Topic: My rebuilt bike is trying to commit suicide !

[newcbguy]

I just got my '71 cb750 running again, took it for a few test rides and all was well. I went to start the bike today and gave it a little gas, backed off and the bikes rpm kept climbing. I quickly threw the kill switch. I fiddled with it and noticed that it will  idle about 1200 rpm with the choke all the way on, (eyes burning rich mind you) but when you take the choke off it tried to blow up again. I have reseated my new carb boots with no better results. I have inline fuel filters, but am starting to suspect that there is some trash keeping something open or closed in the carbs? Im not sure. the idle adjustment hasn't changed and the slides are bottoming out where I set them . bench sync'd with 1/8" drill bit.  Anything else I should look for?

[bryanj]

open cable too tight?(maybe only on one lock)
tickover screw wound in too far?

[750goes]

sticky throttle slides or a cable jamming up a bit....check the cable routing and run it with the tank off the bike..if the problem persists wriggle the cables and see what happens....also could be a sticky throttle tube...   

[HondaMan]

Make sure the "retract" cable didn't break in the cable pair to the carbs. (Or, fall off?)

[newcbguy]

well i took the pods off and can visually see that the slides are down and when i turn the throttle, it does close them when I let off. I am puzzled by this one. The charging system doesnt have anything to do with the way the bike idles does it? I seem to be charging at 17+ volts above idle. I have the dohc rectifier/regulator setup. Its almost like the throttle is wide open but its not?!?! the bike kinda started to hang the last time I rode it. I would be cruising along around 3-4k and let off and the bike would take a few seconds to return to idle. I figured the carbs needed to be sync'd. Maybe its something else. I dunno. I plan to pull the carbs back off and have a look this weekend.

[KB02]

The choke isn't installed backwards, is it (Only other thing I can think of)? Regular operating position should be with the choke flaps lying horizontal to allow the greatest air passage.

Sorry if that sounds stupid or insulting, but like I said, it's the only thing that I can think of.

[jsaab2748]

Unrelated note, but 17 volts charging rate is WAYYYY too high. See if your battery has any water left in it.

[newcbguy]

kbo2 im not sure I understand what you mean, I can see when the choke is on that the flaps come down like they should and restrict the airflow. I notice that they have spring closed flaps in the middle of them I guess to allow a little airflow, is this what you mean? They seem to be installed correctly to me. I would think the spring flaps should open inwards towards the carbs to allow air to flow. 

Thanks all for the input! I was trying to troubleshoot the high charging when this happened. The battery is new and fully charged.

[KB02]

yeah, I was just thinking that if the linkage was set up backwards, the choke would actual;ly be closed when the lever was in the open position. Just a thought.

Like I said, it was all I could think of.

[newcbguy]

so the engine must be getting air through the intake boots or worse the head is leaking at the intake port. Is this a correct diagnosis? I have installed new throttle cables and they appear to be doing their jobs. Im out of ideas, short of taking the carbs back off and opening them up. someone suggested spraying wd40 around the intakes, what effect should this have ?

[Short Round]

I would go ahead and pull the carbs  before you did anying else.  Chris

[Bodain]

Bench synced with 1/8 drill bit... .125    Seems way fat to me....

[mattcb350f]

I didn't see in your posts newcbguy, that you tried backing off the idle adjustment screw

Did you?

 Matt.

[newcbguy]

yup I can see daylight between the idle screw and the plate.  Thanks for the suggestion though. I must have a leak at the intake boots or a stuck float or sunk float or something going on. I will have to pull the carbs this weekend if I get time and find out what the heck is wrong. I had her running great and idling around 1100-1200 or so when this started. I knew that things were going too good.

[KB02]

...someone suggested spraying wd40 around the intakes, what effect should this have ?

Basically, the WD40 will fill in the air leaks for a brief period and cause the idle to change. Hence, if the RPMs change with the spray, you know you've got a leak.

[h8r]

Bench synced with 1/8 drill bit... .125    Seems way fat to me....

I bench synced with a guitar string, around .030 or .040. Going up to .125 sounds way too big to me also.

[bryanj]

wind the tickover screw right out and make sure there is play on the cable!

[newcbguy]

I re-read the faq section which says this:

"I need to sync my carbs but have no vaccum meter. How do I bench-sync?
When I rebuild a set of carbs, I always bench sync them before I re-intall them. Basically, I use a 1/8" drill bit as a gage. Here's how you do it: Adjust the idle screw (the main one) until the 1 carb outside of it is open enough to just let the 1/8" drill bit through. Then adjust the _sync_ screws on the other 3 carbs until the drill bit just fits them as well. You'll probably want to close the idle screw before you bolt them back on the bike, 1/8" openeing is pretty big. This isn't a perfect sync, as there are other things that will affect the airflow to a given cylinder, but it will get you pretty close."

It looks like I missed the part where you adjust the idle screw first. I will go back and set the carbs up this way and start from there. I do remember however first starting the bike and having to turn the idle screw to get the bike to stay running. So I will check it out and post the results when Im finished.

-thanks so much everyone for the help!