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No Accelerator Pump

johnw

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Hi, my 1980 750c does not have an accelerator pump fitted to no. 2 carb and I have read on other threads that the European models did not have them fitted.
It is really hard to start from cold and has been since I bought it last year.
The choke mechanism is working OK and following previous advice I have re-set the valve gaps and cleaned the carbs out.
I'll clean the carbs again.
I also cleaned out the tank and fitted an inline filter. There is good flow of fuel to the carbs.
I suspect the floats are sticking and not letting fuel in because as I turn the bike over there is no great smell of unburned fuel.
After a few turn on the starter, it begins to rumble and then eventually will fire up if the battery lasts.
Do you think the floats could be the issue?
johnw
 

amc49

CB750 Guru
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Engine condition figures in big there, people never question it but the biggest determiner of quick cold starting there is.

You're NOT supposed to smell raw fuel unless going way too far trying to light one that won't.

The accel pump really doesn't help cold start as much as you think.

Don't know if they add ethanol to your fuel there but if they do that can be a big part of it. It can stick floats sitting overnight.
 

Pwnypower

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thats why i always fill up with 98 or more, we rarely have ethanol free gas here in the netherlands, i find it a bit unpleasing that 98 is also 5% ethanol. I always go to fuel stations that claim that their 98 is ethanol free. will make the bike much happier. I also put in some lead replacer just because the bikes hardened valves are usually not designed for these kind of lean gasses that are like sandpaper in comparison to the good old days. I have befriended a guy that specialises in fuels and additives and he also recommends sometype of lubricator because the gas nowadays is way more coarse(he calls it schraler, a dutch word for less lubricating)
 

johnw

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Thanks for the response Pwnypower.
Can't get ethanol free fuel in the UK, at least not easliy.
Will consider using additive.
As an update, with help from amc49 I got a rebuild manual for the carbs and discovered that when closing the choke it should act on a seperate lever to open the throttle butterflies a little. I had mistakenly assumed this extra lever was somehow connected to the accelerator pump which is not fitted to my machine.
The one on my carbs is bent out of shape and is not acting on the throttles.
I've got them apart for cleaning now and will adjust the lever to work.
Am hoping it helps my cold start issues.
 

Pwnypower

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when you pull the carbs apart consider getting some rebuild kits for them, makes the job easier and also ensures that your carbs are back to spec(air cut off valves tend to be a bit stiff with a bike this old).

I didn't do mine but checked them when they cleaned my carbs.
 

johnw

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Yes I will rebuild with new parts.
I changed the air cut off last year and it made a big difference on the overrun - less popping / banging.
My issue is very poor cold start.
Will clean, rebuild and sort this choke mechanism and see if it improves things.
 

amc49

CB750 Guru
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FYI Euro models do not have accelerator pump as they have a slightly richer idle/off idle setting than US ones do. US ones also had idle limiter caps to stop people from adjusting the idle mixture. The aircuts figure in that too, they lean the idle an additional amount and I tend to defeat them when I rework carbs. When you do that you open the mixture screws a bit more to make up for the extra high vacuum decel richness you lost when losing the aircut working.

The aircut if working right only really does anything at high vacuum above idle like the overdraw situation you get when at deceleration with throttle closed. The engine runs lean enough at that time to not fire right, some of the mix then afterburns to pop in the exhaust pipe. With the aircut activated the idle mixture switches from two idle air jets to just one to richen the mix only when needed then when the high vacuum drops off it switches back to the super lean two idle air jets to lean out the normal idle for emissions reasons.
 

johnw

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Hi all and thanks for the previous responses.
I have rebuilt the carbs with rebuild kits and replaced all the air cut off diaphragms etc.
Great results - bike starts first time on the button, really strong power (noticably better than before) and no banging / popping on the overrun.
Much Better!
However, when warm, it won't settle below 2,500 rpm at tickover.
I have the tickover knob turned all the way out so it doesn't even touch the arm.
I have adjusted both cables so they are not hanging up.
Any ideas of what to do next?
 

pidjones

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Hi all and thanks for the previous responses.
I have rebuilt the carbs with rebuild kits and replaced all the air cut off diaphragms etc.
Great results - bike starts first time on the button, really strong power (noticably better than before) and no banging / popping on the overrun.
Much Better!
However, when warm, it won't settle below 2,500 rpm at tickover.
I have the tickover knob turned all the way out so it doesn't even touch the arm.
I have adjusted both cables so they are not hanging up.
Any ideas of what to do next?
Did you align everything before tightening down to the cross-stays? All plates have to be fully closed before (and after) tightening the carbs to the stays. Otherwise, one closed throttle plate can hold the others open. Then the choke cam should be checked to make sure throttle plates close off fully when the choke is OFF. Finally, all air leaks from the throttle plate on must be eliminated.
 

Spag48

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Thanks for the response Pwnypower.
Can't get ethanol free fuel in the UK, at least not easliy.
Will consider using additive.
As an update, with help from amc49 I got a rebuild manual for the carbs and discovered that when closing the choke it should act on a seperate lever to open the throttle butterflies a little. I had mistakenly assumed this extra lever was somehow connected to the accelerator pump which is not fitted to my machine.
The one on my carbs is bent out of shape and is not acting on the throttles.
I've got them apart for cleaning now and will adjust the lever to work.
Am hoping it helps my cold start issues.
Esso claim their Premium 99 fuel is Ethanol free.
 
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