B20 idle and low vacuum

I've been chasing idle issues with my B20 with HIF6 carbs. I rebuilt the carbs, surfaced the manifold, replaced all intake/carb gaskets, and checked that the brake booster is not leaking. Float level is 3/32 gap with carb body. Choke is only on the front carb. It's got a 123ignition distributor, timing is 10 degrees BTDC and advances correctly. Carb B20 with an early F head.

I've read this thread at least ten times.

Here's a very long checklist of what's going on:
- I can never ever even come close to passing the lift pin test. Even after 3+ extra turns rich, engine bogs and stalls. When turned way richer, it takes longer before the idle drops, but stalls nonetheless.
- At the "base" 2 1/2 turns, acceleration is a little bit hesitant and jerky. Turned 2 turns richer, it accelerates much more smoothly.
- I have to back the idle screws pretty much all the way out to get the idle below 1000rpm.
- Manifold vacuum reads ~14in (the late ign timing part of the gauge). Advancing the distributor raises vacuum, but not enough before idle becomes too high. Raising the idle via idle screws raises vacuum just a little.
- I have a metered port off the manifold connected to a vented oil cap. If I plug that port, the idle lowers a bit. Seems like this is just acting like a vacuum leak?
- When I rebuilt the carbs, the pistons only marginally passed the drop test. Not sure if this affects idle or just acceleration.
- Choke and Hot start valves are adjusted correctly and don't interfere with the throttle/idle adjustment.
- I've sprayed carb cleaner in every spot I can reach and appear to be no vacuum leaks.
- There seem to be two different specs between different heads... I've tried both. Currently at the tighter option.
- Compression test was ~135-140 across all four cylinders. I bought the car 5 years ago with a bad head gasket and replaced it immediately. Rob at Vol-tech took one look at the head and handed me "the thicker gasket". I took his word for it, and I'm assuming/hoping that the consistent low pressures are due to the thick gasket.

Spark plugs. I had them out a few months ago when I took this photo... I think they're in order 1-4, but might have gotten them mixed up. The 1-2 manifold runners are much darker than 3-4, which I always chalked up to the front-only choke.
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Video of Vacuum at idle

This has driven me nuts for years, and as you can see I've checked a lot of potential causes, but haven't come up with anything. I fantasize about the lift pins just making the idle raise then settle nicely. Anything stand out?

To be honest, I gave up on the low vacuum fight on mine years ago. I've had dozens of people think they know the magic problem until I tell them I've already BTDT. Mine in stock form and modded form, doesn't matter k-jet or MS, stock head and good F cam or VPD head and cam, it's been 12-14", period. Everyone says it should run like crap, but ya know what, it really doesn't, and at 9:1 it's putting down 135rwhp, not too shabby, with a lot more to be had from it.

gsellstr said:

To be honest, I gave up on the low vacuum fight on mine years ago.

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My number one issue is I'm tuning the carbs blind (can't use the lift pin test, and have to guess how many turns richer I go). Number two is setting idle rpm because backing the idle screws all the way out sometimes gets me 500rpm and sometimes 1000rpm. These are not supposed to be that complicated, and I am aware that when carbs need a lot of finicky adjustments, it's often a problem somewhere else in the engine. I thought maybe the low vacuum (and kinda low compression) could point me in the right direction, or at least an explanation.

At this point can only really tune by reading plugs.

You've got oil in the dash pots?

Thankfully I don't have that issue, but tuning in MS was a little interesting. lol

svinkle said:

My number one issue is I'm tuning the carbs blind (can't use the lift pin test, and have to guess how many turns richer I go). Number two is setting idle rpm because backing the idle screws all the way out sometimes gets me 500rpm and sometimes 1000rpm. These are not supposed to be that complicated, and I am aware that when carbs need a lot of finicky adjustments, it's often a problem somewhere else in the engine. I thought maybe the low vacuum (and kinda low compression) could point me in the right direction, or at least an explanation.

At this point can only really tune by reading plugs.

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you have leaky throttle shafts. it will most likely never stay where you set it, and will constantly change while driving.

Also, what exactly do you mean tuning blind? Are you are visually impaired? or just have no idea how they function?

Just to be clear, the (original conjoined secondary butterfly???) exhaust manifold pre-heat flap isn't leaking at its shaft?
Or someone pounded a brass plug in it (seen that a few times)?

Valves seal/aren't rising/have hardened inserts and come out at the right height?

Tests done with all vacuum lines blocked off?

Cam lobes aren't flat/rise all the way up? Original C-cam? Cam timing/ignition timing sounds like its right.

You rebuilt the crabs or what exactly?

Smoke machine is so difficult to have access to in these post-crab days for those with the itch...
...tho if you can find one, someone that has it probably uses it so seldom they'd sell it cheap/let you use it as an excuse to make sure it works/test it out?

Original pierburg fuel pump not over-pressurizing the crabs/fuel shuts off @ float?
Fuel tank charcoal can/vacuum mess volvo revised a bunch of times working 'right'?

Warmer days can be way worse on crab cars even if all check valves/fuel tank venting works 'right' on a 140; fuel tank expansion chamber not especially generous & finicky, you can have fuel get forced past the float valve in some cases in the heat.

Make sure engine's healthy, convert to D-jet?
Better economy, emissions, long term engine wear/longevity, but more electrical to leave you stranded?

Catch-22 of crabs; wrong amount of fuel all the time...but...all the time/few electrons involved...

Even if the 1971 high compression engine's tight/HIF are perfectly sealed along with the conjoined/double butterfly manifold doesn't leak, it's real touchy to get it all balanced...timing has to be right at 10, fuel has to be fairly potent etc.
Carb needles are touchy, dashpot oil is touchy.
It's annoying at best.
HS-6 they didn't try to make the carbs more temp/altitude compensating automatically w/ the airbox feeding the carbs what was supposed to be regulated ~80-85 degree air at all times (assuming the airbox pre-heat/wax pellet's any good).

Original 1971 crab head is arguably a better matched set with the original C-cam/no advantage to the late low-compression FI head in that stock driver setup.

Difficult to get it all to sing in concert on gas that's got a bunch of alcohol in it/not very potent on cars mixed/matched around to keep them going 50 years later, but not impossible...
...at least the new iowa farmer lobby ethanol snake-oil scam BS doesn't eat your pierburg fuel pump/hoses/stromberg diaphragm like the MTBE toxic waste that preceded it did maybe eh?

12-14" of idle vacuum on horrible/minimally potent CA gas with the low compression F head sounds pretty believable...passes emissions nicely/closer to all alcohol though, but octane & energy density, not so much heh.

They never really had these design constraints in mind with modern E10/15 gas with HIF-6 & B20Bs 50 years later and HIF-6 don't always wear well/rebuild easily/match up nicely.

FI, at least you can atomize the fuel finer/the brain calculates these things based on the temp sensor/MAP sensor.

That kind of resolution with dual crabs all but impossible.
Hence in the emissions era, B21A isn't meant for power at all and has just the single SU HIF-6 (which still wears out ), has a real mild cam/compression and there isn't a B21B option.

This isn't just volvo though, trend generalizes; single carb, million vacuum lines/pre-heat contraptions/air pumps etc, or FI & newer clean slate engine design late 70s & up.

18-20" on the T-cam B21FT at sea level/manual trans is about all I get anymore on a 90K mile B21FT at the time, 60-70 degrees out, 40% humidity.

Hotter/thinner air/elevation don't help.
Even with constant idle control, wyoming or high desert on a 110? day can cause some interesting running abnormalities.

Still think you probably have a vacuum leak or somewhat uneven leak-down/compression & dashpot dampening that's subtle and all but impossible to find/sort out though.

Had better luck getting the rebuilt HS-6 to run nicely on a healthy engine/modern gas than the HIF...maybe it's just that the needles don't touch/try to compensate for temp & the like?
Or hot weather/emissions controls they added doesn't play well?

Have to re-adjust for large elevation changes on the HS, but that's about it/mostly work otherwise.

Late HS-6 on the conjoined dual downpipe manifold no double butterfly has been the most fuss-free to setup all else healthy of the factory dual carb options 50 years later.

Or strombergs if you can get a quality replacement rubber diaphragm for them (german copy of SU minus dashpot oil variable/matched piston riddle) were less fussy than the king of fussy HIF/double butterfly emissions control manifold combo.

It'll only take ~6 parts cars to recombine the factory rare/obscure 140/164/1800E junkpile to eventually factory EFI, power steering and some 684001 wheels with modern 195 tires, don't worry...

FrankBooth said:

You've got oil in the dash pots?

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Yep.

cwdodson88 said:

you have leaky throttle shafts. it will most likely never stay where you set it, and will constantly change while driving.

Also, what exactly do you mean tuning blind? Are you are visually impaired? or just have no idea how they function?

Click to expand...

I replaced all seals including throttle shaft seals. I wasn't very impressed with them... kind of a loose fit ID and OD. I've read a bunch about HIF issues on brit-car forums, and saw some folks using rubber grommets instead of the SU seals. This week I replaced the seals with grommets that were a much more snug fit, but do not interfere with the throttle closing all the way. No leaks per carb cleaner test.

By tuning blind, I just mean that everywhere references the lift pin test. Seems very straightforward (lift it, if it bogs, it's lean...). Mine just always bog no matter how many turns richer I go. I agree that it sounds like a symptom of throttle shaft leaks letting "unmetered air" in, but I think I've ruled that out.

I don't claim to be a carb whisperer, but I understand in principal how HIF's work, they're just not behaving the way I expect, which is my I began to suspect other issues on the engine side of things.

Have the throttle shafts been fitted tightly into hardened bushings/inserts or sealed bearings?
Or still just rattling in ogged out aluminum with years of use?

You'll never get it to work right if the latter is the case, even if the engine opens/closes/seals all the valves evenly with even compression/leak down.

Leak down gauges are dirt cheap, measuring push-rod lift is easy enough?

Perfect balance between carbs while nice isn't the end all be all; at idle they share the manifold/aren't isolated & in theory the mixture goes around the hot exhaust.

You're already letting computers in/EMP vulnerability with the 123 dist, what's the full distance to full fuel-infection/D-jet EFI?

Kjets On a Plane said:

Just to be clear, the (original conjoined secondary butterfly???) exhaust manifold pre-heat flap isn't leaking at its shaft?
Or someone pounded a brass plug in it (seen that a few times)?

Click to expand...

I have the B18 separate alloy intake, with the dual port exhaust manifold.

Rebuild was all new seals, jets, float needles, and metering needles. Got it all from Joe Curto and he recommended the needles, not sure which ones they are.

I have not taken a look at or confirmed that the valves and cam are behaving properly.

I don't know about the condition of the valve seats. This motor was supposedly rebuilt at Vol-Tech 10+ years ago, then never really run much before I got it (other parts of the project never got done so it sat).

Maybe this is all just the modern fuel making things behave weirdly, and I just need to be happy with them running pretty well off idle and get over it.

Kjets On a Plane said:

Have the throttle shafts been fitted tightly into hardened bushings/inserts or sealed bearings?
Or still just rattling in ogged out aluminum with years of use?

Click to expand...

I thought that was much more of an issue with HS carbs and not HIF?

Leak down gauges are dirt cheap, measuring push-rod lift is easy enough?

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Can't argue with that.

Perfect balance between carbs while nice isn't the end all be all; at idle they share the manifold/aren't isolated & in theory the mixture goes around the hot exhaust.

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I've got a sync tool/gauge/meter and it's pretty easy to get airflow balanced.[/quote]

You're already letting computers in/EMP vulnerability with the 123 dist, what's the full distance to full fuel-infection/D-jet EFI?

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The next cheapskate owner of this 140 can explore that avenue.

^
Gotta leave something for the next guy?

Had HIF have wobbly throttle shafts too, especially pushing on the double-butterfly manifold (as intended from new) at the spring-loaded crank/cam there wears out the carb throttle shaft clearance ever worse.

Many aluminum manifolds are cracked in sneaky places by now & need a weld & subsequent resurface after the weld-repair. Don't care for those things for a fuel-efficient DD.
Conjoined/built in pre-heat idling really is kind of better for longevity/idling quality.
As is working airbox pre-heat/regulated thermostat.

YMMV. Those things are fussy/less potent gas doesn't help, but vac leaks or other engine issues make them impossible to tune too.

The seals can seal the throttle for a vac leak(ish) per carb cleaner/unlit propane torch test well-enough (even though seal quality these days is dubious), but the throttle blades can shut inconsistently if that makes sense with wobbly shafts?

Really makes for a miserable struggle if the thing won't shut consistently/in exactly the same position & use the idling air bypass properly even if the seals prevent an external vac leak at the carbs themselves.
The shutting of the idling screws is kinda the tell to me...if the throttle blades are hanging up in the bores/not stopping in the same position consistently or you're having to move the mixture that much, something's not right carb/manifold/vac leak wise probably.

Hiperfauto sells brass plugs for the 1968+ double-butterfly conjoined manifold to get rid of the secondary air blades/butterfly, presumably you've got one of those from the parts car to effectively make that into an all-iron conjoined manifold.

How have you hooked up the engine venting piping and piping to the servo? Couple of pics would be helpful. NRV on the line to the servo, fitted and working?

svinkle said:

Yep.



I replaced all seals including throttle shaft seals. I wasn't very impressed with them... kind of a loose fit ID and OD. I've read a bunch about HIF issues on brit-car forums, and saw some folks using rubber grommets instead of the SU seals. This week I replaced the seals with grommets that were a much more snug fit, but do not interfere with the throttle closing all the way. No leaks per carb cleaner test.

By tuning blind, I just mean that everywhere references the lift pin test. Seems very straightforward (lift it, if it bogs, it's lean...). Mine just always bog no matter how many turns richer I go. I agree that it sounds like a symptom of throttle shaft leaks letting "unmetered air" in, but I think I've ruled that out.

I don't claim to be a carb whisperer, but I understand in principal how HIF's work, they're just not behaving the way I expect, which is my I began to suspect other issues on the engine side of things.

Click to expand...

I wasn?t ever impressed with the SU shaft seals for the hifs. I had found some really nice radial shaft seals from a bearing supplier. I just gave them id and od and they figured out what to do.

The hif has some interesting things about them. The little temp compensation deal has always been good, but sometimes they do goofy things. While it was apart, did you check out what needles it?s running? What size jet?

Then there?s the lift pin... they tend to lift it further than what they were intended or not enough if there?s gum or a binding spring in there. So a stack of feeler gauges or a measured thickness gauge pin is the ticket. 3/32.

With the air cleaners off I use a flat screw driver to lift the piston 1/8" to check the mixture, never mind the lifting pin.

cwdodson88 said:

I wasn?t ever impressed with the SU shaft seals for the hifs. I had found some really nice radial shaft seals from a bearing supplier. I just gave them id and od and they figured out what to do.

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There is an updated ?later? seal but seems like it?s only available in the UK. Maybe Joe Curto can get them, but I never checked. The rubber grommets seem to fit just right.

The hif has some interesting things about them. The little temp compensation deal has always been good, but sometimes they do goofy things. While it was apart, did you check out what needles it?s running? What size jet?

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I replaced the needles and jets when I rebuilt them a couple years ago. Just did whatever Joe recommended.

Then there?s the lift pin... they tend to lift it further than what they were intended or not enough if there?s gum or a binding spring in there. So a stack of feeler gauges or a measured thickness gauge pin is the ticket. 3/32.

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Thats the first I?ve heard of that. Good info!