Topic: Help! 76 CB750 Dies At 60mph! WTH??????

[dusterdude]

hammer time, bob.

[eldar]

We are but the basic operation of engines has not changed as far as the mechanical aspect goes. It is like some of the new electronic ignition systems for our bikes. The advance is computer controlled on the dyna 200 I think. This allows spark to be timed to the engine better.

[HackNslashCustoms]

I removed the petcock and the tube screen/bag. It had a lot of rusty gunk caked on. I cleaned it off with a soft brush and gasoline.
Is this the only screen?

I do not see any type of vent in my "flip up" gas cap.

While taking apart the petcock valve, one of the screw heads broke off so I was not able to reassemble everything and see if this was a contributing factor. I bought some Stainless allan head screws to replace the old beat up ones. Now to easy out or drill the old one out geesh!

I again looked for a fuel filter and did not find one. I will be installing one asap.

[Bob Wessner]

I do not see any type of vent in my "flip up" gas cap.

Look around the middle of the flat round portion underneath. It looks like someone started to use the worlds smallest can opener and stopped just after piercing the metal. It's a very small hole.

[cd811]

just my opinion but those petcock filter are a pain...void it and install an inline filter

about the advance debate(lol)...I had a broke spring on the advance that give me very irregular advances...just to confuse things...lol

[Dennis]

I removed the petcock and the tube screen/bag. It had a lot of rusty gunk caked on. I cleaned it off with a soft brush and gasoline.
Is this the only screen?

I do not see any type of vent in my "flip up" gas cap.

I again looked for a fuel filter and did not find one. I will be installing one asap.

If you found all of that rusty gunk cakes on the screen, did you bother to rinse out the tank? I know I would (actually have done just that).

As far as other screens, Steve F already told you that they are above the fuel needles in the carbs. Remove fuel bowl, remove float and needle. Remove needle seat. Remove little screen from needle seat, clean and reassemble.

I use the in tank screen plus an inline filter on my 750F.

[HackNslashCustoms]

If you found all of that rusty gunk cakes on the screen, did you bother to rinse out the tank? I know I would (actually have done just that).

As far as other screens, Steve F already told you that they are above the fuel needles in the carbs. 

I did rinse out the tank. I will again look for the gas cap vent hole. And thanks for the clarification of which screens were in question.

Thank you all for your help and patience. I'll keep you posted. I hope I get time to work on it but it looks like I have to help family with hurricane damage and repairs because of SOB looters.

[HackNslashCustoms]

Don't forget to check the tiny screens mounted to the float needle valves. They can get clogged too.

That is where I'm at now. I took out the petcock, cleaned the rust colored gunk off the filter bag, broke the screw off when dissasembling, paid $21 to the machine shop to get broken screw out and tap the threads. I reinstalled it, put in the gas, started it and went for a test run.

NOW it bogs like crazy! I can't get over 3k rpms without it sputtering and bucking like a mule. The bike shut off on me about 1/4 mile away and we had to push it back.

I'm looking throught my lousy clymers manual for the float needle valves locations or diagarams. No luck, if anyone has any pics, or diagrams, It would be greatly appreciated.

Where are they located before I open this up? Please bear with the motorcycle/carb noobie. GRRRR... :-D

  Pictures taken from: http://www.cyclemaintenance.com/mechicscorner/cb750_carbs.htm

[Bob Wessner]

Have you rechecked the timing, to include verifying with a timing light, that the advance unit is functioning properly?

[mrbreeze]

Hey Hacknslash,I was just wondering if you are running your carbs with that hose looped on your vents like in your picture.If you are,then thats your problem. you have to vent your float bowls.

[HackNslashCustoms]

Hey Hacknslash,I was just wondering if you are running your carbs with that hose looped on your vents like in your picture.

Oh sorry I modified the post. That picture isn't mine I'm using it for reference so somene can tell me what to open up and inspect before I totally monkey with the wrong thing.

[Dennis]

Don't forget to check the tiny screens mounted to the float needle valves. They can get clogged too.

That is where I'm at now. I took out the petcock, cleaned the rust colored gunk off the filter bag, broke the screw off when dissasembling, paid $21 to the machine shop to get broken screw out and tap the threads. I reinstalled it, put in the gas, started it and went for a test run.

NOW it bogs like crazy! I can't get over 3k rpms without it sputtering and bucking like a mule. The bike shut off on me about 1/4 mile away and we had to push it back.

I'm looking throught my lousy clymers manual for the float needle valves locations or diagarams. No luck, if anyone has any pics, or diagrams, It would be greatly appreciated.

Where are they located before I open this up? Please bear with the motorcycle/carb noobie. GRRRR... :-D

Alright, as I said before remove float bowl, that's the "cover" on the very bottom of the carb with that big clip around it. Just gently pry the clip back until it clears the bowl and carefully lower the bowl. The float will be right there in front of you.

I don't really understand why you are complaining about the pictures in the Clymer book.
My Clymers has a pretty good drawing in the section titled CB750 K1-K6 and F Removal/Disassembly in the Fuel System Chapter. Float Item #16, pivot pin un-numbered, #13 & 14 identified as float valve set (Honda Terminology) are actually the fuel needle and seat. There is a small screen (not shown here) attached to the top of the seat on some models.

After you have remover the fuel bowl, remove the pivot pin -VERY CAREFULLY- SO AS NOT TO BREAK THE "LEGS" which it passes through. I use a tiny pin punch (drift pin or whatever you want to call it) and a very light weight hammer of even a small pair of pliers to tap the punch. Some people use very small nails as a drift. Remember to tap very gently, that's all it should take to move the pin. Any hard tap can break the carb. Use spray carb cleaner or penetrating oil to loosen the pin. When the pin is removed the float will be free and the needle will fall out. Do not attempt to adjust anything until you check carefully and determine that this is really needed. Floats do not usually get out of adjustment all by themselves. Just clean and check for now.
Check the little plunger in the bottom of the needle to be sure that it moves freely, it is spring loaded.
If I remember correctly the seat is a hex brass style shich screws out . I use small 6 point sockets.
Check the seat and the screen located above.

ANY QUESTIONS???

[HackNslashCustoms]

Awesome thank you! I'm sorry you had to dummy it down. But, unfortunately thats where my skill level is on these at this point. Don't worry I do learn though. Thanks again! I'm going to get on em in the morning.

[Dennis]

I'm sorry you had to dummy it down. But, unfortunately thats where my skill level is on these at this point. Don't worry I do learn though. Thanks again! I'm going to get on em in the morning.

OK, be sure to take another look at the pictures. And be very careful not to break anything which does not come apart easily. I already warned you where you can get into EXPENSIVE trouble.

[enforcer]

SeaWeb:

Where did you get those handlebars?  They look a little bit different than the clubmans that Carpy uses.  How do you like them?

Thanks,

Enforcer

[HackNslashCustoms]

Have you rechecked the timing, to include verifying with a timing light, that the advance unit is functioning properly?

OK, here is where I'm at:

1. Checked gas cap vent, no obstruction.
2. Removed & Cleaned Gas tank.
3. Removed petcock (had crud on filter screen) checked fuel flow & reinstalled.
After reinstallation it would not rev over 3k sputtered and died. (Pushed bike home)

4. Cleaned the Float needle valve screens. Mild rust colored crud on them. [when removing the float bowls, the left 2 had fuel in them, and the right 2 had none. Just curious if this is normal or indicated another probleml]
Bike still won't stay running. I kick start it (electric start issues I haven't gotten to yet), it will run a second or two and then die. If I turn the throttle while it is trying to start, it totally *coughs* at me and won't start.

Under troubleshooting the honda manual suggests a clogged slow jet as a possible cause. Is this something that can be inspected without having to resynch the carbs or cause other problems? It says it can be removed with a flat head screwdriver.


My thoughts are: 1. To drain the tank again and put in new gas. There could have been moisture in the gas can I drained the fuel into since it was borrowed.
2. Reinspect fuel lines and flow up to the carbs.
3. Pull and inspect spark plugs
4. Then Check the base and timing if I can get it to run. If you have any suggestions thanks, as I'm learning as I go on this one. I'll catch up soon I promise.

Oh and I take back what I said about the Clymers book. The carb diagrams and a few others were cut out and tucked in the last few pages of the book. Now I can make sense of it! Woo hoo!

[TwoTired]

4. Cleaned the Float needle valve screens. Mild rust colored crud on them. when removing the float bowls, the left 2 had fuel in them, and the right 2 had none. Just curious if this is normal or indicated another problem

Oh, that's a problem all right.  Doesn't it seem unlikely two cylinders would work if they didn't get gas?

Under troubleshooting the honda manual suggests a clogged slow jet as a possible cause. Is this something that can be inspected without having to resynch the carbs or cause other problems? It says it can be removed with a flat head screwdriver.

You won't have to resynch for a change there.  But, if they aren't getting gas, who cares (yet) if they are plugged or not?

My thoughts are: 1. To drain the tank again and put in new gas. There could have been moisture in the gas can I drained the fuel into since it was borrowed.

Water in the gas will not combine.  It will stay separate and collect at the bottom of any fuel reservoir.
But, you can't expect the engine to run on water.  The oil companies would be pissed if it did!

[mrbreeze]

You either have blockage in your tee fitting going from carbs 1&2 to 3&4 0r your floats are stuck closed on 3&4.Thats if your set up is like mine...one hose from the petcock.

[seaweb11]

OK, I’ve watched this thread for a while now.

$200.00 towards your local Honda shop will probably cure your ills. Get your wiring worked out 1st though.

It's not that I don't wrench on my bike...I DO!  When it breaks...........it is after all a K8.
But at a certain point, it just seems good karma to get it fixed so I can ride and smile , and sleep at night.

Sometimes it takes a Mechanic, admit it, your pissed and not loving your bike right now. Imagine the grin on your face when you pick it up and ride off into the sunset

There will be a lot more issues you WILL be able to sort out,  for years to come.............



Just my thoughts on these troublesome prblems that come up.

[Dennis]

OK, here is where I'm at:

1. Checked gas cap vent, no obstruction.
2. Removed & Cleaned Gas tank.
3. Removed petcock (had crud on filter screen) checked fuel flow & reinstalled.
After reinstallation it would not rev over 3k sputtered and died. (Pushed bike home)

4. Cleaned the Float needle valve screens. Mild rust colored crud on them. [when removing the float bowls, the left 2 had fuel in them, and the right 2 had none. Just curious if this is normal or indicated another probleml]

Under troubleshooting the honda manual suggests a clogged slow jet as a possible cause.


Oh and I take back what I said about the Clymers book. The carb diagrams and a few others were cut out and tucked in the last few pages of the book. Now I can make sense of it! Woo hoo!

 

It is not normal to have varying amounts of fuel in the different carb bowls. The clogged pilot jet is not even worth looking for until you have determined that you have fuel.
You have demonstrated that there was a fuel problem when you first removed the carb bowls.
Are you getting adequate fuel flow now? And is it getting into the carb bowls?
Open drain screw on each carb bolw and be sure to see a steady flow. Then shut off petcock and drop each bowl to see amound of fuel and check for particulate.

[mrbreeze]

Exactly!!! Don't worry about clogged slow jets. First worry about getting fuel to the bowls. If it gets to 1&2 ...Then why the hell ain't it gettin' to 3&4?

[mrbreeze]

I'll tell ya what Seaweb....If you're gonna run these old bikes...You better mechanic up because when you take a 70's model SOHC to a Honda shop...Probably 3/4's of the techs aren't familiar with it and they will charge you up the ass to work on it. The best bet is to find you an old timer on the side that is familiar with it.

[seaweb11]

I'll tell ya what Seaweb....If you're gonna run these old bikes...You better mechanic up because when you take a 70's model SOHC to a Honda shop...Probably 3/4's of the techs aren't familiar with it and they will charge you up the ass to work on it. The best bet is to find you an old timer on the side that is familiar with it.

As you well may know by clicking on my link, I do get my hands very dirty. I  also know when to get on with it.
I have both a mechanic at the dealership that loves my bike and an old dude accross the street with years of wisdom.
Not to metion all the help I had here when I was getting it together.

All I was saying is that there is a time to get it fixed before you go Mad!  There will always be another reason to remove the carbs and get aquaited with them again.  After 45 posts and it still isn't working or sinking in, perhaps there is a better way.

That's all

[Tony CB750K6]

Did you check your petrol filter already? ( if fitted).
I had the same problem: filter seemed to be clean enough, but after replacement the problem was sorted. Tony.

[KB02]

Okay, so this post is basically a Sum-up of what everyone else is saying: You have a fuel delivery problem. Something is clogged up somewhere.

Replace all your fuel lines. When you have the lines off, and with the float bowls removed, spray some Carb Cleaner into the fuel inlet on the side that is not getting gas and make sure that it is coming out  of both carbs.If it is not, pull 'em off and go through 'em.

And to defend SEAWEB (not that he really needs defending), if it's getting you frusrated enough, the local shop might not be the worst thing to thing about.

It all makes sence, though: When the engine needed the extra fuel to hit around 60, it was starving out; i.e. - the clog was getting bad. After doing some work, the clog moved or had gunk added onto it and it got worse; i.e. - the problem got worse. Now you have no fuel delivery at all.
Then again, sometimes these bikes defy logic and common sence.