Topic: Just a fair warning and another ?

[kjdelaney]

I want to put it out there that i am not a mechanic ( I'm a carpenter) and I'm learning this as i go. I've always wanted to know more about how motors work and how to troubleshoot them. So what better way than buy an old motorcycle tear it apart, clean it and rebuild it. So please bare with me if i ask allot of ?'s as I'm sure they will come!

I've already tore down and I'm in the process of rebuilding my 1977 CB750k. I just finished putting on my new exhaust and it turns out i had a leak ( see my previous post). I cranked down the bolts and the leak has gone away. It sounds much better and there is no ticking on the top. However, the #3 and #4 cylinders are running MUCH hotter than the 1 and 2. I'm afraid to run the motor to long. It seems to me to be to hot. the last thing i want ti st seize the engine. Any suggestions on how to begin troubleshooting this?

[kjdelaney]

Here's another thought too. I do not have a center stand. I have my bike on a 2x4 that the side stand is resting on. The bike leans to that side so is there a chance that all the oil is dumping to that side and leaving the 3 and 4 side drier?

[Gordon]

The oil is distributed by the oil pump, but the gas is gravity fed to the carbs.  With the bike leaning to the left while running the 3 & 4 cylinders could be running lean (hotter) due to the 3 & 4 carbs not getting enough fuel. 

[bollingball]

I gotta ask. You do have a fan on it right? and where is it on the floor. I use one on each side not one of those box fans but a sure enought fast fan.
Ken

[seanbarney41]

motorcycle engines generally get too hot to touch after running for a few minutes...what makes you think that 3 and 4 are significantly hotter?

[Rob69]

I want to put it out there that i am not a mechanic ( I'm a carpenter) and I'm learning this as i go. I've always wanted to know more about how motors work and how to troubleshoot them. So what better way than buy an old motorcycle tear it apart, clean it and rebuild it. So please bare with me if i ask allot of ?'s as I'm sure they will come!

I've already tore down and I'm in the process of rebuilding my 1977 CB750k. I just finished putting on my new exhaust and it turns out i had a leak ( see my previous post). I cranked down the bolts and the leak has gone away. It sounds much better and there is no ticking on the top. However, the #3 and #4 cylinders are running MUCH hotter than the 1 and 2. I'm afraid to run the motor to long. It seems to me to be to hot. the last thing i want ti st seize the engine. Any suggestions on how to begin troubleshooting this?

Are you sure that it's actually running on the number one and two cylinders?

[cgswss]

First, just for the heck of it, If you have a spare plug, pull the plug leads on at a time and hook it to the spare plug- which you hold against the head to see the spark. (of course the engine has to be running, or at least turning over).  That will at least tell us that you are getting spark.  Next, pull the plugs and take a look at them.  We won't really be able to tell anything about mixture at this time, but we may be able to spot a problem.  You didn't say if the bike was running before you took it apart.  If it wasn't, there is the real possibility of a bad cylinder (or two).  If you pull out an oily black plug, that my be a problem.

[kjdelaney]

First, just for the heck of it, If you have a spare plug, pull the plug leads on at a time and hook it to the spare plug- which you hold against the head to see the spark. (of course the engine has to be running, or at least turning over).  That will at least tell us that you are getting spark.  Next, pull the plugs and take a look at them.  We won't really be able to tell anything about mixture at this time, but we may be able to spot a problem.  You didn't say if the bike was running before you took it apart.  If it wasn't, there is the real possibility of a bad cylinder (or two).  If you pull out an oily black plug, that my be a problem.

The engine was running before i took it down, although it was an oily mess. Then again it is a 35 year old bike! I checked compression on all cylinders and they all checked out around 25 lbs (give or take 3 lbs... I think the lowest was 23). How long untill residue will show up on a plug? I replaced all the plugs so they are all very new. Tomorrow i will pull the plug leads and test on a spare. However it sounds like they are hitting on all 4. Ill post video of the results. 

[kjdelaney]

motorcycle engines generally get too hot to touch after running for a few minutes...what makes you think that 3 and 4 are significantly hotter?

i can briefly touch 1 and 2.  3 and 4 i can only tap with my finger before i get burned.

[seanbarney41]

motorcycle engines generally get too hot to touch after running for a few minutes...what makes you think that 3 and 4 are significantly hotter?

i can briefly touch 1 and 2.  3 and 4 i can only tap with my finger before i get burned.

assuming your talking about the pipes, they should all get that hot within a minute or two of starting...unless you have burn proof superman hands!

[kjdelaney]

I gotta ask. You do have a fan on it right? and where is it on the floor. I use one on each side not one of those box fans but a sure enought fast fan.
Ken

No fan but that did cross my mind since it is air cooled. I'm going to put one on it, but it still doesn't explain why it is hotter on the right side. It was so hot that i could feel the heat radiating off that side. The other side was tolerable. I'm not certain that air moving over it would fix the problem. It was a substantial temperature difference.

[MCRider]

At 25ft lbs I'm surprised it would even start. 80 is about minimum and 160 is healthy.

My recent rebuild puffs at 220+.

PLEASE put a fan on it and stand it up as close to vertical as safety permits.

The CB750 can run pretty well on 2 cylinders so you've got to prove taht they are all hitting by pulling the plug caps one by one and listening for it to drop that cylinder.

But i hasten back to the compression. Did you remove all the plugs, hold the throttle wide open and crank it with the electric?

[kjdelaney]

motorcycle engines generally get too hot to touch after running for a few minutes...what makes you think that 3 and 4 are significantly hotter?

i can briefly touch 1 and 2.  3 and 4 i can only tap with my finger before i get burned.

assuming your talking about the pipes, they should all get that hot within a minute or two of starting...unless you have burn proof superman hands!

I do have calluses on my hands!!! lol i guess you could say that they are super man hands!!! HA HA! Im going to test the spark tomorrow.  Just to make sure they are firing!

[seanbarney41]

if your compression checks are valid (please detail the process and equipement you used), you have some problems, otherwise, probably a plugged idle jet on the cooler cylinders

[kjdelaney]

motorcycle engines generally get too hot to touch after running for a few minutes...what makes you think that 3 and 4 are significantly hotter?

i can briefly touch 1 and 2.  3 and 4 i can only tap with my finger before i get burned.

assuming your talking about the pipes, they should all get that hot within a minute or two of starting...unless you have burn proof superman hands!

I do have calluses on my hands!!! lol i guess you could say that they are super man hands!!! HA HA! Im going to test the spark tomorrow.  Just to make sure they are firing!

I just pulled the 1 and 2 leads... There was no change in the way the motor was running. It is only running on 2 cylinders. # 3 and 4...Suggestions? New coil?  I'm going to inspect the connections tomorrow. Perhaps that is the issue.

[seanbarney41]

if your compression checks are valid (please detail the process and equipement you used), you have some problems, otherwise, probably a plugged idle jet on the cooler cylinders

it's not your coil/s, points, or condensers, one coil fires 1 and 4, one coil fires 2 and 3...have you done a valve clearance adjustement yet?

[78whiteorbs]

compression test but more likely  check juice to each coil wire with a test light wit key on ,,bet one is dead or also trim a small amount off you spark plug wire and reinstall the cap.


sorry misread ,do you have spark on problem cylinders?

[cgswss]

Great! you have a compression gauge.  OK lets find out a LOT about your engine.  Pull ALL the plugs.  Test each cylinder with the compression gauge.  Hold the throdle wide open an use the started to crank the motor until the reading stop going up.  Next go back an put a little oil in the spark plug hole an test the compression again.  If you have a large difference in the two reading, its semi goo news.  That would indicate most likely have good valves an just need rings.  While compression gauges vary, anything under 100 would indicate you should pull the jugs, give them a good hone job, and replace the rings.  No sense in doing the dry wall if you've got rot in the studs.

[cgswss]

Using that spare plug to check for spark may eliminate a lot of guessing

[kjdelaney]

if your compression checks are valid (please detail the process and equipement you used), you have some problems, otherwise, probably a plugged idle jet on the cooler cylinders

it's not your coil/s, points, or condensers, one coil fires 1 and 4, one coil fires 2 and 3...have you done a valve clearance adjustement yet?

No. not yet. That did cross my mind though. I just tested with a spare plug to see if all plugs are working and they are. When i pull #4 it sputters and then stalls. Plus, i used a spare plug to see if it was sparking and all were. Could that be a valve thing? Perhaps timing?

[kjdelaney]

Great! you have a compression gauge.  OK lets find out a LOT about your engine.  Pull ALL the plugs.  Test each cylinder with the compression gauge.  Hold the throdle wide open an use the started to crank the motor until the reading stop going up.  Next go back an put a little oil in the spark plug hole an test the compression again.  If you have a large difference in the two reading, its semi goo news.  That would indicate most likely have good valves an just need rings.  While compression gauges vary, anything under 100 would indicate you should pull the jugs, give them a good hone job, and replace the rings.  No sense in doing the dry wall if you've got rot in the studs.

My father in law has the compression gauge. Look like i need to have him come back over for a few beers!!! lol

[cgswss]

OK we can leave the electrial alone for a while-all spark.  Is there any chance you have some starter fluid?  We need spark and fuel plus compression.  Again we can short cut a lot of testing by spraying some starter fluid in the air box and seeing if suddenly we have all 4 running.

[kjdelaney]

OK we can leave the electrial alone for a while-all spark.  Is there any chance you have some starter fluid?  We need spark and fuel plus compression.  Again we can short cut a lot of testing by spraying some starter fluid in the air box and seeing if suddenly we have all 4 running.

No I dont have any. But now I'm going to get some. What should I do?

[Rob69]

OK we can leave the electrial alone for a while-all spark.  Is there any chance you have some starter fluid?  We need spark and fuel plus compression.  Again we can short cut a lot of testing by spraying some starter fluid in the air box and seeing if suddenly we have all 4 running.

No I dont have any. But now I'm going to get some. What should I do?

You can use carb cleaner as well, if you have any sitting around. Remove the airbox lid/cover and spray it into the airbox while the engine is running. In the bike comes to life all of a sudden, it can be assumed that you have a fuel delivery problem. If I recall, those have a petcock with two outlets, one for the 1 and 2 carburetors and one for the 2 and 4 carburetors. That's where I'd start.

[Really?]

A temp gun from HF is nice to have so you can check the temp of each pipe.

[Duanob]

Let s try the basic things first. Does the 77 750 fuel tap have two drain pipes? If it's anything like the 76 550 you have one pipe feeds the 1&2 and the other pipe feeds the 3&4. If the gas tank is fairly empty and leaning on the side stand there is a chance that two carbs aren't getting enough fuel. Easy enough to check, put a bowl under the fuel tap and pull the lines and open the tap to see how much fuel flow you get. *It might just need more gas in the tank. Cold cylinders only means they are not firing it doesn't mean an electrical problem.

*Trust me it happens. I had a shrug shoulders/slap forehead moment just like this (right after pulling the carbs and recleaning them, going through the entire ignition system and doing a compression test!) All it needed was more gas.

[kjdelaney]

Let s try the basic things first. Does the 77 750 fuel tap have two drain pipes? If it's anything like the 76 550 you have one pipe feeds the 1&2 and the other pipe feeds the 3&4. If the gas tank is fairly empty and leaning on the side stand there is a chance that two carbs aren't getting enough fuel. Easy enough to check, put a bowl under the fuel tap and pull the lines and open the tap to see how much fuel flow you get. *It might just need more gas in the tank. Cold cylinders only means they are not firing it doesn't mean an electrical problem.

*Trust me it happens. I had a shrug shoulders/slap forehead moment just like this (right after pulling the carbs and recleaning them, going through the entire ignition system and doing a compression test!) All it needed was more gas.

No it has one hose. It goes to the 2 carb. It is the master and the others are slaves. I just did the starter fluid thing on 1 and there was no change. As of right now if i pull the wire to the 1 cylinder there is little change to how the engine is running. if i pull the 4 wire the motor dies. As far as my compression I misspoke. If my memory serves me it was @ 85. However i want to check it again just to be sure. All carb bowls have fuel. Honestly i believe they are all running but the 3 and 4 side runs hotter.

I just went out and started the bike. Let it run for a minute or so and touched the 1 and 2 pipe and they were hot. The 3 and 4 side was by far WAY hotter. So much so that the heat radiates off that side. Honestly im afraid to let it run too long out of fear of seizing it up!

I don't understand why if i pull the 4 wire the engine dies and if i do the same to 1 it barely makes a change. There's a slight noise difference but i had to do it a couple times just to pick up on it.

[cgswss]

It simply means that the engine is running off of #4, but #1 is not doing anything.

When you use the started fluid, it is taking the place of any gas in the bike.  That's why I use it- to see if the cylinder is ready to run, but just isn't getting gas.

Do you have an inductive timing light?  I'm not interested in the timing, I want to make sure the #1 cylinder is still firing under compression.  Also, if you have enough length on the spark plug wires, just for grins, you might switch the wire going to 1 with the one going to 4.

Do we know that spark plug in #1 is a good one?  If you have another, you might want to switch it, or just swap the #4 plug to #1 (as we know that one is working)

[kjdelaney]

Do you have an inductive timing light?  I'm not interested in the timing, I want to make sure the #1 cylinder is still firing under compression.  Also, if you have enough length on the spark plug wires, just for grins, you might switch the wire going to 1 with the one going to 4.

Do we know that spark plug in #1 is a good one?  If you have another, you might want to switch it, or just swap the #4 plug to #1 (as we know that one is working)

I do not have a light. i swapped the plug wires and the same thing continued to happen. However this time, after I swapped the wires back the way they should be, i pulled 4 and it kept running. All the plugs are brand new. I'm at a loss. I do think its over heating though. After having it run for a couple of minutes it started to just bog down and die (it did have gas) ..... Something tells me i'm going to have to pull this motor out and give it some love. i don't want to but i guess that can be next winters project.

[LesterPiglet]

After reading this I am now confused. It would be a good idea to establish you have the correct ht leads going to the correct cylinders now.

[kjdelaney]

After reading this I am now confused. It would be a good idea to establish you have the correct ht leads going to the correct cylinders now.

You and i both....It is all hooked up as it should be. I'm just trying everything that gets thrown my way. Bottom line is i know something is "off" the bike is running but it gets hot. Too hot on tht right. I'm going to try to get the carbs dialed in. See if that helps. It may be too lean @ 3 and 4. I know its a stab in the dark but it needs to be done.

[LesterPiglet]

What speed is it idling at?

[kjdelaney]

What speed is it idling at?

@1500 rpm. My idle speed is another thing i haven't really brought up. I have to force the accelerator forward to keep the idol where it should be. One thing at a time i guess... any thoughts on that? Ive been looking for a good link for info about the carb adjustment and syncing but i haven't really come up with much.

[MCRider]

What speed is it idling at?

@1500 rpm. My idle speed is another thing i haven't really brought up. I have to force the accelerator forward to keep the idol where it should be. One thing at a time i guess... any thoughts on that? Ive been looking for a good link for info about the carb adjustment and syncing but i haven't really come up with much.

Usually, when you have to force the idle down, the cables are misrouted or otherwise too tight. Since the b cable doesn't have an adjuster, it may be installed with both nuts oon the sme side of the bracket, causing everything to be too tight, or its OK and the A cable is too tight. Assuming the idle screw in the rack is down far enough to let it idle.

One way to tell is to disconnect the B cable altogether and just work with the A. There should be a little slop in the twistgrip, then it should pull open smoothly and upon release, snap back to idle without any help.

Then add the B cable back in. There should be no difference.

[kjdelaney]

Just a quick update! I synced my carbs last night and the 3 and 4 carbs had way less vaccuum. They were way lean. Now it is running alot better. Except for some minor backfiring (poping in the pipes) any suggestions as to where the idol mix screw is on PD carbs?

[bjatwood]

Good news on the carbs & syncing. Now check the timing, adjust the valves,  and THEN do another compression test and let us know what kind of numbers your getting.....