Got the bike 3 months ago and have replaced all elements of the ignition.. coils, complete new ignition backplate with new points and condensors (NOS honda part) plugs and caps. On fuel side! Fitted new sets of jets, needle valves etc etc. I have done a carb sync and adlusted air screws to give best tickover, though it is still slightly lumpy and the clutch is a rattling a bit with it. So when i hold the accelerator at about 1/8-1/4 when riding it , it seems to have fuel starvation.. cant think it to be anything else causing the stutter and momentary poor response. When i wind the throttle to accelerate, it goes like a rocket and accelerates very very well indeed without a hiccup. I can cruise along at higher revs with the throttle more open without the stuttering. Any ideas on what may still be causing this despite my best efforts?
Reading you statement is confusing but I gather that the problem is occurring when at 1/8 to 1/4 throttle and you accelerate that there is a flat spot or a hesitation in the rpm increase
If that is what you are trying to say then it indicates a carby problem
In a normal carby there is a accelerator pump that delivers a prolonged squirt of fuel to compensate for the change in air velocity ( falls of and vacuum drops off)over the main jet in the venturi of the carby
So what you have to do is determine if there is such a pump or method or injection that squirt of fuel. Could be a vacuum operated diaphragm
If the clutch and gears are rattling that indicates that the idle rpm is too low
This would place the butterfly in the wrong position and will also contribute to a flat spot by not uncovering the transition holes in the venturi at the correct time
Another point that may be contributing to the flat spot is ignition timing When acceleration occurs , the vacuum advance moves to retard the timing until the vacuum builds back up and then it advances it , so making sure that the ignition timing is exact for the engine and not where you should think it should be is important
Remembering that point gap is set by a dwell meter to get maximum coil field saturation so what you have set it on using feeler gauges can be up to .003" out of required and that will affect the timing mark
Lastly the float level is very important for carby operation as if it is too low then when the vacuum drops off during rpm increase , there will not be enough air speed through the venturi to suck up the fuel until that air speed picks back up
For information
vacuum in a manifold of any engine is set by the throttle position and influenced by rpm
so at idle with the throttle closed it will be around 15 " of mercury and on sudden acceleration will drop to almost zero and on deceleration from high rpm will go as high as 25" of mercury
Vacuum is related to air speed through the venturi so keep that in mind as you search for the solution
Testimonial: "It praps wasnt as clear as it could have been... when travelling along 'cruising' withe throttle about 1/8-1/4 open... it almost as if there is intermittent almost momentary drop ipower.. as if it is suddenly running on 3 out of four cylinders... and if i dont accelerate stay at the same revs, it begins to feel like its stuttering as a result. Initially, i thought it could be a plug lead or a coil, as it feels like a 'lost' spark ... but i have replaced all the ignition side.. so cant think of anything else that could cause it on that side of things. So it must be a fuel issue?... am confised cos it only happens when throttle is open to this degree... once its opened more, or wound back to accelerate quickly, it really does go like the clappers. So i dont think there is anything majorly wrong.. just a setting, or an unseen air leak may e.. something i have missed?.. just thought i would ask the question as a newbie on here to see if anyone could help... thanks for your input..."
It praps wasnt as clear as it could have been... when travelling along 'cruising' withe throttle about 1/8-1/4 open... it almost as if there is intermittent almost momentary drop ipower.. as if it is suddenly running on 3 out of four cylinders... and if i dont accelerate stay at the same revs, it begins to feel like its stuttering as a result. Initially, i thought it could be a plug lead or a coil, as it feels like a 'lost' spark ... but i have replaced all the ignition side.. so cant think of anything else that could cause it on that side of things. So it must be a fuel issue?... am confised cos it only happens when throttle is open to this degree... once its opened more, or wound back to accelerate quickly, it really does go like the clappers. So i dont think there is anything majorly wrong.. just a setting, or an unseen air leak may e.. something i have missed?.. just thought i would ask the question as a newbie on here to see if anyone could help... thanks for your input...".. sorry, i think i put this text as a testimonial for you... can sure tell i am a newbie..
yep -- different explanation to your original statement which requires more thought on the problem
IN principle an d engine regardless of bike car,boat or stationary will run consistently at any given any rpm if the folowing conditions are met
Air fuel ratio is correct for the rpm
ignition is consistent
exhaust is clear and appropriate
load is consistent
valve clearance is consistent
Now from your statement that it is only at a certain throttle setting RPM reading when it happens would be a big help as well as the throttle is a bit iffy indicates something not meeting requirements
I am thinking that it is coming from a harmonic problem occurring at that rpm range
things like a worn distributor shaft bush, worn rocker bushes, worn valve guides, yes possible vacuum leaks
Can you supply make and model of carby in use so that I can see the workings and see if there is something that is not correct
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Sounds like lung failure rather than fuel, If it is lumpy at idle it is usually air/fuel mix may also be sucking air at carby vibration mount.Maybe a full test before replacing anything else will tell the story. see points below to concentrate on.
Check inlet and exhaust valve clearances, they are very finicky.
Secondly the fuel tap at tank sometimes gets shellac inside and restricts flow. have you done:
a) a fuel flow test?
b)cylinder compression test
c) Exhaust emission /flow test?
Air leak test at carburettor manifold mounts - Aerostart, ether spray or lpg sprayed at mounts while running. WARNING these are flammable so just quick bursts. If concerned fine mist of water from spray bottle will do the job. What you are looking for is a change in revs while spraying at joints.
Good luck
AJ
Cheers, i am looking into these suggestions... particularly the air leak possibility. I hav since noticed a bit of 'gungy' brown residue coming from three of the inlet rubbers where they mount the head. Could be meaningful.. seeing as it shouldnt be happening??... i am having a full stripdown over the coming winter months to replace cam chain and check internals.. upon rebuild we will have new gaskets and seals etc so that may help.. as well as grinding valve seats in and re-setting valve clearances etc.. am looking forward to a busy winter. Aint stripped a 400 down for 30 yrs !!.. it wont have changed in that time .
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SOURCE: When accelerating the engine seems to stutter.
Try unplugging your Mass Air Flow Sensor (MAFS) - you'll find it on top of the metal body immediately after the air filter. I went through an 18 month ordeal - replaced fuel filter, fuel pump, dropped fuel tank and cleaned it, ran 55 gallons of fuel injector cleaner through the system - nothing! After reading some blogs on this car/engine, gave the MAFS unplug trick a try. The car instantly corrected itself (when you unplug this sensor, the engine control module reverts to factory default, allows sufficient fuel into the engine for unitterupted performance.) However, if this corrects your problem, it's only a temporary fix. Run it about 3 days - it you notice soot in the tailpipe, the factory default is running too rich. Replace the MAFS.
SOURCE: car stutters at stoplights lexus ls400 1990
I had the same issue with my SC400, it was due to low compression in one of the cylinders, due to a leaking head gasket... I was in the process of swapping out the engine for a new 1uzfe, so it wasnt a huge problem, I had been expecting it due to the fact that I had 301,000 miles on it when I finally replaced the engine last week.
SOURCE: 2003 rio 1.3,car stuttering at take off,at full
If this an automatic replace your throttle cable located in the same place with that of the accelerator cable. Keep me posted.
SOURCE: 2003 Honda Shadow Spirit VT750DC - Stock carbs,
Goto a bigger set of jets untill you find a set the cures the bog up.
The recommended jets are only a starting point.
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/sean_54c3a3dc48a7773c
SOURCE: I HAVE A 91 SEDAN
Sounds like a malfunctioning egr system. Too much egr gasses at once will lean out the air fuel mixture and cause hesitation in an engine. Just my 2 cents.
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