MGB & GT Forum

My MGB with SU carburetors has been running perfect. After sitting about a month I went to start it. Pulled the choke and it slipped out of my had and went back in. Pulled again and fired right up. But then ran as if the choke was on even though I pushed it all the way in. Black smoke and flooding. I unhooked the cable and made sure the choke linkage was fully pulled down. Sounds to me like the choke inside the carburetor or carburetors is on. Anybody know anything about this? Thanks

I fixed my 1974 B yesterday.
My rear carb was flooding.
I installed new needle and seats, new floats. And changed out the choke o-rings.
Make sure to change the oil afterwards, it can get contaminated with fuel.

Joe - Post a few pics of the carbs, front and both sides. If you mean literally "flooding" (raw gas visibly flowing), the choke (enrichment mechanism if HIFs) cannot cause flooding. Rather as mentioned above the fuel flow into the float bowls are not functioning properly.

Joe my 30 second diagnosis was to remove the vent lines and turn the key on. Gas game out of the rear vent.
This indicated to me that there was either dirt holding the needle valve open or a float that was too heavy.
On inspection the viton tip showed some wear. The floats are high failure items. So they were replaced.
I also found a missing throttle shaft seal, so those were replaced.
The only dirt I found was in the choke assembly.
Since the carb was pumping fuel into the charcoal canister the canister will need to be serviced. You can find the instructions in the tech library.

A very common cause of fouling the float needle valve is deteriorated hoses' internal surface sloughing off rubber particles. Use fresh hoses after the fuel filter and between carbs. hoses

Thanks. Sounds like something like that. I had ran some regular fuel with ethanol in it. Probably did something to it. Probably needs the carburetors rebuilt. Not sure if I can do it. I can take them off.

Sounds good. I have had boats and jet skis that ethanol fuel ate the inside of the fuel lines up. Probably need the Carburetors rebuilt. Do you know how much that normally cost? I have seen the rebuilt kit on eBay for $150
But not sure if I can do it myself.

Joe, if you are even remotely mechanically inclined, you can do the carbs yourself.
- Purchase quality parts from an MG parts house (Moss, Victoria British, etc)
- work on one carb at a time as you don't want to mixed parts between carbs (like pistons)
- make sure you have a good shop manual (Haynes)
- Do not use Grose jets even though they are "new and improved" many members here have had issues with them

These carbs are incredibly simple devices without the usual tiny jets and microscopic holes and myriad passages.

When you are done, follow directions and take your time balancing them. It help to have a flow meter (Uni-Syn on amazon for $26)

BTW, your problem, like Barry said, is most likely a stuck needle valve or (less likely) a failed float, both of which you replace when you rebuild your carbs

Good luck, you can do this

Thanks for all the help. I’m normally a Chrysler /Plymouth Guy. I sold both of my Plymouth’s and ended up with this MGB by accident. It belonged to a Client of mine. He hade it in his basement and practically gave it to me when I sold his house. The good thing about it is it doesn’t take up much room in the garage. I have rebuilt old dirt bike carburetors years ago. Looks like I’m going to have to try these. Another option I thought about is converting it over to a down draft Weber. The kit is under $400. I had seen on YouTube a guy saying that Weber makes good grills. And not to change over. Anybody ever made the conversation?

Joe if you had a rubber bumper car with the single carb the Weber would be an upgrade.
If you can do a dirt bike carb then you have the skills to repair an SU carb.
But ever if you did go to the Weber you have to open up the carbs to check for rust from the tank or water contamination.

Joe - "Weber makes good grills". Do an advance search of this forum and read somewhere between a thousand and a million comments/views of Weber carbs, especially as a substitute for OE twin SUs.

Joe, several of the members here (including me) have either recently, or are in the process of, switching our MGs from Weber’s back to twin SUs. With a 74 you should have HIF4s which are a breeze to work on. There are multiple sites with step by step instructions and YouTube videos out there. If you need help, we’re all here for you. After all, you live in my hometown of Gainesville, GA. Gotta stick together!

---

My MGB is like a former girlfriend:

Beautiful on the outside, a little broken on the inside...

...often topless

Just to be sure, can you confirm for us that you HAVE HIF carbs?

That's what would have been fitted from the factory, but they can be interchanged with HS type carbs without too much trouble.

The "enrichment circuit" on HIF carbs-to OVERLY simplify things-is a cross-drilled bolt that when aligns the cross-drilling with a secondary passage through the carb body that provides a secondary path for gasoline from the bowl to the throat(in addition to the primary jet). When the enrichment circuit is off, this cross-drilled hole is not aligned, which has the effect of shutting off this secondary circuit. To me it's a beautifully simple design, but one that stumped me when I overhauled my first HIF(admittedly the HIF6 on my Marina, not an HIF4 as would have been fitted to a B, but it's really just a larger version) and I though it wasn't working when I reassembled it on the bench as I couldn't see anything externally moving when I moved the lever by hand and it also doesn't "feel like" you're doing anything. IIRC, there's an O-ring or two and maybe a fiber washer that's normally replaced as part of a rebuild, but that's about it, and it's unlikely for the circuit to actually stay on.

HS type carbs have the jet hanging out the bottom of the carb, and when you activate the "choke" a lever physically pulls the jet down. This has the effect of "increasing" the size of the jet by moving it to a smaller diameter part of the metering needle. BTW, this obviously takes a decently heavy tug on the choke cable or when activating the mechanism with your hand, especially compared to the HIF way, which is why someone use to working on these might be confused when dealing with an HIF. If the lever that moves the jet is a bit out of adjustment, or there's something else going on in the carb(i.e. sitting for a while and allowing the gas to gum up) the jet can stick "down" when the knob is released. The set on my car now was just enough out that it would occasionally happen to me, and I'd feel it stumble and idle rough once warm. That would necessitate a road-side stop where I'd hop out, pop the hood, and just reach up under the known problematic carb and touch the jet. I'd feel it "pop" up smartly, and the car would immediately run correctly. It rarely took more than touching the jet "just right" to get it back in place. I finally pulled the carb and carefully adjusted the lever until it would retract reliably.

I only mention this because even messing with the linkage up top on an HS won't "fix" the jet normally. There's a good chance you don't have HSs, but the car has had nearly 50 years of who knows how many people working on it, and the change between the two carb types is probably the least drastic/least involved carb swap you can make(there are pluses and minuses to both designs).

Aside from that, the other points are good, and pay particular attention to the one about deteriorating rubber fuel lines. One often-overlooked one is the one that ties the two together. On I think Mk II and later cars(I may be wrong on the exact changeover date) with either SU carb type, the hard line comes up the front left wing, where it transitions to rubber, goes into the fuel filter, and then into the front carb, The front carb has a direct pass-through with another hose attached to it that feeds the rear(not that it matters in this case, but early cars reversed this feed order). When I bought my car, I replaced all the rubber from the hard line to the front carb. I'd occasionally get a stuck float on the rear carb despite having rebuilt both with new floats and needle valves. The stuck float would of course cause the rear to flood out, with a more extreme version of the stuck choke symptoms I described above and also often with a gasoline smell. Hopping out and tapping the bowl lid with whatever tool was handy would usually fix it right up. Finally, it occurred to me to replace the cross-over between the two, and I haven't had a stuck float/needle valve since then.

Attached is a photo of the carburetors

The MGB is a 74 with Crome bumpers.