Nc29 5,000-6,000 rpm hesitation help needed

Forum rules
Please can you post items for sale or wanted in the correct For Sale section. Items / bikes for sale here will be removed without warning. Reasons for this are in the FAQ. Thanks
Post Reply
User avatar
royster81
NWAA Supporter
NWAA Supporter
Reactions:
Posts: 3774
Joined: Wed Apr 30, 2008 8:04 pm
Bike owned: VFR400R-L NC30 CBR400RR-R NC29
Location: Belfast,Northern Ireland
Nc29 5,000-6,000 rpm hesitation help needed

Post by royster81 » Wed Jul 25, 2018 7:08 am

I'm looking a little advice in to where to start looking to cure this out even diagnose it so I know what parts are needed to fix it.

The bike has sat for a few years but the carbs have been cleaned, it's had new plugs, correctly gapped to 8mm, the bike has a standard airbox lid with dunno jet carbs, a K&N filter,a Graeme France exhaust ( I'm led to believe) with an ART can.

The symptoms are,

If I'm cruising along at or below 5,000 rpm and I slowly or gradually accelerate with say 1/8-1/4 the bike starts to hesitate with a deep in power but after 6,000 rpm it clears and the bike accelerates as I expect a CBR should.

Or

If I'm coming off a set of lights and give it full wide open throttle it boys slightly, clears and accelerates through the revs cleanly.

I was thinking of checking the carbs again, especially the slides or pilot jets?

Other thoughts where coils, but I can't understand why it revs freely above 7,000 rpm ?

Airbox to carb rubbers ( not carb to engine) , not sure if a leak in this area would cause hesitation?

Other than that I'm open to suggestions.

I know there is a certain exhaust that gives poor midrange but I'm almost 100% I've a Graeme France system, pictures to confirm later.
It's not having what you want but wanting what you've got....Loud ,Proud and Modified ....

CB1pod70
NWAA Supporter
NWAA Supporter
Reactions:
Posts: 375
Joined: Thu Apr 09, 2015 1:55 pm
Location: Gravesend
Re: Nc29 5,000-6,000 rpm hesitation help needed

Post by CB1pod70 » Thu Jul 26, 2018 1:35 pm

I had a similar issue with one of my NC23's which was down to a Delkevic 4-1 exhaust system. Swapped it for a 4-2-1 system and it solved the problem

sa.racing
Site Supporter
Site Supporter
Reactions:
Posts: 124
Joined: Fri May 02, 2008 7:08 pm
Bike owned: NC29 Race
Location: Surrey
Contact:
Re: Nc29 5,000-6,000 rpm hesitation help needed

Post by sa.racing » Thu Jul 26, 2018 9:39 pm

Post a picture of your exhaust let's eliminate that first

Secondly get the bike warmed up to temperature, let the bike sit at idle for arguments sake say 1500RPM give the throttle a little blip don't go too mad just to about 3/4k and see what happens to the idle,

Does it hang high for a few seconds then drop back to 1,500
Does it drop below 1,500 then raise back to normal idle (1,500)
Or does it drop straight back to 1,500 clean and crisp and purrr like a kitten?

Try it and let us know

User avatar
royster81
NWAA Supporter
NWAA Supporter
Reactions:
Posts: 3774
Joined: Wed Apr 30, 2008 8:04 pm
Bike owned: VFR400R-L NC30 CBR400RR-R NC29
Location: Belfast,Northern Ireland
Re: Nc29 5,000-6,000 rpm hesitation help needed

Post by royster81 » Thu Jul 26, 2018 11:11 pm

I sent pictures to Graeme france and he confirms it's one of his road systems.

I'll try the steps sa.racing has suggested and get back to you.

Thanks for the help so far
It's not having what you want but wanting what you've got....Loud ,Proud and Modified ....

just4kicks
Settled in member
Reactions:
Posts: 15
Joined: Wed Aug 30, 2017 12:37 pm
Re: Nc29 5,000-6,000 rpm hesitation help needed

Post by just4kicks » Fri Feb 08, 2019 7:50 am

sa.racing wrote:
Thu Jul 26, 2018 9:39 pm
Post a picture of your exhaust let's eliminate that first

Secondly get the bike warmed up to temperature, let the bike sit at idle for arguments sake say 1500RPM give the throttle a little blip don't go too mad just to about 3/4k and see what happens to the idle,

Does it hang high for a few seconds then drop back to 1,500
Does it drop below 1,500 then raise back to normal idle (1,500)
Or does it drop straight back to 1,500 clean and crisp and purrr like a kitten?

Try it and let us know
I'm having similar issues with my nc23, std jets 110 keihin, std headers, Leo Vance end can and spongy air filter. At startup and idle, the fumes smell rich but dont have access to a reliable AF sensor to confirm. Also, if you lean out the bottom end (below 5000rpm) to fix the rich idling, will it make the top end to lean as well?

What does the first 2 options above mean and how do we get it to option 3 which I presume is the idle option?

sa.racing
Site Supporter
Site Supporter
Reactions:
Posts: 124
Joined: Fri May 02, 2008 7:08 pm
Bike owned: NC29 Race
Location: Surrey
Contact:
Re: Nc29 5,000-6,000 rpm hesitation help needed

Post by sa.racing » Wed Feb 13, 2019 5:24 pm

just4kicks wrote:
Fri Feb 08, 2019 7:50 am

I'm having similar issues with my nc23, std jets 110 keihin, std headers, Leo Vance end can and spongy air filter. At startup and idle, the fumes smell rich but dont have access to a reliable AF sensor to confirm. Also, if you lean out the bottom end (below 5000rpm) to fix the rich idling, will it make the top end to lean as well?

What does the first 2 options above mean and how do we get it to option 3 which I presume is the idle option?
Sorry for the late reply I don't get on here much these days, generally speaking your idle mixture screws control the idle circuit low RPM cruise and high RPM small throttle openings (about 1/8 turn ish) so if your problem is in this area this post maybe relevant,

What exactly is the problem your having? the way a carburetor works is in sync, everything works together although you adjust different aspects to achieve different results (midrange, low RPM cruise etc) if something is not quite right it can throw everything out of sync
110 main jets sounds a bit low (lean) to me, from memory a stock NC23 with an aftermarket can usually sits around 116/118 but we need more info before we can give anymore advice as numbers on jets don't mean much (especially on older bikes that have passed through many hands/owners/workshops/blokes down the pub) as some tuners if they don't have the correct size in stock will actually drill out the stock jets so you may well have the correct size main jet.

in answer to your above question the idle test is a good indication if your pilot circuit is operating correctly if your too lean the revs will hang for a few seconds then drop back to idle too rich and it will bog down and drop below idle then rise back up again, if your getting option 3 (as you called it) its a good indication your in the ball park

have a read of this website its a fantastic source of information on carb tuning and what affects what any more questions just let us know and we'll see if we can help

Si

http://www.factorypro.com/tech/tech_tun ... gines.html

just4kicks
Settled in member
Reactions:
Posts: 15
Joined: Wed Aug 30, 2017 12:37 pm
Re: Nc29 5,000-6,000 rpm hesitation help needed

Post by just4kicks » Mon Feb 25, 2019 10:58 am

sa.racing wrote:
Wed Feb 13, 2019 5:24 pm
just4kicks wrote:
Fri Feb 08, 2019 7:50 am

I'm having similar issues with my nc23, std jets 110 keihin, std headers, Leo Vance end can and spongy air filter. At startup and idle, the fumes smell rich but dont have access to a reliable AF sensor to confirm. Also, if you lean out the bottom end (below 5000rpm) to fix the rich idling, will it make the top end to lean as well?

What does the first 2 options above mean and how do we get it to option 3 which I presume is the idle option?
Sorry for the late reply I don't get on here much these days, generally speaking your idle mixture screws control the idle circuit low RPM cruise and high RPM small throttle openings (about 1/8 turn ish) so if your problem is in this area this post maybe relevant,

What exactly is the problem your having? the way a carburetor works is in sync, everything works together although you adjust different aspects to achieve different results (midrange, low RPM cruise etc) if something is not quite right it can throw everything out of sync
110 main jets sounds a bit low (lean) to me, from memory a stock NC23 with an aftermarket can usually sits around 116/118 but we need more info before we can give anymore advice as numbers on jets don't mean much (especially on older bikes that have passed through many hands/owners/workshops/blokes down the pub) as some tuners if they don't have the correct size in stock will actually drill out the stock jets so you may well have the correct size main jet.

in answer to your above question the idle test is a good indication if your pilot circuit is operating correctly if your too lean the revs will hang for a few seconds then drop back to idle too rich and it will bog down and drop below idle then rise back up again, if your getting option 3 (as you called it) its a good indication your in the ball park

have a read of this website its a fantastic source of information on carb tuning and what affects what any more questions just let us know and we'll see if we can help

Si

http://www.factorypro.com/tech/tech_tun ... gines.html
No worries @sa.racing , I also generally only check in about once a week or so if time allows.

The exact issues are somewhat 2-fold. On launch, from stand still to normal pull way, say at a traffic light, i need to keep it at about 4000-5000 rpm, while sort-of riding the clutch in order to pull away smoothly. If I rev any lower, the bike stalls even riding on the clutch for the pull-away. If I keep at the higher rpm's 4k-5k, but release the clutch to quickly, the bike bogs down for a few seconds during pull-away, kind of like a small car trying to tow a massive caravan. Once its rolling though, no issues, I can even do parking lot maneuvers without the bike stalling at sub 4k rpm.

Part 2 of the issue is, my end-can is pitch black, no thick soot though, just a light dusting. Even after long trips, this doesn't clear. Generally I rid Mid - High RPM as I stay in traffic, so keep the revs mid so cars can hear me, and over the mountain pass, high rpm as i live and work on 2 opp ends of a mountain pass.

FYI, my bike is quite heavier on fuel compared to my buddy's nc30, he has had the extra whole drilled in his carbs, rev-derestrictor fitted etc but he's is much more efficient on fuel. He advised if I go bigger jets my bike will be even more heavier on fuel though.

Any and all advise greatly appreciated :pray: :pray: :pray:

sa.racing
Site Supporter
Site Supporter
Reactions:
Posts: 124
Joined: Fri May 02, 2008 7:08 pm
Bike owned: NC29 Race
Location: Surrey
Contact:
Re: Nc29 5,000-6,000 rpm hesitation help needed

Post by sa.racing » Sun Mar 10, 2019 4:39 pm

Sounds to me you have a pilot circuit (fuel screw) issue, you say Mountain pass, where are you located? could be an altitude issue as you know the air gets thinner the higher you go, fueling is very dynamic and an ever changing factor so what works in one part of the world can be miles off somewhere else,

have you tried the above mentioned idle test? if so what were the results?

does sound like the bike is running a touch rich, don't let the colour of the end can fool you though, you wouldn't believe the sh*t they put in petrol these days, if you can stretch to a visit to a local dyno it would save you a lot of time and guess work, if not I would suggest the above mentioned idle test and let us know the results and we can go from there.

Si


Post Reply

Return to “CBR - Workshop”