• 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

MGB High rpm problem

Share this page

Carlbanan56

Carlbanan56

Senior Member
Country flag
Offline
I have a question regarding my MGB form 1975. The engine won't go down in rpm while idling and I have backed out of the idle screw. I could come up with two theories for this.

The first one is that the timing might be wrong. I have set the timing myself with a strobe light but I am not sure I have done it right. When I set the timing I lined up the indent in the flywheel (Check the picture) with the TDC mark.
Then I marked the TCD and about 10 degrees BTDC on the flywheel. Then I used the timing light to line up the marks (with advanced timing removed). I have got some backfiring as well and that's a symptom of bad timing.

The other theory is that I have an air leak and therefore the engine is sucking in air somewhere. I am not sure how to test this.

Alla answers are greatly appreciated.
 

Attachments

  • IMG_0035.jpg
    IMG_0035.jpg
    1.4 MB · Views: 70
It is unlikely to be timing. Even if not perfect, it is good enough.

It is relatively easy to test for air leaks with the engine running, spray starter fluid or WD40 around the intake manifold and where the carb connects to the manifold, if the idle speed changed you have an air leak.

That said, I wonder if you have an issue with choke or butterfly valve? What carb do you have on the car? By 1975 MGs had a lot of funky emissions stuff. Does it have a manual choke? If it is stuck on (or partially on) that can make a high idle.

Has it run properly before?

let us know
 
O
JPSmit said:
It is unlikely to be timing. Even if not perfect, it is good enough.

It is relatively easy to test for air leaks with the engine running, spray starter fluid or WD40 around the intake manifold and where the carb connects to the manifold, if the idle speed changed you have an air leak.

That said, I wonder if you have an issue with choke or butterfly valve? What carb do you have on the car? By 1975 MGs had a lot of funky emissions stuff. Does it have a manual choke? If it is stuck on (or partially on) that can make a high idle.

Has it run properly before?

let us know
Click to expand...
I have HIF4 carburetors with manual choke. Before the winter the car did not have this problem.
 
I would also check and verify that the throttle cable is sliding freely. If it isn't fully returning or binding somewhere, then adjusting the idle screws will have no effect.
 
YakkoWarner said:
I would also check and verify that the throttle cable is sliding freely. If it isn't fully returning or binding somewhere, then adjusting the idle screws will have no effect.
Click to expand...
I have checked the carbs. The throttle cable is free and if I lift the carburetor piston it looks like the throttle disc is completely closed.
 
On the air leak, you should also try grabbing the carbs and trying to rock the manifold. If loose, you will hear a change. I kinda favor the throttle cable. When one strand breaks, the end will seize on the sheath at some point.
Bob
 
Now I have tried to rock the manifold and sprayed WD40 and nothing changed the rpm. Any advice would be appreciated😁
 
Going more into the carbs; the small axles that the bufferfly plates rotate on can have worn bushings where they come thru the side of the carb. That can easily simulate a partially open throttle.

Check the vacuum line going across to the distributor - that can become an open vacuum leak if cracked/broken. Any of the small vacuum hoses connected to the carbs or the intake manifold itself can become a leak.
 
Can you forcibly rotate the throttle shaft to reduce the idle? can you disconnect the cable end and see if idle returns to normal?
Bob
 
YakkoWarner said:
Going more into the carbs; the small axles that the bufferfly plates rotate on can have worn bushings where they come thru the side of the carb. That can easily simulate a partially open throttle.
Click to expand...
If butterflies are closed and idle is elevated, Yakko's on to something. Spray some WD40 on those areas and see what happens.
 
Mickey
Post 7 says he did the WD. I suspect the throttle system. Maybe when Doc gets here he will have another idea. Carlaban says the problem developed itself, apparently without tinkering,
Bob
 
bobhustead said:
Mickey
Post 7 says he did the WD. I suspect the throttle system. Maybe when Doc gets here he will have another idea.
Bob
Click to expand...
Saw that, Bob, but would ~rocking~ the carbs/manifold affect the throttle shafts if they're loose in the bushings?
 
Carlbanan56 said:
O

I have HIF4 carburetors with manual choke. Before the winter the car did not have this problem.
Click to expand...
If the car didn't have the problem before the winter, it is unlikely a worn shaft or any such thing. Out of curiousity what is it idling at?

I suspect it could be a binding throttle cable or a sticking choke cable. Assuming the idle isn't too too high I might lube the cables and take the car out for a good run to blow out the cobwebs - Or take it for a good run and then lube the cables.

Oh and maybe - maybe - something fell off through the winter - vacuum hose? plug.

If you haven't done anything to the car and it was fine before you put it away, don't start taking things apart - you are already very close to having it right
 
Carlbanan56 said:
Before the winter the car did not have this problem.
Click to expand...

As JP says, shafts and hoses don't fail if the car has been sitting idle for a Swedish winter. But - an old vacuum hose can split if cold and suddenly in use.

Throttle is closed, but idle is high.

What rpm is that idle speed?
 
When I get home after school I will be taking the carburetors off and make sure the throttle valves are fully closed.
 
Carlbanan56 said:
When I get home after school I will be taking the carburetors off and make sure the throttle valves are fully closed.
Click to expand...

We thought you had already said the throttle is closed?
 
Back
Top