Throttle/Carburator 71 CB 750

[KarlM]

I have a carb/idle/throttle problem. Carbs are clean and seem to be set OK but not vacuum ajusted yet!
The problem is after the bike is heated up....3-5 mile ride.  The throttle does not return to idle setting of 900 for a few seconds.  It almost sounds like I'm speed shifting. Well.....it's not quite that bad... but that helps with my eplanation. The idle only goes back to about 2000-2400-- rpm after an immediate throttle "snap". The best description I can give is the throttle "hangs up" at a higher than normal RPM and than goes back down to set idle after 10-20 seconds.   I think it's heat related because it never happens when the bike cold.  When It is cold I get the normal response with a twist of the throttle.
You guy's have been a great help in the past!
Thanks
Karl

[TOOLS1]

Start with disconnecting the cabels, and checking, for smooth operation in the cables, and the carbs. However it still could be a vacuum leak that only shows up when the engine is at operating temperature.
TOOLS

[KarlM]

Thank you.

Cables seem smooth.
How do I find the leak?
Thanks
Karl

[KarlM]

Tools,
Would the cables change the way they function due to temperature change?
Thanks again
Karl

[LukeM]

I would not think so.  It's possible the return cable may be hanging up somehow.  Check the cable routing as described in the Factory Service Manual (you _DID_ download it from our home page, right?).  Maybe one of the cables is near something hot, and changes a bit.

A better guess would be to examine stuff that would change a bit due to heat:  carb boots (check the clamps on both ends, should be very tight), for one.  A remote cause would be a throttle return spring that gets soft when it gets hot (a SWAG, but possible).

If it is a carb boot issue, you can diagnose it with some spray carb cleaner and a pair of goggles.  If there is an air/vacuum leak, the engine rpm will change if the carb cleaner is leaking in.  Spray around the carb boots, and at the intake ports of the engine.  Sometimes the boots can be revived with a diluted mixture of wintergreen and water.  Do a search on wintergreen on the home page.

Also, check the ports that would be used for carb synchronizing.  Sometimes they work loose, or the little washers or gaskets will fail.

I hope this helps.  I'm sure others will chime in.
Luke M

[KarlM]

Thanks Like.
 I did read something about the boots and checking with carb cleaner.  Good suggestions.
Karl

[rrgunslinger]

What are the air screws set at?  

[KarlM]

PROBLEM SOLVED....Thanks for the suggestions.  I re-tightened and adjusted all the boot clamps so they were in the factory indentations.  I did a half ass job when I reinstalled the carbs after cleaning.  Well....not really half ass'd just not done at a professional level! Problem SOLVED!!  
I have my air adjustment at one full turn ......except I'm having a problem with #4 backfiring  on deceleration so I 'm trying another 1/4 turn but that doesn't seem to help.  
I do have a new muffler on that cylinder but it is a new Honda #300 part!  Any chance the new baffle is less restrictive than the one made in 1971?  
Any suggestion on "backfire" from #4 cylinder?
Thanks
Karl

[Lucky 1]

All of the mixture screws should be set the same.
You need to get those carbs sync'd.

[KarlM]

Thanks Lucky.  I have the guages.  I just have to do it!
Karl

[KarlM]

Luke!
I new I could get it right one of these posts!!!
Karl