1979 Honda CB750F Super Sport (not running)

[RedLeaf]

Hello everyone! I just purchased my first CB750 yesterday and was wondering if I could get some tips? I have no experience with engines or motorcycles. I tried to start it, it turned over once and then no more (besides a few quiet sounds) I was wondering what could the problem be? I've tried to do some research but there is just so much to tackle I have no idea where to begin this trouble shoot. I'm planning on buying a fresh battery (as I was recommended to do) since this one is pretty week. What should I do after that? is there some stuff I should check first to make sure it's all connected? Is there some stuff I should just replace like the battery? I unfortunately don't have any volt testers or any advanced tools like that. Any help would be appreciated! (including other posts, some sort of manuals/step by step guide, and videos)

Something I noticed, in the picture I included there is a black tube that ends, and a smaller red transparent tube from the top. they are both disconnected and I believe they were meant to be in each other at one point. Does anyone know what the purpose of them is? could that be contributing to the motorcycle not turning over?

https://gyazo.com/c4f57aa66dc2d9d435374866a84b458e

 

[MickMick]

Hello everyone! I just purchased my first CB750 yesterday and was wondering if I could get some tips? I have no experience with engines or motorcycles. I tried to start it, it turned over once and then no more (besides a few quiet sounds) I was wondering what could the problem be? I've tried to do some research but there is just so much to tackle I have no idea where to begin this trouble shoot. I'm planning on buying a fresh battery (as I was recommended to do) since this one is pretty week. What should I do after that? is there some stuff I should check first to make sure it's all connected? Is there some stuff I should just replace like the battery? I unfortunately don't have any volt testers or any advanced tools like that. Any help would be appreciated! (including other posts, some sort of manuals/step by step guide, and videos)

Something I noticed, in the picture I included there is a black tube that ends, and a smaller red transparent tube from the top. they are both disconnected and I believe they were meant to be in each other at one point. Does anyone know what the purpose of them is? could that be contributing to the motorcycle not turning over?

https://gyazo.com/c4f57aa66dc2d9d435374866a84b458e

Hiya, the black pipe is a breather pipe that connects to a T piece in to the top of the airbox. Would not cause your issues. If the engine will not turn over, would normally be electrical. Normally you would use a battery tester or simply try and charge it. The battery you have is one that you need to maintain the electrolyte levels so may be low or end of life if left too long with low fluid. I am assuming that the solenoid is clicking when you try to start? If the battery and solenoid are good, it is down to the starter motor but these are quite robust but may need a service if burnt out or low bushes.
On the battery side, I changed mine to a maintenance free one as I was sick if taking the caps off during charging and topping up the electrolyte.
That ping pipe looks more like a fuel or vacuum pipe but I do not know those carbs so others may be able to advise.

 

[RedLeaf]

Hiya, the black pipe is a breather pipe that connects to a T piece in to the top of the airbox. Would not cause your issues. If the engine will not turn over, would normally be electrical. Normally you would use a battery tester or simply try and charge it. The battery you have is one that you need to maintain the electrolyte levels so may be low or end of life if left too long with low fluid. I am assuming that the solenoid is clicking when you try to start? If the battery and solenoid are good, it is down to the starter motor but these are quite robust but may need a service if burnt out or low bushes.
On the battery side, I changed mine to a maintenance free one as I was sick if taking the caps off during charging and topping up the electrolyte.
That ping pipe looks more like a fuel or vacuum pipe but I do not know those carbs so others may be able to advise.

Thanks for the reply! I jump started the motorcycle today and it cranked, and sounded like it was almost starting, but it just wouldn't. I have Murray Carbs which look relatively fresh, so im hoping it's not an issue with them. Could this be a problem with spark plugs and no spark? really hoping that it's a simple fix, and not one that will be so complicated I need to send it off to a mechanic.... I also wanted to mention, the oil light on the display is on. Could that contribute to the problem? could that BE the problem? As I said, the bike is fresh in my possession. Haven't touched the oil at all.

 

[MickMick]

I am afraid you need to do a whole lot of diagnostics. If it has been sat for too long, then changing the oil and filter is a must before attempting to start. Then take the plugs out and rest them on the engine and check for spark on all 4. If that is OK and the timing is good, it can only be fuel. Have you checked there is fuel in the carb bowls? As before, try and use your hand on the air filter box inlet when trying to start to try and increase the vacuum in the venturi chamber in the carb to help it suck out some fuel

 

[dirtdigger]

Before you do ANYTHING, check compression with a good gauge and if lower than 110 120 psi then you need to address that first. These engines are notorious for having the valves too tight holding the valves open and burning the valves. You can have all the spark and fuel in world but these things motors will never run or hardly run on lower than 110 psi. They need to be at least 150psi to run well and 170 to run right.

 

[RedLeaf]

Before you do ANYTHING, check compression with a good gauge and if lower than 110 120 psi then you need to address that first. These engines are notorious for having the valves too tight holding the valves open and burning the valves. You can have all the spark and fuel in world but these things motors will never run or hardly run on lower than 110 psi. They need to be at least 150psi to run well and 170 to run right.

Did an oil and filter change this week, friend checked spark plugs, they seemed fine (didn't check for spark since they looked like they were in good condition) we ran a compression test and here are the numbers: Cyl4 105 - Cyl3 25.... - Cyl2 115 - Cyl1 80 this was his first time using that compression tester, no sure if that would play a role in the results. What would you recommend I start with? I prefer start with the little, easier stuff first in hopes it's a smaller issue. So far I heard I only have 2 options. Valve adjustments or a cylinder rebuild/engine swap..

 

[dirtdigger]

If it is a good thread in compression tester then you have some major issues. Also make sure throttle is held wide open when doing test as well as a fully charged battery so you have consistent cranking speed. put just a small amount of oil in the cylinders and see if the readings change. simple thing first would be to check valves, if you have some that are tight you can adjust them so they are correct but it is no guarantee that damage hasnt been done to the valves/and seats.

 

[CDEK]

If it is a good thread in compression tester then you have some major issues. Also make sure throttle is held wide open when doing test as well as a fully charged battery so you have consistent cranking speed. put just a small amount of oil in the cylinders and see if the readings change. simple thing first would be to check valves, if you have some that are tight you can adjust them so they are correct but it is no guarantee that damage hasnt been done to the valves/and seats.

Isn't it best to do a compression test with the engine warm? I guess when first looking into the bike, it would be to see if the numbers are consistent?

 

[dirtdigger]

yes if the engine runs a warm compression test is good but this one doesnt so you have to start somewhere. This thing could be 200 degrees and the compression readings wont be any better anyway. You generally dont find much more than 5 psi difference warm to cold anyway if engine is in good condition.

 

[Motogrady]

Put a fresh battery in it before you try anything.
Just go to wal mart and spend 50 bucks.

Mine, I've had charging issues.
They start to get weak, battery wise, they are hard to start and run like crap.

A fresh battery, mine starts right up, and runs like a bat outa you know where.

Compression on #3 is a concern. 105,115, 108, not bad. But then a 25, not good.

Good luck guy.