Engine and Transmission Tech

I drove my MGB GT V8 from Colorado to Lincoln Nebraska this week. It was running GREAT - until shortly after I pulled into the Sinclair gas station by the Lincoln airport and gassed-up on their "Premium". Within fifteen or twenty miles of that, the car's off-throttle/idle performance totally went to hell. The car still seemed to start easy and run fine at any other engine speed, so I figured the problem would go away after a tankful or two of better fuel. Well... ~600 miles later I'm starting to get concerned.

Nebraska is a field corn producing state. I don't know how much ethanol was in Sinclair's premium gas, but by guess is that it was more than the usual ten percent. One clue - I didn't notice this until after I'd filled-up - both Plus and Premium were priced below Regular at that station. Watching more carefully after that, I found that Plus is cheaper than Regular at lots of Nebraska gas stations and it's usually labeled as ethanol enriched - but Premium is usually premium-priced. That Sinclair station apparently isn't typical of the overall market.

I thought maybe they had a lot of water in their fuel... so in despair I tried one bottle of "Heet" fuel dryer with the next fill-up. That product contains methyl alcohol, so it should have soaked up any remaining water droplets. It didn't seem to affect performance one way or the other, and anyhow I'm a couple tankfuls of gas past that now.

When I got back home to Colorado, I (#1) pumped the fuel tank dry, (#2) drained the fuel line, (#3) emptied the float bowls of old fuel - incidentally, there was no sign of water droplets or debris - (#4) temporarily removed the idle air screws, (#5) sprayed carb cleaner through every oriface and jet I could see to spray, (#6) verified that my jetting was 100% normal for Colorado, and (#7) put fresh gas from my usual hometown gas station in the car, and... no improvement at all!

The engine runs great at any speed and any amount of load... except idle. At idle speed, the RPM bounces around. I have an AEM air/fuel ratio gauge and it reads WAY TOO LEAN at idle speed. With just the slightest touch of the throttle pedal I can make the air/fuel ratio drop from above 17:1 to 12.x:1 or thereabouts.

I'm about ready to smash my Edelbrock carburetor with a sledgehammer and get to work on installing electronic fuel injection... but before I do that can someone please tell me what I'm missing here?

Almost sounds like a vacuum leak?

That's exactly what I was thinking.

Another thought is maybe you have some trash that got into the idle circuit blocking fuel flow?



Edited 1 time(s). Last edit at 06/24/2012 11:01PM by rficalora.

The Nebraska fuel would have contained ethanol so any water would have been absorbed even without the Heet. Ethanol tends to damage rubber parts so all of the carb gaskets, diaphraghms, seals and o-rings are suspect. Might want to throw a kit at it before going any further.

Jim

And where the heck is Fred?

Any chance you might have put E-85 in it? The only reason I ask is that it would extremely out of the norm for premium to be cheaper than the lesser grades in this area.

Also the pump and regulator could have been damaged as well.

Jim

Pretty sure the Sinclair pump wasn't labeled E85... but maybe it was mislabeled.

My fuel pump seems okay at every other load level. I don't have a separate regulator.

This morning I started the engine and then sprayed ether starting fluid all around the carburetor and the vacuum advance hose to the distributor... but engine RPM didn't increase as I'd expect it to if there were a vacuum leak. I'll put a vacuum gauge on it when I have another moment.

The oxygen sensor is on my righthand exhaust, but as soon as time permits I'll move it to the lefthand side bung and see if it reads the same. The super-lean A/F gauge reading worries me more than the rough idle.

Curtis, Try spraying carb cleaner in the mixture screw holes. Idle circuit may be plugged as Rob said.

Curtis,
warm the engine up. take the air cleaner off. rev the engine up to about 2500- 3000 then put your hand ove the top of the carb to close off all the air flow and let the carb lenkage drop to the idle possition.
this will put lots of vaccume on the idle circ and may clear it out.

Steve

Removing the mixture screws and shooting carb cleaner down their holes might work better if I could get my finger down the venturi to block the exit hole there... but without that blocked off, the carb cleaner just sprays into the venturi. Everything seems so clean.

Steve's method is fun... but in this case it didn't seem to help. I tried it eight or ten times at least.

When I look down into the carb barrels at idle, I see a lot of fuel sputtering out of the driver side venturi booster but hardly any out of the passenger side one. I've had this trouble before in hot weather with ethanol enriched fuel and the problem was solved then by adjusting float level. So, I took the top off the carb and checked that. Can't remember what I set them at last time - ten years ago? - but they matched (about 5/16" from the cover when inverted), so I lowered both floats 1/16" to see if that would reduce the sputtering. No detectable change. Should I try going even lower? Could it be that I have too much fuel pressure? I think a carb rebuild kit is in my future and I resent it because I've been saving pennies toward EFI.


p.s. 100F here today, and the radio says we'll have the same for five days in a row.



Edited 1 time(s). Last edit at 06/25/2012 08:40PM by Moderator.

Curtis, floats on the Edelbrock.should.be 7/16 when inverted.

I rebuilt the carb today with Edelbrock kit p/n 1477 (~$50 locally.)

I didn't find anything conspicuously wrong inside the carb. The float level measurement I mentioned in an earlier post wasn't really off; my measurement method was just different than Edelbrock's. The droop measurement was off by quite a bit (maybe 1.25" vs. prescribed 15/16"), but I don't expect that should effect idle.

Everything now seems to be back to normal. (The engine idles at 800rpm with a/f ratio around 12.3:1.)

Had to have been something in the idle circuit that you finally flushed and just didn't see it. I'm sticking with carb on my build.