Outer Limits 4x4 Board

My mate's 1989 Maverick petrol engine has a constant miss at idle. It has a pionts distibuter . History since he had it is. The head blew had that redone. Tuning. New plugs, leads, filter. The mechanic who did the work can't solve the missing problem. It does it on LPG and petrol.
Any suggestions?

constant miss ....... ,
Questions

is it the same cylinder or does it change from cyl to cyl ?
to test, let it idle and after a couple of mins "feel" the exhaust manifold temp at each port.... a cooler port would indicate a single cylinder problem..... ( best done with a cold motor)

have you done a compression test and is there any large variations ?

has the distrubitor been looked at .....
a bent distributor shaft can cause a miss at idle ( very prevelant in V8s)

any air leaks in either the inlet manifold or carby/LPG setup ?
to test lightly spray "Areo Start" around joining surfaces and carby/LPG setup...... an air leak will increase revs slightly for a secound.

electronic ignition module..... try swaping it with another unit that is working properly on another vehicle

Kingy

Burnt. bent or missadjusted valve....

take it to a mechanic with a tune up machine with an osscilliscope and a 4gas aniliser they should be able to tell whether its a single cyl or not or a vacuum leak or a dizzy problem easy.

possibly a crack in the bakerlite plate , under the dizzy cap !!!!

Thanks all, I'll pass it on

while you are checking the dizzy, bent the rotor button up about 3mm with a screwdriver. makes a world of difference, and just may fix the problem.

Wendle wrote:while you are checking the dizzy, bent the rotor button up about 3mm with a screwdriver. makes a world of difference, and just may fix the problem.

Wendle, my mate did what you suggested and reports it runs much better. only the old slight miss now. Some else told him it was a fault in that model and one chap had something welded to the distributer rotor to make it higher on the tip. the bending the end seems an easier fix.

i found this problem while running lpg

the gap between the rotor and the cap is too big

there is about 6mm between them you need to close this gap about 5mm

this will fix many problems eg missing under load, rough idle, fuel econ, coils burning out.

to fix this i used a piece of stainless sheeting cut down to the width of the rotor and enough to pop rivet and give me 5mm overhang from the original rotor..

just pop rivet the new piece on top of the rotor and make sure it does not touch the cap.. this took a little grinding to get the right shape but it was well worth it.

also if running lpg close the plug gaps from .8mm to .6mm