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.