Unless you are racing, Champion N12YC or if running solid core spark plug wires, RN12YC.
Hmmm. Maybe a problem here. I’m running RN12YC with carbon core wire. I have some N12YC plugs. I’ll clean them up and see what they do later today.
You can run resistor plugs with carbon core wires. It is for radio suppression, not engine running issues. Look elsewhere.
You mentioned in your first post that even if you lean out the carbs, you still have sooty plugs…are you talking about the jet adjustment screw OR the idle bypass screw? The idle bypass adjustment has nothing to do with mixture strength(rich/lean), only mixture volume. (Unless the 1.75" HDs I have have a different idle circuit scheme than the 2" HDs you have?) Either way, I would completely close the idle bypass screws and see if you can get an improved idle the old fashioned way…adjusting throttle plate openings and jet height adjustments…just like you would on an “H” series carb. Don’t know if this would help but it would eliminate one variable.
Seems to me you may well have fuel droplets forming in the intake due to poor vacuum readings, running in rivulets toward the intake valves, but they will not burn as a liquid. I’ve see this in wildly cammed V8s with low vacuum. Also, your situation is bound to improve once the engine is broken in UNDER A LOAD. The compression and vacuum should both rise once you have full ring seat…the engine needs a load on it for this to happen properly. So, hate to say it, but you might wanna consider getting the car drivable even before the current situation is completely sorted out…unless you have access to an engine dyno?
Ok, progress! First, I did change the plugs and the terminal ends, with no improvement. No surprise. Lee, my HD8s won’t run with the idle bypass closed. There is no throttle plate adjustment - It’s normally closed all the way, so there must be a bypass to flow air and fuel. In my case, the bypass adjustment had to be wide open to run at all, and there was still not enough AIR at idle. So, I drilled the throttle butterfly plates, and it’s a new ball game.
I started at .075 and progressively increased to .090, then .105. My idle went up each time, and the carbs became adjustable. I was able to return the timing to 10 BTDC (previously required 16 for even a poor idle). I wouldn’t say the vacuum is great at 14-15, but I expect some improvement with further tuning, and after running in under load, as you suggest, Lee. Plugs were still sooty at .090 (top photo), but much better at .105 (bottom), still a bit black. I’ll attach some links to YouTube videos for anyone interested. Video 1 (pre-drill) and video 4 (.105) would be the most pertinent. Also interesting that I’m now getting plenty of water out of the tailpipe, which had been dry.
So, the bottom line is that the engine just couldn’t get enough air/fuel mixture through the carbs. Walt was right when he said that soot doesn’t necessarily mean over-richness. I will check the lift on the cams tomorrow to see if I might have gotten a hotter cam than I ordered. I’m going to put my UO needles back in, because I’m pretty sure that I will be lean at speed with the current UM needles. The front and rear carbs balance with quite different settings right now - front jet probably a bit off center fwiw. If that persists, I may think about increasing the aperture of the manifold baffle between the carbs - Bill Terry suggested that Jaguar had to do that when going to 2" carbs. Anyone know anything about that? Will not increase the throttle plate holes any more yet.
Thanks again for your interest, everyone. It’s been really helpful, and I’ve learned a lot.
https://youtu.be/gSHdQEGRCrE
So glad you have made progress…the duration of the cam is what I would check since that has the biggest effect on idle quality. Though it seems the problem IS the idle circuit. Looks like the idle bypass circuit is just not big enough for an “enhanced” 3.8 with only 2 carbs…and probably barely adequate on warmed up 4.2s with 3 carbs. The A/F mixture has to make SIX, 90* turns before it reaches the intake…very convoluted and fuel droplet inducing.
Can’t remember off hand, but do the HD carbs have a “flat” at the throttle spindle boss for a potential throttle stop/adjustment screw to rest against?..that would certainly be my preference since we are working around/eliminating the idle circuit anyway. “H”-style throttle lever parts would work, if you have them. Looks to me that the idle circuit was introduced as a convenient fine tuning tool, but is easily overwhelmed by engine modifications.
E-types have a manual enrichment mechanism that drops the jet and opens the throttle plate that could be adapted, if you could get the parts - probably direct from Burlen, and I considered that, but the hole is much simpler, and puts the bypass port adjustment screw into a range where it works. As Dick pointed out, once the hole is there, you can enlarge it without disassembling the carb again. I have a set of 420G tricarbs on my 68 e-type with the starter carb plumbed into the knock outs on the series 1 e manifold - the solenoid is wired to the choke light circuit at the dash lever through a relay and it works great. It’s a stock 4.2 and the flow is perfectly adequate through the bypass ports at idle. What is different in the 120 application is the low vacuum, which is still not fully explained. To get adequate volume at low vacuum, I needed bigger ports in the carbs.
I’ll check the cam specs soon, but will wait until I’m done with tuning, since it will be easier with the radiator off - access to the crank bolt is limited by the narrow spacing. I’ll post again after that.
Many years ago I worked on a car which had a single SU with a steel ball trapped in a spring/cage on the valve side of the throttle plate, can’t think of the application now, but did ponder about it at the time…I noticed on your first video the hesitancy of your starter to crank on the first revolution, always a sympton of over advancement in my experience …
That’s interesting about the slow crank - I hadn’t noticed. With the timing at 10 BTDC, seems better. I don’t think I can dial it in until the car is roaded, but it’s supposed to be stock timing with this cam.
A couple of things.
If you are using 2" HD8 fitted with ASC (all Saloons), unlike E-types with manual choke
There is a narrow drilling at the very bottom of the float bowl to the ASC body (1 carb only)
If this gets partially/blocked, which it can, your vehicle will run poorly
also possible for the idle circuit and needle tip to become obstructed or damaged.
re the 0.100 vs 0.125 needle& jets …as far as I know HD8 have 0.125, later HS8 have 0.100
I believe the needle & jet can be swapped as a pair
Interestingly to me. same needle is specified for dual or tri-carb setups, which is UM on MKX/420G/ S1 XJ6
I now have dual choked HS8, and wish to convert to tri HS8, so hoping no need to change needles, just add the other carb
So, to wrap this up, I am going to accept what I’ve got until I can run the car. The cams are a bit more aggressive than I was led to believe, with a total duration of about 275-280, at 0.050 about 230, lift about 4.10 plus .10 valve spacing. I think that I’ll have to accept a lit irregular idle, but that the increased power will be worth it.
I might increase the lash to .012 on the intake side and try to retard the timing to 8 BTDC, as well as add-ons to enhance the spark, but it’s good enough to run, and I think it’s going to make good power on the road. I do intent to add bungs for O2 sensors in the exhaust and will tune the car further based on fuel air ratios when I do get to drive it.
Again, I appreciate the help that has been provided.
Ron
230 @ .050" IS a lot of cam…even a big block V-8 will have a noticeable idle with those cam specs…
Just making one more post to update the data. I added a couple thousandths to the intake clearance, making it .012. Might have helped a tad. I retarded the timing to 8 BTDC, checked total advance (about 30) and retuned the carbs. I installed an MSD ignition module and tried this with two different coils, one a Pertronix Flamethrower, the other a standard coil. Surprisingly to me, the engine runs significantly better on standard points than with the MSD. I lost 1-2 mm Hg vacuum and the idle and rev significantly worsened. It’s possible that there could be some cross-talk between the plug wires with the MSD, as I did not remove the plug wires from the stock conduit in the valley of the head. With the standard points and Flamethrower, I’m at 15 mmHg. Finally, I removed the exhaust and welded in O2 sensor bungs. The A/F ratio at idle is 12.5-7, about where an SU carburetted car should be, I think. On unloaded revving, it was in the 13-14 range, but I’ll have to evaluate those numbers when the car is roaded. At least, it’s not dangerously lean at higher rpms at this point.
At any rate, I have an engine that idles pretty well, revs great, cools and lubricates well, so I think it’s time to shut it down for now, get the body on, and start putting things together.
Ron