Check your Choke Tech
Originally posted 10/00 R. Kwas Revs on-going. [Comments added]
added Choke Adjustment Notes and Checking the Mixture 10/01
added Start this @#%$% thing in the cold (Brickboard response reprint) 01/02
added Choke Adjustment 02/05
Choke Adjustment Notes
added Jet Lubrication and Bowden Cable details 5/2016
The original Check You Choke tech article was really only intended to make owners aware of, and cover the pulled cable sheath problem, and as time passed, I just attached other choke and carb related items here…Since it’s now wintertime (up here in the northern Hemisphere anyway), the topic has naturally come up again, as owners try to start their cars in frigid January temperatures. The choke function is once again on many vintage Volvo owners’ minds.
Appendix 1 Lubrication
Appendix 2 Modifying Cable Sheath Clamp with Barbs to assure grip and prevent slipping.
Choke Adjustment Notes
SU Choke Return Spring
Presuming your SU carburetors are maintained and in generally good working order (which is enough of a subject for another article), this article is intended to help in making certain your choke will work properly for you in the next cold season.
On frigid mornings, not too far in the future now, a good working choke is absolutely essential for getting our SU equipped SUV (Swedish Utility Volvo) started. This, along with among other things: A fresh, fully charged battery (see Alternator Upgrade kit and Restored Fuseblock); clean, tight electrical connections - especially on high current starter wiring-and don’t forget the chassis to engine strap (see Gas-Tight-Joint article); and a Start Switch Kit to eliminate that “I-wonder-if-the-key-will-twist-off-today” feeling (do you see a pattern here)?
A simple check that you might want to do on the choke and it’s adjustment before those arctic mornings arrive, follows: Have an assistant operate the dashboard control handle slowly (engine off) while observing motion of the linkages and carburetor jets in the engine compartment.
Picture 1. Choke Linkage including Fast Idle Cam / Follower...no that's not a wooden intake manifold, but a mounted, "training carb" which allows a close, clean, inspection at Nino's Italian Restaurant Technical Sessions...after Lasagna and Vino!....molto benne!...and it makes an interesting paperweight when not being used as a teaching aid.
Picture 2. Jet in Choke (lowered) position
With the Choke Control pulled out to it’s limit (about two inches), one should see the jets of both carbs lowered about .25 inches from their normal, raised positions [see Picture 2], and the cams of the fast idle mechanism contacting their adjustment screws and lifting their respective throttle plates slightly off their idle adjustment screw stops. [We’re presuming everything is reasonably well lubed and operating smoothly - if not deduct 2 points for neglect and see Appendix 1] Any less motion at the jets will probably not give sufficient enrichening of the mixture to get a start on a frigid morning. This motion is a function of the cable stop position adjustment, and can be achieved only if the full two inches of control travel is transferred all the way to the carbs. If the two inches of cable travel are not detectable at both carbs, a sticky, unlubricated cable sheath may simply be causing the slipping of one or both of the sheaths in the clamp at the dashboard end...more on this below.
Picture 3. Choke Cable Clamp / Choke-On Detect Switch (Under Dash)
I have found that a good preventative (after well-lubricated cables), is a sheath clamp, modified with some barbs [see: Appendix 2], which will grab the rubber covered sheaths of both cables more positively and prevent slipping. Once the choke is working as it should, it is a joy to behold its honest functional simplicity. Plus, you can amaze your friends and coworkers with your dependable starts saying “the colder it gets, the more it likes it…I guess it thinks its back at home in Sweden!”
Choke-On Detect Switch
Also, how’s your gas mileage? If you tend to forget to disengage your choke after a minute or so, or after the temperature needle leaves the peg, you might appreciate a Choke ON indicator light or chime which will remind (nag) you that your choke is still ON, and supplying way more fuel than the engine needs after it’s warmed up. I developed one for the snow weasel [see above], after I had gotten to work a few times and realized I had forgotten about the choke…it did wonders for my mileage! See upgrade kits for Choke On Reminder Kit you can install into your Amazon…not available for 1800 owners…their babies are probably under a cover in the garage after November anyway-except maybe Irv G’s-who's probably trying to figure out a way to drive to Sweden to get to that second million!
Choke Adjustment (See also Choke Adjustment Notes below)
A fully charged battery, good electrical connections for the entire current path of the starting current, a healthy starter, and a well working ignition system in a good state of tune are all necessary, but without a lot of enrichening (commonly called Choke because it was derived from blocking off the airflow on “normal”, downdraught style carburetors) there is little hope of starting a beSU’ed engine. The easiest way to check this is to check the jets at the carbs and this is covered in the Check Your Choke Article . Once you have assured the choke cables and mechanisms are indeed giving about 1/4” of jet lowering, the enrichening will be sufficient to allow starting in even the very cold conditions (other tricks like engine block heaters, timing retardation, and spark plug preheating may be required if you happen to be starting your BV202 Bandvagn at the artic Research station in Thule Greenland, but ULTRA-COLD starting is beyond the scope of these notes).
Once started with extra fuel enrichening, the mixture, the Fast Idle (FI) then sees to it, that the engine idle is raised somewhat in order to assure that it keeps running. Without it, at normal idle speeds, the (gobs of) additional fuel which choking adds, would surely foul the plugs, causing them to misfire in short order and eventually stall the engine.
Because it is dependent on many things like air temperature, Relative Humidity (RH), basic mixture setting, choke adjustment is typically a pretty subjective adjustment set to someone’s preference. Letting a shop make the adjustment (if you can find a qualified one!) means they set it by the mechanics’ judgment and preference, valid for conditions when he happened to be working on the car. Your preference, influenced by conditions when you do your starting and driving may cause you to modify his settings…as they say: Your results may vary!
Pull the choke control out all the way, keep you foot off the throttle, and energize the starter…it may take a while of cranking until there is sufficient mixture in the cylinders for ignition. This is because atomization of fuel is quite poor at the slooooow venturi velocities at slow cranking…and the fuel temperatures don’t help. I will typically have slightly thinner 10W-30 weight oil in the crankcase for the winter season which helps with cranking, but the bottom line is that a lot of fuel needs to be dumped into the cylinders at this point.
Once started, the idle must be raised enough to keep the overly rich mixture from fouling the cold (and unable to burn away combustion products) plugs, eventually bogging the engine into stalling (and the fouled plugs wont help attempts to restart). This is where Fast Idle (FI) comes in.
Fast Idle (FI)
The FI is accomplished by a screw riding on an eccentric cam actuated by the choke cables, and since the FI cam is actuated by the choke linkage, FI RPM depends on how far the choke control is pulled out. I don’t recall want the manuals call for, but I like the FI to be adjusted to raise the idle to 1800-2000 RPMs at fully pulled choke. Adjusted this way, I find that I can adjust the amount of choking and FI RPMs well with the last 1/2” of Choke control travel.
Adjust the (sensitive) FI screws equally on both carbs in increments of 1/8 turns while listening for the RPM increase.
Throttle Response (TR)
After 30-60 seconds of choked warm-up, or after a trip around the car with a scraper to clear the greenhouse and defroster air intake of snow or ice, I’ll move away slowly, generally driving like the preverbal “little old lady”, until I see signs of life from the temp needle (and only then is it time to set the cabin temperature/defroster control to the warm position).
Driving when the engine is cold but choked does improve the “throttle response”, but since we should be driving fairly gently until the engine is somewhat warmed up, TR is really not the right term, and it’s not what we should be checking at this point. Drive with gentle throttle inputs when the engine is cold and before the temp needle lifts from the cold peg…and if you are opening the throttle fast enough to get a noticeable bog or even a lean misfire, you might want to enrichen the mixture a little by pulling the choke a little more, and go just a bit easier (or try Decaf, and getting up just a little earlier!)!
Deactivating the choke
Even though the jets are lowered against spring return force of a spring located at the carbs (a couple of versions were used), but even with nicely lubed cables, there is enough friction which prevents the control from returning on its own.
At the latest, you should return the choke control to its rest position when the temperature needle is well off the peg and in the cold end of the gauge range. Subject engine to max load (flooring) only after cooling system temp indicates in the normal range. I personally wont subject the engine to maximum load until long after that, since the oil temperature lags behind the cooling system by minutes, so true engine thermal equilibrium occurs much later (if at all in artic conditions!).
* Link to interesting Brickboard Choke thread: http://www.brickboard.com/RWD/index.htm?id=912001
Appendix 1 Lubrication
Lube Choke Cables: Thick oils are OK I guess, light WD40 alone is worthless in the long run as it is mostly solvent which dries leaving little behind, but Lubricating Particulates are the way to go! Try this: Thin anti-seize (graphite filled grease) to pouring consistency (a few fluid ounces should do it) using almost any solvent in your garage (including WD40), disconnect cables from carbs (noting adjustment so this can be restored upon reassembly), hold cable ends up high, and pour mix into cable sheaths (catch potential mess under dash with a rag wrapped around cable ends there), and keep pouring until it does come out at the dashboard end. When solvent evaporates, graphite, nickel, copper solids remain to permanently lube cables…and while you’re at it, a dab of anti-seize into the threads of the cable stop during reassembly will keep these from ever corroding into uselessness. This is a bit of a messy procedure, but only necessary once a decade!
Lube Carb Lnkages: White grease is probably the best over oils which just don’t last, and anti-seize which I like for its Particulates, but which can be messy, resulting in the dreaded Tin Man Syndrome (not on the car - on you!).
Jet Lubrication: As a moving mechanical assembly, the Jet on Jet Bearing Assembly are moving surfaces of brass on brass, a bit of lubrication is in order. This can be applied to the exposed area of Jet when it is in the lowered, Choke Position, and should be applied to area between Cork Washers when Jet Bearing Assembly is apart and being reassembled. This lube is captured between Cork Gland Washers.
Graphic of (early) SU Jet Bearing assembly...later ones are functionally similar, but not nearly as complicated!
Appendix 2 Modifying Cable Sheath Clamp with Barbs to assure grip and prevent slipping.
Modify (all) clamp with barbs to prevent sheath slipping…punched in with a sharp center-punch held at a shallow angle to the axis of the cable while the plate is well-secured in a vice [see picture 4]). This will displace the metal into a hole with an adjacent sharp barb.
Picture 4. Cable Sheath Clamp being Modified with Barbs
Reference Information: From a popular reference site: A Bowden Cable is a "...flexible cable used to transmit mechanical force or energy by the movement of an inner cable relative to a hollow outer cable housing." See also: https://en.wikipedia.org/wiki/Bowden_cable
Bowden Cable Detail:
Cable slips when Cable to Sheath Friction exceeds the Clamp's ability to hold the Sheath as Control is moved. By adding Barbs to Clamp, and Lubing Sheath, proper operation with no lost motion at working end of cable is assured!
PS. Do you get the feeling my cars are a rolling advertisement for anti-seize? I guess you’re right! I think the stuff is great - it permanently lubes, it permanently protects from corrosion, it even allows your wife to be able to remove lug-nuts with the hand-wrench to change a flat tire-or at least the helpful guy who stops to assist!
Cable Terminations at the Carbs:
Choke Adjustment Notes:
E-mail question by Eric M.
I just read the "Check your Choke" article on the SWEM website. Coincidentally, I've been trying to get the SU's on my '67 1800 synchronized and chokes adjusted correctly for the past week.
I've got at least 2 questions:
1.Everything is pretty well set, but I'm not sure where to set the fast idle cams. Right now if I pull the choke lever the cams are set such that the idle will increase about 800 or so RPM before the jets start to drop, then another few hundred as the jets drop. Is this close to right?
2.You say it should be possible to deactivate the choke as soon as the water temp gauge starts to move. NO WAY on my car. Even when the gauge gets up to 200 degrees, if it's a cold morning and I deactivate the choke the car will stall at idle. Once fully warmed up it idles at about 1000-1200 RPM (that's a guess - the tach reads about 1500, but I know it reads several hundred high). Advice?
Regarding Choke adjustment Qs, it sounds as though your fast idle may be adjusted to activate too early (that is BEFORE
jets drop?...it is intended to raise the idle by opening the throttles a bit, AS the jets are lowered (or even after!) enrichening
the mixture, to prevent fouling with a rich mixture at normal idle revs.). Each car is different based on a bunch of factors
(including and possibly most important, metering needle position in dashpot - see
below*), but I usually make the adjustment so that it raises the idle from 1500 - 2000 revs at full choke (jets .25" lowered). Yes
that's a bit high, but I'll never worry about the engine dying from fouling during warm-up! When you back fully off the choke, this will result in a good bit of
space between those fast idle cam follower screws and the cams.
With regard to Q2, this clearly suggests to me, that you are most probably adjusted on the lean side, and need that enrichening even longer...one way to verify this is to blip the throttle (or momentarily fully open the throttles) [see also: mixture checking]...if you get a big hesitation before revs pick-up or even a lean backfire, it pretty much confirms you are lean...you should do this test (and not get a backfire) when cold (under choke), partly warmed up (part choke), or fully warmed up (no choke). I adjust my cars so that I NEVER get a backfire like that...maybe a bit rich but I prefer it, with the downside of being adjusted rich is not nearly as bad as the downside of being adjusted lean...(can you say burned valves or perforated piston!).
I would also do this check after the car is fully warmed up first (and you've verified the position of the metering needles as *needle shoulders FLUSH with dashpot bottom), I expect (in your case) you may need to lower the jets a bit to enrichen...reset your idle...and drive the car with this setup for a couple of days (starting, warming, running, to see how all operating conditions are), adjust further as necessary...until you're satisfied with everything. Other than the fast idle, the choke operates pretty much independently from the normal adjustments (but it's better to start with those being satisfactory). THEN set your choke...don't forget, SUs are great in their simplicity, BUT they are not nearly as smart as a modern engine management computer which sniffs all of the operating conditions by way of a bunch of sensors and then optimizes the operating adjustments for you about a hundred times a second!
Adjustments: I find that I also need to adjust between
summer and winter by three faces (half turn) on the adjustment nut (richer in
cold season obviously, and incidentally, this is a 15mm nut, one of the
few other places on a vintage Volvo - besides the Bosch supplied electricals -
where you will need your metric wrenches), I find it gives me a lot better
throttle response just off idle...I then turn
over the (double-sided, punched) summer/winter tag under one of the air filter
bolts when making this adjustment.
Summer/Winter tag identifies the current mixture adjustment.
Other than that, I really
don't touch my carbs much at all except maybe to fill the dampening oil
(ATF) once in a blue moon or to keep the linkages lubed!
The important thing is: If you are happy with the starting, warming, running (including acceleration) performance of your
setup, that's what's right, period! Don't make yourself crazy trying to achieve the exact idle RPM specified in the
manual. With a little "false air" getting past slightly worn throttle shafts and
the carbs additionally subject to all
sorts of things including temperatures (block and outside air), humidity and
lord knows what else, making the idle adjustments a tricky operation, resulting
in not even particularly repeatable operation...you could drive yourself to drinking...and not for
pleasure! The way I feel about it, (and since most of my cars are known to have
slightly worn throttle shafts) I purposely don't make myself nuts with
idle adjustments since I figure idle is a temporary condition used only in the driveway, at stop lights, and traffic jams etc....I
want my adjustments to result in satisfactory running when I'm DRIVING!
Hope that helps.
Checking the Mixture:
I was also never too
impressed by the function of that little "mixture checking pin" which
lifts the dashpot a bit...I don't think I EVER got that RPM result they so
neatly showed a graph of in a number of the manuals!
[Probably my carbs were pulling False
Air by worn throttles!] In the end, I always tune for
most satisfactory road performance ("Proof of the pudding is in the
Mixture checking at Idle, using the Lifting Pin.
DON'T do this if throttle shafts are worn, expecting meaningful or repeatable results!
See also: Mixture Optimization and the Gunson Colortune
Reprint from Brickboard in response to: start this @#%$% thing in the cold 120-130 1967 posted by Phred.
Remember, you enrichen to choke (duh)...the SU is quite a bit different from typical down-draft carbs in that it (you) chokes by (manually) lowering the jet, and since the dashpot effectively is raised by flow through the carb (which during cranking is very low) it is down, and thus takes the place of the closed "choke flap" you may be used to from the down-draft carbs.
SUs and fuel in general also atomize pretty poorly at low temps, so the rule is to choke fully (jet 1/4" lowered), apply NO throttle (which would feed more air and undo the enrichening you need), and keep cranking (hopefully from a full battery with good voltage). The wet plugs you see are typical when choked and not started (no surprise there)...you'd be amazed how much fuel they want to start...charge your bat., pour a little fuel in each carb and try again (and NO THROTTLE this time)...you might be surprised! Let us know how you make out.
Please see also tech article on this very subject at: http://www.intelab.com/swem/checkchoke.htm
The "retard the timing trick" developed for super high compression engines might also help when its REALLY frigid and cranking is so slow that you can actually hear the cranking get hauled down to nothing when a cylinder firing occurs before TDC. Under normal running conditions, after ignition occurs, the flamefront in the cylinder takes a small (but finite) time to develop and make pressure, so its OK to fire at 16 deg. BTDC. The crankshaft is turning fast enough that by the time the pressure peaks, the piston is on the way down and benefits from this force (power stroke). But when cranking is dead slow, the flamefront actually beats the piston crown to the TDC and actually stops the cranking dead since it pressurizes the cylinder while piston is still on the way up (and working to compress) and before the it has gone "over the hump"!...that's when it's time for some 5W-20 and/or a block heater and the timing retard trick. Naturally, a good pre cold season ignition tune-up to assure that the everything is right on the firing side is a good idea too.
My list of things to check (any one of which will cause a no start in frigid conditions):
1. Are you getting full choke action AT THE CARBS?
2. Are all electrical (starting current) connections clean and solid?
3. Is ignition system in good state of tune?
4. Is thick (summer) oil or marginal battery or connections causing slow cranking?
Non-working Choke causes "B18 to run rough
SU Choke Return Spring:
Choke Return Spring applies rotational force to Choke tube by way of a linkage arm, and thereby lifts the Jet into the Jet Adjustment Nut and unlowered, unactivated position. [Correction and a more accurate description of the action: The CRS applies counter-rotational force to the Choke Bellcrank Shaft, which is in-turn connected by an arm to the Choketube, which controls its vertical position. With minimum friction in Choke Cable, CRS and motor vibration will tend to return Choke linkage to the unactivated position]
Action of CRS when unhooking from locating peg cast into housing. When released from the holding peg in Carb Housing, ClockWise springforce will rotate the end more than 180Degrees as it relaxes.
CRS (Orange, viewed from below, both ends also highlighted Orange, shown released from its normal position, and relaxed.). Yellow highlights slot where one end of spring should be located in service.
One end of CRS in place on Choke Linkage at Orange, the other end hooked by a copper wire to allow pulling it (Greens), applying preload, in preparation for hooking it.
Copper wire applying tension (Greens) and pulling CRS into position towards holding peg in Carb casting (Blue).
CRS pulled beyond holding peg (Blue) and secured there with the help of a needle nose pliers.
A drop of oil on the linkage pivot where CRS is located will keep everything lubed, and test activating the Choke the Linkage (without a cable connected) results in the Jet being lifted back to its unlowered position by the force of the CRS.
Related thread: (Choke return spring installation) https://www.volvoforums.org.uk/showthread.php?t=290730
External material sources are attributed. Otherwise, this article is Copyright © 2000-2020. Ronald Kwas. The term Volvo is used for reference only. I have no affiliation with this company other than to try to keep its products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of its products here. The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion. As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles!
You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source. If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you! As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment.