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Quick Jetting Question


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I have received my rejetting kit today so I can change my jets tonight.  I just wanted to make sure before I get to it.  Is it ok to mix and match jets to air corrector jets?  For ex: can I leave my air/ corrector jet (180)  and just change main jet a step down?  Or is it recommended to take a step down or up on both?


I will be taking a step down on both mains and air correctors, but I was just curious about this.  Is there any positives/ negatives to having them mix matched?


Thanks In Advance

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yes - you can just change a Main without changing a Air Correction




note how the car runs corrently.

especially how,  and if,  it pulls cleanly in 2nd and 3rd gears at 4,000

to 5,500 rpm.


the Air Correction jets come into play at those high rpms


ther's a lot more to it


come back with more input as you jet and road test


IGNITION TIMING using a timing light FIRST! - and leave it.

Don't change it during your jetting changes

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Thanks CD!!!


I haven't had a full good run since this is a full engine overhaul/ upgrade.  But I can get it to rev pretty high at idle, cannot rev much under load.


I am happy with the ignition timing and quick rev response so it will stay.  Looks like I will definitely be changing the air correction jets since that is my issue.


My extra parts will be sent to you, which you can toss into your neighbor's pool. :blink:

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You can get a engine to free rev to 7000 with the carburetor from a large lawn mower with no load on it. How it responds to throttle without load means almost nothing about the jetting. It will do it all with just the idle circuits. Give us some idea what you have for an engine (displacement, cam, compression ratio, ignition timing) and what you have for carbs (40's, 45's, 48's? Choke size, Aux venturi size, Emulsion tube, main jet, air corrector, idle jet, inlet valve the one in the bottom of the float bowl) we should be able to get you close with your initial jetting. Things like exhaust system, air filters and cold air supply will make a difference too. Like CD said, you need to get it so it runs well below 4000 and part throttle FIRST. This will be done with Main jets,emulsion tubes, idle jets, pump jets, float level, ignition timing. Once you have that working well then start working on Full throttle and the 4000 on up in the rev range. This will mostly be done with Air correctors. Remember bigger number on the main jet= richer, bigger number on the Air Corrector=LEANER

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Ok so this is my engine specs and what i have done tonight.



Dual 45's

302 IE Cam

Dual Valve Springs

All Coated Bearings

10:1 Forged Pistons (90mm)

1mm Oversized intake and exhaust valves

Stage 2 IE Exhaust

Stock exhaust after that

Slightly ported

Bosch Blue Coil

Have MSD 6A but not installed yet (wanted to keep it simple at first for easier diag)

8mm Wires

NGK plugs

Timing: 35 degrees at roughly 1100-1300 rpm


All sizes are stock to the carbs, or what came off when changing jets.

Venturi- 36mm

Aux. Venturi- 4.5mm

Main Jet- 145mm

Air Corrector Jet- 180mm (Weber diagram says 155mm, Redline mustve changed it)

Emulsion Tube- F16 (whatever that means)


My Tests Tonight


Stock Setting M- 145, A- 180 

Test 1 M- 140, A- 180  (better, still stumbles up slight hill)

Test 2 M- 140, A-170 (better than above)

Test 3 M- 130, A- 170 (better or same than 2)

Test 4 M- 130, A- 180 (better than 3)

Test 5 M- 130, A- 185 (better than 4)


Overall, i could barely make it up my hill and would stumble very early, would have to short shift perfectly.  Now I can drive it like a normal car, but still not revving as high as it can and no power.


All of your help is greatly appreciated to get this car back on the road after being sitting for 10 years.  Hopefully onto the track!!!


Thank You!!

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I can not stress how important the right idle jet is to tuning DCOE's or IDF's. Everything from idle to about 3000rpm is dependent on these. Your idle mixture screws should be no more than 1.5 turns out. If they are that much or more you need bigger idle jets.


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If your timing is set to 35 degrees at 1100-1300 RPM, you've got it too far advanced.  Full advance should be 35-36 degrees, which should  occur somewhere around 3500-4000 RPM, depending on your advance curve.


Your attention to detail and tuning methodology are good, and keeping a log of your results will not only help you to get it dialed in, it will teach you a lot about DCOEs.  


Questions for you:


1. Which version of the 45 DCOEs do you have?  45 DCOE [fill in the blank here]  Are they Spanish or Italian?  The Spanish carbs are more recent, and will likely have more progression holes.  I suggest you remove the progression cover on one throttle body, and post a photo of the progression hole pattern, as it will contribute to the ever growing body of information regarding the various progression hole patterns available on 45 DCOEs.  The progression hole pattern (and the number of progression holes) has an impact on the tunability of the carburetor to suit variations in the motor.  The early 45 DCOE 15/16s, which were used on the TiSA 1800, have only two progression holes per throttle body, whereas the most recent 45 DCOE 152s have up to 5 progression holes per throttle body. 


2. What idle jet are you using?

Edited by williamggruff
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William is correct, if you have your timing set at 35 deg BTDC at 1500rpm that is WAY TO MUCH, you should be at 34-36 deg when the distributor is at full advance (usually they are all in by 3500) Start there. What idle jet do you have? My recommendation is a 45F9 idle, you need to check the float level if you have plastic floats set them at 12.5mm. I would think a 135 main jet and a 180 air corrector on the F16 Etube should be a good place to start. once again work on the part throttle and lower engine speeds by changing JUST the main jet and once you have that working well then do a full pull and see how it runs, if you need it richer go to a smaller air corrector or leaner with a larger air corrector. Pulling a spark plug and looking at it can give you a idea of the full throttle mixture, you want a light grey (NOT WHITE!!!)

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John- I will double check and see where my idle mixture screw is and report back tonight.


William & Preypy-  I had it timed to 17 BTDC and I would start stumbling at 3500 ish at a stand still.  The more I advanced it the more revving I can do.  This is before I started jetting the car.  But I will rev it at 3500 and see if the timing is still around 34-36 BTDC.


I have 45 DCOE's 152 (spain)

The progression hole cover shows #71 on my picture but im not sure, can you verify?  This helps have more of a visual for me too.



I am not sure if Redline changed out the idle jet or not, but stock it is .55mm.  I will do my best to get a # off of it though.


My kit only jumps from 130 to 140 then to 145 so I don't have a 135 main jet.  Should I start with a 140/180 combo first then??  Or keep it at what I have now?  Because I have tried the 140/ 180 and have seen improvement with the more trials I have done.


This is the best I can answer them right now.  When I get home from work I will check timing @ 3500 and play with the air jets some more.  I will hopefully get a visual confirmation on what idle jets I have as well.

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Yes, #71 is the progression hole cover.  When you remove it, you should see 3, 4, or 5 small holes.  Please post a photograph to the forum.


Though the "stock" idle jet for this carb may be a 55F8 or 55F9, the only thing that's relevant is what is in your carbs.  I suggest you check each of them, as it's not unheard of that they get mismatched during assembly.

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So I checked my timing.  It roughly 34-36 @ 1100-1300 rpm.  Revved it up to 3500 and it was still at 34-36.


Harv important info for you and me obviously.  The idle mixture screws were turned all the way up.  I turned them downward until there was resistance, and slowly adjusted them up to 1.5 turns and it will not idle or start.  Air bypass screws are all the was closed (what instructions said).  So i guess first step is changing idle jets for bigger ones correct? And how do the idle jet #s work.  The higher the number the smaller or vice versa or what?


Idle Jet Size = 60R8 on all 4


Test 6  M-130, A-190  Same or a little better.  From around Test 4 to current they are good under light load, but stumble under heavier load.  Test 6 is currently the biggest Air Jet and the smallest main jet.



I have not pulled the plugs or checked the float levels yet. 



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That is a VERY rich idle jet, try some 45f9 idle jets. If your distributor is fully advanced at 1500 rpm you need to stop messing with the carbs and fix the distributor FIRST! You will never get it to idle or progress off idle with that much advance, you should be closer to 25deg at 1500-1800. You say it will not start or run with the idle mixture screws 1.5 turns out from fully closed? You need t be very carefull when you turn the needles fully closed, BE VERY GENTLE AND STOP TURNING AS SOON AS YOU FEEL ANY RESISTANCE, if you tighten them down you will damage the carburetor body and they can not be repaired. You have the later carbs and the needles are a much more gradual taper you might need to run them as much as 3 full turns from fully closed but start with the idle jets I recomended.

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Ok will do, I will find them and order them tomorrow.  Also see what is in between so i'll grab some at one time.  I should easily be able to set it at 25deg at around 1500-1800.  I did it before and im pretty sure i can rev it up to at least 4k which is a start.


Spark plugs are black, just checked them.


And yes, I closed the screws all the way (when resistance was felt, aware of possible damage) then backed them off 1.5 turns.  I backed them out probably roughly 3 turns or more and it idles there.



So tomorrow I will set the jets back to 130 mains and 180 airs and then adjust the timing and retest vehicle and report back.  Just to let you guys know at least if there was a change or not by just timing.


I also look foward to see what William has to say.  Always great to see what others have done or have experienced.


All of your knowledge is appreciated!!!



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