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I continue to struggle with my carbs and was hoping to get some guidance from the more experienced SU gurus out there:
1. Per the Haynes manual, I am setting jets to a baseline of 0.060" below the bridge, measured with the tail of a vernier caliper. This results in 24 flats down from fully up for the front carb, and 20 flats for the rear. Two questions: a) is it normal for the two carbs to differ by 4 flats? b) 20-24 flats seems like a lot, since I've seen 12 flats referred to as the standard baseline?
2. Using the baseline settings above (20-24 flats), I'm finding that I can adjust the rear carb by 2-3 flats so that lifting the piston results in a rise-then-fall in RPMs. However, lifting the front piston doesn't seem to do anything until I lean the jets (5+ flats) to the point where it starts to sputter and stall. FYI, floats are set to 3/16" (not sure if that affects mixture settings?).
3. I notice that when I pinch off the line from the intake manifold to the PCV valve (this is a B18), the engine stalls - but otherwise, the engine won't stall, even with idle screws all the way out. I've changed the PCV valve and lines and cleaned the flame trap. Is the PCV system behaving correctly (ie, should I be looking for problems elsewhere)? FYI, I have tested throttle shafts with carb cleaner (and even rebushed them), and tried re-centering the throttle plates (although, I dunno if I've done a good enough job).
In general, my gas mileage stinks -- both figuratively and literally: I get about 10 mpg and the car smells like fuel whenever it runs. (Although, this may have been due to overfilling float bowls, which have been remedied thanks to some new float bowl lids purchased from Agent Strangelove.)
A rundown of what I've tried so far can be found on a blog I'm keeping:
http://65brick.blogspot.com/search/label/carbs
Thanks!
Jerry
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Howdy,
FYI, floats are set to 3/16" (not sure if that affects mixture settings?).
According to the Green Book, the SU carbs are "relatively insensitive" to float position. BUT, the SU shop manual and "Tuning the SU carburetter" book both say that float level is very important to a proper mix. Which makes sense: the height of the fuel in the bowl IS the height of the fuel in the jet. If too high, too rich. If too low, too lean.
Phil's procedure is excellent. The only thing I do differently is to set the mechanical adjustments first. I also back the high idle screw completely out and disconnect the choke cable. Then after the carbs are tuned I adjust the chokes.
--
Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)
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Nice write up on the carburetor work you've completed.. I'd not seen that blog.
That said... to quote you:
"Lesson learned - it's impossible to tune SU's without having the rest of the motor in good order."
So, that's a good place to start, and that PVC thing sounds amiss.. I would pull the line at the manifold and plug the hole in the manifold.
Then, Get it running as good as possible, and go through ignition again. Mainly, check points and timing again.. another quote:
"6. timing - OK, I fibbed. I still need to get my hands on a timing light."
Did you get your hands on a timing light?
After knowing that all issues beyond the carbs are good, I would grab your Haynes manual again, find all pages that relate to carburetor tuning, tear those pages out and throw them away.
I'd say considering all that you've done, you know your carbs better than the book... Fine tuning is almost always done 'by ear', 'by butt', and by fuel gauge. (that goes for timing as well, but it's good to have a light to get it in the ball park)
I don't like the '21-25 flats' to get to that .06" baseline figure.. I'd be suspect of your measuring technique.. perhaps you aren't getting the back of those calipers quite flat... If you are dropping them down 2x the 'normal' baseline, that'd cause your problems for sure.
Also, lifting that piston by only 1/32"... I'd think that's rather small. Heck, the added air allowed by that much is offset by the width of the screwdriver you used to do the lifting..
Look for fuel leaks... even if running horribly rich, your car shouldn't smell like fuel. If it smells like unburned hydrocarbons from the tailpipe.. that's more likely running rich, but that's not quite what you said...
You'll get it, and you know that.. Best of luck!
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-Matt I ♥ my ♂
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don't have time to get into it but basically the haynes shows 2 different systems of PCV. Look into it, it is possible that a po has mix and matched the 2 together causing who knows what problems. basically there are 2 different directions of flow possible...
--
Patrick, '68 220, '92 Eurovan (work truck) '53 PD4104 (conversion).
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Pinching off the PCV line should have basically no effect, so that's the first problem to address -- you're not going to get the carbs to tune with that going on. Is the PCV valve in the right way around? Fat end goes down, closest to the teardrop.
I crank the jets all the way up, and then down 12 flats as a baseline. This is almost always too rich, but it's easier to go from rich to lean than vice versa. I have no use for the piston lifting thingie, so here's what I do from there:
Sync the airflow at idle with the idle speed screws, so both carbs sound exactly the same. Use a meter if you prefer.
Bring the jets up equally until you get smoothest/fastest idle. Then bring one of them up further until the idle degrades, back down until the idle degrades again, then back up a flat or two from there. Repeat for the other jet.
Readjust the idle speed, keeping the carbs balanced. Check the jets again, one at a time.
Then adjust the cranks on the linkage between the carbs so both pistons lift off at the same instant as you open the throttle slightly by hand. I'm more worried about them starting simultaneously than rising to identical heights.
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Phil,
When you're adjusting the jets for the best idle, do you blow out the carbs after each adjustment of the jet(s), i.e. by giving it a blip of throttle? Or do you not touch the throttle and just move the jets?
To the original poster,
I would agree with the suggestion to try running at 12 flats. It's hard to imagine a functional SU running too poorly at that setting. Also make sure your PCV lines aren't leaking anywhere. Perhaps replace the hoses and add some zipties to the connections, to prevent small air leaks. If you've got a brake booster running off the same line as well, make sure it isn't leaking air either.
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I don't usually blip the throttle while adjusting idle mixture.
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Thanks to everyone who has chimed in on this thread -- lots of great advice! I didn't have much of a chance to tinker this weekend, but did observe a few things:
I'm pretty sure Phil and others correct - there seems to be something wrong with my PCV system. When I remove the PCV valve and plug the nipple on the intake manifold, the idle dips down and smooths out. I suspect that the mixture will lean out significantly, too (hope to futz with the mixture/balance soon).
Can someone confirm whether the flange on the PCV valve (side with the smaller diameter hose nipple) is supposed to go to the flame trap, or the intake manifold?
Also, I checked my timing - it's around 17-18 deg - my Haynes manual says 19 deg -- does 1-2 deg make a big difference? (I'm worried about introducing more things I could screw up, so I've been hesitant to adjust it.)
BTW, I have to put a plug in for Phil's excellent article "All About Ignition" article on VClassics -- after reading it 5 times, I _almost_ understand the basics of automotive ignition systems!
--
Jerry | www.65brick.blogspot.com |
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The smaller end of the PCV valve goes towards the intake manifold, the larger end towards the oil trap teardrop.
Set the timing so the advance tops out at 34 degrees BTDC when you rev the engine. The last mark on the pulley is 30 degrees, but you should be able to "guesstimate" where 34 is. It's best to do this before you finalize the carb tuning.
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