Topic: 1974 CB350F - Fixin' Her Up for Daily Riding - Plugs fouled

[carnivorous chicken]

I pulled the boot off of the lead to cylinder #1 and took a photo. Is this what you would expect to see? Or should there be more exposed wires? I'd hate to trim off 1 or 2 centimeters only to find out that my leads are now too short haha. For whatever it's worth, the spark on cylinder #1 (when using a brand new plug) looks strong.

I'd trim that, maybe start with .5cm and see what it looks like. But yes, you should see more wire.

[Trevor from Warragul]

I've had my CB350F for about 30 years, so I might know a little bit about these bikes...

Mine ran beautifully for years and years, then leaded petrol was phased out. On modern unleaded petrol it ran perpetually "rich". In the end I went down one size on the main jets, and it's "pep" was restored.

Try running yours without the air filter lid, if the fouling lessens, then it's too rich.

Also, the float height is critical. The only way to correctly set them (IMHO) is to use a clear tube glued into a drilled float bowl drain screw. I ended up fitting socket head cap screws to my float bowls, so they can be removed without taking the carburetors off.

Trevor 

[themrbruceguy]

So I got a couple free hours last night to work on syncing up my carbs. It was a pretty straightforward process (and I love the Morgan Carbtune compared to my old blue-dial Chinese set) but I still have some hesitancies/concerns about whether or not this is all looking correct.

Upon starting the bike and idling around 1,200-1,400 rpm, the Carbtune was barely registering any vacuum at all. See photo below:


The bike was idling well, but wasn't yet warmed up. You can see the Carbtune struggling to show any indication of vacuum at this time.

So I decided to set the idle to ~2,800rpm which helped raise the stainless steel rods high enough to take a good reading. I am not sure if this is recommended/acceptable when syncing carbs, but I couldn't think of anything else to do (turns out that once the engine warmed up, the Carbtune started getting decent vacuum at idle. But I didn't learn this until I was basically done syncing lol)

Once the rpms were set to ~2,800, the rest was fairly straight-forward. I started out with cylinders #2 and #3 being pretty close together (within 2-3 cm Hg), but cylinder #1 was pretty low and cylinder #4 was pretty high. I brought cylinder #1 up to match #2/#3. And then brought cylinder #4 down to match the remaining cylinders. I am happy with how the synchronization looked at the end; all cylinders were easily within 2 cm Hg. See photo below:


All cylinders synced pretty well. This was taken after I dialed the rpms back down to ~1,400rpm.

However, my main concern (that I thought of after I was finished syncing) is whether or not the average vacuum value is acceptable? Please note that the photo above was taken with the rpms set at ~1,400rpm. I looked at my Clymer manual and it specifies that the average vacuum value should be between 16-24 cm Hg. My average value was about 15 cm Hg, which seems pretty low. Would it be worth it to re-sync the carbs and adjust the average value to ~20 cm Hg per the Clymer manual? Also, could the low vacuum be contributing to the bike's rich condition?

I've had my CB350F for about 30 years, so I might know a little bit about these bikes...

Mine ran beautifully for years and years, then leaded petrol was phased out. On modern unleaded petrol it ran perpetually "rich". In the end I went down one size on the main jets, and it's "pep" was restored.

Try running yours without the air filter lid, if the fouling lessens, then it's too rich.

Also, the float height is critical. The only way to correctly set them (IMHO) is to use a clear tube glued into a drilled float bowl drain screw. I ended up fitting socket head cap screws to my float bowls, so they can be removed without taking the carburetors off.

Trevor 

I can try this on my ride home from work later today, thanks for the recommendation The bike definitely seems to be running rich. Several things make me think this:

• The bike starts first kick no problem, oftentimes without the need of the choke.
• While this isn't necessarily a direct indicator that my bike is running too rich, it does make me wonder... If it's starting first kick no problem without the choke, perhaps the current (rich) fuel mixture is replicating the purpose of the choke?
• The very fouled condition of the spark plugs
• The fact that my past couple fill-ups have been an average of 28.6mpg
• I feel pretty silly riding the frontage roads all of the way to work (the little CB350F doesn't like interstates speeds very much) only to get basically the exact same fuel economy as my Subaru Outback. Hopefully the fuel economy improves a little bit after getting this rich condition sorted out.

I will follow up in the thread about whether or not removing the air filter lid seems to help improve the operation of the bike.

Thank you to all who have helped so far! I am learning quite a bit. I go back and forth between enjoying the project and then just wanting it to be done. It would help if I had a 2nd bike that was in good condition while I worked on the CB350F I've had my eye on some '83 GL650 Silverwing Interstates... We will see.

[Robbo]

I would re-sync them higher.  With the vacuum within spec, it’s one less thing to wonder about.  Set it between 20 and 24.

What you could do is take a test ride with it set at 15 and then re-sync it to spec and test ride again.  You should feel a difference in the throttle response and how the bike accelerates.


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[Trevor from Warragul]

I average about 45 miles per gallon...

[carnivorous chicken]

Dunno if anyone's asked yet, but what's your elevation?

[Trevor from Warragul]

I'm at 1,000ft elevation in Kansas City, MO if that helps at all.

[Trevor from Warragul]

I average about 45 miles per gallon...

I filled the tank today & worked out my fuel consumption. 58 mpg. The fuel consumption on all my bikes has improved since I moved to the country (no stop-start traffic).

[nor_al_67]

I had fouling problems with my 350F and switched to a Denso 4099 Spark Plug -X24ES-U.  I think it's a little hotter than the NKG. 

[Robbo]

Hi Jake, made any progress on the bike ?


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[themrbruceguy]

Hi Jake, made any progress on the bike ?

I must apologize for ditching this thread. Life got busy last year and house projects took priority with my wife being pregnant with our first child at the time. My daughter is now almost 9 months old! Crazy how time flies.

I got around to the Honda again this week and woke her up from her slumber. Went on a ride and filled her to the brim with 87 non-ethanol gas. Rode about 25 miles in total, 10 of those being on the interstate (which I haven't done for several years). She sounded really good when revved up to 6,000rpm+. Had a ton of fun and found myself smiling and giggling in my helmet. It was good to get out on a ride.

The spark plugs are still pretty fouled and I would imagine that my gas mileage is probably still terrible, so I still need to dig into that.

I would re-sync them higher.  With the vacuum within spec, it’s one less thing to wonder about.  Set it between 20 and 24.

What you could do is take a test ride with it set at 15 and then re-sync it to spec and test ride again.  You should feel a difference in the throttle response and how the bike accelerates.

I can't remember if I resynched the carbs to a higher setting or not last year. I will have to break out the Carbtune again to check.

[themrbruceguy]

Also, the float height is critical. The only way to correctly set them (IMHO) is to use a clear tube glued into a drilled float bowl drain screw. I ended up fitting socket head cap screws to my float bowls, so they can be removed without taking the carburetors off.

I have seen pictures of this method, but I'm curious to see what your setup looks like. What kind of hardware assortment does one need in order to perform the clear tube method for checking the float height?

[HondaMan]

In my experience with mercury-filled tube vacuum gages versus dial-type vacuum gages: the mercury-filled type must lift the weight of the mercury and they tend to read much lower numbers than the dial gages do.

It is also highly dependent on how long the air lines are to the carbs, how big those lines are, and how stiff (or not) the rubber of those hoses is: stiffer reads higher numbers, softer the opposite. Long hoses read lower numbers, short hoses higher numbers. All 4 hoses must be the same length, too.

There should also be restrictors (tiny holes, one in each vacuum line) in each hose, near to the gages, to reduce fluctuations in readings. The mercury type gages are a little more forgiving of not having them.

[carnivorous chicken]

Also, the float height is critical. The only way to correctly set them (IMHO) is to use a clear tube glued into a drilled float bowl drain screw. I ended up fitting socket head cap screws to my float bowls, so they can be removed without taking the carburetors off.

I have seen pictures of this method, but I'm curious to see what your setup looks like. What kind of hardware assortment does one need in order to perform the clear tube method for checking the float height?

You can remove the drain screws with the carbs on the bike -- as long as the drain screws are seized in place. Whenever I rebuild carbs I always make sure the drain screws come out -- usually all it takes is a little heat to free them up from the varnished, gummy gas.

To make the fitting, it's just another drain screw or one the same size, drilled out through the center. I JB welded a little brass tube in there and then bought some clear tubing to stick on the end. Take out the drain screw, put in the one with the tube, turn the gas on, and it will show you where the fuel level is once the carb fills. No crazy hardware needed. If I remember I'll post a photo of mine when I get around to it.