Topic: 78 honda cb750 running problem, help!

[drmeloh]

I recorded what my bike does. it does it more in the second video.
when I turn it on I have to adjust the idle screw so It can rev to approximate 1200rpms and be kept on.
I take off the choke once the engine is hot, then I rev it, it tends to stay stuck at 2000-3000 rpms so I adjust the idle screw again to 1200 rpms.
it also tends to accelerate/rev on its own when its warming up so I adjust the idle.
and once I get going and hit 1st gear it does the same thing so I have to adjust again. it does this for a while throughout the ride but then it becomes somewhat normal after adjusting the idle screw for several times.
HELP PLEASE!!!

https://www.youtube.com/watch?v=g8W7rttkz2A
https://www.youtube.com/watch?v=nOAvRirCKfA

[grcamna2]

Try removing your points plate & then removing the advance mechanism;it probably needs to be fully serviced..,such as taking it apart, removing & greasing the main pivot shaft,etc.The info for doing this can be found here on the forum.That's where I'd go first. 

[KB02]

Question #1 - If that's a '78 CB750, why did you post on the "Other Bikes" section? 

Question #2 - Do you have the stock '78 carbs?

The older 750's did not have an increased idle cam when the choked was pulled. It was expected that that the rider would keep the engine speed higher throughout warm-up with slight application of the right wrist. in '77 (I think) Honda added the idle cam so you didn't have to get a sore wrist until you were ready to ride.

SO - If you have the stock carbs on your '78, it would be a good idea to pull them off and make sure that the idle cam is working. If you don't, you are creating your own problem by constantly adjusting the idle screw.

Question #3 - When was the last you synced the carbs?

[drmeloh]

i was told to post them there so i can get more results but it didnt happen 

#2 i highly think they are the stock carbs.

#3 they were synced and clean twice within 6 months or so

[KB02]

Post a picture of your carbs. That could help us make sure we're helping you in the right direction.

[drmeloh]

I will post a picture.
Should I post it here or on another post?!

[toytuff]

Have you checked the rubber boots for leaks?

tt

[KB02]

I will post a picture.
Should I post it here or on another post?!

Post here and keep this thread going.


MODERATORS! Should we move this thread? 

[dragracer]

Have you checked the rubber boots for leaks?

tt

I agree with this question. Sounds  like a lean condition.

If you find no leaks at the boots then I would recommend putting in brand new plugs. Start the engine and let idle in the manner you use now to keep it running but DO NOT rev the engine up, let it idle only. Let it run for a few minutes and then pull the plugs. Read the color to see if any are lean. Odds are you will see at least one that still looks new with no color. Either clean that carb again or check the jetting on all of them. If you have a non contact thermometer, shoot the pipes to see which one
(s) is the hottest. Again, this will indicate what cylinder is lean.

Just my 2 cents!!!

[drmeloh]

my bad, I been so busy with school and work I haven't had a chance to update this.

i had the pod filters for a while and i switched it to the stock air box cus i was told that a lot of air was going to.
when i switched to the air box it did the same thing so im back with pod filters.

it seems like the boots are fine. and i have cleaned the carbs twice in less than 6 months.
i believe all the jettings are "stock", i might have to buy new ones to set them at different levels to see if that works.

i will try to post some photos by tomorrow or this weekend. 

[dragracer]

If it has the original PD carbs on the bike and pods, you will need to rejet to avoid lean conditions, especially if there an aftermarket exhaust. PDs have press in pilot jets and only a few companies offer then. Sirious Consolidated or JetsRus carry all you might need.

Before you invest in jets, I still recommend that you start with a fresh set of spark plugs and read them at idle. Typically a wandering idle is related to not enough fuel.  But be sure all the slides are in the same position based on the non-adjustable #2 carb and you've properly adjusted the mixture screws to factory specs.

I'm sure our resident carb experts will offer more indepth advice than i can offer but i'm sharing my own personal experience with the problem you mentioned, taking into account excluding the need to reclean the carbs. There was an old post on this site detailing issues to look out on the PD carbs.  Do a search and you'll find some good information on the accelerator pump circuit also.