Left pot miss fire back fire

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Jet
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Left pot miss fire back fire

Postby Jet » Thu Aug 03, 2017 2:41 pm

I was out on my R80 last week and the old girl was going really well, even surprised me with her acceleration a couple of times when I opened her up to over take. About 6 miles from home however and the left pot started miss firing and back firing badly. I got home basically on one cylinder working and one continuing to back fire and miss fire all the way.

I've swapped the HT leads round, the coils round and the spark plugs round, but this seems to make no difference. The carbs were cleaned and new diaphragms fitted last year, so I don't think it's them, although I haven't had them apart yet.

Does anyone have an ideas?
1979 R80/7 also known as "Helga"

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Re: Left pot miss fire back fire

Postby george baker » Thu Aug 03, 2017 3:44 pm

Hi
valve clearance slipped?

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Re: Left pot miss fire back fire

Postby windmill john » Thu Aug 03, 2017 9:34 pm

Assuming valves okay, all electrics swapped over and you are running points, check throttle cables and carbs.
Yes I know they were done a year ago, but could be many things, dropped needle, jammed float, blocked jet, crud.....

Does it tick over okay?
Is issue under load?
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Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Left pot miss fire back fire

Postby boxerman » Thu Aug 03, 2017 10:12 pm

Checkbasics first. My first thought would be either restricted fuel flow to that carb or a flooding carb.
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Re: Left pot miss fire back fire

Postby Jet » Fri Aug 04, 2017 8:52 am

Thanks for the suggestions everyone.

In answer to Windmill John, the back and miss fire happens as soon as the choke is backed off a little, so when she's just been started. I've got her off choke a couple of times, the back and miss fire are apparent at idle and when I blip the throttle. I reckon I get a back fire every minute or so (sometimes less) and the pot doesn't heat up like the right side. It's like the timing is way out, but just on the left.

I'm going to check the diaphragm today, and maybe get the rocker cover off.
1979 R80/7 also known as "Helga"

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Re: Left pot miss fire back fire

Postby PhilO » Fri Aug 04, 2017 8:46 pm

Do you have an R80/7 with flat top carbs?
Some pattern diaphragms are too thick and cause the piston to hang up.
Always fit OE rubbery on these carbs.
You can check by removing the inlet elbow and shoving the piston slide right up to the top, extract your digit and the slide should return smartly to the closed position. Later models had a spring to make sure this problem was never going to be an issue.

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Re: Left pot miss fire back fire

Postby Jet » Mon Aug 07, 2017 3:16 pm

I checked the diaphragm and it looks OK, although is was a bit of a job getting it back in. I've adjusted the tappets today and it's better. She's not back firing now but still wont rev on that left pot and making a "shhuuuuuk" sound so I'm thinking it's new carb time.

They are the flat top Bings Philo, so I am disappointed at the thought of replacing them as I like the way they look on the bike, but she never was going to be a restore, more of a custom job really. They have done 126000 miles along with the rest of the bike, so fair enough, new Mikunis are just under £400. Ouch!
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Re: Left pot miss fire back fire

Postby windmill john » Mon Aug 07, 2017 5:09 pm

If there are no broken bits, you should not need to replace the carbs.
You may not get perfect tickover sometimes, but with good and clean internals, you should be okay.
You say the issue is just after choke, you might need another carb strip, new gaskets etc, but check your idle circuit for cleanliness.
The fact you are getting the backfire, do check cable sync just after checking idle setting.
Once you have warmed the bike after at least 15 miles, get your mixture and idle correct. Do not even think about balancing your cables until you've got a good idle / mixture sync.
If they are not synced, or the air mixture is not right, you'll be chasing your tale if you try to balance the cables.
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Left pot miss fire back fire

Postby PhilO » Mon Aug 07, 2017 7:48 pm

Windmill John has his thinking head on. Get a carb kit and do a careful re-assemble . Do not assume that it should all go back the way it came apart. Starting carbs(chokes) can be misassembled and float levels can be problematic.
Rubber o rings can chew up when assembled dry. Airways are very fine and need careful examination and scrupulous cleaning. Granted, flat tops are not the best carb ever invented but they are capable of performing...after all they were fine when they left the factory.
Things which make the carb scrap are excessive spindle wear. Worn main jet and needle wear is an easy fix but do both carbs together of course.

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Re: Left pot miss fire back fire

Postby Jet » Mon Aug 07, 2017 8:46 pm

Thanks fellas, you have given me renewed hope to keep going with my Bings.

I've checked inside the carb thisarvo and the piston slider moves freely up and down, the float bowl was full and the floats moving freely too. I did notice that there was an oily/petrolly residue on the inside of the inlet side so I sprayed some carb cleaner in and wiped it clean. The butterfly is moving OK too. also noticed that the plastic air pipe was warm when it was running. Replaced a bit of petrol line that looked like it may have been restricting flow as well.

Started her up and the "shhuuuuuk" noise had gone but the back fire is back. I'll let you know what happens next over the next few days, thanks again.
1979 R80/7 also known as "Helga"

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Re: Left pot miss fire back

Postby windmill john » Mon Aug 07, 2017 9:39 pm

You will see an oily residue in the inlets, this is blow back from the crankcase. The crankcase breather feeds into the carb inlets. This, over time, can cause crud to build up in your jet ways.
I've fitted a remote reservoir to one of my bikes which ensures I dint need yo flesh the carbs as frequently. (EDIT - Or without predictive text, that'll be I don't need to clean my carbs as frequently!!)
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Left pot miss fire back fire

Postby Galactic Greyhound » Mon Aug 07, 2017 9:45 pm

......... I did notice that there was an oily/petrolly residue on the inside of the inlet side ........

The emulsion chamber and/or its Air Inlet bore from the Air Filter side may be blocked so shutting off the air supply to the Emulsion Chamber.

This can happen if the bike is kept on the Sidestand whereby excessive oil inside the Air Filter box drains into the LH Carb Inlet Duct and into the Emulsion Chamber Air Inlet Duct.

You will likely need to clean out the Inlet Bore, Emulsion Chamber and the Emulsion Tube which will likely be clogged with a frothy foam which will harden with time.

See cutaway diagram below:
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Bing cutaway - emulsion air supply.jpg
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Re: Left pot miss fire back fire

Postby Jet » Sat Aug 12, 2017 9:50 am

Helga spends her life on the side stand she doesn't have a centre stand, so I'll be looking onto cleaning the inlet Bore, Emulsion Chamber and the Emulsion Tube as you have suggested GG, thanks.

I spent ages swapping over the coils and HT leads again to see it that makes any difference and the results are sporadic to say the least. Sometimes there is a small improvement, sometimes not, guess the next think is testing with a multi meter.

Using a light gun gave some confusing results, the right pot seemed to be firing OK the left not so, but swapping the coils over didn't seem to make any difference. In fact firing the strobe into the timing window showed OT on lowish revs (1000-1500 ish) and F on higher, 2000 or so on the right pot, but nothing was visible from the left pot. When I changed the coils and or HT leads round, the results remained the same! This suggests to me that the timing may be a little off, but nothing to write home about, but how can timing be OKish one side and not the other? It's been explained to me that the bike has one set of points and 2 coils that pulse together, one spark being "wasted" each time each side, so how can the timing be as it is?

Feeling frustrated!
1979 R80/7 also known as "Helga"

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Re: Left pot miss fire back fire

Postby Eddie Barnes » Sat Aug 12, 2017 3:29 pm

I park my bike in my garage on its surefoot side stand with the off side next to a wall. I put a block of wood under the stand so its almost upright to prevent oil trickling into the left side carb.

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Re: Left pot miss fire back fire

Postby SteveD » Sun Aug 13, 2017 10:03 am

Have you checked for a blockage in the main jet on the dodgey side? That gave me your symptoms once.
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Re: Left pot miss fire back fire

Postby PhilO » Mon Aug 14, 2017 7:52 pm

What about a new set of spark plugs? But a word of caution here because even new plugs have been known to be duff. The original R80 plugs were non-resistor types with the resistor being built in to the special cap and leads.

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Re: Left pot miss fire back fire

Postby Memphis Twin » Tue Aug 15, 2017 1:47 pm

I'm willing to bet that this is due to blocked jets. I had no end of problems with my carbs until I fitted a pair of in-line fuel filters, and not a single problem since.

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Re: Left pot miss fire back fire

Postby Jet » Tue Aug 15, 2017 5:35 pm

Spark plugs were new last year and apart from the left one being a bit wet, seem OK, I've swapped them over at least once with no difference being made! Thanks for the suggestions of main jet block from a couple of you, I think that will be the next job.
Last edited by Jet on Tue Aug 15, 2017 9:30 pm, edited 1 time in total.
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Re: Left pot miss fire back fire

Postby windmill john » Tue Aug 15, 2017 5:39 pm

One plug bring wet could still be not just jets, but not firing on that side. But as per all comments, this being an Airhead, check out those carbs.
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Left pot miss fire back fire

Postby New Haven Neil » Tue Aug 15, 2017 9:25 pm

Stuck/burnt valve? Sounds like the left pot isn't firing at all, the backfire is unburnt mixture popping off via the balance pipe.

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Re: Left pot miss fire back fire

Postby PhilO » Wed Aug 16, 2017 9:52 am

It takes a Manxman to point us in the right direction. A compression test should be easy enough to do even if you don't have all the swanky gear. You can feel the resistance, or lack of it, as you pull the rear wheel round by hand.
If you pull the coil lead off and crank the engine over you can sometimes hear the unevenness of the compressions and an ear or hand over the end of the exhaust pipe will give different sounds.
Plan 'd' is to put the cylinder on compression and remove the plug then get a short length of garden hose on the plug hole and blow down it. You will feel and hear any differences.

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Re: Left pot miss fire back fire

Postby Galactic Greyhound » Wed Aug 16, 2017 10:22 am

.....Plan 'd' is to put the cylinder on compression and remove the plug then get a short length of garden hose on the plug hole and blow down it. You will feel and hear any differences.

Good one - a nice simple 'leak-down' test! \:D/
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Re: Left pot miss fire back fire

Postby New Haven Neil » Wed Aug 16, 2017 7:28 pm

It takes a Manxman to point us in the right direction. A compression test should be easy enough to do even if you don't have all the swanky gear. You can feel the resistance, or lack of it, as you pull the rear wheel round by hand.
If you pull the coil lead off and crank the engine over you can sometimes hear the unevenness of the compressions and an ear or hand over the end of the exhaust pipe will give different sounds.
Plan 'd' is to put the cylinder on compression and remove the plug then get a short length of garden hose on the plug hole and blow down it. You will feel and hear any differences.
Ah, he's an ex-pat Geordie though!! :grin:

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Re: Left pot miss fire back fire

Postby Airbear » Thu Aug 17, 2017 12:00 am

Have you checked for a blockage in the main jet on the dodgey side? That gave me your symptoms once.
My experience too.

Bike would start and idle but under load would immediately run very rough, with missing and backfiring on the left side. After worrying about all sorts of expensive things I eventually found a tiny black crumb (of fuel hose rubber, I think) was being sucked into the main jet orifice of my left carb. Quick and easy to check with the carbs on the bike.
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Re: Left pot miss fire back fire

Postby Jet » Fri Aug 25, 2017 3:25 pm

Thanks Airbear and the others of you that suggest I look at the jets on that pesky left carb. I've squirted carb cleaner up the jet (using the little red straw attachment on the aerosol) and well, basically everywhere to be honest. The float bowl had a surprising amount of particles in the bottom of it.

I've spent most of the rest of the day today testing coils (they seem ok at 6000 ohms each and HT leads 1200 ohms each) getting the left carb and jet as clean inside as I can without actually removing it from the bike, tightening jubilee clips to eliminate air leaks and changed the throttle cables and she still won't run right.

The left pot takes longer to warm up and backfires, not every stroke, but enough. When I pull the throttle cable on the right carb it picks up OK, when I pull the throttle cable on the left carb she tries to pick up but seems to be restricted in some way, she won't rev freely, if that makes sense. I've also had the situation where she won't return to idle but stays around 2500-3000 revs until I hit the kill switch and then restart her, then she may do the same. I changed the springs on the advance/retard mechanism last year.

She's done 126000 miles so am thinking again perhaps new carbs are required, but at this point I'm wondering how I can get her to my local bike mechanic for a couple of hours tinkering.
1979 R80/7 also known as "Helga"


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