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Re: 1948 Iron 4G Coil Ignition

Posted: Fri Aug 16, 2019 6:09 pm
by chris.shearwood
This diagram from Bacon's book might help you.

Re: 1948 Iron 4G Coil Ignition

Posted: Fri Aug 16, 2019 8:12 pm
by GuyH
This may or may not be relevant but I have a '39 4F with the non bi-starter Solex. I've had similar problems as you describe and it's always been caused by dirt in the carb. I found that cleaning each component and even putting them through the ultrasonic cleaner didn't cure it. Only completely dismantling the carb and blasting through each component with a proper high pressure airline cured the problem. It seems that even microscopic bits of fluff can interfere with starting and running.

Re: 1948 Iron 4G Coil Ignition

Posted: Fri Aug 16, 2019 10:33 pm
by simon.holyfield
paul.jameson wrote:This still sounds like a carb problem to me, although I did have some spectacular problems with the coil ignition on my own 1948 4G from time to time. As one last check on the Solex carb, have you undone the 4 screws which hold the float chamber section to the rest of the carb? This joint is vertical. (I don't have a bi-starter carb here at present so that description is from memory). When you remove these 4 screws, you should be able to see the passage which takes fuel into the bi-starter. There is a hole in the gasket corresponding to this passage. Could the gasket be fitted such as to block this passage? I do emphasise that I am working from memory here, but it is worth a quick check. Presumably the errant silicone could also have got into this passage?

This might help - there is a picture of the four-screw joint in this post: ... ebris.html

Needless to say, you don't want a leak in this joint.

It must be some thing fundamental - I must say my bike is a very reliable starter. Do you have the bi-starter assembled correctly? Is the disc valve sitting flat on the surface of the carb body? Is the starter well in the float chamber casting filling with fuel?

Re: 1948 Iron 4G Coil Ignition

Posted: Sat Aug 17, 2019 6:31 am
by nevhunter
Aerostart and such Have ether in them and a very low octane figure. You could get a bit of dieseling 'clacking detonation" not particularly good for head Gaskets and engines generally A small piece of rag soaked in petrol will do as well as long as it doesn't go into the engine. It doesn't need to run for long even if it fires up for a few seconds it will prove lack of fuel as the issue The main part of the carb will work of you strangle it a bit with your fingers over the intake and kick the engine with about 1/4 throttle. Nev

Re: 1948 Iron 4G Coil Ignition

Posted: Tue Aug 20, 2019 7:57 am
by chris.matthews
Hello Everyone,
Let me give an update.

With Solex off the bike, BiStarter assembled and the throttle body open - in Full Choke position I was able to blow through the hole in the throttle body; in half choke it was more difficult to blow and in No Choke it blew through OK. I fitted a new Throttle Body Gasket.

I refitted the Solex. My bike has the remains of the Magneto and the Dynamo, so there is minimal space between Solex and Dynamo, so I cannot simply drop the float chamber to see if its filling. But if I ease off the two bolts in the top of the Float Chamber I see some leakage. In addition if I ease off the Starter Jet in the Float Chamber I get a good flow. I feel the Float Chamber is filling OK.

I checked the Ignition Timing - I think it should be 8mm BTDC with Full Advance - I find it very difficult to get this exact - it always moves when final tightening. Now its at 7mm BTDC - but my system retains the manual Advance / Retard, so this might be close enough.

I used Local Easy Start into the Front End of the Solex - not a glimmer and also put some in #1 cylinder - nothing.
I had another go with Easy Start - I pushed the spray tube well into the inlet manifold, well past the butterfly. At each kick on the Kickstart there was one isolated 'pop'. This process was done with the Easy Start spray tube under the butterfly (so throttle was part open),
I repeated with the spray tube out and throttle closed - not a glimmer of life.

After a Coffee Break I repeated the two steps using Easy Start with exactly the same results.

I verified sparks at plugs - this is with the original coil, sparks seemed OK - but not like 'real strong' magneto sparks. I fitted a new coil, much the same result - minimal improvement in sparks is probably wishful thinking. I did the same Spray Start routine - with the same result - single 'pop' per kick with spray tube under the butterfly and nothing with it out.

My Ignition setup is NEGATIVE earth; the ignition Power comes from ignition switch to Positive Terminal (+) on Coil where I see 6.5V; the LT signal comes from the Contact Breaker to the Negative Terminal (-) on Coil. On same negative terminal is the condenser connection and the condenser body is earthed.

I really am at a loss - a couple of months ago it was starting and running very well - It all went downhill when I had Leaking Carb Inlet; Timing Slipped; Float Needle Valve and Dynamo Setup. Some folks here, believe that when I set up the charging circuit and flashed the dynamo I might have 'cooked' the Solid State Voltage Regulator, or the Coil or the Condenser. I'm not convinced - but any thoughts are most welcome.

With Best Regards
Chris in Tokyo

Re: 1948 Iron 4G Coil Ignition

Posted: Tue Aug 20, 2019 8:21 am
by Richard Kal
Have you changed the plugs?

Re: 1948 Iron 4G Coil Ignition

Posted: Tue Aug 20, 2019 9:18 am
by simon.holyfield
Chris, regarding float chamber filling. Obviously you have fuel in the float chamber, because you have cracked the starter jet plug and seen it, but can you verify that the starter well (which is above the jet) is filling?

You will have to remove the float chamber and manually fill it to see the fuel coming up the starter well.

The other thing that caught my eye was that you said you could blow through with the bistarter knob in (choke off). Is that as it should be? IIRC, there is a big port in the starter disc valve for full bistarter, a small one for the mid position and none(?) for choke off.

Re: 1948 Iron 4G Coil Ignition

Posted: Tue Aug 20, 2019 10:24 am
by john.whiting
A coil ignition spark should be a strong fat blue white spark at crankover speed,any orange or red is a capacitance problem..........but having said that,spray will start a Villiers on a pump I have,where the spark is near invisible ,except in the dark,due to flywheel mag weakness........if the timing is close ,spray will cause a motor to fire.......although ,in some countries the spray is said to have no ether.(claimed online) Aus ,the spray is 25% ether.......a few pops or a burp would say to me check leads for right position .

Re: 1948 Iron 4G Coil Ignition

Posted: Tue Aug 20, 2019 10:31 am
by john.whiting
As to air leaks,the spray will start and run a 6 cyl diesel with no carby ,and no fuel in the carby defects wont stop a motor running on spray........ A really flooded petrol motor wont start ,and generally needs to be cranked with the plugs out,which will blow out liquid fuel,then it will start easily with it seems the problem is ignition related.

Re: 1948 Iron 4G Coil Ignition

Posted: Tue Aug 20, 2019 12:29 pm
by brenton.roy
I agree re with John re the spray - it should run without fuel - but didn't you originally say it was running for a minute or so, then 10 seconds for each start after that?

What has changed? Have you changed any ignition component settings or is there a reason to think the ignition timing has changed?
If not, I think you are still at the carby - or at least, I wouldn't change anything elsewhere until I was sure the carby wasn't the problem.

Having said that, sometimes a weak spark can completely disappear under compression, when everything else seems ok. Can you borrow another battery? If every thing else the same and the symptoms are getting worse, the battery could be failing. If it's not sealed, you might be able to get hold of a hygrometer or have it load tested.