Lean idle. Rich cruise.
Dyno'd it when built. Engine dyno. Locked.
The only curve that even was close was 10 degree bushing with two light springs. 25 initial, all in by 2500(35) It hated that the least. I tried every spring combo possible. Went all the way to 2 heavy springs. 25 degrees initial and all in (35), at 4150RPM. Also every friggin setup was rich cruising to the local "spot".
Weeeeeee. I gave myself a gold star for remembering you had your engine on the dyno.
I’m going to hold off on the timing curve a bit and talk about some of the other stuff you’ve posted.
as stated earlier my carb is the XP Ultra 850.
MAB is 34. I went down to 30 and it was richer, just like I thought.
On an 850 my starting point for a MAB is .026 and no bigger. To start. There are times to go bigger but most times .026 is the best starting point.
I get that it went richer on the smaller MAB but it only does that when it’s on the booster (pulling from the main jet). In other words, until the booster started, it SHOULD have been leaner and probably was.
That’s because the SMALLER main air bleed STARTS the mains (booster) later than a larger MAB.
So it should have been left in there (that .030 was too big too) and then it would have been leaner, longer because the smaller MAB delays the start of the main jets (and by that I mean the booster and I use jet and booster interchangeably so don’t let my double talk confuse you) and the opposite is true.
Let’s say you made a big swing the other way. Let’s say it’s lean after the T slot or better yet it’s lean transitioning from the T slot to the mains.
To clean up that lean spot in transition you can start the boosters (and by that I’m saying the main jets) sooner. That’s adding main jet fuel EARLIER in throttle opening.
So we drop in a honkingly large MAB of a staggering .048 thou!! That big mother will start the boosters way sooner than a .026 MAB because it gets main jet fuel to the engine sooner in the throttle opening.
The question is why and how does that work. Why does the MAB affect the start of the booster AND why does it affect WOT differently?
That’s because at low throttle openings (and therefore low air flow) that MAB wants to THINK or better yet wants to ACT like an emulsion bleed. Both are similar but different.
That’s because like I said that MAB acts like an emulsion bleed in that adding in atmospheric pressure through the bleed at low throttle openings makes the fuel in the main well lighter, and therefore it flows out to the booster and starts fuel flow through the boosters. That’s the BIG MAB we are talking about.
That same big air bleed, once the fuel is moving through the boosters and air flow through the carb is increasing it starts acting like an air bleed again and it then “bleeds” off booster signal and leans out the upper part of the fuel curve more than if the air bleed was smaller.
Back to the .026 MAB example, we see the opposite effect. That smaller bleed by its sizing allows LESS air to the main well (did I mention the MAB is directly linked to the main jet well?? Don’t think I did but it does and that’s important to say and remember) and therefore the the fuel is heavier (air mixed with fuel is lighter by volume than solid liquid fuel) and it takes a greater pressure differential at the booster to get the fuel moving.
When you screwed in the smaller air bleeds it would have been leaner BEFORE the booster starts and then richer at WOT because the smaller bleed doesn’t kill as much signal as a bigger MAB.
So… if you want it LEANER at low throttle openings you use a SMALLER MAB and if you want it RICHER at low throttle openings use a larger one. Then to correct WOT A/F ratio you change the main jet.
If I’m not clear the MAB controls when the booster STARTS and how much of the “signal” is bled off at WOT.
If I was working on your stuff before I ever touched the distributor I would correct the MAB first, make my best WAG (Wild Assed Guess) as to what main jet it will need, erring to the slightly rich side.
Then I look at the emulsion stack. Very few carbs EVER want 4 emulsion bleeds. In fact some of my tunnel ram stuff has just come emulsion bleed of .026 thou.
Saying that, I’m saying you have too much emulsion. At least DOUBLE what you should have IMO. I mean two .028’s is plenty. Having FOUR open emulsion holes is too much.
So what happens when you have all that emulsion?
You get what’s called “slugging” and you can see this in the A/F ratio. It’s unstable and looks like a saw blade.
That’s because you want to introduce small air bubbles into the main well. Remember emulsion is to get fuel flowing to the main well at the right time and as the float level drops.
The harder the booster “pulls” on the main jet the less emulsion you need. And a smaller main jet to compensate.
With too much emulsion the bubbles will grow and coalesce into a big bubble. When that happens you get a big slug of air (engine goes lean) and then you get a big slug of fuel (engine goes rich). And you get the saw blade effect on the fuel curve. It goes lean/rich, lean/rich constantly.
You can’t develop a tune up doing **** like that.
I hope this is making sense because I’ve spent a lot of time here because IMHO your carb is off far enough that it’s going to affect every other part of your tune up.
Now would be the perfect time for
@Hysteric to come in and discuss why this carb work should be done BEFORE you do any timing curve set up.
It’s an about prepping the fuel BEFORE it gets to the chamber for the most complete burn you can get and how fuel size in the chamber affects tuning.
I just don’t want to type that much and he gets it. We just have different ways of getting there lol.
Ok, so I’d start with a .028 emulsion bleed in the top and third hole down, and block the second and fourth holes. You can do that with 6-32 brass set screws 1/8 inch long.
That will clean up the slugging.
Then make DOUBLE DAMN SURE the idle feed jets are in the low position. When looking at your metering blocks there shouldn’t be ANY brass above the top emulsion hole. If there is, it’s the idle feed jet and it’s to get moved to the low position.
You may have to drill and tap the lower hole to 6-32 but it’s easy. Go slow and use a lot of lube (that’s what she said) and it’s down where it should be.
Like all those open emulsion bleeds, having a fuel jet ABOVE the fuel. It just pisses off the carb at idle. That’s the kind of thing that makes people lock out timing and stuff like that.
I’m not even saying you are wrong. I’m saying what I would do long before I locked out the timing. But if your carb is as you say it is, and no o e here should doubt what you say I expect that some of your issues are carb related.
Summing it up, we fixed the emulsion, lowered the idle feed jet and trimmed the main air bleed way down. For your 850 I would START (and tune from there) with an .028 idle feed jet (restriction) and a .070 idle air bleed. We have a .026 MAB bleed installed and we have a primary main jet of .078 and a power valve channel restricter of .058 with a 6.5 power valve. A secondary jet about .084 is a nice start.
At those settings you should be on the rich side everywhere.
Now we can start looking at your timing curve. Almost forgot that your T slot restricters should be .068-072 and tune from there.
Emulsions are brass, all open. Never measured them but they appear to all be the same size. Seems ridiculous to me. Back in the day we didn't see that on the old 750 DP, or even the old Dominator 750. Which I thought were EASILY tuned.
And forTT5.9, my wideband data logger is my nephew with a cell phone.
As for checking each mod, I use a Draggy. All passes were close in temperature and RH. Really didn't need it though. 35 locked is dramatically different than all 9 curves(spring combo's) I tried.
I may be a little carb stupid, but I've been at this game a loooong time. Just trying to get it as perfect as I can. Been running around rich at cruise for a few years and just figured what the hell, it's great at Idle, great at WOT no big deal. Now.....I'm irritated as hell that I cannot get it to 100%.
Personally, I think I'm on the main jet at cruise RPM. 80's up front, 86 in the back. 2 4.5 P valves. idles 10 inches vacuum at 1150 RPM.
Look guys, I know this is rambling but I really would like to get things a bit better. I do plenty of cruising around, sometimes for nice long rides. If it was a total drag car, I would leave it alone.
Back to your timing. I’m supposing you had every curve in by 3500 rpm or less. And locking out the timing is essentially all in on the starter.
So how can I say this is wrong the vast majority of the time? I know I can’t give a percentage because if I do some freak will say I talk in absolutes. I don’t. There are times you lock out the timing but I don’t think you are one of those cases.
I can say the curves you tried are probably wrong because all of them had the curve in before the engine reached peak torque. And that’s a bad thing as I will attempt to explain.
Hold on, **** just cropped up. I’ll finish this later.