Topic: 1975 CB 750 idle circuit

[kammery]

  The idle is slightly rough , Any lower than 1,300 rpm's and the engine will start to idle very rough and then stall . The air bleed screws are almost seated . At 1/4 turn out and the slight backfire is present at the exhaust . I have taken each carb apart and blown air and cleaner thru the idle circuit . My low speed jets are new and clear (# 40 ) The float levels are set to 26 mm . I have brass floats . My rubber tubes from carbs to engine are new and tight . The points are set to .013 and the ignition timing is set . No gas is present in the oil . The spark plugs are new from Honda . The carbs are in sync with 8 inches of vacuum at each port . The have read posts about correctly setting the floats and have done so . Is the float setting different for brass floats ?

       Are plastic floats set at 26mm and brass ones different ? The air bleed screws do they have rubber o-rings on them , mine did not have them when disassembled . Also on the fuel line I'm running it in between #1 and #2 carbs .Does not seem right as the angle to the fuel petcock does not seem relaxed . Have been looking for a picture to check . I do not see the hole in the plate to run the line thru . My petcock is on the left side . It would seem if I raised the fuel level the engine would run with the air bleeds out a full turn . Have done many automobile carbs with no problems . Any input would be welcome .

[harisuluv]

Stock is one turn out on the air screws.  What type of air setup do you have?  From your avatar it looks like you have the stock exhaust?

[lucky]

  The idle is slightly rough , Any lower than 1,300 rpm's and the engine will start to idle very rough and then stall . The air bleed screws are almost seated . At 1/4 turn out and the slight backfire is present at the exhaust . I have taken each carb apart and blown air and cleaner thru the idle circuit . My low speed jets are new and clear (# 40 ) The float levels are set to 26 mm . I have brass floats . My rubber tubes from carbs to engine are new and tight . The points are set to .013 and the ignition timing is set . No gas is present in the oil . The spark plugs are new from Honda . The carbs are in sync with 8 inches of vacuum at each port . The have read posts about correctly setting the floats and have done so . Is the float setting different for brass floats ?

       Are plastic floats set at 26mm and brass ones different ? The air bleed screws do they have rubber o-rings on them , mine did not have them when disassembled . Also on the fuel line I'm running it in between #1 and #2 carbs .Does not seem right as the angle to the fuel petcock does not seem relaxed . Have been looking for a picture to check . I do not see the hole in the plate to run the line thru . My petcock is on the left side . It would seem if I raised the fuel level the engine would run with the air bleeds out a full turn . Have done many automobile carbs with no problems . Any input would be welcome .

 You SAID,
"have taken each carb apart and blown air and cleaner thru the idle circuit . "
Can you see daylight through each idle jet?
Just set the floats like the book tells you too.
Thats all you have to do.
You do not tune and engine with float adjusting.
Make sure no fuel lines are kinked at all.

Set all the mixture screws to one full turn open.
Then re sync the carbs by adjusting the height of the slides.

Set the idle screw so that it keeps running.
 DO NOT TOUCH THE MIXTURE SCREWS.


Vacuum 8-10 millibars.

[kammery]

 Yes , I do have the stock exhaust on it . I'm not trying to tune the engine with float adjustment . Just asking is the float setting different with brass floats . When I bought this I had to correct the previous owners blunders . There were alought of things not right on this one . So if I'm reading the other post correctly .Set air bleeds 1 turn out and set the idle speed . Yes I can see daylight thru the idle jets . Also check the SYNC on the carbs . One of the rods (#4 ) that connects the slides to the throttle arms was bent .I found one on E-BAY , but it was to long . The rubber bushings were the rods slide thru are in good shape . I need to find the correct rod . Thanks for the input .Will give it a try .

[kammery]

 Also , does anyone have a picture of the correct fuel line routing . I do net see a hole in the mounting bracket to slide the line thru .

[DH]

If this is the bent rod you speak of, you will indeed have sync issues.
the needle plate may also be bent, especially if someone tried to open the throttle while the throttle valves were varnished/stuck.
The red arrow points to the needle plate.  The plate keeps a pre load on the spring. If the plate is bent upward, (spread apart) the spring is no longer effective, and throttle rod looseness results,
and the carb will never sync  correctly. If you take it apart, it'll make sense. If you suspect this scenario, compare it to one from another carb. BE CAREFUL if you do, those plate mounting screws are very prone to stripping out. Hopefully the phillips heads aren't too rusty.

[DH]

Also , does anyone have a picture of the correct fuel line routing . I do net see a hole in the mounting bracket to slide the line thru .

If you have a k 5 with fuel valve on the left side of the tank, there should be a single hole in the carb rack bracket between carbs 1/2.
After it goes thru the bracket hole, it tees off, one short hose goes down to feed 1/2, and one long hose runs paralel to the top of the airbox, and ending at the feed pipe for 3/4.

[harisuluv]

DH makes a good point about those screws for the rod.  They are often times rusty and can be stuck.  If you don't have a JIS screwdriver I would recommend one, as it will fit the screw better.  Often times with that screw you might only get one shot at it.  If you strip it there is not a lot of room to work in there or to get a needlenose vice grips in there. 

Just be mindful, hopefully they will come right out.

If you need a new, straight rod, I have spares that I can get to you.  If you are interested you can PM me.

Also I was going to ask you how the replacement rod that you got off of ebay was too long.

There is a difference in rods that I noticed where the threaded end of earlier models has a squared ending and the later models had somewhat of a beveled threaded end.  Regardless, they should both work fine.

Post a picture of the two side by side, I'm curious to see, thanks.

[kammery]

 Tried suggested turning air bleeds out 1 full turn , setting idle so it stays running  . Started surging between 1,500 and 2,000 rpm's .  I suspect there is something amiss with the entire cab assembly . The previous owner installed incorrect throttle spring .Put correct one on and the throttle sticks and binds . I can get a complete carb ssembly . And use the old for for spare parts . 8 inches of vacuum seems low . Also the mentioned hole for the fuel line is not there . This leads me to believe this may not be the correct carb assembly or parts of it .

[harisuluv]

Maybe it's time you posted some pictures of the carbs and also give us the casting numbers on the carbs. 

The surging idle is often a right idle condition.  You can adjust the mixture screws and see if that makes a difference.  Try a quarter turn at a time.  Remember it might not have an immediate affect so wait 20-30 seconds to assess.

[kammery]

 harisuluv , Thank you for the offer . I held the old rod and the one from E-Bay and there was a noticble difference . Evan though the old one was slightly bent , there was a difference . When I take a picture to post on here a screen comes up saying the file is to large . I will try again . Thank you everyone for the input.

[kammery]

 Looked at my carb setup last night .The round hole in the plate the carbs are attached to is on the right side . My petcock is on the left side . The build date for 750 is 8/74 . I suspect previous owner has installed  a earlier carb assembly .  I'm going to start looking for a 1975 carb assembly . In addtion I need to correct my binding throttle .This may solve both of my problems .

[harisuluv]

What is the casting on the carburetors?  It could just be the correct carburetors for the year with an earlier backplate.  I assume you are talking about the fuel lines in the backplate...

The carb bodies themselves are functionally identical, it's the jetting that is different.

Again...  Each picture is worth a thousand words..

[kammery]

 I have took pictures with my camera . When I try to post them Screen reads "file to large" Need to figure out why this happens .As soon as I do   will post pictures .

[Rookster]

Host the pictures on photobucket.  Its free to join and they automatically shrink the file size for you to forum acceptable levels.  Otherwise crop the pictures you took.  That should remove some of the file size.

Scott

[lucky]

Yes , I do have the stock exhaust on it . I'm not trying to tune the engine with float adjustment . Just asking is the float setting different with brass floats . When I bought this I had to correct the previous owners blunders . There were alought of things not right on this one . So if I'm reading the other post correctly .Set air bleeds 1 turn out and set the idle speed . Yes I can see daylight thru the idle jets . Also check the SYNC on the carbs . One of the rods (#4 ) that connects the slides to the throttle arms was bent .I found one on E-BAY , but it was to long . The rubber bushings were the rods slide thru are in good shape . I need to find the correct rod . Thanks for the input .Will give it a try .

You are on the right track.

You keep mentioning...
"Are plastic floats set at 26mm and brass ones different ?"
Did you substitute the earlier brass floats for plastic floats???

Remember the float level cannot be so high in the float bowls that the gas is constantly over flowing out of the over flow tubes.
The float level cannot be so high that if you just wiggle the bike that gas goes into the over flow tubes.

The bike would be a FIRE hazard.

You also mentioned the fuel lines. and I keep wondering if you have connected the fuel supply lines to the vent lines. Or mixed them up.
You should have FOUR vent lines going overboard.

Sure would be nice to have a photo.

[lucky]

General thinking...LUCKY

If you do not fully understand how some part works, it means that you could have  a problem causing the bike to not run correctly.

[lucky]

General thinking...LUCKY

If you do not fully understand how some part works, it means that you could have  a problem causing the bike to not run correctly.

EXAMPLE:  The float needle. If it has a little spring and plunger, have you really looked at it to see what that spring and plunger do?
Does it open and close something ? NO it does not.

[kammery]

 The fuel lines are mounted correctly . I have one hose on each of the float bowls and one on each vent all tied together and pointed to the ground . When I bought this it had brass floats . It still has brass floats . Since my petcock is on the left side and the round hole is in the right side . I was suspecting the carbs where not the original . Does it make a difference ,  this is the question . There where several things not done correctly on this CB750 . I worked for Honda about 30 years ago ,trying to remember some things at times is impossible . Where I live at I have yet to see another CB750 . I will try the suggestion for the photos .  Winter is near so I can disassemble some things and start over  . Thanks to everyone for the input .

[kammery]

 Found the number on my carbs ,the number is 7A this is the only number on the body .  from looking around on EBay etc. It looks like I have to run the fuel line from the right side of the carb assembly to the petcock on the right . As far as my binding throttle assembly there is a carb set for parts only on EBay .Going to bid on it . Have not found a chart or any info if this is the right carbs for my year . Will try this weekend to take and upload pictures as suggested .

[DH]

Hose routing for a K5.....Instructions from a hose kit bought from ebay seller dgs38. A nice kit.

[lucky]

If this is the bent rod you speak of, you will indeed have sync issues.
the needle plate may also be bent, especially if someone tried to open the throttle while the throttle valves were varnished/stuck.
The red arrow points to the needle plate.  The plate keeps a pre load on the spring. If the plate is bent upward, (spread apart) the spring is no longer effective, and throttle rod looseness results,
and the carb will never sync  correctly. If you take it apart, it'll make sense. If you suspect this scenario, compare it to one from another carb. BE CAREFUL if you do, those plate mounting screws are very prone to stripping out. Hopefully the phillips heads aren't too rusty.

the plate mounting screws get stripped when you do not use the correct size screwdriver.
Use a #2 phillips bit. Long screwdriver.

[bjbuchanan]

As far as I know the 750's didn't use orings on the air screws, I know that my 72 and 76 rack don't use them. Also a rack for the "proper" year isn't gonna magically fix what is wrong with the bike. You could buy the rack and have the same issues and then what?

Stuff like that usually comes down to dirty carbs, the holes are tiny and shots of carb cleaner doesn't clean the bodies.

[DH]

If this is the bent rod you speak of, you will indeed have sync issues.
the needle plate may also be bent, especially if someone tried to open the throttle while the throttle valves were varnished/stuck.
The red arrow points to the needle plate.  The plate keeps a pre load on the spring. If the plate is bent upward, (spread apart) the spring is no longer effective, and throttle rod looseness results,
and the carb will never sync  correctly. If you take it apart, it'll make sense. If you suspect this scenario, compare it to one from another carb. BE CAREFUL if you do, those plate mounting screws are very prone to stripping out. Hopefully the phillips heads aren't too rusty.

the plate mounting screws get stripped when you do not use the correct size screwdriver.
Use a #2 phillips bit. Long screwdriver.

They can also strip when the screw heads are so rusty, they're nearly desintegrated
In any case, CARE should be exercised, using proper tools.