Topic: 1977 Cb750k Carb Issues

[ZCook]

Hey guys, new member here! Im working on a Cafe build with a 1977 Cb750k that i had purchased a few months back. The bike has about 14k miles on it, two owners, me being the 3rd. I purchased the bike for $400 knowing it didn't run. I replaced the points and condensers, oil filter, coils, fuel filter, and then soaked the carbs for about a week, i bought the rebuild kit off of dime city and went through the carbs replacing all the O rings, accelerator pump, and cleaned the bowls and any holes i could. Since then I've had the bike running, at first it would only idle with the choke on. After taking the carbs on and off multiple times and cleaning them up more, I've managed to get the bike to idle fine without the choke, The problem I've run into now is my accelerator pump is not squirting. Ive read endless forums on fixing this and have done everything, I'm at a standstill now and I'm really not sure where to go from here.

Ive taken the accelerator pump apart, i took the nylon retainer out and cleaned the little ball inside there. I did the same thing to the retainer at the bottom of the carb and cleaned that out as well. If i spray carb cleaner into the hole that disperses the fuel to all 4 carbs it sprays. Im assuming theres something in my accelerator pump I'm missing, I've also considered the float level being too low.

Any help would be awesome! I'm near the point of ordering CR carbs with the headache I've had with this lately
Thanks in advance!

[martin99]

First thing I would check is the size of the o ring that sits on the check valve (little ball and nylon retainer) in the #2 float bowl. This is more critical than you may think. Too fat, and it will compress and spread when the bowl is mated to the carb body and will restrict fuel flow to the carbs. If you've used aftermarket repair kits, this is highly likely to be the cause of your woes IMO.
Good luck

[Wobbly]

Check your installation against manual. Most parts are still available from Honda--and that is the way to go rather than the aftermarket junk.
http://www.discounthondaparts.com/fiche_section_detail.asp?section=2451882&category=Motorcycles&make=Honda&year=1977&fveh=131105

[flybox1]

First thing I would check is the size of the o ring that sits on the check valve (little ball and nylon retainer) in the #2 float bowl. This is more critical than you may think. Too fat, and it will compress and spread when the bowl is mated to the carb body and will restrict fuel flow to the carbs. If you've used aftermarket repair kits, this is highly likely to be the cause of your woes IMO.
Good luck

+1 to this!!!

[harisuluv]

If i spray carb cleaner into the hole that disperses the fuel to all 4 carbs it sprays. Im assuming theres something in my accelerator pump I'm missing, I've also considered the float level being too low.

What am I missing, it sprays, all good, no?

On a side note, buying CR carbs will just give you many more, different headaches.  They are not bolt and go, they require a lot tuning and expertise and are made for racing, no daily driving.

[ZCook]

If i spray carb cleaner into the hole that disperses the fuel to all 4 carbs it sprays. Im assuming theres something in my accelerator pump I'm missing, I've also considered the float level being too low.

What am I missing, it sprays, all good, no?

On a side note, buying CR carbs will just give you many more, different headaches.  They are not bolt and go, they require a lot tuning and expertise and are made for racing, no daily driving.

When i spray carb cleaner into that hole it disperses to all 4 carbs and sprays out the the brass nozzle. When i put the carbs all back together with the accelerator pump, the actual pump isn't pushing fuel to the nozzles.

And thanks for all the info guys! I've read about the O ring, I'm going to check the O ring today, ill post my results later!

[harisuluv]

Well your problem is pretty simple then.  You have verified that the passages from beginning to end are clear.

Your issue is probably not the o-ring, if you were losing pressure there it would be leaking and you would probably notice it.  Basic accel pump install stuff--the holes need to be lined up--in the diaphragm and the corresponding bowl holes.  Assembly goes assembly pump in, then spring, then cap.  If you got the spring on the wrong side it won't work.

The accelerator pump is actuated by the throttle.  So follow the rod to where it is physically pushed down and verify that it is making contact and that it is actuating properly.  There are many different part numbers for the pump and if you get the wrong one it might be too short to reach.

Next and probably most likely for you is that the check valves are not operating correctly.  You have added a variable to this as you have taken them apart, which is ok but takes some expertise.  Remember, the check valves are supposed to operate only ONE way.  You might be able to spray carb clean one way, but it needs to not allow flow back the other way.  If it does then you will never build up pressure in the circuit as once you introduce it, pressure will leave again, you will never be able to "prime" the circuit.  Remember there is a check valve in the bowl AND the bowl accel pump cap.

[kerryb]

When I sprayed carb cleaner in the accelerator pump, the cleaner destroyed the rubber diaphragm.  After purchasing a new diaphragm from Sirius, (and the right o-ring at the bowl junction) it worked fine.  I don't use carb cleaner on carbs much anymore, too hard on the rubber parts.

[flybox1]

you can fill your #2 carb bowl(and accel pump installed) with liquid, and actuate the diaphragm plunger.
If the system is working, you'll get a nice spray out of the check valve.

[riffman12]

if you have the later carbs with the accelerator pump, I believe you can not soak the entire assembly. there is a felt piece in the pump that can be easily damaged depending what you soak it in. If you search, you should see some threads about the felt piece.

Could be part of your problem.

[flybox1]

no felt in the accel pump system, but as mentioned, the newer aftermarket diaphragms will melt under some carb cleaners.

[shotty750]

no felt in the accel pump system, but as mentioned, the newer aftermarket diaphragms will melt under some carb cleaners.

Is this true of the K&L accelerator pumps? Just purchased one and still need to clean up the carbs. Using Valucraft carb cleaner which I've heard is pretty tough #$%*!

[flybox1]

yes.  do not expose it to carb cleaner.  you'll end up with a gooey black popsicle.
if you need to clean it, use simple green.

[shotty750]

Phew! flybox saves the day....those little f*ckers are expensive!!

[scottly]

Make sure both of the "Mickey Mouse" ears in the pump diaphragm have clear holes in them; there have been problems with aftermarket diaphragms lacking one of the holes. 

[brewsky]

Make sure both of the "Mickey Mouse" ears in the pump diaphragm have clear holes in them; there have been problems with aftermarket diaphragms lacking one of the holes. 

+1

[AintNoEasyWay]

Make sure both of the "Mickey Mouse" ears in the pump diaphragm have clear holes in them; there have been problems with aftermarket diaphragms lacking one of the holes. 

+1

+2. Exactly what I was about to say.