Topic: Popping in carbs and white smoke

[Sething]

I’ve been reading about my 1977 CB750K carburetor issues in that I have popping coming from the carbs and some white smoke coming from the exhaust when it starts to heat up. It also starts to rev up pretty high on its own with no throttle when the choke is pulled. I’ve set the air/mixture screws to 1.5 turns.

From prior posts, my guess would be too lean a mixture given I have KN air filter pods now and a 4into1 exhaust.

I’m thinking of turning the mixture screws another half turn and see what I get. I haven’t had time to check the spark plugs for color yet. My timing is electronic and I’ve checked that twice and it seems good to go. I’ve also checked my float/gas levels and that seems to be ok as well.

Am I on the right track?
Thanks











Honda 1977 CB750K

[Jarrett_Honda]

Sounds like it's too lean and the rising in rpms without input could mean you have an air leak

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[Sething]

Thanks. I’ll try to riches the mixture and see what happens.
I did the starter fluid around the rubber connectors and I was able to verify no change in rpm to suggest an air leak between the engine and carbs. (Forgot to mention I checked that)



Honda 1977 CB750K

[Stev-o]

Popping on decel also suggests possible exhaust leak.

Need to do a plug chop.

[TwoTired]

White smoke is almost certainly water vapor.  Water is a byproduct of combustion, and the flow though of humidity in the air.  Ignore it unless it persists for a half hour after the engine is at full operating temp while driving about.

Popping on decel is either a leaky exhaust or too lean an idle mixture setting, coupled with low exhaust back pressure for the mixture delivery.

You did put in new crush washers for the new exhaust, right?  You did hang the rear of exhaust before tightening the header bolts, on the crush washers, right?  Even new crush washers can be damaged/deformed upon installation.

Cheers,

[Sething]

I neglected to change the crush washers. I will change those before I do anything else.


Honda 1977 CB750K

[Jarrett_Honda]

Have you checked the float height?

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[Sething]

I’ve been working very hard on the floats and  I’ve been able to get my fuel levels to 2-3mm below the gasket line.


Honda 1977 CB750K

[Sething]

I was finally able to get some time away from the toddlers and work on the bike today. The crush washers were installed. The mixture screw has been placed at 1.75 turns out to make the mixture more rich. I still had some popping at the start of running it. I also noticed that it was running better with the choke half on.  After messing with the idle screw a little and letting it run for a little while, the popping stopped, I was able to have it idle without the choke, and the white smoke I noticed before from the exhaust stopped. I noticed that it was still starting to rev up high at times without throttle which leads me to believe it’s still running a little lean or the floats still need some work. It definitely ran better when warm.
I’m wondering for the 1977 CB750K if the idle jets need to be larger to increase the richness at idle. I’m concerned that going two full rotations with the mixture screws is too much. It still has the original idle jets. I put larger jets on the main.
Not sure what direction to go at this point but sure feels good to have it idling and revving nicely at times.




Honda 1977 CB750K