Topic: 1977 cb750 throttle hanging

[Grebnaws]

Hi Everyone, I'm trying to help a friend with a bike problem and came here for guidance. I own a 1982 cb900f and have some general experience with the 79-82 dohc bikes and CV carbs but I have no experience with earlier Hondas. The short lived dohc F bikes seem to have some peculiarities that may not directly relate to my friends problem and I do not want to make things worse.

What we're dealing with is a very nice 1977 750. I do not know if it's an F, C, or K model. It has had the carbs rebuilt (I believe the entire bike was taken in for the job) and the bike is in good running shape. Last summer this bike completed a long trip without any reliability issues except the rider had to deal with the throttle hanging.

The bike starts easily and settles down very well when warmed up. Carbs appear to by sync'd perfectly and the idle is dead smooth when warm. I have a Morgan carbtune to verify and adjust if necessary. The problem is when blipping or releasing the throttle. The RPMs hang around 3000rpm and very slowly creep back down. To complete the long motorcycle trip an extra strength throttle return spring was installed as a band-aid. Even with a strong return spring to snap the throttle back the RPMs will hang. After the throttle returns to rest naturally and the RPMs are slowly floating down you can cause the RPMs to drop almost instantly if you continue to force the throttle forward. I can feel some forward play in the throttle grip after it comes to a natural rest, and I can feel a spring pushing the throttle back towards me.

Does this describe what I am feeling? Although the throttle returns to a resting position I can still force it to close further by twisting the grip away from me, but a small spring will then roll it back to a resting position which is further back.

The throttle pull and return cables were just replaced and properly adjusted but did not help. The throttle return spring is not stock (I was told that part is unobtanium) and the currently installed spring feels a little soft but it works.  I don't know if the valves were adjusted or if the ignition timing is off. I don't have the experience to help with electronics but I have done my own DOHC valve adjustments.

I do not know anything about this model of carbs and have attached a photo. Are there standard idle mixture screws below? Are they a likely cause of rpm hang? I believe I could remove and reinstall the carbs if necessary (it was hell on both my F bikes) but the rubber on this cb750 seems exceptionally soft for it's age.

I have the impression that the throttle return stop is adjusted improperly, but why is there still room to roll the throttle forward when it comes to a rest. I know there is something pushing it back after being forced forward. Can anyone provide some up close photos of what I'll be dealing with if I have to remove the entire bank of carbs?

[brandEn]

Might be worth looking at the throttle tube and grip. I have bar end mirrors and found a combination of the bar end mirror rubbing on the grip and the grip over hanging the throttle tube a bit and causing enough friction to do exactly what you described. I was chasing my tail like you adjusting cables and routing ect and in the end it was just a simple fix.

removing the PD carbs is not that bad. Carbs probably need a good cleaning anyway. Disconnecting the cables is the hardest part.

[Mandic]

I had a real problem with a sticky throttle that would slowly creep back down when I got my 77 750F.  Took off the throttle cables(both of them) and found them both pretty whooped.  A bunch of TLC with a Cable Luber and they work smooth as glass now. 

Not sure how someone would put a heavier return spring on a PD carb setup since the return is a spring wound around the linkage.

[Old Scrambler]

Two sources of help so far.............I'll add to the mix..........One of the carb slides may be hanging slightly open until pushed down by the extra effort from the return cable. May be a sticky cable.........may be a sticky or worn slide..........maybe a rich idle-air mix..........could even be a slightly botched carb sinch.

[wrenchmuch]

You might check the spark advancer unit to make sure its not binding . Take the points plate off and turn the points cam by hand . As you turn it the weights should move out and when you release it it should snap back quickly .

[HondaMan]

From your picture: if the engine is original, it appears the bike is a CB750K7 model.

The throttle rack has an integral clockworks-style spring that lets you force the slides fully closed regardless of the idle stop screw. This design was from Honda's safety concerns of the 1970s era. Speaking an an ex-racer, I found it handy for increasing compression braking once in a while (but, I digress...).

There are 2 things I know about that can cause the symptom you are describing, provided the cam is properly timed inside the engine. One is the spark advancer, like Wrenchmuch points out: either sticky from rust and lack of lube, or broken spring(s), or excessive timing advance. In particular, check the advance when at idle. In the last few years there have been some ignition points around from a company in China called Daiichi: these points are impossible to set correctly, usually resulting in too much advance at idle.

The second thing: one or more severe vacuum leaks in the hoses that connect the carbs to the head. If the clamps are loose, this might be an easy fix, but if they are fully tight and the hose can still be turned on the head and/or carb, they need to be sealed up somehow. Since they are unobtanium for the post-1976 CB750 bikes now, you can soften them up overnight in a 50/50 mix of wintergreen oil (Amazon.com) and xylol (from Ace Hardware). you might have to switch the hose clamps to wider, non-Honda ones, too.

[jakec]

Interesting fact, that the throttle can be jammed forward of the resting position by design. I had adjusted my cables and tried to fix this problem (obviously I couldn't). So I have the idle set at 1200 rpm on my 400F. And then I can jam the throttle forward a bit to bring it down to 1000. I thought I was going to try adjusting the cables again, but I won't bother now, thanks.

The reason I was reading on this topic today is because I noticed that my idle was creeping this morning, as I pulled up to work. I sat there and watched it for a minute to confirm what I was seeing:

Bike idling at 1200 rpm, then it slowly climbs to about 1800, pauses, and slowly falls back down to 1200. Then pause, climb back up. It kept doing this until I got bored.

This didn't start until the bike got fully warmed toward the end of my 15 minute ride to work. I am already planning to check the hose clamps on the carb boots when I get home.

[beemerbum]

Looks like the 77-78 carb holders and trumpets are available from CB750 Vintage.

[Coredozer19]

i was having this same issue and it helped when dousing the throttle slide linkage with oil, i think its sticky throttle linkage.

[jakec]

Looks like the 77-78 carb holders and trumpets are available from CB750 Vintage.

Hey Beemer, thank you but I am actually hijacking this 7 year old thread to ask about my 75 400F. The boots are in pretty nice supple condition.

[jakec]

I rode home and tightened the hose clamps on the carb boots while everything was still warm. I got about 1/4 turn or less out of all of the screws. Then I started it back up and the problem had gone away.

However when I rode in to work today, the problem was back although not as bad. When i parked in the garage and watched the idle, it would hover at 1200, then climb a bit to 1350 (previously would climb to 2000). Then fall back again. Like the bike was taking deep breaths.

[Paintedseat]

You might have a bad float needle

[jakec]

How do you mean?