Carb jetting issues after light head porting

[jefferybond]

Hi folks,

I've recently done a bit of very light head and manifold porting, basically just a clean up and a matching of the ports to the gasket. After putting the engine all back together, it now seems to be running lean.

It's a 9.75:1CR, 3.5 SD1 engine, with edelbrock performer manifold and edelbrock 1404 four barrel carb.

Previously it was running great using the stock jetting (.086 main, 65/52 needle), but now it's a little hesitant over 3000rpm (only slightly!), and the plugs are quite clean and white looking.

I know many people say this carb overfuels with the stock jetting, but before the head work I was getting good performance, reasonable economy and nice biscuity brown plugs.

Is it usual for head work to require mixture changes afterwards? I find it hard to believe that the stock jetting on a 1404 can run lean! (nothing is blocked in the carb, and it is almost new).

I'm going to try a richer needle or jet if I can get hold of one, but is this normal?

Cheers,
Jeff

[kiwicar]

Hi
Head work can and will effect mixture as you have hopfully increased gas flow into the head for a given throttle setting (otherwise it ain't working right! ) I can't predict how because that depends on too many things.
If the thing is leaner than "normal/usual" for the type of carb have you checked to see if you have ant air leaks on the inlet side after the carb? it may be it was "leaking just enough" before?
Mike

[jefferybond]

Hi
Head work can and will effect mixture as you have hopfully increased gas flow into the head for a given throttle setting (otherwise it ain't working right! :shock: ) I can't predict how because that depends on too many things.
If the thing is leaner than "normal/usual" for the type of carb have you checked to see if you have ant air leaks on the inlet side after the carb? it may be it was "leaking just enough" before?
Mike

Don't think I've got any air leaks (even though I re-used the carb to manifold gasket). The idle speed and smoothness have stayed the same as before.

Hopefully the reason is because I have better airflow, and now need more fuel to match?

Jeff

[kiwicar]

what air filter are you using?
Mike

[jefferybond]

what air filter are you using?
Mike

It a 'Mr Gasket' 14" x 2" open element job.

Jeff

[kiwicar]

OK, Just wondering as I know you used to have to re jet alot richer going from a paper element to a K&N set up.
Mike

[jefferybond]

OK, Just wondering as I know you used to have to re jet alot richer going from a paper element to a K&N set up.
Mike

Might try a richer needle then, maybe a 65x47 or a 63x47. It seems that cruise mode is just fine, nice throttle progression and pulls well from low revs, it just then when you open it up a bit it kinda holds back (relatively speaking in a 700kg car!)

Anyone know where the best place to get edelbrock jets and needles from? I've tried RPi in the past but they only seem to stock certain ones separately, unless you buy the whole calibration kit.

Cheers,
Jeff

[sidecar]

Hi folks,

I've recently done a bit of very light head and manifold porting, basically just a clean up and a matching of the ports to the gasket. After putting the engine all back together, it now seems to be running lean.

It's a 9.75:1CR, 3.5 SD1 engine, with edelbrock performer manifold and edelbrock 1404 four barrel carb.

Previously it was running great using the stock jetting (.086 main, 65/52 needle), but now it's a little hesitant over 3000rpm (only slightly!), and the plugs are quite clean and white looking.

I know many people say this carb overfuels with the stock jetting, but before the head work I was getting good performance, reasonable economy and nice biscuity brown plugs.

Is it usual for head work to require mixture changes afterwards? I find it hard to believe that the stock jetting on a 1404 can run lean! (nothing is blocked in the carb, and it is almost new).

I'm going to try a richer needle or jet if I can get hold of one, but is this normal?

Cheers,
Jeff

Its quite normal to have to recalibrate your carb after doing this sort of work to your motor. When I went to stage III heads I had to wind the pilot screws out 3/4 of a turn more just to get a good tickover. (A total of 2+ 3/4).
I bought a calbration kit from Real Steel then spent quite a while fiddling about with my carb. You can by just a pair of needles but from other places but they cost about 12 quid, by the time you've bought a few pairs and some jets you may as well have bought the kit.

There are some old posts kicking about where I've explained an easy way to compare jet and needle combinations. (It was something that V8 Developments showed me). It's worth finding the post, if you have problems I'll dig it up.

I am very supprised that your car ran OK and not rich with the stock setup, I've never heard ANYONE say that it was too lean...there's always a first time I suppose.

Edit...heres the post:

http://www.v-8.org.uk/forum/viewtopic.p ... c&start=15


Pete

[jefferybond]

I am very supprised that your car ran OK and not rich with the stock setup, I've never heard ANYONE say that it was too lean...there's always a first time I suppose.

I was surprised too. I did try RPi recommended jetting .080 jet, 6252 needle, but that was lean as hell. I even tried a setting between stock and RPi's recommendation, using a .083 jet, which was better but still not great. I've played with all combinations of the above and stock is always best.

Maybe it's something to do with my nice tubular manifolds and straight through silencers? The manifolds have a nice 12" straight run of 1.5" pipe straight from the head before encountering any bends.

Thanks for the advice,
Jeff