cough at 3500RPM
04-18-2013, 08:44 AM
#1
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cough at 3500RPM
I've been following other threads on carburetor issues hoping to glean some information to an issue I've had for a while. The problem: the bike "coughs" even after warm up at about 3500 RPM in 2nd gear. The symptom arrived after I removed the PAIR system. I've gone back over everything several times looking for mistakes but everything is where it's supposed to be. I've balanced the carburetors carefully, adjusted the TPS to 490 ohms and checked fuel hoses and vacuum hoses. I did hawks carburetor enhancement, #48 pilot jets, remove shim from front carb needle and epoxy one lift hole on the front slide. I realize I need to play with the shims to get best performance but the "cough" has not gone away. It's like a backfire through the carbs, I can hear it under the tank in the air box. What next?
04-18-2013, 08:47 AM
#2
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It's time to use the choke as your trouble shooting tool. Crack it open slightly, do the bike run better or worse?
If better, you're lean and you need to raise the needles a shim or open the pilots 1\4-1\2 turns.
If worse, you're rich and you either need to drop the needles or close the pilots a little.
If better, you're lean and you need to raise the needles a shim or open the pilots 1\4-1\2 turns.
If worse, you're rich and you either need to drop the needles or close the pilots a little.
04-18-2013, 09:19 AM
#3
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It's time to use the choke as your trouble shooting tool. Crack it open slightly, do the bike run better or worse?
If better, you're lean and you need to raise the needles a shim or open the pilots 1\4-1\2 turns.
If worse, you're rich and you either need to drop the needles or close the pilots a little.
If better, you're lean and you need to raise the needles a shim or open the pilots 1\4-1\2 turns.
If worse, you're rich and you either need to drop the needles or close the pilots a little.
Last edited by twist; 04-18-2013 at 09:23 AM.
04-18-2013, 10:14 AM
#6
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It's actually a fuel mixture screw on these carbs. Turing it in= less fuel. This adjustment affects the mixture of fuel going into the pilots, which are the heaviest influence on the lower RPM's. Adjusting the needles doesn't do much until you're in the mid-higher rpm's when the slides open.
04-18-2013, 10:38 AM
#7
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Cough
If it's a backfire into the airbox then logically the intake valve is still open when it occurs which may mean preignition or retarded intake timing. When did it start doing this?
04-18-2013, 10:38 AM
#8
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got it! Now I need only follow up on my intention to find an easier way to get to those screws while the bike is running. My experience is limited to Mikuni TM 38 flat slides, those are very easy to adjust and not as finicky. So far, my experience with the carburetors on the VTR is, they are touchy to very slight changes and difficult to balance perfectly. That's probably just me tho.
04-18-2013, 10:40 AM
#9
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seems to me that it began just after I removed the PAIR system. It's a "cough" but it sounds and feels like it's coming through the air box.
04-18-2013, 01:40 PM
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I have a stuttering issue in the same RPM area I have been trying to resolve since reverting back to an almost stock setup.
I had some carb hicups but they were resolved. I am moving forward with shimming the needles to be richer as suggested in my "Carb Fine Tuning thread". Are you also having popping on Decel?
I have not been able to adjust Idle Mixture screws while the bike is running due to the location and my lack of the CRAZY TOOLS REQUIRED!.... ^.^
Step by step, week by week, eventually it will run perfect and then I'll post the bike on Cragslist HAHAHA....just playing.
I had some carb hicups but they were resolved. I am moving forward with shimming the needles to be richer as suggested in my "Carb Fine Tuning thread". Are you also having popping on Decel?
I have not been able to adjust Idle Mixture screws while the bike is running due to the location and my lack of the CRAZY TOOLS REQUIRED!.... ^.^
Step by step, week by week, eventually it will run perfect and then I'll post the bike on Cragslist HAHAHA....just playing.
04-18-2013, 01:57 PM
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If it makes you feel any better I had my carbs off yesterday to do a pilot screw adj. The last tweak to get things right again after changing velocity stacks.
So while it can be a pain the reward is worth the effort IMHO
So while it can be a pain the reward is worth the effort IMHO
04-18-2013, 02:10 PM
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As the Chuck Norris of carbs, I thought you just stared them into perfection........whats with this "removal" nonsense?
Last edited by WhOrD; 04-18-2013 at 02:21 PM.
04-18-2013, 02:28 PM
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Well my pilot tool was in the tool box that walked away a few years back with most of my "special" tools in it.....
and it is one that just hasn't been replaced yet.
Besides you don't have to disconnect anything on the carbs, just pop them out of the boots so its 45mins tops for the whole job.... and that is taking my sweet assed time
Also in case you missed it Carl pointed out a good way to "fine tune" the pilots.
Set the idle @1200RPM or as close as you can get it.
Now "blip" the throttle . you only want the bike to rev to 2K or less and then watch the tach.
Does the needle go directly back to 1200RPM? if so you are good
If the needle hangs up at 1400RPM or so and then settles back down to 1200RPM, then you are lean or open the pilot screw a little bit.
If the needle drops below 1200RPM and then climbs back up, you are rich and need to close the pilot screw
and it is one that just hasn't been replaced yet.
Besides you don't have to disconnect anything on the carbs, just pop them out of the boots so its 45mins tops for the whole job.... and that is taking my sweet assed time
Also in case you missed it Carl pointed out a good way to "fine tune" the pilots.
Set the idle @1200RPM or as close as you can get it.
Now "blip" the throttle . you only want the bike to rev to 2K or less and then watch the tach.
Does the needle go directly back to 1200RPM? if so you are good
If the needle hangs up at 1400RPM or so and then settles back down to 1200RPM, then you are lean or open the pilot screw a little bit.
If the needle drops below 1200RPM and then climbs back up, you are rich and need to close the pilot screw
04-18-2013, 03:07 PM
#14
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Well my pilot tool was in the tool box that walked away a few years back with most of my "special" tools in it.....
and it is one that just hasn't been replaced yet.
Besides you don't have to disconnect anything on the carbs, just pop them out of the boots so its 45mins tops for the whole job.... and that is taking my sweet assed time
Also in case you missed it Carl pointed out a good way to "fine tune" the pilots.
Set the idle @1200RPM or as close as you can get it.
Now "blip" the throttle . you only want the bike to rev to 2K or less and then watch the tach.
Does the needle go directly back to 1200RPM? if so you are good
If the needle hangs up at 1400RPM or so and then settles back down to 1200RPM, then you are lean or open the pilot screw a little bit.
If the needle drops below 1200RPM and then climbs back up, you are rich and need to close the pilot screw
and it is one that just hasn't been replaced yet.
Besides you don't have to disconnect anything on the carbs, just pop them out of the boots so its 45mins tops for the whole job.... and that is taking my sweet assed time
Also in case you missed it Carl pointed out a good way to "fine tune" the pilots.
Set the idle @1200RPM or as close as you can get it.
Now "blip" the throttle . you only want the bike to rev to 2K or less and then watch the tach.
Does the needle go directly back to 1200RPM? if so you are good
If the needle hangs up at 1400RPM or so and then settles back down to 1200RPM, then you are lean or open the pilot screw a little bit.
If the needle drops below 1200RPM and then climbs back up, you are rich and need to close the pilot screw
04-18-2013, 03:10 PM
#15
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Set the idle @1200RPM or as close as you can get it.
Now "blip" the throttle . you only want the bike to rev to 2K or less and then watch the tach.
Does the needle go directly back to 1200RPM? if so you are good
If the needle hangs up at 1400RPM or so and then settles back down to 1200RPM, then you are lean or open the pilot screw a little bit.
If the needle drops below 1200RPM and then climbs back up, you are rich and need to close the pilot screw
04-18-2013, 04:25 PM
#16
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Well #!@$, nobody mentioned this to me. I'll verify this when I shim my needles 2nite.
Wow a lot of people playing with/adjusting carbs recently.
zxbud is correct in what he's saying as timing can be an issue but carb coughs can also be a sign of being too lean. The first thing I would do is make sure there are no intake leaks or blowing exhaust gaskets/clamps as this can be an issue as well And in twist's case check over all the PAIR removal and make sure everything is OK there and nothing has been missed. IIRC the PAIR system has a hose that runs to the bottom of the airbox. Did this hole get filled otherwise you may loose a bit of pressure in the airbox.
When putting the 48pilots in did you adjust the fuel screws to compensate for larger pilots. You will probably find with the 48's you will need approx 2.5 turns out on the front and 2.1/4 rear give or take anywhere between 3/8 turn. You would be surprised how much of a difference even 1/8 turn can make. If you had 45's then you would be around 3.5 turns out and if
50's about 1.5 turns. So larger pilot = less and smaller = more.
The pilot jets mainly affect the idle circuit as 7mmore said but it also affects the needle/slide and mains too. It starts heaviest at the bottom (idle circuit) and tapers off to it's smallest at the mains, so the mains get a trickle.
So far, my experience with the carburetors on the VTR is, they are touchy to very slight changes and difficult to balance perfectly. That's probably just me tho.
I use the following for adjusting the fuel/mixture screws.
Carb Tool 90 Degree w/Bits | Motion Pro
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