MGB & GT Forum

I've been piddling with a 72' mgb that hadn't run in 12+ years. In spite of whoever touched it last, I've managed to get it running. I've set the valves, changed points, rotor, cap, condenser. New coil.
I rebuilt the carbs with nice rebuild kits that included shaft bushings, etc.

The problem I have now is that I can't get the car to run at less than 3000 rpm's. By that I mean, the car literally shoots straight to 3000 and won't come down below that. I've got the butterflys closed as far as they'll go and there is oil in the dampers. If I lean the carbs enough I can get them to shut off, but nothing in between.

I'm left searching for a vacuum/air leak, is that the correct approach? One thing I'm unsure of is the function of that black canister on the Right side engine compartment that has a hose to the carbs and a hose to the engine vent. None of that is hooked up so the lines are basically unattached. I did try completely blocking the line to the carbs which resulted in the engine not even running. I messed with enough stuff that I'm not certain that is still the case, but is it a clue?

Any suggestions would be appreciated, especially if that mystery canister needs to be hooked up.
-Brad

I would look at the linkage that connects the carburetors to each other. A common source of this problem. And double check that the throttle plates are truly centered inside the carburetor bores. RAY

Brad,

Die the butterflys still have the poppet valve in them? It those are not closing completely it will be impossible to control speed.

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To start, get a manual as well as a Moss, or other catalogue, so that you can see normal setup. At worst -
http://www.cardiagn.com/mgb-workshop-manual-pdf-download/.

You likely have a massive vacuum leak. If the 2 rearmost ports on your HIF carbs are open to the atmosphere, you got the worst. They need to be connected to the front tappet cover with a "Y" pipe. You should be able to run a hose between the 2 ports just to end that leak on a temporary basis. After a car has set for extended periods of time, gasket shrinkage is common. A propane torch "unlit" directed to manifolds, hose fittings, etc can often identify 'leaks'.

You've only just begun!

The mystery stuff is the evaporative controls system. It is described in the manual and should be hooked up correctly. It manages both carb bowl evaporation/vent and crankcase blowby emissions. It does nothing detrimental to the performance of the car but reduces emissions greatly. The carb balance is set up assuming the system is in place.

The thin lines are the carb vents They must not be blocked. It should run with the fat lines blocked as the normal leakage to there should be minimal.

Are you sure you got the choke/enrichment stuff back in there correctly? Backed out the choke screws so they don't touch the cam? Set the jets the baseline 2 1/2 turns? ( 060"

Be sure your ignition is not way too advanced.

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Cogito ergo sum periculoso

1. no poppet valves on the butterfly's.
2. They seem to be installed correctly but I've made mistakes before, that's my next step. That said, looking into the bores they look fine.
3. All new gaskets, including to the intake.
4. HIF carbs
5. that hose with the big ports currently has a "T" that routes it to the canister that I think says something about charcoal or servicing...I don't recall. I think I'll see what I can do to plug that but when I plugged it last time, the car wouldn't even start. That makes no sense to me so maybe something else was messed up at the time.
6. As a side note, the vacuum advance is coming off the manifold and unplugging it to atmosphere makes it run terribly bad.
7. Both carbs behave the same if I try to cover them up. IE: the engine wants to stall if I completely over one or the other carb but both behave the same so the leak doesn't seem to be associated with one or the other carb.
8. any possible opening on the intake that I can't see?

Like I said get a manual and read it. You need to start with the basics. Any port on the intake must be plugged or connected to something that does not allow entry of air. On your '72, if OE, you have a restricted line from the manifold to the Vac Advance unit on the distributor. Confirm that the vac adv does not have a perforated diaphragm by sucking on the line where it attaches to the intake manifold. If you can draw air, plug that manifold port. You should not have an ARO valve, so not port to that. The only other connections are to the gulp valve, if installed, one large and one small. Hopefully, the gulp valve and air pump have been correctly removed. If present, please tell us. It would/could be helpful to us to post some pics of the engine bay.

Lets try and deal with the seeming vacuum leak before we attempt proper carb tuning. With HIFs, leaking worn throttle shafts should not be a problem. Cold start mechanism certainly could be contributing.

Like they told Dorothy start at the beginning.
https://rockymountainroadsters.com/pretune.php
A good guide getting from here to there.

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brad- pm sent

Good advice above. One that took me some time to figure out.

One I had "self-healing" float. It had allowed some gas to seep in, and could not bring the idle down. This was for a 1973 MGB with HIF4 carbs.

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Shareef Hassan
Unique Parts for Four Cylinder MGB
www.safetyfaster251.com

I guess that you have the vacuum system misrouted.

As said above. Plug all the holes in the manifold except advance to the dizzy.

Plug the large ports on the carbs for now. The smaller vents on the carbs (float chamber vents) should be open.

The breather (side cover) should be open to atmosphere for now.

After that go back and make sure you have the throttle links and choke links setup right.

Set initial timing to around 10*BTDC.

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My throttle return spring was weak on my 65 causing high idle problems, also the timing was off.

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High idle speed means high air flow. Either a big vacuum leak(requires a rich mixture adjust to compensate), or throttle plates not closing all the way. This should be apparent by having one carb piston riding higher than the other. Throttle plates have to go in with the bevel correct and they must be finessed into place with screws loose and throttle closed to assure sealing. There is enough clearance in the throttle plate holes to allow this.

my next pass is the butterfly valves, maybe I didn't install them correctly. They look OK and both carbs seem to be opening the same amount. I'd be surprised that such a small amount would have this affect.
Attached are 2 pics of the mess I have. Keep in mind that I'd never seen under the hood of an MG until this one followed me home. So far, almost everything I've found was screwed up before i go it so I never know what is correct.
yes, it came with 3 shop manuals but sometimes they don't really help all that much.

I appreciate all the suggestions. it's hard to reply to all of them but I do check out all the suggestions. If you see something in the pics, please let me know. I'm getting pretty close....