'86 XJ6 Shaking At Idle

Sorry ignore all the carb and points stuff. I just saw 86 in the thread title. I did look but didn't see year in your post and the text size on an iPhone is borderline for me these days!

Hello and welcome. Can't argue with a free car. What year is your Jag?

You say it's smooth at 800 rpm but when it's idling lower it shudders. When it's shuddering and you carefully lift the revs, at what rpm does it smooth out? If it's something like 700-750 and you are really broke I would start by raising the idle. Then I would remove the plugs and inspect and clean them before I thought of buying new ones - it's what we all did in the old days before we got rich. Remember to check the insulators for unusual staining or cracks as you clean the outside. You can check the gaps - thumbnail thickness is about right if you have no feelers. The center electrode should be square and if you have pliers to fold the side electrode away you can dress the centre square with a fine file and remove any pitting from the ground electrode before folding it flat and parallel again. It's tedious but cheap and you will have saved ten dollars to put in the fuel tank or buy some leather food for the interior.

I would also look under the bonnet someplace dark at night and look for blue corona discharges indicating an HT wire problem. It may be that a good wipe down and rearranging the plug wires helps - also free. If you invest in another can of anything make it WD40 (or a cheap equivalent as it's only 95% kerosene anyhow). spray the wires and distributor cap after you clean and check the connections. Don't omit to remove the cap and clean up the likely corroded posts inside with a sharp-edged screwdriver blade or similar - just scrape until you see shiny metal and then blow/wipe the crumbs away and spray with WD. After 1994 they stopped using plug wires and distributors.

If it's a carb car then possibly it still has points ignition. Today they throw them away and fit new, just like plugs, but we used to clean and dress them. I've cleaned up corroded points by taking them apart and rubbing the faces on smooth concrete when I was a kid, or the side of a box of matches. You might have a fine file or piece of wet and dry sandpaper.

If your car has carburettors you can take the float bowls apart and clean them. Put a drop of engine oil in the dampers if they are low after standing, and check the air filter element which can be given a new lease of life by tapping the exterior face down on a hard surface and maybe using a vacuum cleaner to remove some of the dirt from the external folds. Squirt some WD on whatever type of throttle opening mechanism you have and look around for signs of loose wires or tubes - your eyes and brains are priceless as well as free. Look for weeping or cracked/bulging water hoses that might cause trouble in future and tighten any loose clips and plan to replace bad hoses or frayed belts sometime.

You may be a poor kid at college but you will also learn a great deal from your fine car that a modern one with too many computers could not teach you - and there are no tuition fees! Look in the Shoppe here and equip yourself with the relevant textbooks for this course in classic car upkeep. We look forward to grading you straight 'A's in enthusiasm, persistence, inventiveness and money saving. You can do a huge amount by improvising and refurbishing instead of throwing new parts at the car.

Pete

Agreed on checking for vaccum leaks and basics such as spark plugs, etc.

Many places for vacuum leaks, You'll have to be thorough. Among everything else carefully examine the ducting between the airflow meter and the throttle body.

Partially clogged injectors can cause a rough idle. I recomend removing them for professional cleaning. All three of my Jags have had some sort of idle problems. Having the injectors professionally cleaned has aways helped.

Make sure the throttle body is clean and the throttle blade is set at the proper .002" gap.

Click the "Rough Idle Checklist" list below for more reading on the subejct.

Cheers
DD

david when it is cold it is using the cold start injector. I suspect a vac. leak so that when at speed it is overcome by the engine but at idle is running a bit lean.

Do all the simple little things first like change the spark plugs and plug leads. If this does not help, then you will need to spend some money on diagnosis, or rum on premium gas until you do to minimise the effects of what ever ails the beast.