Poor Running Under Load

[roundyuk]

Been a while since I was on here, over on the Polestar forum after changing from the V60...but my dad has just bought a 122S :-)

We're having some problems with running though, she starts and idles fine, even drives ok with very light throttle; but when under load she can really stumble and kangaroo!

Checked the choke was returning closed, all ok and then pulled the plugs - really wet and very very black; my assumption is she's running very rich? In fact you can tell she is as the black mess coming out the back is very wet! The plug gap was also huge, adjusted them.

We did check the timing and this was a bit too retarded, so tweaked that a little and things have improved from where they are, but she's still stumbling under load when you put your foot down.

It's got a cone filter and intake setup and the engine was recently rebuilt; points and cap look ok. I guess the next thing is to reset the carbs to base and go from there?

[Othen]

I'm guessing it has a B18D motor. It does sound like setting up the SU carburettors from scratch would be the most sensible thing to do. A link to the green book is here, the SU carburettor is a pretty simple instrument so this should be all you need:

https://volvo1800pictures.com/docume...ne_B18_B_D.pdf

The Ron Kwas guide is also very good:

https://www.sw-em.com/su_carbs.htm#T...djusting%20SUs

Good fortune,

Alan

[blueosprey90]

"engine was recently rebuilt".

By whom?

If the engine had been rebuilt by me, and if the carbs and ignition timing seem to be more or less in spec, you would suspect the camshaft timing is off.

You would also check compression and then move to a leak down test because who knows what other mistakes I might have made in the rebuild.

I've had a similar problem - undiagnosed misfire under load at high speed - for two years after rebuilding my MGA. I finally suspect a porosity in the water jacket in the engine block as the most likely candidate and have pulled the engine. Replacing with a "refreshed" spare. Hope it runs!

[jimbo1593]

Start with the simplest and cheapest things first.
Also dump those plugs and get new ones correctly gapped. Sounds more like fuel than spark, but do check points etc.

SU set up is actually really easy if you follow the various guides. Be methodical, start at fuel tank...fresh fuel, furl getting to filter. Filter new, clean, right way round. Pump ok, check your hoses are compliant with e10 or just use superunleaded. Check for leaks. Set up your carbs.
See where you are after checking that lot.

[Othen]

Originally Posted by jimbo1593

Start with the simplest and cheapest things first.
Also dump those plugs and get new ones correctly gapped. Sounds more like fuel than spark, but do check points etc.

SU set up is actually really easy if you follow the various guides....
See where you are after checking that lot.

... and a set of new spark plugs is only £7.10 delivered to your house:

https://www.ebay.co.uk/itm/301980009...53.m1438.l2649

I use these in my Volvo 122S (Aunt Maud).

:-)

[142 Guy]

"pulled the plugs - really wet and very very black"

If the wet appears to be from fuel, then combined with the "very very black" would point you in the direction of the carbs delivering too much fuel. In fact, if the plugs are wet, it might not be a 'mixture' adjustment problem; but, a problem with the float level causing flooding of the carb.

If the 'wet' appears oily, then you have a problem with the rebuild.

I will riff off of blueosprey90 and ask what was included in the rebuild process. B18 / 20 engines are notorious for camshaft / lifter wear. If the cam and lifters were not replaced and the rocker shoes reprofiled, you need to check the lift of the valves using a dial gauge to confirm that you do not have a cam with some flat spots on it. If the camshaft is good, do a compression test to confirm that it is in spec (if this has not been done already). There is no point in trying to precisely tune the fuel mixture if the engine is well past its best before date.

"It's got a cone filter and intake setup " - does that imply an oiled element style filter such as a K&N, Filtron, UNI, ....whatever? If so, my personal opinion is that these are usually a poor choice because users inevitably over oil them which results in restricted air flow (and rich fuel mixture on a carb). Unless you have a motocross bike or are racing to Dakar stick with a paper element air filter. Does 'intake setup' mean that you have a different carb?

Before attempting the fuel mixture tune make sure that the valve lash is correctly set and that the ignition is set to spec. Adjusting fuel mixture should always the last step in the process. SU carbs represent the 'dark arts' to me so I will let others advise on their set-up.

[roundyuk]

Thanks for all the feedback guys, my dad has an SU carb tool kit from his old MGB; so I'll get him to run through the basics!