Author Topic: Engine "coughing" at low revs  (Read 303 times)

Offline braaap!

  • Posts: 20
Engine "coughing" at low revs
« on: Sep 27, 2018, 10:49:48 »
Hi all

Bonneville T100 2004

since a week about, engine has been having difficulties at revs under 3000. It performs better if choke is on.

Engine is stopping sometimes at idle
Some backfires
Seems to skip fire at times

It's really like it's too lean.

I checked spark plugs and verified distance between poles. I also verified carbs synchronization - still bad.
I removed carbs last night to clean pilot jet and main jet but to no avail.  Also, I don't know if this is normal but pilot jet is totally screwed to the bottom - I wanted to measure how many turns it was from bottom but couldn't screw in any further.

Any suggestions as to what I should look?

Thank you!

Offline DanielK

  • Posts: 57
Re: Engine "coughing" at low revs
« Reply #1 on: Sep 27, 2018, 14:25:41 »
How is your fuel filter? Have you changed air filter and/or exhaust?

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CB650 -80

Offline jpmobius

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  • Posts: 1229
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Re: Engine "coughing" at low revs
« Reply #2 on: Sep 27, 2018, 20:20:41 »
there are numerous possibilities, but likely the pilot system is dirty.  Cleaning the carbs correctly will eliminate most of the possible problems.  The pilot system is not simply the pilot jet, but a sort of mini system within the carb.  The thing to grasp is that this system is comprised of an air inlet component and a fuel inlet component and a sort of reservoir that mixes the two together before that mixture is delivered to the main throttle bore of the carb.  In other words, while you may think the pilot jet delivers a set amount of fuel into the throttle bore, it actually delivers a fixed amount of fuel into the pilot system, which adds air, mixes it into an emulsion, and then that emulsion is sent through those tiny holes just under the throttle plate.  Looking at those (I think 4 on your bike?) teeny tiny holes will give you a good idea as to how easy it is for the pilot system to get dirty, so you have to look at much more than the pilot jet itself to have any hope at all of getting things sufficiently clean.  On the positive side, you only need a can of carb cleaner with a straw, an understanding of what you are doing and some patience to properly clean the system.  You will have to learn the various passageways on your carbs so that you know where the air comes from, where the fuel comes from, where they mix, and where that mixture enters the main bore (those 4 little holes).  The main system works on this same principle with a variable fuel emulsion aperture (instead of the 4 holes) which is your needle and variable air aperture (instead of a pilot screw) which is your throttle plate.  On your CV carbs their is a vacuum operated slide/needle assembly controlled by the vacuum behind the throttle plate which (hopefully) keeps the engine from stalling with too sudden a throttle opening, but this extra contraption has no impact on fuel air mixture.  So look at the parts involved, and figure out where the passages are that connect them.  There is an air entry port within the main air intake bell that will lead to passage above the pilot jet.  The air and fuel begin to mix at this point and continue to flow through another passageway.  Down stream there will be a metering needle, which is the pilot screw and eventually a passageway leading to the four tiny holes and into the main bore.  All you have to do is make sure all these passageways are perfectly clean and you can cause a smooth clean stream of carb cleaner to flow through all of them in both directions. As the passageways connect to each other, you will have to block off with your finger different openings at various times to direct the cleaner where you want.  It is very rare on an otherwise functioning bike for this process to not be sufficient to absolutely, positively guarantee that all is well inside your carb as far as cleanliness goes.  Clean the main system and the rest with the same process and you should be fine.  If you still have trouble look for intake boot leaks or damage to the slide diaphragms.  I seem to recall these bikes having trouble with the intake boots sealing so make sure the clamps are doing their job.  good luck!
« Last Edit: Sep 27, 2018, 20:24:47 by jpmobius »

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1973 RD350 Yamaha build

Offline braaap!

  • Posts: 20
Re: Engine "coughing" at low revs
« Reply #3 on: Sep 27, 2018, 22:33:47 »
Thanks a lot - I will reclean again tonight to make sure all thoroughly clean!

I haven't change anything lately to justify rejetting. The only fuel filter is at the petcock - I haven't check it out - I will - but why would I be running ok above 3000 RPM?