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Discussion Starter · #1 · (Edited)
Hey all,

this is my first post on the forum, hoping you all will have some wisdom to help me out here.

I have a 1975 CB350, it was given to me by my uncle and it has sat in my parents barn out of sight and out of mind for the last 4 years or so while i have been working on my degrees. I have been making an effort to get it running again. So far i have replaced the battery, cleaned the carbs, and put in new plugs. The bike at this point will idle with a little convincing, but it dies if you give it the slightest amount of throttle. I noticed that the exhaust coming from the left side is cool, while the right side is warm. I have tried cleaning the carbs a couple more times thinking it was a lack of fuel supply, but nothing seems to help. My dad noticed the crankcase was under pressure when it was idling, and he think that the rings in the left cylinder are stuck to the piston, so it is not getting any compression. I do not have a compression gauge, nor do i have access to one. Does this seem like a likely diagnosis? I am wroth to drop the engine and do a rebuild or disassembly. I have a tough schedule and will be transferring to a school where i will not have access to a garage or shop to store the bike, so it will go back to my parents till next summer if i do not get it working.

Any thoughts are much appreciated, or things to try that might help.

edit: just pulled the drain plug in prep for an oil change, quite a bit of fuel was in the crank case. To my knowledge this reinforces the lack of compression theory?

thanks,
zeke
 

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Take one plug out and using the kick start note the effort it takes to kick the engine over. Put that plug back in and pull the other plug and do the kick start comparing the effort needed for each side. It isn't definitive but if the compression difference is large you will certainly be able to tell.

Check for spark at both plugs and swap plugs to see if the problem follows the plug.

Welcome to the forum and good luck with your bike ..... it might be best to park and store it properly if you will have limited time or space to work on it.
 

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Welcome to the forum. Find the cause of the extra fuel going into the crankcase. I've read many posts of damage to the cam shaft bearing blocks from lack of lubrication (which fuel fouled oil causes). Replace with fresh oil and crank it with the kill switch on first to get the oil up there, then try again to start it.
 

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Compression Gauges are cheap. But if you buy one look for the shortest hose.
The Total capacity in the cylinder of these motors is small and you have to compress the air in the hose as well as the cylinder.
Long hoses tend to give false readings.

Also - ASK your local parts stores - Many Auto Parts Stores will Loan or Rent one cheap for the day. (expect to asked for a deposit)

A Proper Gauge really is the only way to tell if the motor has compression or not.

Honda 350 Spec's:
170 psi (+) = Good
160 - 150 psi it will run but not be as strong as desired.
150 psi or less = Rebuild
More than 10% Difference = Problem.

To do the test correctly:
Both Spark Plugs removed.
Choke - OPEN
Throttles WIDE OPEN.
Kick or crank with the starter motor until you get the highest reading
Record it.
Rinse and repeat on 2nd side

Add 1 TBLS of Motor Oil directly into the cylinder thru the spark plug hole
Repeat the test above.

A cylinder with bad and/or stuck rings will typically jump way up in compression as the oil sits around the rings and helps seal.

Conversely - a Cylinder with a Valve issue will typically not respond to the oil - No difference in compression before/after.
The oil will flow DOWN onto the Cylinder & Rings.
The Valves are at the Top the oil will not flow to them.

If you find the OIL helps - I would recommend you purchase some SEAFOAM or PB Blaster and Allow the Weak Cylinder to soak applying a treatment to it twice a day for several days.
It has been known to loosen up stuck rings.
It's worth a shot.

If you had fuel in the OIL your Carbs may be leaking fuel into the intakes.
This happens when the overflow tubes are blocked and the floats get stuck or don't seal well.
I would do all of your compression testing with the Fuel Petcock turned OFF and the Carbs Bowls drained.
Note: The Floats on these carbs were never the most reliable and it is recommended the Petcock ALWAYS be turned OFF when the bike is not running.

Also Turn the KILL Switch to OFF when doing these tests.
You do NOT want power to the coils unless the motor is running.
They WILL overheat and self-destruct.
 
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Discussion Starter · #5 · (Edited)
Thank you for the quick replies. I went ahead and picked up a pressure tester, and tested both sides. I got 160-157. This pretty much rules out a stuck ring unless i am mistaken. the non-firing side was 160. Went ahead and put new oil in it, not sure what else to try. Both sides have very strong spark, and both carbs have been cleaned thoroughly.

edit: just thought, maybe spark timing? is this an issue that comes up often or even a possibility? if so, is there a way to test it?
 

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More than once I have Tested for Spark and seen a Good Bright Blue Spark only to find the Plug would not fire under compression.
Most recently with my weed whacker.
Really ticked me off my arm was getting really tired from pulling and listening to it blip.
New Plug and it roared to life.
Lesson leaned - Again, you would think I'd listen to my own advice once in while.

Spark Plugs are cheap and you are going to need them sooner or later..

Go get a fresh set make sure they are gapped correctly and try again.
 

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Discussion Starter · #7 ·
This is pretty much a new set of plugs, put them in a week ago.

Tried swapping them anyways and the issue persists in the same cylinder.:confused:
 

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Discussion Starter · #8 · (Edited)
quick update, tried swapping condensers, and coils, but i got the same results. it seems like the left cylinder fires every once in a while, maybe every 5th time it should. but it is intermittent, so like 3 in a row then nothing for 15 or so.

edit: noticed one exhaust looks tinted blue and the other silver, perhaps this side has never run well, and therefore not as hot?

edit 2: tomorrow morning i will go check timing and gaps on the points. ran it for about 30 minutes tonight at idle and it seemed to improve slightly but not enough to rev above 2500 or so.
 

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Do you have a Timing Strobe Light to check the timing ?

I ask because you can see if the Advance is working.

It's possible that the advance mechanism is stuck.
It might be OK at Idle but as you increase RPM and it needs advance might be nothing is happening.
 

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Try pulling the plug cap up of the plug a quarter inch, for a stronger spark(forces the coil to build more voltage). With the air cleaners off, spray a shot of aerosol carb cleaner into the carburetor throat of the dead cylinder. If it picks up some rpms some more carb work is needed.
 

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Discussion Starter · #11 · (Edited)
Okay, timing is now set, checked, and double checked. same exact issues, maybe slightly smoother idle, but left cylinder firing inconsistently, and right firing always. still dies after reving to 2500 or so.

I do not have a strobe light, tried pulling plug wires off part way and couldn't get any different results without just killing it.

seems like advance mechanism is working appropriately from what i can tell by taking it off, cleaning, and lubing.

Starting to rip hair out of this one haha, pretty much stumped again. Any other possibilities?

Thinking about swapping jets/diaphragm between carbs to see if that changes anything, but they all seem to be clear. maybe ill swap whole carbs if i can get them to fit on opposite sides.
 

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Discussion Starter · #12 · (Edited)
okay, swapped carbs and the left cylinder started firing now. Definitely carb causing the issue!!! ill start swapping components between the two and if i cant get that to work ill buy a new carb and swap it out. thanks for the help all!!

Swapped jets, and the upper diaphram assembly, but still had persisting problem with the left carb. any ideas? i have soaked it in mineral spirits already, but i guess i could try again, if not, does anyone know of a good place to get a new carb?
 

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Did you read thru the Sticky POST in the FUEL Section for 350's on rebuilding the Carbs?

They really need to be soaked in TRUE Carb Cleaner not just Mineral sprits.

Also Re-Oil and/or Replace if needed the FELT Washers that Seal the Throttle shaft.
 
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