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'78 750K slow throttle return and it's not the cable

Hisvett

CB750 Enthusiast
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New to this model of carbs...meaning no experience. I pulled the tank to check the cables for any binding, and didn't see anything. The carb/slides close completely when motor is not running but when running, it takes a good 5 seconds or more to return to idle from 3500-4K revs. I'm guessing I have a vacuum issue that is holding the slides up? And if so, what needs to be done to find and stop the vacuum leak.

Second, what the heck tool do you use to adjust the choke fast idle cam screw? Anyone happen to have a photo of the tool?

Thanks from DFW
 
To find a vacuum leak, spray carb cleaner around all the rubber boots while it is idling. Your slow return to idle can also be due to carbs being out of sync
 
Make sure the cables have some slack in them. so they arent fighting each other. Then is carb sync good? And as I say many times on here make sure the ignition advancer is working and is moving absolutely free moving. If it is sticking or the springs are weak it can cause hanging idle....especially if idle is set above 1200ish rpm.

The slides are not vacuum operated so vacuum has no effect on slide movement.
 
The T Cable slack was the first thing I looked at; engine not running, no problem returning to idle stop. According to the paperwork, the carbs did have a sync job some time back but didn't post mileage at that time so??? I'm looking at a vacuum sync tool, should have it next week. I'll check the timing advance in the mean time.

Thanks for the lead dirtdigger.....
 
"Make sure the cables have some slack in them. so they arent fighting each other."
I also had throttle return troubles until I gave some slack to the return cable. My cables were brand new and well lubed, but it was the slack that helped.

Accelerator Pump
If you don't find the cause, it might be worth your time to examine the condition of your carburetor's accelerator pump diaphragm. It should be supple. If it gets hard, then your bike will act poorly on deceleration, especially near idle, and especially when the throttle is "snapped" or twisted abruptly. I don't know that it will prevent decel. for a full 5 seconds.

The accelerator pump is located on your #2 carburetor (#2 is the second from the left, as you're sitting on the bike).

I'm linking a video of a skilled fella tear apart and rebuild the same carburetor your bike has. The value is that you can see all the parts in color, as opposed to the crummy B/W images in the manual.

 
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Thanks Qaxe; good video (part 1 and part 2), the guy is a little hard to understand at times....unless you're from "Down Under". I now have a way better picture of how the carbs are tied together and see that they are way more complicated than my K2 of decades ago. What ever happened to "KISS" engineering????

I managed to remove the ignition cover (screws were only finger tight :doh:) and checked the condition of the points. They have way more miles on them than the information I received (recent tune-up prior to selling:sneaky:). I guess we all have our own definition of "Recent". The points cam was dryer than a Pop-Corn Fart....so that kinda speaks volumes right there. I'm sure when I check the advance weights I will find some issues.

Other than the slow throttle return, the engine runs really good. Having said that and seen a few things that doesn't match sellers information, I thing I'm going to take my time and start with square one; I'm going to replace the points with pointless ignition (any one brand better than others??) Make sure the advance is up to standards and lube well and of course confirm timing.

By then I will have my new carb sync tool to check out just how well the last guy did his job. If that doesn't fix my issues, then I'll get into dismantling the carbs.....I was looking for a winter project anyway :rolleyes:
 
Touched on already-carb balance worth checking.
So (are you confident) its a "hanging idle or a binding issue??....does it return to normal idle without touching anything (eg. without touching the throttle)
 
Touched on already-carb balance worth checking.
As stated before, the throttle returns to the stop without the motor running, every time and with no hesitation. It's only running that the return is slow. I can return it to idle quickly by closing the slides manually by rotation the hand throttle closed. Yes, it does point towards a sync issue and once my sync tools are in hand we will answer the question for sure.

Thanks,
MK
 
Up-date; took the timing advance all apart, it was a little gummy but not to the point of sticking or slowing down the advance/return. It's all cleaned, oiled and added a touch of lube on the points (where they run on the cam). My carb sync tool came in the mail today so gave it a workout too. They were not out all that far but while in the neighborhood, I did a little adjusting. Here's what they looked like prior to adjustment;

I also found a really good video for rebuilding '78 carbs; check out "Doc Jones Garage" This guy is really good at his work and you can understand him too.

Best of all, after watching Doc Jones work on my carbs, or ones like mine, I found that some dummy did a real Micky Mouse job on clocking the return spring for the carbs. I'm more than sure this is why my idle return hasn't been up to par. Not sure how I'm going to get it clocked correctly without at least removing the airbox....project for tomorrow.

Thanks for the comments,
M
 

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Good job on the sync.
When you say clocked the spring, what do you mean?
The return spring that wraps around the throttle shaft is hooked on one end by the idle adjustment stop and the other end should be hooked to the lower (front) side of #3 carb. The s#!$head that worked on this last attached it to the top of #3 (cover screw) so I'm missing about 3/4 of a turn on spring pressure to close the slides. This is why the idle takes its time to come down from 3-4K.

Hopefully I can get it fixed today, the weather is warming back into the 70's and 80's, great time to put a few miles on old girl!
 
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