Jetting for Dellorto carbs
Hey all, well, I need some advice here, here's the story, I bought Jim K's book about a year ago and performed all the modifications to the head of a nord 1.6 engine, all but bigger valves really, i'm using a cam that has 246º at 0.05in and 10.92mm lift, compression is about 9.5:1 i'm guessing, the thing is that after much fiddling with fuel pressures and setting the fuel float level as per the manual, i've found that I need bigger jets, now, does anyone have any idea on wich size for air and fuel jet would be good, how many sizes up should I go? currently the main jet is 130 for fuel and 180 for air...any ideas?
Main jet is about 4-4.2 x Venturi size. The airjet for this application (I assume these are the old Gp1 cams or CB47, with 290-293* seat duration)will range from 160-200. Do you have headers? They will influence your choice also.
The best way to find the right jets is to find a long uphill stretch and do repeated timed runs, stopwatch in hand: to find the main jet, floor it from 3krpm to 4.5krpm and note the time required. repeat to make sure, then change one size up or down and do again. continue until you get the quickest time. For the air jet, follow the same procedure but from 5krpm to 6.5krpm. Results will be conclusive.
Leave fuel pressure at 4-4.2psi and don't bother no more with it.
Jim K.
The best way to find the right jets is to find a long uphill stretch and do repeated timed runs, stopwatch in hand: to find the main jet, floor it from 3krpm to 4.5krpm and note the time required. repeat to make sure, then change one size up or down and do again. continue until you get the quickest time. For the air jet, follow the same procedure but from 5krpm to 6.5krpm. Results will be conclusive.
Leave fuel pressure at 4-4.2psi and don't bother no more with it.
Jim K.
Hey Jim! many thanks for the tips, i've been reading and re reading your book on the subject of carb tuning, I assumed that even if they are for weber carbs it should work pretty close for dellorto carbs.
Yes these are the Group1 cams, the profile detailed in your book, I had these custom made, I don't have headers, what I did is use an Alfetta 2.0 cast iron header, since I couldn't find any good quality header builder here so far..I plan to bring some from Europe in the future.
I have a 32mm venturi, here's my main problem, these are the stock carbs, that came with the 116 Giulietta 1.6, if you floor the engine let's say in second gear at about 1500rpm's, the car just dies, it doesn't rattle or anything, it just dies, then picks up again, I took out the carbs about a week ago and cleaned them thoroughly, so I know they are clean, I have a holley pressure regulator since the stock one wasn't there when I bought the car, standard mechanical fuel pump, fuel level is at the prescribed stock 14.5mm from what I could find it to be on the internet (i don't have a dellorto tuning manual) got the info from www.racecar.co.uk, back to the problem, if I feather the throttle to full, I don't have that problem, if i'm let's say cruising in fourth or fifth at about 3500 or 4000 rpm's, and I floor it, the car bogs then picks up.., also I found that at low rpm's in gear when I floor it you can initially feel the car accelerate a tiny second (as if using the fuel from the accel pump) and then die as if the mixture was too lean.. As far as fuel press, well, i have at one of the outlets from the fuel pressure regulator an autometer fuel pressure gauge, the other outlet goes to the carbs, I only manage to read about 2psi with the engine running...at 3000 rpm's or idle..weird, maybe i'm doing something wrong.
Also the car feels slow, hehe, but that may just be me being anxious to have the V6 in..hehe
How about the idle jet? should I leave that as it is or re jet?
Yes these are the Group1 cams, the profile detailed in your book, I had these custom made, I don't have headers, what I did is use an Alfetta 2.0 cast iron header, since I couldn't find any good quality header builder here so far..I plan to bring some from Europe in the future.
I have a 32mm venturi, here's my main problem, these are the stock carbs, that came with the 116 Giulietta 1.6, if you floor the engine let's say in second gear at about 1500rpm's, the car just dies, it doesn't rattle or anything, it just dies, then picks up again, I took out the carbs about a week ago and cleaned them thoroughly, so I know they are clean, I have a holley pressure regulator since the stock one wasn't there when I bought the car, standard mechanical fuel pump, fuel level is at the prescribed stock 14.5mm from what I could find it to be on the internet (i don't have a dellorto tuning manual) got the info from www.racecar.co.uk, back to the problem, if I feather the throttle to full, I don't have that problem, if i'm let's say cruising in fourth or fifth at about 3500 or 4000 rpm's, and I floor it, the car bogs then picks up.., also I found that at low rpm's in gear when I floor it you can initially feel the car accelerate a tiny second (as if using the fuel from the accel pump) and then die as if the mixture was too lean.. As far as fuel press, well, i have at one of the outlets from the fuel pressure regulator an autometer fuel pressure gauge, the other outlet goes to the carbs, I only manage to read about 2psi with the engine running...at 3000 rpm's or idle..weird, maybe i'm doing something wrong.
Also the car feels slow, hehe, but that may just be me being anxious to have the V6 in..hehe
How about the idle jet? should I leave that as it is or re jet?
Idle jet for Dellortos may be around the 55 size. For the stumbling problem you may try a few things with timing advance being the first. If std timing is 2-4* BTDC just try from 10-14* BTDC, but you must limit max advance to ~38-40* at 5krpm (with your low 9.5:1 CR). What about acceleration pump delivery? There is a spec for that, if I remember ~8cc for 20 full strokes. You'll have to remove and test the carbs for this. Solve the fuel press. problem though!
Goodluck!
Jim K.
Goodluck!
Jim K.
yes, i've been thinking about the accelerator pump calibration, already tried timing, makes no difference, fuel pressure seems ok since i've tried with a bit too much and the spark plugs get fouled, then too little and it goes way too lean, it did change the bogging charasteristics a little bit, improving with the augmentation of the fuel pressure up to a point, that's why after much trying i've ruled out it being a fuel pressure problem. When I took out the carbs for cleaning I tried the accelerator pumps, they seemed to squirt quite a bit of fuel and it was long, i'd have to measure it though...
I'm thinking it might be jetting, it still easier to change jets than to take the carbs out and check for the accelerator pump delivery, I think I might try rejetting first, if that doesn't improve it then i'll take the carbs out. Oh I just bought a hallmeter with a wideband O2 sensor, I need to get it installed this week, the car is at the shop resolving some driveshaft vibration issues, but with the hallmeter I should have a better idea about the mixture.
I almost damn the day my car didn't come with weber carbs
it would have been just a matter of following your book's instructions..oh well..at least i'm learning in the process! 
I'm thinking it might be jetting, it still easier to change jets than to take the carbs out and check for the accelerator pump delivery, I think I might try rejetting first, if that doesn't improve it then i'll take the carbs out. Oh I just bought a hallmeter with a wideband O2 sensor, I need to get it installed this week, the car is at the shop resolving some driveshaft vibration issues, but with the hallmeter I should have a better idea about the mixture.
I almost damn the day my car didn't come with weber carbs
it would have been just a matter of following your book's instructions..oh well..at least i'm learning in the process! Well I changed the jets, main 135 and air 200, it did improve the performance a little bit but the bogging is still there so it must me accelerator pump, i'm taking the carbs out next week on my days off and calibrate the accel pumps...
Thanks on the tip about the jets, one other thing, i'm thinking about having another set of cams made, what do you reccomend for a cam profile? i'm raising the compression to 10.5:1 with new custom made pistons, it's a street engine with some track use (i'm going to use this engine on a Giulia Super i'm building for club racing, but it's use will be track/street)
Thanks on the tip about the jets, one other thing, i'm thinking about having another set of cams made, what do you reccomend for a cam profile? i'm raising the compression to 10.5:1 with new custom made pistons, it's a street engine with some track use (i'm going to use this engine on a Giulia Super i'm building for club racing, but it's use will be track/street)