In reply to Trent :
I still have the enrichment circuit hooked up on my car, but I don't think of it as a choke in the traditional sense. I have used it to start the car in cold weather.
If I have a chance I'll see about reaching out to Bud at Redline.
Update: Thanks to a fellow GRM'r, I got a selection of jets to test with. I've only played with the idle jets so far, since they seem the most likely culprit (as a reminder, the problem is bucking while cruising at 2000-3000rpm, throttle just cracked open to maintain speed; coasting or accelerating, there's no bucking at all). I started with 50F8s, and ran through a number of others, as follows:
45F9: Bucking eliminated, but power in the midrange seems down a bit
50F9: About the same as 50F8, maybe a tiny improvement
55F9: Bucking worse than originally
55F11: Bucking worse than originally
So it seems that the idle jet needs to be leaner to eliminate the bucking, but I'd rather not lose midrange power as a side-effect. I currently have a F9 emulsion tube installed, and I have an F11 to test with, so I will try that with the 45F9s to see if there's any change. The F number of the idle jet is more important as you reach to high end of the progression circuit, so perhaps if the emulsion tube doesn't change things for the better, I need to get some other permutations of 45 idle jets to see if I can dial it in. There isn't far to go to the rich side - only F6 and F12 are richer than F9 - but there are a lot of levels of lean to try, though I don't think it's common to go beyond the next steps of F8 and F11.
Is the power loss far enough into the midrange that bumping the mains up a size is worth exploring?
Feeling adventurous and have a tiny drill set?
I bought a wide band 02 sensor to tune carbs. I went so far as wiring it to a Megasquirt to log it so I could look closely at the results, not try to watch digital numbers change on a guage. There are probably easier ways now, that was about 20 years ago.
As you know, the butt dyno will lie. A stumble is easy to feel, a surge could be a good spot in a bad tune, but is it lean or rich?
I've tried bumping the mains from 125 to 135 when I had the original idle jets installed - it was not helpful. I can try again now that the idle jets have been scaled back.
A wideband is a very good idea, but since I'm really trying to make an effort to drive the car as much as I can this summer after years of inactivity, it's going to have to wait at least until the season ends. Right now, I'm trying to dial it in well enough to make driving it pleasant.
I do have a tiny drill set, but I am not yet feeling that adventurous.
Okay, progress!
The butt dyno can lie, but if a well-tuned backside detects that the car is happier, it probably is. You said that going from 125 mains to 135s did not improve things, and that tracks if the idle jets were already too rich. You also said earlier that top-end power improved when you went from 125 to 135 mains. It may be worth trying leaner idle jets yet, until the idle is best at 1-1/2 turns out on the mixture screws, and then bump the mains up one size at a time until low-speed behavior smooths out. At that point you can start playing with air correctors and emulsion tubes to tune the upper portion of the power and airflow range.
Before going any further, have you confirmed that the valves are adjusted correctly?
In reply to DarkMonohue :
Agreed that the butt dyno can be a filthy liar, but the bucking behavior is unmistakable. As to power, well, that's another question, but I try to test by driving the same loop of local roads and highway, so at least that's not a variable.
Valves should be fine, as I regularly adjust them, but I have not confirmed since I awakened the car this spring.
Idle mixture screws are not even a full turn out, and going further out lowers and roughens the idle. The 45F9s are the leanest jets I have currently.
Your comment about the mains is making me question what I have in there now. My notes are getting a bit cluttered, so it could be that I left the 135s in. I didn't have the main jets out today, but I'll pull them to confirm when I next have time to do some more testing.
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