Topic: Lurching?? - '77 CB750A - Hondamatic

[keith5]

Hi all -

Have had my A since May 2012 and am enjoying it. One thing I am still having problems with: After she is warm, if I put her into Neutral she revs EXTREMELY high. It's so bad that once I put her into gear (L) she lurches forcefully and my back tire spins. She idles fine in L and D, only high in N.

I know this is one of the downsides of no clutch, but its pretty bad... Once last year I was parked on a hill and it lurched so bad that I dropped the bike... First time Ive dropped any bike in years.

I'm assuming there is an adjustment I need to make? How can I adjust this without losing a nice idle? If I dont keep my idle relatively high she stalls out.

I found a post where someone had this problem when their A was in "L" gear, but mine is fine in L and D, this only revs extremely high in Neutral.

Thanks.

[salukispeed]

There are several here that can chime in on this. There are two idle speed adjustments on the 77-78 bikes. the one on the engine side of the carbs is the netural /base idle screw. the one between 3-4 carbs is harder to see and is often covered with a cap. It is accessed from behind the 3-4 carbs just under the gas tank. The base idle screw should be set so it idles normally in netural and when put in drive the screw that sets the "in gear" idle should be adjusted so it idles normally then also. But it should not be such of a difference between them. Will the bike idle properly in netural if you turn down the speed to around 1,000 rpm or does it die out. If it does not idle cleanly/properly at low speed there could be idle circuit issues with the carbs or possibly manifold vacuum leaks . Have you done a good tune up including valve adjustments?  Tight or leaking  intake valves can give you a similar result. My 77 will idle in gear and in netural without the speed up system working but the difference is noticable but not as bad as you describe.

[keith5]

Thanks for the help -- This is very interesting because I have always only adjusted ONE screw.

I'll start with what I have NOT done:

The bike has not had a good tune up since I got it. It was in shape but I did a lot to it to get it safe and running well... took brake and calipers apart and cleaned them, changed brake fluid, changed fork oil, cleaned/lubed chain, changed oil & filter, changed air filter, cleaned breathing element etc, replaced old/worn fuel lines and other lines, new tires & balance, etc.

I have not done any valve adjustments. Is this difficult to do? When you say valve adjustment, are you referring to the "Valve Tappet Clearance Adjustment" on page 53 of the Owner's Manual? What else do I need to do as part of a good tune up? Im intimidated by taking the engine itself apart to do much. I've done carb cleanings etc on single cyclinder bikes before but nothing on a bike this complex.

What I HAVE done:

I have adjusted what the Owner's Manual calls the "throttle stop screw" on page 62 of the Owner's Manual. Its a very large screw that I can reach under the gas tank with my fingers and adjust with my thumb and index finger if I have to. Only after I perfected where this screw needed to be would the bike idle without stalling out or idling way too high.

Not sure which of these you're referring to is the throttle stop screw. I havent found a second screw to adjust the idle. Am I adjusting the right one? Where is the other screw? I have only been adjusting ONE idle screw, so if there's another idle screw then that may be the problem.

One other note: The engine only idles high in Neutral after I've ridden for a bit, after its been given some gas for awhile and really warm. If I stop at a gas station for example and go into Neutral after a bit of a ride, it idles extremely high. It does not idle high in Neutral when I first start up the bike.

Thanks.

[raymond10078]

Please download the service manual from this site.  All of your answers you are seeking are detailed there.

If you've only been adjusting the single large thumbscrew - then you've been adjusting the "in neutral" setting.

There is another adjustment for "in gear".  The "in gear" setting relies on the "in gear" speed adjust system to be working.

If you can clean carbs - then you can adjust valves, too.  Download that manual and give it a read.

The "lurch" on a properly maintained auto bike is just like in a auto car - not much.

[salukispeed]

Kandrtech is correct,  having a manual in front of you is incredably helpful. I did throw a lot of questions at you  for starters. Yes the tappet clerance is the valve adjustments I was asking about. The screw you can turn with your fingers is the netural idle adjustment and hopefully you can get a nice stable idle around 1,000-1,100 rpm Warmed up. That will be the first hurdle to overcome. If you can get that then you can hopefully get the In gear idle to match( adjustment between 3-4 under tank ) It is there to  bump the idle just a little when in gear. Many here have run into the "in gear" idle system  has quit working or only when it feels like it. Stay with it and you will get there.

[keith5]

Thank you both --- I have the original owner's manual and a Clymer CB750 SOHC4 1969-1978 that I have used to do all my work so far and always have in front of me while Im working, but it looks like the PDF for the CB750A from this site will be more helpful since this is specific to the A.

I have a nice idle in neutral, when I start the bike and it warms up I can have it sit in idle without it stalling etc or revving too high. So its odd to me that after I ride for a bit and go back into neutral it idles so high. It starts off idling perfect, so I think that my adjustments on the neutral idle thumbscrew are good. The bike also idles fine in Drive, so I question whether I need to adjust the gear idle... Both idle good, its just going from Drive to Neutral after riding for a bit that the bike revs high in Neutral and wants to take off as soon as I go back to drive. Its almost like the carbs are stuck open in Neutral or something after the bike is really warm.

Maybe its a carb/tappet issue. I will see what I can do. I will adjust the tappets, but as for carb cleaning, any thoughts on using SeaFoam for a solution thats not as good as carb cleaning but helps?

When you guys say idle 1,000-1,100 rpm etc, how are you determining this without at tach? Are you attaching an inductive tachometer to a spark plug or something? Ive always just adjusted idles on bike by ear, never had the luxury of a tach on a bike in my life.

I love reading this stuff so I will take a look at the service manual, see what I can do with the tappets, and update this post when Im done in case it helps someone else.

[raymond10078]

I'm not a carb expert, but based on your symptoms - idle speed is high after the bike is warm, I would tend to think you have a carb issue, an advance issue, or a valve clearance issue.

Do the easiest first.  Check the valve lash.  Second, if you have a timing light, check the advance - or better yet, pull it, clean it, lube it, and put it back on.  Third, I would check the emulsion tubes.  Unfortunately, you can't clean these without pulling the carbs.  I have read that others have had luck with Sea Foam, but I've never gone that route - just pulled the carbs and cleaned them.

Anybody else have a suggestion?

As far as the "how to check idle speed", I use an automotive tach/dwell tester, hooked up the points.

[Enzofure1]

Did You ever figure this out I am having the same issue. Thanks

[Feragos]

Hi, I know it's an old topic but where I can find the service manual for the cb750A?, it doesn't show under the downloads tab. Many thanks and safe rides.

[CycleRanger]

Hi, I know it's an old topic but where I can find the service manual for the cb750A?, it doesn't show under the downloads tab. Many thanks and safe rides.

It's on this site in pdf segments
https://sites.google.com/site/hondamaticmotorcycleclub/Resources/cb750a

You can get a reprint here:
http://www.helminc.com/helm/product2.asp?Make=AHC&Model=CB31&Year=&Category=1&class%5F2=AHC&mk=Honda+Motorcycle&yr=All+Years&md=CB750A&dt=Shop%2FService+Information&module=&from=result&Style=helm&Sku=6139300&itemtype=N

They come up on ebay from time to time as well.

[HondaMan]

I have not done any valve adjustments. Is this difficult to do? When you say valve adjustment, are you referring to the "Valve Tappet Clearance Adjustment" on page 53 of the Owner's Manual? What else do I need to do as part of a good tune up? Im intimidated by taking the engine itself apart to do much. I've done carb cleanings etc on single cyclinder bikes before but nothing on a bike this complex.

If you have done the Singles, this is not different, there are just 4 of them. If you're comfortable with the valve settings via "singles" method, just move the timing mark to "1-4 T" to do either cylinder #1 or #4 (whichever has the loose rockers), then turn the engine 360 degrees and do it to the other one of the pair. Then, do the same with the "2-3 T" mark, for the 2-3 cylinders.