Honda CB350 - No spark and rattling/knocking

Hi all,

I'm very new to all this so please excuse my ignorance.

The short story is I recently purchased a 1973 Honda CB350G. Pre-purchase inspection showed compression in both cylinders was OK (170 PSI in the left and 190 PSI in the right) but I was told the left cylinder was not firing and it was not the plug. Therefore, I did the following:

1. Removed both plugs from the cylinders and kick-started the engine - I only got a spark from the right plug;

2. Swapped plugs to test the left plug in the right spark plug lead and got a spark - so both plugs are good;

3. At this stage I wanted to keep the inspection simple before having to remove the fuel tank so I opened up the breaker point cover (image attached); and

4. I started the bike to view the breaker points in action and only saw a spark coming from the left point (video link: https://www.youtube.com/watch?v=Hv-7_m3ERoU)

Once I removed the breaker point cover it seems like the left cylinder started to fire because the left exhaust was warm to touch. However, I also started noticing a terrible rattling/knocking sound (listen to about 8 seconds onwards in the video - I feel like it sounded a lot worse in person) so I cut the power and haven't ran it since.

Any thoughts or working diagnoses would be incredibly helpful. Thanks for your time.

~Dru

Let's start with the easy things. That left plug cap and lead look a bit shonky. Remove the tape and then unscrew the plug cap and put it close to the motor and kick it over watching for sparks from the end of the lead. If ir sparks, the plug cap is dead.

If still no spark, test the coil for resistance with ignition OFF. primary side should be around 3-5 ohms and the secondary should be somewhere around 12-15,000 ohms.

If that all seems OK, you can do another quick test. Rotate the motor so that the left (forward) points are closed. Then remove both plug caps and lay the HT leads on the motor. Turn on the ignition and flick the points open and watch for spark on both plug leads. Repeat for the other set of points (the rear or right set). As you flick the points open one plug lead should spark.

With the Honda twins, losing spark on one cylinder is a classic symptom of your battery nearing the end of its life. If teazer's tests are inconclusive, trying running some jumper cables from your car (keep the car turned off) to your bike and see if that changes things.

Thanks for the replies.

Sonreir you're right. The battery I have in there is old and although it's holding a charge it's constantly in the low 12v range (i.e. 12.2-12.3v).

It looks like the power lead for the left cylinder (front point) was grounding out on the breaker point cover because as soon as I removed the cover I was getting spark from both plugs.

The rattling/knocking has seemed to have disappeared but the engine seems to be idling very high (unfortunately I can't confirm this because the tach gauge is old, broken, and unreadable).

Also, the bike is now smoking out the left exhaust flange. I've decided to stop running it until I do some serious tune ups. If anyone has any more input/suggestions your help is always appreciated. Thanks!

Sounds like you have a few things going on. High idle indicates leaky intake. Have you been through the carbs and checked all the rubber from the air cleaner to the cylinder - should be soft and crack free.
Since you are changing your battery, I would also go through and clean all the connections with Deoxit of something similar to make sure everything is getting enough juice. Dirty connections = high resistance which can lead to all kinds of quirky symptoms.
The smoke could just be oil burning off if anythings dripped onto the head and it hasn't been run in a while.
Have you checked/gapped your valve clearances?

Thanks jrogers.

Quick update.

Before getting into valve clearances and ignition timing I ran the bike to show my sister's husband (who knows a lot more about bikes than I do) how it was idling high. Unfortunately I can't tell how high it idles because the gauges are broken.

Turns out the throttle cable was set pretty tight, so tight in fact that when you turned the handle bars it would pull the throttle cable. We loosened the throttle cables and tried it again. It continued to rev very high.

With the choke ON it seemed to idle a little lower (which seems odd to me) but would rev erratically, sometimes die, and on one occasion spit fire out the left exhaust.

With the choke OFF the thing was a demon, reved high, and seemed like it wanted to keep climbing. We assumed this was because the bike was running rich and when we pulled the plugs our guess was confirmed - plugs were sooty black.

For air filters the bike has velocity stacks but these are very clean. Nevertheless, we ran it without the stacks and had the same high idle issue. The stock carbs looked good as well. We tried to adjust the air screws and idle screw but still no change. When we pulled the lower float bowls there was a lot of fuel in them but the floats weren't stuck.

Finally, we started looking for a vacuum leak. Turns out the left exhaust pipe had a lot of play. We pulled it off and found the PO had substituted ty wire for the standard exhaust collar...

Anyways, I have a new collar on order. I'm going to pull the carbs, clean them, set them, then I plan on going through the cam chain tension, valve clearance, and ignition timing before I try running her again.

Any thoughts or suggestions?

You can't tell if a bike is rich by looking at the plug unless the engine has been under load. Plugs will look black even if you're idling a lean bike.

You problem sounds a lot like an intake leak to me. Check the rubber intake manifolds and make sure they're in good shape. When you put the carbs back on, apply a thin layer of vasoline to the inside, where the carbs slide in.

Thanks for the advice Sonreir. I'll have a look at the rubber intake manifolds when I pull the carbs. I'll report back once I've got it all together again and let you know how it runs.

+ 1 on both issues that Sonrier raised.