Carb & Exhaust Backfire at the same time

[Ronaldo1973]

Hi guys,

am in real need for a help here this problem is a nightmare and I have done approximately every thing possible.

240z 72

L24, and 280zx 81 dizzy (gives a good spark)

SU carbs ( i have cleaned them very well)

it has been sitting for 3 yrs (and am working on this prob for 2 yrs

shes having a very very very poor idle and a lot of crab backfire and exhaust backfiring at the same time and during cranking (Not idling)

The crabs backfire like a flame thrower.

The exhaust backfires like a grenade.

I have tried adj the carbs but no luck,

I have tried adj valve backlash but could not got em right (8/10) cold

I have check the dizzy timing and it is 100% right (I even got cyl 1 at tdc and check each wire)

I have used electric fuel pump to ensure enough fuel

check compression all cyl 130-145

timing chain is tight

Am considering replacing the engine

[beermanpete]

It sounds like the timing is 180 degrees (at distributor) out. You might have timed it while at TDC while on the exhuat stroke (valve overlap) rather than TDC on the compression stroke. Try timing at the "other" TDC.

[Ronaldo1973]

It sounds like the timing is 180 degrees (at distributor) out. You might have timed it while at TDC while on the exhuat stroke (valve overlap) rather than TDC on the compression stroke. Try timing at the "other" TDC.

As i know that the distributor could not be installed 180 deg out (since there is a tooth on the dizzy's shaft that fits with oil pump shaft and this tooth is not centered so that it should fit correctly )

and regarding the TDC, I have timed Cyl 1 @ TDC on compression stroke with the valve cover is off !!!!

[240ZGL]

Timing problem

[Bruce Palmer]

Or a bad case of crabs...... sorry....

[zKars]

Its a timing problem almost for sure. You have convinced yourself some aspect of the time system must be ok, but it is not. Do not be afraid to challenge every detail of your assumptions.

I suggest you start from absolute square one with the timing. Really.

Remove the valve cover, remove the oil pump, remove the oil/distributor drive shaft. Put the engine at compression TDC, confirm with both #1 cam lobes "up" and look in spark plug hole for piston at top dead center.

Then re-assemble the oil pump/drive shaft lining up the marks on both. Notch on the side of the oil pump just below where the drive shaft enters, and a punch dot on the drive shaft. Put the pump back on the motor, straight in. It likes to go in a tooth off, might take a couple of tries.

Wish I could find a post that shows the correct position looking into the dist. mounting hole from above that shows the correct narrow/wide side of the drive tang relative to the the engine, I think its tang aligned to 11/5

Now put the distributor back in. Put the rotor back on (not the cap yet). Where is the rotor pointing? Better be forward toward the rad. The advance diaphram is pointing the same way. Look at the cap. The #1 tower is marked with a line cast into the cap. Put the cap on. That tower better line up EXACTLY with the rotor inside.

Now put the plug wires on, one at a time, remove them all from the cap, starting with #1. Go counter clockwise, 1-5-3-6-2-4 (not 153426!! Anyone who knows me knows that story....)

There are many many posts here describing this proceedure in excruiating detail, do a little searching, you will find very descriptive pictures.

Report back what you have found. You will overcome this problem.

Edited October 27, 2011 by zKars

[zKars]

I knew you couldn't resist Bruce....

[FastWoman]

EVERYTHING that Kcars said! However, I'll just throw in a couple of other items:

Pull off your cap and rotor, and try rotating your distributor's breaker plate by hand. It should rotate smoothly, without binding, and it should be under spring tension. The mechanism often rusts and/or falls apart and gets stuck in some random position of advance. I doubt whether that could result in problems as extreme as you're describing, but it might be part of the overall picture -- quick to check.

Another part of the picture could be the valve lash. You say you tried adjusting it, but you couldn't get it right. To me that's a big red flag. WHY couldn't you get the valve lash right? (It's just an adjustment, after all.) Perhaps your valves are sticking? I'd suggest doing a compression test. You'll find out very quickly whether that's the case. Possible reasons for valves hanging are: They're bent. They're rusty. They're gooped up in varnish. You can address the latter problem with a solvent like SeaFoam, but the head might have to come off if it's either of the former two.

It's tempting to think of this as a single problem, but on an old car that's probably seen lots of neglect in its life, MULTIPLE problems are more typical.

[LeonV]

You don't need to take the pump or distributor out, and you don't even have to pull the valve cover if you don't feel like it. Pull the #1 plug out, disconnect the coil wire, put your thumb over the plug hole and stop when you feel the compression stroke (you'll feel it). Mark (or memorize) where the #1 plug wire is on the distributor cap and take it off. Look to see if the rotor is pointing there. If not, adjust the plug wires on the cap so it is, and make sure the firing order is correct.

If you're going to check valves, the valve cover will come off anyway which makes visual inspection of the cam the preferred option.

[zKars]

Leon there are clearly short cuts to dianosing his timing problems, but given the history, I felt it best to really get back to basics and to do a full "from square one" process to maximize his chance of finding the core issue.

Fast Woman, good catch on the valve lash settings being a possible contributing factor. The settings are 0.008 inch and 0.010 inch just to be clear, in case there is a metric/english conversion confusing you Ronaldo. It must also be done with each lobe pointing nearly straight up to ensure you are measusing between the base circle of the cam and the top of the rocker.

And I'm ZKars not K-Cars. I'm a Gemini, not an Aries

[FastWoman]

zKars, I regret I'm a bit (or maybe more than just a bit) dyslexic!

"Dyslexics of the world, UNTIE!" ...anon

[LeonV]

Leon there are clearly short cuts to dianosing his timing problems, but given the history, I felt it best to really get back to basics and to do a full "from square one" process to maximize his chance of finding the core issue.

Right, I understand that, but dropping the pump and distributor is a bit overkill IMO. If you can get the job done without doing so, I don't see a problem with it. Only if the problem lies in a broken gear or spindle would I drop the pump and dizzy. Otherwise, it's as easy as finding TDC, setting #1 plug wire to where the rotor points, putting the rest of the wires in the right order, checking with a timing gun and you're done!

And I'm ZKars not K-Cars. I'm a Gemini, not an Aries