Topic: I Have a 1972 (i think ) CB750 that wont tick over

[elly]

I have a 1972( I think ) Honda CB 750 chopper

http://i172.photobucket.com/albums/w16/robelly/d_resize.jpg

http://i172.photobucket.com/albums/w16/robelly/5c7e_12.jpg


It runs great but wont tick over the carbs I believe are a early type which have a cable into the top of  each one. I have been told these are not original and when the bike was turned into a chopper parts have been removed  so the carbs would fit around the top frame i.e. some kind of throttle stop
I was wondering if someone would know for definite if these are original Honda carbs or not ( I will take a better pic when I find my camera tomorrow)
It makes it difficult to ride as it has no indicators due to its old school style and signalling right with your arm and keeping it running is a nightmare

[mcpuffett]

hi elly and welcome, was your bike originally built by prisoners from one of our majesty's establishments? i have seen this bike or one like it in magazines  ,    cheers mick.

[cleveland]

Good looking bike!  Those carbs look right from a distance, but a closer pic would help.

[gerhed]

Hard to tell, but they may be Mikunis.
The nut on the top of each one could be used to raise the idle rpm.
They should all lift at the same time--then the twist grip itself would hold the carbs open.
If there's a throttle stop screw--it would be used to hold the throttle pistons in the proper idle position.

[Magpie]

Hi!
What's the serial number on the motor (left side near the starter cover)? If those are the early carbs it may be an interesting motor!
Cliff.

[flatblack]

Yeah, they look like round slide Mikunis from here...

Could be as many as three idle speed adjustments:

1. Throttle cable (at the grip).  Look for the hex-shaped sleeve and lock nut.  Loosen the lock nut and turn the sleeve to take up any slack in the cable.  Tighten the lock nut when you're finished.

2.  Earlier poster mentioned the brass nuts/lock nuts on the carb tops.  Same principle as the sleeve at the throttle grip.  You'll have to do all four by the same amount.

3.  Somewhere between the carbs and the throttle grip there has to be a junction of some sort splitting the single cable into four.  Might be some adjustment there, too.

HTH...

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[elly]

Hi all
Many thanks for the advice on my bike
http://i172.photobucket.com/albums/w16/robelly/BikeRight.jpg


Here are a pic of the carbs and cables but I guess from the earlier advice the best idea is to adjust the tick over on top of the carbs as if I do it on the throttle end it puts strain continuously on the top cable and the tick over adjusts slightly  if the handlebars/top cable moves when turning etc

http://i172.photobucket.com/albums/w16/robelly/Carb.jpg

http://i172.photobucket.com/albums/w16/robelly/Cables.jpg

http://i172.photobucket.com/albums/w16/robelly/2carbs.jpg


mcpuffett
Yes it is the bike you are thinking of built at Full Sutton Security prison.
I know it’s a long shot but I don’t suppose you know which mag it was in

Thanks again

Rob

[elly]

Could anyone advise me if these are the kind of gauges i would need to set my carbs

http://www.carbtune.com/

[Bob Wessner]

Those will certainly do the trick. I use them, but quite a few folks use vacuum dial gauges that can be had for about half the price, if cost is an issue.

[flatblack]

Could anyone advise me if these are the kind of gauges i would need to set my carbs

http://www.carbtune.com/

Great tool.  I have one, and it's easy to use.  One problem, however, with your carbs (I can't tell if those are Mikunis or Keihins): There may not be a nipple/port you can use with the Morgan Carbtune to synch all four carbs.  Also, I don't see a visual synch port on the slide with which to synch them using drill bits.

In that event, you'll need to do some broken-field running.  You may want to consider getting a rack of used carbs and, perhaps, a new throttle cable for that bike...

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[flatblack]

Here are a pic of the carbs and cables but I guess from the earlier advice the best idea is to adjust the tick over on top of the carbs as if I do it on the throttle end it puts strain continuously on the top cable and the tick over adjusts slightly  if the handlebars/top cable moves when turning etc.

Yeah, you've got all kinds of adjustment between the throttle grip, the four cables to the carbs and the carb tops.

I might set and forget the adjustments at the twist grip and the top of the carbs and focus only on the four cable adjusters running between where the cable splits (under the tank) and the carbs.  I'd adjust the nuts on the carb tops to the same distance/number of threads/turns from bottom and leave them alone.

That way, you know what you've got and are only dealing with one adjustment, four times.

Also, following up on my post about the Morgan Carbtune, if you can't find a way to connect the Morgan to the carbs/boots, maybe investing in a laser thermometer would be a way to go.  I've never done it on a bike, and there are a few mixture-related variables, but you should be able to synch the carbs quite nicely by measuring each pipe's temperature with the laser thermometer and adjusting the cables to get each carb/pipe as close together as possible.

As always, you'll want to dial in the ignition timing and points gap before synching carbs.

Good luck, and HTH...

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[mcpuffett]

Hi rob, i think the article i saw was in either classic bike or classic bike guide ? sorry can't be more helpful good luck with the bike   ,    mick.

[elly]

I have tried but with no look to adjust the idle on the cables which ever way I try to do it the cables just don’t release quick enough or back to the exact same place every time. Im not surprised this is the case as due to th shape routing etc of the cables there is to much friction involved.
Looking at the workshop manual I have downloaded it shows my carbs  (Keihins)in the fuel section and they look exactly the same as mine apart from the fact they have a throttle stop screw does anybody know which model these would be off
Or a compatible set of carbs I could look at fitting not forgetting that my top frame comes down between the two banks of carbs so

http://i172.photobucket.com/albums/w16/robelly/Carb.jpg

cheers rob

[flatblack]

Where are you?  Someone who's done this before needs to take a look...

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[elly]

Hi and thanks for all the help
i made and fitted a new top cable and it would appear to have done the job it seems the old cable was sticking and not allowing the revs to return correctly each time

My next question is this when you set up the carbs with a set of balance guages what is it exactly you are setting is it simply to make sure all the slides are at the correct hight on tickover or is it more to do with setting the air mixture scre or do you just set that to 1 1/2 turns out

Many thanks for the help


Rob

[flatblack]

My next question is this when you set up the carbs with a set of balance guages what is it exactly you are setting is it simply to make sure all the slides are at the correct hight on tickover or is it more to do with setting the air mixture scre or do you just set that to 1 1/2 turns out

Ideally, carb synch is the last step in tuning the bike.  Some of the others steps include:

1. Ignition timing
2. Jetting
3. Idle speed/mixture

When synching with a set of balance gauges reading vacuum, you'll be looking at how much air/fuel is going into each cylinder.  The theory is if this is done at idle, with all other carb settings roughly equal, including throttle cables, the carbs will be in synch throughout the engine's operating rpms.  The absolute value provided by each gauge is not important.  The relationship of each gauge to the others is.  Basically, you want each cylinder to read as close to the same value as you can get them.

Do it at idle, but blip the throttle occasionally to eliminate any hung cables/slides.  I'd also check that the carbs are synched at high idle and/or at mid-rpm, if your gauges will read those values.

Of course, the bike needs to be warmed up and at operating temperature.  Depending on the conditions, it might be a good idea to set up a fan in front of the bike to blow cooling air on it during the procedure.  With your carb set-up, I'm not sure, but most (all?) SOHC Hondas require the tank be removed to access all the adjustment screws.  To get fuel to the engine during the procedure, I usually position the tank much higher than the engine and run a longer hose to the carb rack.

HTH...

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[750goes]

synching carbs is getting the slides to all be open to the same extent - but all pulling the same .... so this depends on valve gap, points timing, dwell, good plugs, clear carburettor passages, fully warmed up motor, and all idle screw/mixture screw settings on each cylinder performing equally - it also helps if each carb is breathing the same amount of air, through a clean air filter...plus all manifold and carburrettor boots, air box rubbers being properly sealed.....then the carb sync will be the final piece in the puzzle...

sorry I forgot camchain adjustment as well

get all the others right and final synching will put the icing on the cake..................

if you have not got all the others right, then synching is really wasted effort....because you will have to do it again...