Thanks for the input on this thread everyone - learning a lot from you clever chaps as always. More tweaking this weekend i think
Please keep us updated with the jets you end up using as I think we have similar set up. I have twin Weber 34 ICTs with Main jet 155, Air 175, Idle 55, Needle 175, Pump inlet valve 40, Emulsion F6 on an ex FI
@Chrisd when i finalise a setup that works best on my motor i'll be taking off the exhaust with the bung and the sensor gubbins - you'd be welcome to borrow this if you wanted to test your own setup, the PE kit just needs a live 12v and install the app on your phone, super easy.. but yes i will update this thread with my final config
So an update after some weeks of tweaking and testing - As suspected it was running very rich - i've gradually reduced main and air jet sizes to get it down to where it needs to be. I'd previously thought that the bogging i get on long hills was overfuelling, but having done a bunch of tests with the smaller jets i can see from the gauge that it was suddenly going really lean, back off for a few seconds and try again and it would be fine. I'm guessing with the big mains it was draining the float bowls (and the small needle jets couldn't keep up?) so i tried bigger needle jets and this seems to have resolved now. Started out with wayoutwesties recommended 165 mains 185 airs, 150 needles and f6 emulsions Tried 160/180, then 155/175, then 150/170, then back to the original jets that came with the carbs, so 145 main and 160 air. This gave me the best afr readings but as above, still ran lean on long hills, so back to the bigger 175 needles. At this point i was getting the afr readings i wanted but it seemed to be holding back. I'd set timing some time back at 28 (3,000 rpm vac off) as this was at the low/ conservative end of the suggested 28-31 degrees range that seems to be the consensus.. re-did this again but set to 31 and it seems to be great now. AFR didn't seem to change much with the timing adjustment but it drives much nicer and seems more eager. I guess with the mixture being right its more sensitive to timing. So, with 145 mains, 160 airs, 55 idles, 175 needles, f6 emulsions and timed at 31 degrees i get: WOT: 12.8-13.2 so bang on the nose of what i was aiming for 65mph light throttle on a flat road, hovers around 14.5 - 15 or so which should be ok? Its driving nicer than it ever has before and i hope will use less fuel. As its a bit perkier now i'm finding i don't need to mash my right foot to get up most hills. Its never going to be a rocket ship but i think i'm getting the best that's possible with these little ict carbs.
Instead of changing the needle valves, maybe you could have eeked up the fuel pressure a tad? That sorted mine, but I think ICTs have a rep for the needles valves being overcome by too much pressure so... I'm just guessing. If you could monitor your fuel pressure under load you would certainly see it plummet from it's idle setting, I've seen that on a rolling road where it was set under full load - not to any particular pressure, it was set to be stable just the right side of dropping. Sounds great! I worried that 14.5 at 65mph wasn't ideal, but the important part is "light throttle" and in fact mine also runs really well like that! If I jet it richer I need more throttle so there's another clue that this is in reality just dandy.
Always suspected way out westy was jetting in a wierd way. His recommendation for your engine is 2 litres 165 mains, 185 air, 55 idle, F6 emulsions, 150 float jets. Looking at his recommendations for various engine sizes he just ups the sizes as the engine size gets bigger which doesn't gel with my own experiences. When I moved my carbs from a 2l to my 2.4l I did nothing to the jets initially and it ran just great. Even the mixtures could have been left alone but of course I adjusted them to find that out. When I upped the venturi size I had to increase the main jet to suit that. This is why I find his jetting odd because the venturi size and emulsions are constant. Both my engines had similar valve sizes. He does maintain the same difference between his mains and airs but the ratio between them gets closer as the engines get bigger which would suggest his jetting gets richer as his engine sizes increase but who knows?
One thing about fuel pressure is with the demand valve type pressure regulators (think Malpassi Filter King) is that at zero flow, they produce the pressure you measure on the gauge. In order for there to be any flow, there has to be a pressure drop on the outlet, plus a pressure difference between the input and output operating on a diaphragm to open the needle valve in the regulator. So if you have a fuel pump producing only maybe 5 PSI and you try to regulate it to 3.5 PSI thats only giving you 1.5PSI to open the valve in the regulator. It probably requires more drop than that to open fully for full flow at wide open throttle. So that then means the output pressure drops until the difference is enough to support the flow. If you had a higher pressure fuel pump that would help solve the pressure drop requirement for full flow, but make it more likely the regulator starts leaking at low flow producing too much pressure. Result : some combos of fuel pumps and regulators result in starvation at high flow, and dont necessarily stop overfuelling at low flow when they get older. The Carter rotary on the other hand pump fuel through a bypass valve that opens at e.g. 3.5 PSI, and that means that as the demand for fuel increases, the amount of fuel bypassed drops until you reach the full pump output. The pressure does not drop as much with flow. Imagine its a dam with a lot of water running over the top, and you are tapping water off the bottom . Fuel injection systems also tend to use a bypass type regulator, as it is more stable.
Thanks for the update. Out of curiousity, do you know what what your compression ratio is set up as? After my standard rebuild, I measured my CR as 7.6:1 with dished pistons. It would be really interesting to know if yours is similar as my carb set up is slightly bigger on the mains and air. (Main jet 155, Air 175, Idle 55, Needle 175, Pump inlet valve 40, Emulsion F6) but is running well and I get between 20 to 22mph...but we won't mention mph will we Are you by any chance coming to TE to play?
yep, that's a good point re the pressure - i have a low pressure pump (huco) so i can't adjust that, but i'm guessing it not exceeding 2.5 psi or whatever when its pushing against closed needle valves - what it gives when the system is wide open maybe isnt enough for the rate of flow that a 2.0 on full chat would need.
Sounds to me like you're both pretty close. 20-22mph isn't very fast though. So you don't have a separate pressure regulator? Now I get it with the needle valve changing.
