Starting problems

Hi Mike,

I am no carb expert, so i would suggest speaking to Amal and telling them what set up you have and confirm with them all the settings. It has been known for new carbs to have swarf or a blockage in the idle passage !

I think you are looking in the right area with the float level. This can be adjusted by warming the float bowl and tapping the brass float needle seat up or down until you have the desired position. You must be careful not to damage the actual needle seat and a tiny adjustment will make a big difference to the level of the float. I would aim for a level about 3mm below the top edge of the bowl. Press down on one of the ears that locates the float needle with a scriber to seat the needle, then carefully invert the whole assembly, this will take out the backlash / slop in the float spindle and show you where the float sits when it's lifted by the fuel. just doing it it's normal orientation does not take out the slop in the spindle and will give you a false reading.

Previously david_evans wrote:

I think you are looking in the right area with the float level. This can be adjusted by warming the float bowl and tapping the brass float needle seat up or down until you have the desired position. You must be careful not to damage the actual needle seat and a tiny adjustment will make a big difference to the level of the float. I would aim for a level about 3mm below the top edge of the bowl. Press down on one of the ears that locates the float needle with a scriber to seat the needle, then carefully invert the whole assembly, this will take out the backlash / slop in the float spindle and show you where the float sits when it's lifted by the fuel. just doing it it's normal orientation does not take out the slop in the spindle and will give you a false reading.

I agree with David that the float level might be your problem. An alternative to adjusting the float level by moving the needle seat (which can be rather fiddly) you could obtain a 'Stay-Up' float which is much easier to adjust (and ethanol resistant).

Dear Mike,

If the choke makes no difference then it tells you something. I would check that when choke is on that there is a small amount of slack in the cable; if not then adjust till there is some - yes you may have to cut some of the outr from one end.

My 750 is set lean like yours, take choke off too soon and it spits back from card and tries to stall - but fine once hot.

regards Martin.

I purchased a new premier 932 carb, earlier in the year. It came with a 220 main jet which I was informed would be fine for use as a single carb on my 750 Commando. I querried this as my Atlas needed a 270 main jet on a Mark 1 single carb set-up. A 220 will be fine, I was reassured as these new carb are made to much closer tolerances.

The reality was, my engine was difficult to start from cold unless the choke was on full, the carb flooded and the engine kicked over at least twice below switching on the ignition.The engine would then fire up but I could not drive off unless the choke was kept on at least half-way for about 2 miles of riding. Even then the engine would often stall when shut down at a junction. I must add that this was on a rebored engine so I did not expect perfect running to start with.

After 200 miles of moderate running I checked the plugs, which were fairly white. So I upped the main jet to 240 and added a K&N cone filter. I also reset my Pazon ignition back from 31* to 29*. This solved all my starting and running problems. My pilot air screw is 1? turns out.

The cold start symptoms point to weak mixture - possibly due to the choke seemingly having no effect which points to it not functioning correctly as mentioned previously. Engine hot and it runs ok again points to weak mixture when cold.

I wouldn't go altering the float height until you've established the fuel level. Amal recommend 0.21" + or - 0.04" below top edge of float bowl measured using a clear pipe attached to a spare drain plug.

Phil, I'm suprised that the main jet affected your starting. it doesn't come into play until the top end of the throttle range. Your starting cure may have been down to the retarded ignition. The float levels quoted will be different once the carbs are mounted on the tilted manifolds. I guess you can only measure accurately with the carb body vertical. What's your take on that Simon?

Welcome back by the way Simon, see you next Thursday?

I'd thought about the fitted angle of the carbs on a Commando and decided measuring the fuel level in line with the main jet, side on to the bowl. Don't know what Amal suggest. I guess it's not too critical anyhow with such a wide tolerance but may as well get it as close as one can to the manufacturers recommendations.

Thank's for the invitation, I'll more than likely see you there. Will there be anyone else on a Norton? Ho ho.

Mike,

When the engine is running well and warm at 5,000rpm what do you estimate is the amount of throttle opening? i.e is the bike on the road in top gear (lots of throttle - probably on the needle or main jet) or in neutral (small throttle opening still on the slide) ?

The symptoms still point to weak mixture (I could be wrong). Does the choke have an effect when the engine is warm?

Previously simon_ratcliff wrote:

Mike,

When the engine is running well and warm at 5,000rpm what do you estimate is the amount of throttle opening? i.e is the bike on the road in top gear (lots of throttle - probably on the needle or main jet) or in neutral (small throttle opening still on the slide) ?

The symptoms still point to weak mixture (I could be wrong). Does the choke have an effect when the engine is warm?

A very silly point but often over looked. How old is the fuel? if it is ethanol laden then it goes off in a matter of months and causes the exact problems you are suffering with. Just a thought.

Mark.

Previously mike_chizlett wrote:

Hi Simon

I havenât had the bike on the road yet. It takes a good half hour or so to get it to tick over and respond to throttle openings. I have only momentarily blipped it to around 5000 rpm but when warm it picks up well and revs freely. I have ridden it around the garden at walking pace and the engine runs smooth.

The motor has been rebored with new pistons, valves quides and seats and the crank reground with new shells so I want to treat it gently to start with.

Iâve tried a number 3 and 31/2 slide but noticed no improvement.

Petrol is fresh. (thanks Mark) Itâs the same as I use in my Domi which I can leave for 6 months or more with no effect.

Iâve not checked if the choke makes a difference when the engine is warm but it doesnât seem to help when cold.

Since the bike will run, eventually, the ignition timing should be about right, although yet to be strobed. I have a dual coil which sparks both plugs at the same time regardless if the motor is on compression or exhaust stroke. I assume, therefore, that it doesnât matter which cylinder is used to set the timing, or am I wrong?

I was thinking of putting my domi carb on just to see what happens. Obviously itâs far too small (itâs a 626) but at least it is known to operate without fault and I would have thought it should get the commando to run in some fashion.

I too think its inlet related and Iâm waiting for a selection of guitar strings to turn up so I can poke the carb in all the right places.

Iâm pinning my hopes on a malfunctioning pilot jet but a blast of air suggests all in that department is ok.

I've dicovered that the manifold I have is quite old in that it only has the 2 outer fixing holes. I have drilled 2 extra holes to clamp the middle section but again no improvement. I'll see if I can checkfor any air leaks in the casting.

Here's hoping!

I had a lot of problems getting my commando to run last year after rebuild. A friend in the NOC helped diagnose problems by tickling the carbs while I held the bike and controlled the twist grip.

Results diagnosed:- intermittent Boyer black box, old float brass needles sticking. Crud in drillings in the blind side of pilot jet.(carbs had been ultrasonicly cleaned.

Cure:- new ignition, stay up floats with new Brass float needles (tried Aluminium float needles could not stop flooding when engine running).

Drilled out blind side of pilot jet as per Bushmans Amal and cleaned out crud. Bike now running ok and have finished running in.

While messing around with carbs kept wet towels and Fire extinguisher close to hand.