Sprint misfire at lower rpm, especially under load.
Posted: Sat Dec 31, 2016 6:23 pm
I've got a problem with the sprint I don't understand. It started okay yesterday on choke, but seemed to need it for a bit long: more than the few hundred yards that seems normal. But, at this stage it was just a bit lumpy at lower rpm under load. But after about a mile or so, with the temp gauge at about 1/4 it started to really misfire, popping and banging and refusing to tickover, with the choke having no real effect. I thought I was running out of fuel, but the gauge reads half and the low level light only comes on if I brake hard. There is, of course, oil and water in it.
I checked that there was fuel pressure at both carbs, fuel in the float chambers, and not overflowing, etc., and could find nothing. I also checked the point's gap and that the dizzy looked secure and not like it had slipped, but couldn't see a problem. I managed to get it home, during which time it did not smoke, overheat, or make any unusual noises, other than the misfire. I let it cool overnight and tried to start it this morning.
It didn't really start, and coughed, popped, spat, and kicked-back. I checked the fuel pump at both outlets to the carbs, pumping into a coke bottle on the starter. That seemed to be fine. I looked in both float chambers, and they're filled ok. Both carb pistons are free and the dampers are filled (now). So I pulled the plugs and checked the static timing. The plugs were rather sooty, but I'd been playing with the choke trying to get it to run, so that might not be a general mixture issue. I admit I've not checked them for a couple of months - the last time after I fitted solid butterflies. But it's been running really well, apart from needing the point's gap opening up a few thousand miles ago. Anyway, I swapped them for some others, not actually new, but they were clean and dry. That made no difference.
I also checked the timing, both at the flywheel and cam. The mark on the cam lines up with TDC on the flywheel with the engine going forward. I should, perhaps have checked the slack in the timing by seeing where it lined up with going backwards, but I forgot. The static ignition timing was about 8-10 degrees before, again going forwards. I should actually set it correctly, but my low profile 7/16ths socket has done a bunk, and I'll have to get another re-ground. But I think I'm sure the dizzy's not slipped anywhere.
Anyway, none of this has made a ha'porth of difference, and it still misfires and pops and bangs under about 2000rpm. Get it over that, and it seems to run smooth, though there's a clear shortage of power. It feels like fuel to me, but I can't find a problem there.
I though I had a spare set of new points and condenser, but, if I did, they've gone walkabout. But I've another couple of dizzys here and there, and one should have a good set. I'll do some checking of the condenser tomorrow. I don't have a spare coil to hand, and I haven't yet done anything to look at the spark, but I'll do that before I look at the condenser tomorrow. Think I've a spare for the herald that might fit.
Anybody think of anything obvious I've missed or can try with what I might have to hand?
Graham
I checked that there was fuel pressure at both carbs, fuel in the float chambers, and not overflowing, etc., and could find nothing. I also checked the point's gap and that the dizzy looked secure and not like it had slipped, but couldn't see a problem. I managed to get it home, during which time it did not smoke, overheat, or make any unusual noises, other than the misfire. I let it cool overnight and tried to start it this morning.
It didn't really start, and coughed, popped, spat, and kicked-back. I checked the fuel pump at both outlets to the carbs, pumping into a coke bottle on the starter. That seemed to be fine. I looked in both float chambers, and they're filled ok. Both carb pistons are free and the dampers are filled (now). So I pulled the plugs and checked the static timing. The plugs were rather sooty, but I'd been playing with the choke trying to get it to run, so that might not be a general mixture issue. I admit I've not checked them for a couple of months - the last time after I fitted solid butterflies. But it's been running really well, apart from needing the point's gap opening up a few thousand miles ago. Anyway, I swapped them for some others, not actually new, but they were clean and dry. That made no difference.
I also checked the timing, both at the flywheel and cam. The mark on the cam lines up with TDC on the flywheel with the engine going forward. I should, perhaps have checked the slack in the timing by seeing where it lined up with going backwards, but I forgot. The static ignition timing was about 8-10 degrees before, again going forwards. I should actually set it correctly, but my low profile 7/16ths socket has done a bunk, and I'll have to get another re-ground. But I think I'm sure the dizzy's not slipped anywhere.
Anyway, none of this has made a ha'porth of difference, and it still misfires and pops and bangs under about 2000rpm. Get it over that, and it seems to run smooth, though there's a clear shortage of power. It feels like fuel to me, but I can't find a problem there.
I though I had a spare set of new points and condenser, but, if I did, they've gone walkabout. But I've another couple of dizzys here and there, and one should have a good set. I'll do some checking of the condenser tomorrow. I don't have a spare coil to hand, and I haven't yet done anything to look at the spark, but I'll do that before I look at the condenser tomorrow. Think I've a spare for the herald that might fit.
Anybody think of anything obvious I've missed or can try with what I might have to hand?
Graham
, personally i would switch to electronic ignition also as it's a more reliable system.