Topic: Introduction and CB350F lean issue

[mlake01]

Hi everyone,

By way of first post introduction, my name is Mike and I have a '73 CB350F that I'm having some issues with - looking for some tips/reassurance/help here.  Sounds like an intro at an AA meeting, doesn't it?  Anyway, I'm no stranger to bikes or mechanical things, and can usually figure most anything mechanical out.  This one is puzzling me a bit though.  I think I know where to go with it, but just looking for ideas or confirmation from the experts here.

Background:
Bought the bike a few years ago for a song.  Box stock except for a 4 into 2 Jardine exhaust.  Less than 8,000 miles on it and in cherry shape except for it hit a deer...  I stored it until a couple of months ago, and have begun the resurrection.  Crash damage has been fixed, now it's time to make it run good.

The symptoms:
The bike is running super lean, especially under load at around 1/2 throttle (~4,000+ rpm).  It starts great (although it likes choke, even on 85 deg days), idles great, revs to redline quickly in neutral, etc.  Put it in gear and take off down the road however, and it flattens off at ~1/2 throttle and will not rev beyond 4,000 to 5,000 rpm in any gear.  While running along this way, I can give it ~3/4 choke and it fixes everything - runs great with the choke artificially richening things.  You have to modulate the choke a bit as rpms change, but you can fiddle with it and make it run super good all the way to redline.  A plug check shows some snow-white electrodes..!  I live at ~3,500 elevation, so most things run a tad rich here.

What I've done:
Adjusted valves - .002/.003
New points - .013"
New condensors
Advance unit working fine
Timing set and checked dynamically with light
New D8EA plugs
Rebuilt fuel petcock, verified flow
Carbs super cleaned, every part removed and cleaned, emulsifier tubes cleaned, new float valves, needles, pilot jets, main jets, o-rings, gaskets, etc.  Carbs are super clean and full of new parts.  Particulars are - 75 mains, 35 pilots, float level 4mm below flange with clear tubes, needle clip in middle notch, etc.
Bench synched slides and choke plates - have not vacuum synched yet.

I have checked the spark plug caps with the following results:
1 - 8.80k ohms
2 - 9.93k ohms
3 - 11.30k ohms
4 - 9.01k ohms

New plug caps are on order (5k), but not here yet.

I have a hard time believing that plug caps alone would cause this dramatic of a flat spot?  Is it time to just believe everything that the bike is telling me (give me fuel!), and raise the needles about two notches and/or put some 80+ mains in there?

Thanks in advance for any ideas you may have,
-Mike

[scottly]

Welcome Mike. Did the bike run well before you hit the deer? I also have a hard time believing the plug caps would cause your symptoms.

[tomkimberly]

Just a guess based on experience, is the air intake blocked?

It's located at the rear of the seat and in my case, I had blocked off the air flow with a plastic bag holding the insurance papers. 

[mlake01]

The previous owner hit the deer - I can only assume it was running well at the time, but no guarantees there...  I have the tool kit and tray removed while testing, so the intake isn't blocked other than by the air filter (which I should probably replace), but I think a restricted intake would make it go rich, not lean?

Forgot to mention - I replaced the intake boot o-rings too..

Thanks for the ideas so far!




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[scottly]

Yes, a restricted air intake would make it run rich. Looked again at your thorough list of what you've done so far, and the only thing that stands out is the 4mm clear tube test results, and even that is not that far out of line. Maybe try resetting the floats to 3mm and see if it helps?

[steam-powered man]

replace the paper air filter, or maybe try testing without it just for s/g's? 

[iron_worker]

What position is the needle clip set at? What is the stock setting?

Is 75/35 stock for jet sizes?

Something is causing it to go crazy lean if it is completely stopping it from revving in the mid range.

 Fuel restriction somewhere?

Did you put the needle jets in upside down? (not familiar with the 350F carbs but the 750 carbs have press in needle jets that look like they could go in either way but the needle taper is only on the one side. Not sure if that would cause the lean condition you describe.

Fuel line blockage?

Major air leak somewhere?

IW

[camelman]

You didn't mention the airbox lid. It sets atop the filter box, but below the tool tray. The bike won't run well without it. It will be too lean. Many are lost during the life of the bike.

How are the intake boots?  I struggled with a few of these old engines before realizing the intake boots were leaking. The leak internally, not at the bolted flanges. The leaks occur when the old rubber begins to pull away from the internal metal sleeve.

DSS has replacements for all these parts. But new if you can.

[mlake01]

Needle clips are in the middle notch, and I'm pretty sure 35/75 is stock jetting.  I have verified fuel flow through the petcock, fuel filter, and into a coffee can.. ;-)

Airbox lid - now THERE is a good suggestion!  The lid is missing off this bike, and that would certainly make things go a bit leaner than with the lid.  I'm a little surprised it is lean enough to just level off and stop revving - even stops firing if you open the throttle further.  Anyway, excellent suggestion and I will be going home tonight and taping a piece of cardboard over the airbox for testing!

Intake boots are in cherry condition - no cracks, splits, separation from the aluminum core, etc.

Thanks for the help and I will be sure to let you know how it runs.

[tomkimberly]

Are you sure you're running lean? Can you post a pic of the plugs?

It's hard to imagine a lean running bike not reving over 5k.

Tom

[mlake01]

Are you sure you're running lean? Can you post a pic of the plugs?

It's hard to imagine a lean running bike not reving over 5k.

Tom

Yep, it's lean - like, super lean - choking it improves things.  Here are the plugs after a ~3 miles test run:


Following an earlier suggestion from this forum, I used a piece of aluminum sheet and some duct tape to seal up the top of my airbox (where the missing lid would have been).  This made the biggest difference to date - easily a 60% improvement!!

It will now rev right up to about 6,500 or 7,000 rpm in every gear before it levels off.  Adding about 1/3 choke brings it on up redline, whereas it used to need 3/4 choke.  Pretty impressive improvement for a simple airbox lid!  Kind of makes me wonder if a tank of real gas might help (we have 10% ethanol in The People's Republic of Oregon) too?

Here are the plugs after the airbox lid modification:


Better, but still a tad lean.  I think my next step is to give it the fuel it wants - raise the needles 2 notches and put in some #80 mains.  Too bad it's such a &#%$!@ chore to raise the needles...

Thanks again for all the continued help.  I hope my pictures come through..

[tomkimberly]

Looks like your on the right track.   

[mlake01]

That was it!  I slipped in a set of 80 mains and raised the needles one notch (I know - change one thing at a time... ;-), it ran WAY better except just a tad rich around 1/4 throttle.  Removed the duct tape airbox lid I had installed during previous testing and it's near-perfect!  One ever-so-slight rich burble just off idle as you are letting the clutch out, then perfect all the way to redline.  Probably should drop the needles back to the mid position (mains alone were enough), but it's such a PITA to adjust the needles without pulling the carbs I'll probably leave it right there for a little bit as I live at 3,600 ft ASL.  When I ride over to the valley (~500ft elevation) it'll be perfect.

Thanks everybody for the help!  I thought I knew where to go with it, but the collective wisdom and experience here was very helpful.

Best,
Mike