I'm new to the Jag fraternity, recently buying myself a 77 XJ6. She is a rare thing of beauty.

As much as I love her, a week into our relationship, she refuses to go more than two kilometres without loosing all power, back-firing, and then cutting out.

Sadly, I am no mechanic and am at a loss

 

The "could be" list is long but I suspect a fuel-related issue.

Empty the contents of the fuel filter into a clean container. What you see might dictate your next steps. Rusty or otherwise contaminated fuel is common with older Jags and doubly so if the car sat unused for long periods of time....as old Jags often do.

Cheers
DD

 

Allow me to disagree with Doug. I suspect ignition problems. The backfire is caused as:

1. The engine continues to turn and draw in the air/fuel mixture
2. The faulty ignition does not, well, ignite the mixture.
3. When the ignition cuts back in, the unburned air/fuel in the cylinders/exhaust ignites, causing the backfire.

This is similar to the "key-bang", where a mischievous teenager (usually) turns the key off while driving, waits a second or two, then turns it back on. The result is a fantastic backfire for the reasons listed above.

I think your car, as a '77, has an electronic ignition, but I would start with the usual suspects, such as:

1. Plug conditions and gaps
2. Cap, wires, and rotor - If they haven't been replaced recently, swap them out for new ones and keep the old ones as spares (provided that they don't turn out the be the problem)
3. Check for loose wires in the distributor, to/from the coil, etc.

My $0.02,

Tom

 

Of course, now that I have replied, I just realized that the location of the backfire is essential to diagnosing this.

1. If the backfire is coming out of the exhaust, see my earlier post.
2. If the car is backfiring out of the intake, then listen to Doug, as that is a sign of a lean condition caused by a fuel delivery problem.

Does your car have carbs? I do not know when the EFI was introduced.


Tom

 

Thanks for your responses. Wow, I'm really stoked that there are people out there who are keen to help, just for the sake of helping.

THis is the carb version. No injection.

 

Glad to provide advice, though remember that it may only be worth what you paid for it...

The good news is that your problem is almost certainly one of three things:

1. Not Enough Air
2. Not Enough Fuel
3. Not Enough Spark

Since #1 is awfully unlikely, that leaves #2 and #3, and they are both pretty easy to diagnose, though complicated somewhat by the fact that it seems intermittent.

You haven't provided a lot of information, and I am still wondering if the car is backfiring through the intake (noise coming from under the hood/bonnet) or through the exhaust (noise coming from the rear of the car)?

Also, now that I have re-read your original post, I am thinking Doug was likely barking up the right tree. If the car runs fine for a short distance, you might be running out of fuel up in the carbs. This could be either a fuel-pump that works OK at first and then dies (only to start working again later), a sticky float/needle in the carburetor, or debris in the tank/line/filter.

I once had a similar problem with my MGB. It would start up and run fine, but then sputter and die a half-mile down the road. It turned out that the needle was sticking in one of the carbs, preventing fuel from getting into the fuel bowls and starving the engine. If I let the car sit, it would eventually un-stick itself, allowing the pump to resupply the carburetors. Then it would stick again and I'd get another half-mile out of it.

I would consider the following course of action:

1. Check the fuel filter(s) as Doug suggested and look for debris, tiny rust particles, etc. If you find any, you need to locate and eliminate the source of the debris.
2. Open the trunk and remove the panels that cover the spare and the fuel pumps.
3. Start the car. As it runs, verify that you hear the pump periodically clicking (every few seconds or so) to replenish the supply up at the carbs.
4. If not, try switching to the other fuel tank (assuming it works and has clean fuel in it) and see if that pump is working.
5. If neither pump seems to do anything, take the handle of a screwdriver and gently rap on the lower portion of both carburetors. If a needle is stuck, this might free it. If so, and provided you still have the key on, you should now hear the pump clicking away as it refills the fuel bowl(s), and you have your answer.

Please get us some more information and I'm sure we can get to the bottom of this.

Tom

PS: If the car dies on you, I would recommend trying #5 right away with the key in the "on" position. If that's your problem, you may even hear the fuel rush into the carb.

 

You are on the money here! What happened to your MGB is exactly what's happening to my jag. I must admit that it seems worse on inclines.

I guess I will be looking into the fuel now.

I'll keep you posted

 

Perhaps it's worse on an incline because the car is working that much harder?

Of course, you should keep in mind that 85% of carburetor problems can be traced back to the ignition system...

You still haven't answered these two questions:

1. From whence does the backfire come?
2. When was the last time you replaced "tune up" items (Plugs, rotor, cap, wires)?

Tom

 

Hi Tom

I assumed that she was working harder on the inclines, and therefore needed more fuel, resulting in the sudden lack of power, and eventual cut out.

I cannot honestly answer these questions, as I have owned the car for a month now, but I can see no harm in giving the old girl new filters and fluids. Yes, and plugs and I'll no doubt look at the rotor too.

 

Well, keep an eye on it. If the popping and sputtering is from under the hood, you almost certainly have a fuel issue.

If you are uncertain when a tuneup was last done, then it's pretty cheap money to source a cap, rotor, points, wires, and plugs, and you'll have a set of spares when you're done in case any of those items ever fail. I don't know if the XJ6 still had the Lucas 22D distributor when your car was built, or if it had switched to a 45D, or perhaps even something else, but those parts should be pretty cheap and plentiful.

Besides, you never know what you'll find. When I bought my MGC, I went to replace the ignition rotor with a new one, intending to keep the old one as a spare. Much to my surprise, I lifted the cap to find a 4-cylinder rotor in a 6-cylinder car, with a set of HEAVILY worn points, to boot. Even if your ignition system isn't the source of this problem, a little attention in that area just might let you avoid your next problem.

In the long run, you should probably talk to Jeff at Advanced Distributors about having your distributor rebuilt and recurved, plus converted to Vacuum-advance if it is a Vacuum-retard unit. His prices are generally quite reasonable, and the distributor comes back better than new. Vacuum units can be pricey, though.

If the car is running most of the time, it probably wouldn't hurt to throw a bottle of carburetor/injector cleaner in the tank(s) to help clean out any accumulated gunk.

Keep us in the loop as you move forward!

Tom