Topic: 1978 cb750k carb question

[tmtrebor]

Hello,

New to this forum, but been lurking a long time on various bike forums...  2 year ago I bought a (semi) running cb750 from a guy's barn... It ran, but it leaked oil everywhere, needed to be cleaned up, and was in medium condition... I guess what you'd expect from a running barn bike.  Anyway, over the course of these two years I've learned a lot about this motorcycle.  I tore the entire thing down, and rebuilt it up, replacing what seemed to be out of spec, finding missing bolts (1 engine mounting bolt, and the engine hanger bolt at the bottom), and building two harnesses due to stupidity (Had no idea how much I'd have to rewire if I bought everything up front...). 

Anyway, the bike now runs... kinda.  I think I have the problem narrowed down to the carbs, which was expected, and I think it's running lean, as the engine is extremely hot.  It also only idles about 15-30 seconds before dying, unless I continuously rev the bike hard (activating the main jets).  If I slowly turn the throttle as if to increase speed normally, the bike dies.  I have bench synced the carbs, but I cant actually sync the carbs as they wont idle.

Now, I know this has been asked a ton, but I've had a couple people look at the bike, and heard multiple things, so I was hoping someone could point me and/or talk me through the right one.  I have pods (of course) and a 4-1.  I have 125 main jets, stock pilot jets, and cleaned carbs.  They are the PD(?) style carbs with the NON round top and press in pilot jets. 

My question is here... would upgrading the pilot jets to #40 or #42 or #45 fix this?  If not, what's a good idle mix to setting to get these things to idle?  i've done everything I can think of.  Also, am I fighting a losing battle, and would it be worth it to just upgrade to the mikuni carbs and get it over with? 

Thanks guys!

[flybox1]

Hey TM, Yes, you are too lean, but its easily corrected.
Start with 42's and 1 turn out on the fuel screws, and a new set of NGK D8EA plugs. 
.....or go back to a stock airbox and stock jetting.

www.jetsrus.com  for OEM Keihin jets.

[tmtrebor]

Thank you!

[madmtnmotors]

Not enough information...

>Have you checked for vacuum leaks? Are you using new carb boots, or the old hard cracked originals?
>Running hot? Are you measuring temps with an infrared thermometer? What are your readings?
>If you are running hot, is the crankcase vented? The vent at the back of the valve cover must be open to the atmosphere. Since this is normally connected to the stock airbox, where was this vent routed when the pods were installed? It's not capped off is it? If it is capped, this will cause temps to spike tremendously.
>Do you have the stock airbox? If so, put it back on to try and eliminate the pods as a variable.
>You "tore the entire thing down"? Are the pistons stock or overbore? Did you replace the cam? If so, with what?

Would it be worth it to just upgrade to the mikuni carbs and get it over with? 

Where are you? If you just want to throw money at it then bring it over to me and I'll get it running right. I have two 78' CB 750K's and one 78' CB750F that all have the "PD" carbs and I have no tuning issues with them at all. I would figure out the problem first, these PD carbs work well for me and I believe the bad rap given to these carbs is undeserved. Earlier carbs may be simpler, but they all work like they should when properly assembled/adjusted.

[tmtrebor]

Not enough information...

>Have you checked for vacuum leaks? Are you using new carb boots, or the old hard cracked originals?
>Running hot? Are you measuring temps with an infrared thermometer? What are your readings?
>If you are running hot, is the crankcase vented? The vent at the back of the valve cover must be open to the atmosphere. Since this is normally connected to the stock airbox, where was this vent routed when the pods were installed? It's not capped off is it? If it is capped, this will cause temps to spike tremendously.
>Do you have the stock airbox? If so, put it back on to try and eliminate the pods as a variable.
>You "tore the entire thing down"? Are the pistons stock or overbore? Did you replace the cam? If so, with what?

Would it be worth it to just upgrade to the mikuni carbs and get it over with? 

Where are you? If you just want to throw money at it then bring it over to me and I'll get it running right. I have two 78' CB 750K's and one 78' CB750F that all have the "PD" carbs and I have no tuning issues with them at all. I would figure out the problem first, these PD carbs work well for me and I believe the bad rap given to these carbs is undeserved. Earlier carbs may be simpler, but they all work like they should when properly assembled/adjusted.

I have checked for leaks by spraying water on them, but i cant get the thing to idle long enough for it to really matter (I think).  No change in RPM when sprayed.

By hot, I mean the exhaust heats up in 15-30 seconds on idle to "impossible to touch."   It also has backfired through the carbs 3 times.

The vent (i'm assuming the vent is the open nipple at the top) is open to atmosphere and routed down to where the carbs drain.

stock airbox is still in my garage, but it has a huge hole in the side from being in a barn for so long... I'm assuming an animal lived there.  That's why I chose to go with pods.

Pistons are stock, , cylinders are honed, piston rings were new and expensive, compression is 144-150 in all cylinders (Harbor freight compression tester, so it varies).  The cam and chain were not replaced as it seemed to be fine.  by tore down, i mean down to the timing at the bottom (top 3 pieces removed and cleaned) and all gaskets replaced and stuff inspected.

 I'm in Louisville KY and dont mind throwing a little money.  Everyone says these are just way harder to tune and parts are harder to find than the mikuni carbs. 

Does this help?

[flybox1]

these are air cooled bikes, if you allow them to idle without airflow, they will get really hot.  always employ a box fan if you need it to run on its stand.
these carbs are easy to tune.
one way to tell you are lean with your current setup...throw in a new set of plugs and let it idle for 5 min under a fan.  No blipping of the throttle.
if the plug insulators are white, swap in 42 pilot jets jets, 1 turn out on the fuel screws. 
plugs back in and check plug color again after idle. they should be light tan at the tip.
if they are barely showing color, go to 1.5 turns out on the fuel screws and test again.

[madmtnmotors]

I have checked for leaks by spraying water on them, but i cant get the thing to idle long enough for it to really matter (I think).  No change in RPM when sprayed.

By hot, I mean the exhaust heats up in 15-30 seconds on idle to "impossible to touch."   It also has backfired through the carbs 3 times.

The vent (i'm assuming the vent is the open nipple at the top) is open to atmosphere and routed down to where the carbs drain.

stock airbox is still in my garage, but it has a huge hole in the side from being in a barn for so long... I'm assuming an animal lived there.  That's why I chose to go with pods.

Pistons are stock, , cylinders are honed, piston rings were new and expensive, compression is 144-150 in all cylinders (Harbor freight compression tester, so it varies).  The cam and chain were not replaced as it seemed to be fine.  by tore down, i mean down to the timing at the bottom (top 3 pieces removed and cleaned) and all gaskets replaced and stuff inspected.

 I'm in Louisville KY and dont mind throwing a little money.  Everyone says these are just way harder to tune and parts are harder to find than the mikuni carbs. 

Does this help?

That helps a lot so we know what the demands of the engine might be.

How's the timing? Did you static time the ignition, or just "let it ride" until you got it running in order to set the timing dynamically?

As for the vent "routed down to where the carbs drain" means open on the end? As long as it's open and not connected to the crankcase vent at the back of the crankcase (which should be connected to the oil tank) then you should be fine as far as the vent goes. Some people will put a filter on the end of the vent to reduce the amount of oil mist expelled.

The PD carbs can be more of a "challenge" but far from impossible and should be able to easily serve the stock engine. Even with the gaping hole in the side, the stock airbox with a $10 oem style filter will help determine whether the pods are the culprit or if the problem lies elsewhere. I suspect either the carb to head rubber boots are leaking, or the carbs are not cleaned/adjusted/assembled properly, or any combination of those. The idle jets are pressed in and are difficult to clean thoroughly. They can be removed with care and inspected for debris. Compressed air can also give a basic indication of their condition without removing them.

By all means do not take them to a "shop" unless someone you know can verify first hand that they know how, and have the patience to work on these older carbs.

[tmtrebor]

I have checked for leaks by spraying water on them, but i cant get the thing to idle long enough for it to really matter (I think).  No change in RPM when sprayed.

By hot, I mean the exhaust heats up in 15-30 seconds on idle to "impossible to touch."   It also has backfired through the carbs 3 times.

The vent (i'm assuming the vent is the open nipple at the top) is open to atmosphere and routed down to where the carbs drain.

stock airbox is still in my garage, but it has a huge hole in the side from being in a barn for so long... I'm assuming an animal lived there.  That's why I chose to go with pods.

Pistons are stock, , cylinders are honed, piston rings were new and expensive, compression is 144-150 in all cylinders (Harbor freight compression tester, so it varies).  The cam and chain were not replaced as it seemed to be fine.  by tore down, i mean down to the timing at the bottom (top 3 pieces removed and cleaned) and all gaskets replaced and stuff inspected.

 I'm in Louisville KY and dont mind throwing a little money.  Everyone says these are just way harder to tune and parts are harder to find than the mikuni carbs. 

Does this help?

That helps a lot so we know what the demands of the engine might be.

How's the timing? Did you static time the ignition, or just "let it ride" until you got it running in order to set the timing dynamically?

As for the vent "routed down to where the carbs drain" means open on the end? As long as it's open and not connected to the crankcase vent at the back of the crankcase (which should be connected to the oil tank) then you should be fine as far as the vent goes. Some people will put a filter on the end of the vent to reduce the amount of oil mist expelled.

The PD carbs can be more of a "challenge" but far from impossible and should be able to easily serve the stock engine. Even with the gaping hole in the side, the stock airbox with a $10 oem style filter will help determine whether the pods are the culprit or if the problem lies elsewhere. I suspect either the carb to head rubber boots are leaking, or the carbs are not cleaned/adjusted/assembled properly, or any combination of those. The idle jets are pressed in and are difficult to clean thoroughly. They can be removed with care and inspected for debris. Compressed air can also give a basic indication of their condition without removing them.

By all means do not take them to a "shop" unless someone you know can verify first hand that they know how, and have the patience to work on these older carbs.

Well, I let the timing "ride" as I was planning on adjusting everything once it was going again.  I can add this to the list of things to check though.  Is this procedure in the clymer manual?

Yes, the vent is open at the end of the tube.  I will check to see where these two things are going and report back.

I will throw the airbox on as well.

As for the pilot jets, can i just blast them with an air compressor to try to clean them out?  Will this hurt them at all?
Ok, i have to get back to work, but I will report back by next week on the status of this thing. 

[madmtnmotors]

Well, I let the timing "ride" as I was planning on adjusting everything once it was going again.  I can add this to the list of things to check though.  Is this procedure in the clymer manual?

Yes, the vent is open at the end of the tube.  I will check to see where these two things are going and report back.

I will throw the airbox on as well.

As for the pilot jets, can i just blast them with an air compressor to try to clean them out?  Will this hurt them at all?
Ok, i have to get back to work, but I will report back by next week on the status of this thing. 

You will find a link to static and dynamic timing procedures in my signature.

The pilot (or idle) jets typically can't be "cleaned" with just compressed air. Compressed air will not damage them. Blowing compressed air through them, and checking whether the air is flowing through the idle port in the throat of the carb can indicate whether they are blocked. This method would be difficult to determine if they were partially obstructed. The only way to know for sure would be to carefully remove and inspect them.

I guess I should get back to work as well... 

[meangreen]

 When I bought my '74, it had been sitting for at least two years with fuel in it. I got it going by taking a wire brush, bending a bristle 90 degrees, and using the bristle to clean each jet one at a time. I had to remove them to do this, but it's easy to get to everything when you have pods. You can verify that the blockage is gone by looking through each jet at a bright light.
 Bike carbs are super fussy. I personally wouldn't even have tried to run it with pods and stock jetting. Looks like some knowledgeable guys have chimed in on jet sizes, and you really need to get those in there before you go any further. It's amazing how much change even one size can make.
 Also, that timing needs to be as close as you can get it statically. A hot engine and backfiring through the carbs tells me the timing is way off, probably after TDC. Static time it, make sure your plugs are good, make sure the valves are set properly, get the right jets, and I bet it fires right up.

[tmtrebor]

OK guys, Sorry it took so long, but I'm back.

So, i took the pilot jets off and... yup. black.  Totally black throughout.  I let them all sit in carb cleaner for 1 hour, seafoam for 4, and got a guitar string and popped a hole out of the darkness, and now there is light.  I'm going to drop them back in the carb cleaner  today, bike tomorrow (monday at the latest), and get back with you, but I think that's the problem.

The jets are #35... which seems weird, since I have read they are #40.. My next question is, how "clean" do they need to be?  I cant tell how thick the gauge is, but I know only the thinnest guitar string I have fits... I believe it's a .015.  Would propane torch melt the brass?  And what about kerosene....

Thanks for your help guys, this is what i needed.

[flybox1]

#35 pilot jets are stock.   The cleaner the better. Soak in carb cleaner or use an ultrasonic cleaner.
The hole thru the center and the 8 holes in the tip all need to be clean and clear.
Dont forget to inspect the emulsion tube under the main jet, and all of the accel pump system.