank you in advance.

Matt
 

steveg

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Matt,
It sounds like your front throttle shaft bushing is worn out, admitting air. In which case, your carbs need to be rebuilt as they'll never run right. Rebuilding OEM carbs is usually preferable to buying new.

You can have your carbs re-bushed to use the standard diameter shaft which is the best, least expensive way for a variety of reasons.

In the USA, Tom Bryant can fit lifetime Delrin nylon bushes. Joe Curto can rebuild them with stock bushings.

In the UK, Burlen SU can probably rebuild them. They're on the internet.

There is an Austin Healey club in France - have you been in contact with them? They might have a favorite carb rebuilder.
 
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matt78

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Thanks for your reply Steve, but I wasn't clear enough : the leak is on the BODY, where the small cam pivots (the cam which opens wider the throttle when the choke is engaged).
I was talking about the throttle shaft only in case I buy a new body, for I don't know what the bushings' ID would be, so if my current shaft will fit...

Nonetheless, I agree that a complete overhaul would be simpler, but nobody to trust for that because there are some machining operations are required that I can't perform.
For your information, the club you mention promotes the workshop I cited. I'll find another solution, no problem ;-)

Matt
 

red57

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Matt,

Congrats on finding the source of you issue, good job.

I'm not sure but I think the boss that the cam pivot bolt screws into should be a blind hole threaded for the bolt.

If the boss somehow got damaged/cracked previously or the treads stripped, someone may have tried to epoxy the boss back on and/or they may have bored the hole on thru into the throttle bore to get fresh threads, thereby allowing a vacuum leak around the threads. If something strange like this happened, you may be able to pull the bolt and seal it in with Loctite to prevent the vacuum leak. The only thing that cam does is open the throttle a little when the choke is pulled, so if there are weak threads or other issues, you could disconnect the link to the cam and still drive it as long as you use your foot to increase the idle speed when choking - the rest of the time it does nothing. I would consider this a temporary fix and start looking for another carb body - currently ebay, here in the US has 2 or 3 ranging from $40 to $140.

As Steve said, Joe Curto here in the US is a possible source (he may have used ones as well as new to sell).

Dave

Guess I'm too slow at typing - you and Steve were much faster than I :smile:
 
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matt78

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Thanks Dave.
In fact, it's worse than you think : it appears that the boss itself isn't original, and didn't come from the initial casting :livid: Hence my comments...
It's a long road :joyous:

Matt
 

red57

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Another good argument for doing it all yourself IMO - that way at least you know what was done and why.....

It's been a long time since I worked on an H type carb but looking at the Moss H type page, it looks like that choke idle cam is only found on the front carb. If that's the case, is it possible someone used a rear carb for the front and tried to get creative with the idle cam? Maybe, if you get a front carb body, that's all you need and everything else is fine? Hope springs eternal and all that.

Dave
 
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matt78

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Dave,

For sure, it smells like a big SU bitza :mad-new:
I will soon dismantle all this stuff to check in case of another hidden surprise, before deciding what to do : temporary aluminium powder-stuffed epoxy if the crack isn't structural, brazing or maybe source another used body (if not already worn out...).

Kind of a hobby !

Matt
 

Dandare

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Just looking at a pair of H4s the boss is on both carbs but only tapped on one of them (rear in this case).

Danny
 
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matt78

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Thank you for the picture Steve.

Danny, you're right.
Having had a look, I noticed that :
1) there is a long boss (threaded inside) on the left side of my two carbs, but they can't be used because of the float bowls preventing any access...
2) there is a short boss on the right side of the two carbs, and it appears that the front carb's one has been extended to support the cam's pivot... What a mess !
I don't know what is the original definition : are there two symetrical bosses of the same length on each two carbs sides ?

To make it clearer (if possible...), the layout from front to rear of my car is as follows :
(float bowl) (unused long threaded boss) (front carb body) (short boss modified to support cam, and leaky as previously mentionned) - (float bowl) (unused long threaded boss) (rear carb body) (unused short boss).

I'm not used to SU carbs, and still have to dismantle them (I put my hands on new needles and rebuild kit...), but I'm afraid I couldn't modify the bowls layout for example, in order to get an access to a better cam support...
Am I right ?

Matt
 

steveg

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Matt,
I did a Google images search for "Healey 100M carb" and came up with many images. These replacement carb pictures seem to be typical.
See how the long bosses are on the rear side of the carbs.
Can you post a picture of your setup?
screenshot.2195.jpg
screenshot.2194.jpg


Currently on eBay - according to the seller, this H6 body is from a Jag XK120/140. The long boss would be on the front side if this carb were mounted on a Healey, but the rear side on a Jag. It would appear you have the Jaguar H6, not the Healey.
screenshot.2196.jpg


...or possibly a Triumph:
screenshot.2197.jpg
 
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