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Discussion Starter · #1 ·
I have a 72 cb350 with Mikuni vm30's that I've never gotten to run right, but have definitely gotten closer than it is now.

Pulling it out from the winter, it fired up with choke, but only on the right side. After a few minutes, gas was spraying out of the cold left exhaust.

Spent today starting with oil change, then valves, then setting carbs with drill bit and syncing slides, then timing points, then checking all the electrical connections. At one point, with the tank off, it was running on both sides, both pipes getting equally warm. I put the gas tank back on, and started buttoning it up again, and it went back to a dead left side again. I figured the gas tank must be pulling a connection, so I took the gas tank off again and started going through it again. Battery is brand new, and showing 12.74 after all the testing.

Then I noticed it seems like little puffs of smoke are coming from the left spark plug hole. I can't be 100% sure, but it's definitely from the area. The plug is in tight, but the left side does have a helicoil in it. I ran a compression test, and got back 150.

I was in and out of the plugs this morning, and wonder if I just got lucky one time with a good seal that led to good fire, but all the other times there's a leak at the plug hole?

Is it okay to run more or softer washers on the plugs? I noticed my compression tester washer is smaller than the washer on the plug, and I wonder if that's making a better seal.

Tearing my hair out here, and will take any suggestions or help. Thanks.
 

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Can’t hurt to try. Too many washers will raise the plug up too high into the head. It could be cross threaded as well, so check the threads out carefully. The helicoil could be put in improperly as well, or failing.
 

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Discussion Starter · #3 ·
It could be cross threaded as well, so check the threads out carefully. The helicoil could be put in improperly as well, or failing.
I don't think it's cross threaded, as plugs go in smoothly by hand until the last 1/4 tightening. But the "failing" is what I'm worried about. It's actually one I'd put in last year to replace the one put in by the PO. I used red Loctite according to instructions, and everything seemed to go smoothly. Are there any other hints that it gives that it's failing? The last one just popped out on the plug. I didn't even know it was there until it came out.
 

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Discussion Starter · #4 ·
I'm trying to work through some of the assumptions I'm making as I troubleshoot, to be sure I'm not overlooking other possible problems.

1) if the plug is wet, and there is fuel blowing out the exhaust, then the carb is obviously getting fuel and the carb is not the reason for it not firing.
2) if the timing strobe lights up in a nice steady rhythm with timing spot on, it means the electricity is at least running to the caps, so the coils and condensers and points are not the problem, including any possible shorts in the points.
3) if I swap the coils and condensers and the problem doesn't follow, it confirms the coils and condensers are not the problem.
4) if I run a colortune, and can see the spark in the chamber, with all the above true, the electrical shouldn't be the problem.

If any of that is false, please let me know.

Thanks.
 

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#1 is FALSE.... A flooding carb COULD be the problem......The drowned plug then refuses to fire even if the coils are delivering spark.....

The FIRST thing I would do is check the oil for fuel smell ...(fuel thinned oil causes many problems up to and including catastrophic engine failure)
 
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Discussion Starter · #6 ·
#1 is FALSE.... A flooding carb COULD be the problem......The drowned plug then refuses to fire even if the coils are delivering spark.....

The FIRST thing I would do is check the oil for fuel smell ...(fuel thinned oil causes many problems up to and including catastrophic engine failure)
I changed the oil first thing today because of concerns about this. It was hard to tell if there was fuel in the oil, as the fuel is literally dripping out of the joint between the header and the muffler. I don't think there is though. Considering how many miles I've put on since the last oil change, the oil was practically new coming out.

Now are there signs of a flooding carb vs just regular gas getting sucked into and out of the cylinder? As well, when using the colortune, I can see the spark in the cylinder. Would it still spark if it was drowned? Lastly, if I left it with the petcock off and spark plug out, for everything to dry out, wouldn't it run for at least a little bit the next time around before it flooded again?

Thanks a ton for your input.
 

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Now are there signs of a flooding carb vs just regular gas getting sucked into and out of the cylinder? Regardless of why or how it gets in, unburnt fuel coming out the exhaust valve is undesirable....It is either just flowing in (carb flooding), or being drawn in during the normal intake process...or both...As well, when using the colortune, I can see the spark in the cylinder...Just seeing a spark does NOT mean it is timed correctly, or it has an available,suitable fuel/air mixture to burn, or it is properly compressed to explode... Would it still spark if it was drowned? Probably not, but maybe...No way to be sure, and could be intermittent.... Lastly, if I left it with the petcock off and spark plug out, for everything to dry out, wouldn't it run for at least a little bit the next time around before it flooded again?...Not necessarily....

The 350's are notorious for the points cover shorting out one of the point sets.....Particularly if using aftermarket points.....That alone could be your problem


Thanks a ton for your input.
See Blue comments above, and feel free to phone to discuss further....540-525-5199....Steve

 

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Discussion Starter · #8 ·
Hello Steve,

I've been running it with the points cover off to avoid the possibility of a short, and was hoping that the consistent strobe while strobe timing it was a sign against a short or electrical problem. I'll smell my oil tomorrow, go through all the simple bits one more time, then give you a buzz around noon, if that's okay. We're in the same time zone, and if a different time works better, let me know.

Thanks again,
John
 

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Sensei
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Assuming your 150 compression number is correct, and your spark is indeed occurring and correctly timed at LF on the left compression stroke, then the only reason there would be liquid fuel dumping out the exhaust would be such a great preponderance of fuel that there is not enough air (oxygen) involved to create a burnable mixture.... Remember, only fuel in the vaporous state and mixed with enough oxygen will burn or explode, in liquid state it can put out a lit match... (Fuel must be cooled enough for this experiment that it is still 100% liquid @ room temperature if only briefly...DO NOT TRY THIS AT HOME...)
Logically, if we are assuming normal carb function, then the volume of air passing through the venturi and lifting the fuel up and out of the jetting should create a burnable atomized mixture, since that is what the carb was designed to do.....Since the volume of fuel reaching the cylinder far exceeds that, the carb is obviously dumping in the extra fuel....It's either Flooding, or the controlling jet fell out and no longer limits the amount that can be drawn out through its passage.......

Easy test would be to swap carb positions......
 

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Surely the easiest test for a leaky carb float is to turn off the fuel and run it. Put in a good clean or new plug. This will prove a flooding carb if it runs ok (until the fuel level in the float chamber runs out, of course). This has all the hallmarks of a pinholed float or, as I have found on mine, the needle jet seating that has some damage. Mine had surface corrosion that allowed a dribble of fuel past the float needle.
 

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While that's true, I don't think a leaking float is the problem.....But it could be....

There is a specific reason I suggested swapping the carbs positionally.......
 

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Discussion Starter · #12 ·
Surely the easiest test for a leaky carb float is to turn off the fuel and run it. Put in a good clean or new plug. This will prove a flooding carb if it runs ok (until the fuel level in the float chamber runs out, of course). This has all the hallmarks of a pinholed float or, as I have found on mine, the needle jet seating that has some damage. Mine had surface corrosion that allowed a dribble of fuel past the float needle.
Just woke up to this, and it gives me so much hope. When I mentioned it ran fine with the tank off, the tank was literally off, so it was just running through what was in the float bowls. So I inadvertently ran your test, and I guess it points to flooding carbs.

The carbs are Mikuni vm30's, which are new in terms of use (probably less than 300 miles), but not in terms of age (about 7 years). I'll give it another once over today, especially the needle valve.

Thanks so much.
 

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Discussion Starter · #13 ·
...Since the volume of fuel reaching the cylinder far exceeds that, the carb is obviously dumping in the extra fuel....It's either Flooding, or the controlling jet fell out and no longer limits the amount that can be drawn out through its passage.......

Easy test would be to swap carb positions......
I'd been ignoring the carbs, because it was #1 on my list of assumptions. Thanks for setting me straight on that. I'm pretty comfortable with getting in and out of the carbs, so I'll give them a deeper once over today. If a cleaning doesn't work, I'll swap the positions.

...
1) if the plug is wet, and there is fuel blowing out the exhaust, then the carb is obviously getting fuel and the carb is not the reason for it not firing.
2) if the timing strobe lights up in a nice steady rhythm with timing spot on, it means the electricity is at least running to the caps, so the coils and condensers and points are not the problem, including any possible shorts in the points.
3) if I swap the coils and condensers and the problem doesn't follow, it confirms the coils and condensers are not the problem.
4) if I run a colortune, and can see the spark in the chamber, with all the above true, the electrical shouldn't be the problem.
Are there any other assumptions I have wrong?

Thanks,
John
 

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Discussion Starter · #14 ·
Went through the left carb, paying attention to the float valve. Nothing obvious jumped out, but when I got it back in, the left side is now running. Right side pipe is cooler now, and getting some backfires. Assuming I need to sync things up again and go through it all with a fine tooth comb. Now that I know both sides CAN run, I just have to get them to play nice together.

Steve, thanks so much for your insights and offer of help over the phone. I'm going to postpone that call for now, and get back into the garage to see what I can do. I've been close for 2 years now, so I'm sure I can use your knowledge again very soon.

jjdugen, thanks for the tip. Along with Steve's insights, it pointed me in the right direction I was overlooking.
 

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How old is the battery? I was getting a miss and a few backfires or running on one cylinder. I could see the sl350's neutral light struggling too. Even with the charging system going (3k+ RPM), the bike was failing. New battery fixed it for me. Battery was charging ok on the charger but after 4+ years, it was unable to handle the load.
 

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Discussion Starter · #16 ·
Brand new battery this spring. The old lead acid was showing 12.5 after charging and I didn't want to play with maybes. Picked up an agm (yt12c, it's a perfect fit).

With a carb sync, both sides are running great at idle. I'm back to chasing my stutter while cruising. Started a new thread in "electrical" and hoping I'm making some headway there.
 

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I ran into this on my CB77. I to noticed puffs of smoke and oil coming from the spark plug on the right side. to make a long story short it turned out to be combustion gas leaking out, not through the spark plug threads but next to them. The head on a 305 has a cast iron liner / combustion chamber inside the aluminum head. Over time the two metals may separate causing oil and combustion gas to leak around the spark plug hole. To check I would use a magnet and see if the spark plug hole threads are magnetic if so then the 350 may also have the cast iron combustion chamber and the known separation issue as the 305s, 250s and other early bikes have. I bought a kit from a guy Arie Boers, [email protected] on the Honda305 forum, he made a special cone shaped washer that was sealed with JB weld that solved the issue. This could be what is going on with your bike.

PS: I found this info on the issue http://www.honda305.com/forums/oil-at-spark-plugs-t7635326.html



This info is via Bill Silver:

"SL #65 6/1/66 HONDA MOTORCYCLE SERVICE BULLETINS
OIL SEEPAGE NEAR SPARK PLUGS IN ENGINES WITH ALUMINUM CYLINDER HEADS

American Honda receives occasional complaints of the tendency for certain models with
aluminum cylinder heads to seep a small quantity of oil near the spark plugs. In some cases,
dealers have replaced cylinder heads in an effort to eliminate this seepage. This bulletin is
intended to clarify our policy in relation to this problem, and to suggest some countermeasures
that we have found to be effective. In cases where it can be confirmed that the spark plug sealing
washers are not leaking, accumulation of a stain, or oil residue, near the spark plugs can be traced
to oil seepage from the joint between the combustion chamber "skull" and the aluminum cylinder
head casting. Since the spark plugs are threaded into the iron skull, rather than into the soft
aluminum casting, this joint must "come to the surface" near the spark plugs. Although it is no
mystery how oil reaches the outside of the engine, it is difficult to determine the source of the oil.
Apparently, oil reaches the joint from the oil-bearing chambers through internal porosity in the
aluminum casting; such porosity is extremely difficult to avoid. Once oil enters the joint, it has an
almost unimpeded leak path to the outside because the skull is not bonded to the head casting.
Our studies have shown that machines experiencing this problem can be graded into three broad
categories, based on the severity; each category should be dealt with in a different manner:

1. A stain or oily residue collects near the spark plugs over a period of several days or weeks.
Seepage of this magnitude should be considered a normal, inescapable consequence of the
cylinder head design, and no repair should be attempted. Customers complaining of such seepage
should be assured that no defect exists; suggest more frequent cleaning of the engine.

2. More severe seepage causes definite accumulation of liquid near the plugs; following a hard
run, oil droplets or streaks can be found on the air cleaner covers, etc. Although a "defect" is not
necessarily indicated by this seepage, countermeasures are often necessary to satisfy customers.
We have found that seepage can be slowed or stopped in the following manner: a) Remove spark
plugs and completely clean the region around the plug holes so that the joint between the skull
and the casting can be seen. b) Using a dull punch, punch a ring of depressions, tangent to each
other, in the aluminum immediately outside of the joint.

3. Liquid oil "bubbles" from the skull/head casting joint, puffs of vapor can be seen when the
engine is suddenly accelerated. In these cases, a definite defect is indicated, i.e., actual
separation of the skull and head casting. In most cases, however, the cylinder, rather than the
cylinder head , is the faulty part. We have found this problem to be most commonly related to
"sinkage" of the cylinder sleeve in the cylinder casting, such that the upper surface of the cylinder
sleeve is below the upper surface of the cylinder casting. When this "sinkage" occurs, the
Honda 250-305 Super Hawk Restoration & Maintenance Guide
combustion chamber skull is not forced into intimate contact with the cylinder head casting and
severe oil or vapor leakage, near the sparkplugs can occur. The recommended repair is, of
course, to locate and replace the faulty part.

Source: SL #68 4/29/66 HONDA MOTORCYCLE SERVICE BULLETINS"

Phil
 

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Discussion Starter · #18 ·
This is why I love these forums. Thanks Phil, this is amazing information.

I think I fall into the #2 situation, where it's not necessarily a defect in normal running, but I wonder if having the cylinder full of fuel was forcing the "puffs of vapor". Now that I've gotten what I assume was a bad float valve straightened out, the puffs seem to be gone. But I've always had a few drips of oil after every ride, which I always thought was a bad gasket or top end job. Actually, it probably still is. Go with the simplest answer, right?

I don't understand the dull punch/ring of depressions though. Any idea what the thought process is behind that?
 

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Ya the dull punch was to expand the aluminum around the spark plug hole to seal up the leak. In my investigation I didn't find anyone that recommended that solution in fact I found plenty of people that said it didn't work. and caused damage to the head. That is why I used Arie Boers's fix. Mine was leaking a lot of oil and vapor. it has been dry since then.
 

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Heliocoils are not meant for sprakplugs. It can work but a thread bush works way better.
 
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