S1, 2 ltr engine, surging when accelerating

[JimUSA]

I have recently completed the restoration/rebuild of my 1955 Series 1 (2 ltr petrol engine) and am working the bugs out. It was running great, then last night it started "surging" -- engine goes up and down in RPMs every few seconds even though the accelerator pedal is not moving. It does not happen at idle, but happens at every engine speed above idle.


Timing and points are good -- just had them professionally adjusted. I have checked/cleaned all fuel filters, blown out fuel lines, checked the fuel pump, and checked the jets on the carb (the ones removable from the outside) -- all clean and good.


I think it is something inside the carb (Solex 32 PBI-2) but not sure where to start. I have a rebuild kit but have not fiddled much with carbs. Only have about 1200 miles on the carb since installation. Any suggestions before I start taking the carb apart? Many thanks.

[JDNSW]

I can think of a few possibilities. Is there a loose manifold or other variable air leak on the intake? Sticky centrifugal advance in distributor? (unlikely)

Or is the engine moving due to loose engine mounts and interfering with the throttle linkage? Note that this could be exacerbated by incorrect assembly of the linkage, or part of the linkage actually on the carburetter loose.

But as you suggest, perhaps the most likey thing is something inside the carburetter. I find it difficult to get my head around how this Solex actually works, so I am not sure what is likely to cause it, but I would suggest one of the jets is either loose or partly blocked, or there is some foreign material floating round in one of the passages.

Hope this helps,

John

[Tank]

How's the vacuum advance, is it working, Regards Frank.

[JimUSA]

Thanks for the replies. I believe I have sorted out the problem.


Turns out one of the wires inside the distributor was broken -- the wire that connects to the coil terminal. Why this was causing the described symptoms is beyond the troubleshooting abilities of this amateur mechanic.


But of course that is not the end of the story. After repairing the wire and putting it back together I tried to start it and nothing -- it turned over but would not start. A bit of troubleshooting confirmed there was no spark. After consulting with a real mechanic (who has worked on the vehicle before) it turns out when I reassembled the distributor I did not connect the wires properly.


The wire from the coil plus the condenser wire are connected to a post which also anchors the point spring. But there is a little plastic piece which makes sure the wires (and point spring) are isolated from the post and thus ground. If not, the wires are grounded and no spark for you. Swapping the parts around such that the wires contact the spring but NOT the post means it now works. Perhaps this hard won knowledge will help some other schlub.


And another lesson learned -- various troubleshooting web pages for surging problems kept saying to check electrical problems before getting into the carb. I resisted this advice but decided to check the electrics one last time before ripping into the carb and glad I did. The carb (and vacuum advance, and fuel pump, and fuel lines, and manifold) are all fine -- it was the stupid wire. Cheers for now.

[JDNSW]

Thanks for the replies. I believe I have sorted out the problem.


Turns out one of the wires inside the distributor was broken -- the wire that connects to the coil terminal. Why this was causing the described symptoms is beyond the troubleshooting abilities of this amateur mechanic.

........

What will have been happening is that the broken wire was making contact, but as soon as the vacuum advance moved the breaker plate it disturbed the contact.

John

[JimUSA]

Yes, very plausible. It did seem like the wire was making light contact. The breaker plate moves, the wire loses contact, the engine slows down, breaker plate moves back, wire makes contact again, repeat. Thanks for the insight. I guess if I learned something it's not all bad, right?