Loss power at warm/unwillingness to rev

Good evening gents,

I feel a bit embarrassed sometimes as I keep leeching here but can not provide any help yet. I will learn more and hope to really lend a hand or some thoughts to new owners one day.

So my 1971 series 1 xj6 had just covered the first 1000 miles after I took ownership six months ago. It runs a 4.2lt equipped with twin SU HD8 carburettors.

I use it mainly on mild hillclimbs in the weekend, doing 20~50 miles every week. She has been running very smoothly since I bought her, however an issue of revving difficulty and power loss happened recently and I have to keep her in the garage for now, which is not ideal especially in our autumn here, which I believe is the most beautiful season to drive through the hills and valleys.

This problem happens only when the engine is warm/hot and it is getting worse as I use the car more. The first time when it occurred, I was doing a long hillclimb. The big cat suddenly lose power while climbing, and the engine refused to rev higher than 1500rpm even with the accelerator floored. Shifting it down one gear temerally increased the rpm to 2000 but dropped back to 1500 in 2 seconds. I then turned around and went downhill. The car had no problems making the 20 miles trip home. The second time when I encountered the same problem, I was on my way to a weekend valley drive. I had to drive the car in limp mode into a shopping centre carpark, waited for about 15 mins (I thought it was overheated) and drove home without a problem. After that, this power loss and rev hesitation problem got worse. I was only able to do a few loops around my house before the engine started to struggle. When the car is in gear, flooring the accelerator can only keep the rpm @1500~1800 maximum and the engine feels much weaker than a normal running xk engine at 1800rpm. If I don’t keep my foot on the gas pedal, rpm will drop to lower than 500 and engine would almost stall. There was no smoke. Engine was not overheated. And it could be restarted straight again showing a normal warm idling at about 850rpm. Until I put it in gear other than “P” or “N”, rpm dropped and the engine struggled again.

I searched earlier posts in this forum with filters “stalling” “power loss” “heat” “warm” “hot” to go through the related information. There are some suggestions from you more experienced owners, including the following:

  1. Electronical problems:may originate from ign amp, coil and distributor which can be affected by heat.
    I have replaced all spark plugs recently but can not remember if I did it before or after the problem occurred. During one episode of power loss, I parked the car and touched the coil and found it to be cool. Engine speed did not drop violently either. Just the engine felt much weaker at the same rpm.
  2. Dirty air filter: One PO suggested a very dirty air filter could play a role if the grease blocks the air flow badly. I had the air filter replaced and the problem was not solved.
  3. Problems with fuel system: More discussion was found on possible fuel starvation. There are two fuel filters in the car. Fuel filters #1 in boot and #2 before carbs. I am not sure about the tanks condition. The previous owner did mention he experienced fuel delivery problem due to dirty tanks, he then changed them and had new liners put in tanks with receipts shown in the folder. I tried to switch tanks several times when the power loss problem happened, however there was no noticeable improvement.
  4. Blocked catalytic converters: One owner suggested the contribution of blocked cats. I am not sure with the condition. I placed a hand over the exhaust pipe and I could feel strong puffs of gas.
  5. Oil pressure problem: In one post, oil pressure problems related to blocked breather was mentioned. I opened the breather, found thin coat of dark oil, but the F tube going to the intake flows freely.
  6. Carburettor problems: one owner had similar problems but all solved after a carburettor service.

To summarise, the engine loses power (has rev difficulty) after various driving distances – this distance is getting quite short now. I did not notice any misfire/bang or smoke when the power loss happened. It is more obvious when the car is in gear other than “P” and “N”.
It happened only when the engine was warm. The engine was not overheated as indicated by the temp gauge also by an IR thermo gun.
If I only start the car in garage to warm up the engine, such a problem has not been observed in some 20~30 mins period after the engine is operating in normal temperature. The engine can rev at 3000 rpm freely.

Best regards

Honestly, I have a '72 built identical car with same carbs and it sounds like exactly the issue I had from firsty purchase, which was down to fuel starvation. Your symptom conditions coincide exactly with when the car requires maximum fuel flow.

So check all the filters in the system, in trunk (boot), at carburettor inlets and possibly in tanks and also check the pump outputs at the carburettor end (disconnect hose and check flow rate into a can - not recommended while smoking a cigarette). This cleanup is also the easiest to do and so should be checked before any of the other stuff.

I ended up cleaning and replacing fuel filters a few times and replacing one fuel pump which was giving weak flow. But the issue is sorted and she revs cleanly now.

There should be no catalytic convertors in your car!

1 Like


"new liners in the tanks. "The material is peeling away from the inner walls of the tanks. The detritus clogs the pumps and the filters. and who knows what at the carbs.

Install an in lie fuel pressure guage. Operate the car in various modes. A nice steady 6 - 8 pounds. Just a guess at the value. But is it steady or wavering or much too low…


1 Like

Hi Fan,

first step is to isolate the problem.

Does it only happen under high power (Best example is accelerating uphill)? Then the first and clearest solution is a lack of fuel during high demand - check filters. There are filters inside the tank, switch tanks to see if pump or these filters are at fault. Then check the main fuel filter and don’t forget the small filters where the line comes into the float bowls.

Does it only happen when hot and at higher RPM? Suspect the ignition. Condenser or coil, but neither are near my area of expertise.

You don’t have (to worry about):
Ignition amplifier, catalytic converter, spark plugs if it works well otherwise but you can inspect them for signs, air filter, oil pressure which by the way is not influenced by the breather. And you said it was good in an earlier thread, right?

My diagnosis is that you have a fuel delivery issue, that is somewhere there is a restriction and the higher fuel demand of a hard working engine can not be satisfied - it will probably idle well as long as the float bowls are full, and rev up in neutral just fine.

Good luck and I hope you get to autumn driving soon, best season,

1 Like

Hi Fan, I would be checking the carbs for a blockage. I wonder if it could be the valve in the float chamber, perhaps just on one carb and then the other carb would not be able to supply enough fuel? If I read your post correctly you have checked the filters?
The carbs are an easy place to check out in the first instance!

1 Like

I would be starting at the tanks, drop the sump plug out to check that the screen(s) (if fitted my left tank didn’t have one) next I would be opening the fuel pumps my left one was full of rust particles that would stop the flow after a short time.

1 Like

Thanks to all you gents taking the time to go through the long list!
I have very limited (almost none) experience in mechanics, so I may do the diagnosis suggested by you guys from the easier/more accessible tasks, then move to the more demanding tasks.

I will 1) change the two fuel filters first once the ordered parts arrive, 2) put in inline filters between the tanks and pumps, 3) drain the fuel tanks to check the debris build up, fix that if required, 4) check the pump conditions, and 5) check and clean the carbs.

As David suggested, there could be a problem due to the ign, condenser or coil. This power loss problem happened the first time when the car was doing hillclimb. But the later episodes occurred at cruising speed of 1500~2000rpm. So I will leave this step to the last if all the fuel system checks have not improved the condition.

Thanks again for all your help

The good news is that you can rule out the fuel pumps and in-tank filters if the problem does not go away almost immediately you switch the fuel tanks.
I would think the pumps are okay internally if they stop clicking (or get close to) after a while. In that case they can build up pressure.

No additional filters yet, please. They are not needed and they don’t do anything helpful to fix the problem. Draining the tanks (there will be a bit left inside even on an empty tank!) can’t hurt.
A new main fuel filter never hurts (the paper one needs to be changed, the other versions can be washed, same for the float bowl filters and the in-tank ones)

A stuck float valve maybe but wouldn’t that influence idle as well, not fix itself on restarts and also persist when cold?

Added check:

  1. Float levels in the carbs too low. OK for low RPM. Inadequate for open throttle.

Another story:

In my teens. 31 Studebaker big six ate too much gas. Rear axle making eiire noises!! Shopped for a smaller car. Found. Nice little 34 Chevrolet tudor. Standard with a straight front axle. Not the troublesome “knee action”. Black with red wire wheel. Nice DIY interior. haggle worked. Ran nice, but the choke had to be out all the time??? Gee, what a neat little 1/4" copper line from the fuel pump to the carb!!! Off with it. On with plain steel 3/8" pipe Eureka, More power and choke not needed except on cold mornings… Lots of adventure in the little stove bolt…


1 Like


I had similar issues on a carbed car, first totally intermittently - on/off -, then gradually worsening. Initially, I put in a new coil as the negative incidents mostly occurred during hot weather conditions, with good results. Unfortunately, the issues came back and were rectified with a thorough tuning of the carbs. So far, so good.

In your case - in particular as this is a relatively new car (to you:-) I’d check the fuel issues first: have a look at the filters (clean or with residues of rust, flakes, gum, gunk, baaah?) and check function of the pumps (although with switching tanks you should have excluded this possibility; you have the stock dual pumps, don’t you?).

If the problem might be heat related, the coil is a cheap suspect to check out. What does your tach say during these incidents?

From your last description it surely looks like my own case where ultimately the rear carb was diagnosed as being totally out of whack - please don’t ask why its state changed from perfect running to this particular condition. During cold running the aux carb may compensate for some failure, but with a hot engine poorly tuned and synced carbs will give a lot more trouble than slightly suboptimal performance or increased fuel consumption.

Good luck


75 XJ6L 4.2 auto (UK spec)

1 Like

Thanks Jochen,

I may have read about your experience in the earlier posts and included it as item No.6.
I will get the filters replaced this weekend, and see if I need to move to the step with carbs. I have to get professionals to handle the carb work if required. Too easy for me to cause more damage than good.


The HD8 are the best carbs to adjust, mainly because you rarely ever need to. Almost certainly not in this case. I‘m sure it’s a fuel delivery Problem, not a fuel mixture Problem. The problem is somewhere between the tanks and the floats if Carl says so. Easy things first and report back with your findings after you have attended to the filters…

Gents, so my ordered parts arrived several days ago and I feel today is warm and bright outside to start checking the fuel system of the car.
I open the boot and remove all parts in the way until I can get to the fuel filter bowl inside the spare wheel well. I then notice this, what the hell~~~~. Just a transparent glass bowl with some tiny debries at bottom. Shouldn’t there be a paper filter in there and some mechanism to keep it in place?

Looks like the car is only relying on the little filter before the carbs to filter out all debris. So should I source a used unit from a wrecking yard to put in? What should I do now? Kind of amazed by what Peter, who is the mechanic used by PO, has done to the car.

I believe the paper filters are not original. The sediment bowl is, or should be sufficient to filter the crap out before it gets to the carbs. Of course, adding paper filters won’t hurt, but you could just drain, clean and reinstall.

1 Like

Thanks Kevin. I was surprised to find the fuel filter set up to be quite different to what is shown in the workshop manual.

So, I did replaced the fuel filter before the carbs. This is what the contents in it look like. There should be still more inside the filter.

I thought I may have fixed the fuel delivery issue and took the car for a rest run around the house. After fully warmed up, the car idled much smoother initially, however after 5 mins of driving, the fuel starvation problem reoccured. Only improvement was that the engine could rev to 2000rpm other than 1500rpm.
I got the car back in garage in limp mode. Checked the now transparent fuel filter and found very small amount of fuel in it. I left it to cool off, turned on the ignition but not cranking. Then I noticed some fuel smell and heard some water flowing sound. A quick check found the front carb leaking from the overflow tube. Hmmmm, more problems to solve~~~~~~

The glass bowl was the original pre-73 set-up and did include a disposable paper filter, Fan Y, to be replaced at regular service intervals - but it may not be readily available? The post-73 filter, in the spare wheel well, likely used the same disposable filter?

However, with the low flow/pressure of carbs, the filter is not really essential. Water and debris will sink to the bottom, and as Kevin aptly says; cleaning out the bowl is likely sufficient - it worked perfectly on my old Cortina. The tiny filters on the carb inlets are just backstops - to be cleaned as required…

The ‘T’ on top is a stopcock, to be closed before the bowl is removed…

xj6 85 Sov Europe (UK/NZ)

1 Like


things seem to sort out. Indeed, as Frank wrote, the glass bowl filter is correct for ealy SI cars. Maybe you spot a good used SII filter, as they are mounted similarly and the paper filter elements seem to be readily available (at least I was able to buy one off the shelf from one of the usual suspects not too many years ago). They look like that

Maybe it was also helpful to open the hood as I’d be inclined to attribute the loss of power to a flooding of the carb. Did you, by any chance, open the hood and smell check the issue when the problem occurred before?

Good luck


75 XJ6L 4.2 auto (UK spec)

1 Like

Thanks Frank and Jochen. The fuel filter picture in my workshop manual looks exactly like the picture Jochen attached.
At the moment, the only fuel filter has been replaced. The fuel starvation problem (at least I think it is starvation) is still here. So next step I will check the pumps. Currently I have got only the left tank half full, right tank to lower than 1/4. But the gauges are never very accurate in this car. If I switch on ignition with the left tank connected, it clicked a few times, fast to slow, then stopped. If I connect the right tank, the pump clicks fast and sound like it is never gonna stop. I may get 10 liters of petrol in it and try again tomorrow.
After that, I may move to the front carb overflow problem. A lot of earlier posts suggest it is due to a stuck needle valve or a floater problem. I need to go through some more reading and youtube videos before touching the carb.


You may have two different pumps, Fan Yi - but the likely cause of the ‘continuous’ pumping may indeed be a leak. Preventing pump reaching the required pressure to shut off…?

But as another thought. If a carb is overflowing, or with excessive fuel level; the stumbling and starting differences, particularly with a hot engine, may be caused by overfueling rather than starvation. The symptoms of ‘too rich’ and ‘too lean’ are overlapping…

xj6 85 Sov Europe (UK/NZ)