New carb continued....

A few other things I've noticed now I'm running the Eddy Thunder AVS 1805 carb (650cfm) over the 1406 Performer (750cfm).

I checked it over before I installed it to adjust the floats and idle speed to standard (floats a little off out the box) and checked what rods and jets were in. Compared to the 1406 Performer carb its a little more difficult to change the rods. Not a lot, but things are a little more cramped at the top of the carb so its more fiddly. The piston that the rods fit into doesn't quite clear the screw of the little screw-down cover plate without a bit of care taken which turns a 2 minute job into a 10 minute job.

Here's what I mean:

1706 Performer, loads of room here to swing the cover plate out the way and get the rod and piston assembly out easily.

1805 Thunder AVS has a different shape surrounding the choke, almost reversed. There is a high lip that goes across the centre which means to get the rod and piston out you have to completely remove the cover plate and screw which adds a bit of time, and adds to the risk of dropping bits down the carb. I much preferred the simplicity of undoing the screw a couple of turns and swinging the plate out the way.

You can also see above the Thunder has got dual fuel feed (one on either side) unlike the performer which is UK driver side only. Seeing that I already had the fuel line plumbed in for the drivers side this didn't bother me but is useful to know. I don't yet know how the car will behave in hard turns but with the previous carb it would bog rich on very hard turns and hard stops. 

One benefit of running an AFR in the 13's at cruise instead of waving around between 14.5:1 and as high as 16:1 is that the engine runs 5 degrees cooler. Previously I was seeing that at prolonged periods of 30 or even at 40mph when I'm forced to do 2000rpm to get a smooth ride the temp gauge needle would creep over 90°C to approx 95°C. Its hard to tell exactly as there are very few increment marks on the gauge. It now sits just below 90°C in all conditions, dropping a little more if cruising over 60mph.

We brought the throttle pedal end stop up a couple of mm to prevent the throttle cable being stretched and the butterflies being damaged. This is a bit awkward, its just an M12 bolt through the front of the footwell but needs someone in the footwell and a helper with their hands stuck between the headers and the brake servo in the engine bay. The travel required on the cable on the 1805 for some reason seems to be about 3 or 4mm less than on the 1406. Not quite sure if this is actually a difference between the carbs or if something has changed on the cable route somewhere. Might look into this a little more.

Still a couple of other things to give attention to - I happened to look down the primaries with the engine running at idle and I could see a couple of drips coming from the primary nozzles which means the throttle is open a little too much at idle. The transition slot should appear square when the throttle is closed but mine isn't perfect as it needs a more throttle to idle because of the cam.

The engine runs on (diesels) ever so slightly, just one or two turns after switching off the key so these two things are likely related to the idle throttle position just being a hair too much. I think there is some leeway on the idle speed - now that the carb responds properly to changes in tune, I can revisit my timing and then coming back to the carb again. Perhaps I could add another degree of timing at idle which would allow me to close the throttle a little more. I'm not currently 100% confident in my cheap timing light so I've borrowed a good snap-on one from a friend to verify the timing.

Carbs - dont go too big! (running problems solved)

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!

Nice little run out

Now I've been really bad recently for constantly bemoaning the poor running of the car and I haven't really driven it much because its never running right.

Well this afternoon after the F1 was finished I decided to stop being mr grumpy and Naomi and I went to Welwyn to see some friends who helped us build the car. It was about 1hr 10mim away and gave a very good opportunity for some good lomg steady state cruise data gathering.

Drilling out the idle jets has made no noticeable difference, its still terribly lean below 2000rpm. Interestingly cruising at 2400rpm (70 in 5th) the AFR dropped to about 12.5:1. It would appear that all my rod and jet changes for cruise mixture have been pointless because all the time I was evaluating the changes I was never truly on the cruise circuit, I was still in the troublesome lean idle/transition circuit!

I think I've just about had enough with this carb. It simply will not meter the fuel correctly at light throttle.  Even with my fairly hot cam it should be better than this, anything below 2000rpm and its bucking and surging at 15:1 or worse AFR regardless of what we do to the mixture screws or idle revs. Time to procure a smaller carb!

Back on the road!

Battery charged overnight, turns out it wasn't the battery that was the problem. It was my overly enthusiastic crimping. I swapped the tacho signal wire for shielded wire in an effort to cure my flickering speedo, exposed the shielding and crimped a wire over to connect to earth. Unknown to me, unfortunately the ferrule thingy I chose for crimping had bitten right down through the shield into the signal wire, effectively earthing the coil, which would explain why on trying to start some of the needles on various gauges were flickering.

Stripped the wire back, left the shield disconnected and it started first time! The shield was still connected at the tacho end, just not at the dizzy end, which seems to be good enough as the speedo is now steady so I hope that's cured now.

We had no real idea what the total mech advance from the Moroso advance kit was, the paper booklet that came with it said 23° but I was skeptical of this as various sources I had read said the weight and centre plate combo in the Moroso kit gives around 15° mech advance. I tried to measure it with pen marks on some tape stuck to the rotor and reckoned it was about 18-19ish. I was hoping that whatever it was, it was towards 15 so I could use more initial advance. I made one last check before getting the timing light out that the advance mechanism was working smoothly and not binding on the inside of the rotor like the nasty Accel kit that I fitted. Everything seemed fine.

Naomi is now an expert setting timing, I was in the car watching the revs and she was on the light. We fired up the car and set the initial at 16°, had to open up the idle speed a hair and richen up the idle screws and it just scraped 10 in.hg at 900rpm. I then raised the revs in roughly 500rpm stages so we could see what the curve looked like. My spring combo was weak/medium. 15° was steady at idle (so the centrifugal advance wasn't coming in at idle revs = springs not too light). The timing started coming in at about 1100-1200rpm and peaked out at around 29/30 degrees at 2800rpm. We took it briefly to 4000 (very loud in our single car brick garage!) to make sure there wasn't any extra surprise timing hiding in the mechanism and it had truly levelled off and it was still 30° at 4000rpm.

We've got new neighbours who have seen the car but never heard it, fortunately they approve!

This would suggest the 15° figure I had read somewhere (can't remember where) was correct. Seems odd that on the booklet that comes with the kit it says 23°. Interestingly Naomi checked the timing on the damper marker (no dial-back on the timing light) and got 14° but when she dialled in advance on the light to get the marker to 0 on the damper, it read 15/16°. I'll try and find a garage with a good timing light to verify things.

I know that 30° all in is a bit short of the optimum for peak power but at least it was running and it hadn't turned a wheel for nearly 6 weeks so I went for a short drive. I got to the end of our road (50 yards) and promptly ran out of fuel, d'oh!

5 litres of British Petroleum's finest 98 octane and we were rolling again. With the exhaust leak now fixed, the true AF ratio revealed itself to be... very rich! It was running between 12.5 and 12.8 cruise. Very boggy, sounded like pan of water on a rolling boil. Not much zip, felt a bit sluggish but at least it works.

The lean spot on part throttle is still there but I only drove it 6 miles or so I can't yet decide if drilling the idle jets has worked or not, first thing is to verify the timing, get it to something more like 34° all in, lean off the primaries to something sensible, then I reckon I can close the idle speed back down and see if the transition circuit is performing any better.

One thing I did notice while driving it, the water temp is still pretty high when below 40mph, it was mid 90's (celsius). I know my rad is good enough, all my hoses are the same as most others but I'm pretty sure it should be able to run 88-90°C (195F). I have previously played around with my infra red temperature gauge and I compared the top and bottom temps of the rad which showed the rad is definitely doing its job. This lead me to think there could be a restriction in the system somewhere. One thing to think about is - I wonder if my heater bypass might be too much of a restriction. It came off a Ford Puma and probably has a much lower flow rate requirement. The other alternative is that the bleed-off back to the header could be too large so the water is bypassing the rad. Anyway, more pressing matters at the mo, back to the carb setup!

Edit - high temperature was caused by retarded timing. I set the timing up correctly and the temperature came down.