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Odd misfire

RootesRooter

Donation Time
I took my SV w/standard Weber carb out for its first spin in several months. It ran perfectly at first. Smooth. Good power. A mile from the house, just starting to warm up, I headed down a fairly steep windy road. As I approached the bottom of the hill and let off the gas I noticed the rpms were staying very high, about 4000. I pulled off, turned it off and checked the linkage. No apparent binding.

I restarted and almost immediately felt the engine running on what felt like three cylinders, but no more excess rpms. I limped it back up the hill. I had to keep in a lower gear than normal, as it would make a strong but irregular ping below 3000. But at higher rpms 4000+ it would occasionally backfire.

When I got it home I checked the plugs. All clean. Dizzy not loose. Cap looked good. Replaced the rotor after the old one broke while prying it loose to inspect it. It feels like the dizzy springs are moving normally. But no change in how it ran.

Strong oil pressure-no change. No internal rattles before or after. Intake gasket is new. Intake bolts tight.

I didn't think my set-it-and-forget-it Weber could be subject to catastrophic failure, but I'm running out of ideas.

I still have to check the points. In the past, closed points would cause weak or no spark, but never irregular like this. Similarly, a bad condenser would either be off or briefly back on, never in between.

Any suggestions?
 

Gordon Holsinger

Diamond Level Sponsor
I took my SV w/standard Weber carb out for its first spin in several months. It ran perfectly at first. Smooth. Good power. A mile from the house, just starting to warm up, I headed down a fairly steep windy road. As I approached the bottom of the hill and let off the gas I noticed the rpms were staying very high, about 4000. I pulled off, turned it off and checked the linkage. No apparent binding.

I restarted and almost immediately felt the engine running on what felt like three cylinders, but no more excess rpms. I limped it back up the hill. I had to keep in a lower gear than normal, as it would make a strong but irregular ping below 3000. But at higher rpms 4000+ it would occasionally backfire.

When I got it home I checked the plugs. All clean. Dizzy not loose. Cap looked good. Replaced the rotor after the old one broke while prying it loose to inspect it. It feels like the dizzy springs are moving normally. But no change in how it ran.

Strong oil pressure-no change. No internal rattles before or after. Intake gasket is new. Intake bolts tight.

I didn't think my set-it-and-forget-it Weber could be subject to catastrophic failure, but I'm running out of ideas.

I still have to check the points. In the past, closed points would cause weak or no spark, but never irregular like this. Similarly, a bad condenser would either be off or briefly back on, never in between.

Any suggestions?
Possibly a bad plug wire or wires, or hairline crack in the distributor cap
 

mxp01

Platinum Level Sponsor
Probably worth changing the fuel filter(s) if you haven’t done that already.
 

Tim R

Silver Level Sponsor
Are your plugs genuine? There are a lot of very convincing fake spark plugs around, they are packaged and sealed to look like the real deal but are much worse quality and they fail quickly.
Tim R
 

Gordon Holsinger

Diamond Level Sponsor
I took my SV w/standard Weber carb out for its first spin in several months. It ran perfectly at first. Smooth. Good power. A mile from the house, just starting to warm up, I headed down a fairly steep windy road. As I approached the bottom of the hill and let off the gas I noticed the rpms were staying very high, about 4000. I pulled off, turned it off and checked the linkage. No apparent binding.

I restarted and almost immediately felt the engine running on what felt like three cylinders, but no more excess rpms. I limped it back up the hill. I had to keep in a lower gear than normal, as it would make a strong but irregular ping below 3000. But at higher rpms 4000+ it would occasionally backfire.

When I got it home I checked the plugs. All clean. Dizzy not loose. Cap looked good. Replaced the rotor after the old one broke while prying it loose to inspect it. It feels like the dizzy springs are moving normally. But no change in how it ran.

Strong oil pressure-no change. No internal rattles before or after. Intake gasket is new. Intake bolts tight.

I didn't think my set-it-and-forget-it Weber could be subject to catastrophic failure, but I'm running out of ideas.

I still have to check the points. In the past, closed points would cause weak or no spark, but never irregular like this. Similarly, a bad condenser would either be off or briefly back on, never in between.

Any suggestions?
I recently read a post on this forum about pattern parts being a problem. Is your coil breaking down?
 

Tim R

Silver Level Sponsor
Gordon,
I made a video recently showing the problem with pattern, usually Far East made, rotor arms (but the same issue applies to points, coils etc etc). The tolerances were so big that the pattern part was over 1mm smaller than the original and the gap between the rotor arm tip and the distributor cap electrode was huge. As a result the car would churn and churn before starting, if it even did start.

As soon a I fitted a high quality British made reproduction rotor arm and cap the car started immediately and has continued to do so. The new parts were three times the price of the alternative but they are the correct size and they work.

Just because something says on the box that it is made by 'Lucas' or 'Champion' doesn't mean that it is and with some manufacturers, even if it really is made by the same company, it is not made to the same standards that they used to use.

It makes life very hard because old parts need replacing and if you can't get decent new bits the pool of useable parts is constantly shrinking.

Tim R
 

Gordon Holsinger

Diamond Level Sponsor
Chasing ignition problems can be a pain. I learned about pattern parts a long time ago. Coil problems can be a challenge to pin point. If the coil is old it can be breaking down internally works fine until they get hot, voltage rise time! Distributor, shaft wobble the dwell angle goes nuts. Points bounce weak spring, advance springs, crack in the cap carbon tracking, bad plug wires, points gap closed up, points arcing. You have to chase it down item by item.
 

Eleven

Platinum Level Sponsor
Just for fun, start it up and then hold a dollar bill at the exhaust pipe outlet. If it sucks it back in occasionally, probably lost a valve.
 
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