1969 volvo amazon B20A engine issue
 

Hi

About 1,5 years ago i bought a well restored volvo amazon.
It has recently developed a engine issue.
I can best describe the issue as the engine holding or pushing back when accelerating.
To me it seems that the fuel mixture is igniting before the cylinder is at top dead center.
The engine can be completely fine one day and the next day this issue can occure.
I often drive my car (sits for max 2 weeks)
Things i have already done:
Replaced oil
replaced condensator and spark distributor (low spark issue)
this last item was done by a garage, they also tuned the pre ignition
after that it was fine again but reoccured after a week or two.

Any suggestions?

Thanks in advance

Do this first. SPLIT CARB DIAPHRAGM. Replace even if you don't see a split.
Weak mixture.
No oil in dashpot.
Too far advanced. Check timing properly. First set the static timing by the book and then check with the engine running using a strobe light. Pull vacuum pipe from the distributor when doing this and block it off to stop the air leak to the manifold. It may need to be retarded by a couple of degrees more than the book due to modern fuel. Initially, you could just rotate the distributor slightly to retard it. 1º of a turn at the distributor = 2º of ignition timing. Rotate anti-clockwise to retard.
Do each step separately and then test. I don't say this will cure your problem but all worth a try.

Jens;

Welcome to this forum!

I presume Derek deduces the single Zenith-Stromberg carb, from the B20A designation, but I'd ask for confirmation of this...if confirmed, the torn diaphragm seems to be a common failure mode of those carbs...although I have to question if that can produce symptoms one day and not the next ("...completely fine one day and the next day this issue can occur...")...I might expect failure symptoms to be consistently repetitive once diaphragm is torn...

"replaced...spark distributor (low spark issue)"...strength and intensity of Spark is a function of Ign Coil...Distributor (Cap and Rotor) then direct it to the correct Spark Plug...I question this value and effectiveness of this action...and it seems not to have addressed or corrected symptoms...

If carb is OK and symptoms persist, I would return to Ign Sys...you can try Derek's recommended Ign adjustments, but timing, as checked on a timing strobe should be consistent and return consistently (within a degree or two) after blipping throttle...a stuck or inconsistently returning Centrifugal Advance can certainly cause symptoms often described as feeling like engine is being held back on throttling up (Check CA action and assure it is lubed, see: http://www.sw-em.com/Volvo%20Ignitio...e_Spark_Timing )

Good Hunting!

Splits in the Stromberg diaphragm can be small and hard to spot. Also means that when they are very small their effect on the piston movement can be inconsistent. The diaphragm can also become distorted which also affects piston movement. This distortion is probably more likely now due to ethanol fumes.
Ron's comment "direct it to the correct spark plug" also means the correct plug type for your engine.
I agree with checking the advance and retard mechanism. The rotor should spring back when turned against the mechanism and released. Check that the vacuum diaphragm moves the breaker plate. Sucking by mouth on the pipe is usually enough to see if it's working.

I've had intermittent fault similar to this with a faulty coil, although it was over days rather than weeks. You could try jury rigging an ordinary coil to see if it makes a difference.

.a stuck or inconsistently returning Centrifugal Advance can certainly cause symptoms often described as feeling like engine is being held back on throttling up (Check CA action and assure it is lubed, see: http://www.sw-em.com/Volvo%20Ignitio...e_Spark_Timing )

This!

Check the ignition side properly ,
Kay

Had an issue with similar symptoms in a GM engine about 50 years ago, tried everything for over a month, finally it came to an almost (but not quite) plugged fuel filter. Worth to check it.

Hi sorry for the late reply,
i'm currently in my final exam period.
i've checked both centrifugal and vacuum advance systems as you described they are completely fine. which leads me to believe the issue is either ignition or carb. i replaced the spark distributor because the electrodes were pretty heavily burned but i agree it wasn't the problem.
Thank you very much for the replies.
Things i'm going to check:
-static timing
-carb diaphragm
-Ignition coil
-fuel filter?

Since the post i've driven my car about 100km without any symptoms.
although at higher speeds (120kmh) i can still feel it, but centainly not as much as before where i had trouble reaching 60kmh.