New to the biker world

[sam9580]

Hey everyone my name is Sam and I just recently bought a 1981 honda CB750 it's my first bike and in need of some work. The body is in great condition just needs a fresh paint job. However, when I purchased the bike it ran just, not so well. I was wondering if maybe you guys might be able to help.

The issues:
When the bike starts you have to constantly rev the engine otherwise the bike dies out.

Since purchasing it when I go to turn the bike on you hear a click when attempting to turn it on but the bike itself never turns on.

The bike backfires. I have a hunch that has to do with what the bikes idle is set to but Im not quite sure how to adjust it.


Thats about all mechanically that is wrong with it other than that its just some electrical work like hooking up the turn signals and the brake and front light which ill deal with later but for now if anyone has any imput on those problems I would greatly appreciate the help.

Also I am looking for the repair and or service manual for this type of bike im not sure if its a f or a k class all i know is that its a 81 cb 750.

And I know im asking a lot here but does anyone know how to customize my seat cover. The seat has a few tears in it covered by duct tape. Although thats a classy look I was hoping to put maybe a white colored material on the seat instead of the classic black.

Thanks!
 

[Cabreco]

sam9580 wrote

Hey everyone my name is Sam and I just recently bought a 1981 honda CB750 it's my first bike and in need of some work. The body is in great condition just needs a fresh paint job. However, when I purchased the bike it ran just, not so well. I was wondering if maybe you guys might be able to help.

Welcome to the forum Sam, I recently joined myself & also have an 81...great bunch of guys here LOTS of info.

sam9580 wrote

The issues:
When the bike starts you have to constantly rev the engine otherwise the bike dies out.

Since purchasing it when I go to turn the bike on you hear a click when attempting to turn it on but the bike itself never turns on.

The bike backfires. I have a hunch that has to do with what the bikes idle is set to but Im not quite sure how to adjust it.

My first hunch would be this is a carb issue. Dirty, gummy, out of sync or all of the above. (don't rule out bad gas in the tank.)

sam9580 wrote

Thats about all mechanically that is wrong with it other than that its just some electrical work like hooking up the turn signals and the brake and front light which ill deal with later but for now if anyone has any imput on those problems I would greatly appreciate the help.

Also I am looking for the repair and or service manual for this type of bike im not sure if its a f or a k class all i know is that its a 81 cb 750.

The Service manual is here, you just have to choose which model addendum you need for your model (check the neck plate on the bike):
http://honda-cb750-s.456789.n3.nabble.com/NEW-Repair-manuals-April-26-2011-Updated-td685918.html
That was a godsend to me, I also bought the Clymer manual.
I have put the 750 Custom & K model owners manual in that thread as well. Mine is a K,
 

assuming that no one has modded your bike if it's a K it will have spokes, a rear drum and no tail piece.

The F looks like this:

 

sam9580 wrote

And I know im asking a lot here but does anyone know how to customize my seat cover. The seat has a few tears in it covered by duct tape. Although thats a classy look I was hoping to put maybe a white colored material on the seat instead of the classic black.
Thanks!

I would recommend skinning it gingerly, blowing the stitches & making a pattern. Once you make a pattern, anybody with a heavy duty sewing maching could sew you up a new skin. (ask some dry cleaners/ tailors who work on leather coats)

[sam9580]

"My first hunch would be this is a carb issue. Dirty, gummy, out of sync or all of the above. (don't rule out bad gas in the tank.)"


The carb seemed ok when I took it apart I couldnt clean out all the jets because some of them were just way to small so maybe that might be the reason why the bike dies down....Maybe the bike isnt getting enough fuel through the carb? is that a possibility?

Also why wont the bike start now? That has to be related to the starter mechanism. When I push the starter I hear a click but the bike doesnt start.

[Rodgil]

How good is your battery? When you only hear a click, it usually means your battery is low on voltage and is operating the starter solonoid, but doesn't have enough voltage to operate the starter motor. You would have to check it with a voltage meter.

[Cabreco]

IDK if you've been running the bike at idle, & starting it on & off but I found out (the hard way) that these bike will actually deplete the battery quickly in various ways. The batteries are NOT like car batteries, they are 16 amp. The manual says to hit the starter for 5 sec wait 10 sec before trying again, the draw must be big. The charging system only charges the battery over 1700 rpm, so at idle it's actually depleting the battery. If you shut the bike dowu using the engine stop switch &  leave the key on the headlight is still on...draining the battery, and the list goes on & on.
I hate the fact there is no kick start!

Chances are you killed the battery. You should get a battery tender or a 1 amp trickle charger. If you use a 10 amp car charger, only attach it for a few hours & not too often as you will cook the battery.

I wound up replacing the battery which was 4 years old.

[cheesehead]

If your bike has battery charging issues, let me know. I just got a 1981 K model about a month ago that wouldn't charge. I'd be happy to walk you through the process of testing each part of the charging system and diagnose the problem. All you'll need is a multimeter and some basic tools. As for any other issues, I am not (yet) your guy, but there are plenty on here to help you out.

[Cabreco]

Thanks Brian I appreciate it.

She seems to be ok. At first, when I was cleaning up her & not really riding her, I would kill the battery weekly. I'd shut her off & not turn the key off or I'd let her sit at idle after starting her a lot. After I started riding & I changed the battery, so far she's been ok with no problems.

But the checking procedure would make a great write up for the forum!

[sgtslag]

There are many things to do to clean up your ride, but let me start with the simplest.

1)  Get some SeaFoam (any auto parts department, or store, typically $9/16 oz. can), put 1 oz. per gallon of gas, into the tank, and run it for several minutes, to get it to circulate throughout the fuel system (jump start the bike, if necessary, as this will also help identify whether the starter motor is good, or not...); let it sit overnight, then start, and run it down the road, if possible, to push SeaFoam'ed gasoline through the fuel system -- it will dissolve varnish deposits in the fuel tank, floats, and the carbs.

2) Add 1 oz. of SeaFoam, per quart of oil, to the crankcase.  Check the color of the oil first!  If it is already dirty, then change it, first.  After adding the SeaFoam to the crankcase, run the bike, on its centerstand, for around 5 minutes, shifting through all five gears, while idling!  The SeaFoam will dissolve, and liquify, sludge, and carbon deposits, within the engine, clutch assembly, and the transmission.  After five minutes of idling, shut it off, and check the color of the oil:  if dirty, dark, or black (most likely...), change both the filter, and the oil, immediately!

3)  Check the voltage on the battery, resting (1+ hours after shutting the bike off):  if it is less than 12 Volts, your battery is discharged -- charge it fully, then repeat this test after it has rested, off of the charger, overnight.  If it again is lower than 12 Volts, your battery is toast.  I would recommend an AGM replacement, over lead-acid -- more money, but will last 2-3 times longer, won't discharge over the Winter, unless you have accessories that drain it (clock, or some other powered device running after the key is off), no bother with water levels.

4) Check the electrical charging system by connecting a volt meter across the battery, with the bike running at 2,000 RPM:  if the voltage is less than 13 Volts, you have a problem with the alternator, or the rectifier/regulator, or the battery cables.  DO NOT RUN THE BIKE WITHOUT A BATTERY CONNECTED!!! (Use a Digital Volt Meter, typically less than $10 at local DIY stores, Menards, Mills Fleet Farm, Lowes, etc., in the Electrical Department.  DVM's are far more accurate, and useful, than an analog Volt Meter.)

After that, come back, and let us know what you found.

If well maintained, these bikes' engines are good for 70k+ miles, without major work.  Add in an oil cooler (can be somewhat complex, and challenging), and you could double their life.  The oil temperature, running down the highway, two up, with luggage, on my '79, ran at around 250 F, which is the limit for dino oils, before they break down into carbon sludge...  Synthetic oils won't break down until around 450 F!  I added an oil cooler, and my oil temperature, same load, same conditions, dropped to around 210 F (meter is not terribly accurate) -- hot enough to boil off any water.  Synthetic oil may extend your ride's life by twice, or more, and it is a lot easier to add, compared to an oil cooler.   Shell Rotella T6 Synthetic (diesel oil, also rated for cars...), is around $26-$29 per gallon, at Wal-Mart; no friction modifers to affect your clutch; rated SM for automobiles; 5W-40 viscosity rating, for easy starts in any temperature.  You can also extend the drain time to 7,000 miles -- longer with oil analysis.  Just some food for thought.  Cheers!

[Cabreco]

The Seafoam was a GREAT idea. I did it to Fiona & pulled a multitude of sludge out of it. The downside is she developed a slight weap. (ok she piddles in the garage <LOL>)
The only thing I would add is, when you let it idle for 5 minutes or so, especially with the current heat in weather, get a house fan an let it breeze over the engine to keep it cooler. (My bike did overheat & stall because I did it in 98 degree heat) Scared the crap out of me.

BTW
Sarge, Any chance on you doing a write up (with pix??) and a parts list on installing an oil cooler?

[LukeM]

Are  the "piddles" oil or gas?  Oil would be OK, as SeaFoam will unsludge an engine, and sometimes the sludge acts as a seal. The good news is: SeaFoam may be able to swell the seals and gaskets in an engine, which means it may correct itself.  If it's the gas that's leaking, you'll want to fix that pretty quick. I just got done replacing the float bowl gaskets on my Shadow, as the one carb was leaking gas onto the exhaust pipe. Probably won't happen for the CB750, but leaking gas is a bad thing.

Glad "Fiona" is running better.  SeaFoam is great stuff.
Luke M

[Cabreco]

The piddle is definitely oil.
Fiona is just marking her territory in the garage...she has dillussions of being a Harley Knucklehead.
It's not bad at all, just a drop overnight. I put a cardboard box under her & it is negligable.

Winter project.

[Re-run]

If you know where it is from, you might be able to tighten. For instance, my oil plug would leak a bit because the threads were cracked. I was able to use a washer with a rubber coating to seal it.
You may just need a new crush washer or something.

The well maintained mileage is good but age is the other killer. My bike has about 20,000 and it needs a new head gasket I think. I have not tried torquing yet so that might do it. I am afraid to do it and snap a headbolt though as that would end riding for me this year as I usually have little time to work on things like that. So I let it weep just slightly and while it does not drip anywhere, it is on 2 fins and is a small trail, so not to bad.

[LukeM]

An old mechanic friend had a mantra: "If there's no oil ON them, there's no oil IN them."

It kinda works, but who wants to ride around on an old oil-smelling bike?  Bad enough the SeaFoam in the gas makes it smell like a diesel locomotive. :-)

It's not that bad.  
Luke M

[Cabreco]

LukeM wrote

Bad enough the SeaFoam in the gas makes it smell like a diesel locomotive. :-)

That's what that smell is???? I thought it was my truck!

[TOOLS1]

That leaking between the fins is the round rubber head bolt covers. They are under the camshaft, and are not too hard to replace (if you have a matic).
TOOLS

[sam9580]

Alright so I took the carb apart to look at it and also took it to a honda dealership just to take a peek at and the carb is in great condition so it cant be that. Any other ideas to why the bike isnt staying in idle and why its just shutting off. Also I cant get the bike to start i think its an electrical problem.

[Cabreco]

Start with the basics.

Gas
Spark
Juice

1. You have gas flowing to the carbs? If I am not mistaken petcocks have screens that clog. Gas filter, clog fuel line.

2. do you have spark at the plugs? Did you pull the plugs yet? , Bad coil?

3. I am assuming the battery is fully charged since it's an 81 with no kickstart.

Ruling out the battery & carbs next I would test the coils.
Before you do any testing make sure you have a fully charged battery. Test the ignition coils.
Measure the resistance between the primary (low-tension) wire and ground or ground terminal. It should be .5 to 1.5 Ohms. The primary wire should be the small wire going to the CDI box or points. Next measure the secondary (high-tension) wire and ground. This should be high, like 6000 to 13000 ohms. If the coil is out side the specs given in the shop manual the coil might be bad.

[LukeM]

Quick checks for electrical problems:

Start with a full charged battery. If you have to supplement your MC battery with one in a car, DO NOT RUN THE ENGINE WITH THE JUMPER CABLES CONNECTED. Car systems can put out more amperage than the bike can handle, and at the least, fuses will blow.  At the worst, other things might go *boom*.

Verify the battery post connections are tight, but not too tight.  The metal used for the posts is fairly soft, and I've seen a few broken posts over the years. Also make sure the main ground for the harness is up against bare metal on the chassis, and the terminals and hardware are good and clean.  After securing that bolt, it's nice to coat it with some anti-corrosion stuff.  I've used petroleum jelly, there's some spray-on stuff, and some goop you can get from the auto parts stores.

Verify the connections at the starter solenoid are good.  This involves a short heavy gauge red wire straight from the battery, a similar heavy gauge wire going to the starter, and 1 or 2 control wires coming from the START button.  When the key is on and you close the START switch, the solenoid should make a loud click, which means the contacts on the high current side have closed.  If not, it's possible to take the solenoid apart and clean the contacts.  Possible, but probably not advisable.  Replacements are not hard to locate.  If the solenoid doesn't click, you may want to check the continuity of the START button/switch.  

To test the starter, you can remove it from the engine, and connect it to the battery (bike battery if it's charged, a car battery IN A NON-RUNNING CAR is an alternative).  There WILL be some sparks, and the motor will torque around as it spins up, but it should spin freely.  If not, it's either electrical (bad brushes are a culprit) or mechanical (frozen or dry bearings).

It's a fairly simple system and not a whole lot to go wrong. The FSM has some good procedures as well.

Good luck, and I hope you get the carb problem sorted out.  I'm wondering if it's a carb sync issue: maybe the carb assembly is REALLY out of adjustment.  Better to get it idling first, then we'll worry about that.

Luke M