Topic: 78F2 gearbox noise? headache

[jus750]

Hey everyone, Can anyone popint me in the right direction to fix my problem.
Here it is, 1 year full rebuild on my 78F2
details.
 Dyna ignition, coils etc,leads,plugs. timing seams to be spot on
900 DOHC pistons Ithink that m,akes it about 850cc
stock air box, new filter
Kerker 4 into 1 exhaust(think its a kerker) its a 4 into 1 anyway
thouroughly cleaned carbs(not sure on jet size) mixture screw all 1 3/4 turns out, ( have not messed with them as i am not sure how)
Full tank of premium 98oc fuel with lead additive.
Synced carbs, all the same vacumme.
Bike starts perfect, choke works well also, when running it runs perfect, plenty of power and pretty smooth, does splutter a fair bit on deceleration, which i dont mind.
Idles all over the place which i need to fix, when it is warmed up it idles at about 1300 rpm but it sounds rattly and can hear backfiring inside the carbs. I have to keep reving it at the lights cause it feels like it is going to stall.

how do i fix it, what should i be looking for, any help would be awesome

[joeb]

you may want to check for air leaks around the intakes.   

[TwoTired]

Air leaks in the induction?

carb bowl level the same for all carbs?

Is the accel pump circuit leaking into the carb throat or have check valves that aren't sealing?

You may have to readjust the pilot screws to compensate for the exhaust.  The book settings are for bikes that are all stock.  Mess with stock, mess with carb settings.

You don't need more octane than regular if you haven't increased the compression ratio.  You don't need lead additive unless you have experienced pinging.

When it approaches stall is it gradually losing RPM, or speeding up before stall.  Prior is a rich indication, latter is a lean indication.  Both should show up on spark plug deposits.

Cheers,

[jus750]

I only am running the higher octane to get away from the ethanol fuel, next tank I will go to 95 and stick with that. will that be ok?
Not sure on fuel levels in the floats, should I try the clear tube method in the drain hole or should actually check the floats and measure them ?
Do you reckon for the exhaust I should make it richer or leaner ? It has a baffle but is pretty loud
I am pretty sure there are no leaks on the intake, I used the wintergreen method and new hose clamps and the fit is very tight, around the motor and carbs.
I noticed on the screws you take out to put the vacumme guages in there are no whashers or O ring to seal around the screw, do you think air could be getting in there. ? do they need crush washers
the idle is varley constant, i mean once it has reached a rpm it does stay there, it just does not sound that happy, there is definatly some popping inside the carbs, what is this. ?
when i get home from work I will have the whole night to work on it, should i take thge carbs off the bike and check if they are clean or should I try and tune them again, what do ya reck,  thanks for the replys this site is the best

[jus750]

Also just thinking bout it , how do I test to see if fuel is leaking in, Is this easy to do, will testing the fuel level, tell me if this is right.
One thing I should add, when i was syching the carbs with the guages, At one stage the bike suddenly started to idle perfect, very smooth, I was over the moon, It only lasted about a minute and then it ran how it is now, whtdo ya reckon this could of been.

[TwoTired]

I only am running the higher octane to get away from the ethanol fuel, next tank I will go to 95 and stick with that. will that be ok?

Yes. I'd avoid gasalcohol if I could, too.

Not sure on fuel levels in the floats, should I try the clear tube method in the drain hole or should actually check the floats and measure them ?

The goal is the correct fuel level, regardless of the actual float setting.

Do you reckon for the exhaust I should make it richer or leaner ? It has a baffle but is pretty loud

A less restrictive exhaust usually requires more fuel for the extra oxygen in the chambers.

I noticed on the screws you take out to put the vacumme guages in there are no whashers or O ring to seal around the screw, do you think air could be getting in there. ? do they need crush washers

I'm used to seeing aluminum washers on those port plug for tight seal.

the idle is varley constant, i mean once it has reached a rpm it does stay there, it just does not sound that happy, there is definatly some popping inside the carbs, what is this. ?

A problem to solve.

Also just thinking bout it , how do I test to see if fuel is leaking in, Is this easy to do, will testing the fuel level, tell me if this is right.

It is a reasonable step in the investigation.

One thing I should add, when i was syching the carbs with the guages, At one stage the bike suddenly started to idle perfect, very smooth, I was over the moon, It only lasted about a minute and then it ran how it is now, whtdo ya reckon this could of been.

Is your fuel clean?  Little bits that temporarily get in the jets or adjust orifice might cause this.
So, could it be if the there are bits holding the float valve open or blocking full closure.


Cheers,

[jus750]

Massive day at work, Im going to start now on the bike.
1 question where should the fuel level be. I have read about 2mm below the gasket. Is this right. I know they should all be the same. Is the clear hose on the end of the drain hole the best way to check this?, I am thinking it would be ?
I have some orings to put over the port seal.
and I will check to see if any gunk is in the carbys as well.
Ifr I say riched the carbs up a 1/4 turn to allow for the exhaust do you think it would be a good starting point?
thanks again

[Terry in Australia]

G'Day mate, where are you located? You're talking 98 and 95 RON fuel, so not in the US, I guess? F2 carbs are buggars of things to get right, but when they are working well, they're as good as CR's, in my humble opinion. Did you pull the "pressed in" idle jets out and give them and the idle circuits a thorough clean? In my experience, poor idling on F2's almost always goes back to blocked pilot jets and idle circuits.

I wouldn't start mucking around with the mixture screws as yet, CB750's are not as sensitive to slight variations in mixture as other bikes, even though the F2's were set up pretty lean to keep the EPA Nazi's happy, they made up for it with accellerator pumps to cover any flat spots. Any "popping" is usually caused by air leaks. Do your carbs leak fuel? If they do, it's probably the tiny O rings around the mixture screws, which might be affecting the idle too. Don't use O rings oin the vacuum take off plugs, like Lloyd said, use aluminum or copper washers. Cheers, Terry.

[jus750]

cheers terry, two tired, joeb, I'm located in Sydney Australia
here is where I'm at now, pulled the carbs off the bike and stripped them down
there was actually a bit of crap in all the float bowls, mainly rust sediment that must of got through the filter i put in.
pulled the slow jets and found 2 where partly blocked
the jets are 115 main and 35 slow. Is it possible I have k7 carbs, they are the same size jets as the k7. don't know ? If so is the factory mixture for these 1 1/2 turns instead of 1 3/4.
question 2 can i just put clear tube on the nipple on the bottom of the carbs and crack the drain plug. ?( man, it would be cool if i could do this, I didn fancy drilling a hole into a drain plug and soldering a nipple on)
Just to clarify should the fuel level be 2mm below the gasket ?
where can i get the copper or aluminium washers from ?
would love to start up my compressor now and finish cleaning the carbs,I'm sure the neighbours would love it.

[TwoTired]

cheers terry, two tired, joeb, I'm located in Sydney Australia

You might think about putting that info in your profile.

here is where I'm at now, pulled the carbs off the bike and stripped them down
there was actually a bit of crap in all the float bowls, mainly rust sediment that must of got through the filter i put in.
pulled the slow jets and found 2 where partly blocked
the jets are 115 main and 35 slow. Is it possible I have k7 carbs, they are the same size jets as the k7. don't know ? If so is the factory mixture for these 1 1/2 turns instead of 1 3/4.

I don't know for sure what Honda sent to Australia, or if you have an American model.  If American, find the number stamped on the carbs and compare to this site:
http://www.motorcycleproject.com/motorcycle/text/specs.html

Rust particles can block full closure of the float valves, allowing the fuel level  to rise and allow droplets to wet the intake runners.  The popping might be from this.
The factory stated mixture screw setting is only accurate if you have the stock induction and exhaust.  Otherwise you have to engineer the proper setting along with the other changes made.

question 2 can i just put clear tube on the nipple on the bottom of the carbs and crack the drain plug. ?( man, it would be cool if i could do this, I didn fancy drilling a hole into a drain plug and soldering a nipple on)

Sounds like another example of misleading info from Clymer.  With the PD carbs, a clear tube on the bottom nipple and opening the drain screw should flood the tube and the level will rise to whatever the internal level is.

Just to clarify should the fuel level be 2mm below the gasket ?

I think somewhere in the 2 to 4 mm range is acceptable.  They do need to be the same across the bank, ie, if you measure 3 mm on one, they should all be the same.

where can i get the copper or aluminium washers from ?

In OZ???   I don't know.    But, I just went out and checked my F bike's PD carbs and the port tap plugs don't have any washers on them.  (My other bikes older style carbs do.)  So, you probably don't need them.  One way to check is with machinists dye (dychem Blue) coat the mating surface with the blue, install and tighten, then remove and look at the mating surface for continuous contact marks.  You could probably use a felt tip pen ink with the same effect.

Cheers,

[jus750]

well Im home from work, I reckon by the end of the night I will hopefully have the carbs back on the bike. Thankyou everyone for your help.
I have some clear plastic tube so once I get these suckers back together  can make sure the fuel levels are right, I reckon I have got about 3 hours to use my compressor b4 I will have to shut it down due to noise

[jus750]

ah no luck, put the things back together, hooked up the clear hose and all the levels are out, 2 carbs actuallyt overflow after a while, will have to try again tomorrow

[TwoTired]

What do you mean "no luck"?  You found something wrong that can be the cause of the problem.  Now you just have to find a fix the problem.  Sounds like progress to me.
 

Are your floats installed upside down?  The "rounded" side points up.

Cheers,

[jus750]

Good point. I will check when I get home, it is possible. Now I just gotta get home to check it out

[Terry in Australia]

Lloyd's right mate, you've discovered that you've probably installed your floats upside down so you've made a lot of progress I'd reckon!

[bender01]

Any pics of this Hot Rod?  Did the fender fit?

[jus750]

ok, have found the fuel tap needs to off on the bike otherwise the carbs overflow, having a bit more luck now, have found 2 carbs levels are now a bit low, im not stopping until I get this fixed. Here's some pics of me bike. Thanks for the fender Bender it looks awesome, just gotta get the little ding out

[jus750]

Ok finaly got the levels all the same, It helps when you turn the fuel tap off b4 opening the drain screw to get a level.
they are roughly 11mm below the gasket level, 3 where already pretty close and the third was much higher prettyu much on the gasket level, so i dropped it down to match the others and fine tuned the rest.
 Thing is, will this be ok ?
It just didn seam to make sence to change all 4, I had the carbs set up in a vise to measure, so they were tilted a little back, Im thinking when they are on the bike the gap will lift a little, They are however all the same. Tommorrow will tell, wish me luck. 

[CB750F2]

I am assuming that you have the KEHIN PD41B carbs - these are OEM for the F2. You need to ensure that the vents in the body above the fuel bowls are clear otherwise you will not be able to set the float levels correctly using the clear tube method. This is because the stub at the bottom of the fuel bowl three purposes - vent, overflow and drain. So, when you connect the clear tube to stub and open the drain screw the tube and the stand pipe fills with fuel. Thus any venting must come from the vent holes above the fuel bowl. If these are blocked then the floats will not shut off when they should resulting in high fuel levels.
I suggest htat you set the float levels to the manufacturer's spec. of 14.5mm, ensure that the vent holes are clear and then use the clear tube method to check that they are all even. I think that it is a good idea to do them on the bench. You will need to setup a temporary fuel supply at height that will give a similar head pressure to what a full tank of fuel would give. The 11mm measurement that you mentioned seems very high. I cannot remember mine exactly but it was a lot less then 11mm.
Regards, Pat from Nth Qld.

[jus750]

Cheers pat. I'm pretty sure the vent holes are clear. I am not sure how to  measure the 14.5 mm height, that's why I am using the clear hose method. I really only adjusted I bowl to match the others, hopefully they are
 close to the 14.5, I will have to look into how to measure them.

[CB750F2]

I suggest that you get hold of a service manual for your model bike. I am pretty sure that you can download a copy from this web site. It explains clearly how to set the floats. Pat from Nth Qld.

[jus750]

Ok here is where im at. pulled carbs back apart and the carb I was having trouble with is a problem,
I had a nightmare time stuffing with it to get the levels right and found the tab is now bent nearly all the way in, It seams there is hardly any adjustment in it, could the needle be stuffed ?it looks fine and the seat looks clean as. The other 3 carbs are set at about 14.5mm the other is out all the way and it already pushing on the pin on the seat. Was hoping to get this back together for the weekend.  anyone know where is a good place to new needles in OZ. I will prob replace all 4

[Terry in Australia]

Here you go mate, try this EBay seller: http://cgi.ebay.com/ebaymotors/HONDA-CB750K7-F2-FLOAT-VALVE-SET-NOS-CB-650-SOHC-CB-750-_W0QQcmdZViewItemQQitemZ270626250782QQptZMotorcyclesQ5fPartsQ5fAccessories 

[jus750]

Awesome, howd you find that. weird I dont think its on the aussie ebay.
thanks for the link this should see me sorted.( if i win that is) How much are they worth ?
Is that your set of cr carbs on ebay, if so they look sweet as, Hmm thinking...... Na Im going to stick with mine, might be back on the road by next weekend

[Terry in Australia]

No worries mate, and yeah, the CR's are mine, they're great carbs but too small for the 1060cc engine I'm building, so I've got some 33mm Mikuni smoothbores modified to go straight on. No idea what new needles and seats are worth, I haven't been in a bike shop since I discovered EBay in 1999! Cheers, Terry.