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.