Back-firing carbies, engine breaking down

[cj260znut]

Hi, I have a 240z with the L28 conversion with twin Hitachi carbies & Im pretty sure they're series one. Have a Bosch GT40R coil with pertronix electronic ignition, new rotor cap & button, new plugs & leads. Have set timing to 8 degrees btdc, have balanced carbies, have got engine idling at 700rpm & have used colour-tune to set the fuel ratio, which all seems ok. My problem is that the engine brakes down & back-fires out of the carbies at about 2,800rpm. Have tried enrichening & leaning off carbies, have tried adjusting timing, still no difference. The vacuum advance seems to be working, any ideas as to the cause??

[chris240]

Does it do this constantly ? ie fully warmed up engine...

Also what "hotness" number are you running on the spark plug, is it a no 5 ?

[d3c0y]

Sounds like an ignition problem to me, usually i would blame the distributor. I've heard of a few pertronix sensors failing too, thats where i would start anyway.

[Linton]

sounds like either fuel or timing I have spent days on mine plugs were black and sooty in the end I took off the su and put on some hitachies that have not been used for 15 years runs fine now with another set up new plugs,

work out which plugs is missing and slide the lead off the end of the plug/s until the lead is almost off the plug (make the spark jump) if the engine clears then it is fuel and plug related, if you get a boot then you jump. ha ha

[cj260znut]

temp of motor seems to make no differece , am running NGK BP6E S

[cj260znut]

maybe i should try puttin the points back on ;(

[nizm0zed]

If its not too hard, putting the points back in would be a good way to diagnose the condition of the pertronix sensor.

While you have the dizzy apart, it would be a good idea to physically look at the vac advance breaker plate, ensure its in good working order, make sure the plastic bearing carrier isnt cracked and the bearings are still there (and lube it)

[mick74]

I had a similar issue many years back, replaced points, leads and plugs, adjusted timing etc. Nothing fixed the problem, worked out to be low octane fuel. Now I only run 98 octane fuel. Try a bottle of octane booster to see I that helps then I would chase electrics or carbs.

[d3c0y]

And just in general it doesnt have old fuel in it does it? Nothing like draining the tank if its got fuel that is over a year old in it.

[Gordo]

Yer like Jake says, sometimes the old Zeds sit around in the garage for a while and the last thing we think of is when was the last time I put fuel in this thing. I have fallen for this one it was three years and she didn't run to good . Change the fuel just to be sure.

[fluegel]

Years ago I fitted SUs to replace the flat tops in my 260. Not long after they would backfire when starting cold as did the original flat tops. Recently I fixed the problem, I took them apart and cleaned them and found the rear jet was seized so there was no "choke" for that carby the cable simply slipped in its attachment. On the front carby the lever would only move half way because the cable was attached incorrectly. I freed the rear jet and fixed the front cable so that now the jets go all the way down when the choke is pulled, the engine starts like it is fuel injected, instant start, no backfiring and smooth idle when I back off the choke. On the other hand I don't think this is your problem ,but it is worth checking.

 

Richard

[Daedalus-Z]

Try hotter plugs! I found the BP6ES' would foul up really quickly for me (didn't help that the carbs where a bit buggered and poorly tuned too), but the 5's stood up FAR better. That's using a pertronix igniter on an L26. The plugs were quite new so it was one of the last things I tried to fix the missing and rough running... but given the price of plugs, it's easier to try a swap early on than it is to say, try a different set of carbies

 

 

[cj260znut]

Ok sorry guys been a bit busy lately, but anyways, car is still doing the same thing at 2800rpm, backfiring and shooting flames out of carbys (only noticed this at night lol).

Anyways since my last post i have rebuilt the carbs, put the points back on, and pretty much re-set everything, with still no luck. I was talking to a person at a local car club meet, and they mentioned because it was sitting so long (20yrs since being registered) that it may need new valve springs??? could this be a problem?? is there anyways to check the tension?? Are they easy to remove?

[nizm0zed]

you can check valve springs, but you do need some specialised equipent.

Easiest way to do that is to take them all into an engine shop and have them check them.

Of course, then you have to dissasemble the top end, either leaving the head on the engine and setting up a way to stop the valves from dropping into the motor, or removing the head completely.

either way, with the work involved, your pretty close to the point where its easier just to take the whole head in and get a complete service on it.

To me, it sort of sounds like your mate 'thinks' he knows a lot more that is actual fact. (nicest way i could put it)

Do you have any air leaks around the manifold? You can check it with a squirt bottle of water, spray it around the gasket seals and listen for a change in the revs or steady idle of the motor.

Its a simple enough check to do, however i would be hesitant to suggest that it is actually the problem (usually shows at idle, rather than running)

Do you have a vacuum gauge?

If not, you can get them pretty cheap. It may be worth putting that on the motor to see what its doing at particular points.

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm (that'll help you decipher the readings)

 

generally, backfiring through the carbs is due to incorrect timing, or really lean fuel mix.