Newly purchased CB750K revs reset at 4000 RPM

[new_750k]

Hi All,

I recently bought a 1980 CB750K. The bike runs but there are numerous problems with it, most importantly the fact that it is losing power at about 4000 RPM. This forum has been very helpful for getting an idea of some of the work I will need to do so thanks to everyone here.

The seller had taken the carbs out and had them rebuilt but they were never sync'd. Apart from that I know very little about the history of the bike. I have no idea whether the tank was cleaned prior to this but what little I can see of it seems relatively clean and rust free, I will be putting a fuel filter in ASAP.

The problem is that when it gets up to about 4k, it's as if I just turned off the throttle, the bike jerks as it loses power and the throttle does nothing until I reset it back to nothing and start increasing it again. There are definitely some problems with the throttle cable so I will be replacing those but I doubt it could cause an issue like this?

I have set the petcock to RES just in case it was gummed up but nothing really changed.

Also, there is some engine noise, mostly at the top end, and I think this is likely the cam chain rattling due to the unsync'd carbs (the noise gets better when I pull the clutch in)?

I have no idea when the valves were last adjusted but it is definitely on my todo list. Could anyone tell me what parts I would need for that? i.e Do I need a new gasket?

Thanks

[TOOLS1]

Does your bike still have the stock air box, or has it been switched out for pods?
TOOLS

[new_750k]

Wow, TOOLS has replied! Thank you.

No the bike is entirely stock as far as I know, it definitely still has the stock airbox.

I've never encountered a problem like this before but when reading other posts about people's issues I always thought of the common problem being that the bike just didn't have much power above X RPM not that the revs actually dropped off completely... Is this unusual?

[TOOLS1]

Does the idle fluctuate? If it does it could have a vacuum leak. Also the pilot jets could be switched with the main jets.
TOOLS

[new_750k]

The idle doesn't fluctuate much. On starting with choke it fluctuates but otherwise it's fairly steady.

I have no idea of the quality of the mechanic who rebuilt the carbs so I guess that the jets being switched would be a possibility. Would mine be the only symptom though?

[motogrady]

Man, carb city this week.

Sounds again like switched jets, or clogged main jets, or the passages behind them.

First 1/4 turn of throttle, you are sucking fuel thru the pilot, or idle, or slow jets,
everybody calls them something different.

Half turn and up the main jet's kick in, right around where you are losing it.

Or, and Ive had this also, a clogged petcock or fuel filter, you run it hard it
starves for fuel, falls onto itself until the carbs fill again, runs great, starves for fuel
again, etc.

[new_750k]

Ah, that makes sense. Thank you.

I also had the same thought about the petcock, I tried switching to reserve to see if that cleared it up but no dice. I guess I will take that out and try to give it a clean before I start pulling the carbs off.

Is there any way to check the jet diagnosis without taking the carbs apart? Would it be possible to modify the point at which they switch somehow and then see if it changed from 4k?

Thanks

[motogrady]

May be a screen in the petcock, with both on and reserve going thru it.

I think Lucky or Tools has a way of dropping the float bowls on the carbs
whilst still on the bike.  Beyond me.

Ya, the pulling of the carbs, first time I did i, it was all weekend.
Now, it takes maybe an hour.
Seat, rear fender, battery and battery cage, pull the airbox back, cables and they be out.

I dunno, the passages behind the jets, especially where they dump into the airstream,
are the size of a thick sewing needle.  A bike sit's, the gas dries out and leaves a gum
or varnish, sometimes even worse, get's hard.  If the bike ran good when parked.........

You gotta get that crap out of there.

A guitar string, 100psi of air, a carb repair shop, boil them, ultrasonic vib tank for a day or so,
there's a few ways guys go at them.    good luck

[motogrady]

What about SOS?  I've never had to mess with one, but there's a few here that have.

Spawn of Satan, something like that.

[new_750k]

OK, I'll see if I can get the petcock out and try to clean it out.

My problem at the moment is a severe lack of tools. I've just moved to the US and I'm still waiting for my tools to arrive. At the moment I'm using the bike's toolkit :(

Quick question about pulling the carbs. I didn't think it was possible to move the battery cage? Isn't it welded to the frame? I may be remembering this wrong...

I would like to get the carbs apart even if that's not the problem. It would be nice to know that everything is actually in good shape in there.

As for cleaning, I can do the guitar string and boiling them, no compressor or vibe tank though. Is it worth getting some kind of carb cleaner? Any recommendations?

[new_750k]

Yes, I saw some posts about the SOS, the automatic petcock I think? I really have no idea what that is though...

[new_750k]

I actually found a pretty good description of the process here, just in case anyone else finds this thread while looking...

http://honda-cb750-s.456789.n3.nabble.com/DOHC-carb-cleaning-procedure-td867809.html

[motogrady]

The cage bolts out, the cables, when you get that far, slide the bank of carbs out
the right side, turn them sideways and they unhook kinda easy.  Take a pic so you know
how things go back.

[new_750k]

I just tried to take the petcock out but I have no idea how to remove the filter. It is stuck inside the hole in the tank and I don't want to try to force it out and damage it. Does anyone have any ideas?

The Clymer manual doesn't help much.

[TOOLS1]

Before taking it apart I would have taken the fuel line loose from the carb and let the gas run into a container to check the flow.
TOOLS

[new_750k]

I did actually check the flow from the hose briefly, it looked reasonable but I don't really have anything to compare it to.

Can you think of any way to remove the filter Tools?

[TOOLS1]

Most people destroy them while removing them. However it does not take much more then a steady 1/8 inch stream of gas coming out of the line to feed one of these bikes.
TOOLS

[new_750k]

Motogrady. I'm trying to pull the carbs at the moment, it seems a little tight to do without removing the battery cage so I'm trying to do that. I can't seem to work out how to remove the rear fender though... Also do i need to disconnect all the electricals?

[Rodgil]

If you undo the clamps on the rubber boots between the carbies and the head, you can move the carbs towards the head to give you more room to remove the air cleaner.

[motogrady]

hey New, yes, the electrics unplug, the red wire from the battery comes off also.
Don't get discouraged, they plug right back in.

the cage itself has 4 10mm (socket size) bolts on the top.
when the wires and battery are off, and unbolted, the cage comes out the right side also.
it's tight, just try a few different positions, it'll come out.

The fender, look underneath, I think it's just 4 bolts also.
Grab it and move it, you should be able to feel where it's secured to the frame.
the black underfender just sits there, get that out also.  it can be tricky getting it
back in properly, it kinda pinches or is wedged in the frame.

once that stuff is out, clamps off the rubber boots at the airbox.
with the rear fender off, the airbox will slide back all the way.
to keep it from moving forward, tighten the bolt on top.
airbox stays in.

The first few times I went in there, I pulled the airbox boots also, but you don't
really have to.  If you pull those, watch it, they're different, mark of note which goes where.
doing this gives you a lot of room, was worth it to me the first few times.

once all that stuff is good, loosen the clamps on the front boots.
get over the top of the bike, grap the carbs with both hands, and wiggle up and down while you kinda pull.  If they havn't been out in awhile, it might fight you a bit.
getting them back in might be a bit hard also, old rubber getting hard,
lately I use a big rubber mallet to set the carbs back in all the way,
working them left and righrt till the seat fully in the boots.

don't get overwhelmed, you get frustrated just walk away from it for a bit.


much luck guy