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Tabasco

Joined:

May 09

Posts: 215

Tabasco says:

Intermittent firing problems

Grrrrrrr

Hi there,

I purchased an old vfr nc21 10 weeks ago which had stood outdoors for 8 long years. It took me 2 weeks to get parts together and finally get it going. Ran lovely.

Day after running rough then wouldn't start. Carbs were taken off and cleaned out. Pilot and main jets blocked.

Put back together and started nice. Now some 9 weeks later I have a miss fire problem on the front left cylinder. It kicks in and the down pipe heats up like the other with 3/4 choke??? (i'ts not the tank. Its been cleaned and por15 sealed and all fuel hoses are new)

All new plugs, new needle valve on that carb (carb seat filters have come away on this carb but float bowl still fills up). I checked and the plug sparked.

Right the day after it run lovley on 4 and I went for a short 5 mile ride no issues at all and huge grin on the face from that v4 sound.

Today, some two days later I just started her and she is only running on 3 again and I just can't work out why. I swapped the front coils over to see if the missfire changed sides and it did not. I swapped the two front ht leads over and it run great on 4 for ages but now is back onto 3 again. I put it back in my garage, closed the door and walked away.

Has anyone got any ideas of what could be causing this? I don't mind taking the carbs off for the 4th time in as many days as I have a spare set of inlet rubbers and clamps just in case one splits. I'm getting very good at putting these back on now.

I have (wait for it) spent over £900 on it so far and have to carry on or i'll lose the lot. I plan to keep this classic sports bike for good as I'm not doing it to make money, more of a rescue job.

I just can't work out if its fuel related of electrical. Heeeeeelp.

Thanks.

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  • Posted 2 years ago (20 May 2012 18:32)

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jaffa90

Joined:

Mar 09

Posts: 8604

jaffa90 says:

firing

You say "i swapped the two front h.t. leads over and it run great on 4 for ages"

Have you a multimeter to check the resistance/continuity on these leads whilst bending/twisting them slightly?

 

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jaffa90

Joined:

Mar 09

Posts: 8604

jaffa90 says:

tap

Also does the fuel tap have any pipes going to that missing cylinder?

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Tabasco

Joined:

May 09

Posts: 215

Tabasco says:

Yes

I checked the resistance on all 4 coils and they were the same. I tried to check the ht leads but could not get a reading. I did not want to force the multimeter prongs in too far as they were slightly wider than the hole in the ht lead coil end.

The fuel tap splits into two and feeds the left and ride side carbs respectively of the v setup. Brand new internal fuel tank filter, new vacuum pipe, diaphram perfect, 4 float bowls full. I can actually hear the cylinder in question fire every now and again. I'm in two minds whether to make up a set new ht leads. I dont know if I can just buy them off the shelf anymore.

 

Hmmmmm baffled.

 

I am thinking electrical though.

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jaffa90

Joined:

Mar 09

Posts: 8604

jaffa90 says:

missing

You say,"some two days later i just started her and she`s only running on 3 again"

Are you sure that vacuum pipe has no petrol in it ?

Can you smell fuel at the end of the exhaust pipes or anywhere?

Story,years ago bought a vauxhall astra running on 3 cyls at an auction cheap,it was a risk i know.

Dreading it being a valve job or a bigger one i started to drive it home,after 3 miles it kicked in on 4 cyls and i was chuffed.

Found out which cyl it was and unscrewed the plug cap,i had to chop 20mm off  the lead to get to the good part of the lead.

All because of somebody pulling the leads off with the lead and not the plug cap.

Don`t let this story mislead you.   

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Tabasco

Joined:

May 09

Posts: 215

Tabasco says:

Yeah

some times when I start if it'll run on three and I can hear the forth trying to kick in, other days it'll not fire at all on the forth and yesterday I went on a 8 mile round trip and she run like a dream. Never missed a beat. Was not lumpy or anything.

I swapped the coils and leads over on the front two cylinders thinking if one was dodgy the problem would swap side but its still the same cylinder thats got the problem.

Dad said to check the valves but then why would they stick sometimes and not others and be perfect on a ride out?

 

There is deff no fuel in the vacuum pipe, I checked. All that side of things is working nicely and I can not smell petrol in the exhaust.

From the ride out yesterday to todays problem, all I did was spend 5 hrs cleaning her and then started it up to put it back in the garage and noticed only one front pipe was hot and my ears are tuned to the sound of it missing now.

 

I wonder if both front ht leads are playing up. They are original and 26 years old. I just can't understand the intermitance of the problem and like I say, it never shifted pistons when I swapped over the running gear to the other side.

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jaffa90

Joined:

Mar 09

Posts: 8604

jaffa90 says:

kick in

Not an easy one,if it`s trying to kick in and no smell of fuel anywhere then it`s short of fuel or too much air,if you could remove that plug and see if it`s dry that`s a bonus.

When it runs on 3 have you tried it above 3,000rpm to rule out the pilot jets or used the choke for a few seconds?

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Tabasco

Joined:

May 09

Posts: 215

Tabasco says:

Hi guys

Right, above 3000rpm it still does the same. The other day I slowly put the choke on as it was running (was warmed up) and as it got to around 3/4 on the cylinder then fired up and ran ok. When I took the choke off, it went back to 3 cylinders.

The day after is when I went for the ride and it ran like a dream even on tickover, no choke. I went up to 8000rpm and no problems.

 

I'm struggling to find ht leads that will have a plug cover to block the spark plug hole up to stop road crap getting into the hole and dropping in the bore when I remove the plug (access is a hug prob on my hands. Its a rad forward job all the while).

Right today after work I took the ht lead off the plug and tried it on the original old old plug and it fired nice and bright. I have not taken the new plug out yet today.

I am still worried that there maybe fuel problems so I took the carbs off an hour ago for the 4th time. I checked the pilot jet with a very powerful eyeglass and it is lovely and clean. The main jet on that carb is the same, clean. No muck anywhere and a brand new float valve fitted friday. The only thing I know of is the little filters on the back of the float vavle seat have come away from the body and I can not for the life of me find a replacement in case this is restricting fuel flow so I might take these out and reply on the brand new tank filter and fully cleaned and sealed tank.

I checked the pilot screws and they were all different from each other and turned out more turns than the manual says so I cleaned them and set them back to 2 turns out.

I'm trying to think what else to check or look at while the carbs are off on my kitchen floor. I'm getting a little peed off at this now.

If it was a straight "it doesn't work" problem I'd be happy but one min it works and the next it does not is so frustrating.

I'll keep you guys informed. I'm going to borrow some ht leads off dad just to eliminate.

Thanks for all your help.

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Tabasco

Joined:

May 09

Posts: 215

Tabasco says:

I

have noticed like you say kcmc that if it doesn't start after first or second try it bloody floods the plugs like my mates old xj1300 and i have to either dry them or try again after maybe half hour. Not ideal really grrr.

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jaffa90

Joined:

Mar 09

Posts: 8604

jaffa90 says:

carb

I can`t help any more but the choke idea is with throttle off for a few seconds,also the inside rear 2 cyls  should have a richer mixture with different jets.

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Tabasco

Joined:

May 09

Posts: 215

Tabasco says:

Ok

just wrapped up for the night now but what I found was this....

I took the float chamber covers off and all seemed clean and ok like the other 3 times they have been off in 10 weeks. Took the pilot out the problem cylinders carb and it was blocked. I also noticed that someone had put the main jets the wrong way round front to back, back to front as per the manual info??? Also when I tap the pilot jet gently on some clean white paper with my fingers little black bits come out so I have them all soaking in petrol over night in the back garden. I am going to buy some of that petroleum based carb cleaner tomorrow and give the carbs a dam good clean out and a balst of clean compressed air.

I got the float valve seat filters out, cleaned those and connected them back to the float valve seat as they had fallen off the back. It def needs a dam good clinical clean out which it will get tomorrow after work. I'll keep you guys updated on my progress. I do not want to miss this nice sunshine coming up this week haha.

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