79 cb750 rattle at low rpms

[DreamCrusher]

Hello, recently purchased a 79 cb750k. 15k mi. 1 owner. good shape and garage kept. When I got it home it did not run, put in a new battery and drained oil. Found over 2 gallons of gas in engine case. The previous owner said he has been trying to start it since last year. Pulled carbs and found #68 jet in the bottle of bowl # 1.  Cleaned carbs really good, put in new plugs and new oil. also replaced passage plugs where the jets are pressed in since they did not look healthy. Bike started up no issues. Smoked a bit for a 5 minutes, then it cleaned up..  I have two issues I am dealing with...

1.) I hear a rattle at low rpms and it sounds like it's coming from the front of the engine more so on the left side. at 1500-2000rpms the rattle is reduced significantly. it has a 4 - 1 exhaust.

2.) When riding the bike, deceleration produces a spitting/popping sound.

I am not an expert, but I got myself this far with no assistance. However, I know my limits and I am in need of assistance. Any suggestions?

thanks!

respectfully,

dennis

[Re-run]

For the rattle, put in neutral and pull in the clutch. Does the sound diminish? Most likely it will and if it does, the carbs need to be synced. Any time you pull apart the carbs, this would need to be done to even the craw on all 4 carbs.

The popping on decel is most likely caused by 1 or 2 things. The air cutoff valves are not working right, or the idle mix on the carbs needs to be adjusted. An easy way to check is to pull the plugs and see if they are white or black. White plugs mean the mix is too lean, black means it is too rich. They should be about a light tan color.

[DreamCrusher]

Thank you very much for your support. I will check the plugs.. As for the carb sync, I have never done this and i do no have the gauges to perfomr it.. I will look around if i can find a shop to help with this..

Thanks!

[DreamCrusher]

Is the idle mix screw the one at far end from the float hinge? (When looking from the bottom of the carb with bowl off)

[TOOLS1]

You can buy the sync gauges off ebay for around $75.00. They are well worth the price. Since they will pay for themselves the first time, you use them. As for syncing the carbs. It is a whole lot easier then rebuilding them.
TOOLS

[DreamCrusher]

Tools,

Here is the breakdown of the spark plugs I just pulled.

Cyl # 1 (white/little browning inner) with a white/gray upper tip

Cyl # 2 ( charcoal inner) with a gray upper tip

Cyl # 3 (black inner) with a gray upper tip

Cyl # 4 (black inner) with a black upper tip

Let me know what you think..

[DreamCrusher]

All the pilot screws were at 1/2 turns out. I turned them 2 1/2 out to see if that would make the engine run more lean since the plugs were really bacl except cyl # 1.

it didnt stop the rattle at low rpms, but it only spit one time on the decel when I went over 6k rpms. I will check the plugs after they cool.

any suggestions, let me know thanks!

[DreamCrusher]

typo.. the pilot screws were at 1 1/2 turns out before i made the adjustment to 2 1/2 out. Should I go to 3 turns out?

[Re-run]

Check the manual for sure but I am almost positive your carbs need to go the other way. Out = richer, in = leaner, 99% sure. Aim for the plugs to be about #2 or so.

and get them synced.

[DreamCrusher]

ok, I ran the bike today at 2 1/2 turns out on the pilot screws. Pulled the plugs and they seem to be running more clean. all have white ashy outter tips. except # 1 which is really clean all over. I agree, I need to buy a carb sync kit. Can I borrrow one from somebody? I live in central pa.

Thanks!

Dennis

[seestheday]

Yep, out = richer on the DOHC's (79 and above - I have an 81).

What do you mean by aiming for the plugs to be about #2?

[DreamCrusher]

I didnt say that, the other guy made a comment and said: try to shoot for your plugs to be like # 2. However, I rotated the pilot screws out from 1 1/2 to 2 1/2 and the plugs cleaned and up and do not have as much black build up on them. So wouldnt that show the carbs running leaner, with less fuel? I thought if it got richer, it would cause more black build up showing the cylinders to be loading up. Or do Iunderstand it incorrectly. More fuel= black build up on plugs=richer mixer.... Less fuel=lighter colored/non build upplugs= learner mixer.  YES???

 

If above is true, then turning the pilot screws out made the engine run more lean/no build up.

 

Please educate me if I am wrong.

 

Thanks!

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From: "seestheday [via Honda CB750'S]" <[hidden email]>
To: "DreamCrusher" <[hidden email]>
Sent: Wednesday, August 1, 2012 3:40:19 PM
Subject: Re: 79 cb750 rattle at low rpms

Yep, out = richer on the DOHC's (79 and above - I have an 81).

What do you mean by aiming for the plugs to be about #2?

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[seestheday]

Hmmm... Not sure why your plugs are reading like that, but I'm 99% sure that out = richer. There is a good explanation on what is going on here: http://www.mailinglistarchive.com/[hidden email]/msg21143.html

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "DreamCrusher [via Honda CB750'S]" <[hidden email]>

Date: Wed, 1 Aug 2012 14:22:44 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: 79 cb750 rattle at low rpms

I didnt say that, the other guy made a comment and said: try to shoot for your plugs to be like # 2. However, I rotated the pilot screws out from 1 1/2 to 2 1/2 and the plugs cleaned and up and do not have as much black build up on them. So wouldnt that show the carbs running leaner, with less fuel? I thought if it got richer, it would cause more black build up showing the cylinders to be loading up. Or do Iunderstand it incorrectly. More fuel= black build up on plugs=richer mixer.... Less fuel=lighter colored/non build upplugs= learner mixer.  YES???

 

If above is true, then turning the pilot screws out made the engine run more lean/no build up.

 

Please educate me if I am wrong.

 

Thanks!

---

From: "seestheday [via Honda CB750'S]" <[hidden email]>
To: "DreamCrusher" <[hidden email]>
Sent: Wednesday, August 1, 2012 3:40:19 PM
Subject: Re: 79 cb750 rattle at low rpms

Yep, out = richer on the DOHC's (79 and above - I have an 81).

What do you mean by aiming for the plugs to be about #2?

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NAML

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