Hello! I need help! My father's CB 750C has difficulties taking the gas.. It wont rev properly.. sputters and bangs .. Is it false air? Must we clean the carburetor?
Ignition is pretty good, new coils and the timing is set OK.. but there is oil at the pulser generator coils.. should there be oil?
We tried to videofilm it, so you could get a feel for it ..
The night before with would rev up to 6000 pretty fast when we did the timing, but when we went for a ride it almost wouldn't rev.. but if we drove carefully and low rpm it was OK .. And the next day it sounded so awful.. like something is hitting something inside the motor..
Sorry for bad explanation.. Im not that good at english, but I hope you get the picture..
Welcome to the forum. We have a softcopy of the Factory Service Manual (FSM) on the home page. Feel free to download it. It's in English (sorry about that). You'll find all sorts of info about your bike.
I would recommend going to the Tune Up section and run through that.
Looks like it's the stock airbox, and a 4 > 1 exhaust, probably similar to the 750F set up. It shouldn't be too far from the 750K set up.
I'd recommend going back to the factory settings on the carbs for mixture. I believe it was 1.5 to 1.75 turns out from the lightly seated position for the mixture screws. That's a good baseline to start.
Double check the electrical connections for your coils. Make sure the spark plug caps are securely on the wires. Sometimes they'll be a little loose, and you won't get all the voltage for the spark plug to ignite in the cylinders.
From the sound it's making, my first guess would be the carbs are out of sync, as you suggested. Before doing that, though, I'd probably pull the carb bank and give the carbs a GOOD cleaning, especially if the bike has been sitting a while. Varnish and gum can play havoc with these carbs. Re-Run says you can't get them clean enough.
Again, welcome aboard, and keep us informed. We'll help you if we can.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
Did the bike sit for a while, possibly for a year of more without starting? It seems fuel related so if you can, maybe dump some seafoam into the gas, probably half a can or so and a few gallons of gas.
Before doing that, pull out the spark plugs and see what they look like. Look at the insulator tip.
It seems like the bike is running pretty lean and if the insulator is really white, then that would be your indicator. I am guessing your dad never opened the carbs up before so I an betting they are dirty from old fuel.
If it still does this after a few good miles, then a deep cleaning is probably in order. When doing this, check all rubber parts for any splits or cracks. Those carbs have an air cutof valve if I remember correctly and if it has cracks in it, it can cause popping.
Check any fuel filters too to make sure they are in working order.
It is hard to tell from the vid but it also sounds like some metal contact but it could just be the audio.
When was the last oil change? Might be time for that too. if you can, drop your oil pan when you do the change and just check it for metal debris. Just in case.
We should have the manual here to download, so do that too.
The ride IS the adventure. The destination is just to get gas!
Wow thanks alot for the quick answers! We will try to read about the carbs and the stuff you said.. I dont live close to my dad, so it may take some time between the work on the bike.. and the updates to this thread :( But I'll post when we have (good) news :)
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
Messed up/out of sync carbs will make an engine sound like it is ready to fly apart. I would start with a good carb cleaning/re-build, and sync.
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."
1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
Sorry if you already posted it, but what year is the bike?
Do you have an auto-petcock (it's a mess of tubes sitting on top of your carb bank)? If you do, you might try bypassing it and just running fuel straight into the carbs. If your auto-petcock has gone bad then your engine probably isn't getting enough gas.
On Mon, Mar 12, 2012 at 11:33 AM, seestheday [via Honda CB750'S] <[hidden email]> wrote:
Sorry if you already posted it, but what year is the bike?
Do you have an auto-petcock (it's a mess of tubes sitting on top of your carb bank)? Â If you do, you might try bypassing it and just running fuel straight into the carbs. Â If your auto-petcock has gone bad then your engine probably isn't getting enough gas.
1981 CB750K
If you reply to this email, your message will be added to the discussion below:
Its my dads bike, so Im not sure what year, but I think its a '81.. And yes its got thing on the carbs.. looks like this
Is that the auto petcock? Or is it the accelerator pump? Because under on the number 2 carb there is no accelerator pump.. So I guessed they replaced it? Or am I way off? :)
Its my dads bike, so Im not sure what year, but I think its a '81.. And yes its got thing on the carbs.. looks like this
Is that the auto petcock? Or is it the accelerator pump? Because under on the number 2 carb there is no accelerator pump.. So I guessed they replaced it? Or am I way off? :)
Yes I think its stock air filter...
I will keep you updated :)
If you reply to this email, your message will be added to the discussion below:
Its not to see it on the picture, but the fuel line goes from the tank the "pump" and a line to the carbs.. and a 3. line goes to carb number 2 (Think its a vacuum line)..
Unless I miss my guess, that is a fuel manifold valve. It sits between the outlet of the petcock and the input for the fuel rail for the carbs. There is a third line that should go to a vacuum source (possibly near carb #2). The idea is that when the engine is running, fuel flows to the carbs, and when the engine stops (no vacuum) fuel stops. This allows you to leave the petcock in the ON position all the time.
You _can_ bypass it, but you you'll need to switch the petcock off when the bike sits to keep the carbs from overflowing. I had one on my bike, and it was only a problem if the bike wasn't run for 3-4 days. If it's ridden daily, it's not too annoying.
You might also want to put an in-line fuel filter between the petcock and the valve (or the input to the fuel rail, if you bypass the valve) to keep dirt out of the carbs.
So, is it running any better?
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
Sorry, I probably wasn't being clear. I meant to try bypassing it as a way to troubleshoot if it was the problem. Removing it completely has other implications as you state.
For what its worth, mine failed (rubber inside disintegrated), so I just removed it.
Sent from my BlackBerry device on the Rogers Wireless Network
Unless I miss my guess, that is a fuel manifold valve. It sits between the outlet of the petcock and the input for the fuel rail for the carbs. There is a third line that should go to a vacuum source (possibly near carb #2). The idea is that when the engine is running, fuel flows to the carbs, and when the engine stops (no vacuum) fuel stops. This allows you to leave the petcock in the ON position all the time.
You _can_ bypass it, but you you'll need to switch the petcock off when the bike sits to keep the carbs from overflowing. I had one on my bike, and it was only a problem if the bike wasn't run for 3-4 days. If it's ridden daily, it's not too annoying.
You might also want to put an in-line fuel filter between the petcock and the valve (or the input to the fuel rail, if you bypass the valve) to keep dirt out of the carbs.
So, is it running any better?
Luke M
Back on 2 wheels after a 30+ year break. Used to have a 1979 CB750L, sold it as a parts bike, now riding a 1984 VT700C. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
If you reply to this email, your message will be added to the discussion below:
Now we had time to test some more and we found out that the ignition amplifier (guess its also is called CDI) may be broken. The Haynes manual says it should show 0 V and 12 V when performing the test, but they both are showing 1,7 V and 12 V..
When we testet the ignition timing with stroboscopic light, it blinked very irregularly and almost stopped blinking when we rev'd the engine. So we dont suspect the carbs right now..
As I said we found out that the CDI boxes are broken. They are very expensive (310 dollars a piece) so we would like to know if anybody has a cheaper trick/idea to use something else.. maybe DIY or some other CDIs that can be used?
There is a trick about replacing the boxes with GM (General Motors) HEI coils. I'm not sure if it works on your application but it is definitely a cheap alternative. I'll see what I can find out
Locked
