MGB 1977 MGB will only run on full choke

[Mickey Richaud]

Before you do the rebuild, check the float levels one more time; critical with the SUs. Also, sounds like you're still 2 turns down on the jets. Run them up to see if things change.

 

[Lavo]

Cheers Mickey, I'll try that now, one other thing I have just noticed is that the rear carb is pulling a much better vacuum than the front one. Only noticed when I put my hand over each intake to see if the engine note went up...it didnt by the way, the engine just died.

 

[Mickey Richaud]

AH! Are you adjusting them independently - that is, with the linkage between them disconnected? If not, then do so and adjust one at a time, obtaining good suction on one, then matching with the other. Probably will take a back-and-forth process.
Hang in there - you're on the right path!

 

[Mickey Richaud]

Oh, and blip the throttle periodically during the process to clear any fuel.

 

[JPSmit]

I feel like we are getting closer on this - I would not suggest a carb rebuild right now - let's see if we can get it closer to correct first. Besides cars typically run lean with worn carbs (too much air through worn shafts).

what RPMs is the engine idling at with full choke? when you release the choke - can giving it gas keep it running?

 

[Lavo]

Cheers guys, the car idles on full choke between 1500 to 2000rpm. Once the choke is reduced the engine dies no matter how much you press tje accelerator.

 

[DrEntropy]

That takes us back to massive intake leak or lack of fuel. Is there a filter that might be only partially restricted? Maybe one down near the pump?

 

[JPSmit]

Full disclosure I am not a mechanic and am somewhat clutching at straws, but, the only thing that I can possibly think that could fall out of tune in the car is the distributer. The carbs don't change their settings, fuel pumps work and then they stop. Filters get plugged and flow stops.

However I have seen/ heard of distributers working loose and shafts getting worn. I am not enough of a mechanic to know if this could be a factor - though I imagine if it was slightly off that it would only run under certain conditions. I don't want to add another confusing factor but...

 

[DrEntropy]

fuel pumps work and then they stop. Filters get plugged and flow stops.

SU pumps sometimes half-work, filters sometimes become partially restricted. Forty-five years for rust to develop in a tank, sediment can cause some strange conditions.

 

[Lavo]

I've followed your instructions and have managed to get it to run off choke but I can't get it to idle below 1100rpm. I'm not sure how it runs under load yet as I'm currently blocked in by a van but I'll try it tomorrow. Can't thank you all enough for all the help.
Dave.

 

[Mickey Richaud]

I've followed your instructions and have managed to get it to run off choke but I can't get it to idle below 1100rpm. I'm not sure how it runs under load yet as I'm currently blocked in by a van but I'll try it tomorrow. Can't thank you all enough for all the help.
Dave.

Make sure the fast idle screws aren't causing the linkage to bind.

 

[JPSmit]

I've followed your instructions and have managed to get it to run off choke but I can't get it to idle below 1100rpm. I'm not sure how it runs under load yet as I'm currently blocked in by a van but I'll try it tomorrow. Can't thank you all enough for all the help.
Dave.

great news. In terms of idle, it took me a long time to learn to set it with the engine hot. good luck and keep us posted!

 

[Lavo]

Started it up today intending to take it for a run, right back to square one. Won't run without choke. I've managed to get it to run on half choke but it's very lumpy. Changed plugs and leads, checked balance on carbs, all ok and still no joy.

 

[DrEntropy]

Rats. Sorry to hear that.

WAG suggestion: As it's "idling" at 1000 RPM, pull up and hold a second or two then push down the same on the carbs themselves. A change in RPM would indicate an air leak past the carbs someplace along the manifold.

 

[Lavo]

I'll give that a try, thanks

 

[Gliderman8]

WAG …. Is there a chance that the manifold is loose allowing air to get in?
A quick check of the nut’s securing the manifold to the engine should tell you quickly.

 

[Lavo]

I'll take a look at that today.the carbs are coming off again anyway to check again. Cheers.

 

[pdplot]

So - starts on full choke and runs good until you push choke halfway home, then it speeds up and dies. Right? before you go tearing things apart, get yourself an SU (or maybe even a Stromberg) trouble-shooting manual and go through it page by page. Those are "variable choke" carburetors with no real choke plate like an American classic carburetor. What spark plugs are you using? Plugs may be too cold and fouling at low rpm. Were they wet with fuel when you took them out?

 

[Lavo]

I've checked the manifold nuts,all seemed OK there. I'm using ngk plugs and they seem dry but sooty aftrt the cars been running for a short time. I have again managed to get the car to run without choke but runs rough. I'll look for a su troubleshooter book today see what I an find. Thanks

 

[Gliderman8]

Do your carbs have diaphragms?
If you do, you might check for tears in them.