• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Carb Problem

Share this page

S

Skip & Lynne

Jedi Warrior
Country flag
Offline
Guys I replaced both lower diaphragms in my carbs as they were split and rotten when I did the rear carb and started the car it ran fine except for the front carb pouring gas. I pulled the front carb and replaced the diaphragm and now it wants to stumble and spit back out of the rear carb. I can hardly get it over 1,000 rpm does anyone have any suggestions
 
Sounds like one or more carbs are running way lean; did you ballpark the mixture setting/jet height? Possibly a float needle got stuck.

Did you precisely center the jets? If not, the jet needle can hang up in the jet and, long-term, can result in the jet orifice getting ob-longed and making tuning difficult. This little kit can be surprisingly helpful, it has a 'mandrel' for centering the jet and the wire thingies show piston travel:

mossmotors.com

CARBURETOR TOOL KIT, SU

Carburetor Tool Kit
mossmotors.com mossmotors.com
 
Bob_Spidell said:
Sounds like one or more carbs are running way lean; did you ballpark the mixture setting/jet height? Possibly a float needle got stuck.

Did you precisely center the jets? If not, the jet needle can hang up in the jet and, long-term, can result in the jet orifice getting ob-longed and making tuning difficult. This little kit can be surprisingly helpful, it has a 'mandrel' for centering the jet and the wire thingies show piston travel:

mossmotors.com

CARBURETOR TOOL KIT, SU

Carburetor Tool Kit
mossmotors.com mossmotors.com
Click to expand...
I did none of the above I just swapped out the torn ones with the new ones, I`m ordering the kit right now. I have built a truckload of Holly and Edelbrock carbs but none of these. Looks like I am about to educate myself
 
Be sure to check your carb floats if they are the original brass ones. They tend to get pinholes and ingest gas.
 
You guys got me to thinking and I went back out to the shop to check and see if I had a stuck needle and seat and I have 2 different sets one is a solid seat with no holes with a plastic needle and one is a seat with holes in it and has a solid metal needle it also looks like the fuel level in the front bowl is lower than the rear bowl by about 1/4 inch. I have an extra pair of carbs so I can change out the top of the float bowl and use the seats with the holes and metal needles if you guys think those are the better ones
 
Pics?

In theory, as long as the levers are adjusted properly--depends on the car model--it wouldn't matter if the float needles and seats are different (though I'd want them to be the same). It's a bit of a trick to get the levers set correctly, at proper height above and parallel to the top of the bowl (so the needle doesn't get cocked sideways). The needles should have a small, spring-loaded pin at the bottom. AFAIK the lever adjustment should be made with the pin extended; I think it's basically a shock-absorber for the bouncing float. If the valve has a small metal ball it's a Grose Jet-type.

Also, the jet needles need to be inserted 'just so' into the vacuum piston; it makes a surprisingly big difference and, of course, there are needles marked at least a couple different ways.
 
There are no springs in these needle and seats I can change the arm that the needle rides against to change the fuel level in the bowl I never removed the needles on these carbs I just removed the 4 screws on the bottom removed the bad jet and diaphgram and installed the new one like I said the car ran fine after I changed the one on the rear carb but when I changed the front one and started the car it went lean
 
Had the same problem. Checked my brass floats and could find no leaks . . . but found that the pin holes were opening once the floats got hot. Replaced with the plastic ones and problem went away.
 
I went back through the carbs this weekend and the car is running fine but still lean no matter what I do. I have no idea who worked on these carbs before I restored it. I`m wondering if it may have BJ8 needles in them and now that I have installed BT7 jets and have the wrong needles in them. I have looked at them and there are no numbers on them is there anyway I can check the needles and see if they are the correct ones.
 
Skip & Lynne said:
I went back through the carbs this weekend and the car is running fine but still lean no matter what I do. I have no idea who worked on these carbs before I restored it. I`m wondering if it may have BJ8 needles in them and now that I have installed BT7 jets and have the wrong needles in them. I have looked at them and there are no numbers on them is there anyway I can check the needles and see if they are the correct ones.
Click to expand...
Skip,
The carb numbers are normally on the end that fits into the piston. If you remove the piston and loosen the screw that's accessed from the side of the piston, you can carefully pull the needle out. The numbers may be wiped out if the needle was installed with the numbers facing the screw. The needle part numbers for a 3000 MkI are:
  • RD: Rich
  • CV: Standard
  • SQ: Weak
Needles fitted to a BJ8:
  • UN: Rich
  • UH: Standard
  • UL: Weak
 
BobHaskell said:
Skip,
The carb numbers are normally on the end that fits into the piston. If you remove the piston and loosen the screw that's accessed from the side of the piston, you can carefully pull the needle out. The numbers may be wiped out if the needle was installed with the numbers facing the screw. The needle part numbers for a 3000 MkI are:
  • RD: Rich
  • CV: Standard
  • SQ: Weak
Needles fitted to a BJ8:
  • UN: Rich
  • UH: Standard
  • UL: Weak
Click to expand...
Any suggestions as to how to get them out I removed the screw and pulled really hard and they dont come out of either carb. I have them soaking in carb cleaner in hopes of getting them to turn loose. The needles in my spare carbs are marked CV I`m gonna swap dashpots and pistons and see what happens. Amazing I swapped out the dashpot and piston and it runs great. Engine has a little lope to it but I can adjust that out of it tomorrow. I think I am going to check the piston on the other spare carb and if it is marked the same I will change it out too
 
Last edited:
What I'd do: Spray them with your favorite penetrating oil--I favor Kroil (note: WD40 is not a penetrating oil)--repeatedly for a day or so (squirting into the screw hole should be especially effective). You might try a little heat, say, with a heat gun on 'low,' but don't overdo it. They do get plenty warm when running, but too much heat might distort the aluminium.

I'm curious about how the needles are set in either set of carbs. This scan shows two types, but all the ones I've seen had a full groove and not just a taper; it can make a surprisingly (to me) significant difference in mixture strength. I've run STD needles in my BJ8 through decades of California gas insults with no issues.
 

Attachments

  • IMG_0001.jpg
    IMG_0001.jpg
    1.6 MB · Views: 74
On my spare cabs I removed the screw and they pulled right with my fingers the other carbs they are stuck tight. I have soaked them overnight in carb cleaner and I am going out to try to remove them later today. I hate to ruin a needle by using needle nose pliers to hold it while I pull on the piston but that may be a last resort
 
Back
Top