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Old 09-26-2018, 11:32 PM   #1
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Join Date: Feb 2012
Default SU needles / tune with ported B20 head

Hey, I'm putting my B20 back together for the hundedth time and I wondered if my jet needles are a little too conservative for what I'm doing. I just got the head ported and I'm expecting to flow a little more air at the top end. Previous torque curve with all but the head porting was enormous midrange surge that dropped off somewhat suddenly.

Stock 1969-1970 B20B bottom end
Isky street performance cam/lifters/pushrods
Cloyes timing gear set
Shaved and ported B20 head, not sure which but probably the less desirable casting
Rebuilt SU HS6's with "DX" needles, big honkin velocity stacks, stock intake
iPd 4-1 header and 2-1/4" glasspack
Stock B20B cast iron 078 distributor with vacuum retard unplugged

Do I need richer needles to support the increased head flow? Or is it not likely to be that dramatic?
I'm also curious what the ignition curve looks like from the stock distributor, and how much I have to gain from more timing.
EDIT: I have an 0231 153 003 ready to throw on, I've been told that 003 is best for performance applications short of using electronic ignition. Read up on the 078 and it's not terrible but flattens out quick.

Last edited by smncutler; 09-27-2018 at 04:09 AM..
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Old 09-27-2018, 10:15 AM   #2
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Join Date: Jan 2016

The hotter the cam the more initial advance and less centrifugal it needs. No one will know how much timing you need and where. You've made yourself a nice engine but now it's time to spend some money and have it tuned by someone who knows what there doing, especially with the SU's
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Old 09-27-2018, 10:28 AM   #3
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You’re probably going to want to see what the o2 looks like under load, but the sudden drop off would point at the 078 advance curve. How high are you going to be revving? The 003, is that strictly centrifugal advance? If so it’s pretty easy to swap springs (be careful when doing this) or lighten the weights and bump the base timing up. The result is a slightly broader curve that more closely follows the torque curve provided by your new cam. It’s a fiddly bit of tuning, but it can net good results if done correctly.
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Old 09-27-2018, 12:25 PM   #4
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Join Date: Feb 2012

003 is strictly centrifugal, yes. I read in the archives someone on here had a huge KG cam that idled at 30 degrees and only advanced about ten, if I recall correctly. Didn't really consider that fully. In the grand scheme of things my Isky probably isn't that big.
I feel like I should take it to a hot rod shop and get a full on tune with a dyno and clip on o2 sensor etc.
I'll be revving it to 6k some of the time, 4-5k mostly. I just need my AFR's timing and head flow to support my cam between 4500-6500
For different spring choices I should talk to Glenn Ring.
In the meantime it'll have 14 degrees base timing and be tuned slightly rich.

Thanks guys

Last edited by smncutler; 09-27-2018 at 12:45 PM..
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Old 09-27-2018, 12:47 PM   #5
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File a SMALL flat on the base of the needle about 3/8" down the needle this will enrich the mixture at idle, so that you don't over enrich the mixture once past part throttle. With a big cam this helps. Total timing is the key. Base, plus distributor I'm guessing 30 to 36 degrees.
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Old 09-27-2018, 03:10 PM   #6
Join Date: Jul 2018

I don't want to derail your thread, smncutler, but since your specs are so close to mine I'm interested in your results and whether I can use anything you learn.
Here are mine:

Late B20B head with the larger intake valves- ported and matched to an early intake (I have both of these--they're not on the car)
Early intake with the ridges smoothed out on the engine side.
Separate stock exhaust manifold and 2" exhaust (on the car)
Mechanical advance distributor (003?) (on the car)
SU HS6's (on the car)
D cam
VP performance lifters and push rods
Cloys timing set

I was planning on a complete rebuild when I can afford it but with ~130,000 miles on the engine maybe I can just do the head and cam. I think the compression was around 160 on all cylinders when I checked a year ago.

Which brings me back to this thread. I need to check which needles are in the SU's.
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Old 09-28-2018, 01:17 AM   #7
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Someone on an archived thread recommended 003 distributor and 34 degrees total timing, you were right in the ballpark!
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Old 09-28-2018, 01:29 AM   #8
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Nice setup Frank! I would be wise to look into a later head (next project, too late now). I don't know whether I have an early intake but Tom Bryant (really cool guy) told me the carbs were from a 67-68 140. I believe he put in a new set of the same DX needles.
I should do a good gasket matching. The ridges on the engine side of my manifold appear to have been ground down a bit, don't know if stock or not. I've been told that there are a lot of things that come into play when considering air scavenging, both intake and exhaust. Anyone feel like schooling me?
I have 380K miles on mine and all I've done is the head/cam/etc (twice now). After the overheating episode that brought about the latest head work, my oil change was pretty gnarly, I'm due for bearings at the least. I'll get it running as proof of concept and then pull it apart myself and do rings and bearings in a few months.
How would you describe your B20's power?
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Old 09-30-2018, 08:42 AM   #9
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Location: E(Seattle!Vancouver! San Francisco!LA!) Helsinski

KN or KD needles works nice on tuned B20, and I'd get 123 Distributor, even it cost money.
I helped one guy to install 123 on his 122S, and even his dist was in ok condition, the improvment was huge. Smooth running on 4th gear almost from idle. Starts immediately and so on.
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Old 10-01-2018, 09:50 PM   #10
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Before assuming anything about flow improvements from the ported head, have it tested on a flow bench and compare your results with stock heads. Then you'll be better able to gauge your needs in terms of carb needles and advance curves.

You can also get there by experimenting, which is fun but takes a LOT longer.

Also, not all ported heads are even remotely similar. Who did the work on yours?
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Old 10-02-2018, 12:56 AM   #11
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Thanks, I'll look at those needles. I'm sticking with points because I'm enjoying learning to hot rod the car like it's 1965.
I had TEM Performance in Napa CA do the head. With some tweaks it's now driving really well, power keeps going where it used to drop off, just what I was aiming for. I meant to take a picture of the casting number but forgot.
Currently at 53-54 degrees dwell, 22 degrees advance at 1000 RPM fast idle, and 38 degrees total. The 078 distributor can give you tons of advance but it gets there slowly. Today I realized I had been driving it around with 49 degrees total advance...idled weird but wasn't too bad. I'm looking forward to a steeper advance curve from the 003.

-Readjust valves
-Sync carbs with UniSyn
-Swap distributors and aim for 38 degrees total
-Make sure my AFR's are decent without buying the gauge
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Old 10-02-2018, 08:30 AM   #12
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Location: E(Seattle!Vancouver! San Francisco!LA!) Helsinski

B20A distributor 085 has been successful choice for most tuned pushrod Volvo's. Vacuum unconnected. Little fine tuning with static/basic tuning, and that's it.

When checking sync, do that not only at idle. Open the throttles so both throttles are activated by linkage, then check that sync again. May need helper with wooden leg. Part throttle sync improves response.
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Old 10-02-2018, 04:47 PM   #13
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Re: needles...

If you really want to understand the answer to your question, get the SU Haynes manual, which lists the exact dimensional 14 stage profile of each needle used with most any and every B18/20 motor.

You'll find that amongst those choices i.e. ZH, KA, SM, KN, DX, KD (roughly from leaner to richer) that there are 3 significant areas to consider: idle, mid-range, and top end.

The DX needle, used ONLY with the post-'67 (integral secondary throttles) "emission control" intake manifolds, is weaker (i.e. thicker) around idle and lower midrange than either the original low-compression motor needle i.e. ZH, or any other of the common choices.

The KD is the richest choice throughout the full range, somewhat thinner at idle and mid-range, and way thinner from there down the needle than any other choice other than the DX.

With enough compression and advance, none will be quite rich enough...
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