SU HD8 Question

[JOEL]

This is going to be very difficult for me to try to get my question into words. Hope it is understandable.I am going to use the factory shop manuals names of the parts. 25 years ago when I first rebuilt my carbs is seemed like when I unscrewed the mixture adjustment screw (C) until it was flush with the bridge of its carborator the jet was flush with the jet bearing. If you look at the cutaway picture on page C.3 I think you will see what I mean. Three years ago I rebuilt the carbs again. This time when I unscrew the mixture adj screw to its proper spot the jet is resesed in the jet bearing about 3/16 inch. I cant get it to come up any more. If you remove the piston and look down at the hole that the needle goes into, this is what I am talking about.The car runs very rich and I cant seem to stop it. Did I get the wrong parts from Terry's? Or is this normal. I have loads of experience on carbs, from SU's to the webbers in my Ferrari, so I hope I didnt make a mistake in my rebuild. But to do the same mistake three times. Any thoughts on the issue would be appreciated. Thanks.

 

[Bugeye58]

Joel, are you sure that the jet bearing is screwed all the way up into the body, flush with the bridge? The only other thing I can think of is that the jets themselves are too short.
I'm puzzled.
Jeff

 

[Basil]

Joel,
To add to what Jeff said, the only two other possibilities come to mind. One, either you have the wrong Jet installed for that carb (i.e., to short as Jeff said), or two the jet is not properly centered such that the jet is binding on the jet bearing thus preventing it from coming all the way up. My Bentley's "Copmplete Official Jaguar E" Manual describes the process for centering the jets. I suspect you book does also.

Finally, since I am just up the road, I would be happy to meet with you with one of my carbs (which are currently off the car) and we could do a comparison of the jets, etc.

Basil

 

[Bugeye58]

Basil, I think that the jet centering procedure really only centers the jet and needle. With the piston and needle out of the picture, the jet should still come all the way to the top of the jet bearing, even if it isn't centered.
I suppose another possibility, and this is kind of out in left field, is that the jet bearings are incorrect. But, most people don't replace those as part of a normal rebuild.
I wish I could be there to help figure this out, as I haven't been to Albuquerque in awhile, and it's always been one of my favorite places. I used to love going TDY from Luke to Kirkland.
Keep us posted,
Jeff

 

[JOEL]

Thanks for your help. The question still remains, is my set up not right. Thanks.

 

[Bugeye58]

Joel, as I said, I wish I was there to take a look. My advice now would be to take Basil up on his offer, compare the carbs, and see what's different. A lot easier than trying to figure something out on-line.
Jeff

 

[Basil]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Bugeye58:
Basil, I think that the jet centering procedure really only centers the jet and needle. <hr></blockquote>

True that is the primary purpose of centering the jet, but since the jet bearing is a hollow tube and the jet is also a hollow tube that moves inside the jet bearing, I propose that if the alignment procedure isn't correct, the inner tube (jet) could bind within the outter tube (bearing) if the jet itself is not adjusted properly. Also, when reading the centering procedure, it seems that if the diaphram is off-kilter when attached with the four screws to the bottom of the jet chamber, that could cause the jet to be also off kilter with respect to the bearing tube, thus causing it to bind. I'm not saying this is necessarily Joels problem, but if the jet is the correct size, this would be one easy thing to check.

Basil

 

[Bugeye58]

Here I sit, abashed!

You are probably correct, and I have to admit, it's been awhile since I've been into an HD8.
Jeff

 

[Basil]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Bugeye58:
Here I sit, abashed!

You are probably correct, and I have to admit, it's been awhile since I've been into an HD8.
Jeff<hr></blockquote>


Mind you, I have not ever seen this problem, I'm just pre-supposing that it could happen based on the way the thing is put together. Its equally likely that I am out in left field. The best way to find out is if I could meet with Joel and do a comparision between our two carbs since I have the exact same setup that I know is correct (at least it was last time it was on the car).

Cheers,
Basil

 

[Bugeye58]

My suggestion to Joel exactly. Now, just don't get the front carb mixed up with the rear!
Jeff

 

[piman]

Hello Joel,
the mixture adjustment screw only works in one direction, to lower the jet, it is raised by a spring below the diaphragm. If the jet does not come up to the top of the bridge when you unscrew the mixture adjustment screw then the likely cause is that the spring which lifts the jet has corroded away. You will need to remove the bottom section of the carburettor to check this, but I have found this fault on more than one occasion. Fortunately it is an easy and cheap repair.
BTW, jet centering is only for alignment of the jet relative to the needle.

P.S. The HD8\HD6 have the same basic mixture set up as most other S.U. carburetters of two full turns of the mixture adjustment starting wih the jet flush to the bridge

Alec

[ 11-16-2003: Message edited by: piman ]</p>

 

[Basil]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by piman:
<snip> the likely cause is that the spring which lifts the jet has corroded away. <snip> <hr></blockquote>

That makes sense!