Spit back through carb and backfires

[Colsbeeza]

I have been absorbing this topic with interest, as I have just completed a 33-year restoration of my 1961 Golden Flash.
I fitted the tank, knee grips and badges last week. She looks beautiful.! Well- I did say LOOKS!
After a complete rebuild of the motor about 20 years ago as a fairly inexperienced mug, I got a local British bike enthusiast bloke named Phil  to strip the engine down and check it over. Fortunately, he found nothing wrong except that I had the LHS conrod round the wrong way. He started it in October 2015 and it ran sweetly on the stand, except for some smoke from the LHS cylinder which went away after 30 or so seconds. Certainly no back-spitting.
It has taken me 18 months on and off to get the wiring done - much head scratching.
Now to startup this week.
It started second kick, but would not run for more than 10 seconds due to so much back spitting that it would not pick up engine revs.
I have sofar re-checked the timing and found that the points on RHS opened about 18mm BTDC. Ahah thinks I.!  Knowing Phil's high work standards, I suspect that the ATD slipped on the taper. It came off fairly easily but I think it was tight enough!

Reset to  5/8" BTDC after much reading, and although it started immediately, the spitting was exactly the same.
The magneto was overhauled about 25 years ago which Phil thought was a bit long, so I had it re-tested last September 2015 by a well-recommended Sydney magneto specialist. He found a couple of minor problems and re-magnetised it, although he told me that the bloke who rewound it back then had had a few failures, so to be wary.
My logical next step is to strip and clean the carburettor, as there was some fuel left in it for a few months after the October 2015 startup before I got around to draining it. My first suspicion is a blocked pilot system.
If that doesn't work, I may be back to you blokes for advice.
Colin

[Colsbeeza]

To intercept any questions about my resetting the timing, I did take a short cut, being reluctant to remove the LHS drive cover.
I first checked the points setting. The RHS corresponded with the points fibre pad being at the bottom of the camring. It was set at 12-13 thou, closer to 13. The LHS was closer to 8-9. The camring is new, so that was surprising.
I removed both plugs, refitting the RHS one about 1 thread so I could hear the hiss as the piston rose on the compression stroke. As the engine is new, it took both hands to turn the back wheel, so no spare finger to feel the compression.
After finding TDC, I used a plastic straw down the hole and marked the position with a Sharpie pen. I then backed it off, and raised the piston again to take up any gear slack until the straw reached my BTDC mark, then chocked the ATD at full advance with a wooden peg. I set the RHS so that I could just pull out a 1.5 thou feeler gauge. The heel of the points was just beginning to touch the edge point of the ramp. Previously, it had been about 1/4 inch further advanced round the camring.
I checked the points opening for the other cylinder and it was in about the same spot relative to the camring ramp. So all seems OK. Now to the shed for the carburettor clean.

[RichardL]

There is a lot to say here, but let's start with the BTDC measuremnt. 5/8" is too much. Without knowing much about the bike, I think it is still safe to say you shuold be at 3/8" or maybe 11/32".

As for identifying compression stroke, with both plugs out and in neutral, use kickstart pedal to slowly turn over until you get pressure on your thumb (or finger), then, in 4th gear, turn rear wheel (with plugs out, of course; yes, I know you know that) to manipulate to desired BTDC.

Rather than fumbling with a feeler gauge, you would probably be better off using the fag paper method. My approach is to tear a thin strip of fag paper and put one end between the closed points and attach an alligator clip lead to the other end. Then, let the weight of the lead pull out the paper from between the points to signal when they open.

If you have the means to print a measuring graticule to scale, I would recommend printing one out with a few inches divided into 1/32" increments. Then, glue or tape this graticule to your straw. It will give you much more confidence in your measurements. Glue or otherwise fix a screw or nail in the top of your straw as a weight to be sure of positive movement. If you think it will help, rig some kind of guide that keeps the straw straight up and down.

Once you are convinced of the timing, move one of the pistons to the timing point and look into the top pickup hole of the mag (may require a mirror). If you see the brass in the slip ring, connect the top lead to the cyclinder currently on the compression stroke and vice-versa if you don't see the brass.

I'm going to step back now and let others fill in what I've missed (which will be plenty). As we say here, apologies if telling you any of this stuff is like teaching gramma how to suck eggs.

Richard L.

[RichardL]

So, let me add in this separate post:

FIRST PLACE, BLUE RIBBON, GRAND CHAMPION of PERSEVERANCE for completing your restoration in 33 years.  

[RichardL]

Oops,

One more thing, for now. Don't forget to check the timing again once the pinion is tightened down.

Richard L.

[Triton Thrasher]

Spittiing back is often a sign of weak mixture.

An air leak into the inlet manifold at the carburettor flange, is a common cause. 

Or possibly a blocked pilot jet. The Monobloc pilot jet is removeable for cleaning, but may be stuck.

[Black Sheep]

If you don't need the choke for a cold start you are running way too rich on the first 1/8 throttle opening. Either your pilot jet is too big (how many turns out is your pilot air screw?) or the slide cutaway need to go up a number. What's your cutaway? 3, 3 1/2?  Either that or your carb float is not seating properly and the carb is flooding. As for spitting back on the move, that's a sure sign of weakness. Drop the needle a notch and see how that goes. What main jet do you have? It strikes me that the P/O just chucked a carb on with no regard to the actual settings required.
Most unlikely to be a sticking valve. 

[Triton Thrasher]

As for spitting back on the move, that's a sure sign of weakness. Drop the needle a notch

Raise the needle to enrich!

Although if it won't run for 10 seconds, the needle taper probably isn't coming into play at these small throttle openings.

[Colsbeeza]

Richard, Thanks for the Congratulations.!. I did a few bikes in the interim, got involved in Lilac Flat Twins and restored a 1965 Model 500 Magnum. I have already washed my mouth out with soap in case that is suggested.
I like the alligator clip idea, which would only work with fag paper. I used to use fag paper, but I didn't have any at the time, gave up the fags years ago. I read that fag paper was about 2 thou, but would work better with a clip attached. I just realised I said 5/8" BTDC. I meant 5/16" which is about 30-32 Degrees. This is 8mm. I did set at at 5/16". Certainly no sign yet of backfiring through exhaust, but it is early days. The straw did tilt a bit, which made it a bit harder, so will make up a guide next try. Yes, I had plugs out, 4th gear, still took two hands on the back wheel.
I didn't know about the idea of looking through the mag pickup.I'll do that next time. For the moment, it should be close enough to sort out the spitting if it is the carb. I'll clean that first and them move on.
Titon Thrasher - As it ran well 18 months ago, I did not recheck the carb bolt tightness, assuming that nothing would have changed. An obvious check I need to do, as the gasket (or is it an O-ring?)may have dried out and shrunk a bit. The pilot system is a more obvious check which I will do. I have to get the carb off anyway.
Black Sheep - It is fitted with a choke, as I imported the bike from Sussex in 1984 from Andy Tiernan when he was first starting out. They generally were not fitted in Australia, and I have checked that it is secured fully open each time I have started it. I'll probably end up removing it, but in the meantime it may be useful to check Main Jet size on the road. All the settings and sizes are as per the specs for this model. The carb was in exceptional nick when I received it, I suspect it was a recent replacement before the bike died way back then. I just cleaned it thoroughly back then. It ran very nicely in October 2015, so I will go through the cleaning first before chasing other possibilities. My first goal is to get it to start and run for a minute or two without konking out due to spitting at low revs.
Cheers

[chaterlea25]

Hi,
Problem sounds like stale fuel to me

John

[RichardL]

Col,

This is a link to an old post of mine showing my graticule tube with guide/indicator. http://www.a7a10.net/forum/index.php?topic=8301.msg58614#msg58614

All well and good. I love to talk (too much) about A10's and share my ideas, which are not always (or, "not often") original, and I am miles (figuratively and literally) from the top mechanics here. (Also, I use too many parentheses.)

Richard L.

[Triton Thrasher]

Someone mentioned looking at the brass segment in the slip ring.

That reminds me- a dirty slip ring can cause very erratic running.

[Colsbeeza]

Hi Gents,
I finally got the 1960 BSA Golden Flash running properly today. I went through everything, learnt a lot more than I ever knew. I was quite satisfied with the Carburettor, valve clearances, etc, so had to suspect the magneto. I did learn that a #25 pilot jet is 16 thou, and a #30 jet is 18 thou.
To recap, I had the magneto rewound etc about 25 years ago, and remagnetised and bench-tested in July 2015 as it had been sitting so long. It was firing properly when the motor was completed in October 2015. I spent a very intermittent 18 months wiring, indicators etc etc, and when trying to start it early this year, it would not fire. On the last attempt, the left exhaust got near white hot. At that point I had had enough. I sent the magneto down to Sydney for checking to the same bloke who bench-tested it in 2015. After waiting 2 months for the queue to shorten, he found about a 9 degree difference between cylinders, a bent armature shaft, replaced the slip ring and points, and took all the slop out of the points pivot pin by bushing it. How it got like this in 18 months sitting idle cannot be explained.  Electrically it tested OK.
I got it back on the bike today. It sounds very nice indeed.!
I set the timing to about 31.5 deg BTDC, and it started and ran properly, although plenty of smoke from the LHS pipe. I'll work on that later. Slight occasional spit, but I have a #25 pilot jet and as yet untuned.
I can now get on with the fettling for the road, and I do hope the smoke goes away.
Just goes to show that the old adage that "most carburettor problems are the magneto" is true.
Thanks all and Cheers
Colin

[TimK]

Hi everyone.

I use a variation of RichardL's straw:

Glue or otherwise fix a screw or nail in the top of your straw as a weight to be sure of positive movement. If you think it will help, rig some kind of guide that keeps the straw straight up and down.

Instead of a straw I use a kebab stick, which is stiffer and less likely to bend. I attach it tightly (so it can be pushed up, but won't drop down) to the head steady with a small cable tie. This way it's pushed up to TDC, holds that position and can be marked against the top surface of the head steady to provide an accurate BTDC measurement if it's pushed back down before the engine is rotated to the next firing stroke on that cylinder.

cheers

Tim

[Black Sheep]

And to think I wasted a couple of quid on a tdc gauge. The shame.