1953 Plunger A10 starting woes

[BrummieJon]

I am currently rebuilding an 1953 A10 that I have owned since 1980. It suffered a spectacular engine failure in 1996 (left hand big end oil starvation, the sludge trap was full).... The mag was rebuilt only a few months before the engine blew up.
It has taken me until now to give the bike a full rebuild, the engine has been rebuilt over a scattered period of 15 years. The rest of the bike has been a Covid lockdown project.
I have got to the stage of starting the bike..... It fires, revs, then stops.... I'm pretty sure the timing is good, it is firing on both cylinders but just will not run continuously or tick over.
When I bought the bike in 1980 it had a monobloc carb.... I never used to have any trouble with carbruration  so I never changed any jet sizes, I never needed to. During the rebuild I dismantled and cleaned the carb and bought a rebuild kit (with the original jet sizes) and carefully treated it to a full overhaul....
I'm now at a loss as to why it will start but not run, both cylinders are firing (hot exhaust both sides) but I can't keep it running or ticking over.... I've checked everything I can think of and have run out of ideas..... 
I'm reaching out to the experts for help now.....

Jon

[Spaceman]

Hi Jon. Can you explain the symptoms a bit more? For example, does it fire into life and then refuse to rev and die as you open the throttle or does it run ok and rev for a short time and then die? Will it start again afterwards and do you have to tickle the carb again?

[BrummieJon]

It fires into life then dies straight away.... No backfiring

[BrummieJon]

Actually, it fires into life, revs and then dies, it won’t continue to run, rev or tickover. It will fire again without tickling but float level is not something I have checked so that’s a job for tomorrow.

[EDDIE SIMPSON]

hello,
impossible to diagnose that fault from emails.
i ve heard the saying on forums that most carb issues are electrical and vise versa.
i am having similar ongoing issues with richness even with electronic ignition and suspect its the latest fuel.
have you checked the fuel flow from the petrol tank is good? new petrol? is the petrol being contaminated with water from the petrol tank? drain the carb and check the jets are clear.
the next option is to substitute a good working carb and magneto.

[Spaceman]

Had this been a fully functioning bike that had just started to give a problem, it would have been much easier to diagnose the cause. However, because the engine has just been rebuilt, it unfortunately makes this much more difficult.

From what has been said, it looks as though the magneto and the carb were working properly previously and so there should not be a fundamental problem there. Nevertheless, it would be worth double checking both have been assembled correctly - with the magneto, that all the electrical connections are 100% - with the carb that everything is as it should be including the float needle valve, throttle needle, fuel flow into the carb and the mounting flange is perfectly flat with a good O-ring.

Unfortunately, if the magneto and carb look good, you may have to consider that there may be a problem with the engine itself after the rebuild. The compression is easy enough to test but it may be worth double checking the valve timing which could cause problems similar to that being experienced if it is out. Also worth checking the tappet adjustments.

Hopefully, it is just a simple problem which the tests will reveal?

[corneg2]

I had a similar problem as this when I re-built my A10, a 1952 plunger. At that time I was running a magneto too. I went back through the timing process again and this seemed to sort the issue. However, I think this can also be a symptom of a failing magneto. I know you said yours was re-built, but... My magneto started to fail last year and the symptoms were rough idle, poor pick up and occasional back firing. I have since switched to electronic ignition and this has transformed the running of the bike.

[DAVE BRADY]

Hi,

As Spaceman says.  Check everything over again.  You have a spark and some fuel is getting into the combustion chamber so it almost definitely down to settings and adjustments. 

Dave.

[Mike Farmer]

 

I cant remember from my last A7/10. But it sounds Mag? Does this mag have a condenser, Cos thats a good place to look with your symptoms. Certainly if I was doing a big old tractor engine that would be an early port of call.

Mike

[Mike Farmer]

 

Quick follow up. Get someone to hold the tickler down straight after it starts. Might give you some indication about fuel.

Mike

[bikerbob]

Always go for the simple things first this could simply be a blocked or partialy blocked pilot jet. I have had similar problems with my A65 and it was a partially blocked pilot jet, you have overhauled the carb  some years ago I assume you are using fresh fuel but after standing for so long there could be sediment in the fuel tank or fuel lines that could cause a blockage in the pilot jet. It only takes a few minutes to remove the jet and clean it of any obstruction.

[Mike Farmer]

 

Hi. Any joy with this one yet. Be interesting to know the cure and the outcome.

Mike 

[Dean Southall]

blocked filler cap vent hole

[BrummieJon]

Still no joy with this....... Stripped and cleaned the carb, checked float height ok,  blew out with compressed air..... did the timing again.....same issues.....starts easy, revs then dies.... Firing on both cylinders..... just did a compression test and I've go 100psi on both cylinders (which seems about right on 7:1 pistons..... my auto advanc unit seemed like it doesn't close properly so I've cleaned that and put new springs on the Bob weights..... wish me luck 

[chaz]

my plunger has electronic ignition, does not get used and when the wet sump is drained, battery charged and clean fuel always starts within a couple of kicks. the B40 was temporamental to start after rebuild but is easy now Ive fitted electronic ignition.
I did try new bob weights and springs but with all the play electronic proved so much easier.

if its racing away , that would suggest air leak.
one thing I did find, has the tank been coated inside? I have a jerry can that I kept oily old fuel in, then while draining some last week, a big sheet of paint from the inside came into view. could be its starting and the vacuum letting the fuel flow, coating covers the inside of the fuel tap.