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I had a 1966 TR6R that was firing only one cylinder, and when I loosened the tank, it would run fine. One of the coils was shorting against the tank bottom.
 

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Discussion Starter #42 (Edited)
Hi Peg,

Swap your new plugs just in case, measure the resistance in the lead between the plugs, or measure the voltage at each coil with the ignition on.
Swapped plugs. No change. Measuring ~13V at coil on new battery.

If the misfire is proving to be immovable, then maybe it is in an immovable part of the engine,
Inlet tract, —leaks or blockage
cylinder, -valve leaks, ring leaks, cylinder head gasket
exhaust - blockage if not a crossover type.
I tried the WD-40 trick to check for a vacuum leak on the intake manifold. Couldn't detect anything. Regarding the cylinder, I suppose it could be something mechanical here. I did the compression check which looked good and I can see the intake and exhaust valves opening as expected. How else can one diagnosis a bad valve seal, piston seal, or cylinder head seal? The exhaust is the crossover type.

Hi Andy,

What brand / number spark plugs are you using?
Champion N3C. I've tried a new set and swapping the old set side to side. So 4 different plugs have been in the non-firing cylinder to no avail.

Could you try connecting the white Boyer box lead direct to the battery? I am assuming your machine is positive earth, so connect direct to battery negative.
That was an interesting test! Immediately when I connected the white lead to the battery negative it frontfired, scaring the crap outta me! I was able to get the bike fired up, and it ran, albeit on one cylinder, for about 5 seconds then the fuse blew. Not sure if that was supposed to have happened, but it did not get the other cylinder to fire.

Neal.
 

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Discussion Starter #43
Finally, made some progress today. Looks like a carb issue all along. I only had the left spark plug cable connected and was able to get the bike fired up with about 50% throttle, so this confirmed for me that there is compression and spark in that cylinder.

When I swapped the carbs to the opposite side several weeks back, I did not swap the throttle slide/needle. Just the carb bodies. Essentially my laziness ruled out everything on the carb except the slide/needle. I think that was the source of my problems. Perhaps I damaged the slide or the needle while replacing the cables. At least this is my working hypothesis.

Thanks for everyones help in getting this narrowed down.
 

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Neal
have a look at my post (rbe) today on 'what i did with my triumph today'
seems very similar to my problem
rory
 

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Discussion Starter #45
Just seeing your post Rory. I thought too that maybe the needle clip was hanging on the spring somehow or I damaged the needle. So I swapped in new ones and have been over that at least half dozen times now and am convinced it is seated correctly in the carb, and the carb is not the culprit. But I'm glad to read it solved your problem!

Long story short, still running one one cylinder. Only way to get the LH cylinder to fire is to flood the carb or crack the throttle >50% open. I've tried everything I know related to fuel delivery and electronics (see the previous 2 pages of troubleshooting in this post) but no difference sadly. Even replaced the petcocks last week cause one wasn't flowing quite right -- that was my hail mary.

Maybe the timing is completely whacked? But don't know how to verify if bike doesn't run, and further, don't know why this would just spontaneously occur.

Bike is out of commission unless there are more ideas. And man could I use a good ride right now...
 

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Discussion Starter #47
Hi Stuart,

Compression was reading decent when I checked a few weeks back:
  • The cold/dry compression was 125 lbs (for both the left and right cylinders) and the cold/wet compression was 130 lbs (again, for both cylinders). This almost exactly matches numbers I got four years ago, the last time I checked compression when the engine was running well as a baseline measure. It does take a few kicks to get to the max pressure.
My mind is on mechanical these days. I checked that the valves are opening and closing, and the lash is within spec. Before I strip the engine down to inspect everything inside the cylinder I of course want to rule out all other (easier) issues, such as timing.

Neal.
 

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It’s confusing. Did you swap the carbs, complete with slides, over, or just some half effort with one carb?

How sure are you that the pilot jet is clear? How do you know?
 

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Hi Neal,
The difficulty I have with this is getting past the idea that it is the carb.
The symptoms just point to it.
The Cylinder was not firing, so you add fuel by flooding the carb- cylinder fires.
All you did was add fuel-no electrical changes, no ignition timing changes, no changes to the compression.
If you add fuel and it runs, it is hard not to speculate that the fuel was missing in the first place

There is also
Cylinder fires at above 50% throttle.
If the piston speed is faster, then any losses of compression through leaks become less relevant. Reving the engine can compensate for a leaking valve or head gasket, where the compressed gasses have less time to escape.
Also to consider
The Amal carb is like 4 separate carburettors in one body, fuel is regulated by:
Pilot jet at zero to small throttle opening
Slide cutaway at small to 1/3-1/2 throttle opening.
Needle at 1/3- 3/4 throttle opening.
Main jet at above 3/4 throttle opening.

If the fuel is missing for the lower throttle openings-it will only run on higher throttle.

The fuel is picked at large throttle openings (over half) directly from the bottom of the float bowl. But at small throttle openings it is picked up via a transfer port in the float bowl. This port passes into the carb body through a hole in the gasket between the carb and body. If this is blocked/misaligned or has an air leak the fuel will not be lifted into the pilot circuit. I think it might be worth:
a) checking the fuel height in the float bowl.
b) checking that the sealing between the float bowl is not compromised, especially where the fuel transfer port is located.

Another check that might help; when the engine is running on one cylinder, a small squirt of easy start into the non running carb help pinpoint the fault.
If it is fuel related, the second cylinder should momentarily fire, if it is not fuel related it will not fire.

regards
Peg.
 

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Discussion Starter #50
Did you swap the carbs, complete with slides, over, or just some half effort with one carb?

How sure are you that the pilot jet is clear? How do you know?
This is what led me down the path of not believing it was the carbs. I swapped them side to side. The only component I did not swap was the cable/slide. The body and the needle were swapped. After swapping the fault did not switch sides.

I don't know for sure that the pilot jet is clear, but I followed the directions of using a #78 drill to open the jet. It did not meet much resistance, and was able to pass completely through. Blasting some carb cleaner into the jet shoots out the front out as expected.

I think it might be worth:
a) checking the fuel height in the float bowl.
b) checking that the sealing between the float bowl is not compromised, especially where the fuel transfer port is located.
A shot of carb cleaner into the carb when running on the opposite cylinder does get the non working cylinder to fire. I agree Peg that everything points to fuel delivery, but I just can't wrap my head around why swapping carbs did not transfer the fault. Nevertheless I am willing to do a carb rebuild because it is relatively painless compared to tearing down the top end of the engine.

Will report back!
 

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Hi all,

Finally a warm enough day here for me to fire up the 77 bonnie for the first time in a few months and try out some of the repairs and tweaks I've done. After a few kicks, bike fired up, ran very rough, then died. I did a few quick tests and determined the left cylinder is not firing. I don't think it is electrical (good spark, swapped cables) or valve train (clearances correct), so that leaves me with fuel/carb. I tried flushing out the float bowls and putting in fresh fuel to no avail. The only thing that seems to get the left cylinder intermittently firing is if I give the bike lots of gas, or I hold down the tickler to flood the carb.

Two things I did recently that I think are relevant: change the throttle cable and remove the choke slides from both carbs. I noticed that the throttle cable has excessive slack in it: so much so that it jumps off the mounting point on the grip. So I may need to shorten the cable. I did check that both carbs were in synch by the twist grip when I installed it. Interestingly, the previous throttle cable was not in synch with the slides on each carb: there was about 1/8 inch difference between the left and right slide, so I'm wondering if that is relevant -- as in, did the previous owner compensate for a known issue. Or could this be because I removed the chokes (I have never once used them), which I know lots of people have done, in which case, do the carbs need to be adjusted?

Any suggestions as to how I should go about narrowing this down, so I don't end up chasing a red herring?

Thanks.
Neal.
Sounds like your carb has gummed up passages. 2 products that will work extremely well for cleaning out varnish from old gas are; "Berryman B12 that you can pour into the tank. If you have the carb off the bike, their spray carb cleaner will melt the gunk in the carb very fast. The 2nd option is to put some Marvel Mystery oil in the gas tank. As long as the bad carb is pulling some fuel through, it too will melt that varnish.

Have used both products for over 30 years on bikes, cars, lawn mowers and snow blowers. cheap stuff for fast cure.
 

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Hmm, I did a quick battery test on my way out this morning. Ignition and headlight on for ~1min dropped the voltage by ¼ - ½ V, so it was reading 12V after this brief test. I'm inclined to replace the battery cause I'm running out of ideas pretty quickly.

Rusty1: Interesting to hear that you also had a one cylinder misfiring issue and turned out to be the battery. Fingers crossed this is the same issue I'm having.
If the battery does not hold a charge at 12.2 volts, the battery is dead or dying and can't be revived. New battery.
 

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Sounds like your carb has gummed up passages. 2 products that will work extremely well for cleaning out varnish from old gas are; "Berryman B12 that you can pour into the tank. If you have the carb off the bike, their spray carb cleaner will melt the gunk in the carb very fast. The 2nd option is to put some Marvel Mystery oil in the gas tank. As long as the bad carb is pulling some fuel through, it too will melt that varnish.

Have used both products for over 30 years on bikes, cars, lawn mowers and snow blowers. cheap stuff for fast cure.

Forgot to ask...is it the LEFT carb/cylinder that is the problem? My suggestion would be more likely if it is. When the bike is on a side stand the fuel in the left carb tends to sit longer than in the right carb. That would cause the gunk buildup in the passages.
 

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Neal I have found over the years if you change something and get a problem. The best thing to try first is go back to how things were before. Personally I have never understood why it always seems to be the left side ? Perhaps it's like Williekiote says with sidestand.but please reinstate and try again costs nothing but time.cheers
 

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Discussion Starter #55
Hi Williekiote -

If the battery does not hold a charge at 12.2 volts, the battery is dead or dying and can't be revived. New battery.
Tried this -- got a brand new battery, no difference.

is it the LEFT carb/cylinder that is the problem? My suggestion would be more likely if it is. When the bike is on a side stand the fuel in the left carb tends to sit longer than in the right carb. That would cause the gunk buildup in the passages.
Yup, it is the left, although bike is parked with a brick under the kickstand so it is more or less level. Nevertheless, will proceed with stripping and rebuilding the carb. If that one carb was gummed up, though, it should have transferred the fault to the right cylinder when I swapped carbs, which it did not. So either the tolerance of the right cylinder for a really lean mixture was higher, or it is not the carb and something mechanical in the left cylinder.
 

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I remember once I replaced air filters on my T140E next ride battery fried and dumped acid on to silencer unconnected I thought turned out I had knocked wire off zenner took bloody ages to find that.
 

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Me again its got to be fuel or spark really can't see what mechanical failure it could be.dont strip motor yet have you checked floats on bench making sure they aren't sticking up or down flooding or starving.
 

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Discussion Starter #58
have you checked floats on bench making sure they aren't sticking up or down flooding or starving
Hi -- this is next on my to-do list. I'm thinking of just replacing the floats with the Amal stay-up float which seems to be more resistant to ethanol-based fuels. As long as I'm stripping and cleaning the carb, might as well replace some parts in there that will give long life.
 

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Just a shot in the dark I always open both Petcocks do you? have you made sure hole in gas tank isn't blocked.it is just something small keep trying things !
 

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Hi Neal,

thinking of just replacing the floats with the Amal stay-up float
This would be wise, given modern fuel.

Risking telling you something you know already, stay-up floats must sit in a different position to give a similar fuel level in the bowl - the top of a stay-up float should be level with and just (1 mm.?) above the top of the bowl.

Hth.

Regards,
 
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