Topic: HELP 1977 cb750f bogging

[Bennie1977]

Need some serious help. Just picked up a 1977 CB750F and it ran alright the first couple times I rode it. Now I'm having serious issues. Bike boggs when I put it in first like it's not getting gas or air.  Checked fuel lines, fuel filter, and petcock, and it's getting gas to the carb. Took the pods it came with when I got it, off to see if it wasn't breathing. Carbs are soaked in gas. When I get it into high RPMs it runs "normal" but sounds like a lawnmower. Has no balls when it's rode until it hits high RPMs at full throttle. Bike dies when I ride it, and locks the back tire up when coming to a stop. Terminals are tight and that was a problem in the past. I love the bike, but if I cannot get it running properly, I will have to get rid of it due to lack of knowlage. Very new to bikes, I didn't exspect to have a project right off rip. I'm assuming I need to pull the carbs off and clean them. If I do that, I would like to put bigger jets in, but that is something else I don't know about, and don't want to get into just yet. Any help is appreciated.

[seanbarney41]

First you gonna have to learn how to confirm all maintenance issues are good.  Got a manual?  Then you are probably gonna have to thoroughly clean your carbs, which will most likely include replacing several rubber parts.  Put the airbox back.

[Sprocket75]

Hey Bennie...
Sean is right, you need to first go through a maintenance checklist, otherwise we will all be chasing our tails trying to help you.

..."Bike boggs when I put it in first like it's not getting gas or air.
Is this when you start letting the clutch out or simply when the transmission is put into 1st gear? Does it bog in any other gear?  (*Need a little more info here)

...Took the pods it came with when I got it, off to see if it wasn't breathing. Carbs are soaked in gas.
Do you mean pods are soaked in gas...? Or did you really mean carbs?

...When I get it into high RPMs it runs "normal" but sounds like a lawnmower. Has no balls when it's rode until it hits high RPMs at full throttle.
Hmmm...sounds like a lawnmower? Have you verified that each cylinder is firing? Checked spark plugs? Checked the temperature on each header pipe at the bend? Does the exhaust seem cool or hot when you put your hand over the outlet? Have you checked the timing? Have you checked/replaced/cleaned the points? Condensers?

...Bike dies when I ride it, and locks the back tire up when coming to a stop.
Im a little puzzled by this one. What specifically do you mean when you say "locks the back tire up when coming to a stop"? Do you mean the brake is locking up? Or the transmission? (*need more info here)

..."I'm assuming I need to pull the carbs off and clean them. If I do that, I would like to put bigger jets in, but that is something else I don't know about, and don't want to get into just yet.
Why do you want to put bigger jets in? Do you know what size jets are in there now? You certainly do not want to go changing things out before you even get the bike running properly. Thats just adding more frustration, in my opinion.

You really need to first get familiar with your bike, and get a manual. They are easily downloaded on this site. You absolutely MUST do the 3000 service before going any further. Also, for your own safety, I probably would't ride it anymore until you get it into safe, proper running order. Additionally, you could be causing serious damage to the bike.

You will get a ton of help and great information on this site...but please be willing to listen to what everyone here has to offer.

[ekpent]

 Both Seanbarney41 and I are from the Kalamazoo area, what part of Michigan are you at ?  Welcome to the joys of old bikes    and the forum.

[Grabcon]

I have a 1976 F that I just went through a similar issue. My bike has the round top carbs. I went through everything ignition and carb wise. I found that the pipes for cylinders 3 and 4 were cold. (Carb issue not ignition) After I went through the carbs twice I found that the breather tube for carbs 3 & 4 were plug. Not only the rubber tube but the passages that connect the carbs together.

Also thanks to Hondaman I found that the points plate was non OEM and I could not time the motor properly. I dug out an old OEM points plate and this allowed me to get enough movement in the plate to time it. The slots on the points plate were significantly different between the OEM and after market.

[flybox1]

what carb model do you have?
3000mi tuneup items done?
Pods or airbox?
what exhaust do you have.
post a picture....it'll help us see what youre working on, and might clue us in on a few things youve left out.

[Bennie1977]

Ekpent, I'm from Davisburg small town in the middle of Oakland county. It's not a joy just yet. Big pin in the ass.

Sprocket75, bike only Boggs when I let the clutch out. Pupils, and sputters like hell when it's not in gear and runs at high RPMs. I mean the boss are covered in gas, like I'm gettting way to much fuel. Just changed every plug, but I shoulda checked each plug for spark. Like I said. I'm a noob. Went with the NGK-D8EA plugs. Still have stock coil packs though. To be more specific, the back tire locked up from the trans when I was coming to a stop. Yes, I know how to downshift. Breaks are just fine. I have no manual, because I didn't foresee the problems so early.

Seanbarney41, I'm hoping like hell I don't need to replace any parts in the carbs. I've heard these things are a nightmare to sync back together.

As for the pictures, I didn't put the Harley parts on it, just bought it for the platform.

[Bennie1977]

Also, no idea what size jets are in it, was told bigger = faster. Idfk, I'm an old school car guy. Don't have the air box. No idea what a 3000mi tune up is. I can talk cars, not bikes.

[seanbarney41]

That bike needs new carb boots at least...nla from Honda, repros are available but pricy and suspect longterm quality.  Probably also need to replace accel pump parts in carb and all o-rings, that is another $100+  ..on top of that performance will always be compremised, if not totally  unrideable, with those cheapo pod filters...really just doesn't work without LOTS of tuning by someone knowledgeable.   I doubt the incorrect front brake set-up can be made to work properly.

Hate to be Debbie downer, but I would not waste anymore time or money on that bike.  Plenty of much more viable projects available in Michigan for less than you would spend to make that safe and dependable.  I know I have passed up several under $1000 750's just this summer because I already have too many.

[seanbarney41]

Also, no idea what size jets are in it, was told bigger = faster. Idfk, I'm an old school car guy. Don't have the air box. No idea what a 3000mi tune up is. I can talk cars, not bikes.

car knowledge is both a helpful stepping stone, because its all internal combustion, and a knowledge barrier because, it's only two wheels so  both wheels are REALLY important, thus handling and braking dynamics are TOTALLY different and much more important, these engines are tiny and will not tolerate hamb-fisting in the slightest, and as one result, were already engineered to maximize performance from the factory.  This is why the old "flip the air cleaner lid" for more air flow type tricks don't really work with japanese motorcycles.  You are basically dealing with very economically priced 1960's grand prix level technology here...

[Grabcon]

Have you checked the individual pipes to see if they all are hot? If 1 & 4 or 2 & 3 are cold then look at ignition issues. If 1 & 2 or 3 & 4 are cold then look at fuel.

For you since you are new the ignition systems on these bikes have cylinders 1 & 4 and 2 & 3 working off the same points and coils. If either have failed then it will show as those pipes being cold. The carbs at least on the round tops are in banks 1 & 2 and 3 & 4. The fuel supply and the breather tubes are independent for each bank causing the pipes for the bank to be cold.

What is a cold pipe? A pipe you can hold your hand on after the bike has run a bit, a few minutes. They will always have some warmth due to heat transfer from the head but they won't be so hot that you cannot touch them.

[Bennie1977]

Already have about 1100 into it. It ran pretty solid for about 3 weeks after I got it. Not great, but rideable. Really like the platform though, and would love to completely customize it for me, not nessicerrily wanting to sell it, so what I put into it isn't a huge concern. Before I keep reading this and assuming the worst I'm going to just clean the carbs and see what that gets me.

Anyone know of a good aftermarket carb set up with pods? Maybe a good 4 carb rebuild kit , and vaccume sync system too? Not familiar with bike part brands. Links would be appreciated.

Been doing some research, and the project seems absolutely doable over the winter.

[Bennie1977]

Update, come to find out the set screw on carb 1,3,4 is screwed all the way in. Going to buy a sync system and see what happens if I tune it. Worst case I'm still in the same boat and need to put more money into rebuild kit.

[calj737]

Bennie - I'm not a 750 guy, but Sean and Erik are, and they're closer too. Here's a few links for you:

http://manuals.sohc4.net/cb750f/
http://forums.sohc4.net/index.php/topic,65293.0.html
https://www.westernhillshondayamaha.com/oempartfinder.htm

The 3,000 Mile Service is described in the manual in the first link. It will walk you through the "maintenance" section of the bike and familiarize you with the systems, locations, and proper settings. You're in for some teeth gnashing as Sean stated due to pods on your bike.

The second link is for an indispensable book written by a member here, Hondaman (Mark). I know of no one who has a 750 project that hasn't bought it, used it, and depended upon it. Make the investment and you will quickly come out the other side of frustration and intimidation. And you'll save yourself heaps of pain, money and anguish by throwing parts at a bike.

The last link is for a Honda dealer online Parts Fiche. These too are invaluable. They will show the factory assembly, specs on certain parts, and the correct name/part number for searching. Just scroll through the different sections as needed.

And lastly, here's a link for the Clear Tube method for verifying the fuel level in your carbs. This is more accurate than a measurement, and may/may not be completely viable with your carb model (I don't know the model you have). Given you bought a "customized" bike, I would encourage you to use the links above, and go thoroughly through the carbs, ignition, and brakes. For your safety and sanity.

Relax, maybe you didn't want a project, but you have one now. And winter is coming, so you have ample time to correct and perfect this bike.