260z su carbs

G

gls50090

Well-Known Forum User
hi ive fitted su carbs to my 260z the car runs fine,colour tuned carbs now the problem is( or not a problem) when I let of throttle im backfireing through exhaust not carbs (overrun?) is this normall considering I have pancake filters,dont seem to be running rich or lean the car did this with the old flat top carbs since I changed backbox to 4 " sportsbox but even more now lol does the sound normal/acceptable? please tell
 
Exhaust backfire is a sign of a lean mixture I think. did you check oil in the carbs? I know one of them was empty when I packed it, that might leanness on overrun. Perhaps :)
 
hi carbs are fine well pleased thanks I topped up oil dampers I had the backfiring before with old carbs just seems a bit more now but they rev and idle fine
,can you remember roughly how many turns out on mixture screws
thanks gary
 
No memory of that I'm afraid, but I'd have set it with my colourtune too, so expect you're right on idle.

It shouldn't significantly back fire on overrun. Any chance you could stick a recording up of it on YouTube?

Have you checked the cam and ignition timing too?
 
I have standard cam im still on points everything set as in manual,ill check timming again my strobe is nearly impossible to see lol the backfireing only happens on deacceleration the higher revs the bigger the backfire/crackling ill uplaoad a vid after tea
 
jonbills said:
Exhaust backfire is a sign of a lean mixture I think. :)
Click to expand...


Jon, how does that work!? I though rich would mean unburnt fuel and lean would be hot :confused:

I have a similar experience both with old 240z carbs and new z-therapy ones! Haven't had time to check with colour tune but it smells a little rich.

I have Janspeed racing headers and a non standard exhaust + standard cam; it's always popped like a load of popcorn going off as if in a pan in the back box. Any gear, any rev over 2krpm and lots.

Gls50090 does yours sound like that or louder?





Ali K
 
yes sounds same reving over 2000 and changing gear popping crackling
car runs good checked points, timing ,dizzy all good I richen carbs half a turn today and its made it a lot better which dosent make sense more fuel more backfire
sounds good if its not going to cause problems im happy but theres nothing else I can do myself it seems to me the sports backbox must be the culprit if Im right running rich causes the backfire also running lean does the same,
im out of ideas on what to do next other then leave it,would feel better if I new it was normal
my mate has a 200sx same backbox his car cracks and pops like mad 2002 injection ecu so its not his carbs as he has non lol
 
Gary,
I see you have got yourself some carbs so I pressume you don't want the ones I have but it is okay if you don't.
My suggestion is to get a compression check on all of your cylinders to see if they vary much or are down on compression.
Back firing and popping on over run may mean your exhaust valves are burnt or not seating correctly. Are your valve clearances gaped correctly ?
I don't think it is a mixture problem but the mixture screws should be wound down to about two and a quarter turns.

Richard.
 
Richard said:
Gary,
I see you have got yourself some carbs so I pressume you don't want the ones I have but it is okay if you don't.
My suggestion is to get a compression check on all of your cylinders to see if they vary much or are down on compression.
Back firing and popping on over run may mean your exhaust valves are burnt or not seating correctly. Are your valve clearances gaped correctly ?
I don't think it is a mixture problem but the mixture screws should be wound down to about two and a quarter turns.

Richard.
Click to expand...

still want carbs from you if ok my dad has Datsun also
 
Since I put the side pipe on mine it's like a Rice Crispy car, as soon as it warms up snap crackle and pop on any sort of over run, low gear and gentle throttle is the only way to keep it quiet past nervous old ladies and babies, that kinda figures the lean mixture theory, still will see when the air fuel ratio gauge goes in. Running triple 40 DCOE Webers with a 2 1/2" pipe going to a Side Pipe with a generous 1/2" glass wrapping of 2 1/2' (feet)
 
done valves every thing good if I richen carbs it dosent crackle as much but still a lot
runs perfect,im going to leave as is it seems its not just me with the popping/crackling it must be ok as theres nothing else I can do
I think its time to drive lol
 
Sounds like a plan. Put a 100 miles on it and see what the plugs look like!
 
OK now I HAVE to get the colour tune out and put my mixture right!! :bow:

Jon, (or anyone else) I still don't get it! How does lean equal unburnt fuel popping in the exhaust!? What is the mechanism/theory behind it? Just seems counter intuitive! :confused:

Ali K
 
As it's mainly under deceleration, is it not because you are lifting off the throttle but the air intake is still high, so what was a lean mixture becomes an even leaner one with no throttle. It then becomes so lean the engine can't ignite it and then it quickly becomes a build up of unburnt fuel which then ignites in the exhaust?
 
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