Hi

I have a 76 xjs v12 that has an issue between 2500-3000rpm. Its more noticeable when the car is cold, and the car will backfire if put under load. Once it warms up its less noticable, but it still lacks power between 2500-3000rpm. Outside of that 500rpm its fine.

I originally thought it was a fuel issue (and I still do) - the tank was filled with rust. I gave it a good clean out, changed the filters multiple times, replaced the fuel lines, installed a pre filter. It helped, but still not perfect.

The bogging/misfiring/whatever you call it occurs while driving as well as with no load on it in neutral. You can hear the exhaust note change quite drastically as soon as it gets to 2500rpm.

Any ideas appreciated. Thanks

 

Could be fuel injectors. Is the dizzy mech advance working. Plugs cap and leads in good order. We have some pre he ignition parts if u trace it to a particular component.

 

The fact it is OK outside of that small window, I am thinking a dead spot in the Throttle Switch.

I have notes somewhere in the library on testing that switch, and the segments that make it do what it does.

If you need them, let me know and I will find them.

Nearly forgot, WELCOME. When time permits, please do an intro in the "New Members Area".

Also I would add NZ to your location, coz I first thought you were in Hamilton in Victoria, on this side of that ditch.

 

How would I test the dizzy mech? The vacuum hose to the dizzy hold good pressure. Plugs recently replaced, leads are prob a bit old but I can't see and sparks in the dark, and I would have thought that would cause issues throughout the rev range. Same for the fuel injectors, I thought issues would get worse at higher rpm if it was injectors.

 

Thanks for the welcome, I'm new to the jag world. Will change location.

If you could flick the info though that would be great.

I have ordered a fuel pressure guage, I still haven't ruled out a fuel issue. there's still a bit of crap in the tank and it clogs filters a fair bit which makes the problem a lot worse. Also it seems to be getting air into the fuel system when not running... Can't figure out why.

On the other hand if it was fuel I would have thought the bog would get worse in higher rpm and change under load. Maybe I have two issues...

 

Wifey just made an offer I should not refuse, so I will dig that stuff out when I get home later tonight.

You got a few issues me thinks,

 

Here ya go.

Ask away if that dont solve some of the issues.

The crud in that tank/system worries me a tad, but I agree with you that if the main filter and/or the basket filters in each injector are clogged, then the high end would be near impossiible to attain.

PreHE throttle switch testing and resetting.doc

 

Remove the cap and see if the rotor arm moves. It should move about 15 degrees approx.

 

With the cap off, if you "flick" the rotor (anti clockwise) it should move a bit, then spring back............

https://www.youtube.com/watch?v=mNOi...ature=youtu.be

 

Thanks heaps for all the input!

I think shes fixed now - may still be bogging a little, but i may be in my imagination, i need to take it for a proper drive.

I pulled one fuel rail off hoping to find the injector filers mentioned above, couldnt find them, so put it back together. Replaced one seriously cracked injector hose (yes, i will do them all, next thing on the list). Pulled the dizzy cap off, sanded the points, checked the advance (was the same as in video). Cleaned the throttle switch (I could not hear any ticking outlined in the word doc above).

Runs much better now. The dizzy cap is cracked in multiple places so i have ordered another one.

Not sure if I still have fuel issues or not - when i pulled the fuel rail off there was no pressure in there at all and just a little bit of fuel dribbled out.

Anyway, cant thank you all enough for your advice.

 

You have points? I was pretty sure all these were electronic ignition. My 78 has Lucas Opus with a Crane conversion. I had to clean my injectors, prior to that t actually ran better foot to the floor than it did at part throttle. When you get that pressure gauge check pressure at the rail, I thin spec is about 30 psi, I set my pressure regulators higher than that, closer to 40, and it seems pretty happy there.

 

AHA, the mud clears slightly.

Cracked cap, not good when diagnosing running issues.

Cleaning the contacts inside the cap, and the the tip of the rotor is not recommended, as that rotor tip only clears the cap contact by an air gap clearance. Too much clearance here and the cap contacts will arc up even more than normal. OK, as a stop gap to try a diagnosis, is acceptable in my opinion.

When and IF, you replace that rotor, have a look at the underside of the heel of the one you got. It has a magnet imbedded in it. Some are "round" some are "square" in shape, and that shape is dependent on the version of the trigger board fitted to the distributor. Make sure you get the same as you got, and they are hard to find, so be aware of $$ for one.

 

Fuel pressure tester turned up, was faulty. Just got hold of a replacement - fuel pressure was a touch low so started adjusting the fuel pressure as per the suggestion above... One regulator looked brand new, the other not so much... After adjusting the old one it began to leak and i found the thread was stripped and it had been boogied up. Replaced that and it runs even better now... Have new dizzy cap and leads still to go on.

One thing i noticed is the fuel pressure drops to 0 within about 10sec of switching off the engine... is this normal or is the fuel rail supposed to hold a certain pressure?

Thanks for all the help so far!

 

Nah, they drop off fairly quickly.

The books etc state to hold XX pressure for so many seconds/minutes, but none of mine ever have, except the HE which held 5psi below run pressure for an overnight sit.

That rail/s will prime up as soon as that ignition is in the ON position.

Dont forget those fuel hoses in all the excitement, as that thing will ignite is less than a heartbeat. I have redone 15 in my time coz the owners did not pay attention. NONE of mine in that lot.