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Very rough idle and misfire at around 6-7k revs - Moisture?

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Vardypeeps
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Very rough idle and misfire at around 6-7k revs - Moisture?

Post by Vardypeeps on Tue 06 Dec 2016, 5:05 pm

Hi Guys.

I'm having an issue with a rough idle that appears around 3-5secs after revving the Carbed CB, I'm also experiencing intermittent backfire/misfire problems at round 6-7k revs with a fully open throttle although this comes and goes the rough idle always comes back.

With all the foggy weather we have been having could I possibly put this down to water ingress somewhere on the bike like the original fuel cap etc?
I don't think it's the reg/rec as lights brighten up at the right time to show it's charging and the bike doesn't cut out on idle but jumps between 500-1500revs

Stuck choke?

Any help would be great as it's my daily commuter while my other bike is being fixed after a crash.

Thank You! Smile
Rob
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liverpool_f_
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by liverpool_f_ on Tue 06 Dec 2016, 5:30 pm

If it appears after revving and up in the rev range, it may well be time for a carb rebuild/service. Often, parts become slightly sticky inside like the slide and it causes running problems like you have described. It can be intermittent and dependant on loads of factors like heat etc.
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Vardypeeps
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by Vardypeeps on Tue 06 Dec 2016, 5:33 pm

Excellent, one to check if it doesn't go away and I've got all next week off so I can get them on the kitchen table and make the house stink of brake/carb cleaner Very Happy

Best end to remove the carbs from first?

Thank You
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Jameshambleton
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by Jameshambleton on Tue 06 Dec 2016, 10:38 pm

6-7k rpm was a dead spot in my rev range when I had the 35kW restrictors in and it completely messed with the fueling, check to see if there it any restrictor washers between the carbs and the head.
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Vardypeeps
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by Vardypeeps on Wed 07 Dec 2016, 9:15 am

I can confirm there are no restrictors fitted but Thank You. Rather than a dead spot it's a full on misfire.
Bike has been taken through the gears with no issues.
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eternally_troubled
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by eternally_troubled on Wed 07 Dec 2016, 1:44 pm

You say it jumps between 500 and 1500 rpm.....  it shouldn't ever be at 500 rpm :)

Have you had a look at the coils? Might be worth an investigate with the multimeter. They are a bit of pain to get to but easily checked. While you are there it might be worth examining the HT leads and, if you haven't done it already, changing the spark plugs (just 'cos they are cheap and you might as well do it).

Also, given what you were saying about dampness, if you have the time, would it be worth going over various electrical connectors with some WD40 or similar and cleaning out any corrosion etc.

I think I'd check all that before taking the carbs apart...
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Vardypeeps
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by Vardypeeps on Wed 28 Dec 2016, 12:28 pm

Guys, I think this can be put down to Carb Freezing on the 95 CB500 unless its supposed to have a heater?
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Jameshambleton
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by Jameshambleton on Wed 28 Dec 2016, 12:32 pm

Vardypeeps wrote:Guys, I think this can be put down to Carb Freezing on the 95 CB500 unless its supposed to have a heater?

Carbs are heated/warmed though the cooling system of the bike - there is a small hose that is vertical coming from the top of the water pump that connects to the carbs and there is another one at the other side but I can't remember off the top of my head where that one goes.
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eternally_troubled
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by eternally_troubled on Wed 28 Dec 2016, 3:01 pm

Vardypeeps wrote:Guys, I think this can be put down to Carb Freezing on the 95 CB500 unless its supposed to have a heater?


This shouldn't really happen on the CB500 - the cooling (hot!) water is routed through bits of the carbs to warm them up.  I've certainly never had this particular problem - carb icing tends to happen when it is cool and damp rather than really cold, in my experience.

Is it possible that there is something wrong (a blockage?) with your carb heat circuit?  It might be worth grabbing one of the carb heat hoses (there is a photo in the Haynes showing which one it is) and checking that it is getting hot.  Also worth checking your coolant level, just in case.
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Vardypeeps
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by Vardypeeps on Wed 28 Dec 2016, 4:05 pm

Coolant seems ok but most likely needs to be replaced and may help with the issue, Thanks for the tip for checking it, can this same thing be done to check the water pump is working?
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Re: Very rough idle and misfire at around 6-7k revs - Moisture?

Post by eternally_troubled on Thu 29 Dec 2016, 7:43 am

Vardypeeps wrote:Coolant seems ok but most likely needs to be replaced and may help with the issue, Thanks for the tip for checking it, can this same thing be done to check the water pump is working?

The water pump is pretty well-connected to the movement of the engine, so it's quite hard for it to stop working! It's main failure mode is when its oil/water seal goes and it starts leaking in some way...

If you wait until the thermostat has opened (check after a ride) the pipes going to and from the water pump should be a similar temperature to each other and a similar temperature to the radiator pipes (obviously not the *same* temperatures but there shouldn't be any cold spots). It's pretty unlikely there will be any cold spots....

It's worth replacing the coolant and doing a system flush (even if it's only with plain water) to try and get any gunk out - even if just for your peace of mind.

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