jono wrote:In relation to my transition stumble described earlier in the post, I attach a dyno plot from my car with the jetting as it currently is. The engine is mildly modded, including QED 360 cams and some light porting, and the jetting is as recommended by QED on the other attachment (they make no mention of the idle air correctors)
Can you discern anything from this in terms of whether i'm to lean or too rich in the idle circuit/transition
Yup, a communal jet inventory would be very helpful. Given the far-flung nature of the internet, that's probably best done at a local club level.
As far as your question, my simple answer is, "No, I can't". To begin with, you really haven't clearly stated what jets you are using. Especially your Idle Air Corrector... you just talk about going 'leaner'.
As far as the QED Q360 cam in a Twin Cam is concerned, I have no experience with it.
Is your jetting currently exactly what QED recommends? Except for the Idle Air Corrector... which they don't mention.
What Idle Air Corrector are you using? You've talked about leaner, but haven't been specific.
Adjustments made to the Idle Jet and Idle Air Corrector do influence one another. That's why after changing one, you must re-check the other... and repeat the check & re-check cycle until no further change is required. So yes, it's conceivable that you could make the Idle Air Corrector so rich that it 'cures' the usual 3200 rpm lean-stumble. It's also possible for an overly rich mixture to cause the engine to load-up and stumble... a rich-stumble. And in that case, an overly lean Idle Air Corrector 'might' improve that condition. However, by taking the Idle Air Corrector out of it's normal range, how badly have you screwed-up overall Air-Fuel Ratio for the Idle Circuit, and/ or the off-idle performance? It's best to address a problem at it's source, rather than take something else way out of it's range to force a... a what?... another compensating error?
As mentioned in an earlier post, the correct approach is to first adjust the Idle Air Corrector to eliminate an Off Idle Hesitation. Then go for a drive and use a full throttle run to diagnose and adjust the Idle Jet to eliminate a typical 3200 rpm lean-stumble. Even if it's a more uncommon rich-stumble, address it with the Idle Jet. Adjust the Idle Jet so that the Idle Circuit is running out of flow capacity just as the Main Circuit is kicking in.
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You already have a baseline recommendation from QED, and it's normal to do some tuning from there (from the starting point). I do find it curious that QED omits the Idle Air Corrector from their jetting recommendation. If you're following their spec and having trouble, I strongly recommend that you call QED and ask for more details.
I've never messed with the Q360 cam myself, so I'd have nothing to offer that's any 'smarter' than QED's recommendation. That being the case, the following is just a ramble on the broad subject.
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Looking at QED's dyno chart, the O2R x 10 plot is pretty lean at low rpm... ie, at Idle Circuit speeds. Their data sheet on the Q360 cam does recommend a 50 Idle Jet (which is fairly rich), yet the dyno chart shows a lean condition at low rpm, so I'm 'guessing' that the Idle Air Corrector is on the lean side.
Which Idle Air Corrector are you using?
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Is the float height (preferably the fuel level) correctly set? That's a foundational setting, and everything else is affected by it. Make sure it's right before trying to adjust any other jets.
Emulsion Tubes are chosen according to cylinder displacement. The TC is 1558cc / 4 = 389.5cc per cylinder. Dellorto Emulsion Tubes go rich to lean in part number order. In other words, 7772.3 is richer than 7772.4. There is some overlapping of ranges since a very efficient, high performance engine will draw in more air than a less efficient grocery-getter engine of the same displacement.
Emulsion Tubes
Richer / Larger Cylinder Displacement
7772.3 for 675cc and above
7772.4 for 550cc - 675cc
7772.5 for 450cc - 575cc <<< Lotus & QED both specify richer yet - it's a performance engine.
7772.6 for 350cc - 475cc <<< Dellorto generic for 1558 / 4 = 389.5cc - Richer
7772.7 for 275cc - 400cc <<< Dellorto generic for 1558 / 4 = 389.5cc - Leaner
7772.8 for 250cc - 325cc
Leaner / Smaller Cylinder Displacement
Idle Jets
40 for 250cc - 350cc per cylinder.
45 for 350cc - 420cc <<< Dellorto generic for 389.5cc/ cyl. Also Lotus for emissions small valve.
50 for 420cc - 490cc <<< Lotus & QED both specify richer, except Lotus for emissions small valve.
55 for 490cc - 560cc
60 for 560cc - 630cc
65 for 630cc - 700cc
The Twin Cam's displacement is 1558 cc, and QED claims the Q360 cam with 10.5:1 compression plus their jetting set-up can produce 135 bhp @ 6500 / 117 lbs-ft @ 5500. If you take that displacement and power output, and work backwards through the standard Dellorto formulas, you get the following 'Performance' and 'Flexibility' recommendations:
QED Vs Generic Dellorto Jetting Recommendations:
............................................. Dellorto Generic... Lotus Big Valve
Spec Source .... QED ......... Performance ........ UK Dom
Carb ................. Dellorto ... Dellorto ................. Dellorto
Model .............. DHLA40 ... DHLA40 ................. DHLA40
Choke (mm) .... 33 ............. 33 ........................... 33
Main Jet ........... 135 ........... 135 ......................... 120
Main Air Corr... 150 ........... 180 ......................... 130
Emul Tube ....... 7772.5 ..... 7772.6 or .7 ........... 7772.5
Idle Jet .............. 50 ............. 45 ........................... 50
Idle Air Corr ..... 7850._ ...... 7850.2 .................... 7850.2
Pump Jet ............................. 35 / 45 .................... 45
Aux Venturi ......................... 7848.3 .................... 7848.3
Idle Air Corrector Jet Sizes:
7850.5 .. Leaner, going down the list gets richer.
7850.10
7850.9
7850.4 .. 5-10-9-4 are too lean to be good for anything other than emissions
7850.1 .. Normal
7850.3 .. Normal
7850.6
7850.7
7850.2 .. Rich <<<<<<<<< Lotus & Dellorto generic jetting, above.
7850.8 .. Richest
The QED and generic Performance columns start out in agreement on the choice of Choke and Main Jet. But QED's Main Air Corrector and Emulsion Tube are richer than standard. On the other hand, Lotus' UK Domestic spec for the Big Valve agrees with QED on the richer than standard Emulsion Tube. And QED's & Lotus' Idle Jets are richer than generic.
Just for comparison, here are Lotus' Dellorto jetting set-ups. They're from a 1974 Technical Service Bulletin, so may vary slightly from any earlier manual publications. Also note that all small valve Twin Cams use 30mm chokes, while the Big Valve TCs use 33mm chokes for the UK domestic market, and 32mm for the Euro ECE-15 emissions standard markets.
Lotus Technical Service Bulletin -- Number 1974/1 -- Date: 12 Feb 1974
(1974 - ie, looking back with the benefit of hindsight.)
Amend Dellorto carburetors settings to:
................................ ------ Small Valve TC ------ ... -------- Big Valve TC--------
................................ UK ............. Euro ECE-15 ... UK ............. Euro ECE-15
................................ Domestic... Emissions ....... Domestic... Emissions
Throat.. (mm) ........ 40 .............. 40E .................. 40 .............. 40E
Choke (mm) .......... 30 .............. 30 ..................... 33 .............. 32
Main Jet ................. 115 ............ 120 ................... 120 ............ 120
Main Air Corr ........ 160 ............ 200 ................... 130 ............ 160
Main Emul. Tube... 7772-l.28 ... 7772-5 ............. 7772-5 ...... 7772-5
Idle Jet .................... 50.02 (?) .... 45L .................... 50.02 (?) ... 50L
Idle Jet Holder ....... 7850-2 ....... 7850-4 .............. 7850-2 ..... 7850-1
Pump Jet ................ 40 ............... 33 ..................... 45* ............ 33
....................................................................... * Use 35 if necessary to improve driving
Starter Jet ............... 70 ............... 70 ..................... 70 .............. 70
Starter Emul Tube.. 7482.1.28 .. 7482-1.28 ....... 7482-1.28 .. 7482-1.28
Needle Valve .......... 150.33 ....... 150.33 ............. 150.33 ....... 150.33
Float Ass'y (P/N) ..... 7298-01 .... 7298-02 .......... 7298-01 ...... 7298-02
Float Wgt (grams)... 10 ............... 8.5 ................... 10 ............... 8.5
Float Hgt. (mm) ...... 14.5-15.0 .. 16.5-17.0 ......... 14.5-15.0 ... 16.5-17.0
Air Horn Lgth (mm).. 40 .............. 40 .................... 40 ............... 40
Idle Speed (rpm) ..... 900-950 .... 900-950 ........... 900-950 ..... 900-950
Camshaft ................. C-Cam ...... C-Cam .............. D-Cam ....... D-Cam
Sorry, just rambling... no direct, simple answer for you.
Tim Engel