Rough Idle solution?
 

I have a question. My 144 B20A idles like a pig, engine shakes the whole car shakes, hunting up and down between 700~800rpm and the vacuum gauge needle vibrating between 20~25.

However, when I block the hose from the oil filler cap to the manifold, the engine smoothens out and runs steady at 700rpm and the vac needle gauge is holds at 20. What would be consequences of plugging the hose from the oil filling cap to the manifold long term?

About the car: 1972 144DL, B20A, Auto, 140k miles, 123ignition, compression 145psi on all cylinders, not burning oil, oil pressure 60psi, spark plugs are dry and look normal, timing set at 10°. Unmodified head and using unleaded additive religiously. Getting around 26~27 mpg which I think is very good.

I removed the head recently looking for a solution to the idling the problem thinking it was valves or guides. All the valves looked good but a few leaked with a water test. Cleaned them up, lapped them in and retested, no more leaks. There didn’t appear to be any noticeable ware on the valve guides. Cleaned the head and the pistons, there was no lip in the cylinder walls. Cross hatch on the cylinder walls was faint though.

Service history is compresive from 1974~1986; Had major work done in the mid '80s.

** May-1983 @ 126k miles “Decoked, new valves, springs, guides, points, kick down cable, engine oil, transmission oil, rear axle oil, lubysil added. New plugs, water pump, fuel, air, oil filter”

** April-1985 @ 128k miles “Removed and rebuilt the engine, new piston rings, main shell bearings, big end bearings, oil filter, automatic oil pump”

Last service entry was in Dec-1986 @ 132k. I got the car in 2018 with 137k and I have put a further 3k on it.

I'd go for a vacuum leak. Hoses, hoses clamped, brake booster nrv, carb isolating plate cracked etc.

But first check timing is correct. Set at 1500rpm with advance hose blocked off.
Check valve clearances.
Check vacuum leaks.
Check oil in dashpot. I use 80/90 gearoil.
Replace carb diaphragm.

I'd go for the quick and easy checks and fixes first - dashpot and diaphragm. I always carried a replacement diaphragm as part of my 'essential spares' kit when I was running 1 and 2 series cars in period. :regular_smile:

Regards, John.

Me too. There are two diaphragms you can get now, the proper one from Burlens is a thin pliable rubber and the other is not as pliable. Go for the one from Burlens.

Blocking off the line from the oil filler cap to the manifold would result in you losing the 'positive' air flow for the crankcase ventilation system. This will result in the faster accumulation of moisture in the oil with potential sludge build up. However, most cars up to the early '60s used this non positive ventilation so it will work.

Since the crankcase ventilation hose seems to affect your problem, make sure that you have the hose connected to the correct nipple on the intake manifold. The correct nipple will have a restricting orifice in it (I think its about 1/8") which limits 'false air' flow into the intake manifold. If you use an unrestricted nipple you will have too much air flow and won't be able to properly set up idle.

You said you set the timing at 10 deg. According to the 1971 Volvo service manual the B20A ignition is set to 21-23 deg BTDC with the engine running at 1500 RPM and the vacuum line to the servo on the distributor clamped off. I don't know what the values are for a non North American 1972 B20A so perhaps confirm the correct values for your 1972 B20A. As described by others, make sure all the base tune settings are correct before trying to diagnose further.

On an off chance, since you have a 123 distributor, make sure that the advance settings are correct for a B20A . In North America the up to 1971 B20A engines used vacuum advance as opposed to retard. I don't know whether the 1972 versions continued to use vacuum advance so make sure you haven't mixed that up if you have a 123 that is programable.

Where did you get the 123 dizzy from? My car is a little modified with increased compression ratio and a slightly hotter cam but it ran horrible on the B20A ignition curve. I have a Amazon cars 123 dizzy. If the other things suggested don’t make any difference try the B18B curve as well.

If you've got a 123 from Amazon Cars it should have their own custom curve for unleaded (number 3 I think).

I think the 10 degrees might be static timing.

Renew the seal on the oil filler cap and also inspect the hose on it as well. Often the hoses perish/go soft so while they seal when cold, once warm they go soggy and let excess air in.

Thank you for your input. I did a full overhaul on the the cab with a kit from Burlens, plus new needle and jet.

Yeah but also has the standard B20A map. I am using the B18B, like it better than the Amazon cars map.

I preset 123 from Skandix and set it for B20A as per the documentation.

I got a preset 123 from Skandix and set it up for B20A (I think #4)

Thank you for your indept reply. I've combed through my 1972 green service manual and the only timing setting all I can find is 10°BTDC @600-800 rpm for 97 octane in the specifications on page 0.9. I will try your settings mentioned abover and see how it runs.

I did confirm that the hose was connected to the correct nipple with the restriction orifice. Also add that I have a new break booster, NRV and hose.

My 123 distributor is the preset type from Skandix. I confirmed the correct setting for B20A.

The car drives okay, easily cruses at 55mph anything over that the wind noise is unbearable and its returing 26-27mpg(UK).

Rough idle maybe something I have to live with. :confused_smile:

Check your oil filler cap and the gasket. It should be airtight.

How have you set the idle mixture? Did it idle nice after you rebuilt the carb and reinstalled it? Did you replace the seal and bushes on the butterfly throttle shaft? What oil have you got in the dash pot?

My 71 B20A specs are 21 to 23 deg at 1500rpm for ignition.

My isolation plate between the carb and manifold was cracked and my spare was also cracked. It idled nicer after it was replaced.

It should idle lovely. I think they will always fluctuate a little with the Stromberg with the carb piston but nothing too much.

It appears that 10 deg timing is for B20B, and 21-23 deg is for the B20A.

https://www.volvoclub.org.uk/manuals...ualOct1970.pdf

https://volvo1800pictures.com/docume..._B20A_B20B.pdf

Its a new gasket and the cap goes on snug.

What about the hose like i advised earlier? :thinking:

It must be a missprint on my '72 volvo service manual green book. The Haynes manual also says 14 deg @ 600-800 rpm (which would be close to 21-23 @ 1500rpm as in the owners manual)

I also found a service bulletin 20-16 issued in '74 for timing adjustments for the different octane ratings of the day. There is some interesting documents on this webpage with the help of google translate from Swedish.
http://www.240.se/litteratur/sm20-16.pdf
http://www.240.se/

The hose appears to be in very good condition and seems to fit snug on the cap nipple, I will try it with a hose clip at the cap end anyway.

Thanks for your reply. It was a full overhaul kit from Burlen (Diaphragm, Shaft, butterfly, seals, float valve, gaskets etc. plus a new needle and jet. My isolation plate wasn’t cracked but I replaced it anyway just in case with new gaskets. The needle and jet are the fixed type and fitted the needle flush with the bottom of the piston and the pressed in the new jet 2.5mm (100 thou) from the top of the internal surface of the carb. I tuned the carb by the book. The idle trim screw fully closed seems to give best idle results.

The old seals/shaft were well worn and needed replacing anyway. I could have bought the 3 special tools needed but couldn’t justify the extra £100+ for a once off job. I was thinking there may have been an issue with the temperature compensator, I only cleaned it and installed it with new seals without fiddling with it.

Overhauling the carb, 123 distributer, new coil, replacing leaking break booster, new hoses, head off, cleaned & lapped valves, decarbed the cylinders, 2mm head gasket improved the idling but didn’t cure it. The only thing that’s worked is sticking my finger in the hose and like magic she runs smooth and steady @ 700rpm

The uncertainty with those documents is the application to the 1972 B20A. The owners manual is for a 1971 (I recognize it because I have the same booklet in my glove box) and the Volvo green book appears to be for earlier B20A & B20B engines because of the CR listed for the B20B. In North America, the 'tunes' changed for the engines in 1972 because of emission requirements and the 10 deg at 600-800 RPM might be correct for a 1972 B20A.

What did the car run like before you did the carb? What needle are you running? I replaced just the needle first off and the idle was horrible. I then fitted a adjustable jet kit so I could adjust mixture at idle, made the world of difference.

When you say you block the hose from the filler cap, did you also block the manifold nipple?

Maybe a couple of pics of your setup might help.

As a bit of a Hale Mary suggestion, try clamping the line from the manifold to the port on the distributor to make the vacuum advance function in the distributor inactive. Ignition timing affects engine speed - modern ECUs advance and retard ignition to stabilize the idle speed. If you already have some small idle speed fluctuations causing changes in manifold pressure the distributor may be picking those up and amplifying them by changing the timing which makes the speed fluctuations worse.

If clamping the vacuum line improves things, your problem may be related to the advance curve being used in the 123 distributor. You may be able to operate without the vacuum advance function with a slight sacrifice in your fuel consumption or you may be able to resolve the problem by selecting different advance setting.

Easy to do and doesn't cost anything to try.

It ran horrible for a number of reasons when I got it. The engine and trans mounts were completely gone, the break booster was whistling and sucking in air. The carb throttle shaft was worn flopping around and a carb and dripping petrol from the cold start. I used a B1CC (for 3573 carb) needle from Burlen and their rebuild kit. I fixed the cold start (choke) leak by polishing the disk surfaces and putting a small ‘o’-ring on the inside of the shaft as there was nothing in the rebuild kit for it.

By blocking the hose, I mean pulling it off the fill cap and plugging the end with my finger. There are no vac leaks on the hose or at the manifold nipple.

A smaller orifice in the breather nipple on the manifold might help, looks to be about 3mm or 4mm... maybe make a little disk with a 2mm hole and place is inside.

Thanks for the suggestion, I might explore that. I haven’t played around with the other settings on the 123dist and assumed it was already tuned with the correct settings in the presets for the B20A. My current fuel economy is 26-27mpg which I think is very good considering it’s an auto.

The hose from the oil filler cap should go to the air filter. Clean air enters the crankcase from the filter. If you have it going to the inlet manifold the engine sees it as a massive air leak.
The nipple on the manifold with the restrictor should route across to the side ear just behind the fuel pump. That hose should have a one way valve just above the ear. In this cross pipe there usually is a T piece that runs a hose to your servo. Check all of the illustrations in the workshop manual, hand book or what ever you have. This is a common problem. You showed evidence of it in your first post when you mention clamping off the hose from the oil filler cap.
Go back to square one and correct your hose routing.

Hi Derek, he’s got a B20. Think a pic or two would help diagnose. This is my setup with a single Stromberg.

This depends on the vintage of the engine. Volvo reversed the direction of air flow in the PCV system on later B20s. I am not exactly sure when the change occurred; but, on my 1971 (and later) the air flow is from the air filter, into the oil separator box on the side of the engine, out the oil filler cap and into the intake manifold. Later engines did not use a PCV valve in the venting arrangement. Air flow is controlled by the all important restricting nipple in the PCV flow arrangement.

On my B20E there are separate ports for the connection to the brake booster and the PCV system (with different nipples on those ports). Later carb manifolds also had separate ports so no requirement for a Tee to the brake servo.

Mine is the standard configuration for a '72 B20A.

I'm wondering if you've hit the nail on the head and the direction of flow has been inadvertently reversed? Would explain a lot if so!

OP - could be worth swapping the hoses around so the oil filler hose goes to the air cleaner and the vac stub on the inlet goes to the oil separator (going from memory of what was on my last 140 many moons ago and the pic above)

Unfortunately, the hoses are configured correctly as per the attached diagram from the service manual.

I will explore placing a metal orifice disk with a smaller orifice inside the manifold nipple which may restrict the vacuum flow whilst still function to extract gasses from the crankcase.

It might be right for a 1972 car but what if the car is in fact a 1971 car that was registered late or perhaps someone has changed the engine at some point in time for an older chronologically but younger in miles unit?

Also as others have mentioned the vacuum advance was changed about the same time, perhaps to coincide with changes to the PCV system that may have some influence.

Thinking back i remember my last 144 was 1972/L registration and had the later arrangement as you have uploaded. I'd previously had a 1971/K 145 with the breathers the opposite way round. When i got the L reg, i at first assumed someone had put the breathers on incorrectly and swapped them back to how i thought they should be and it wouldn't tickover properly - very similar symptoms to yours?

It seems you've tried most everything else, maybe the time now is to try the silly, stupid stuff and keep it that way?

The nozzle size I think is designed to be correct so I don’t think it should need to be changed.

I just can’t get my head around why taking the hose off and blocking it makes a big difference. Have you cleaned the cap and flame guard pot (pr whatever it’s called).

I think and I could be wrong but the B18 flame trap has a NRV fitted to the intake where the B20 one doesn’t so something to check.

Pic of a B18 setup showing NRV.

I’ve been through everything and it’s all clean as a whistle; the flame trap and the thing it’s attached to on the side of the block, the manifold nipple, the hoses, the filler cap.

I will try making a little friction fit orifice disk with a smaller hole over the weekend and see how it behaves. If there is no change it maybe something I have to live with. There is nothing worse than being stopped at a junction and seeing the bonnet shaking and steering wheel vibrating out of the dash... pulling out the choke to fast idle does hide it. Other than that, the car is mint.

Its a very late Nov'72 (L reg) with flush door handles... must have been one of the very last to be sold as the newer facelift '73 models were already being sold in '72. Hand written in the owners manual that it cost £2,374.98 (£32,047.26 in todays money)

I think bonnet shake is a vintage 140 'thing'. On my 1971 142 the hood (bonnet) frame has a center section and at one time the hood skin was bonded to that center section with an adhesive of some sort. However, with age the adhesive let go and the whole hood skin was free to resonate resulting in different modes of drumming at different engine speeds. Quite interesting / annoying. I re attached the hood skin to the center frame with some seam sealer which eliminated the lower frequency large amplitude vibration modes. After that the sections on either side of the center frame would vibrate; but, with lower amplitude and at higher frequencies. I reduced those vibrations by applying a couple of viscoelastic pads on either side of the center frame. You can see them in the attached photo.

If you have steering wheel shake you likely have problems with the rubber bushes which hold the steering column in the column support brackets. It could be as simple as one or both of the brackets that hold the rubber bushes has come loose. The rubber bush may also deteriorate; but, when I did work on my car a couple of years ago the bushes looked to be in excellent shape. If the steering wheel is vibrating in and out on the column you have a problem with the steering column thrust bearing adjustment at the top and bottom of the steering column. Don't ignore that because the actual thrust bearing cages are out of production and if yours gets damaged you are going to be into looking for a new steering column from a salvage car.