backfiring thru carbs

[maabus03]

I have the early model of a 260z with a 240z engine. It has the distrubiter off the original 260z engine and the 72 SU carbs. The original engine had the same problems but had the 74 flat top carbs on it (which were supposably rebuilt). I had a 240z parts car this engine and carbs came from. Ran great but was a complete rust bucket. So i'm just using this engine temporary til either i rebuild original or convert to v8 ( leaning towards v8 for the power )

The problem I'm having is its backfiring thru the carbs when I give it gas. Usually it won't do this if I "slowly" rev it up. I can get it to idle perfectly at 750 rpm. Sounds and idles smoothly. It also does this with the other carbs ( flat tops). Planning on using the rounds tops only.

I'm pretty sure the carbs are running too lean. When I raise the piston the car will start to die ( same results on both carbs). I've tried adjusting the mixture nuts to fully rich and didnt change a thing. Still tries to die when raising the pistons. Some of the other things i've tried is: Adjusting timing...set it at 7 degrees at 750 rpm like my books shows. New distr. cap and rotor, plug/coil wires, Swapping fuel pumps ( have 3, no difference in any of them), new plugs -gapped at.35, new fuel filter ( Popping got alot worse after changing this. Used to have expensive fuel filter, changed to cheap clear one), changed practically all the fuel lines/vacuum lines with new hoses, compltely took apart carbs ( 72 su's) and cleaned them with carb cleaner, no worn out or damage I could find, adjusted the valves about 20 mins ago (hot). Probably a few other small things I can't think of right now. Still no difference and has only gotten worse after changing that fuel filter. I noticed the rear carb is sucking in way more air then the front one. Front one is barely sucking in any air. Also the rear carb is kinda making a hissing sound coming from it at idle ( same sound as with flattop carbs). Lately its beginning to get worse and worse. Almost sounds as if the carbs are sucking in too much air and not enough fuel? If I give it gas quickly, it just chokes and pops and dies. A few times I've even had good sized flames come out of there. Also I noticed the plugs foul up fast.

I've also done compression check, 125 on first 4, 105 on 5, 95 on 6. Tomorrow I plan on blowing out all the fuel lines and making sure its all free flowing because it sounds like it just not getting enough fuel. I don't have a fuel pressure gauge to test it either.

I'm stumped on what to do, i've searched and searched on the forums for somethin else but nothin has made a difference. So any suggestions or idea's would be greatly appreciated!

[dogma420]

Well, for one thing, go ahead and get the SU tuning video from Z Therapy...www.ztherapy.com

Another thing is, SUs will backfire through the carbs a lot...this is very common...don't be alarmed. They do that basically by design even if tuned perfectly.

Sounds like your compression isn't very even through the cylinders...add some oil to the cylinders (turn over a couple of times, then do another compression test) to see if your rings or your valves are losing compression. If your compression is higher for each cylinder after adding oil, the rings are bad...if compression is the same, your valves have some kind of issue.

You should be able to lift the piston on either carb at idle and it should continue to run. You should be able to block the air inlet one at a time during idle for each carb, and the car should continue to run if the carbs/valves/engine timing are all in synch.

[Curtis240Z]

Your carbs aren't syncd. They are suppose to both be sucking the same. Properly tuned SUs almost never backfire from my experience. Although your #6 cylinder is quite low on compression.

[maabus03]

I adjusted the valves after I did the compression check, haven't checked them since, should have improved it some. 10 of 12 vavles needed to be adjusted quite a bit. About the carbs...it doesn't seem to matter how much the carbs are adjusted, the airflow is the same between them. I have the flat top carbs also, and its the exact same thing with them. So I figured it had to be somethin else. Gonna go ahead and order the video, might help. The piston does freely move up and down.

Another possibility is I think I might have some of my vacuum lines mixed up. Anyone have a picture of the E-33 intake with the 72 su carbs I could use as a reference to verify all my vacuums.

Thanks

[maabus03]

Compression check after valve adjustment showed 120-125 on all 6 cylinders now. Tested fuel pressure, getting 4.5 psi. Also tested alternator, getting full charge from alternator and plenty of juice under load. Ordered the SU video and have a cheap rebuild kit comin on friday/sat, so that might fix it. If anyone has any other idea's i can check until i get those, let me know. Thanks

[240 in OZ]

Enrichen the mixture a bit.

[cremmenga]

Mine did the same thing. I just put in octane boost,and sea foam. I also put in new spark plugs and drove it a bit and got that sea foam through the carbs. Never backfired since.

[maabus03]

I did try to enrichen the mixture...I adjusted the mixtures to fully rich, didn't do much, and did the opposite, to fully lean. Barely any difference, idled a bit higher/lower (can't remember offhand if it idled lower or higher).

I am running on the cheapest gas...I could try to put like the premium in there and see if that makes a difference. Did you put the seafoam in the gas tank or actually pour into the carbs? I put seafoam into the fuel tank, didn't run any different unless it hasn't gotten to the engine yet...doubtful.

[cremmenga]

I did try to enrichen the mixture...I adjusted the mixtures to fully rich, didn't do much, and did the opposite, to fully lean. Barely any difference, idled a bit higher/lower (can't remember offhand if it idled lower or higher).

I am running on the cheapest gas...I could try to put like the premium in there and see if that makes a difference. Did you put the seafoam in the gas tank or actually pour into the carbs? I put seafoam into the fuel tank, didn't run any different unless it hasn't gotten to the engine yet...doubtful.

I put it in the gas tank. And i bought the highest octane gas i could get, 93 octane, and put a whole thing of octane boost in, didn't know if it was to much for the beast but it fixed mine!

[maabus03]

I'll have to give that a try tomorrow, since it'll take til probably saturday until I get that cheap rebuild kit and SU video. I was reading a post a few hours ago about how these cars running poorly on cheap gas. Thanks for the suggestion.

[cremmenga]

you bet chya!