Topic: CB350F rich at idle to ~2K

[mikestebbins]

Hello, this is my first post, but I’ve been reading this forum for a few years now. Thanks for being so helpful! I apologize for the length, but I’m trying to anticipate some of the questions folks will ask.

Situation: 1974 CB350F runs very rich from idle to ~2K. Moving to a smaller pilot jet didn’t help. Adjusting the air screws (all the way from the specified ⅞ turn out to 6+ turns out) doesn’t seem to affect idle speed. New/clean plugs are covered in soot after starting and running in the driveway and even when it’s taken out on some 45 mph streets nearby (trying to run it at 5-8K  to clean them off) for 5 minutes. Whitish-smoke coming from the exhaust. If I pull the plugs after just running it in the driveway, they sometimes have a bit of shine and the smell of fuel on them.

When I get it out on the street and warmed up, it runs fairly well from ~2K on up, except for when I pull the clutch in and let off the throttle (e.g., when approaching a stop sign), it will die instantly, until I release the clutch and the rear wheel starts it up again or if I keep the throttle rev’d a bit it won’t die. When I adjust the idle screw (the knurled thumb screw) enough so that it won’t die at every intersection, it is "idling" at ~3K.

This is now a brat style bike that was effectively stock when I got it. It was running well then and has had this rich idle problem ever since I got the engine put back together. The previous owner (my uncle) ran it at ~5100’ of elevation and I’m currently at ~4800’.

Current set-up and Diagnostics thus far:

Top-end rebuilt:
- New rings, cylinders ball-honed, new gasket/o-ring kit, valves brought to sparkling clean, new cam chain guide and slipper installed.
- Valve lash set to the looser side of 0.002”-0.003”.
- Leak-down test showed all four cylinders between 1.1% and 2.2%.

Carbs rebuilt:
- Ultrasonic cleaned/baking soda blasted, every passage sprayed through with carb cleaner.
- All new o-rings and float bowl o-ring.
- Original brass cleaned and reinstalled: 75 mains, 18234 needles (I think these are original), emulsion tubes.
- New brass installed: new Keyster float needle valve assemblies (fixed some leaking valves when I used the old brass), new 32 pilots (down from the stock 35s I ran for awhile) from Jets R’ Us.
- Float height set to 24mm (higher than spec’d, did this to attempt to lean it out).
- A little smear of fuel-resistant gasket maker on the main o-rings before pushing them into place. There is definitely some corrosion of the “towers” that the main jets push into and the main jet o-rings don’t feel all that hard to push into place, so I wondered if I had fuel leaking around the o-ring and decided to ensure it couldn’t.

Air-in:
- Currently using the stock setup: airbox, airbox-to-carbs piece, and airbox “snorkel”.
- I tried using pods, but decided to take that variable out of the equation after reading many forum posts.
- Put in brand new intake insulators (the carb-to-intake rubber boots).

Air-out:
- Delkevic 4-into-1 headers with Cone Engineering “Quiet-er Core” Reverse cone slip-on muffler
- Before I tore it all down, it had the stock 4-into-4 exhaust
- All 4 headers are hot after running for a bit

Electrical:
- Rewired with NWT Cycletronics X21 controller (basically an M-unit competitor) and hand-built wire harness, all new connectors or soldered connections.
- Moved to Dyna S electronic ignition, as I was wondering if I just couldn’t figure out points; they are new to me and seem to be shrouded in black magic
- New Magna high-output coils, plug caps, plug wires, and many sets of new spark plugs have been tried (both the standard D8EA and a set of hotter D7EA plugs, as I live in town and there aren’t many chances to get it rev’d up for any real period of time).
- Lithium Antigravity AG-801 battery and a lithium-specific regulator/rectifier.
- Battery is in good shape, system is charging it (when the RPMs get high-enough, of course).
- Resistance measurements of stator, coils, spark plug wires, and caps all within spec.
- I have spark, but I don’t know if it’s good enough or “weak” (I’m just not experienced enough to say)

Any ideas about causes or diagnostics I can try would be much appreciated. I haven’t worked on motorcycles for ~25 years (and those were all dirt bikes) so while I’ve learned a lot over the past couple of years and am fairly mechanically-minded, I could still be making a dumb mistake (or five!). As you can see from the list, I've tried a lot, and I'm starting to second-guess whether it will ever run correctly. Thanks for reading this far and for your thoughts!

Mike

[newday777]

Howdy Mike welcome aboard the forum from NH
I noticed that you didn't mention if you have vacuum sync’d the carbs since rebuild???
So you put standard rings on the old bore and pistons? Did you use an internal bore mic and check for out of round and taper in the bores?

[mikestebbins]

Hi Stu, thanks for the warm welcome!

The carbs have been bench-synced each time I reassembled. I have done it both with a 1mm drill bit and with a strong light and a black card on the backside. I can't seem to get the bike idling low enough to do a vacuum sync, at least according to the videos I've watched on it. I own the vacuum sync gauge. Can I do a vacuum sync at 2000-2500 RPM?

I forgot to say in the original post, I have the Chilton’s manual for the bike.

I did measure the cylinder bores and pistons. We have a prototype machine shop at work, so I have access to a 3-point internal micrometer and external mics. Chilton's lists the maximum allowed cylinder diameter measurement as 47.1 mm (1.854”). The largest measurement I took was 47.024 mm (1.851”).

Chilton’s calls out “NA” for taper and ovality maximum measurements (for some reason). I think that this is the reason I didn't specify in my notes what I measured for taper and ovality. But, I can take the max. diameter measured minus the min. diameter measured for each cylinder, across every measurement (I took multiple at top, middle, and bottom locations), and this should represent the worst possible ovality and taper. The largest difference I have in diameters in any one of the four cylinders is 0.014 mm (0.00055”).

The smallest piston diameter measured at 46.94 mm (1.848”). Chilton’s calls out the min. acceptable diameter as 46.85 mm (1.845”).

Thanks for any help or ideas you might have!

[HondaMan]

This is a little harder to measure, but I'd bet $5 it is the source of the problem.

The 350F engine's piston-to-bore clearance must be TIGHT. REAL TIGHT. If it exceeds 0.0008" then the engine has blowby issues that can't be fixed until it has been properly rebored and re-pistoned. When I rebuild these it is with 0.0004" piston skirt-to-bore clearance. There's actually several very long threads around here about other 350F bikes that went thru the same issues. The fix was the same in each one of the posts: tight bores.

[willbird]

Hi Stu, thanks for the warm welcome!

The carbs have been bench-synced each time I reassembled. I have done it both with a 1mm drill bit and with a strong light and a black card on the backside. I can't seem to get the bike idling low enough to do a vacuum sync, at least according to the videos I've watched on it. I own the vacuum sync gauge. Can I do a vacuum sync at 2000-2500 RPM?

I forgot to say in the original post, I have the Chilton’s manual for the bike.

I did measure the cylinder bores and pistons. We have a prototype machine shop at work, so I have access to a 3-point internal micrometer and external mics. Chilton's lists the maximum allowed cylinder diameter measurement as 47.1 mm (1.854”). The largest measurement I took was 47.024 mm (1.851”).

Chilton’s calls out “NA” for taper and ovality maximum measurements (for some reason). I think that this is the reason I didn't specify in my notes what I measured for taper and ovality. But, I can take the max. diameter measured minus the min. diameter measured for each cylinder, across every measurement (I took multiple at top, middle, and bottom locations), and this should represent the worst possible ovality and taper. The largest difference I have in diameters in any one of the four cylinders is 0.014 mm (0.00055”).

The smallest piston diameter measured at 46.94 mm (1.848”). Chilton’s calls out the min. acceptable diameter as 46.85 mm (1.845”).

Thanks for any help or ideas you might have!

So your average piston to wall clearance is .003" roughly ??

[mikestebbins]

HondaMan, thanks for the response, even if it's not what I wanted to hear! Just to be certain, when you're talking about piston-to-bore clearance, is that measurement done in diameter or radius?

willbird, I only jotted down the minimum piston measurement I made (although, of course, I measured all 4 pistons), so I think that it would be fair to say that for at least 1 cylinder, the clearance is somewhere between 0.0025" and 0.003". That's a difference of diameters, not radii. But, even in terms of radii, 0.00125" - 0.0015" is a whole lot more than 0.0004" - 0.0008"!

Something that I'm not sure that I understand that maybe someone can help me out with. The engine was running well before tearing the top-end apart, and these measurement were made prior to me honing/de-glazing the cylinders, so this large piston-to-wall clearance already existed while it was running well. I deglazed the cylinder walls with a flex-ball hone (https://www.brushresearch.com/pdf/FlexHone_Resource_Guide_For_Customer.pdf), per their instructions. They claim that it is a surface-finish tool and removes only a minimal amount of material. Is the idea that between deglazing (perhaps removing a bit more material than they claimed it would and increasing the clearance significantly) and new rings on a newly-surfaced cylinder wall, some combination of those two pushed me over the edge from no or little blowby to enough blowby that it runs poorly?

Thanks again! mike

[newday777]

How much a hone takes off depends upon how many passes you did. You should always do measurements before and after.
Diameter difference is what you measure not radius.

[mikestebbins]

Stu, Thanks for the confirmation on diameter vs. radius. I do realize now how useful it would have been to measure after the deglazing procedure ...

Would a compression test or a leak-down test not show symptoms of blowby? I'm guessing that neither test creates as much pressure as the combustion process, but I want to ask and be certain of that, instead of assuming.

Does anyone have anything else that I should try, before pulling the top-end off? Even if it's a low-probability of success, if it doesn't take too long or cost too much, I'd rather try it before I remove the top-end. Thanks!

[denward17]

Leak down test I think would show the current condition of combustion chamber, whether it be valves and or rings.

[Mark1976]

Leak down test I think would show the current condition of combustion chamber, whether it be valves and or rings.

+1
Do a compression check while your there.
What da ya got to lose...

[HondaMan]

An 'old timer' test I used in my shop for the smaller engines (not just the 350F, but many 50cc-125cc single-piston engines) was to change to 40 weight oil and put it in, then run it to see if it got 'better' somewhat, before tearing down an engine for a rebuild - or not (I kept 2 gallons of this "Test Oil", as it was labelled). If this increased compression significantly (like by 10-15 PSI) after warming up the engine, then we'd "call" it as needing rebore due to too much piston-bore clearance, which the thicker oil produced. While admittedly a backwoods test as it is somewhat subjective, it worked for the smaller bores in Singles and Twins. The 350F is definitely one of those! (I never used it for a 750 test as the engine is a dry-sump design - not enough oil splash-up is available then.)

The clearance is measured between the front or rear piston skirt against the front or rear of the bore, at the top of the piston's stroke (from underneath, so to speak). If we could get a 0.0010" feeler gage in there, they were needing rebore. We'd do it with the cylinders upside down, trapping the feeler gage between the piston skirt and the bore as we pressed the (ringless) piston in from the bottom, to see where in the bore the piston stopped and fully trapped the 0.0010" gage (since we didn't have any thinner ones). If the piston's ring lands cleared the top of the bore without trapping the feeler gage, it was considered REAL worn out.

We did it this way for lack of having an expensive set of accurate bore gages and micrometers, and I don't trust vernier calipers for measuring this sort of thing - too many uncontrolled variables are involved there.

This was adapted from an old Ford test for the tractor engines (170 & 200 CID) common on the farms in the area in those days. (Those much larger pistons used 0.0020" piston clearance if new, 0.006" if worn out).

[mikestebbins]

Mark1976, denward17, I've already completed a leak-down test after the top-end rebuild. With 90psi input, the highest leaking cylinder I see sits at 89psi, or a 2.2% leak. Which, from what I've read, tells me that rings, valves, cylinder head, and cylinder gasket are all doing great.

HondaMan, I like the 'old-timer' test! If it pointed me in a direction besides top-end overhaul, that 40 weight oil would be worth it's weight in...$20/bottle wine?!   I understand and agree on not trusting calipers for measurements under 0.002"...what's making me grimace even more than you all might imagine I am already is that I'm a Mechanical Engineer...it probably goes without saying that I have never had the pleasure of engineering in internal combustion engines!!! Still kicking myself that I didn't think to remeasure the cylinders after the ball-hone...

Where I am still a little confused is if this piston-to-cylinder-wall clearance overage would or wouldn't show up in a leak-down test. That's what I meant when I asked if the pressure in the cylinder at the time of combustion is just so much greater than the 90psi I'm using for leak-down testing, that that's why the engine seems "tight" according to the leak-down test, but significant blowby occurs when the pressure gets much higher during combustion. If I'm headed down the wrong path, could someone give me the one-two sentence answer, or even better point me to some reading to better understand blowby and the part that piston-to-cylinder-wall clearance plays in it?

Thanks, everyone! I genuinely appreciate you taking the time to help me out here!

[scottly]

Situation: 1974 CB350F runs very rich from idle to ~2K. Moving to a smaller pilot jet didn’t help. Adjusting the air screws (all the way from the specified ⅞ turn out to 6+ turns out) doesn’t seem to affect idle speed. New/clean plugs are covered in soot after starting and running in the driveway and even when it’s taken out on some 45 mph streets nearby (trying to run it at 5-8K  to clean them off) for 5 minutes. Whitish-smoke coming from the exhaust. If I pull the plugs after just running it in the driveway, they sometimes have a bit of shine and the smell of fuel on them.

Mike, you have a carb problem! It's amazing how quickly a thread can go sideways lately?? Are all four spark plugs sooty? Verify the actual fuel level in the float bowls using the "clear tube" method; it should be about 3mm below the top edge of the float bowl. Are the o-rings that seal the main jets in the carb body doing their job? Are the main jet retainer springs in place?

[jonda500]

Hi Mike,
   really feeling your pain. Please excuse my ignorance as I try to understand your problem.
Are the plugs blackened with fuel or burnt oil? I think you are describing fuel soot on the plugs?
Does it start easily and then straight away want the choke off?
Will it start (cold) without choke?
Have you considered trying even smaller pilot jets?

   I don't get how excessive piston to bore clearance can make it run rich..
My 500 has low compression and burns some oil for sure, (all four exhausts emit smoke at idle and have black oily residue inside) yet it runs and idles fine - but I do have two sizes smaller than stock pilot jets (#40>#35) and one size bigger mains. (+New valve stem seals!)
John

Edit- scottly beat me to it - I say +1 to the carbs being your problem.

[Herr Supersport]

"Mike, you have a carb problem!" – I agree:

Could it be that your float needle valve is sticking and causing the fuel level to be too high during operation?

You have installed Keyster valves. In the German CB forum, a good guy measured the Keyster and Keihin valves [pic: post #15]. http://www.cbfourclub.de/cbfourum/viewtopic.php?f=1&t=10081

In summary: Keyster needles have more play and are shorter than the original ones. This makes them more prone to jamming than the Keihin parts.

Best regards,
Daniel

[willbird]

Hi Mike,
   really feeling your pain. Please excuse my ignorance as I try to understand your problem.
Are the plugs blackened with fuel or burnt oil? I think you are describing fuel soot on the plugs?
Does it start easily and then straight away want the choke off?
Will it start (cold) without choke?
Have you considered trying even smaller pilot jets?

   I don't get how excessive piston to bore clearance can make it run rich..
My 500 has low compression and burns some oil for sure, (all four exhausts emit smoke at idle and have black oily residue inside) yet it runs and idles fine - but I do have two sizes smaller than stock pilot jets (#40>#35) and one size bigger mains. (+New valve stem seals!)
John

Edit- scottly beat me to it - I say +1 to the carbs being your problem.

I would think that was the last possibility too, piston to bore.

[Mark1976]

Situation: 1974 CB350F runs very rich from idle to ~2K. Moving to a smaller pilot jet didn’t help. Adjusting the air screws (all the way from the specified ⅞ turn out to 6+ turns out) doesn’t seem to affect idle speed. New/clean plugs are covered in soot after starting and running in the driveway and even when it’s taken out on some 45 mph streets nearby (trying to run it at 5-8K  to clean them off) for 5 minutes. Whitish-smoke coming from the exhaust. If I pull the plugs after just running it in the driveway, they sometimes have a bit of shine and the smell of fuel on them.

Mike, you have a carb problem! It's amazing how quickly a thread can go sideways lately?? Are all four spark plugs sooty? Verify the actual fuel level in the float bowls using the "clear tube" method; it should be about 3mm below the top edge of the float bowl. Are the o-rings that seal the main jets in the carb body doing their job? Are the main jet retainer springs in place?

+1

[mikestebbins]

All, thanks for the additional input, it's much appreciated!

I was feeling pretty down about the top-end until reading these posts this morning. I read through markreimer's post "CB350F: two failed top end rebuilds, stock bore and 393cc" last night. Even if it does turn out to be the piston clearance issue (which I imagine it could), it at least boosts my spirits a bit to have a few simple things to consider before concluding that a second rebuild is in order.

Scottly, yes, all four plugs are sooty. When I get it out and run it for a bit and then pull the plugs, they are dry and sooty. If I just run it at idle with a few throttle blips in the driveway, and the pull the plugs, they are sooty and a bit wet. It looks more like gas than oil if I drag one across my hand, but I'm not 100% on that. I read on another post last night that someone put a flame to the wet plug to see if it's gas or not. I'll try that next time. I have not done the clear tube fuel level verification. I will do that this weekend and report back. The O-rings that seal the main jets are from an inexpensive carb rebuild kit (I did that before I knew better, but now I've put back the OEM brass where I could). I did put a layer of fuel-resistant gasket maker around them the last time I had the carbs apart, then put the main jet retainer springs on, and bolted up the float bowls and let them sit for 24 hours before letting fuel in. All that to say that I think that 4 leaky o-rings would have been solved when I did that. FWIW, I plan on buying "good" o-rings to replace my gasket maker bandaid.

jonda500, I answered some of your questions above. Additionally, it starts cold on the first or second kick every time (the starter motor is fried, and it starts so easily that I haven't bothered to replace it yet). I never have to use the choke; in fact, when I try to use it, it won't start. I have not experimented much with giving it a little bit of choke instead of just on or off (basically because it starts so easily w/o). When I looked at Jets R' Us, size 32 pilot jet (one size smaller than stock 35) was the smallest they offered in that configuration. Based on what I've read, that place is almost the "end-all be-all" for high-quality jets, so I haven't looked elsewhere. Also, the only significant set-up change to the bike since it was running well is the 4-into-1 exhaust. Needing to drop more than 1x pilot jet size due to that change seemed odd to me (and makes me think something else is going on, perhaps fuel level or fuel valve, etc.), but of course, I shouldn't assume that it's out of the question. I'm just unsure how to go smaller.

Daniel, that's great info on the Keyster float valves, thanks for the link. Unfortunately, the forum isn't letting me register (it helpfully says "The email address you provided may not be used.") I suppose that the valves could be sticking/jamming, but I haven't had a fuel overflow out of the drain tubing ever since I put them in. I would expect that would be what happens if the valve sticks/jams. Either way, I will check the carb fuel level with the clear tube method this weekend to get a better understanding of what's going on in there. Do you have any other thoughts on what to check or do here? Was the end consensus from this person's thread that the Keyster float valves are junk, and they should be replaced with Keihin?

[scottly]

Oil doesn't leave dry soot like gas does. If the soot is dry, it means the cylinder is still firing: it takes fire to make soot. If the plug gets fouled bad enough, the carbon "shorts" out the spark, and the plug gets wet. Also, be aware that once fouled, a plug will tend to stay fouled, even after the original problem is corrected. I burn the black off with a propane torch.
Also, what is the condition of the air filter? A plugged paper filter can cause a rich condition, but it usually manifests itself more at higher RPMs.

[Herr Supersport]

What I wanted to say: even if the carburetors are not overflowing, the fuel level can be too high. Some of the air holes in the idle jets are then inactive, and the mixture becomes very rich.

Sorry, I didn't realize that the German forum is only accessible with registration. I'm stealing the essential pic and try to post it here, hoping the author doesn't mind.

The fuel-tube method gives you clarity about what is happening in the float chamber.

[Herr Supersport]

End consensus was: Keyster valves stick when they are fully extended.

My own experience is that a precise adjustment of the fuel level was not possible with the Keyster parts, and my 400 always ran rich.
After installing original valves, these problems were resolved.
BTW: Fuel level with Keihin brass [set at 21mm] was lower than 3-4mm below gasket seem.

[jonda500]

You could try putting the original exhaust system back on?
Seems like for some reason you have not enough air or too much fuel...
Following this with much interest, good luck, John

[scottly]

You could try putting the original exhaust system back on?

John, maybe we should wait for Mike to do the clear tube test before making guesses?

[mikestebbins]

Scottly,
I've been cleaning plugs with a short soak in Easy Off oven cleaner followed by scrubbing with a metal bristle brush, then clean with carb cleaner. Good call on the air filter. It looks to be in good shape and clean, but I know that that doesn't mean a lot. I haven't worried much about it because I've had the same problems when I was running with pod filters and with nothing on the back end of the carbs, so I assumed the air filter wasn't the real culprit. But, I will stick a new filter on, might as well eliminate that variable.

Daniel,
Thanks for providing that picture, the explanation, and your experience.

John,
That is a good idea, but, unfortunately I have already eliminated that as a possibility. The exhaust is the one place where I broke the cardinal rule that you should never sell a part that you're replacing until you're done with the project. The exhaust was in good shape, I knew I wanted a different pipe, and I saw what they were going for on eBay... I would change this decision, if I could

[jonda500]

You could try putting the original exhaust system back on?

John, maybe we should wait for Mike to do the clear tube test before making guesses?

Good call to go through the steps in the most logical order doing one change at a time.
- I thought as long as the fuel levels in the bowls were anywhere between 0 and 4mm from the top that it wouldn't be able to make it run that rich? If the level is 1mm or more higher than the seam it should piss out the overflow tube.
John
B.T.W. There are many very knowledgeable people here, but Scottly I have to say, you are in my opinion the wisest person on this forum.