Carb Issue, Check My Thinking Please

[Downeaster]

Okay, "Tater" (It's a Yam, ya know...) is in the tuning stage.

The carbs were disassembled, cleaned and reassembled using the Carb Bible from this site.

I have an intermittent problem on the left cylinder. The first time I went through the carb adjustment procedure, the left cylinder wouldn't run unless the idle mixture screw was WAY out...like 5 turns out.

I took the carbs back off, checked the float levels (they were within spec @ 25mm, but I tweaked them to 24mm while I was there) and put them back on.

Same problem. It seemed to run better with the choke on, so I quickly covered the carb throat with the palm of my hand (air boxes off at the moment to facilitate carb removal/replacement) and once it cleared the surge of gas, it ran perfectly and I was able to adjust the mixture via the dead cylinder method. Shut it down and came in the house for a bit.

Went back out to start the bike and check the charging rate, and the problem was back. Left cylinder won't run unless the idle mixture screw is backed way out.

Ratzenfratzenfrickenfracken...

So, looking at the diagrams, reading the bible and scratching my head, I figure I must have missed something in the left carb's pilot circuit. Plan at this point is to take the carbs off and tear the left one down again and double check all the passages with particular attention to the pilot circuit.

Anything else I should look for while I'm in there?

 

[5twins]

It does sound like an idle circuit problem, like maybe the bypass holes are plugged.

 

[jayel]

electrical (spark) problem masquerading as a carb problem? what ignition, new plug wires, plug caps, plugs, coils? had a honda 450 for a while it would start on one cylinder had to lift the plug wire on the non-firing side due to weak spark once it got some heat into the cylinder it would run fine

 

[gggGary]

I'm with Jayel here points ignitions on the very best day are marginal. Throw any blip in the system and sufficient spark can be an issue. Have you checked the charging system? Is it putting out about 13.5 volts just off idle? I keep a jumper to plug the coils straight to the battery or even a separate battery during "back from the dead" trouble shooting. Poor connections, switch contacts, old parts, all can add up to an intermittent or weak spark masquerading as a carb issue. I am guessing your weather is not condusive to a nice long high speed run on the highway................

 

[Downeaster]

I'll certainly look into the ignition. TampaSVT sent me a good spare set of coils and a condenser (Thanks again, Tampa!) and I swapped them in this morning. No change there.

Swapped plugs side to side, no change.

Battery voltage goes DOWN with the engine running,from 12.5 with engine off and key on, to 12.0x with the engine running, even at high idle. Battery is new and kept up with a battery tender, but may still have electrical issues.

I've got a couple of spare batteries floating around, I'll try the direct hookup and see if that changes the symptoms.

Problem does ease off significantly when the bike warms up while running in the garage with me blipping the throttle occasionally.

Couldn't stand it any more and went for a short putt up to the corner and back. Thing pulls like a tank in mid range (no room to try a WOT run, and too damned cold anyway...) but pops and farts when decelerating and still idles like shit.

I roughly synched the carbs and tweaked the idle mixture again and got some improvement, but I'm thinking I still need to pull that left carb apart again.

I'll try the ignition tweaks first tho.

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ETA: Thought about the plug wires when I was putting the coils in, didn't keep track to see if I swapped them side-to-side or not. I'll try that too, and may just cut some new ones. Carbon core should be okay, right?

Also, put new brushes on the alternator, not sure why I'm not getting better output. Rectifier/regulator next, I guess.

 

[jayel]

stranded copper wire not carbon core, I like to run the coils off a 30A auto relay gets full battery voltage to them

 

[Downeaster]

Thanks, jayel. Existing wires appear to be carbon.

 

[5twins]

Use solid wire core wires, not carbon core. The carbon core are resistor wires and the ignition system is weak enough as it is. I even switched to zero ohm plug caps when I had points. You can usually R&R your original wires in a pinch by just cutting about 1/4" off each end so you get to some fresh wire.

You say you get barely 12V at idle? How much when revved to 2500-3000 RPMs? If the output doesn't climb to like 13.5 or 14V as you rev it then you have a charging problem. You may just need to adjust your voltage regulator. I did mine last summer and got a nice improvement to the charging output.

Edit - if you have carbon core wires AND resistor plug caps on there now, that may be part of the problem - too much resistance in the circuit. Higher RPMs pump more juice through the plug wires and can probably overcome the increased resistance. Lower speed running and idle maybe can not.

 

[Downeaster]

Okay, I tried the hot battery direct to the coils thing. Noticeable improvement but still not "perfect".

5twins, I definitely have charging issues. I ohmed out the alternator, 5.8 ohms between the slip rings, which is in the ballpark.

I'll try tweeking the regulator as I get NO increase at any RPM.

I'm also thinking I'll go with the Chrysler regulator and home-made rectifier option. Gonna do a little checking, if everything (or most of it) is available locally, I'll pick up new wires and caps, the regulator and the parts to make the rectifier in town tomorrow.

 

[5twins]

Well, if you're not charging, don't even bother with the carbs yet. It's pointless and there may not even be anything wrong with them. A regulator adjustment may not help you at this point. The regulator does as it's name implies - regulates or varies the charging rate. You have none at the moment to regulate, lol. I would still pop the cover off and have a look inside. Maybe it's all corroded and the contact points are stuck shut. Here's what you'll find inside .....

I found my points gap (#4) to be at about .20". Resetting it to within the .012"-.016" range brought my charging rate back up to where it should be. I was seeing just over 12V at idle and just over 13V revved. Now I get near 13 at idle, near 14 revved.

 

[Downeaster]

Yup, that's what I'm thinking. I'm about to toddle out to the groj and see if I can tweek the regulator.

 

[retiredgentleman]

Okay, I tried the hot battery direct to the coils thing. Noticeable improvement but still not "perfect".

5twins, I definitely have charging issues. I ohmed out the alternator, 5.8 ohms between the slip rings, which is in the ballpark.

I'll try tweeking the regulator as I get NO increase at any RPM.

I'm also thinking I'll go with the Chrysler regulator and home-made rectifier option. Gonna do a little checking, if everything (or most of it) is available locally, I'll pick up new wires and caps, the regulator and the parts to make the rectifier in town tomorrow.

Downeaster;

Since your bike is a 79, your wiring and alternator are set-up to run with one brush grounded. It would be simpler and easier to use a regulator that is designed for "one brush grounded", such as the Standard VR-115.

Going with 2 new bridge rectifiers is a good upgrade to the old rectifier.

 

[gggGary]

Tossing in here that the brushes need to be long, anywhere near the suggested service limit is replacement time.
To point at (or away from) the regulator run 12 volts to the brush that would normally go to the regulator see what your system voltage looks like when it's running. Don't do this for long or rev very high if it's charging.

 

[jayel]

see how much trouble I can cause just by throwing out a hypothetical question?
electrical (spark) problem masquerading as a carb problem?

 

[gggGary]

see how much trouble I can cause just by throwing out a hypothetical question?
electrical (spark) problem masquerading as a carb problem?

 

[Downeaster]

gggGary, I put new brushes in the alternator. Double-checked them today, they look fine.

retiredgentleman, is that Standard VR-115 available through NAPA?

5twins, I pulled the cover off the regulator and cleaned the points, but didn't think about doh checking the gap. Tweaking the adjustment screw (#10) didn't have any effect. I'll go back in a bit and check/set the gap.

jayel, you done good, cuz I was putting off dealing with the charging issue, and now I've got to fix that before I move to the next step. I'd have been trying to tweak the carbs or frig with the ignition first.

 

[jayel]

I went thru this with my 74 a couple years ago, fighting carbs, then fixed the ignition and the carb problem went away so it is possible to be barking up the wrong tree

 

[Downeaster]

Okay, now I'm chasing the charging issue.

I did the "slap test"...Nada.

I checked voltage at the positive brush on the alternator - good to go.

The tech sheet "how-to" on the charging system says:

"3. On the solid state regulator models all you need to do is locate the green wire at the regulator plug and make a jump from it to ground. That bypasses the regulator and allows full battery current to flow through the brushes out of the rotor through the green wire to ground. That causes the rotor to make a stronger magnetic field which in turn causes more current to flow in the stator. If your battery terminal charge voltage jumps up to 14.5VDC when you rev the engine then the regulator or the ground connection for the regulator is your problem. "

Problem is, I don't have the solid state regulator. Can I still do this test?

eta: With the alternator cover off, there is some residual magnetism on the rotor shaft/retaining nut, but not much and it doesn't change key on/key off.

 

[Downeaster]

Okay. Channeling Emily Latella here: "Oh. Well. Never Mind"

See the "I'm an Idiot" post for details, but essentially I had the alternator brushes wired wrong.

Charging problem fixed, back to the carbs...