Topic: Bike no longer no longer starts

[Floshenbarnical]

Hi friends,

1977 CB750 SS projected was completed on Tuesday (happy 4th!) and it started first time. I adjusted tappet clearance, cam chain tensioner, vacuum synchronized carburetors, checked oil pressure, rode the piss out of it all day Tuesday and half of Wednesday. On my way home on Tuesday night, the bike basically #$%* the bed and totally lost power and I had to cruise to a stop by the side of the road, where it refused to start until I switched it to reserve tank. I figured that I was low on gas, and went home.

I live near Watkins Glen racetrack so I took a ride up there yesterday. Halfway there the same thing happened - bike #$%* the bed, I cruised to a stop and took a look at the carbs. Gas was flowing out of #3 drain tube. Gas was leaking out of air filter from #1 carburetor. I guessed that a float bowl was stuck or something so I opened the drains, whacked the carbs with the heel of my boot a few times, then closed the drains. Bike started, we got home. On the ride home, if I let my RPM drop to cruising speed (50mph traffic) then the exhaust sound would get "weird" and power would stumble. If I kept my RPM higher then the exhaust sounded normal. If I rolled off the throttle a bit, I would get some popping from the exhaust. This mostly cleared up by the time I got home.

When I got home I took off the carbs and adjusted the float height a few mm leaner. I didn't have much time so I didn't do clear tube test.

However, it doesn't start. It turns over, so its not hydrolocked. The battery has full charge. The float height is the *only* change I made.

Facts:

- 1977 CB750F2
- 4-1 exhaust with very open muffler
- K&N pod filters (airbox won't fit without undoing PO's frame modification, that's a project for next month).
- #40 idle jets (#35 is stock)
- #120 main jets (#105 is stock)
- Fuel screw 1.5 turns out
- Properly gapped NGK plugs

What am I missing? I'm really hoping I didn't wash my cylinders or get gas in my oil. Please help, and try not to give me too much #$%* about the pod filters - I don't have another option right now.

[scottly]

What do the spark plugs look like?

[Floshenbarnical]

#1 & #4 look like this

I should add - I put the float height back to where it was but the tiniest bit leaner

[Scootch]

Something to maybe check... Gas cap venting?

[scottly]

That plug looks pretty lean.

[newday777]

1st thought. Rust in the tank, even light surface will hold float valves open when they should be closed.
Is there rust in the bowls?
Is there rust in the tank?
Do you have in line fuel filters? If so, Are they under the carbs?(not good as flow will be cut off/slowed greatly,  at speed)

These pics are a K4 tank with light rust and the rust in the carbs from that little surface rust.
And after properly cleaning the tank of rust. I'm just putting the carbs back together after cleaning them.

[Floshenbarnical]

I have an in-line filter and the fuel going into it is clean as a whistle. The filter is under the carb, as with the petcock I have there isn’t space to get one in-line

[scottly]

Are you still running with the petcock set to reserve? Did you ever add any gas?

[Floshenbarnical]

Tried in both positions, tank is full of non-ethanol gas

I haven’t pulled plugs 2 or 3 yet, I wonder if they’re too fouled. Will report back

[Floshenbarnical]

Well I finagled hosing to allow the filter to be above the carb. I changed the spark plugs, all of which look a little lean except for #3 which was perfect oddly.

It’s alive! Again

So should I be putting in larger jets or opening my fuel screw to solve this lean running condition?

[scottly]

If you had to finagle the hose to get fuel into the carbs, it may have caused the lean condition. Also, you should verify the fuel levels in the float bowls with the clear tube. The mixture screws mostly affect the idle and very low throttle openings.

[Floshenbarnical]

Yes sir, that is on the menu tonight. Right now I have to put some food into my body

[Stev-o]

I have an in-line filter and the fuel going into it is clean as a whistle. The filter is under the carb, as with the petcock I have there isn’t space to get one in-line

- Inline filters have been known to cause issues
- Fuel is gravity fed, run the fuel lines as vertical as possible
- If/when it starts to stumble, open the gas cap to rule out venting issue

PS. Watkins Glen is a great race track and area. Was there a few years ago, and also hiked the Glen!

[Floshenbarnical]

Filters causing fuel starvation issues? I’ll take it off. There’s no grit or sediment in the filter paper and the fuel is crystal clear so I’m not concerned about that. My tank is very clean and I don’t let it fall below 50% filled really. There’s a non-ethanol pump open 24/7 a mile from my house.

Where is the best place to buy float bowl gaskets? I’m finding the quality of the rebuild kits to be quite poor even from “good” stores, and I only need gaskets. Most of the kits come with all sorts of nonsense I don’t need.

[Floshenbarnical]

Something to maybe check... Gas cap venting?

I missed this. What do you mean?

[Scootch]

The gas tank needs to be vented via the gas cap or there will eventually be a vacuum and fuel will not flow to the carbs. When the machine acts up or stumbles pop the gas cap to see if the issue clears up.

[madmtnmotors]

Has the filter screen inside the tank been removed or has it been abandoned in place (possibly packed with crud)? I favor a new in tank filter screen as opposed to the in line filters. I highly recommend Honda filters instead of aftermarket.

[Floshenbarnical]

Brand new petcock + filter screen.

Air does pass through the gas cap but I don't think it's very much. Previous owner lined the tank, with the gas cap on. I thought I got most of the crud out of the vent holes but perhaps not enough? I'll take off the O-ring and soak it in acetone tonight.

While I'm at it, perhaps you can answer my question I put in a different thread.

My cognito moto oil tank does not have the connection for the oil breather hose that attaches to the spigot on the back of the crank case

Should I attach a hose to this, run it up vertically a little ways, and cap it off? My theory is that a little length of hose will allow the vapors to collect and drip oil back down the hose into the crank case. I read from other threads that the hose that attaches to the spigot on the stock oil tank does not connect to the main oil reservoir. Currently it's misting oil onto my tyre and I'm going through oil awful quick

[willbird]

Brand new petcock + filter screen.

Air does pass through the gas cap but I don't think it's very much. Previous owner lined the tank, with the gas cap on. I thought I got most of the crud out of the vent holes but perhaps not enough? I'll take off the O-ring and soak it in acetone tonight.

While I'm at it, perhaps you can answer my question I put in a different thread.

My cognito moto oil tank does not have the connection for the oil breather hose that attaches to the spigot on the back of the crank case

Should I attach a hose to this, run it up vertically a little ways, and cap it off? My theory is that a little length of hose will allow the vapors to collect and drip oil back down the hose into the crank case. I read from other threads that the hose that attaches to the spigot on the stock oil tank does not connect to the main oil reservoir. Currently it's misting oil onto my tyre and I'm going through oil awful quick

I have kind of struggled with tank venting myself, I have a new 4:1 cap on my bike now and it is so so at venting.

Reply #87 in this thread shows the internals of that fuel cap...I thought I had it settled but it still acts goofy sometimes and I have to pop the cap open.

http://forums.sohc4.net/index.php/topic,192484.75.html

There is WAY more going on inside those caps than I originally thought when I soaked my complete original cap in EvapoRust.

[Floshenbarnical]

How hard is it to get a new one, lol

[Don R]

I have an in-line filter and the fuel going into it is clean as a whistle. The filter is under the carb, as with the petcock I have there isn’t space to get one in-line

 I've documented inline filters refusing to flow fuel. My theory is that moisture in fuel makes a gel that coats the filter media and gravity can't overcome it. Remove the fuel line after the filter and check for flow. I actually blew into my gas cap to make one flow to get home. (I looked both ways first, no one was watching).
 One more thought, the fuel level is often affected by grit under the float valve needle and seat. An F carb sometimes has a tiny screen above the float valve too. Those may need cleaned on occasion.

[willbird]

How hard is it to get a new one, lol

If you are talking about my gas cap, the new repro one acts exactly like the OEM one that I soaked in ERvaporust ;-). It looks prettier while not working correctly though LOL.

Bill