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MGB-GT Erratic and Sporadic problem with '74 GT

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BoneIdle

BoneIdle

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Well, the GT made the 6 hour trip from Mechanicsville, IA to lovely Winona Lake, IN back in late March. Prior to the trip, the car had been down for about a year while I figured out what to do about replacing the gas tank.

I ( and yes, I mean "I") replaced the gas tank, then the fuel pump ( it was leaking and I didn't have the nerve to drive 300+ miles that way), removed and disassembled the carbs (SU HIF4 X2, reinstalled, etc.

I had no problem starting the car, I adjusted the mixture and the idle - the thing was running like a top. I drove about 20 miles the night before the trip and the engine was running smoothly.

On starting the road trip, I drove approximately 15 miles before I began hearing and feeling small pops in around the resonator. This wasn't a big deal, but about the time I reached the middle of Illinois, as soon as I decelerated on an exit ramp, the engine was misfiring. I could keep it running,but there was no power - it almost felt and sounded like a fouled plug.

There were no symptoms ,such as increased engine temp, decreased oil pressure, and oddly enough, no exhaust smoke ( that I could see).

The problem seemed to resolve itself after the car was switched off ( e.g. at a service station), or when it was being driven at a steady speed ( 60 mph and corresponding RPMs).

So, this occurred pretty much every time, I'd slow down.

As soon as I could get the unpacking under control, I began working on the GT again. Once again I pulled the carbs ( this was yesterday). Everything was relatively clean, so I reinstalled, readjusted, and drove up to the convenience store to fill the tank. No problem.

I decided that I was hungry and there was a McDonald's about 1/4 mile away, so I drove there. No problem.

I was sitting in the drive thru line for about 5 minutes. Big problem. The misfiring began. I could barely keep the car running.

After a couple of minutes of revving the engine, I was able to drive away, but it was still as if more than one plug was fouling.

As I headed reached home ( about a 2 mile trip), I noticed that the engine was running normally again.

I've about given up on the idea that this problem has anything to do with fuel delivery.

Does anyone have suggestions? BTW, this is a standard ignition system, not Electronic.
 
Start at the coil clean the connections then move to the distributor Check the cap and rotor if good move to the points and check/set points if good then a new condensor if it still continues on new plugs and wires. Check all the wiring as you may have one chaffed.

If you need ignition parts jeff at advance distributors seems to have a good supply of quality parts for the 25d distributors. Or send it in for a rebuild and have a distributor thats better than it ever left the factory in your car.
 
Thanks for your suggestions, Stewart.
I've replaced the condenser, the rotor, checked all connections from the coil to the distributor, and still no change in symptoms. Today, I'm pulling the distributor. I have a feeling that there might be a bad wire or loose connection in there. I figure I might as well pull the distributor since at one point ( before I got the car), the heater control valve had leaked on it, so some cleanup might be in order.

I'm just not seeing a slew of other possibilities. If it were the coil, which has gone bad before, I wouldn't expect the car to start. The points look good, and as I've said, the condenser and rotor have just been replaced.

If anyone has any other suggestions,now's the time, especially if they involve pulling the distributor.
 
Sounds to me as if on over-run (boot out of the throttle at speed) or idle you have too much fuel entering the holes. Plugs foul then clear. What did you use to determine the mixture adjustment?

IOW, I kinda think if it were the iggy system it'd be happening all the time, not as it's described. Just my WAG.

BTW: Glad to know it made the trip! :savewave:
 
Well, I used the manual to determine the mixture ( Haynes - Weber and SU).

Here's a breakdown of yesterday's fun:

Changed the condenser.

Started car and let it idle while moving the truck out of the way.
Drove to Autozone ( figured I'd get a spare set of points). The distance was approximately 5 miles.

Went in. Looked around. Figure 15 minutes.

Started car and left.

Drove approximately 1.5 miles @ 35 m.p.h. and the problem occurred.

(When this thing happens, sometimes there's a burp, or a miss, but other times the sound of the engine is similar to the sound of a diesel tractor's exhaust when it's shifted to a lower gear to decelerate.)

After struggling to keep it alive through two traffic lights in downtown Warsaw, I hit a 35 m.p.h. zone once more. I managed to get the car to about 40 ( okay, so give me a ticket) and the problem, while still present, became less pronounced (i.e. just burping and missing).

So, at this point, all I know is: it occurs after the engine is warm

It could be the mixture, and I'll gladly adjust that again, but if it's not the mixture, I don't think there's a lot left in the fuel system that it could be.

It does seem to survive long trips, doesn't it ?
 
Sounds to me like a fuel delivery problem...like too little fuel in the float bowls at times. Could be sticking float or floats....could be an intermittent fuel pump issue...could be trash in the fuel line...

Just my guess....
 
The problem seems to be resolved:

I adjusted the mixture once more, this time running the jets all the way in and out before I started the adjustment and that seems to have corrected the problem.

I'm not sure what was going on. It's possible that I wasn't gauging the height of the jet well enough before.

at any rate, I took about a 15 mile drive through rural Kosciusko county and experienced nary a burp or worse.

Now, I have to adjust the idle a bit - and clean the interior of the car.
 
....and mebbe treat it to a new set of plugs. :laugh:
 
Yeah. What's the latest, best-est plug for the little critter?
 
Don't fuss with success. RN9YC Champions for my money. :wink:
 
Arrgh! I spoke too soon. However, this time I had the opportunity to observe a bit more.

The tachometer keeps pace with the throttle action and when the problem occurs, it doesn't indicate that anything is wrong - engine speed is steady according to the tach.

The problem started about 100 feet from the driveway this time and continued off and on until I returned home ( ironically, the trip was to the BMV).

I do occasionally pick up a whiff of gasoline, but whether it's relevant or not...

There's no discernible frequency to the problem.

Got to be something simple, but what?
 
Not likely the problem but be sure the coil is wired correct. It will run with the coil backwards but some cars will cut out at odd times when it heats up. The negative terminal goes to the distributor on a negative ground system. (white with black tracer?) Bob
 
Today, I changed out the points.

On the test drive, while turning onto US 30 via a sharply curved ramp, as I accelerated at the end, I noticed a minor version of the symptom occurring, but only for a period of about 5 seconds. After that - nothing.

I did look carefully at the distributor cap and there is considerable wear on the inside, so I'll replace it tomorrow.

Other than that, there's been nothing that's waved a flag at me.

I think the coil is correctly wired,BTW, at least as far as I could see.

Sueggestions?
 
I think we're done, but of course, I'll be driving around waiting for a recurrence.

I wish I could say that I found a piece of grit the size of the Hope Diamond, or that one of my floats was welded into position, but I can't.

After changing points, rotor, condenser, and plugs ( I'm waiting on the distributor cap) and having no success, I removed the carburetors, tore them down as thoroughly as I could, cleaned everything, adjusted he float heights, reassembled them, and reinstalled them on the car.

So far I haven't had a burp. My fingers are crossed.

Ironically, I remembered where I'd seen the symptom before: it was in Iowa, after I had replaced the float in the rear carb. I had not been careful enough when i reinstalled and the linkage to the rear carb wasn't quite connected. The engine ran the same way as it has here, in the last week or so, except for the fact that it was idling normally ( there was gas in the bowl).
The upshot is that the symptom points to a cause of running on one carburetor.

I know several people have made the statement that "these cars don't like to sit." Most recently, I heard it from Dr. Entropy.

Heeding that phrase, I decided to check a few other items out before putting away the tools. I discovered that I was almost out of brake/clutch fluid, my battery had grown interesting crystals on its positive terminal, and that my thermostat is apparently sticking since I'm coming up to probably 180 and never dropping ( that's what I get for replacing the temperature sensor).

Now, I'm looking forward to a nice drive where all of my woes will have migrated from beneath the hood to a location 10' behind my rear bumper ( "Dang Mabel! You ever seen such a puny car?").
 
Yay. Pleased to learn it seems to be cured. You're thorough so the carb R&R/cleaning has likely removed whatever it was. "Stuff" builds up on piston/chamber, jet/needle, needle/seat. A combination will cause grief and not be immediately noticeable.
It's fixed. Don't second guess! :jester:
 
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