Topic: 78 cb750 having problems - idles but stalls with throttle

[kine8282]

I just picked up a 78 cb750K. It idles fine but as soon as I pull the thorttle back the engine, it doesnt surge up it just dies. I pulled the airbox to check since it almost sounds like the motor is making a "sucking" sound. Not obstructions so I tried to run the motor sans airbox and the bike idles but again dies as soon as i try the throttle. Any ideas before I get into tearing the carbs down?

The bike was the PO's daily or atleast that was what I was told. I've worked on a 75 cb550 before this cb750 so i've got some experience but the 750 are different so if anyone can give me a steap by step as to what to check that would be awesome...

[thrownchain]

Check to see if something is living in the exhaust system. If not, next time it's running, spray carb cleaner or something similar around the carb boots, between the carb and head. If it revs up your carb boots are leaking air and need to be replaced.  Also check the ignition timing just to make sure that's with in spec. Do that then get back to me.

[kine8282]

ill try that...interesting, i thought that the carbs were not the constant velocity types...like on the older Yammys

[eldar]

They are not cv carbs. The spray trick is to check for air leaks. Have you done any work on the bike? If not, then you need to start there. Give us a history of the work you have done. Your carbs are most likely all crudded up.

[kine8282]

They are not cv carbs. The spray trick is to check for air leaks. Have you done any work on the bike? If not, then you need to start there. Give us a history of the work you have done. Your carbs are most likely all crudded up.

i havent done any work on the bike and the PO said is sat for a few months...the only thing ive changed so far is the battery. ill try the spray test tomorrow and will let you guys know. stay tuned

[Hush]

Urgggg sounds like someone will be tearing out a set of carbs very shortly.
But don't let me pre-empt the logical checks first!

[KingCustomCycles.com]

A leak at the boots will show up as hard starting and no, or high, idle speeds. the engine will rev just fine if you can get it there.  I'll let two tired explain why that is if he hasn't already somewhere.  Your problem is not leaking intake boots.  Check to see that all exhaust pipes are equally hot.  I have had your symptoms with a plugged idle jet or two.  If you have an accel pump diaphragm, check its condition and verify it is squirting fuel when the throttle is opened quickly.  Also, the statement of the PO is almost always suspect and the problem could be needle jet adjustment or the wrong jet sizes.  If it is a completely stock airbox and exhaust, that requires completely stock jetting.  I hope this helps.

[TwoTired]

Sounds rather like your accelerator pump isn't working.  It's supposed to squirt raw gas into the carb throats when you twist the throttle.  Removing the air box will make the "wheeze" worse.

The pump lives at the bottom of #2 carb bowl.  You can remove that bowl and check for operation with the bowl full of gas.  There are two inaccessible check valves for the pump.  They can be checked for operation with canned carb cleaner with those red tubes.    They should only allow fuel to go one way through their respective passages. The diaphragm should be checked for cracks, too.  And, the mickey mouse ears should have holes the same size as the the one they mate with.

Finally, there are posts at the carb inlets.  These are the accellerator pump squirters with tiny exit holes.  With #2 carb bowl installed and full of fuel, these squirters should push a nice shot of raw fuel into the carb throats each time the throttle is twisted (after the tubes are primed with a few pumps).

Cheers,

[eldar]

Bottom line, clean the carbs. Do it and get it over with. Do it good though. Do not really bother with anything else until you do this. Many find that they continue to have issues until they clean things out.

[kine8282]

thanks dudes...atleast getting the carbs off a 750 is ALOT easier than on a 550, not more skinned knuckles. wish me luck

[eldar]

Remember to split the airbox. Makes removal much easier for that as well.

[kine8282]

thanks everyone...tore through the carbs this afternoon and found that the rubber tubes between the carbs that the accel pump feeds the fuel through was clogged up. after servicing those and cleaning up the rest put the carbs back together and slapped on the bike and it was gold. Except for a slight delay with the throttle response but i noticed that the accel pum was leaking and i found that i forgot to put back the o-ring on the accel pump so i suspect that this is why the throttle response is sluggish. its funny how little things cuase big problems...gotta take the carbs out again to make sure i didnt forget to put anything else back!

1 day 1 bike part at a time...

[markreimer]

Hopefully someone can help me out here, I'm having the same problem on my '78 k8. It's my first bike, so keep in mind I'm a bit green. I bought the bike running but it idled poorly and when you twist the throttle quickly, the engine would die.

I took the carbs 100% apart, had the bodies cleaned in an ultra-sonic cleaner as well as a soaking in carb cleaner, replaced the main jets, needles, adjusted the floats, cleaned the slow jets (could pass a thin guitar wire through), I had some advice and i'm positive the the carbs are totally clean. I replaced the acc. pump as well, but I'm still having this problem. The bike runs and idles considerably better, but the quick twist issue is still here. It's worse when the bike has just been started. After ripping around for a bit it's better, but still dies if I really hit the gas. I've also synced the carbs with some gauges and am running new premium fuel, a new air filter, and the plugs look ok. New battery as well.

When I took the acc pump out it never had an o-ring with it. The new kit included an o-ring and a little rubber boot (looks like a miniature version of a fork boot), but I can't see where it would be installed. If I slide the rubber boot down the acc pump shaft the spring doesn't have enough tension to push it back down. Could this be my problem? How do I test if the acc pump is working properly?

I have a video of how the bike is running now on youtube, seems I can't post a link here directly but if you search for my user-name ( mnomad45 ) you can find it as my latest upload. any advice would be greatly appreciated.

[TwoTired]

Stock air filter/exhaust?

[markreimer]

The air filter is stock but it came with a 4-2 exhaust, turn-out style. Unsure of what brand it is.

[Trav-i]

Still sounds like clogged slow jets.  Did you actually pull them out?  I know they're press fit, but they do come out pretty easily with pliers and someting to pad the jaws.  Then I prefer to use a carb clean to wash the passage, and some compressed air to really get the debris out.  Then check the holes in the slow jet and make sure they're clear, then lightly tap them back in with something soft.

I have had to clean the carbs on my 78F countless time before it ran like it should, as a matter of fact I just cleaned them Monday with that same procedure, and it runs awesome.  Ask anyone with PD carbs and they will tell you it will take multiple cleanings before thigs run nice and smooth.

Hope some of this helps.

Good Luck

Travis

[markreimer]

Yep, I pulled them right out, cleaned them in an ultra-sonic cleaner, and was able to slide a wire through the slow jets easily.

[gnarlycharlie4u]

Hopefully someone can help me out here, I'm having the same problem on my '78 k8. It's my first bike, so keep in mind I'm a bit green. I bought the bike running but it idled poorly and when you twist the throttle quickly, the engine would die.

I took the carbs 100% apart, had the bodies cleaned in an ultra-sonic cleaner as well as a soaking in carb cleaner, replaced the main jets, needles, adjusted the floats, cleaned the slow jets (could pass a thin guitar wire through), I had some advice and i'm positive the the carbs are totally clean. I replaced the acc. pump as well, but I'm still having this problem. The bike runs and idles considerably better, but the quick twist issue is still here. It's worse when the bike has just been started. After ripping around for a bit it's better, but still dies if I really hit the gas. I've also synced the carbs with some gauges and am running new premium fuel, a new air filter, and the plugs look ok. New battery as well.

When I took the acc pump out it never had an o-ring with it. The new kit included an o-ring and a little rubber boot (looks like a miniature version of a fork boot), but I can't see where it would be installed. If I slide the rubber boot down the acc pump shaft the spring doesn't have enough tension to push it back down. Could this be my problem? How do I test if the acc pump is working properly?

I have a video of how the bike is running now on youtube, seems I can't post a link here directly but if you search for my user-name ( mnomad45 ) you can find it as my latest upload. any advice would be greatly appreciated.

the spring is supposed to go underneath the acc pump between the diaphragm and the bowl.

as for that boot thingy. put it in the hole where it belongs and slide the acc pump shaft through it after it's in place. this makes it easier.

edit:

also did you do the valve repair/clean on the acc pump?
http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/page1.html

[Trav-i]

Hmm... The last time my bike had that same symptom, my slows were almost completely clogged.  After a good cleaning it ran like it should.  The only other thing I can say is to check your carb boots for air leaks.  But it still sounds to me like dirty carbs.

[TwoTired]

What setting are you using for the Idle Mixture Screws?  The exhaust change (less back pressure) may require them turned out farther than a stock setting.

The little accordian dust boot goes on the accel pump shaft, keeps dirt out of the dry side of the diaphragm.  They go on before the bowl is attached to the body, if I recall correctly.

There are two ball type check valves associated with the pump.  They are held in with nylon plugs and quite difficult to remove a clean behind.  regardless, they must be "proven" to allow flow in one direction and completely block flow in the other.  I do this with aerosol carb cleaner using those red tubes, (and safety glasses).

You'll note the new pump diaphragm has two "mickey mouse ears" with holes.  These holes need to be matched to the hole sizes in the metal bodies.  I've seen them squashed closed or nearly so, restricting full flow.  Opening them up fixed the last of my throttle response issues on the CB750F2, even with the old diaphragm reused.

The squirt/jet posts at each carb mouth have tiny holes with an odd shape.  Make sure they are clear and the hole shape unaltered. (Inspection mirror).

Do check for a "colder" head pipe while idling.  It's a sign that all cylinders aren't pulling equal weight, and directs focus toward a cure.

Finally, have you seen the Honda shop manual section on adjusting pump arm to pump rod clearance?

Cheers,

[markreimer]

 To clarify, I slid the acc pump in first, then the spring on top, then bolted the bowl down after, so the spring is pushing the pump rod out of the assembly. If I put the boot over the rod, the spring cannot push the rod out of the assembly nearly as far.

Here is a photo of what it looked like PRE cleaning. I assembled it with the new pump the same way.

I have not tried the valve repair/clean mentioned, that's a great tutorial. I'll definitely try that.

[markreimer]

Idle mix screws are 1.5 turns out, as per the cleymor stock spec. Haven't tried adjusting those yet...

I'll try installing that accordion boot and cleaning out those check valves. Seems like that is the most likely culprit at this point. As for adjusting pump arm/rod clearance, I believe I know what you're talking about: There is a set spec for the distance between the acc pump rod and the pump arm, and you adjust it by bending the arm up/down as necessary, right?

I've checked the headers, all seem to be equally hot to the touch.


Can you clarify what you mean by the jet posts? Are these the brass rods that the main jet threads into, with several small holes along the sides?

[TwoTired]

Idle mix screws are 1.5 turns out, as per the cleymor stock spec. Haven't tried adjusting those yet...

Yeah, I'd like to Claymore that book. 
The exhaust change makes it not stock, right?  You have to "tell" the carbs there was a change.

I'll try installing that accordion boot and cleaning out those check valves. Seems like that is the most likely culprit at this point. As for adjusting pump arm/rod clearance, I believe I know what you're talking about: There is a set spec for the distance between the acc pump rod and the pump arm, and you adjust it by bending the arm up/down as necessary, right?

Yes. Zero to .008 In. clearance.

Can you clarify what you mean by the jet posts? Are these the brass rods that the main jet threads into, with several small holes along the sides?

No, you are describing the emulsion tubes.
The squirters/jet posts are at the carb entrance mouth right next to the choke plates.  The choke plates have holes in them where the squirters shoot through when the choke is closed.  Hard to see those jets without a small inspection mirror.

Cheers,

[markreimer]

Hm, any recommendation on the mix screws? I've read up on adjusting them but I don't have a good tach to connect and test.

Well I can say from memory that there is almost certainly more of a gap than 0-.008 inches between the rod and arm. I remember more like 2-3mm... If I have way to big a gap, that would mean it's not even being actuated, correct? That might be the problem right there.

Ok, I remember the jet posts you're talking about now, the choke plates rotate right up against them right? You can just see them poking out here? I didn't realize they squirted fuel out.

[TwoTired]

Get confidence your accel pump circuit is working well first.  Then turn the IMS out to maybe 2-2.5 turns.  I'm just guessing here, as I have no idea how much of a back pressure change your exhaust has over the stock exhaust.  Custom exhaust = custom tuning.

From the Honda shop manual....