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MGB Can't get idle down

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drooartz

drooartz

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So I've got my newly rebuilt carbs (HS4) back and installed, and I've run into a problem. I can't get the idle down at all -- lowest idle I can get is about 1800RPM if I want to keep the carbs synced. I can lower things a bit by backing out the idle screw on the front carb, but the screw on the back carb is already all the way out -- won't lower any more at all.

Here's what I've done so far:

  • Distributor just rebuilt by Jeff, back to points
  • Plugs gapped to .025"
  • Timing is 32* all in with vacuum disconnected
  • Valves all set to .015" cold (most were already close)
  • Needle is #5, jets currently set to .040" down
I've got a call into Hap for advice, but was wondering what else I should be checking for. Seems to be an issue with the rear carb, but that could be a red herring.

As an aside, when blipping the throttle (cables disconnected) it seems to take too long for the RPM to drop back down. Something sticking, maybe?
 
Hey, Drew -

Did you happen to check to make sure the throttle plates are closing fully - especially on the rear carb? Choke linkage fouled?
 
Mickey Richaud said:
Did you happen to check to make sure the throttle plates are closing fully - especially on the rear carb? Choke linkage fouled?
Click to expand...

I didn't check, but that's something I was thinking of checking -- just need to pull the carbs and take a look.
 
Check the linkage joints as well, if they're not loosened already for adjustment.
 
Thanks, Mickey. Just talked to Hap, and I've got a few other things to try. Now I just need to find some time to work on it!
 
If the timing is spot on and the throttle plates are fully closed, then the only thing I could think of is a vacuum leak. Drew, did you have the manifold off? PJ
 
I did have the intake manifold off.

First thing I'm going to try now is to spray some volatiles around the various gaskets and see if there's a vacuum leak from any of those spots. If that's a no, I'll pull the whole assembly and reassemble it, to double check that everything's in as it should be
 
I'm sure it's gonna be something simple - one of those dope-slap-worthy moments!

Let us know what you find.
 
Mickey Richaud said:
I'm sure it's gonna be something simple - one of those dope-slap-worthy moments!

Let us know what you find.
Click to expand...

I'm sure it will be -- I've got enough dopiness for a couple people. Will probably be a few days before I can dive in again, but will report back with what I find out.
 
What is the theory on why a vacuum leak would cause high idle?

Other than that, do you have any other problems?

I have SU H-6 carbs with a Mallory dual point distributor, so no vacuum advance from the distributor. I had (have) a terrible high speed misfire that I thought was a) fuel starvation (but on test, pump provides about 24 oz per minute) b) boiling fuel in float bowls (it was about 95+ degrees) or c) a air leak around the intake of the rear carb (saw traces of moisture on top of heat shield (no gasket).

When the engine was really hot, the idle didn't want to come down. And recently when I started it (cold) the idle was at about 1500 vs. 900, but I was able to reset by turning the idle adjustment screw.

Can it be that my misfire is caused by the air leak theory?
 
Had a couple minutes when I got home, just enough time to fire up the B and try spraying some carb cleaner around to hunt for vacuum leaks. Did not come across anything that I could detect.

Next step will be to pull the carbs off and do a good inspection, make sure I don't have anything binding up or obviously out of place. Simple enough to do with the B. Only bummer is I probably won't be able to get to it until Sunday or Monday.
 
Are the return springs in place? Vacuum won't account for that much change.
 
If you removed throttle plates during rebuild, check that the chamfered edges of the throttle plates are in-line with bores, not reversed.
 
LarryK said:
If you removed throttle plates during rebuild, check that the chamfered edges of the throttle plates are in-line with bores, not reversed.
Click to expand...
Good point Larry, but I think Drew sent the carbs out. Well, it's still a good point, none of us are infallible! (Especially me!) :highly_amused:pJ
 
LarryK said:
If you removed throttle plates during rebuild, check that the chamfered edges of the throttle plates are in-line with bores, not reversed.
Click to expand...

Throttle spring are all in place. My next task is to pull the carbs off and give them a good look over -- I did send them out so I expect they've been put back together correctly, but everyone makes mistakes now and then so it's possible there's an issue somewhere.
 
Update

I had a bit of time yesterday to pull the carbs off and have a looksie. Pretty sure that there's an issue with the rear butterfly. Chatted with Hap for a few minutes, and I'm going to try and swap in a spare I've got just to see if it seals up properly -- if so, Hap will send me a new rear butterfly and I'll pop it in.

Here's what I found:

Front carb, butterfly sits in there pretty nicely.

carb_f.jpg


Rear carb, not so nice at all. Bevels are in the correct orientation, but the butterfly is hanging up somewhere and not closing fully.

carb_r.jpg


You can really tell this as the rear carb has a vacuum port that should be covered completely when the butterfly is closed -- as it is on a spare set of carbs I've got. On these, that port is still visible when the butterfly is closed as far as it will go.

I'll put in a temp spare butterfly tomorrow when I get a moment and see if it closes up properly. Hopefully that's all it is, as that's a simple part to swap in.
 
Drew, very far from a carb guru by any means but is it possible the butterfly on the rear carb is rotated 180 degrees? If the screw holes were off center by 1/16 or even 1/32 of an inch, that could cause the binding? Just a thought since you have to swap it out anyway....
 
Loosen screws, adjust butterfly until it seats, tighten screws, peg bottom of screws with small chisel. See if that works.
 
LarryK said:
Loosen screws, adjust butterfly until it seats, tighten screws, peg bottom of screws with small chisel. See if that works.
Click to expand...

Good suggestion, the shaft and butterfly are the first thing I install during a car rebuild, I get the screw hole lined up and push the throttle plate to a closed position with my hand, then hold the carb body up above my head pointed towards the florescent light above my head, to check air gaps around the butterfly. I pretty good to double and triple check myself, but I'm not perfect, so it could have slipped by me. Drew if this ends up being it, just remember to put a small drop of locktite on the threads of the screw, once the butterfly is aligned you can do this by removing just one screw at a time. Drew and I thought we may have defective butterfly, but the more I think about it, this is probably all it is.
 
I've got some time tonight, will give this a check and see if it clears up the problem.
 
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