Spit back through carb and backfires

[lawnmowerman]

The bike lift has finally arrived so I am going out to the workshop to have a tidy up to make some space, unpack it and get the bike up onto it.
My can of carb cleaner, gasket set and a new pilot jet has just arrived so I am going to strip the carb down and clean out any congealed fuel residue and petseal.
My neighbour who is a Manx Norton specialist reckons that the mag is duff (probably the capacitor or a dodgy slip ring tracking to earth) having heard the bike running with the backfires and spit backs from three gardens away last Saturday. He was bang on when I had problems with a duff rotor arm on my Wolseley last year. He recons mags so I will take it off and get it to him.
I will keep this thread running as it will be good for other members to know the final outcome and to identify what the problem was.
Up to now I have identified three problems:
Partially blocked pilot jet
Undersize main jet
Timing too far advanced 1/2" ish BTDC on full advance.


Jim

[wilko]

As an experiment, you could wire a car condensor into the cutout wire on the end of your mag. An old get you home trick! I'm still skeptical.

[lawnmowerman]

Hi Wilko

That would work if there was an intermittent open circuit on the capacitor but my mate the specialist reckons they usually go intermittent short circuit producing weak and random sparks which would explain the spit backs and backfires - almost like a second set of points opening randomly I would imagine. It has always misfired and run on one occasionally ever since I bought it and is difficult to start when hot - all of which points towards a mag problem. I may ask him if he can put it on his test rig before he strips it just out of interest - he normally runs them at 3000 RPM for three hours with some test gear hooked up to measure the spark strength.
I have resigned myself to a mag rebuild and with the superior insulation materials now available it should outlast me and also rule out ignition problems if the fault remains.
The other thing is that I want to get the bugger running before the sun comes out and we get our usual couple of decent weeks which we call summer.

Jim

[starfield]

Hi,

Not sure if you have resolved your problems, if so ignore the following though its an interesting tale.

Just like you, I have been wrestling with similar problems on my 52 Star Twin - running on one and spitting back. In the end the spitting back proved to be primarily caused by tappets that had been allowed to close up. Resetting them initially seemed to cure the problem. Then the LH inlet valve stuck open and bent a pushrod, almost certainly caused by the carbon build up on the permanently open inlet valve. Naively I though the problems would go away after I stripped the head and did all the valves. Indeed the spitting back was cured but the ticking over on one cylinder and the hiccupping at lows revs remained causing a few hairy moments.

Interestingly, the bike would tick over perfectly as a 250 single on either cylinder so proving which was at fault was difficult. As the carb had already been well sorted by the previous owner, I took the plunge and had the mag serviced. What a good decision that turned out to be. There were numerous issues which were sorted out for a very reasonable cost:
1. The slip ring was badly worn and was probably the major culprit, under test it sparked like a catherine wheel.
2. The pickup brushes were wrong and may have contributed to 1.
3. The oil seal had hardened and was beginning to let oil into the armature
4 The cam ring housing was badly worn and had to be shimmed.

The last point was interesting. My mag had been converted to auto advance and the cam ring had of course been locked by grub screws. The wear had allowed the grub screws to push the cam ring off centre so the points gap differed on each side. I knew there was a problem as I could never get the timing set properly and had to equalize with 8.5mm on one side and 11mm on the other  but had assumed a worn cam ring and did not notice the points gap changing. I am not sure how such wear would affect a manual a/r situation but it cannot help and shimming is probably not an option.

Fitted the new mag, timing now identical on both sides and first kick start. No more ticking over on one, no hiccuping and no spitting back. Totally different bike. Previous owner took it for a ride today and now wishes he had not sold it. Minor moment of panic when it went on reserve on the way home and stalled at a roundabout just to shake my over confidence.

If you still have problems, maybe some of the above helps. I used the guy down at Langton Matravers who advertises in the VMCC mag. He came well recommended and I can see why. Thorough checkover, only replaced that which needed replacing and back in a week looking like new. I hasten to add that I  have no association with the business, just a satisfied customer.

Reminds me of the old story that 90% of carb problems are electrical. For the relatively small outlay compared to the value of the bike I would think a mag checkup by a specialist a good insurance.

regards

Peter

[lawnmowerman]

Hi Peter

A very interesting story. My engine has only done about 500 miles since rebuild so probably does not have the same problems with carbon build up but could possibly have tight valve guides I suppose.
I think my problem is mag related so I removed it today and will get it rebuilt by a friend of mine who rebuilds Manx Nortons.
The timing was just over 1/2" BTDC and the points gap was 15 thou though even on each side so needed retiming. The pinion was tight on the taper so does not look like it had slipped.
At least when the mag is back I can rule out ignition problems and stop chasing red herrings caused by intermittent random sparks and with the modern materials now used the mag will probably last longer than I will.

Jim

[Brian]

Jim when you say your friend will rebuild it is he a magneto specialist? I am not trying to suggest he doesnt mean well but dismantling it and cleaning, new bearings etc will not cure it if its at fault. It has to be rewound and a new condensor fitted plus remagnetised as well as the usual bearings etc.

Unless your friend does this I would suggest you send it to a specialist magneto repairer who comes highly recommended. I am sure others on the forum can recommend someone in your area.

[lawnmowerman]

Hi Brian

Thanks for the good advice. My friend is a first class engineer and I have every confidence in him. He tells me that he was the first to carry out the roller bearing conversion on our bikes many years back. He also works on piston aircraft engines which use mags extensively. He has all the necessary test rig for the mags and although the Manx Norton uses a BTH, he has rebuilt many Lucas mags. He sends off the armatures to be rewound but does the rest himself. He can only remagnetise BTH but knows a guy locally who can remagnetise Lucas mags. He does not advertise his mag rebuilding services as he is always snowed under with work on the Nortons. I am going to ask him if he will put it on his test rig before he strips it just out of interest although he is fairly certain it is a capacitor problem but while it is out it may be a good idea to get it rewound and remagnetised if necessary.

Thanks again

Jim

[Brian]

Sounds like your friend knows what he is doing then, thats good.

The reason I brought it up is I have come across people before who advertise themselves as magneto specialists and who can "fully recondition" your mag but in reality all they do is dismantle them and clean them,none of the stuff that matters.

I have a bit of a thing about magneto's, whenever I buy a bike, even if it has had the maggy rebuilt and there are receipts I take the maggy off and get it rewound and a new condensor.

If the engine is worn the bike will run but will rattle and probably blow smoke.
If the carby is worn or not jetted correctly the bike will run but maybe too rich or too lean.
If the maggy is crook, the bike will not run.

The cost of a maggy overhaul is not a lot compared to the value of the bike and its nice to have bikes that start first or second kick every time, this wont happen with a crook maggy.

[lawnmowerman]

Hi Brian

Absolutely agree. I think the mag is probably the most important component and when they go intermittent they produce random running faults and it is very easy to go off on tangents looking for non existent faults elsewhere. When you look at the process involved in rewinding it is almost impossible to do at home and with the amount of labour involved I think the cost is reasonable. It looks like it is at least a day's work and by way of comparison, the going rate here for electricians, plumbers and plasterers is at least £200 per day.
The only downside is that there are few people who know what they are doing when rebuilding mags so if you want the best you have to look around and be patient as they normally have a big queue of work.

I took the mag off yesterday. I had to get and extractor as it is manual A/R so did not have the built-in puller on the pinion. The HT pickup ring was in a shocking state (sorry no pun intended) with gouges all the way round it and it looked like the brass section had a sort of clear lacquer over it - possibly something the PO had sprayed in there. I really had to dig the meter leads in and scratch the surface to get a reading. I got about 0.4 ohms on the primary and 40-90k on the secondary and the readings were all over the place so it looks like there is a high resistance somewhere.

I dropped it off this morning and I am looking forward to getting it back so I can finally get the bike running as she should.

Jim

[trevinoz]

Jim,
         It is difficult to get an accurate resistance measurement of the secondary winding between the slip ring and earth because the secondary wire end is loosely pushed into the slip ring without a positive connection.
I suspect that there is always a bit of an air gap and the spark jumps it.
         Trev.

[lawnmowerman]

Hi Trev

That explains it. It seemed that when I pushed hard on the probe when it was on the slip ring contact I got variable readings - may have been that the slight movement made partial contact.

Jim

[lawnmowerman]

An update on my troubles.
Still waiting for the mag to come back. Apparently the guy that my mate uses to rewind the armatures for his Manx Nortons does not do many but is very good and has been doing it for ages so worth waiting for. One interesting point is that he uses two capacitors mounted 180 degrees apart to even out the balancing on the armature - never thought of that one!
I have bought an SRM timing disc, mounting nut and piston stop to get the timing spot on - 30.5 seems to be the best setting for a SR. Once set up and checked with a strobe I will make an adjustable piston stop and lock it off at the right setting for future use.
I am also replacing the existing narrow V belt drive on the dynamo fitted by the PO for the 15mm toothed belt version from Mike at Dynamo Regulators. It spins 20% faster than the SRM version as Mike has managed the squeeze an extra tooth on the drive pulley. When I spoke to Mike earlier this week he says they should be ready in a week or two and he has taken on board comments from the forum and is including a couple of tapped holes on the drive pulley to allow removal.

Jim

[lawnmowerman]

Hi all

My mag came back yesterday so I am off to the workshop today to fit it back on and re-time the bike with the aid of my new SRM piston stop and timing disc.
My new dynamo belt drive kit is on its way from Mike at Dynamo Regulators to replace the current narrow V belt.
The plan is to get the bike running by tonight and fit the belt drive when it arrives tomorrow.
Hopefully I will then be mobile as both cars are off the road at the moment and I am relying on Tesco home delivery for essentials (fags, milk and bread )
I will report progress this evening.

Jim

[lawnmowerman]

In for a coffee.

I am impatient to see if the mag has cured the problems so rather than take off the left side exhaust, footrest and drive side cover and use the timing disc I manufactured a timing aid from a fence rail bracket using a large hammer and zip tied it to the barrels. using a piece of fence wire I filed some marks in it - TDC and 5/16 BTDC and I have now refitted the mag, carb and tank (with some new avgas inside) and it is ready to come off the ramp and see if she starts.
Fingers crossed!

More later.

Jim

[lawnmowerman]

SHE RIDES  WOOOOHOOOOO !!!!!!!!!!!!

It is a lovely day here in the Kent countryside - the sun is shining, the cuckoo and the woodpecker in the woods - the baaing of the spring lambs but nothing quite surpasses the sound of a 650 BSA parallel twin through a pair of goldies!

It took a fair few kicks to get her into life after a short burst on the second kick and she ran on one and gave a few spit backs but when she warmed up she did not miss a beat. I need to check the mixture and play around with the pilot jet adjustment but it all looks very promising. Although I cleaned the plugs they were the old ones I was using with the dodgy mag so may have needed to be run and heated up to burn off any residual crud.

I guess the moral of the story is to make sure that your mag is ok before chasing any red herrings - a dodgy mag gives all sorts of strange symptoms depending on temperature, humidity, strength of your right leg etc and mags do die of old age - even if not used.

Jim

[Stu55Flash]

Well done Jim:

The other motto that comes to mind is if at first you don't succeed try and try again. I;ve just got mine running after a mag armature rewind. Wouldn't start at first yesterday so retired to pub to think about it. Came back and swapped the plug leads around and she fired first kick! Some how the slip ring was 180 out from the last time! The weather up here is rubbish so I'll have to wait until next weekend to take her out any distance.

Stu
 

[lawnmowerman]

Hi Stu

It's amazing how a pint (or three) stimulates the brain cells. I was wondering which way round the leads went so I set the points with the heel on the bottom cam section as per the service sheets but nowhere does it tell you which way round the leads go. Fortunately I had some old pics of the engine so copied that arrangement. It looks like spring has arrived here at last so she will be off for an MOT next week - just have the two cars to fix now.............. At least I am mobile now and I can get down to Tesco 

Jim

[bsa- bill]

Well done - result

[lawnmowerman]

Looks like the saga is now over - she passed her MOT today so I went for a ride in the sunshine to the local bikers cafe to celebrate. She did not miss a beat and feels like a different bike. One strange thing was that I got and advisory for the headlamp glass which dips to the right instead of the left. Soon fixed by the helpul MOT guy with a bit of tape on the relevent area of the lens 
I am going to have a look on Ebay for a UK lens.

I will start putting some miles on her now to run-in the engine as it only did 150 miles last year after a full rebuild.

Jim

[BSA500]

Where abouts in Kent. I took my A7 for a buzz yesterday around Tonbridge/East Peckham.
Andy