During acceleration a few days ago, my ‘72 XKE started losing power and I limped home. It took about 5-10 seconds for the power to gradually fall to a low value but no backfires/misfires. I hear the non-return valve singing a bit, the car starts fine and idles ok with no misfires or backfires. It just has very little power, barely enough to slowly move from idle and move the car. Rpms move up very slowly past idle with the gas pedal but can go past 2000. I dare not go more before I know what is causing this.
I got the car on the lift and looked at the timing - about 12 BTDC at idle (750rpm) going to 20 degrees or so at almost 2000 rpm. Somignition seems ok (SNG Barratt ignition conversion).
While the car on the lift, I noticed that the left (driver side LHC) exhaust is cold while the right side gets hot (after about 5’ at idle). No combustion on the left side would definitely lower power!
I am thinking fuel starvation on the left side? What could do this? I will change the distributor cap & rotor and the fuel filter just to see if these can help. After that, I will clean the non-return valve and look into other fuel obstructions that can starve the left side.
I had filled the Stromberg carbs with the prescribed oil recently as well. Is there any such thing as too much or doing it wrong?
Am I missing anything? It’s a strange problem! Any help would be welcome! Thanks.
I did that. Exhaust on left was a tad warm at & after the crossover pipe. I attached one IR image of the header area just forward from the crossover pipe. Right in the pic is the left side. Second pic (in next post) is an IR image of the exhaust after a few minutes of idle (from mid point of car looking forward). This shows the exhaust is clearly cold/different on the left side. Area near the left headers are cold while the right is too hot to touch.
Hi Bill, I’ll check but since the left header is cold, the issue seems above all about why the left bank seems to not have any combustion.
Another clue that nudges me to a left side fuel starvation issue is that I don’t seem to have a smell of unburned gas exiting the left side if this was an ignition/spark issue only.
If three cylinders are not getting warm it is likely not a spark problem so I conclude that proper fuel/air mixture is not getting to the cold cylinders. I would not replace any ignition components at this time. I would start looking for a fuel problem
And since the three cylinders are fed by one carb, I think there is some sort of problem that is stopping fuel or the rubber diaphragm in the carb is broken which gives an air/fuel mix problem
I suggest to check the rubber diaphragm of the offending carb. Torn or holes in it.
If it is fine, I suggest to take off the carb on that side and 1) See if when off there is fuel in the float chamber. Then 2) take out the float chamber fuel valve and clean it and blow air through it checking there is flow when open and no flow when closed. Then test that the float swings freely and floats (ie no full of gas)
Francois - so far you’re getting help from 3 guys with 6-cylinder engines.
I know so little about the 12 cylinder I can only speculate but since you do not smell unburned fuel I would suspect a stuck fuel valve in the carb bowl. Do your pictures suggest that the manifold not warming up promptly is from 3 cylinders (one carb) or 6 cylinders (2 carbs)?
Hopefully a 12-cylinder owner will get engaged here as I am out of my depth.
I wasn’t clear on this…sorry. This is a 12-cylinder engine with the B bank having 2 carbs. To me, both failing and needing work or rebuild seems improbable. It must be something affecting both.
I agree that making ignition part swaps may be focusing on the wrong area.
I’ll change the fuel filter but of course this is common to both banks. I wonder if there is a good way to check the fuel pressure at the carb area. I forgot to add the the linkage to the carbs seems fine and unobstructed. They seem to open fine and in parallel to the right side.
Good point. I’ll check upstream of the merge from the left side exhaust side to see if there are some cylinders undergoing combustion. Since the left exhaust manifold to muffler interface was cold, I doubt it. Even a few cylinders firing that side would have warmed things up.
Stupid question but I’ve done it so I have to askt!!! Are BOTH carburetor linkage arms attached to the Throttle tower at the back of the motor? I forgot to hook one up and the result was embarrassing, to say the least!!! Is all the throttle linkage connected? What was the last thing you did to the motor before this “left side cold” situation happened?
Francois
Thanks for clarifying.
I do not know the V12 so cannot help to troubleshoot it, but you are on the right track to look for a problem that affects a whole bank
Dennis
Yes I checked. Linkages seem correct. Didn’t do much if anything to the car for a while. Problem occurred while driving. I was accelerating harder than usual (carbs opened up quite a bit) and the power loss started then and 5-10 seconds later it became like a limp mode. No backfires or such thing.
The only other thing I feel may have happened is the intake manifold vacuum failed (big leak). I am not up on this engine but is the intake manifold common to both carbs and a 6-cyl side? Would a loss of vacuum from a leak stop pulling fuel/air? I’ll also measure vacuum. Is there a good/convenient place to attach a gauge?
I recognize you have reason to believe the left side is a problem and right is okay. But I am struggling with this smooth idle with the left side doing nothing. It also sounds like when you accelerate, it is smooth, just no power, only 2000 rpms. A limp home mode as you said. I would think even if the left side was out, the right side would get you over 2000 rpms, and it would be rough as can be.
Assuming I am understanding correctly, I would be looking at all the throttle linkage first, then something like majorly restricting intake or as said before exhaust. But maybe it is not idling as smooth as it sounds. Just a thought.
Tom
I would look at the rubber diaphragms inside the carbs on the bad side.
It takes minutes.
Yes, it would be a coincidence that both are torn, but memory tells me that a Stromberg will idle with a torn rubber diaphragm but run terribly at higher rpm.
But V12 troubleshooting I have zero experience with…
Dennis
6 cylinders on his car!
I would pull off the “B” bank air filter and shine a light down the intake and give a few blips of the throttle. This would confirm gas is getting to the carbs, plus the damper is actually working. JS
Changed the fuel filter. Not much debris in the old one. No change.
Cleaned the no-return valve per manual. No change. It “sings” when the electrical is on. Fuel pump clicks at 3-4 clicks per second.
Below the non-return valve I separated the fuel line and purged it for as few seconds. I then connected a pressure/vacuum gauge there to see the fuel pressure. It quickly goes to +2psi and steady. I read this is about right for the carb fuel inlet pressure.
I connected a vacuum gauge on the hose going to the reservac (rear of the driver side rear manifold). Idle vacuum was -12psi, going to about -15psi at 2000rpm. Blipping the throttle for idle dropped the vac to close to zero. Lifting the throttle at around 2000 rpm saw the vacuum go up to -20psi or more before returning.
Not sure if the vacuum numbers and behavior are in line with expectations, but it does seem that the fuel and vacuum are reasonably working. Still sluggish….
I’ll do that later today or tomorrow. Dumb question but what would I look for when I am looking at the carbs that way that would confirm gas is getting there?
Just to be technically accurate, I think you’re mixing up psi and inches of vacuum.
One atmosphere is about 14.7 psi at sea level. It’s impossible to have a vacuum higher than that…
