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MG MGB Technical - Very Rough Idle - 76 MGB

I am hoping someone can give me some help. I'm a novice, only having my 76 MGB for about a month. The PO had it for about 3 years, and only put about 3000 miles on it, so he's no help regarding information. I don't know who the PPO was, but he was the original owner. The car seems to have all the original equipment on it, including the Zenith-Stromberg carb.

When I first got the car, it was running fine. I checked all the fluid levels, changed the oil, air filter, plugs, etc. Still running fine. Up until last weekend, I hadn't run much over 55 mph. My wife encouraged me to run it up to about 65 just to see how it would do, and all seemed fine. We got to where we were going, it sat for about an hour, and then we started up again. I noticed we needed gas, so I stopped and bought some. Shortly thereafter, the car "stuttered." That's the best way I can describe it. It "seems" to run fine at 35 to 45 mph, but at lower speeds, and at idle, it is really rough. I changed the fuel filter, but its not any better.

I've read that it may be a vacuum hose leaking. But I was wondering if this symptom could be caused by bad fuel?

I would appreciate any suggestions. Please keep your explanations as simple as possible. I'm willing to learn, but I'm pretty ignorant when it comes to car engines - probably not good when you're an MGB owner with limited funds. :-)

Thank you VERY much!!

John
John English

John,

It might be 'bad gas' as you suggest. To check to see if that's the problem, this is what I'd do:
Open your trunk and look near the front at the trunk floor level, you'll see a smallish black sheet metal guard on the right hand side. This is the safety cover over the fuel pump. Take it off by removing the two screws. Underneath you will find a spade type electrical connection for the fuel pump. Disconnect it.

Open the hood and disconnect the fuel tubing from the fuel filter on the carb side. Get a funnel and push it into the end of the tubing.

Start the car and let it run out of gas. Get fresh (good) gas and pour it into the funnel until it won't take any more. Start the car and see if it runs better. You can keep topping up the funnel to keep it running until it's fully warmed up and off the choke. That will tell you if it's bad gas or not.

Of course all safety measures should be taken! Gasolene can be very dangerous if not handled with care. If you spill any, clean it up and vent it out before proceeding.

Good luck and let us know how it comes out!
Tom
Tom Sotomayor

Tom, thank you for your idea. I'm starting to think I have a bigger problem than fuel.

Here's an update - I went out this afternoon and tested for vacuum leaks with carb cleaner - I didn't find any. I took it out on a test drive just to see what would happen. Now, whenever I give it gas, I get a LOT of vibration - no matter what the speed - and no power. The problem seems to be getting worse.

Oil pressure is fine, and it's running cool. I'm still looking for ideas.
J.D. English

Update: On a whim, I decided to change out the spark plugs. I must have had a bad plug, because that smoothed out the idle.

I took it down the street, and everything ran fine for about a half a mile - then I lost power. Now it won't rev over about 2000 rpms, I can't get over 35 mph, and I'm getting a lot of blue smoke.

I could really use some help - I think one problem has caused another problem.

Thank you!

John
John English

Do you smell gasoline..like maybe its flooded? Blue smoke or black smoke?
Those ZS carbs will go rich and cause you ro run rich/wet even to the point of the cat converter glowing red due to the fuel burning inside it??? (just throwing out ideas)
Rick Roberts 76 MGB

It's definitly blue smoke. Everything I've read says I'm looking at a complete engine rebuild. I wish I wasn't - even if I could find someone to do it, I couldn't afford it until next year, so I'll miss a whole summer of driving.
John English

John, Start with the basics. Get a compression tester and report results.

Check for spark--remove all plugs and with their wires attached, have an assistant turn the key to "Start". You have plugs oriented on the engine block, so the the threads are touch metal (You're grounding the plugs) and look for a fat, blue spark on the end of each tip.

If compression and spark test are go, then you've eliminatd 2/3's of the problems. Fuel is the last ingredient.

Find where the fuel goes into the carb. Remove that line and wrap a towel over the end. With the help of an assistant, turn the key to "on". Just to the first notch of "on" please, not "Start" as above. Trust me here. Explain this to your assistance twice, or they will start the car. "You said, turn it on", is what you'll hear as fuel pressure washes the front fender. Anyway, if you get a good bit of fuel in your towel in a brief, two second cycle of the key,(off-on-off--two second pace) you've got fuel.

Only three things needed to run the car: Fuel, compression and spark.

Please check for leaks around the gas tank and fuel pump. Line that haven't been replaced in years--if ever--have been spontainiously disintrigrating lately. Those lines should be on your to do list if the look suspect.

But here's the quick fix posibility. Let's hope. Your car is developing a fuel vacuum. Next time it misbehaves, take the gas cap off and listen for the rush of air going into the tank. Yea, longshot.

If you car fails the compression test, there is a second test of squirting oil into the sparkplug holes. This is a wet test. It will determine rings or valves. One is tough but minor (valves), the other is the big crappola.

Lack of spark is usually a cheap if not free fix.

If all three test are thumbs up, then it likely the carburator.

Think positive and you will drive this summer.

Paul Hanley

John,
A long shot, but have you checked the oil level in the carb? It may be this, or some grit induced problem with the carb.
Steve

First, let me thank you all for your responses. Here's where I'm at.

I checked compression: 150, 140, 150, 150 - I think I'm in good shape there.

I've checked to make sure I get a spark at all plugs - I do.

I've checked the carburator oil - it was low, so I topped it off according to the Bentley procedure.

I'm still get blue smoke, and no power.

I've checked for vacuum leaks with carburator cleaner, and haven't found any. My plugs look black, not grey.

I was wondering - even though I'm getting sparks from the plugs, could the voltage be low? Should I try a new ignition coil, or points, wires and rotor? I haven't changed any of those since I purchaced this, and that seems fairly cheap, but I don't want to waste any money if I'm still going to have the problem.

Thank you for your help!! Any will be appreciated!!

John
John English

Check the diaphragm in the carb. A hole will make it run very rich, progressively more so as the hole gets bigger. And when bad enough, it will limit RPM and power. This in itself will cause black smoke, but if it has been going bad for a while, may have diluted the oil to the point that it is very thin, leading to blue smoke. After it runs like this a bit, it will foul plugs, which accounts for the poor idle fixed by changing plugs.
If you replace the diaphragm, be sure to locate the diaphragm correctly in the notches on the piston and carb body. And if this turns out to be the problem, change oil, filter, and plugs if necessary. Also, check the three screws holding the autochoke on the side of the carb - if they are loose, they cause very rich running and danger of leaks and fire.
FRM
FR Millmore

FRM, if you were here, you would get a BIG HUG from a 250 pound, 6'5" MGB owner!!

I checked, and not only do I have a hole in my Zenith diaphragm, I've got a 1/2 inch slit!! Praise the Lord!! Maybe I'll be up and running by tomorrow night!! I've spent more on diagnotic equipment than the part will be!!

John
John English

Yr welome, and congrats - you already know more than a lot of folk. It's one of the few simple and dramatic fixes. Those symptoms -good cold start, poor warm running, no throttle response, and smoke - on a ZS are a dead giveaway. The diaphragms go away after a few years. You may notice that it had little pleats developed, along which it split, also if you try to put it back in, it probably is too big! The rubber seems to be sensitive to some fuels, especially with age. And spraying carb cleaner anywhere it can get on the diaphragm is a no-no. Just be sure to line up the little nibs in the notches- it only goes one way and it has to be correct. Happy motoring!
FRM
FR Millmore

FINAL POST:

Thank you ALL for your help!! I certainly learned a lot.

FRM, thank you again - I put the diaphragm in, and immediately I had more power than when I first purchased it. I ran it around some, and once the oil burned out, NO SMOKE!

Paul, I want to thank you for your long post - especially your "think positive" comment. At the time, I was pretty down, and that really helped.

Hope I can help some of you some time.

Happy motoring!!

John

John English

Yea!! Just love it when it works out. Three cheers and a beer to you for your perserverence.
Paul Hanley

This thread was discussed between 08/06/2005 and 13/06/2005

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