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Discussion Starter · #1 ·
Hi Everyone,

I've been lurking the sites for a while and decided to bite the bullet and join. It's a 1978 CB400Tii Hawk, and was last running about 2 years ago.

I cleaned the petcock,changed the gas, checked for spark and cleaned the carbs over the past two nights and finally got the carbs back on the bike today. On the first electric start the bike kicked over with full choke- I replaced the rubber carb plugs since one was missing when I opened them, and also replaced two O-rings that seated inside the insulators going from the carbs to the cylinder head (other ones were cracked and stiff). We also found the pilot screw set to 3 turns out, instead of 1.5 - so we changed this as well.

With that in mind, we could only keep the bike running with full choke (above 2k RPM), if we tried to lower the choke the bike would stall or if the RPMs dropped below 1800k it would also die. After the bike sat for a little while - the plugs read black. The sounded better but was slow to return to idle when opening the throttle as well - does anyone have any ideas?
 

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Sensei
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Well, ...running on full choke WILL make the plugs black.....You need to find out why it won't run without a lot of fuel.....( I suspect after "sitting" for two years you have "stuck" rings)..... What are the compression readings?

An engine needs only three things to run: Fuel/air mixture, compression, and a spark at the proper time........

You have electronic ignition, so we'll assume the spark and timing are correct.....
You just rebuilt the carbs...(presumably fixing them).....
ALL that's left is compression.....
(could be rings, could be valves, could be the gaskets).....
 

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You did not clean the idle jets when you cleaned the carbs.
If the idle jets were clear, it would idle without the choke on.
 

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fasterspider said:
You did not clean the idle jets when you cleaned the carbs.
If the idle jets were clear, it would idle without the choke on.
That was my first suspicion as well..


GB :mrgreen:
 

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Discussion Starter · #5 ·
fasterspider said:
You did not clean the idle jets when you cleaned the carbs.
If the idle jets were clear, it would idle without the choke on.
When I cleaned the carbs I soaked and sprayed through the primary and secondary jets, the slow air jet, and the pilot screw? Is the idle jet the same as the pilot screw?

Would adjusting tightening or loosening the pilot screw help solve my problem?
 

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Just spraying through the jets isn't enough. You need to remove them to clean them properly. I use carb cleaner spray and a VERY THIN bristle pulled from a cleaning brush. Most people argue with me on the bristle thing but I have yet to do any damage to any jets in all the years I've been working on bikes. As long as you're not forcing the bristle into the jets then you should be ok. It just helps to clear any obstructions. Once you verify that the jets are clean and clear by blowing through them, then reinstall..

It's easier than you may think, but only do one carb at a time to avoid swapping parts on accident. Also, take note of the jet sizes and write it down to be sure that both carbs have matching jets. Sometimes, over the years these bikes get carbs swapped by previous owners or the jets have been changed for the wrong sizes.


GB :mrgreen:
 

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Sensei
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The idle screw is NOT the same as the pilot (slow) jet.... On your carbs, the slow jet is NOT removable...It is under the rubber plug, and MUST be cleaned........
My previous diagnosis was incorrect because you said you had cleaned and rebuilt the carbs, and thus mislead us.....IF you are uncertain , please specify that... we all have areas we don't excel at servicing.......
 

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Discussion Starter · #8 ·
66Sprint said:
The idle screw is NOT the same as the pilot (slow) jet.... On your carbs, the slow jet is NOT removable...It is under the rubber plug, and MUST be cleaned........
My previous diagnosis was incorrect because you said you had cleaned and rebuilt the carbs, and thus mislead us.....IF you are uncertain , please specify that... we all have areas we don't excel at servicing.......
Well I guess the second time was a charm - I thoroughly cleaned the pilot jets this time. Compressed air is a definite must - something I cheaped out on during the first clean.

I'm definitely making progress thanks to all your advice - the bike started tonight and kept running after the choke was OFF! I checked for leaks by spraying WD40 around the seals and seats and the bike appeared to have no problems.

BUT... the bike had bad throttle response as the tachometer was slow to increase in RPMs. And during WOT, the exhaust sounded like they would occasionally backfire? The idle also did not like to be under 2000RPMs or else it would die. Any ideas what I could look at next?
 

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Did you re-synchronise the carbs after cleaning them?
 
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