Carb question 1978 cb750k

[tmtrebor]

Hey guys, I've got my bike almost running, but have some questions about my carbs.

I have rebuilt the bike, done wiring, and now it's time to start the bike, but the bike wont start (well, all the time it wont start).

I can get the bike running off of gas after I've used starter fluid, but the bike will not idle. I have bench synced the carbs (and they look good), cleaned the carbs a few times (myself, and then taken to a shop and dropped 200 for an inspection, float needle replacements/adjustment, accelerator pump fix, and cleaning), and the damn thing does not want to idle on its own.

I am thinking about taking my plugs out to clean/dry/regap, but not sure if this will fix it, especially since the bike fires up with start fluid. The thing blowing my mind is this; cylinder 2 and 3 are "cool" relative to 1:4. No idea why. Compression is 150 on each cylinder (harbor freight tester, but i trust it). And the engine seems to run better if I spray starter fluid on pods 2 and 3 vs 1 and 4.... leading me to think it's a spark issue.

Anyway, bike has pods, 75 jets, and I have screwed the bottom screws on the carbs (pilot screws?) out 3.

any help / recommendations/ points in the right directions would be appreciated, as I know nothing about carbs

 

[irk miller]

Test each plug for spark on the cylinder head if you think that's an issue. Its not odd for 2 and 3 to be cool if you don't have a good charge. If you're having trouble starting it, then there is a good chance you're draining your battery. Charge it on the charger to full charge and try again. Are you using the choke? It's not uncommon for these bikes to struggle to start without choke and they won't run without it until it warms up.

 

[tmtrebor]

Actually not using the choke at all.. i should probably test that too lol. Just wanting to build a list that i can run through when I get off work.

Like I said, totally new at this, so it's a learning process (albeit a long long one....)

Battery power could definitely be an issue, I have it on the charger now.

The reason I'm thinking it's not necessarily the battery is the fact that it runs after it's started, but i have to give it gas and "rev" the throttle a bunch or it dies (if that makes sense). Kinda like I'm keeping it going/ just alive "enough" by doing that.

Here is an example of prior to cleaning the carbs... It is idling a bit, but I was told it needed to be cleaned / sounded weird. Again, started with starter fluid (in my hand) and ran off gas, but wont start gas only...

https://www.youtube.com/watch?v=idYHql5WjYM

 

[irk miller]

Yeah, you're still not describing a situation that refutes battery or choke. The charging system on these sucks. It doesn't charge at all until you get up in the rpm's and it will never fully recharge a battery that has significant loss of charge. 11.8v on the meter is essentially 0% of charge. Having to constantly pull the throttle suggest choke. That said, the idle circuit is notorious for giving people fits who don't know better. Also, make sure your floats are set right.

Tune-A-Fish taught us a great trick to check float levels:

Tip of the day (for those who don't already know) this trick is as old as swimming pools... I think, or its how I used to determine water lines +35 years ago and how I learned it the first time...

Anyway, this is only for checking/verifying float height (clear tube method) all you need is to use the drain tube(s), bike on a stand or upright, crack open the drain and allow some fuel to flow, close and drain out the line, next hold the line up beside the bowl (see pick and open the drain screw, the line will only fill to the same position as the fuel in the bowl, for me on the CB I like it to be a mm or so below the mating surface of the bowl.

Sent from my iPhone using Tapatalk

 

[tmtrebor]

Great! thanks for the info. I will add this to the checklist tonight. And that is a nice trick...

 

[tmtrebor]

OK, sorry for the delay... bike idles fine now, kinda... I think the problem was the choke was connected incorrectly. I took the carbs off of the bracket and gave the spring that connects the two butterfly rod things one more spin to tighten it up, and the choke now open/closes the butterfly things fully.

Anyway, so it idles, but then i started adjusting things and noticed that cylinder 2 was not hot. I believe that's due to the spark plug, but one odd thing....

When the bike was idling, sometimes it would just straight die when I gave it gas. Other times it would not, which was a little concerning. I have the idle air screws out 1 1/4 per the clymer manual, but think I may need to turn them out a bit more. Any suggestions?

 

[crazypj]

The problem is the PD carbs.
Go up at least 2 sizes on pilot jets and re-set float level 1mm higher than stock (I think 15mm works?)
You did check accelerator pump diaphragm and the air cut valves are all moving OK?