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Discussion Starter #1
Newbie mechanic trying to diagnose a problem here . .
2001 bonnie, ridden in all weathers / 24k on the clock.

The issue ;
bike doesnt hold a steady slowish speed, especially in 2nd at 2000/3000 revs - surges and hiccups as if fuel starvation. Intermittently stalls on idle and struggles to find a steady beat, last night firing on one cylinder for a minute but fires back up. Pulls fine with big revs and runs good at speed.

What it isn't ;
cylinders balanced, valves all within spec, petcock and strainers clean, sparks new (iridium), calipers and discs running fine.

I'm going to drain the float bowls and try and check the pick up coil (?) and HT leads but am trying to avoid disassembling the carbs as i use the bike every other day - any suggestions ?
 

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the more cracked rubber I see on my 2yo bike the less impressed I am with Triumph rubber parts overall.

1st. check all the intake rubbers, and caps for vacuum leaks.

2nd, lose the D head idle mix screws for knurled thumbscrews a normal person can relate to. you don't need the D tool, all you need is reamed 1/4" copper tube with a flat spot.
those never should have been invented...a good reason alone to sterilize engineers.

drain the float bowls into a glass jar and look for water and/or rust...if so, drop the bowls and blow the jets out with air. those carbs come off pretty quick if you leave the cables connected.

probably not the tank vent in this case, but see that the BS check valve in the tank vent isn't stuck closed, or the hose at the centerstand isn't full of crud if you haven't done that in a while.

it sounds like a lean flutter...but it's winter, your heaters may not be working and it may be making ice.

check back with your results for troubleshooting page 2
 

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I am having about the same issues. Pulls nicely up to red line but will not run steady at any speed without back firing. Right cylinder is the problem child.

I installed a new Nology coil with no change in performance. I opend the carb drain but did not collect a sample to check for dirt. Cleaned the screen in the gas tank. I'll try some of the other suggestions as well.
 

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Just remember: 90% of all fuel related problems are ignition problems.:D

My recommendation: Start first with ignition--battery connections, coil connections, plug wires, pick up coil gap setting.
 

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Discussion Starter #8
I was fairly sure i had it down to a faulty pick up coil but at £75 for a replacement I bit the bullet and handed it into the shop to get a surefire diagnosis.

Problem turned out to be a clogged tip over valve. This is the small black cylinder about the size of a wine cork half way down the length of the breather tube that comes off the bottom of the fuel tank. I thought i had blown through this tube but guess i must've blown the wrong way ! Its an easy fix and worth checking for symptoms i described at the start.
 

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I was fairly sure i had it down to a faulty pick up coil but at £75 for a replacement I bit the bullet and handed it into the shop to get a surefire diagnosis.
Was that £75 including fitting? I purchased a new pick up coil last year for about £30.
 

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Stewart57, do you blow through the tank or up from the bottom end of the tube to clear the valve? Did you drain the carbs prior to finding the clogged valve and did you have flow from the carbs? I'll give it a try. Thanks

Suas Alba!
 

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Discussion Starter #11
I drained the carbs through a tube into a glass to see if the petrol was dirty (rusty?) but it was as clear as water.

The tip over valve can be pulled right off the rubber tubing for a closer look (put in the bin/replace) - i think this part might've choked a few bikes.

To PAAS - price i presume was for supply only for the original part ? How quickly did the timing coil fail and what were the symptoms (to be aware of) ?
 

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Problem turned out to be a clogged tip over valve. This is the small black cylinder about the size of a wine cork half way down the length of the breather tube that comes off the bottom of the fuel tank. I thought i had blown through this tube but guess i must've blown the wrong way ! Its an easy fix and worth checking for symptoms i described at the start.
>The issue ;
bike doesnt hold a steady slowish speed, especially in 2nd at 2000/3000 revs - surges and hiccups as if fuel starvation. Intermittently stalls on idle and struggles to find a steady beat, last night firing on one cylinder for a minute but fires back up. Pulls fine with big revs and runs good at speed.

what that check valve does is allows air to replenish the volume as gas vacates the tank.

if it's clogged (unvented), the tank goes under a vacuum and gas won't drain by gravity into the float bowls.

the diagnosis doesn't jive with the reported initial issue ...because at speed under increased demand, the tank vacuum doesn't just go away.

one of 2 things.

1. your description is off...(answers are only as good as the information supplied)
or
2. the shop didn't listen to your input and "found" a quick easy "fix".

if it surfaces again, replace the check with an inline fuel filter (for picky, or leave it open) to verify the tank is vented. Running on one cylinder is not the tank vent (unless you have it running on the side stand).

be sure to come back and post...because something ain't right between A & B here.
 

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Discussion Starter #13
Good point modre

Had a look at the clogged tip over valve (or roll over valve) last night which works fine when held plumb vertical but chokes if tipped over to about 10 degrees.

This corresponds with the erratic idle with the bike sat on the sidestand warming up (but good on the centrestand)

Low revs / 2nd gear is generally riding round town negotiating many small roundabouts rightangle junctions etc. on the short straights inbetween the bike hiccups at steady slow speeds - this corresponds with the clogged valve.

At speed the bike is upright so no probs. One fast corner i come through every other day - the bike would hiccup accelarating out of this - again fits the bill.
 

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OK...I'll buy that...hope you nailed it.

my hesitation is it would take a minute for the tank vacuum to effect the float bowls and it wouldn't be an instant on/off symptom. I can only work with what you tell me.

IF that's it...I think an inline fuel filter in place of the check would eliminate that for good.
 

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AGAIN, if that's it, why not fix the fuel tank inward vent which is the fuel tank cap?
 

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Discussion Starter #16
In other words - why does the fuel tank need two breather valves ? One in the fuel cap and one run out from under the tank. I dont know but would be interested to find out. The garage seemed to suggest a few bikes had been in with this clogged valve problem.
 

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Hi there
that stupid tip-over valve has given problems since the Bonnie was introduced:mad: Mine is an 01 build and gave problems early on. Remove the valave and throw it in the bin with the A.I. parts, get a fuel line joiner from your local bike shop and use it to rejoin the vent hose. Problem solved permanently.
The valve is supposedly there to prevent fuel leaking if the bike is on its side. Well mine was on its side yesterday....twice. No fuel leaked out.
Phil
 

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The fuel tank needs two breather valves because it needs to vent 2 directions and to have one valve do this would mean that fuel could also run out when the bike is on its side. As I said before, the fuel cap is the inward vent and the overflow line with the tip over valve is the outward vent for pressure release. The overflow line also serves to run off fuel when one overfills the tank; and I believe this is where the problems arise. Fuel running through the tip over valve can make it sticky, IMHO. On the Tiger 1050 which has a different tank overflow and vent setup, the tip over valve is NOT in the overflow hose, or at least it should not be. The vent line has the tip over valve and the fill-up overflow line has no tip over valve; however, this overflow hose does not have access to the fuel in the tank when the fuel cap is closed. The fuel cap does not vent on the Tiger 1050--at least to my knowledge, it does not vent.
 
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