After being sent away for work and a few other things it's taken ages to get round to giving my 650cfm eddy thunder carb a whirl. This weekend I finally had chance to see if it solves the running problems at light throttle that have plagued us from the very start.
Short answer: Yes! The engine now runs a lot more smoothly at lower revs. It's still not great below about 1400rpm but the improvement elsewhere is night and day. I may need to look at the timing to squeeze the last bit of improvement but for now I'm happy.
Long answer: The carb came with 6847 rods and 095 jets (primary). I set the throttle so the transfer slots were square (as the manual). Floats were a little off but they are easy to adjust. Idle screws starting point is 1.5 turns out.
Put it on the car and fired up, once the float bowls had filled it started easily but wouldn't idle, we had to open the idle speed up quite a bit which concerned me a little as I knew this means the idle transition would be less effective. 10 minutes fiddling with idle speed and mixture we got it idling at 1000rpm but the AFR gauge said 18:1 (max reading) at idle. It would drop if the throttle was blipped but stayed at 18:1 at idle. Even with the idle screws out 4 turns it still said 18:1. I decided this had to be wrong as the engine was idling comfortably with 10" vacuum at 950rpm. It wouldn't be able to do that if it was as lean as the gauge said.
After some headscratching I thought "stuff this. Lets see how it drives". As soon as I reversed off the driveway I knew it was better and once I turned onto the main road I was very very happy. I cruised along in 4th gear, 30mph 1400rpm and it was nice and smooth. The exhaust sounded ridiculous of course, burbling and chuffing away but finally the sub-2k rpm surge was gone!
There's an industrial estate down a quiet road behind our house so I stopped down there for a further fiddle to reduce neighbour annoyance and quickly got the idle to 900. I then noticed that the AFR gauge suddenly started reading something sensible - 12.8 at idle. I have no idea what it was playing at in the garage with the erroneous 18:1 but its fine now.
Chugging around below 2000rpm and the AFR is about 13 but when accelerating very gently it just rises up to 14:1. It's just on the very edge of a lean surge so a tiny bit more tweaking is required but its so much better than it was. Over 2k and its low 13's (as the influence of the idle circuit diminishes and you get full on cruise) then mid 13's on power when the vacuum is below 5". I haven't tried full throttle yet as the throttle travel seems to be slightly less on this carb so I didnt want to stretch the cable. I can adjust the throttle end stop a couple of mm.
I think I can go at least one stage leaner on cruise and possibly lean off the idle a quarter of a turn but it seems to like a rich idle probably due to the cam overlap. When I do that, and sort the throttle stop out I can see what WOT is like and see if there is any bogging. EDIT - I later discovered there is a bog issue. See my Secondary Air Flap post for how to eliminate a WOT bog with this carb.
Over all I'm very pleased. This 1805 eddy thunder carb has worked pretty much straight out the box with the minimum tuning which is explained very clearly in the manual. The smaller primaries compared with the 750cfm mean that the air velocity is much higher and the fuel mixes better ( at least that's how I understand it) and the fuel metering at small throttle openings is much more reliable and stable. It starts to struggle below about 1400rpm but at the moment I don't mind that at all. I can now chug along at 35 in 5th gear and not be thrown out of my seat. Lovely!
Of course this now exaggerates the fact that my intake and cam are designed for higher rpm so flooring it from 1500rpm is a tad disappointing, as mentioned earlier I think there is more to be had from looking at the timing but generally speaking its so much more pleasant to drive now I feel I can finally enjoy the car instead of constantly worrying about never being able to get it running properly.
Thanks to all on the cobra forum who suggested a smaller carb, you were spot on!
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