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HD8 Carburetor rebuild help

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As this rebuild is a learning curve for me, when I spoke to Joe Curto, he suggested I use the (UN) jet needle, which the moss description says rich. Joe mentioned to use that needle because of the newer gas formulation, maybe ethanol?
What's your thoughts on the rich needle UN vs regular needle UH?
 
David Nock also recommends the rich needles. I've run on standard needles for as long as I've had my BJ8--about 35 years--and never had a problem that I can definitively attribute to ethanol in fuel (doesn't mean I like it). You'll get STD needles in your kits--unless there's an option; didn't used to be--I'd try them first then spring for the rich if you think you need them. I bought one rich needle when I put a PCV valve into the hole in the rear of the manifold but never used it.
 
I realize I’m jumping in late...nonetheless I can highly recommend Rhys Kent (Island Automotion Ltd). He has done wonderful SU rebuilds for me. You can find him on the web.

Best,
Crispy
’67 BJ8
’69 Volvo 1800S
Portland, OR
 
I used the rich UN needles from Aug 2016 to a month ago. I was getting pretty lousy freeway mileage around 15+. 11 - 12+ around town. They were doing a good job of staying in the mid 12 AFRs in regular or WOT driving. So I think they're good for power. Jet drops: F - .063"; R - .069"

Then I installed the normal UH needles to try them for a while. What I noticed immediately is cruise shows good power 12s but when you give it the gas the car leans into the mid 13s. Both jet drops set at: .076"

So I think the UH needles are weak on top end. A good setup would be to keep the stations for cruise the same and richen the top end a little. I may embark on this at some point.
 
steveg said:
...

Then I installed the normal UH needles to try them for a while. What I noticed immediately is cruise shows good power 12s but when you give it the gas the car leans into the mid 13s. Both jet drops set at: .076"
...
Click to expand...

What fluid are you using in the carb dashpots?
 
Got any straight 50 you can try just for yucks? Maybe some gear oil? Since you have A/F sensors I'm curious to see what effect it would have.
 
Bob_Spidell said:
Got any straight 50 you can try just for yucks? Maybe some gear oil? Since you have A/F sensors I'm curious to see what effect it would have.
Click to expand...

My understanding is the oil weight is the damper for when the throttle changes. So any effect would be momentary. I'm experiencing a steady (IMO) too-lean condition with the throttle off cruise - as when going up a long grade with your foot into it. EGTs aren't bad - just think not all the power is there because of the 13+ AFRs.
 
Bob_Spidell said:
Stiffer springs?
Click to expand...

My understanding is stiffer springs if they were available would have the effect of holding the needle down (lean) further in relation to throttle opening. Off idle the car would run richer due to less air per unit fuel. More open throttle the car would be a little leaner due to the needle being further down in the jet.
 
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I typically run Marvel Mystery oil in the dashpots. At Snowmass (high Altitude) we went to a lighter machine oil as I recall.
 
Carb rebuilds aren't too bad. Do one at a time so you always have a reference of how things go together. If you take the throttle disks out make sure they close properly (seal without hanging up when closed) when you reinstall. Don't mix parts from one carb to the next, especially the Piston and dome, they are a matched set. Centering the jet sometimes gives trouble, piston needs to rise and fall freely, if you snug it down with the jet fully up (lean) it should work fine, but sometimes takes a couple tries, I have never used a special tool for this. A relatively pleasant task if you are not averse to the slight smell of gas. Nothing is usually corroded on, no massive wrenches or big hammers involved, and , once removed you can do it on a bench or even sitting down with a tray on your lap. Good Luck!
 
steveg said:
My understanding is stiffer springs if they were available would have the effect of holding the needle down (lean) further in relation to throttle opening. Off idle the car would run richer due to less air per unit fuel. More open throttle the car would be a little leaner.
Click to expand...

I may have this all wrong--it's counter-intuitive--but I've been told/read that the purpose of the oil in the dashpot is to suppress the piston from rising quickly when the the throttle plates open. This is (supposed) to create more vacuum over the annular ring around the needle causing a richer condition--essentially, 'choking' the flow--in lieu of an accelerator pump (and also damps spurious movement of the piston). Apparently, the greater vacuum overcomes the (effective) narrowing of the jet opening (while also, paradoxically, limiting mixture flow which, again, I've always assumed is why SUs have less throttle response than, say, Webers). I've seen differing springs available, but never investigated why. Of course, you can also turn down the needles at the point where you'd be cruising and pulling a load.

With your A/F sensors, you can try all kinds of things; let us know what you find :highly_amused:
 
SU Altitude.jpg


From the SU Workshop Manual.

Danny
 
When does the needle and seat need to be replaced?
I'm rebuilding a friends HD8 and decided to look into mine while I'm at it.
I'm at the point of installing the needle and seat.

Both sets of HD8's currently have the (per Moss motors description) "Grose-jet, superior replacement" one installed.
The ones that come in the rebuild kit have the "needle & seat" 2 separate parts, vs the one unit Grose-Jet.
Interesting that the Grose-jet costs less $10.99 vs needle & seat $19.99

I also noticed from the Moss photos, the Grose-jet has a white washer, the needle & seat does not. Should the needle & seat have a washer too?

Is the Grose-jet really better? Why?
How do I tell if the ones we have need to be replaced?
371-055_1.jpg386-390_1.jpg
 
Grose Jets are, in theory, much superior to the needle type. I put a set of Grose Jets in my BJ8's HD8s over 25 years ago and they have performed flawlessly; however, we tried them in our BN2's H6s(?) and the small ball that closes the opening to the fuel line would get stuck occasionally. I heard the company that originally made them got sold; oftentimes quality goes down when a new owner tries to squeeze all the profit they can out of their purchase (pay off a loan, maybe?). If the ones you're now using haven't given you any problems--i.e. flooding when they stick--I'd stick with them.

The needle-and-seat type will sometimes wear a groove around the tip, on the Viton, or other 'rubber' type, and some have all brass needles which will sometimes show wear around the tip of the needle and in the seat. This usually requires many miles of driving; for our occasional runabouts it's probably not much of an issue. The needle-and-seat type will sometimes get stuck, too, but a whack from a plastic or dead-blow hammer on the top of the float bowls will usually free them up. I can see no potential harm in putting a nylon (teflon?) washer under a needle-and-seat type, if you have enough thread and can adjust the float levers properly.
 
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