Topic: 750 K5 Overheating Cause and Residual Effects

[ZB750]

Here is the backstory:

I just got my bike together and running after a 8 month rebuild.  The bike ran when I bought it, so the rebuild mainly focused on the rolling chassis and cosmetics (all new bearings, bushings, seals, brake and wheel rebuild, rewired,  paint, etc).  The only modifications to the motor would be swapping out the stock 4-4 for a Motogpworks 4-1 exhaust system. 

I cleaned and rebuilt my carbs with kits that I bought from 750 supply which contained #100 and #120 main jets.  Since the carbs were equipped with #110 main jets with the 4-4 system, I used the #120s and kept the needle clip in the same position as before, which is the stock (middle) position.  I also adjusted tappet clearance, tensioned cam chain, cleaned and adjusted point gap, and installed new plugs (not sure of heat range).  I synced the carbs with a manometer and adjusted the air screws to ~ 3/4 turns from fully seated (1/4 richer than spec, but within the documented tolerance).  The only thing I did not do was time the bike, either statically or with a gun.

The bike didn't like to stay idling at the 850-950 RPM range, so I set it to 1000-1100, but other than that it was pretty responsive with no noticeable missing.  I rode the bike around my neighborhood for 30 minutes with no real problem. 

The next day I took the bike across town for a 20 min ride on surface streets. It was a around 80 degrees and traffic was pretty normal.  I did stop for a train, but it wasn't that long, maybe 5 minutes tops.  The bike hunted a bit at 1/4 to mid throttle, but it was not uncontrollable.  It also stumbled slightly if i snapped the throttle from idle in neutral.  From what I have read, these are both signs of a lean air/fuel mixture.  The symptoms however, seemed pretty mild, and I figured it wasn't anything a little further tweaking couldn't solve once I got home. 

After arriving at my destination I shut the bike down for ~10 minutes, then started back for home.  It idled for maybe 3-4 minutes before I got back on the road, but once I started out my oil light came on.  I shut the bike down within 30 seconds of seeing the light and let the bike cool for about 10 minutes.  I got back on the bike, but the light came on again a few miles down the road.  This time I let it cool for 25 minutes and attempted to limp it over to my brother's house which was only a few miles away.  I shut down the bike at every light, but it started acting really funky (lurching on take off and losing power).  I shut down immediately and hauled the bike home. 

There were no loud bangs at the time of the incident and the bike starts now, but it idles extremely rough and blubbers and misses badly when I twist the throttle.  I am currently out of town, but I'm trying to form a game plan for sorting this out, hopefully one that does not involve tearing apart the motor.

My questions are these:

What, besides a lean mixture, would cause overheating in standard operating conditions? 
    Incorrect ignition timing?  (Wouldn't that have made it a bear to tune initially?)
    Insufficient lubrication?  (I didn't hear any ticking or other metallic sounds)
    Bad seal on intake boots?

What, besides seizing the motor, are the residual effects of overheating the motor that would cause it to run so poorly once it has sufficiently cooled?
    Could have it been knocked (further) out of time?
    Affected the sync? 

Any tips on the order of operations for trouble shooting?

Should I re-jet to #125 or #130, or should I drop the needle clip a notch first, or both?

Thanks in advance.

[JWExperience]

The light your referring to is a oil pressure light not temp. What besides the light says your overheating? Did u check the plugs and what plugs are they? I would also check the boots on the carbs to make sure there are no leaks. Also tell us everything your running, filter or pods, jetting for pilots etc.
may also want to run through the 3k tune up just to rule other issues out but from the sounds of it I wouldn't jump right to over heating unless you have more indicators

[ZB750]

The light your referring to is a oil pressure light not temp. What besides the light says your overheating? Did u check the plugs and what plugs are they? I would also check the boots on the carbs to make sure there are no leaks. Also tell us everything your running, filter or pods, jetting for pilots etc.
may also want to run through the 3k tune up just to rule other issues out but from the sounds of it I wouldn't jump right to over heating unless you have more indicators

Haha, what does that say about me that I can't read a dummy light.  I guess I associated the light with overheating because the bike felt like it was radiating heat, and I was experiencing loss of power and generally wonkiness (missing and stalling out of first). 

The deets:

Stock air box
Motogpworks 4-1
#40 pilots
#120 mains
Needle clip in middle position
Air screws in the range of 3/4 turn from full seat. 
NGK D8EA plugs.

The problem:

Bike ran great the day before, and when I rode it across town (20 min. ride)  On the way home the light came on, and 10 minutes later it was unridable.

I have not checked plugs or anything else for that matter because I headed out of town the next day for work.  Im going to take your advice and perform the 3K tune up.  I just wanted to arm myself with as much knowledge as possible going in. 

[scunny]

yep, full tune includes timing.

[Black 750K8]

I just got my bike together and running after a 8 month rebuild

How or why could you do this and not do a  3K tune up

[seanbarney41]

I just got my bike together and running after a 8 month rebuild

How or why could you do this and not do a  3K tune up

sounds like he did everything except set the timing...why you didn't do that?...all you need is a 12 volt light bulb and some wire/alligator clips.  Also, these bikes are oil cooled (the air doesn't really do that much)...so if you have low oil pressure, you got problems.  Cb750 should have oil pressure like a car(lots of it), so if the oil light comes on while running, that definately needs to be addressed.

[Nic]

Probably a stupid question, OP didn't say, did you check the oil level?

[RSchaefer]

Yea I mean the big concern would be the oil light, I'm thinking leaking boots, wrong jets, different needle setting, even ignition timing would not be a function of the oil light?

[Don R]

Was it leaking any oil, possibly from the vent hoses?  When you put it together and started it did you re-check the oil level after runnng it ? Do you remember how much oil eyou put in? How fresh/ full  was the fuel?

[lucky]

Cannot help you at all until you tell us what year the bike is.

Need to know what intake system you have.
Just statically time it like the book says and make sure the advance unit is not rusted solid.

QUOTE:
"I synced the carbs with a manometer and adjusted the air screws to ~ 3/4 turns from fully seated (1/4 richer than spec, but within the documented tolerance).  The only thing I did not do was time the bike, either statically or with a gun."

There are no Honda engines that have "air screws" set to 1/2 turn open.
They are all at least 1 full turn open.
I am surprised it idled at all. I hope you were not using "air screws to sync the carbs.
Were you?

All issues like timing statically MUST be resolved before any carb adjustments.


The #120 main is ok depending on what year the bike is and since you were not driving at full throttle anyway it is a non issue.

Pilot jet not mentioned.

You have no proof it is "overheated" yet. NONE

I just runs like crap.
Won't idle.
Poor throttle response.

Brakes and wheels are not affected.LOL...lol

That is called "backpacking" when you start throwing all this non essential info into the problem diagnosis.

[lrutt]

still no statement on oil level, if there was any at all. #1 concern right there.

[ZB750]

still no statement on oil level, if there was any at all. #1 concern right there.

~3.7 qts. of 10-40 and new filter prior to carb tune.  Oil level is fine post incident. 

Cannot help you at all until you tell us what year the bike is.

Need to know what intake system you have.
Just statically time it like the book says and make sure the advance unit is not rusted solid.

It's a 1975 with the stock air box and #40 pilot jets. 

There are no Honda engines that have "air screws" set to 1/2 turn open.
They are all at least 1 full turn open.
I am surprised it idled at all. I hope you were not using "air screws to sync the carbs.
Were you?

I synced the carbs by adjusting the slide heights until there was an acceptable level of vacuum uniformity.  I then adjusted each air screw, all of which were baselined at 1 turn from seat.  I then turned each screw in 1/8 turn increments right and left of base until highest point of RPM was reached, which ended up being in the neighborhood of 3/4 out.  The manual says that point should be within 1/2 - 2 turns from seat.  It's actually stated like this, "If it takes a full turn more or 1/2 turn less than the original setting to change the engine speed check the following...."

The #120 main is ok depending on what year the bike is and since you were not driving at full throttle anyway it is a non issue.

So I get that the main jet primarily pertains to 3/4 - full throttle, but what is it's relationship with needle position?  More specifically, does needle clip position allow a set amount of fuel to pass independently of main jet size?  Or would a #140 be richer than a #105 at 1/2 throttle with identical needle set ups?

You have no proof it is "overheated" yet. NONE

I just runs like crap.
Won't idle.
Poor throttle response.

Brakes and wheels are not affected.LOL...lol

That is called "backpacking" when you start throwing all this non essential info into the problem diagnosis.

I really hope I didn't overheat it.  And yes you are right, boiled down, the problem is : It just runs like crap.  Won't idle.  Poor throttle response.  But this happened suddenly after running quite well, and coincided with the oil light coming on.  I thought those two points were pertinent as well.

Sorry for the "backpacking", I'll try not to let it happen again 

Was it leaking any oil, possibly from the vent hoses?  When you put it together and started it did you re-check the oil level after runnng it ? Do you remember how much oil eyou put in? How fresh/ full  was the fuel?

There was a little seepage coming from the mission cover, but no major leaks.  I put ~3 1/2 qts. in and the level is fine post incident.  The tank is rust free and I put 3 gallons of premium in, along with a hit of Marvels.


At this point I plan on doing the following:

Check plugs
Check if oil is cooked and change if necessary
Statically time and check advance spring
Check intake boots for proper seal
Re-sync
Check oil pressure once running

If anyone has anything else useful to add to the list, I would be grateful. 

[lucky]

Quote:"So I get that the main jet primarily pertains to 3/4 - full throttle, but what is it's relationship with needle position?  More specifically, does needle clip position allow a set amount of fuel to pass independently of main jet size?  Or would a #140 be richer than a #105 at 1/2 throttle with identical needle set ups?"

The needle on the slide goes up and down through the center of the main jet.
The needle clip position sets the point at which the tapered part of the needle allows
more gas to enter the main jet stream.

The needle is controlling mostly the 1/4 to 3/4 throttle and is very important.

The main jet does not really come into full effect until after 3/4 throttle.
That would be when the needle is all the way up and almost none of the needle is in the main jet anymore.

I know what the book says but really just set all 4 pilot air screws exactly the same amount.
Forget that lean best idle for now.

Open all 4 pilot screws the same amount and then do not touch them.
Just open them all one full turn. They will be very close.

You did not tell us if you have the stock air filter. You just said you have the stock airbox.

Please tell us what air filter you have. It makes a big difference.

[phil71]

Take compression. Make sure it's uniform, just to put your mind at ease about the internals.
If the oil light is flickering at a very low and crappy idle, it might not mean all that much .. If you get it spinning at 1000 and it still flickers, I'd look at the oil pump and pickup asap.
   Besides that, the clatter you describe sounds a lot like an engine that dropped a cylinder or 2. 
Dab a damp paper towel on each header pipe. Whichever ones don't steam will lead you towards your problem

[ZB750]

Take compression. Make sure it's uniform, just to put your mind at ease about the internals.
If the oil light is flickering at a very low and crappy idle, it might not mean all that much .. If you get it spinning at 1000 and it still flickers, I'd look at the oil pump and pickup asap.
   Besides that, the clatter you describe sounds a lot like an engine that dropped a cylinder or 2. 
Dab a damp paper towel on each header pipe. Whichever ones don't steam will lead you towards your problem

Will do on the compression test, and if I get that far, hooking up a pressure gauge will inform me as to the oil circulation, or lack there of.  Thanks for the help.

Lucky-

Thanks for the needle jet explanation and advice on the "pilot screws" (which I was incorrectly calling air screws).  As to the air filter, I am pretty sure it is stock.  I will check when I get back home in a few days, hopefully.

Thanks again!

[seanbarney41]

Take compression. Make sure it's uniform, just to put your mind at ease about the internals.
If the oil light is flickering at a very low and crappy idle, it might not mean all that much .. If you get it spinning at 1000 and it still flickers, I'd look at the oil pump and pickup asap.
   Besides that, the clatter you describe sounds a lot like an engine that dropped a cylinder or 2. 
Dab a damp paper towel on each header pipe. Whichever ones don't steam will lead you towards your problem

Will do on the compression test, and if I get that far, hooking up a pressure gauge will inform me as to the oil circulation, or lack there of.  Thanks for the help.

Lucky-

Thanks for the needle jet explanation and advice on the "pilot screws" (which I was incorrectly calling air screws).  As to the air filter, I am pretty sure it is stock.  I will check when I get back home in a few days, hopefully.

Thanks again!

not necessarily...you can have plenty good oil pressure and the oil jets are plugged and no oil is getting to the cam/rockers.  Pull a tappet it cover or two and see how much oil has been splashing around up top...should be really fargin' oily

[lucky]

Take compression. Make sure it's uniform, just to put your mind at ease about the internals.
If the oil light is flickering at a very low and crappy idle, it might not mean all that much .. If you get it spinning at 1000 and it still flickers, I'd look at the oil pump and pickup asap.
   Besides that, the clatter you describe sounds a lot like an engine that dropped a cylinder or 2. 
Dab a damp paper towel on each header pipe. Whichever ones don't steam will lead you towards your problem

Will do on the compression test, and if I get that far, hooking up a pressure gauge will inform me as to the oil circulation, or lack there of.  Thanks for the help.

Lucky-

Thanks for the needle jet explanation and advice on the "pilot screws" (which I was incorrectly calling air screws).  As to the air filter, I am pretty sure it is stock.  I will check when I get back home in a few days, hopefully.

Thanks again!

That name "pilot screws" is the same as mixture screws.
 
Anyway you said, QUOTE: "I used the #120s and kept the needle clip in the same position as before, which is the stock (middle) position.  I also ..."

The stock position of the needle is 4th groove down from the top .NOT MIDDLE POSITION.

No wonder it is so lean. You have the needle one step leaner than stock.
When you lower the clip it raises or richens the mixture.

Maybe it did seize.

Well if it is not making blue smoke, and it turns over, and you do not run it like that again you might be ok. Maybe synthetic oil help save your rings.

But you set the pilot air screws for best lean idle, AND you had the needle clips on the 3rd or middle groove . Both of those things were making it very, very lean.

[JWExperience]

Change your plugs...if you were running lean you could have a plug or two that went. Just a thought because its cheap and doesn't take long to check them. Only problem is that doesnt really fix your oil light issue unless your idle is that crappy and its not getting pressure up, some of these bikes have real low pressure at idle.
 
Like posted earlier check your internals to make sure you have oil everywhere and compression. Hopefully nothing is fried.

About the needles...I read on another site that each size you go up on the main jet adds about 5% more fuel to the midrange mixture with respect to the 3/4-wot increase. Obviously this differs with every scenario but there is a slight increase in midrange fuel mix from what that says. Take that with a grain of salt because I'm not a guru but it does sound like your running lean given your needle position. I wouldn't go over 115-120 mains with your current setup though, just adjust your needles to the 4th clip down for now.

[lucky]

Change your plugs...if you were running lean you could have a plug or two that went. Just a thought because its cheap and doesn't take long to check them. Only problem is that doesnt really fix your oil light issue unless your idle is that crappy and its not getting pressure up, some of these bikes have real low pressure at idle.
 
Like posted earlier check your internals to make sure you have oil everywhere and compression. Hopefully nothing is fried.

About the needles...I read on another site that each size you go up on the main jet adds about 5% more fuel to the midrange mixture with respect to the 3/4-wot increase. Obviously this differs with every scenario but there is a slight increase in midrange fuel mix from what that says. Take that with a grain of salt because I'm not a guru but it does sound like your running lean given your needle position. I wouldn't go over 115-120 mains with your current setup though, just adjust your needles to the 4th clip down for now.

I agree.

The only reason to change the needle to a richer position (fifth groove down),
would be if you have no air filter or a free breathing foam filter combined with open exhaust.

5th groove down would be very extreme.

[ZB750]

Thanks guys.  Ill let you know what happens next week.

[AJK]

If you are not sensing pinging/knocking, my guess is that you are running with excessively retarded timing (instead of over advancing). This will make your motor overheat.
  Overly retarded timing will make the motor run very inefficiently, causing a lot more heat to be generated. Check with a timing light to verify.

Check also that you have the correct amount of oil. I think one of your responses says that your oil level is ok. Oil takes away heat.

Your oil light is likely coming on due to the overheating & thining out of the oil. Likely very watery thin.

If you have 120 mains in there, you shouldn't be running lean, so the heat is not likely due to being lean.


I'd be checking for retarded timing (assuming valve lash & cam chain are set-up correctly & that your carb boots are not leaking air).

[bjatwood]

I can't recall EVER idleing my bike waiting for a train...for 5 minutes. This kind of concerns me with no air to cool an air cooled engine. I personally shut my engine off for a situation like this. Quick push of the starter button and off I go...

[iron_worker]

If you are not sensing pinging/knocking, my guess is that you are running with excessively retarded timing (instead of over advancing). This will make your motor overheat.
  Overly retarded timing will make the motor run very inefficiently, causing a lot more heat to be generated. Check with a timing light to verify.

Check also that you have the correct amount of oil. I think one of your responses says that your oil level is ok. Oil takes away heat.

Your oil light is likely coming on due to the overheating & thining out of the oil. Likely very watery thin.

If you have 120 mains in there, you shouldn't be running lean, so the heat is not likely due to being lean.


I'd be checking for retarded timing (assuming valve lash & cam chain are set-up correctly & that your carb boots are not leaking air).

I agree that improper timing likely had something to do with the over heating. I don't think running one position off from stock needle setting would be enough to cause the bike to overheat. However, that added with improper timing probably would be.

Also, I don't think setting the air screws for highest idle applies to K5 carbs since they have no accel pump and need to be adjusted over rich to compensate. Anyway, at 3/4 of a turn you were a bit richer than the factory setting which is probably in the neighborhood of where you needed to be with a stock airbox and aftermarket pipe.

IW

[HondaMan]

My 2 cents:
After you check the timing and set the carb air screws to 1.0 turn out...

1. Change the sparkplugs: new engine rebuilds often deposit stuff on the plugs, causing them to short out to high voltage. Honda did this with their production bikes: they ran them around a 0.7 mile course, then changed the plugs to new ones and drained the oil.
2. Use 15w40 (minimum) or 20w50 oil. Don't use 10w40 oil in these engines, especially above 50 degrees F. The old, original manuals written before 1974's famous Jinglish mistranslation all said this: after that, trouble began.
3. Check the oil lines for possible obstruction. One of the things that happens if they sat for a long time is: the inner lining of the hose delaminates a thin skin, which can flap partially across the hose and block the suction from the pump. In the last year, 2 other SOHC4 members went through this after their rebuilds.

The 'instructions' you describe from your manual regarding 'how to adjust the carbs' is very curious, and very odd. The air screws only operate in the range of .75 turn to 1.75 turn: further in or further out from there they have no effect. On the PD carbs, this is not true, but on the ones you describe this is 100% true. Also, turning the air screw inward on (all) the 750 carbs leans them out, it does not make them richer.