TB42 Backfires and Runs Rough until warm

[gilly81]

Hi Guys,

Hope someone can help here.

TB42 Carby, running fuel only. Has new plugs, coil, leads, extended rotor button, points.

Choke is functioning normally, fully closed on a cold start and gradually opens to fully open when warm.
I can't see any obvious vacuum leaks.

Basically it'll always fire up first go, idling rough and accelerating like it's running on 4 or 5 cylinders, backfires under a bit more load or if it's been idling for a while and i try giving it a quick stab.
Idle speed is all over the place. Give it a rev and it'll be up around 1100rpm, just breathe on the pedal and release and it'll drop down to 450-500rpm.

Once warm it hardly misses a beat, runs incredibly well at idle and pulls well right through the rev range. Even towing hasn't been a problem. But until it's warm, it runs like a bucket of shit!? Tonight I didn't even drive it, just at idle in the driveway while watching the choke butterfly slowly open and it was backfiring regularly while giving it a quick rev. Also concrete is black from crap out the exhaust - this never used to happen...

Any ideas really appreciated!

cheers
Mark

[the ferret]

Sounds like it's running too rich and the timing needs a timing light used on it.
Did you check the plug gap before installing them?
Cheers, the ferret.

[gilly81]

I agree however it ran perfectly on the new plugs and current timing for a few weeks and has just developed this issue, so I don't want to touch the timing etc until this is sorted out and back running how it should be.

[the ferret]

Put the old rotor button back in and see if that makes any difference, also check for sideways movement in the dizzy shaft, and make sure the new points are not bottoming out. did you clean the laquer off the new points?
re check the points gap and dwell angle.
Check that the little spring loaded carbon pin inside the top of the cap hasn't worn down, it must contact the top of the rotor button.
Try another condensor while yer at it.
Cheers, the ferret.

[gilly81]

Thanks mate, I'll give that a go. Maybe something's not quite right there. It did go a LOT better and smoother with the new extended rotor button.
Actually I'm thinking of going for a Petronix Ignitor system so might do this sooner rather than later.

Just strange that it runs flawlessly when it's warmed up!

[Robo]

Check the plugs and see what's going there, will give you a better idea.
Maybe even be an idea to check em say after running for a min or so to see which particular cylinders are given you the trouble.
Have had faulty brand new plugs a few times over the years.

[gilly81]

Good idea Robo. Cheers

[taslucas]

Hi Mark, i just did a liitle search and found this thread: http://www.nissanpatrol.com.au/forum...hp/t-3251.html
Maybe a few more things to try there.

[97_gq_lwb]

Other things that it could be are the condensor they are just like a capacitor and they react to heat when they are old and stuffed.
You might try disconnecting the noise supressor as well.

[gilly81]

Good idea there too! Can't hurt to throw a new one on. Will give it a go. Cheers.