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Front cylinder not firing at low to mid throttle

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Front cylinder not firing at low to mid throttle

PostAuthor: PatKD » Fri Aug 23, 2019 8:23 am

Hey guys,

So here's the story: I was just getting up to cruising speed on the highway, when the bike suddenly felt super jerky. I got off and discovered that my front cylinder was heavily misfiring at roughly the throttle position I'm at on the highway, i.e. just under halfway open.

Limped home and checked what I could. The front cylinder was definitely only firing as of mid-throttle, anything lower and it runs on the rear cylinder only. I checked:
- Spark plugs (front plug was fouled and wet, I changed it out to no avail)
- Coils (switched cylinders and no difference, rear cylinder firing only from idle to mid-throttle)
- Coil resistance (2.7 ohms on each, didn't manage to measure secondary coil but I figure it's ok if switching them didn't change a thing)
- Pickup resistance (155 ohms on both)
- Battery voltage (13+ V)

I can't tell whether it's the cylinder running way too rich until mid-throttle, or spark timing, or spark voltage, or the TCI, or something else entirely that's causing me grief.

Help!

Cheers
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Re: Front cylinder not firing at low to mid throttle

PostAuthor: bstig60 » Fri Aug 23, 2019 11:12 am

Check the easy things first. Pull the front plug and clean it then ground the tip and crank the engine and make sure you are getting a spark. If so, then turn the pilot screw all the way in (clockwise), counting the turns, then turn it back out to 2.5 turns from the soft seat and see if that helps. Let us know what you find.
Bill
"It´s a friggen motorcycle, it´s not supposed to be comfortable, quiet or safe. The wind noise is supposed to hurt your ears, the seat should be hard and riding it should make you s**t your pants every now and then. "

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Re: Front cylinder not firing at low to mid throttle

PostAuthor: PatKD » Fri Aug 23, 2019 1:37 pm

Thanks bstig, I'll check that out. I'm pretty sure I've got sparking happening at the front cylinder at all rpm, I'll double check. I'll check if the pilot screw does the trick, if not I'll go ahead and clean out the whole carb. Can clogging cause the mixture to become too rich or does it generally lean it out?

Cheers
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Re: Front cylinder not firing at low to mid throttle

PostAuthor: bstig60 » Sat Aug 24, 2019 8:52 am

A bad O ring in the pilot circuit will cause excessive leanness at lower RPM. The other item to check would be the float level setting. There is a gauge on the BDS carbs to show where it should be. I have also heard of the main jet falling out due to not being tightened enough and engine vibration.
Bill
"It´s a friggen motorcycle, it´s not supposed to be comfortable, quiet or safe. The wind noise is supposed to hurt your ears, the seat should be hard and riding it should make you s**t your pants every now and then. "

Please take the time to edit your profile to show your location. City, State/Province, and Country.
It is easier to help you if we know where you are.


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Re: Front cylinder not firing at low to mid throttle

PostAuthor: PatKD » Tue Aug 27, 2019 9:05 am

Ok so I've dismantled the front carb and all the hardware seems to be in order, except for a pinsized tear in the coast enricher diaphragm. It plays a part in the pilot circuit, with the pilot jet #2 in line with it, could it be the culprit for wrong mixture at low throttle? I'm trying to figure out the carburetor section view in the service manual (page 4-2, the enricher diaphragm is in top left of my screenshot) and boy is it hard to tell!

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Re: Front cylinder not firing at low to mid throttle

PostAuthor: bstig60 » Tue Aug 27, 2019 11:27 am

Yes, that's possibly the culprit. Repair it or replace the diaphragm and see if that works.
Bill
"It´s a friggen motorcycle, it´s not supposed to be comfortable, quiet or safe. The wind noise is supposed to hurt your ears, the seat should be hard and riding it should make you s**t your pants every now and then. "

Please take the time to edit your profile to show your location. City, State/Province, and Country.
It is easier to help you if we know where you are.


SERVICE MANUAL DOWNLOAD LINK
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Re: Front cylinder not firing at low to mid throttle

PostAuthor: PatKD » Wed Aug 28, 2019 9:11 am

Being the noob that I am, I hadn't checked the pilot screw before disassembling and it was out 4 turns. I also noticed I had a missing O-ring on the needle jet. The rear carb, which is running perfectly, was at its original position at 2.5 turns. Pretty sure my rich mix was partly due to this. Possibly the PO had problems because of the secondary diaphragm and found some sort of temporary workaround by turning the screw way out. I'm still glad I undid the carbs to gain a bit of insight as to how it works and take the gunk off. I'll definitely change the diaphragm, although it's not cheap.
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Re: Front cylinder not firing at low to mid throttle

PostAuthor: PatKD » Mon Oct 12, 2020 11:49 am

So this issue is finally solved.

I figured out the vacuum hose from the front cylinder was gulping down fuel from the petcock, bypassing the carbs entirely. At high load, there was enough air to ignite the mixture, but at low load the front cylinder was running way too rich. So although the time I passed giving TLC to my carbs was certainly not in vain, the real issue was a shot diaphragm in the petcock. The bike runs well on Prime with the vacuum hose blocked. Man the engineering in the 80s was wild.

New petcock ordered.

KD
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