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Old 09-04-2013, 02:50 PM   #1
Syndacate OP
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Location: Santa Clara, CA
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'85 Honda Nighthawk Carb Help!

Hey,

I'm pretty new to the forum but have been riding for like 4 or 5 years (only 25).

I'm very new to carbs. I understand carbs pretty well, but for the life of me I **cannot** get these carbs dialed in.

I have an '85 Nighthawk CB450SCL. This has the VB22 (or similar) dual carbs for the parallel twin. I cannot for the life of me get these carbs dialed in. I have thrown in the towel a long time ago.

Bike is stock, new OEM filter,OEM airbox, and stock pipes. Pipes are leaking a bit, but the vast majority of the exhaust comes out the end.

Carbs have 2 circuits on this setup (unlike the VB22's on my '82 CM450 which has a secondary (needle) main and a primary main) - it has the pilot circuit and the main - a seemingly simple setup.

Bike has VERY little power like 0-40% throttle, after 40% the bike jerks/jolts to life and has full power all the way to WOT as it should. My guess is that the A/F on the pilot circuit is bad. These are compound circuit carbs, so the pilot doesn't shut off after the main turns on, but since it's not supplying the majority of the fuel it's okay to deal with.

The people on hondatwinns.net have been helping me out a lot, in particular a friend I have up in Redding has been helping me quite a bit.

Whatever the reason, I cannot get these carbs to run right at < 40% throttle when the bike is warmed up. Cold the problem happens, but it's not as bad.

If anybody has suggestions that's great, I'll read and possibly try them, but I really don't care to spend hours working on this anymore, I've invested MANY more hours than I ever care to trying to get these working. On the flip-side, I don't want to turn it over to a shop for them to tell me that the FSM says 2.25 turns is the factory spec and just set it to that.

SOOOOOOOOO

Any shops in the bay area or near it that would know how to dial in a stock CB450SCL?? I was thinking maybe a shop (or even a regular 'ol person who knows these) that does retro or cafe racers or something that would really have a good handle on getting these carbs to run right. I don't want to dump 1000 bucks into a bike I'm not going to own for awhile, but I'd be willing to dump a decent amount of $$ into it for it to run right. I don't need somebody who can tell me that the FSM says 2.25 turns or whatever it is, I need somebody who understands these god-forsaken things and can dial them in...

As side notes:
- Just cleaned the carbs thoroughly, soaked the bodies, jets, etc. in Berryman, it removed some of the 'oddities' and made it run better, but the problem wasn't resolved
- Vacuum slides move up as they should
- Carbs sync'ed by a shop maybe 3 months ago - sync may have changed a bit due to cleaning the carbs, but should be pretty close... No change
- Pulled all CA emissions crap (evap canister, PCV), tied the PCV injection nipples together to help balance, tank bottom vent is venting to atmo. No change.
- air cut-offs and vacuum pump diaphragms all look ok, check valve in accel pump working as it should (no change after unjamming check valve)
- OEM needle isn't adjustable
- OEM plastic floats aren't adjustable
- pilot A/F adjustment 'pegs' have been cut off, so you can tune full pilot A/F from all the way out to seated

EDIT:
I should probably mention that before I dismantled it, my '82 CM450E (which is almost identical) with almost identical VB22 carbs has the **SAME EXACT** problem. I wrote the problem off there, though, because it has straight pipes with no cross-over and no baffles from the previous owner. Main was rejetted. I'm thinking whatever the problem is it's the same with both. Both of these bikes have had this issue since the day I got them..
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Syndacate screwed with this post 09-04-2013 at 03:00 PM
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Old 09-04-2013, 06:07 PM   #2
HAPPYTRAILSINC
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YOU need to clean out the jets!
soaking them does nothing~
take out the jets and use the wire from a wire brush to clear the jet passage! you may have to use a few wires for the main jet or a torch tip cleaner works also!

on some carbs there is a passage on the intake bell that goes to the main jet nozzle-that also gets clogged up!
make sure u can spray carb cleaner thru all passages!
when running/revving up, watch the slides, do move up and down with the rpm's? if not there either ripped or not seated!
after time they (vacume slides) shrink up and don't seat properly, if so u can soak them in gasoline to get them to swell up, or if there to swollen leave them out to dry a bit!
but keep them out of the berryman , it will destroy them!
if u have access to an ultrasonic cleaner, get them in there for a few minutes!
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Old 09-04-2013, 06:39 PM   #3
Syndacate OP
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Quote:
Originally Posted by HAPPYTRAILSINC View Post
YOU need to clean out the jets!
soaking them does nothing~
take out the jets and use the wire from a wire brush to clear the jet passage! you may have to use a few wires for the main jet or a torch tip cleaner works also!

on some carbs there is a passage on the intake bell that goes to the main jet nozzle-that also gets clogged up!
make sure u can spray carb cleaner thru all passages!
when running/revving up, watch the slides, do move up and down with the rpm's? if not there either ripped or not seated!
after time they (vacume slides) shrink up and don't seat properly, if so u can soak them in gasoline to get them to swell up, or if there to swollen leave them out to dry a bit!
but keep them out of the berryman , it will destroy them!
if u have access to an ultrasonic cleaner, get them in there for a few minutes!
On the CM450 (which I noted at the bottom has an identical problem) I straight-up replaced the jets, no change. Here I didn't use a guitar string or similar on them, but I did spray them out with B12 spray. Both bikes, no change :-\.

There's a couple holes for air on the main intake bell, I blew them out on both carbs, no change. The carb spray makes it to all passages AFAIK.

I haven't watched the slides first-hand b/c it requires the intake boots to run right (so I had always put them on). Though that's a good idea, I can pull the air-box boots off and investigate that this weekend.

I don't have access to an ultrasonic cleaner, lol. I wish I did, that would be ideal. Didn't know carb soak would mess up the slides :(.

I'm not 100% sure but I'm pretty sure (seeing as this problem happens in the same way on both bikes, same engine, very very very similar carbs) the problem is the same on both bikes...whatever it may be. I want to say it's an issue with my low end mixture adjustment, I feel that is the most likely cause. Most specifically because I have a hard time following the FSM.

The FSM says to put the idle to 1200 and turn the mixture screw until the highest RPM is reached, then adjust idle back to 1200 and repeat for the other carb. My problem is between the engine stumbling too lean and the engine stumbling too rich I do not get changes in the RPM, it seems to stay steady :(.

How I adjust those low end mixture seems to have a large impact on how potent this problem is, but I can't get it to anything near what I'd consider 'normal'. That's why I'm pretty much just looking for a shop at this point....I figure there's gotta be some older people around who know these things like the back of their hand.


The one for the CM450 was rebuilt by somebody who really knows these and he tested them on his bike w/o issues, so that further backs the point that it's my lack of ability to adjust the low-end mixture... I don't get it, this should be easy to get right...but the raise in RPM which is supposed to happen in the manual just doesn't... :(.
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Old 09-04-2013, 11:29 PM   #4
Kai Ju
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Before you attempt to adjust the carbs you need to make sure that the engine is in good health.
How is the compression ? is it equal side to side ? How about leakdown if compression is low ?
Also, are there any air leaks between the intake manifolds and the cylinder head or carbs ?

Once you've established that the engine is good to go set the air/fuel screws to their standard setting and warm the engine up fully. Again check for air leaks. If ok check carb synch, they should be no more than an inch apart. If not, adjust as necessary. Now do the idle drop where you adjust the low speed mixture. On a clean carb you should be able to starve the engine by reducing the amount of fuel going in and also drown the engine by going the opposite direction. Find the two points where it dies because it's too lean and too rich and split the difference. So let's say it starves at 2 turns out and drowns at 5 1/2 turns out. That's three and a half turns of adjustment. Cut that in half, 1 3/4 and add to the starve setting bringing you to 3 3/4 turns out.

pm me if you have any questions and good luck.

BTW, all of the above only works if the carbs are actually clean and you have no airleaks.
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Old 09-05-2013, 12:10 AM   #5
Syndacate OP
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Quote:
Originally Posted by Kai Ju View Post
Before you attempt to adjust the carbs you need to make sure that the engine is in good health.
How is the compression ? is it equal side to side ? How about leakdown if compression is low ?
Also, are there any air leaks between the intake manifolds and the cylinder head or carbs ?

Once you've established that the engine is good to go set the air/fuel screws to their standard setting and warm the engine up fully. Again check for air leaks. If ok check carb synch, they should be no more than an inch apart. If not, adjust as necessary. Now do the idle drop where you adjust the low speed mixture. On a clean carb you should be able to starve the engine by reducing the amount of fuel going in and also drown the engine by going the opposite direction. Find the two points where it dies because it's too lean and too rich and split the difference. So let's say it starves at 2 turns out and drowns at 5 1/2 turns out. That's three and a half turns of adjustment. Cut that in half, 1 3/4 and add to the starve setting bringing you to 3 3/4 turns out.

pm me if you have any questions and good luck.

BTW, all of the above only works if the carbs are actually clean and you have no airleaks.
Engine is in good health. Compression is good across the board on the CM450E. I don't have the comp tester anymore so I can't test the nighthawk. Even with this problem the CM450 runs fine at WOT and carried me off the end of the speedo without much fuss - and I'm a huge guy - almost 300lbs. That's 450cc's on a 30 year old bike. There's only like 10-15k on both of these.

My nighthawk does the same exact thing. The power is there, it runs fine at WOT. Engine is healthy. Besides, I doubt they hurt the engine the same exact way and somehow I hurt the engine in such a way that hurt it <35-40% throttle but didn't hurt it at >40% throttle.

Given that the air box boots and carb insulators are like 30 years old I'm sure there is cracks that lets air through. I have trouble finding them, though. At idle when I hose it down with starter fluid or carb spray or anything to make it cough I have a problem getting it to cough. I haven't tested these insulators on this bike per-se but I've tested the old ones on this bike and those insulators on the CM450. Same exact problem everywhere, can't get either to cough.

I haven't sync'ed them since I cleaned them, but a shop I trust did so before I did, it didn't change them. I don't have a sync tool :(. I think I'll have to buy a sync tool (was thinking of the edelbrock cover style pictured here) and try sync'ing them again. The FSM says to tune before you sync, but I can try sync'ing first.

I can get the engine to stumble/kill the cylinder too rich and too lean. I'm currently running it by exactly what you suggested (the idea hit me too, ha). By splitting the difference. On a cool day once completely warmed up this technique seems to yield pretty decent results, but definitely no ideal :-\.

I've covered most of these bases to some degree or another before, I read about them CONSTANTLY to try and find something I missed, invested countless hours into reading about them and carb theory in general. This year long vendetta has fueled my utter hatred for carbs (CV in particular). That's why I'm so flustered and willing to turn this heap of junk over to a shop...

Yeah, I haven't been having much luck with this -_-. Maybe I'll just go buy the sync tool and price out new boots + insulators. Throw money at it, something good should happen, ha.
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Old 09-05-2013, 11:59 AM   #6
TUCKERS
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What HE said.

The jets need cleaning. Soaking is no good.

I use 'numbered' drill bits to carefully clean them out. You can blow them, suck them, soak them, shake them....nothing works....they need to be carefully reamed of dirt and debris.
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Old 09-05-2013, 12:08 PM   #7
Syndacate OP
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Quote:
Originally Posted by TUCKERS View Post
What HE said.

The jets need cleaning. Soaking is no good.

I use 'numbered' drill bits to carefully clean them out. You can blow them, suck them, soak them, shake them....nothing works....they need to be carefully reamed of dirt and debris.
I'll keep that in mind. I didn't know the dirt stuck that bad.

On my CM450 though I straight up got new jets, which unfortunately didn't change anything :(.
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