Author Topic: Carb Question**********TT help! (Read 2333 times)

hapakev · « **on:** June 22, 2009, 05:05:41 PM »

I have a 1978 CB750F and recently put a Mac 4to1 header on. The bike will only run with choke, and mixture screws are more that 3 turns out. I am sure that there is a point where the fuel mixture screws stop adjusting anything. I have read here that .020" shims under the needled will work for richening up the mixture. Will this help with idling?

eurban · « **Reply #1 on:** June 22, 2009, 06:41:13 PM »

Shimming the needles with have little effect at idle. The idle mix screws have effective range of around 3 turns out. . . . . .Sounds like your idle circuit is impaired. Did you remove your pressed in pilots and clean them and the passages behind carefully? Sometimes it takes a few tries to get the idle circuit clear. You also might have an air leak. . .

TwoTired · « **Reply #2 on:** June 22, 2009, 06:42:35 PM »

You using an air filter?

hapakev · « **Reply #3 on:** June 23, 2009, 10:20:14 AM »

I have a stock air filter and box. I have cleaned the idle jets twice, but maybe I need to do a more thorough job on the idle circuits? could that also be causing a small leak into the airbox, from the front of the carb?

hapakev · « **Reply #4 on:** June 23, 2009, 03:27:02 PM »

TwoTired · « **Reply #5 on:** June 23, 2009, 04:04:19 PM »

Doesn't really matter how many times you clean something. Until you actually get the jets and entire idle circuit passageways really clean and prove that they flow between all four exit holes from that passage, the cleaning process is just exercise.

The choke applied causes higher throat vacuum. If the idle circuit isn't working, you force the mains and throttle valve to spit more fuel in order to compensate for idle circuit deficiency.

Did you prove the idle circuit was flowing during your cleaning? Or, did you just make an assumption it would?

hapakev · « **Reply #6 on:** June 24, 2009, 11:41:27 AM »

I made sure that the passageways sprayed through with cleaner, but I was not aware the spray would have to come out of 4 passageways at once? I am gonna have to do it again

TwoTired · « **Reply #7 on:** June 24, 2009, 11:45:36 AM »

With the parts stripped from the carb body, there are four holes to consider to the passageway. The air jet, the slow jet the mixture screwhole, and the carb throat exit.

FYI

hapakev · « **Reply #8 on:** June 24, 2009, 02:19:52 PM »

OK thanks, I guess I will be in them again. Is "air jet" another name for the main jet?

TwoTired · « **Reply #9 on:** June 24, 2009, 02:59:32 PM »

Quote from: hapakev on June 24, 2009, 02:19:52 PM

OK thanks, I guess I will be in them again. Is "air jet" another name for the main jet?

No.
At the carb mouth entrance you will find two jets. One feeds air to the slow/idle/pilot system, the other supplies air the mains and throttle valve emulsion tubes.

hapakev · « **Reply #10 on:** June 24, 2009, 04:56:12 PM »

I am 99% sure that both of the copper tubes in the carb throat, along with the slow jet and fuel screw port, were clear because I sprayed carb cleaner in both and saw it shoot out the bowl area, but you are saying that if I spray into one hole then it should come out of the other three?

TwoTired · « **Reply #11 on:** June 24, 2009, 05:41:28 PM »

Quote from: hapakev on June 24, 2009, 04:56:12 PM

I am 99% sure that both of the copper tubes in the carb throat, along with the slow jet and fuel screw port, were clear because I sprayed carb cleaner in both and saw it shoot out the bowl area, but you are saying that if I spray into one hole then it should come out of the other three?

One of those brass tubes gets to the "bowl area" via the slow jet or the hole where it was located and the mixture screw which is slightly ahead of the bowl for the 77/78 carbs. Should also come out into the carb bore. The other of those bass tubes should come out the casting feature that houses the emulsion tube, either end of it actually.

hapakev · « **Reply #12 on:** June 28, 2009, 08:59:34 PM »

OK I am a little cornfused??? at either end of the emulsion tube? thats screws into where the needle protrudes from the slide and houses the main jet? If I spray pressurized carb cleaner into one brass tube, it comes out of the needle/main jet. Spray into the other tube comes out of the slow/idle jet hole, but nothing out of the mixture screw hole. If I spray into the mixture hole it comes out into the carb bore (outlet).

So if I spray into the slow/idle jet hole, should it come out of the mixture screw hole and the brass tube at the inlet?

Also I found no O-rings at the Accel. Pump shaft and fuel outlet, where the carb body meets the carb. Could this be causing my problems?

hapakev · « **Reply #13 on:** June 28, 2009, 09:02:14 PM »

Also, looks like my intake rubber gaskets are possibly not sealing properly at the carb inlet and box outlet. It doesn't appear that they would be leaking badly, but nonetheless.

TwoTired · « **Reply #14 on:** June 28, 2009, 10:00:59 PM »

Main emulsion tube is fed fuel from the main jet, and air from an air jet at the carb mouth entrance. After the emulsion tube mixes air and fuel into a froth, it exits in the the carb main bore via the throttle valve jet, where the slide needle inserts.

A separate circuit, is the slow/Idle/Pilot circuit The jet for this circuit has it's own tiny emulsion tube. This tube is fed fuel from its jet and air from a separate air jet at the carb mouth entrance. This resulting froth is fed to the Idle Mixture screw which regulates the exit in the main carb bore.

If you feed liquid under pressure into any of the holes of a selected circuit, it will exit at its most convenient point, so you have to block the ones you can se flowing to force it to come out of a hole that does not appear to be flowing.

For example, if you feed pressurized liquid into an air jet, and see it come out the jet hole, then block it and it should come out one of the remaining two holes. Block that one and it should com out the last available hole to exit. Now, you have "proven" flow to each outlet in that particular circuit.

The other air jet will feed the main emulsion tube. Pressurized liquid injected there will come out the main jet or the throttle valve jet. Block the one you see fluid passing and it should then exit at the other one.

From memory, The accel pump actuating shaft has a dust boot. But, there should be no fuel on the rod side of the pump diaphragm. I believe there should be an oring for the circuit where the bowl meets body. But, it's been a year since I was inside these.

http://www.cmsnl.com/honda-cb750f-750-super-sport-78-us_model7240/partslist/E++2001.html

Cheers,

hapakev · « **Reply #15 on:** June 30, 2009, 10:23:26 PM »

as I thought, all passageways are clear. I "proved" they were by spraying into air inlet and seeing it spray from slow jet, then plugging slow jet and seeing it spray from the mixture screw port. I believe that is it for the idle circuit. I sprayed in the other air inlet and saw it spray from the main jet. The only other thing I could find was the accel. pump was missing the o-ring in between the carb body and float bowl. If I manually pump it it leaks out the side where o-ring is missing. Also there is no dust boot, but I'm not too concerned with that. Other than that, my intake carb boots may be leaking? they are not hard or cracked, but maybe a little loose or worn out. I guess that's next.

Spanner 1 · « **Reply #16 on:** July 01, 2009, 09:54:09 AM »

Hap, these PD carbs are totally dependent on the acc. pump 'squirting' fuel into the carb throats to get the motor to pick-up above idle...I had 3 out of 4 blocked and total die with ANY throttle applied !.....your previous topic asks about the o-ring, then your last reply here states 'no o-ring, gas is coming out at the bowl/body junction'.....must have that o-ring in place, then prove the acc. pump is working by observing each carb throat for fuel spray EACH time you twist the throttle ( yeah, take the air-box off again ! ). That should do it.

hapakev · « **Reply #17 on:** July 01, 2009, 05:27:28 PM »

I found an o-ring that I think will work. I'm gonna test it tonight. I sprayed cleaner into the microscopic port that goes to all four carbs and it is clear with spray coming from each post. The pump works, albiet I haven't seen it work with the carbs put together, but it pumps fuel out of the delivery port onto my bench. We will see what happens tonight.

hapakev · « **Reply #18 on:** July 02, 2009, 09:50:05 AM »

The o-ring was slightly too big, so its gonna have to be tonight. I will get the right size from work today

News:

Author Topic: Carb Question**********TT help! (Read 2333 times)