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'72 Midget Runs On 2 Cylinders at Highway Speeds

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Hey everyone. I recently got my Dad's '72 Midget (the one with all the switches and buttons) running after sitting for a few years. When I originally parked it it had a running issue that I didn't have time to delve into. I got it running again, and other than a high idle, it runs great even after sitting for a few years. Thread here:

https://www.britishcarforum.com/bcforum/ubbthreads.php/topics/821527/1

The running issue is back, but only on the highway. Sometimes it runs fine, but then usually after a while the power drops off and it sounds like it's only running on 2 cylinders. It also seems to start to happen when going up hill. My first thought is debris in one of the carbs or float, but that would mean that it somehow got past the fuel filter. My other thought is debris in the tank obstructing full fuel flow into the pump / line. The pump sounds strong and runs continuously.

I've tried to duplicate it off the highway by running high RPM's in second or third gear and have had no luck, which stumps me. If it was strictly a fuel flow issue, then wouldn't I be able to duplicate it at slower speed, but similar RPMs. The load on the engine is less, but the fuel flow is similar to a highway situation.

Edit: New Sport Coil, electronic ignition, and spark plugs were installed when I got it running.

Any collective wisdom you guys can pass on would be much appreciated. I just don't want to go down an unnecessary path in the troubleshooting process. Thanks in advance.
 
Coil when running hard? or perhaps needs points changed out. See what the collective wisdom of the list says.
 
The high idle makes me think it is a carb problem, perhaps leaning at cruise for a monster flat spot? Have the carbs been tuned to fix the idle problem yet? I would try to clean the carbs and get them in the ballpark first. Also I would like to be assured that there is not a fuel flow or pump problem as well. Should the pump run continuously if it is a stock pump? IIRC an SU pump will run until pressure is built up and then stop, at least it did on my old XK-120, before you turn the key to start.
 
I would check or just replace the points and condenser if they are old. Might also want to do the whole job and replace the cap and rotor and plug wires if they are old. Use an MGB cap and MGB wires so you have the advantage of resistance wire for less electrical noise in your radio.

Going up a grade in 4th requires that you put the engine under load in high gear. This increased stress and increased cylinder pressures can show up a problem with a weak ignition system that might not be revealed in other conditions.
 
Thanks guys. I initially suspected an ignition problem and replaced the points / coil with a lucas sport coil and electronic ignition. FYI, it did the same thing before I parked it so I don't think it's related to the ignition...

The fuel pump I have runs audibly almost all the time. When I brought the car home I thought it was odd. I asked my British Car guy if that was normal and he said yes.
 
What is the brand name on the pump. Some run all the time some do not.

Lucus does not, only when moving fuel. Pump weak and will not move enough fuel under load. Check the pump first. If Lucus and it runs all the time it is bad.
 
Yeah check pump and fuel filter.

Remove fuel line to carb and place over a container. Turn on key. It should pump at least a pint or more in a minute.

Also, many of these cars end up with unvented gas caps. Remove gas cap and drive car. If it runs better, you have the wrong cap. After that, I'd look at each fuel bowl and see if there's a lot of dirt. My brother's Midget did the same thing and the back carb bowl was very dirty.
 
Is the wiring harness stock and undisturbed?
I know your dad did a lot of custom wiring, so that's why I ask.
I had a problem with the wiring to / from the distributor that I only found by taking it all apart.
Also, does the new ignition stuff you installed not want / need a resistor but you still have one installed?
There are a number of possibilities at this point and you will need to diagnose / investigate one system at a time to find the culprit / culprits.
 
Thanks guys. There could very well be an electrical issue, but that is a 55 gallon drum of worms I'm scared to crack up open.

I'll do the fuel pump test tomorrow and check the float bowls along with the gas cap and report my findings.
 
Alright guys. I couldn't see any name on the pump, but don't think it is a Lucas. Although it is brass colored, which matches my Lucas Sport Coil. It pumped about a gallon in a few minutes. I let it run for a bit just to make sure there wasn't a contaminated filter clogging things up.

I replaced the filter upstream of the pump and the most rust colored fuel I have ever seen came out. It almost looked like brown clay. I also replaced the recently installed filter just upstream of the carbs. I cleaned the float bowls too, which had quite a bit of debris and a lighter rusty sheen. After a thorough cleaning and vacuuming out I filled them full with carb cleaner and hooked everything back up to get to the jets.

I should note that the fuel line and filter I replaced just upstream of the pump are 1/4" not 5/16". It was all the auto parts store and I wanted to see if that made a change. I'm gonna get a 5/16" filter and line in the interim.

Took it out for a drive. No change. I still think it's fuel. Thinking I need to drain and flush the tank next. And get the carbs tuned. It would not surprise me if there is small debris in the jets that could be causing it under high loads.

This is odd, but my dad had at one time installed flame thrower that was fed off of a T in the fuel line just outside the tank. There is a shut off valve at the T that is closed to the flame thrower line. I can't hear any leak, but if the T or the shut off valve do have leaks air could be drawn in by the pump reducing its efficiency.

I had a bad ground one time in my BMW E30 that presented similar, but with the obvious contamination I think I should keep chasing fuel problems.
 
Carefully remove the dampers ("behind" #1), slides and needles (#6) by unscrewing the two damper screws.

When you do this you will be able to see straight down into the jet (#9 and #10).

Remove the float chamber cover with float (#12) and needle/seat assembly (#14). There are three screws holding it (sounds like you already had it off).

Blow straight down throught the jet with something like WD-40 (using the "straw" on the can) The pressure should cause a "geyser" in the bottom of the float bowl. If there is stubborn dirt in there, you may need to use compressed air (but don't use *too* much pressure).

Do this to both carbs.

This procedure generally cleans out the jets pretty well.

SPM-018_1.gif
 
Flame thrower!?! fuel "T"?!? What other James Bond tricks did your dad install?
 
Thanks for the diagram. I thought about doing this, but got scared at digging too deep in the carbs. When I remove the damper and needle is it all self contained or will oil leak out? Just so I have a heads up?

Ha, my Dad had all kinds of crazy stuff hooked up. The flame thrower turned out to be a little too dangerous to keep hooked up, but there is still the plumbing, electronics, blower, and A/C power lines in the trunk. He also has a smoke generator that runs into the exhaust using an old coolant expansion tank and windshield washer pump. That I plan to hook back up some day.
 
If you pull the piston and damper straight up together, the oil will stay in and should not leak out. Sometime the damper will lift off without the piston and sometimes they'll tend to come off together...no big deal either way (just be careful not to bend the needle).

Otherwise, if some oil spills, no worries. Just top it up when you reassemble.
(I'd suggest using a light motor oil such as 5W-20 or even 3-in-One oil.....but you'll get a zillion opinion on this)
 
If very cold weather then lighter oil, if hot weather then a bit heavier.
Most will say whatever you have in the motor should be fine.
 
Alright guys. I'm thoroughly stumped. I pulled the dampers and needles and reverse flowed the jets with carb cleaner and very light air. It geysered into the float bowls. I didn't see any debris, but I sucked out all the carb cleaner from the bowls and re-cleaned them.

I also installed a 5/16" in line pump upstream of the filter and a new 5/16" filter.

I made sure the tank was full and tried running it with the gas cap off and on.

I went for a test drive and this time it started running like it was on two cylinders almost immediately. I didn't mention it, but last time it started running like it was two cylinders a lot earlier in the drive. Also it never stopped. I barely got home and even in the garage after multiple starts it still idled like it was on 2 cylinders and would barely rev at all.

Also of note, was that when I first started it I didn't completely tighten the hose clamp from the carb to the line and it presented a similar symptom during idle to what I've been experiencing on the road.

I tested the fuel pump again and it pumps plenty of gas. I was wondering if it was failing in such a way that the PSI isn't high enough to overcome the pressure in the system. I'm not sure what pressure that is. Is it possible that it is pumping fine when there is no pressure, but when the fuel line is closed and pressure builds it isn't strong enough to get gas in the engine?

The fuel pump is a Napa pump, not Lucas. My local British Motoring shop stocks the solid state replacement pumps, but they are spendy - $250. A replacement Napa pump is like $50. I'm tempted to replace it with a cheap Napa one just to rule out the possibility of failing like I questioned above.

If this isn't the case I'm going to start ruling out fuel and chase other issues. Carbs? Vacuum?

Does any one think the vacuum advance to the distributor line could be a culprit? The fitting that attaches to the carb side of things is easily removed without much pulling. Would a vacuum leak there cause my issue.
 
Pump needs to be low pressure. 3 PSI Max
 
Would that be the result of prolonged exposure to too high of a pressure causing the springs in the needle assembly to fail. It ran without issue for quite a few years on that exact pump.
 
It could be that you have a needle and seat that is worn out and not sealing. If they are old, might as well replace them.
 
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