Won't start - what have I done?

[Mark Wilson]

I totally rebuilt my 1275 Midget engine and I've had it running for a year or so, although only a couple of times out on the road.

Last week it was running okay-ish, but I decided to try and improve it. I had been having problems with the choke mechanism sticking on the HS4 carb, so removed the carb to try and rectify this before starting to play with the timing. (The carb is a converted waxstat, and I finished up adding a spring to persuade the jet to return fully up)

Now I can't get it to start, just the odd short stutter as though it's running on a couple of cylinders. I have what seem to be strong sparks (and I've swapped the dizzy cap, leads and coil just in case). I have the dizzy and clamp marked for where it normally fires ok. The dizzy was overhauled and fitted with an Accuspark unit during the rebuild.

I've gone back to the carb, checked fuel is reaching the bowl, and set the float level as per the book. With the bell and piston removed I set the jet level with the bridge and wound it down twelve flats. I switched on to activate the pump but the fuel level seemed (from what I've read) low in the jet. It's several millimeters below the top of the jet after dropping the jet down fully with the choke, which I thought might indicate that the level in the bowl was too low. After my last attempt to start it, though, (cranking for fifteen seconds or so), I removed the bell and piston again, and found a small quantity of liquid fuel pooling in the base of the carb intake. Is this to be expected here, or have I done something fundamentally wrong?

[simmitc]

I would go back to basics and check that No. 1 cylinder is timed correctly and that the rest of the leads then follow in the correct 1-3-4-2 sequence.

[kennatt]

Dropping the jet via the choke only lets the jet move down to a thinner part of the needle to open up a bigger gap to let more fuel into the carb mix. it has no effect on the fuel level thats set and held via the level in the float chamber .If you've not touched the timing, Initial fault was after working on carb so has to be down to that. Stick some petrol straight into carb intake and seen what happens,will test carb one way or another. Have you checked that the piston and needle is free to rise and fall freely in the bell.

[Mark Wilson]

Thanks for the suggestions so far. I have double checked the lead sequence, on both the two dizzy cap and lead sets I have tried.

"Dropping the jet via the choke only lets the jet move down to a thinner part of the needle to open up a bigger gap to let more fuel into the carb mix. it has no effect on the fuel level thats set and held via the level in the float chamber" I agree the level stays the same, but when the jet moves down shouldn't then the distance between the top of the jet and visible fuel level be less?

I've never gone this far into carb setting before, but from what I've read I should be looking for the jet to be .060" down from the bridge and the fuel level visible just below this. Although I've set the fuel level in the bowl (I changed the float to a metal arm one to make this easier) I'm still seeing the fuel level much further down in the jet. Which I would have thought would mean fuel starvation, but I'm seeing what I take to be carb flooding after trying to start.

The bell and piston are good. How do I go about sticking petrol into the carb - do I need to spray it into the carb mouth?

[kennatt]

Take filter off and just throw a half ish egg cup sized amount of petrol into the carb.

[kevin s]

The fuel level should be below the top of the jet, the suction created by drawing air through the narrow area between the the bridge the jet sits in and the piston draws the fuel up into the airstream and on into the engine. the diagram below (not an HS4 think it's a HIF) shows the principle.

Fuel height2.JPG (21.16 KiB) Viewed 992 times

Assuming the ignition hasn't been touched and was fine before Su's are pretty simple things, as you seem to have fuel and the right float level I would check the following.

-Does the piston slide up and down freely.
-Take the spark plugs out and check they are dry, leave it with them out overnight to dry the cylinders.
- Is the jet servicable, the hole tends to wear eliptical, when this happens they have all sorts of running problems and flood if set to the correct position, if it was screwed up a lot further than the 2 turns down from level to run before it's a good indication this has happened.
- Is the needle set at the right height.

if all the above is right I have always found Su's to be excellent starters.

[Mark Wilson]

Take filter off and just throw a half ish egg cup sized amount of petrol into the carb.

Sorry to be thick - but do you mean throw it in while my assistantess turns the key, or throw it in and then try to start it? (While working under the bonnet I normally apply positive to the solenoid LT to turn the engine, but I'm thinking the sparks I sometimes get wouldn't be a great idea while throwing petrol around....)

[Mark Wilson]

I completely overhauled the carb including new waxstat conversion jet and it has run ok so far, although mainly in the garage. The problems started, when I tried to free the sticking choke - I removed the carb and had to loosen the float bowl, as the securing bolt prevented me removing the operating rod which comes with the conversion kit, but didn't do anything to the flexible tube and at that stage didn't remove the float bowl cover (I've since checked the level and cleaned the needle valve and seat, although it was new).

The needle may be worn, I swapped needles around and can't remember where this came from, but I haven't moved it since it ran ok.

[kennatt]

just throw the petrol in,then get in the car and try to start it, what you are doing is bypassing the action of the choke don't do it more than once(Unless you leave plenty of time between tests ,or you may flood the plugs.. If it starts and then still fails to keep running clear indication of fuel and not electrical problem,could also indicate that the level in bowl is too low,and not getting fuel up to top of jet(Will run for a short time then stop)did you change the needle as well as the choke service ,if so did you make sure the shoulder of the needle was flush up against the piston ,because if the needle is too far down into the jet ,even at full choke ,the thicker part of the needle may be restricting the gap ,preventing full flow through the jet.
once started this doesn't easily show up because the vacuum in the carb lifts the piston and needle to what ever position needed to keep running. But from start up obviously the needle is fully down and stays there until engine runs.

[Mark Wilson]

The lesson is never rule out the possibility of coincidence! Despite me assuming carb problems as the most likely cause, as the last thing I had done before this saga was remove the carb to remedy the sticking choke, I swapped the converted electronic dizzy for a points 25D. Started straight away on static timing, in fact running better than before.

Thanks for all the suggestions. At least I've learned that the original equipment had its good points!

[Mark Wilson]

Incidentally, I had checked the spark (plug held against the cylinder head) and saw what seemed to be a spark, but somehow a bit dithery (technical term). This electronic malarkey is clearly the work of the devil!

[geoberni]

What I don't get is that you originally said ...

Now I can't get it to start, just the odd short stutter as though it's running on a couple of cylinders. I have what seem to be strong sparks (and I've swapped the dizzy cap, leads and coil just in case).

Now you're saying that

I swapped the converted electronic dizzy for a points 25D. Started straight away on static timing, in fact running better than before.

I've never known or heard of an electronic Points Unit giving a weak or intermittent spark, it either works or it doesn't. It's just a switch.

I think there must be something else, associated with that Distributor, other than the actual electronics unit.

[Mark Wilson]

Well - once I moved away from thinking it must be the carb I went back to checking the spark, and found that while it seemed strong and regular at times, at others it was intermittent. The observed intermittent sparking was independent of either mechanical or vacuum advance, the rotor has only been used for a couple of hours of test running, and I'd swapped cap, leads and coil around. What else in the distributor could cause this other than the electronic switch? Are electronic devices incapable of intermittent faults?

[geoberni]

When they are as simple as a 'Switch' there's not much to go wrong, they either work or something fails and they're busted.
Unless you had a poor connection on one of the leads to/from it....

[kevin s]

The wires to and from them can break internally, the old Lumentation used to use a really flexible what looked like woven wire in the cable but many of the modern electronic ignitions seem to use normal cable, the routing inside the distributor also needs to be carefully done to stop straining the wire as the plate the points are normally screwed to moves with advance.