92 nighthawk mods?

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Bamakracker

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I picked up a 92 nighthawk 750 a few months ago for about $500. Didn’t run but after getting it home and cleaning the carbs it started right up. I recently decided to change the look of the bike because honestly the 90’s era of CB bikes ain’t the prettiest. I’ll include some pics, but my problem now is that I cut the air box in half and closed the back off with sheet metal so I could make more room below the seat. Now it’s not running right.this is my first CB and only second inline 4 bike.( the other was a 79 KZ1000) so not sure why it’s acting the way it is but seems like it’s running lean. I’m thinking about jetting carbs with stage 3 jet kit so I can run a ram air setup similar to one a friend’s got on his bike and a new exhaust system. Let me know what you think of this bike so far. All opinions, suggestions, and criticisms are welcome. Thanks.
 

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You just halved the air space the carbs use to feed the combustion chamber. I would expect that to have a pretty big impact on how the bike runs.
You need to put fresh plugs in the bike and let it idle for a few minutes. Then check the plugs to see what they tell you.
 
brettp said:
You just halved the air space the carbs use to feed the combustion chamber. I would expect that to have a pretty big impact on how the bike runs.
You need to put fresh plugs in the bike and let it idle for a few minutes. Then check the plugs to see what they tell you.
Click to expand...
Changed the plugs yesterday. I figured cutting down the air box would result in less air so I put a scoop on left side so when riding it would funnel air in but that didn’t work out. The plugs I took out were black and the bike was running rich. Always smelled of gas. Looking at it now I just made it worse. So I need to get more air?
 
Checked again today after adjusting idle screw and pilot screw? Not sure it was called different things depending on where I was reading about it. Said you shouldn’t turn them unless necessary…. So I turned them. Clockwise to reduce fuel in fuel / air mixture. After riding for about 20 minutes and never able to get over 53 mph pulled the plugs. The one on far right had slight white coloring next plug looks new the2 on left side both still black. Now I have a new problem though. When I returned the screw on bottom of carbs to original position I broke one. Think I’m just gonna quit now before I really mess something up. Best idea I think is gonna be take carbs off and take them to be rebuilt by someone who knows these bikes.(Hopefully that screw can be replaced.) I think I’m gonna have them rejetted at same time so I’m able to run different airbox and 4-1 exhaust. Did manage to get it running halfway ok through 3 gears. Fourth and fifth though just had no power.
 
Sounds like you either don't have spark on the black plugs, or the jetting is so rich the bike bogs. Oh, and that screw can be replaced.
Speaking of rich, does the bike act like it bogs down at 53mph or it is trying to accelerate but something is stopping it?
 
Acts almost like it’s got a governor or something. Last week it would run 110(fastest I’ve gotten it up to) and still be accelerating. Now 5th gear throttle twisted as far as it’ll go 53 is top speed. It’s actually running around 65 now. The air scoop I put on was to close to carbs. I think the air coming in was coming straight across in front of carbs stopping them from pulling in air. I moved it farther back and gained 12 mph. After work today I’m going to block off the opening and move scoop to top facing rearward like the stock inlet was and try it. The old plugs i took out were all black so I figure it was already running rich. Could always smell gas. I’m thinking that it needed adjusting at least maybe carbs rebuilt and me cutting the air box made it even worse. I was planning on checking if the plugs are firing this evening already because both plugs that are black are on same side I’m gonna check the coil it may be bad. Are there any new carbs that work good on these bikes? I’ve been looking at manifolds to run 2 carbs instead of 4.
 
You did a number on that air box. The way you describe it at high speed tells me it is lean at the top end. Your plugs say rich. It might be possible with your airbox edits, you are puling in too much air and fuel at idle but not enough at speed.
How does it rev through the range in lower gears (1st, 2nd, 3rd)?
 
It pulls hard in lower gears( not as hard as it should ) but what actually made me want to check plugs and coil is the sound. Sounds more like the v twins I’m used to riding than a 4 cylinder bike. I haven’t been riding it long but definitely know the sound is off.
 
Ok so all 4 plugs are firing although #2 looks a little weak. Put air box back together and reinstalled it. Put pilot screws back to factory position. Just replaced the petcock on gas tank bc I had replaced the vacuum valve with old gravity one due to diaphragm issues. So now it’s no longer limited to 50 or 60…… but it’s pulsating. When idling and at low speed it’s almost like one cylinder is reving higher. If I accelerate hard it sounds different not sure how to explain.
 
Ok… I did find a rip in the rubber between carb and engine. What would cause the carbs to be out of sync and how do I sync them? Also where can I find rebuild kits for engine? I’ve been looking but it seems everyone only has top end kits for sohc and early dohc cb750s. 90s bikes have nothing.
 
Lots of YouTube vids on syncing carbs. Would take me a long time to write it up. But seeing it in action would be tremendously helpful for you.
Definitely need to replace that boot
 
And I had to learn the hard way. Now if I can get the pulsing straightened out it should be ok. Can the exhaust be changed? Either with 2 shorter exhaust or 4 to 1? Really need those stock pipes out the way
 
Changed the intake manifold that was tore. Don’t seem to change anything. I decided while I had the carbs off to go ahead and clean them again. Bad decision… now I can’t get it to crank. Some how I managed to stop it from getting fuel I assume because still has spark.
 

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