Splutters, bogs, back fires & farts out the carb

[Varastorm]

Went for a ride today & on the way home the bike started to misbehave.

I first noticed it when entering a village, I slowed down as normal & noticed it was surging very badly under 3000rpm. I slipped the clutch & blipped it to avoid it stalling all together but under 3000rpm it was spluttering farting through the carb & backfiring.

From being (very) well behaved for close to a year all this in the space of 30 miles.

I did manage to get it home by slipping the clutch & keeping the rev's over 3000rpm. I must of looked a right tool with it back firing also.

At home I thought I'd look into the problem & thought fuel issues. I stripped the carbs looking for a loose pilot jet at the bottom of a fuel bowl or similar, but all was good. I removed & cleaned the jets whilst I was in there & put it all back together & attempted fired it up again.

Initially, it was very sluggish turning over but it did turn over faster after a few prods of the starter & it managed to fire up but the same poor running as before, popping, farting & very erratic under 3000rpm.

With it getting late & fed up, I decided to packed up & have been reading about similar issues people have had & I am starting to think it might be electrical R/R or stator maybe??

http://www.bikechatforums.com/viewtopic.php?t=283206

I did swap the battery for a new one I had in the shed, but never thought to check the volts as I was fed up by then..

Any help appreciated

[bigtwinthing]

you need to check voltage before start up, then once its running to see if its charging, then see if its overcharging. stick on as many electrical bits as you can, lights, main beam, any accessories etc and check the charge again. make sure you have clean and tight earths, so behind the lugs too. check ya plugs, caps and leads etc. If thats all ok i would change the fuel and clean the tank first, then look at the carbs once you have inspected all hoses.Good luck, some smarty will advise better i am sure. Come on Mac and Tony help a guy out.

[Varastorm]

If thats all ok i would change the fuel and clean the tank first

Good call BTT, never thought it could be crap fuel & thinking about it I put a tenner of "Heron" fuel in when the low fuel light lit up earlier on in the day. It was expensive petrol & planned to fill up else were with Shell or similar.

On the way home, the low fuel light came on again shortly after it started to play up. By which time all I wanted to do was get home.

Thanks for that. I'll empty the dregs & fill it up with premium stuff.

Will update

[Varastorm]

Thought I'd mention that above 3000rpm it runs spot on.

It played up around 30 miles from home (after a mega thrash though), but I managed to get home at 70/80ish (5000rpm) with no issues.

It only runs poor at low revs, 3000rpm or lower. Idling is now none existent & that's what lead me to think it was a pilot circuit problem.

Should of seen me filtering, they must of thought I was a right retarded loon. Revving, popping, farting, backfiring & slipping the clutch at insane rev's. Never been so embarrassed.

I'll update on the tank drain & fresh fuel as BTT recommends

[VTRDark]

it was spluttering farting through the carb & backfiring

This makes think vacuum hose. It may have come off at either end. Also if you have a front vacuum for balancing then there are other connections to consider. Rule out the simple first.

[tony.mon]

Yes, it's the vac hose.
Typical symptoms.

[Varastorm]

This makes think vacuum hose. It may have come off at either end.

Yes, it's the vac hose.Typical symptoms.

Sorry guys, but it has a simple on/off fuel tap connected to a Facet pump.

I have had a look at the charging system & all looks well with the Voltmeter too, 11.8v & 14.8v when running.

Just been for a couple of cans of petrol so will update when tank emptied & fresh installed.

Keep the ideas coming guys, really appreciate all the help & ideas I can get.

Thanks in advance

[KermitLeFrog]

Could be coils and/or plugs. A little known fact is that a more aggresive spark is required at lower revs. Just a thought....

[Varastorm]

Could be coils and/or plugs. A little known fact is that a more aggresive spark is required at lower revs. Just a thought....

It might be, but I feel like it could be fuel so I am playing with the carbs at the moment.

But I haven't a clue tbh. from running perfect to rough as hell in 40 miles

I have just emptied the tank & run the engine with a funnel filled with fuel. It started ok but is farting badly through the carbs.

I have wound the idle up to 1500rpm but is missing quite often. I gave it a few blips hoping it might clear, nothing silly & the rear carb blew off the rubber

I removed the rear plug & noticed it black.

My plan is to strip the carbs & clean the fuel filter, then like you say Kermit, swap some new plugs in & go from there.

I am thinking tight valves also , but I have never seen it happen over such a short period (40 miles).

Thanks for the help guys

[Varastorm]

Right then, After Kermits comments about plugs/coils issues & having read other peoples plug misfire issues he could be right.

I've searched high & low in the shed for my spare set of plugs & found two boxes with only one plug though

I've been looking for plugs on eBay & found these below for £6.70 each. Any one know of any cheaper?

http://www.ebay.co.uk/itm/NGK-5545-MOTO ... xy4fVTFZKQ

[Varastorm]

I've been busy fault finding & can confirm it's still shagged.

The problem seems to be with the rear cylinder, idle is very poor & stalls at the usual 1000rpm so I've wound it up to 1500rpm. Still spitting back through the rear carb, seems to be missing every 4-5 cycles & spits fuel out of the rear carb. Above 3000rpm no issues at all.

I am starting to run low on ideas guys, any help would be really appreciated. Here's what I've done so far.

Not ideal, but only having only one spare spark plug (new one's have been ordered & hopefully arrive tomorrow) I've been swapping them around to see if there is any difference. Still the same..

I've swapped the coils over, front to rear & vice versa (I did use the same HT leads & caps as they are different lengths) still the same...

Checked for tight valve gaps. Opened the rear cam cover & measured the rear valve clearances. Still spot on from when I did the VTR cam swap last year.

I swapped the carbs over (New VTR pair & tps, as in a brand spanks shiny new set, a lucky purchase off eBay a while back), still the same...

To me, this confirms that it can't be a fuel issue. I've been using a funnel as a fuel tank & the only part of the original setup is the facet pump & filter. My thoughts are if it was them I would be experiencing issues to both cylinders, not just the rear.

The only non electrical things still to check is a compression test, then it looks as if it might be only electrical left

One thing I did notice, was that when I removed the rear plug cap & plugged in the spare spark plug & held it on the rocker cover (just to see the strength of the spark) it did seem to spark very often (too quickly), as in not every firing stroke?

If anyone has any other ideas, please chip in, as I am quickly running out of them.

Thanks in advance.

[Cadbury64]

I agree that you've ruled out fuel contamination or supply, and also the coils. It is not impossible that your rear HT lead may be developing some corrosion and you may want to check the impedance of that and compare to the front. Not sure whether the VTR caps have a built-in resister but if so you can unscrew and check that for corrosion and add some dielectric grease.

Have you confirmed that the vacuum take-off port on the intake manifold is still sealed? Maybe also check for vacuum leaks around the rear carb boot.

After that I'd be back into the carb looking for a blockage in a low speed circuit, given the poor running is at low revs but it is OK at high.

[Varastorm]

I agree that you've ruled out fuel contamination or supply, and also the coils. It is not impossible that your rear HT lead may be developing some corrosion and you may want to check the impedance of that and compare to the front. Not sure whether the VTR caps have a built-in resister but if so you can unscrew and check that for corrosion and add some dielectric grease.

Good plan, I'll get the AVO out again tomorrow. Thanks for the tips Cadbury

Have you confirmed that the vacuum take-off port on the intake manifold is still sealed? Maybe also check for vacuum leaks around the rear carb boot.

I have one new intake rubber which I'll dig out & fit

After that I'd be back into the carb looking for a blockage in a low speed circuit, given the poor running is at low revs but it is OK at high.

The carbs that are now on are brand new previously unused items. That's why I am thinking it's going in the direction of electrics.

One more thing, when the rear plug is removed & I look into the cylinder with my torch, it's black/sooty rich looking. It never used to be.

The thing I am dreading is having to purchase a cdi unit for elimination purposes, best part of £100..

One last thing, if the readings from the battery (ignition off 11.8V) (ignition on & running 14.8V) would you say there is no need to look into the charging circuit any more?

Thanks again guys

[KermitLeFrog]

(I did use the same HT leads & caps as they are different lengths) still the same...

[Stephan]

Have you checked the connectors between cdi and coil?