PDA

View Full Version : Dropping a lung on idle



FROSTY
24th December 2009, 18:10
I bought my boik on the understanding it had an interesting characteristic/fault.
86 XJ750
at idle and up to about 2k it regularly drops a lung.
Its always number 3.
Ihavent yet started to haul things apart but I have fitted a new plug and chopped the plug lead back 6mm so its got clean copper.
The XJ750 has 2 coils soI figured if its a coil itll drop 2
Im thinking holey diaphram but cant work out why its fine over 2k.
one thing i notice is that no3 feeds the vacume tap.
Thoughts?

Subike
24th December 2009, 18:40
have you checked the idle jet?
at idle, try spraying a little eazi start around the inlet rubber that could be cracked ,
is no3 plug a lighter colour due to running lean?
Have you tried swapping the plug leads with no 2 to see if the miss moves
Is the float level in no 3 carb right.
Just some suggestions

riffer
24th December 2009, 18:44
Fine over 2K is interesting. If it was fine over about six I'd be considering the needle jet was buggered.

Tell you what it's still worth looking at the needle jet though. Could be it's on a different clip to the others, or maybe got a bit broken or bent.

Mully
24th December 2009, 20:48
Just ride it at over 2K revs.

Problem solved.

warewolf
5th January 2010, 14:39
Im thinking holey diaphram but cant work out why its fine over 2k.
one thing i notice is that no3 feeds the vacume tap.Swap the vacuum feed to another cylinder, see if the problem moves.

There's less vacuum at higher load (revs) so maybe the problem is borderline enough to only be present at the highest intake vacuum levels.

lb99
10th January 2010, 17:56
i have some diaphagms for an 83, laying about, if they are the same and if you want em, let me know.
mine looked like a giant Rd250lc

FROSTY
11th January 2010, 12:11
Im always keen toget XJ parts :-)

F5 Dave
18th January 2010, 15:51
Did you measure the plug cap? Could be that simple. Actually fixed Piglet (a friend's ex cop XJ750) like that.

FROSTY
18th January 2010, 17:02
tell ya the truth ive done fuck all. left bar was bent so I fitted a new one is as far as i've gone with the darn thing.
Too busy churning out LT50 quads and cars