Topic: High Idle in neutral and bogs without choke

[Fran Fitzpatrick]

Hello All,

     This site has been a great help in the past so I'm going to look for help here once again.  I have a 1978 CB750A that I've put about 1500 miles on.  My issue this time is a racing idle once the engine is warmed up and in neutral (choke closed). The other issue is the engine bogging sometimes to the point of stalling once warmed up when leaving from a stop.  If I choke the bike a bit it will leave from a stop just fine.  My last issue for this post is a knocking sound from the engine when leaving from a stop without the choke with the engine warmed up.

Thanks, Fran

[raymond10078]

Without any other information, my first thought is leaking manifolds (causing a too lean condition - which can cause a high idle) - but I could be way off, too.

To help us help you, tell us about your bike.  What carb work or adjustments have been made?  Carbs synched?  Rebuilt?  Carb clamps tight?  Check for air leaks?  Carb idle mixtures set?  Timing checked/set?  Advancer works?  Fuel filter installed?

The knocking sound could be nothing more that an idle that is set too low, or carbs that are out of sync.  Mine will knock with the idle set too low.  Before I synched the carbs, the bike would knock if the idle was lowered too far (setting the idle higher than stock will mask/hide a poor synch).  After the carbs were synched, I could set the idle well below the stock idle setting with no knock.

[Fran Fitzpatrick]

As always thanks for the quick reply.  (Carbs synched?)  Yes. (Rebuilt?) Yes, I had the Carb's cleaned and rebuilt 3 times from two different shops.  The last shop specialized in older Honda's (Charlie's Place in San Francisco).  (Carb clamps tight?) Yes, As tight as the old clamps will go. (Check for air leaks?)  My carb manifolds could be leaking, but I'm not sure how to check that out.  As with most of these older "A's" the manifolds seem to cause problems that have been addressed in depth in this forum.  Any simple ideas on how to check the manifolds for leaks would be a great help. One of Charlie's tech's stated that the manifolds were leaking and I sourced a used set form E-Bay and between old ones on the bike and the old ones from e-bay I replace the units with the best from the two sets.  (Carb idle mixtures set?) Not sure what this is.  Is this covered in the shop manual on this site?  (Timing checked/set?) Yes  (Advancer works?) Again not familiar with "Advancer" is it cover in the shop manual?   (Fuel filter installed?) Yes.  I have read about the idle adjustment in the shop manual, I will need to get a rpm gauge to preform this test. 

Again thanks for all the help you have provided.  I'll start with aspirin before moving on to brain surgery so... If anyone has a simple way to check for carb manifold leaks please pass it along. 

[raymond10078]

Some recommend spraying WD-40 around the carb boots with the bike running - and listening/check for a change in idle speed.

If your clamps are "metal-to-metal" tight - that means that the boots have taken a "set" - and you may want to try the next smaller size clamps.

Yes, the advancer is in the book - it is used to check the timing with the bike revved up (page 14-4 of the Service Manual).

If you are setting the idle - you have two speeds to set - using different screws - one for in neutral, and one for in-gear.  Yes, it is in the manual.

The idle mixture is on page 13-7.  Best to use a special tool - or you hands will get HOT!  But . . . if the carbs were rebuilt - did they install them on the bike?  They SHOULD have made all of these adjustments for you.  Not to say they did - there's a lot of poor workmanship out there . . . .

[shortbloke]

hi, I`ve much the same problem. With the kill switch operated put it slowly into gear. Can you hear both the change relay and throttle opener solenoid working? If they are try checking the throttle opener diaphram. `bogs` means stalls I assume. I`m also posting more info/query as a new post.