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intermittant missing, backfire, loss of power

6K views 30 replies 12 participants last post by  Laser Bob 
#1 ·
I have a little gremlin running around in my 83 Esprit and so far nobody has been able to track it down. Here's what is happening-

My car starts and runs great. After cruising at 3K RPM for awhile, the car starts missing, occasionally backfires, and feels like it's starving for power. After about 5 minutes of feathering the gas pedal, depressing the clutch and gunning the engine, or other similar actions, the problem clears up and the car runs great.

This only happens at 3K RPM. Two weeks ago I was in traffic in 4th gear at 3K RPM and started having problems. I shifted up to 5th gear which brought the RPMs down to 2K and the problem went away. Last Sunday afternoon I was on the interstate, keeping up with traffic at 3K RPM, and the car started acting up. I got off the interstate and traveled the surface roads for the rest of my trip and didn't have a problem.

I've replaced the coil and plugs 3 times, plug wires once, Pertronix igniter twice, and fuel filters numerous times. The fuel pump & fuel lines were replaced last October, both carbs have been rebuilt, and Orlando Lotus did a major belt change in January. It doesn't matter if AC is on or off and the car is not overheating.

The fact that the problem is so specific to 3K RPM and goes away at other RPMs has a lot of people scratching their heads.

Any suggestions? :shrug:
 
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#2 ·
The three things required for fire are:
Fuel
Oxidiser
Heat

I believe with a cable throttle you can rule out air, which leaves sparks and fuel.
If you knew which general mechanism (Of the three) it was then working through it gets easier.

Logging lambda would tell you if it was a fuel issue, but relies on a lambda sensor and logging. Carbs generally do not have high frequency intermittent issues in a steady-state operation.
Do the missing and backfires smoothly come on and go away ? or are they totally sporadic?

Is there a lot of resonance around 3000 RPM?
If so then it is possible that there is some buzzing happening - so maybe it is related to wires or connectors rather than directly to sparks themselves.

It may be worthwhile to change seats and floats for the carbs, and understand fuel pressure.
Maybe the fuel lie is getting hot and you have vapour-lock type of problems?
 
#4 ·
The three things required for fire are:
Fuel
Oxidiser
Heat

I believe with a cable throttle you can rule out air, which leaves sparks and fuel.
If you knew which general mechanism (Of the three) it was then working through it gets easier.

Logging lambda would tell you if it was a fuel issue, but relies on a lambda sensor and logging. Carbs generally do not have high frequency intermittent issues in a steady-state operation.
Do the missing and backfires smoothly come on and go away ? or are they totally sporadic?

Is there a lot of resonance around 3000 RPM?
If so then it is possible that there is some buzzing happening - so maybe it is related to wires or connectors rather than directly to sparks themselves.

It may be worthwhile to change seats and floats for the carbs, and understand fuel pressure.
Maybe the fuel lie is getting hot and you have vapour-lock type of problems?[/QUOTE]

Vapour lock is the first thing I thought of when I read the original post.
 
#8 ·
Same thing happened to me in a Merak (three weber carbs) and it was my fuel pump wasn't able to feed enough fuel to the carbs at 3k rpm so it would cut out,

If I shifted higher and dropped the rpm's it was able to catch up. Make sure there isnt a kink in your fuel line or the filter is not plugged and replace the pump if needed.
 
#9 ·
Thanks for all the suggestions so far. Everyone including myself seems to agree it sounds like a fuel problem.

holmz said:
Maybe the fuel lie is getting hot and you have vapour-lock type of problems?
I thought the same thing. I have the fan on the engine lid on a separate switch and keep the fan on when I am driving so there is constant air blowing down onto the motor. It doesn't make a difference whether the fan is on or off

carbuff said:
Are the carbs supposed to be mounted on vibration-reducing mounts like Thackeray washers or o-rings
Good question. I'll ask around

mieczkow said:
And does it do it only under load? Have you tried just running the engine at 3 k while the car is stationary?
Thanks for the suggestion Tom, I'll check that out today. This is the reason I was asking you about Glenn's carb shop.
 
#10 · (Edited)
I'm just throwing this out there, but could it be turbo related? My car has an aftermarket turbo booster control that was on the car when I bought it (another reason I suspect the previous owner spent a lot of time at the track). Frankly I forgot it was there till this morning and will experiment with the adjusting knob (it's tucked behind the drivers seat) today, however I'm not sure if turning the knob clockwise opens or restricts the turbo. I don't know why, but the previous owner will not tell me anything about the car.
 
#12 ·
The 3k spot is near or right on the carb transition point from idle jets to mains at least according to the rev gauge (2500-3000) on my 83 TE. I've found that the rev gauge can be somewhat approximate as compared to reading rpms directly with an electronic meter. Any "leanness" caused by too small jets or dirt will be pronounced here in a stumble or cut out. It indicates its more than likely fuel/carb related. The dellorto carbs should be mounted on Thackerey washers due to vibration, this is critical as well as the even tension/spacing between flanges at these fastener points (see manual). Did you notice this problem occurring ever since the carb rebuild/refit or just rear its ugly head one day?

I would also check that the fuel regulator is allowing enough pressure to both carbs consistently. If you put your foot into it quickly, the carb accelerator pump jet may mask any problem at this rpm point, but in steady running at this rpm it may show eventual fuel starvation.

As for the aftermarket boost gauge, if this looks like a air compressor pressure regulator knob, Dave Bean used to and may still sell these kits. They can probably send you some instructions or explain which direction to turn for stock boost. If the engine still has cast pistons, using this boost kit is usually not recommended.

Later,

DD
 
#13 ·
Make sure there is nothing in the fuel tank plugging up the suction. At 3K RPM you are using maximum fuel flow and if there is a restriction it will cause a loss of power. On another car (not Lotus) the filter was dirty and under high vacuum the dirt would coat the filter. After shutting the motor off enough dirt would fall away to allow the filter to work enough at low flows. If you find it happening at exactly 3K and it is repeatable, that sounds more like an electrical problem like the rev limiter.
David Teitelbaum
 
#15 ·
If it repeats on a dyno, then they have usually a lambda probe.
high spikes in lambda, means lots of oxygen. This could be due to mis firing and the oxygen not burnt, or just going lean.
Going lean can also cause misfiring.
So it is whether it is trending lean and mis firing or a smooth plot of lambda with high spikes during the mis firing.
If it doesn't do it on the dyno, then that is also good information, but I am not sure exactly how... [stroking-beard]

Does it have a distributor cap, or is it an ECU?
I would change the cap for sure, and a timing light to see if the sparks are stable might make sense. The dwell should be checked as well. A bad dizzy can do these sorts of things.
 
#16 ·
Does it have a distributor cap, or is it an ECU?
I would change the cap for sure, and a timing light to see if the sparks are stable might make sense. The dwell should be checked as well. A bad dizzy can do these sorts of things.
The distributor cap was replaced last fall when the carbs were rebuilt, and Orlando Lotus timed everything out when it was over there for a belt change in Jan.

I'm talking with a different carb shop about bringing the car in on Monday. The first guy I went to is a lot closer to my home but didn't get the high praises the new shop is getting.
 
#22 ·
Update- I dropped the car off at the carb shop yesterday, & verified that it does not have Thackery washers. We'll see what else can be found as they dig deeper into the car & will report back when the car is done.

Thanks to everyone for their suggestions.
 
#24 ·
Interesting- would never have thought the alternator could be the culprit, but I guess it does make sense.

Had a couple conversations with the shop where my car is, in addition to the lack of Thackery gaskets they have found the fuel pump isn't the correct one for my car; all the fuel lines were not replaced as I was told; the carbs were not rebuilt as I was led to believe; and they might be able to create a fuel sending gauge so my fuel gauge will work, sans low fuel light. Plus a couple other issues one finds when the car is on a lift

So far I'm very impressed with how thorough this shop is
 
#25 ·
If not already done, insist that they replace any plastic fuel tees/connectors with metal ones! We don't want any combustion from leaks. -eek-

And the fuel pressure needs to be checked. There is a fuel pressure regulator (as Dief said) that may be going bad. Parts List Sheet 42.03A # 32.
 
#27 ·
Even though yours sounds like a fuel issue another very common problem is the wire that goes to the distributor. Even though you have upgraded to the Pektron inverter :panic: it uses the same feed wire and these fail regularly (typically the small section where it goes into the cap, but this section should have been replaced with the pertronix system , its the plug connection and longer section of the wire to and from that need to be checked out)
 
#28 ·
A-HA!!

I think we found the problem. When the fuel tanks were removed & repaired in 2013 the screen going into the fuel pump intake got sprayed with sealant & was gummed up, impeding fuel flow into the fuel pump.

That, along with the correct fuel pump, seems to have fixed the problem.
:nanner:
:coolnana:
:nanner:
 

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#31 ·
No, they were being thorough and discovered this before installing the new pump. They disconnected the fuel line going into the pump and only got a trickle of gas flowing from the tank. The hose going between tanks is OK, just the fuel pump intake was clogged. The screen was removed with a hole saw and they added a fuel filter between the tank and the pump.

I'm very impressed with them, they even created a fuel sending unit from a TR6 so my fuel gauge now works, sans low fuel tell-tale
 
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