Author Topic: Need help with 78 cb750k (Read 4111 times)

scamper03 · « **on:** December 20, 2011, 06:30:05 AM »

Hi Guys,

I just replaced my motor and it seems to run very strong except I get occassional poofs out of the carbs. Any idea as what's causing it?

knowsnothing · « **Reply #1 on:** December 20, 2011, 06:33:47 AM »

When was they last time you synced them?

scamper03 · « **Reply #2 on:** December 20, 2011, 07:34:56 AM »

about two years ago. When my original motor crap out it only took me two weeks to find a relacement motor. I didn't have any issues with the carbs with the previous motor so I didn't even bother with it.

bollingball · « **Reply #3 on:** December 20, 2011, 08:27:39 AM »

When was the last complete and thorough 3000 mile tune up?
You need to start like you just got the bike and and check everything. Don't trust any thing the PO said has been done and if your carbs have been sitting for two years give them a good cleaning.
what kind of air filter is on it?

Ken

scamper03 · « **Reply #4 on:** December 20, 2011, 08:45:22 AM »

the carb was from my running motor and I am using a mini pod airfilters. Also, it's cold here so I had to sprayed carb clearners on the filters get the bike started. I don't know if that could cause the poofs.

madmtnmotors · « **Reply #5 on:** December 20, 2011, 09:14:33 AM »

Need more info:

3000 mile tune up?
Valve lash checked?
When does this happen (accel/deccel)?
Choke functioning properly? (Full choke should start it, no matter how cold it is outside)

bollingball · « **Reply #6 on:** December 20, 2011, 11:45:48 AM »

+1 for Mad No spray is needed when things are set up correctly. Sounds like valves or timing. anyway a good tuneup should fix it. I'm not clear just when it started doing this and what condition will make it happen. Can you make it happen or is it random?
I have the same bike a lot of people don't like these carbs I think once you learn to set them up they are simple and easy right now like (Mad) said we need a little more info on when it happens

Ken

scamper03 · « **Reply #7 on:** December 20, 2011, 12:53:27 PM »

It happens randomly but I noticed it when it idles. I don't think the choke is functioning correctly as it seems like it doesn't make any differnece. Once the motor is warmed up it doesn't happen as much.

I don't think the tune up was done on the motor in awhile as I sure didn't do it. I just bought motor from PO and installed it.

Bonesaw · « **Reply #8 on:** December 20, 2011, 04:12:16 PM »

did you sync the carbs after you installed the pods? What size jets?

TommyT · « **Reply #9 on:** December 20, 2011, 04:15:04 PM »

Mine was doing the same thing and would cough out the carbs when cruising at slow speed with pods and 4-1 exhaust. Went to a 125 jet size and it fixed it.

scamper03 · « **Reply #10 on:** December 21, 2011, 06:44:45 AM »

I am running stock jets. I've had the pods for a long time and never had an issue with the previous motor. This coughing out of the carb started with the replacement motor. Thanks

madmtnmotors · « **Reply #11 on:** December 21, 2011, 01:48:40 PM »

If the intake valves are too tight they can remain slightly open, poping back through the carb. Ignition timing can do the same thing by firing before the intake valve closes. Can't pin it down until these items are addressed by a 3000 mile tune up in order to find out what it's not.

lucky · « **Reply #12 on:** December 21, 2011, 05:01:06 PM »

Quote from: scamper03 on December 20, 2011, 06:30:05 AM

Hi Guys,

I just replaced my motor and it seems to run very strong except I get occassional poofs out of the carbs. Any idea as what's causing it?

"poofs" do you mean backfire or popping,spitting back?

Make sure that when the engine is off and you look into the carbs with air filters off and you turn the throttle that you see each brass post sticking up in each carb spray fuel into the engine.
If not- your accelerator pump system is not working and it is running way too lean.

See the next post below.

lucky · « **Reply #13 on:** December 21, 2011, 05:07:57 PM »

Go to http://forums.sohc4.net/index.php?topic=94278.25 scroll down to REPLY #46 LUCKY

Read that and the next page(Page#3). PHOTOS and explanation.

CLICK ONCE on PHOTOS to make them larger.

scamper03 · « **Reply #14 on:** December 22, 2011, 12:18:37 PM »

the carb shoots our like smoke or air. Like it's caughing. I haven't had a chance to test out any of the recomendations, but I will be definately working on it this weekend. Now that I think about it the caughing is on the 2nd carb. I could be wrong. I will definately posting again after this weekend. Thanks everyone and anyone else has any other idea please respond as I am looking to try everything and anything to fix the problem. thanks

scamper03 · « **Reply #15 on:** December 26, 2011, 03:29:55 PM »

Lucky: you're right about the carbs. I didn't have a lot of time over the weekend to work on my bike because Christmas but I looked to see if the brass posts are functioning properly. None of the brass posts were spraying fuel into the engine.

Blitzburgh207 · « **Reply #16 on:** December 26, 2011, 09:17:38 PM »

Quote from: lucky on December 21, 2011, 05:01:06 PM

If not- your accelerator pump system is not working and it is running way too lean.

+1
Torn accel. pump diaphram gave me the same symptoms on my 78. Chased my tail for a while, carbs on/off 100 times. Check it CLOSELY - the cracks can be hard to see!
Hope you solve your problem!

scamper03 · « **Reply #17 on:** January 03, 2012, 06:01:20 AM »

Update - Lucky was right, it was the accelator pump that was causing the problem. the little square brass plugger was really hard to get it out, but once I took it out I could see the ball and it was really filty. Once I cleaned it the bike started to run really well and I could visiually see the fuels coming out the brass posts.

Now I got to figure out why the fuel is leaking from the bowl on my #4 Carb. It happened to me before one #1 and #2 and I was told by the local bike shop that the float inside of the bowl can get stuck sometimes and it causes the leak.

As always any suggestions will be appreciated. Thanks guys.

knowsnothing · « **Reply #18 on:** January 03, 2012, 06:10:04 AM »

Give #4 a quick tap and see if the problem stops. If so, something got hung up. If not, you may need to check/set your float heights.

scamper03 · « **Reply #19 on:** March 12, 2012, 08:32:06 AM »

I can't get this bike to run right and I really appreciate all your help and suggessions.

After, I cleaned the accelerator pump, the bike ran good for a little while. I can start the bike without any carb cleaner fluid and it looks like it's it's working the way it should. But here are the issues. I can't stop the carbs from leaking gas. It's usually the number 1 or 2 carb and I took it apart and cleaned it mulitple times. I tried to tap on it a little but it won't do anything. Also, when the bike is idling it starts to caugh out of the air filters again, but it will stop once the bike is warmed up. The bike runs fairly decent on lower gears but once it gets to 4 th gear it starts to ride really rough. It looses power like it's not getting enough gas.

madmtnmotors · « **Reply #20 on:** March 12, 2012, 09:15:36 AM »

Quote from: scamper03 on March 12, 2012, 08:32:06 AM

I can't get this bike to run right and I really appreciate all your help and suggessions.

I can't stop the carbs from leaking gas.

Float level adjustment. May require new float needles (the rubber tipped needle that the float uses to turn off the fuel flow). The brass overflow tubes in the bowl could be damaged (cracked), but you weren't having this issue before, so for this to be the case you would have had to damage them if you weren't careful. My money is on improper float level, and probably the need for new float needles. I chased float levels (leaky carbs) for a while until I finally replaced the float needles on my 750 K8 and have had no problems since.

Quote from: scamper03 on March 12, 2012, 08:32:06 AM

The bike runs fairly decent on lower gears but once it gets to 4 th gear it starts to ride really rough. It looses power like it's not getting enough gas.

You're running out of fuel, just like it feels. My money is on the filter screen, inside the fuel tank, above the fuel valve. Also, check for inline fuel filter if one was added (not stock, but I use one since I removed the in-tank filter screen). Could be partial obstruction of the main jets, but start with the most plausible and easiest to correct (fuel filter(S))

Unless you're running a weak battery, then you could be losing spark at higher rpms.

knowsnothing · « **Reply #21 on:** March 12, 2012, 10:12:47 AM »

I agree with madmtnmotors, its probably a float level issue. One way to check your float levels is to get some clear tubing and attach it to the overflow nipples on the underside of the carb. By holding the tubes upright (so they don't spill) and opening the drain plugs on the bottom of the bowls, you will be able to "see" the fuel level of each carb.

May also want to think about picking up a new set of float bowl gaskets if you are getting stuff. The old ones get hard as a rock and won't seal for #$%*. Also, over tightening the bowl screws can crack stuff......

bollingball · « **Reply #22 on:** March 12, 2012, 11:48:31 AM »

Quote from: knowsnothing on March 12, 2012, 10:12:47 AM

I agree with madmtnmotors, its probably a float level issue. One way to check your float levels is to get some clear tubing and attach it to the overflow nipples on the underside of the carb. By holding the tubes upright (so they don't spill) and opening the drain plugs on the bottom of the bowls, you will be able to "see" the fuel level of each carb.

May also want to think about picking up a new set of float bowl gaskets if you are getting stuff. The old ones get hard as a rock and won't seal for #$%*. Also, over tightening the bowl screws can crack stuff......

On these carbs I like the clear tube much better than trying to measure the float. I have photos of this if you are not sure. I do all of this on a test stand. If on the bike it should be on the center stand. Shoot for a fuel level of 2 to 3 mm below the bowl gasket. You can still get the in tank filter from Honda.
Even though the carbs were synced on the old motor they need to be synced again on the new motor no getting around this.

Ken

bollingball · « **Reply #23 on:** March 12, 2012, 11:53:21 AM »

Look at carb (4) you can just see the fuel level
Click on photo to enlarge
Ken

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Author Topic: Need help with 78 cb750k (Read 4111 times)

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