Spark Knock On B20A ???

[Clan]

Originally Posted by Billy Smalls

Hi There; I seem have what is being termed as ‘spark knock’ when driving under load – up a hill or booting it on the motorway etc.

It’s a B20A with the BW autobox. The best way of describing the noise, is a rattle that doesn’t really follow a set rhythm, sort of tapping like something loose (though I don’t think anything is loose) and it only does it under load. It sounds like an old diesel basically. When cruising on the flat or down hill it's fine, idling it's fine, no overrun when I switch off, doesn't overheat.....

The spark plugs are okay, gaps set and a light brown powdery coating cleaned off recently.

Tappets were set recently.

Points and condenser are new and points set up – 0.5mm gauge will pass through the gap with some resistance.

Using a standard decent Shell petrol.

I want to get this sorted as I fear I may be damaging serious bits of the engine.
Any ideas folks?? Thanks

you need to get the basics right first , for a start the points should be 0.016" using a feeler blade , then the dwell angle ( 60 degrees) should be checked with the engine running . Then the timing should be set at idle with a strobe timing light . none of this has been confirmed as correct . and once you play around with the distributor you have no idea what the Ignition timing is .
Just getting this right is likely to fix it .

Then there is the carb , it has probably not been touched for decades , in which case the needle and jet will be worn with the unleaded fuel we have now . so to get a decent idle the mixture is adjusted at idle ( on a worn needle ) which means the mixture is weak when driving on load . Which can cause pinking .
The Engine just needs a basic set up and a CO meter up the exhaust to see what is going on with the mixture at idle and on load if possible , but always best to start with a new metering needle and jet first if you cant remember when it was last changed . when all this is corrected try some high grade fuel you will find it drives a lot better when it isn't pinking

[kishor kassie]

Originally Posted by Billy Smalls

Hi There; I seem have what is being termed as ‘spark knock’ when driving under load – up a hill or booting it on the motorway etc.

It’s a B20A with the BW autobox. The best way of describing the noise, is a rattle that doesn’t really follow a set rhythm, sort of tapping like something loose (though I don’t think anything is loose) and it only does it under load. It sounds like an old diesel basically. When cruising on the flat or down hill it's fine, idling it's fine, no overrun when I switch off, doesn't overheat.....

The spark plugs are okay, gaps set and a light brown powdery coating cleaned off recently.

Tappets were set recently.

Points and condenser are new and points set up – 0.5mm gauge will pass through the gap with some resistance.

Using a standard decent Shell petrol.

I want to get this sorted as I fear I may be damaging serious bits of the engine.
Any ideas folks?? Thanks

Hi. Check for cracked manifold or damaged exhaust manifold gasket first. I predict it's the timing gear starting to crack around the mounting bolt
Ta
kassie 078 2887 9494

[dingov70]

Get back to basics , start with a cold engine & check your valve clearances are correct. Then set points gap (contact breakers ) , once set run engine until it is at normal working temperature . Clean & mark timing marks on crank pulley with correction fluid ( tippex ) & use strobe timing light to "freeze" timing marks with engine running . Loosen & turn distributor until the correct marks are lined up , ALSO ensure you perform this task with the vacuum pipe removed from the distributors vac capsule if required in service manual .

Now ensure you have have SAE20 oil in dash pot of carb , too heavy a grade & needle rises slowly resulting in a weak mixture , too thin piston rises too quickly also leading to strange running problems .

NOW ! set tickover as per manual , then use small spring loaded pin under dashpot to push up the piston , if the engine revs rise & stay rasied , the mixture is too rich but should the revs rise & then fall , your mixture is too weak . To cure this you simply turn the brass jet underneath the carb body , turn the jet up to weaken the mixture & turn down to richen the mix . You may well feel a slot in base of the jet & a small coin can be used to turn it easier . Once the mixture is right , raising the carb piston using the spring loaded pin wil make revs climb & settle back to what they were before you lifted the piston .


BUT definately make sure you have the right grade of oil in the dash pot . If you want to get all hi-tech , buy a small bottle of motorcycle fork oil of the right grade .

Simple carb , piece of P*SS to set up


Hope this sorts your problem for you

[Billy Smalls]

Hi All and thanks for the advice.

I have Millers ATF in the dashpot as that's what was recommended by Amazon cars and several others.

I turned the distributor anti-clockwise by about 5mm and that has cured the pinging but I need to do the full job soon, just didn't have time this weekend.

I set the tappets only a month ago and the points gap too.

I shall report back with anymore
Cheers

[Volvorama]

Originally Posted by dingov70

To cure this you simply turn the brass jet underneath the carb body , turn the jet up to weaken the mixture & turn down to richen the mix . You may well feel a slot in base of the jet & a small coin can be used to turn it easier . Once the mixture is right , raising the carb piston using the spring loaded pin wil make revs climb & settle back to what they were before you lifted the piston .

Simple carb , piece of P*SS to set up

Unfortunately the B20A carb does not have the simple adjuster described. You need a special tool to raise and lower the jet. Another reason why I tend to remove CD175s....

[Billy Smalls]

Originally Posted by Volvorama

Unfortunately the B20A carb does not have the simple adjuster described. You need a special tool to raise and lower the jet. Another reason why I tend to remove CD175s....

Ahhhh! you see, I did wonder that. I had to remove the plug about a year ago as the seal had gone and it was dripping fuel. I remembered it as a plug more than an adjustable piece, so that makes sense now.

I am going to carefully spray some carb cleaner around the carb and inlet tonight so I can establish if I have a leak or not. Will clean up the carb then start on the timing.

Thanks

[Billy Smalls]

Originally Posted by Clan

you need to get the basics right first , for a start the points should be 0.016" using a feeler blade , then the dwell angle ( 60 degrees) should be checked with the engine running . Then the timing should be set at idle with a strobe timing light . none of this has been confirmed as correct . and once you play around with the distributor you have no idea what the Ignition timing is .
Just getting this right is likely to fix it .

Then there is the carb , it has probably not been touched for decades , in which case the needle and jet will be worn with the unleaded fuel we have now . so to get a decent idle the mixture is adjusted at idle ( on a worn needle ) which means the mixture is weak when driving on load . Which can cause pinking .
The Engine just needs a basic set up and a CO meter up the exhaust to see what is going on with the mixture at idle and on load if possible , but always best to start with a new metering needle and jet first if you cant remember when it was last changed . when all this is corrected try some high grade fuel you will find it drives a lot better when it isn't pinking

Hi, I took the carb off and gave it a damn good clean with carb cleaner, ensuring i didn't rub the needle or brass bits with anything. float chamber and the little filter on the incoming fuel all cleaned out and new gasket fitted. diaphragm is fine.

where about is the metering needle and jet - can you point it out to me thanks alot

[AndrewBrown]

Just to let people know there was a Service Bulletin from Volvo regarding spark plugs and bad running

so depending on how old your car is here is the thing from 10 July 1969