Author Topic: CB750 - Throttle play (and how do you get rid of it) (Read 3708 times)

Zunspec · « **on:** March 12, 2016, 01:10:57 PM »

Hi all,

As a newcomer to the Honda 750 engine I am hoping for some tips on minimising slop/play in the throttle set-up. I like the twist grip to shut with a snap with minimum over travel but I have not been able to achieve this despite fiddling about. If I take up the play (see video clip) the throttle gets stiffer and will not return. Probably something very basic that I have failed to recognise but with the expertise on the site maybe someone can point out the obvious to me

https://www.flickr.com/photos/zunspec/25735186495/in/dateposted-public/

Cheers Geoff

Don R · « **Reply #1 on:** March 12, 2016, 01:46:22 PM »

Looks good to me. If it feels like a problem it might need new cables and a throttle tube

Zunspec · « **Reply #2 on:** March 12, 2016, 02:26:05 PM »

Thanks Don R,

My issue is the amount of play at the twist grip, or overtravel after the slides in the carb hit their stops. It affects my blipping of the throttle on down changes, which is one of the best bits with my silencers

. I can certainly replace cables and tube but I was assuming I just wasn't getting the adjustment right.

Cheers Geoff

Retro Rocket · « **Reply #3 on:** March 12, 2016, 03:33:21 PM »

That play is in the linkage, unless you modify that linkage, the play is always going to be there.

harisuluv · « **Reply #4 on:** March 12, 2016, 03:42:59 PM »

That amount of play looks totally normal and is about what is recommended.

I'm not sure what you meant by it "affecting my blipping of the throttle on down changes." Maybe you can explain this a different way.

If you are under the impression that each carb slide has it's own individual slide stop, it doesn't. They all stop traveling downwards when they hit the idle stop screw on the linkage. In other words, one stop on the linkage affects them all.

If you wanted to get rid of it there should be room for adjustment at each end of the throttle cable.

mystic_1 · « **Reply #5 on:** March 12, 2016, 04:02:31 PM »

There's an overtravel adjustment, performed by rotating the eccentric stop pin.

mystic_1

Desert-SOHC · « **Reply #6 on:** March 12, 2016, 04:08:20 PM »

^^^ beat me to it

Zunspec · « **Reply #7 on:** March 12, 2016, 04:46:56 PM »

That info is just what I was looking for, thank you.

I am a complete beginner with the 750 SOHC engine and it's ancilleraries so lots to learn.

Cheers Geoff

Don R · « **Reply #8 on:** March 12, 2016, 05:52:41 PM »

I've never had that issue. Thanks for asking Geoff, I learned another thing about the CB's.

MCRider · « **Reply #9 on:** March 12, 2016, 07:07:50 PM »

Quote from: Retro Rocket on March 12, 2016, 03:33:21 PM

That play is in the linkage, unless you modify that linkage, the play is always going to be there.

Gotta admit, looks that way to me too. The bellcrank, the part the cables hook into, is moving on its axle, for lack of a better word. OCICBW

Zunspec · « **Reply #10 on:** March 13, 2016, 01:12:07 AM »

Quote from: harisuluv on March 12, 2016, 03:42:59 PM

That amount of play looks totally normal and is about what is recommended.

I'm not sure what you meant by it "affecting my blipping of the throttle on down changes." Maybe you can explain this a different way.

If you are under the impression that each carb slide has it's own individual slide stop, it doesn't. They all stop traveling downwards when they hit the idle stop screw on the linkage. In other words, one stop on the linkage affects them all.

If you wanted to get rid of it there should be room for adjustment at each end of the throttle cable.

Hello harisuluv,

When down changing a blip of the throttle (clutch in) matches engine revs to the lower gears as you go down through the g/box. I like the throttle twist grip to to have the minimum of free play to ensure as direct a connection to the carb slides as possible. At the moment the carb slides come to rest and the twist grip continues for another 5 degrees. Of course when you open the throttle this play has to be taken up first and impairs the direct connection I like. When I try to eliminate this play using the adjustment (at both ends of the "pull" cable) the whole set-up becomes stiff to the point of an open throttle not returning when you release the twist grip. Much easier to see and feel the issue rather than explain it

When I have finished watching World Superbikes I shall head to the garage to try the procedure posted by mystic_1.

Cheers Geoff

MCRider · « **Reply #11 on:** March 13, 2016, 07:02:44 AM »

The play should be in the twist grip, not the bellcrank.

mystic_1 · « **Reply #12 on:** March 13, 2016, 07:12:37 AM »

I suppose I should have posted this bit also, the actual adjustment of the cable tension. This is done after the steps above.

There's also a fine-adjustment-point at the top end of the pull cable, near the throttle grip.

The cables need to have a wee bit of slack in them, otherwise the opposing tension from the cables cause the mechanism to bind up and you get stiff operation. They should be loose enough that when you let go of the grip, the throttles snap closed with some authority.

(note, you should also adjust your throttle's drag shoe, if it has one, and also make sure the twist grip doesn't have crap between it and the bars, otherwise that'll put drag in the system too).

mystic_1

mystic_1 · « **Reply #13 on:** March 13, 2016, 07:21:33 AM »

Quote from: Zunspec on March 13, 2016, 01:12:07 AM

When down changing a blip of the throttle (clutch in) matches engine revs to the lower gears as you go down through the g/box.

<snip>

Of course when you open the throttle this play has to be taken up first

Btw, I don't know that I ever twist the throttle all the way down to the stop when I'm shifting, just back off enough to unload the transmission, and pop the shifter.

What you're trying to achieve is that the input shaft of the tranny is not trying to spin faster than the output shaft, and vise versa. This will be at just a slightly lower throttle setting than if you were trying to maintain a steady speed. When maintaining a steady speed, the engine is putting in just enough extra oomph to overcome friction, wind resistance, and so forth, so if you try to shift gears without clutching, the input shaft is trying to turn faster than the output shaft wants to, so the transmission is loaded.

If, on the other hand, you throttle down all the way, the bike is now trying to slow down, you've got engine braking going on, and now the output shaft of the tranny is wanting to turn faster than the input shaft wants to, so now you have "reverse loading".

Just in between these points is a neutral zone where the shafts are wanting to turn at about the same speed. In this zone, you could basically shift without using the clutch at all.

This is how racing shifters work, they interrupt the ignition momentarily to unload the transmission and then pop the shifter. No clutch shifting. Rider never lets off the throttle.

So, the upshot here is that you don't need to bring the throttles all the way closed in order to get a good shift, in fact if you do this than the RPM drops and you have to bring them back up as you let the clutch back out.

mystic_1

Zunspec · « **Reply #14 on:** March 13, 2016, 12:09:56 PM »

Hi Mystic_1,

I have been doing clutchless up changes since I had my first bike in 1971 lol. It is down changes where you have the throttle fully closed but need to momentarily blip it to match revs for the next lower gear. Having this play/slop at the twist grip does not give the me finesse I like, particularly when braking hard at the same time. A personal riding style trait possibly but it what I am used to (sounds good too

).

I have made progress on the play and it is now almost gone. Still not got it quite right and the cables are not moving at their slickest, despite re-oiling. They may need to be flushed out with a degreasing liquid before re-oiling again (or just get new cables).

Thanks for all for the input though, this is a great site with such an abundance of SOHC experience.

Cheers Geoff

mystic_1 · « **Reply #15 on:** March 13, 2016, 01:24:49 PM »

Ah cool sorry didnt mean to talk down to you or anything, just not always sure of people's experience levels when answering questions. I do tend to ramble on sometimes too

Also totally right that you asked about downshifting and I answered about upshifting.

Glad you've got it adjusted more to your liking now, you could probably throw a sleeve or something over that pin if you wanted to reduce the over-travel even more.

mystic_1

Zunspec · « **Reply #16 on:** March 14, 2016, 07:49:03 AM »

Hello mystic_1,

I know where you are coming from lol

. It is not easy to gauge the experience of someone asking a question when all you have to go on is a few posts. I am a pretty new to the Honda SOHC engine, although I did once help to fit a big bore set of pistons/barrels to a friends Dresda Honda 750 about 30 years ago. My Seeley 750 had been stored for 15 years before I got it so it is still waking up again to a certain extent

. I will take all the help and tips I can get, at the very least it makes me think things through.

If you care to see some of the stuff I have been getting up to just follow the link to my flickr.com photo page https://www.flickr.com/photos/10638107@N02/ .

I could not resist the following photo, my other Seeley (replica frame).

Many thanks

Geoff

PeWe · « **Reply #17 on:** March 14, 2016, 08:00:42 AM »

With Mikuni TMR32 on a CB750 with CB750 throttle mechanism the throttle reaction is VERY quick! A little bit too sensitive. Full throttle is about 1/6 turn on the grip. Very slow turn when start on the street, risk for a wheely. If you like quick responce you know which carbs.

I like a little bit play so the idle is set by the adj screw on carb rack, throttle will not increase when handle the bike in very low speed where the bar is turned max and cables might stretch.

Zunspec · « **Reply #18 on:** April 05, 2016, 06:36:42 PM »

I have appreciated the input on my search for my ideal throttle adjustment.

I ordered new cables from eBay, got it wrong and only bought the closing cable correctly

. Today I fitted it and during the messing about the carb slides were closing really slowly as if they had a damper mechanism

?. I sprayed WD40 into the carbs etc with no difference. I then sprayed the carb tops where the main slide rods emerge. Instantly a positive snap as you released the throttle and with cable adjustments I got it just as I wanted it. Was it gummy slide pull-rods causing the issue all along lol.

Cheers Geoff

evanphi · « **Reply #19 on:** April 06, 2016, 06:20:57 AM »

Yes that is an often-overlooked spot to keep lubricated. I prefer a dab of silicone grease, myself. The WD40 will get gummy again pretty quickly.

mystic_1 · « **Reply #20 on:** April 06, 2016, 10:54:09 AM »

Often overlooked, yes, but it's right there in most shop manuals

Zunspec · « **Reply #21 on:** April 06, 2016, 11:41:17 AM »

Quote from: evanphi on April 06, 2016, 06:20:57 AM

Yes that is an often-overlooked spot to keep lubricated. I prefer a dab of silicone grease, myself. The WD40 will get gummy again pretty quickly.

I have some silicon grease so will get on and do it properly

. Now after all this messing around I guess the art of carb balancing needs to be mastered too (after I get some gauges)

Cheers Geoff

754 · « **Reply #22 on:** April 06, 2016, 01:16:57 PM »

Another thing Hondaman has mentioned;
Loosen top ring of carb at wide open throttle , then tighten the rings. Sometimes the shaft is not centered in the cap and binding can occur.
you can get the carbs fairly close using a rod....I have a tip to make that work even better.

Zunspec · « **Reply #23 on:** April 06, 2016, 02:25:55 PM »

Hello 754,

I will carry out the loosening of the top ring procedure, thanks for the tip.

Cheers Geoff

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Author Topic: CB750 - Throttle play (and how do you get rid of it) (Read 3708 times)