Misfire

SeanW

Registered user
Joined
Jun 18, 2011
Messages
206
Reaction score
0
Location
Rutland
I put my 1979 R80/7 back on the road a couple of months back after a couple of years standing idle. It started up and ran perfectly as soon as I put fuel and new battery in it and I've been doing about 250 commuting miles a week on it with no problems.

Then half a mile form home on Friday it hiccupped a couple of times on an opening throttle, then seemed to settle down. Next time I rode it (this morning) it is now misfiring badly, won't pull without hesitating, cutting out and in, coughing and so on. Just about got it home felt like it was down to one cylinder maybe, barely running.

When I stopped the engine, there was a really odd hissing sound coming from the left hand cylinder head. Might have been head gasket I guess, though sounded like it was coming from within the rocker cover. Definitely not the carb or exhaust anyway. It went on for a matter of minutes and got louder when the bike was upright rather than on the side stand then fizzled out.

Any ideas what the above symptoms might point to as most likely things to investigate?

I'll take the rocker cover off tonight whatever and see if there is anything obviously wrong there. The clearances haven't been checked for some time - as it had been running well, I thought I'd leave them a little bit before checking them.
 
Try the plugs & leads first. You might be lucky. Otherwise symptoms are strange but, in fairness, you don't mention the bike's background, Service History or what work was undertaken on re-commissioning her for the road.

After a long lay-up, piston rings can get sticky, or even crack when restarted, allowing blow-by; performance falls, but the crankcase then gets pressurised so throws up other issues, usually oil leaks & thirst, along with rapid oil contamination. With a boxer twin, parked up say on a side stand long term, it would usually be the LH cylinder as you describe, but I would expect to witness oil smoke on start-up.

Then it could be the valve seats failing to seal - corrosion, clearances or even a broken valve spring. You can check this when you have the lid off.

If that checks out, then sadly it could be a cracked head. I can't speak for an R80/7, but usually they crack across a valve seat, up into either the inlet or exhaust ports. Usually lots of spluttering & farting on start-up, which sometimes settles as the engine gets hot, sometimes gets worse.. The last one I saw was a mates Bonneville - he used a cheap Aldi endoscope to have a looksee up the ports, inside the head, then down the plug aperture, to check the piston crown. It revealed previous welding to the head & a new crack emerging. Annoying, but runs a treat now repaired with a good second hand head.

Lots to consider - hopefully you will get lucky. Nothing that can't be fixed DIY pretty easily on an old airhead if you have the time/inclination. Otherwise a trip to one of our resident specialists.
 
Thanks for thoughts everyone.

More info, it;s an ex police bike on second oversize rebore but before I stopped riding it had had pretty much new or recon everything within the space of a few 1000 miles - bottom end rebuild with used RS crank, new mains, big ends, honed bores, new rings, oil pump innards. New valves and seats.

It had sat for a couple of years though I had drained the carbs before neglecting it. Before starting it up I changed the oil and filter, put in new plugs and replaced a leaking crankshaft front seal and sump gasket. It fired up on about the third turn of the crank and had run faultlessly for more than 1000 miles until Friday.

Electrics may be its weakest point. Alternator output looks ok but for all I know the coils, voltage regulator or other such gubbins I don;t really understand may be original or at the least very old. I've never replaced any of them, anyway.

(oh, it used to look like this http://www.ukgser.com/forums/showth...HOTOS-please?p=3435575&viewfull=1#post3435575 but nowadays looks more like a R100S)
 
I guess you're going to have to give it a good going over, checking valve clearances, spark plug leads, compression test, air intake pipes etc
 
Clearances were nearly non existent so put them right. Who knows what damage that might have done to the valve train already though..

Whatever, it's still doing the same thing - started ok, ran ok and was pulling strongly for about 400 yards then started cutting out, lurching and then just conked out. Had to push it home.

More investigations at the weekend... :confused:
 
Take the fuel cap off and try again. The hissing sound you heard (in your OP) may have been a vacuum in the tank equalising with outside air pressure, caused by a blocked breather in the fuel cap.
 
Float bowls full.

Got the points out to have a good look at them. Gap was out of adjustment- about .3 mm . The faces are pitted but I don’t have the experience to read how bad this is

2rrupz6.jpg


5cglyo.jpg
 
Points are scrap but like Solidstate says sounds like fuel starvation.
Fuel level in bowl should be 1/2" down, not full, only time to check in immediately after the motor stops, by the time the hissing stops the bowls will be at the set level.
 
mikeyboy said:
If it’s still on points or points in bean can I’d certainly be looking at them ,
Click to expand...
I’ve had a few bikes with exactly the same symptoms as you,turned out to be the points closing up,
Try to eliminate one thing at a time ,the points are scrap so replace them and the condenser first.
The hissing might just be the cylinder on compression as it stops
 
SeanW said:
Clearances were nearly non existent so put them right. Who knows what damage that might have done to the valve train already though..

Whatever, it's still doing the same thing - started ok, ran ok and was pulling strongly for about 400 yards then started cutting out, lurching and then just conked out. Had to push it home.

More investigations at the weekend... :confused:
Click to expand...

If the valves have closed up the seats are probably knackered,this too could be the source of your hissing.
 
Forgot about the condenser, they can work OK cold but fail as they warm up.
Last time I had one fail was in 1968 when I was rushing my pregnant wife to hospital in a Morris 1300 and it had been replaced at the service the day before.
Luckily the boys in blue arrived within minutes and my wife made it to maternity in time!
 
Beemerboff said:
Points are scrap but like Solidstate says sounds like fuel starvation.
Fuel level in bowl should be 1/2" down, not full, only time to check in immediately after the motor stops, by the time the hissing stops the bowls will be at the set level.
Click to expand...

That^^. Run it till it dies, pop the full cap off. try again - what happens?

mikeyboy said:
The hissing might just be the cylinder on compression as it stops
Click to expand...

mikeyboy said:
If the valves have closed up the seats are probably knackered,this too could be the source of your hissing.
Click to expand...

^^ both possible of course, but given the OP mentioned the hissing went on for "minutes" and changed when the bike was moved upright (larger volume in the fuel tank and sensitive to movement, as opposed to the cylinder) it seemed, from an armchair diagnoses point of view, the fuel cap/starvation was more likely.

Either way, the OP has a set of diagnostics to run.........including electrics, of course.
 
solidstate100 said:
That^^. Run it till it dies, pop the full cap off. try again - what happens?





^^ both possible of course, but given the OP mentioned the hissing went on for "minutes" and changed when the bike was moved upright (larger volume in the fuel tank and sensitive to movement, as opposed to the cylinder) it seemed, from an armchair diagnoses point of view, the fuel cap/starvation was more likely.

Either way, the OP has a set of diagnostics to run.........including electrics, of course.
Click to expand...
Good point regarding the hissing.
 
Well, she's going again - got me to work no problems anyway. :)

I couldn't detect any blockage in the fuel breather - the culprit turned out to be very loose/ bad coil connections. Cleaned up all the connectors and tweaked a very bad one that wasn't gripping the spade on the coil at all and everything is back to normal now. Actually running a bit better now that the clearances, points gap and timing are all freshly done.

The hissing only happened the once, so that may remain a mystery

I also replaced the big oil seal in the bevel box at the weekend, so now have a working rear brake again, too.

Tickover has gone right up unfortunately I guess as a result of the tune-up. Don't feel so confident about fiddling with carb settings, but maybe it's time to start learning... Though have just looked at another current post and maybe this is sticking ignition advance mech. I did clean the bean can out when I was mucking about with the points...

Thanks to everyone for ideas and help :thumb
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
You must log in or register to reply here.

Similar threads

Mzokk
Checking valve clearances on the Africa Twin Adventure Sports 1100.
Replies
2
Views
192
Mzokk
Mzokk
Want to find a switched live - R100CS
Replies
6
Views
215
casbar
tspivey
Fuel cap stuck
Replies
11
Views
358
pbbanks
King Rat
CS battery
Replies
5
Views
442
The Other PaulG
The Other PaulG
mikeyboy
Recommissioning a “ barn find”
Replies
4
Views
426
bakerlonglegs
bakerlonglegs


Back
Top Bottom