Topic: 78 cb750f started today!!!

[cfue60]

i Finally got my bike started today. I can't keep it on but it runs as long as hold the gas and use starting fluid to start it. It probably needs to have the carbs rebuilt and so on. i do have a question. there are two tubes coming up from the carbs towards the petcock. one is for the fuel in from the pet cock. the other i thought was for a vacuum source that went to the motor to help pull fuel. anybody know what that is for? Also, Is there suppose to be oil in the alternator area. i was reading on a thread that said the jets could be cleanes without bein removed from the carbs on a 78. anybody know if that is possible? sorry for all the question this is the first cb750 i put together.

ciro

[my78k]

Not sure of the differences between the K and F models but on my 78 K there is no way that you could clean the jets while the carbs are mounted...the idle jets are pressed in and need to be yanked (use a rag and vise grips) to have any shot at getting them clean!

As for the 2 tubes I am not sure about that either...mine only has 1 that goes to petcock...wait now that I think about it I think I do have another spiggot on there and I believe it is a vent tube...but I am going from memory (which has been clouded by a few rye and diet pepsi's!  ) and it is too dark outside to go out and have a peek...heck if I have trained the neighbours right if they see someone poking around the bike at night I am liable to have a run in with their 2 rather large and protective Pit Bulls!

I can have a peek tomorrow and say for certain...

Congrats on getting the bike running tho!

Dennis

[cfue60]

thanks for the help. i took my carbs apart and cleaned the jets. i found that the pump diaphragm was trashed. what affect would that have? i ordered a new one and it should be here soon.

ciro

[Lumbee]

there are two tubes coming up from the carbs towards the petcock. one is for the fuel in from the pet cock. the other i thought was for a vacuum source that went to the motor to help pull fuel.

This was on a thread a while back...while no one seemed to know for sure I think we concluded that the other tube was a some sort of vent tube.  I think there maybe an attachement  on the airbox for it.

Is there suppose to be oil in the alternator area.

Yes.  Not alot, but enough to leave a nice pool if the cover is removed, or leak if not good and tight.

the jets could be cleanes without bein removed from the carbs on a 78. anybody know if that is possible?

No.  Space is to tight.  The older carbs can be accessed this way, but not the 77/78.

i found that the pump diaphragm was trashed. what affect would that have?

The Accel. Pump is for...well, acelleration.  It gives a squirt of gas when the throttle is cracked.  Can't really say what it would feel like if it wasn't working, but possibly stuttering when accelerating hard from a stop.

[cfue60]

thanks for the info. i will check the air box for a connection. i will go ahead and put the carbs back on the bike. i did not know if it had to do with idle. i need to get it to stay on and idle.

ciro

[Lumbee]

...I've never connected that 2nd line to anything, so wether its connected to the airbox or not, that should not effect idle.  You said you cleaned the jets.  Did you actually remove the slow jets (not the main jets) and clean them to where you can see light throught them?  Inability to idle usually is related to the slow jets...

[cfue60]

no they were pressed in so i did not want to mess with them because of fear of damage. i sprayed carb cleaner and used small plastice straw like things through them and compressed air. i started it up and rode it around the block but still dont have the ability to keep it on below 2k rpms. i might take them back off and try to actually get them off to clean them better. the idle mixture screw was in to far according to the book. it was only out 1 turn instead of 1 and 3/4 for base settings. that seem to help but still have to have it higher than normal rpms.

ciro

[Lumbee]

...no can do cfue...if the bike sat up for any amount of time chances are the slow jets are clogged.  the only way to get them clean is to remove them.  someone mentioned putting a rag around them so u don't damage them.  i just use pair of pliers and i've never had any problem.  just get a good grip and be sure to twist as you only slightly pull on them.  once out spary carb clener throug them, and you should be able to see through them.  you may need to use something like guitar string or i clip a wire from a stainless steel brush off and use that to poke through the jet to clear it.

[cfue60]

i started it and let i run for a while. the idle has to be high at first. then i lower it to about 1500, after a while it goes real high and i have to turn the idle down. then a little later it goes high again and i have to lower it again. i will try to take the low jets out tonight using a rag and pliers and hope that helps. thanks for all the help guys.

ciro

[upperlake04]

cfue60 - This not an experienced mechanic talking so maybe half points for my advice. 
  From reading posts here , it seems that there could be two other reasons for your present racing idle. One could be that the slides are not all set at the same height  The carb allowing more fuel to its cylinder would  'pull' the others along.  Apparently a small difference can make a big effect.  Also if the rubber boots connecting the carbs to the engine are not perfectly tight and they leak air, the engine rpm will surge.
 Regardless if this helps your problem or not , they still have to be correct for the engine to run well. FWIW

[cfue60]

Well, I took the bike out for a 70 mile shake down ride. it was a great thrill. A few issues came up. the leak from the generator side cover is not from the gasket but from 1 of 2 holes in the bottom of the actual motor right under the generator. they must be weep holes to keep the oil low in the generator, but how do i fix it? also the carbs had not overflowed since I adjusted the floats but when i filled up the gas tank, the carbs started to overflow when I came to a stop. They seem ok when i was riding though. i tell you guys it was still a blast. the bike handled really well. i was going through some nice curves and keeping better lines than my friends '01 Vulcan 800. I had forgotten how much fun i used to have on this thing. I also have an '88 nx650 but it doesn't turn as well as the cb750f.

Ciro

[cfue60]

i had the carbs tuned and the timing set on the cb750f. it runs way better now all the way up to about 75% throttle then it just bogs and almost dies. the only thing i have modded is the back bottom half of the air box was cut off. would this cause that. it runs great the rest of the time except at WOT. the leak by the way was cause by the a set of bolts i missed and left our under the alternator.

ciro

[Lumbee]

i have modded is the back bottom half of the air box was cut off.

   How much of the airbox did you cut off, and why?  If this mod. is allowing more air into the carbs, then you should jet [up] accordingly.  If you are running lean, this could cause you to lose power at WOT throttle...more info please...

[eldar]

So we got a bit of a ratbike here huh? That happens. Well first off, pull the low jets out. They WILL be cruddy. Clean them with a piece of wire or carb cleaning tool. Clean the seat for the jet too. Crud builds up there also. Pull the mains again and reclean with a wire or carb tool too. Replace the accel pump. You can get one off www.partsnmore.com. I got mine there and it seems to work just fine. About $18 which is cheaper than any place else.

Set your idle screws at about 1.75 turns out since you have an F.

I would recommend getting a new airbox, otherwise you may need to turn the idle screws out more and put washers under the E-clip for the needle to richen the mix a little.

Make sure your boots are tight when you put them back on, might need new clamps for this if yours are stretched out.

Clean out for float valves and seats too. If you pee gas while running, you will be asking for trouble sometime down the road. These bikes can pee gas after sitting for a while but not at a simple stop.

Do a bench sync on the carbs also to se the slides at the same height.

[cfue60]

i had the carbs synced and fixed the overflow problems on the carbs. it was only number 2 overflowing so i replace the float needle. i only cut the back of the bottom of the air box. i did not want air bypassing the air filter. i am from the  turbo car world and the first mod was always to have a little more air coming in. the motorcycle has a kn filter and i just wanted a little easier breathing. i have already replaced the accel pump diaphragm and took out and cleaned the low jets. i will try to replace the air box and see how that goes.

ciro

[eldar]

Well thats where you went wrong! These bikes dont really take advantave of extra air like cars do. You may also want to check on other peoples experince with the k&n filters. Not many here use them. Most us Uni if anything other than stock. But at least you have not bypassed the filter like some do. It could be possible that you might be running too lean. The 77/78 years run lean enough the way it is!

[Lumbee]

...how do your plugs look?

[eldar]

well like spark plugs! duh!

[Lumbee]

 

[eldar]

I had to do it to someone sooner or later!

[cfue60]

i guess i will try the stock filter and box. i will have to see if i can buy one on ebay and get the filter from partsnmore. i will pull a sparkplug to see what it looks like.

ciro

[eldar]

Dont get me wrong. I am not saying to dump the k&n filter. just to enclose it in a full airbox. but if you still want the filter, go to your honda shop first. I got an emgo filter for $13 which is the same type of filter partsnmore had.

[Lumbee]

...the reason I said look at your plugs is because after rethinking your first post, the bike really shouldn't fall on its face like you described unless its crazy lean.  So if the plugs look OK (not lean), then the air box is not your problem.  Sounds like it could be a carb issue...I saw you said you adjusted the floats to stop them from leaking/overflowing...did you overadjust them?  Sounds like the motor could be starved for gas.  might want to recheck the float height...

[cfue60]

i will check the plugs. i tried to set,the floats to spec  but had to use a tape measure. i dont have a float tool. bikes are wierd if it was a car
i would say it was rich and gas was putting out the spark.

ciro

[Lumbee]

...check u'r timing too...symptoms you describe could be that too...

[cfue60]

Well here is an update. I put the stock box back on and also hooked up the top hose to the vent hole on the airbox. I thought there was a check valve in there so i blew on it and the air went right into the air box. I did have air bypassing the air filter. The bike runs great all the way to 80% throttle. WOT is still flat but it doesnt almost die when i go WOT. starting and idle is better also. I don't have to put the choke on when it is warm already. I used to have to put the choke up anytime i was starting it , cold or warm. i cleaned my filter screen and learned i need to creme the inside of my tank. the result is the carbs are overflowing, all of a sudden after being fine for 3 weeks. there must be particles getting stuck on the seat where the needle sets.

ciro

[cbjunkie]

don't forget - there are some modern and more reliable alternatives to KREEM...

[Lumbee]

http://www.por15.com/PRODUCTS/COMPLETEKITS/CYCLETANKREPAIRKIT/tabid/103/Default.aspx

[cbjunkie]

a-huh...now who's following who, bud? 

[Lumbee]

...well you baited me into it...how are you gonna suggest "other" alternatives...and not mention what they are!!!