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/forums/archive/index.php/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.

[gilly81]

I've just ordered the Pertronix Ignitor 2 and Flamethrower coil. Hopefully this will help.

[Ben-e-boy]

if all else fails try putting a vacuum gauge on as a diagnostic. I cant remember exactly what each reading means Im sure its online.

[Ben-e-boy]

http://i201.photobucket.com/albums/aa296/moefuzzz/TEMP/vacuumGaugeReadings.jpg

[gilly81]

Thanks - I'll be replacing all the vac lines this weekend so I'll give that a go!

[gilly81]

Ok guys problem solved! Thought I should report back with the fix.

Checked Vac lines, a few had perished right at the ends where they are stretched over the flared ends of the pipe, fixed those, - still rough.
Carby was looking dirty internally so gave it a decent hit of carby cleaner - still rough.
Triple checked points - looked a bit wide so regapped to 0.45mm - still rough

Drove to a mates place and borrowed his timing light - miles off!? It was running sweet a couple of weeks ago and i didn't touch the timing so I never thought that this could have changed itself. Turns out the bolt holding the distributor in correct timing is buggered and allowed the dizzy to move. Set it at 10deg with a new bolt and it's running as sweet as ever! The joys of buying an old car - at least I'm learning what can go wrong as I go.

Thanks to all for your suggestions! Appreciated. Pertronix II and Flamethrower Coil are on their way, that'll be next job!

[gilly81]

I just wanted to follow up on this because it may help someone one day...

The problem came back, although not as bad, no backfiring, idled fine and would actually drive the first few streets like a dream, and then start breaking down BADLY for 3-4 mins, before running perfect at full operating temp.

Tearing my hair out, today I made two more changes. Replaced the distributor cap (which really didn't look that bad anyway) and disconnected the noise suppressor. This one I'm sure is the culprit. The Mav has never run so smooth! From 2 cold starts since - absolutely no problems. Never so much as missed a beat since I disconnected that prick of a thing! Radio has no noise without it connected - yet to try the UHF though, may need to replace it if that's a problem.

So here it is for a search engine to pick up: If your TB42 Carby breaks down or misses and you've replaced points, leads, plugs, condenser, checked vac lines and all the usual, DISCONNECT THE NOISE SUPPRESSOR! It's a 2 second job I really wish I did first. I didn't think it would have any effect on spark delivery but it obviously does. Little bastard! LMAO.

cheers
Gilly