1991 cb750 nighthawk brat

I’m digging the engine start button and that sound....... eager for morning to come to play in the garage again.


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Some highlights as assembly is going on..

udnerseat electrics tray, diy welded aluminum
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custom harness and connectors..
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stop/running light/blinkers location, weld-in nut
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front end status so far..
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digging the motogadget's dial!!!
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breathers
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'airbox'. well sort of.. btw very tight frame on cb750s from nineties, even pods dont fit well.. soon will see how this will work..
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custom downpipe bought by chance from friend of mine, just welded in sensor port..

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exhaust looks version 1. totally wasnt sure on exhaust setup.. since its was a readymade downpipe, didnt mess too much and just fixed the muffler. so far so good.. otherwise would route closer and more parallel to the frame botttom tubing...

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Ok so mechanically bike is ready and its time to tune the carburetor.

I did full disassembly/cleaning/rebuild kit quite some time ago.. Back at the time decided to put 38 (vs 35 stock) slow jet and 120 (vs 110 stock) main jet assuming there will be no stock airbox and straight pipe..

Having innovate mtx gauge with wideband sensor it was fairly easy to set up very solid idle and easy start, but then things got more tricky. As I add throttle slowly, at some point bike just quickly revvs up, from 2k rpm to 4rpm on its own, with throttle stick on same position (slightly below 1/4 of throttle stick). If I then keep rolling throttle, rpm don't go above 5-6rpm anyway and starting from 1/2 stick its starts to spill fire from exhaust and thick smoke. On throttle fully open, just dies and stops. Clearly, mixture is way too rich.. Went back to stock size jets but didnt have time to try today.

Very concerned about this revving on its own, while on low throttle, same position.. I am sure about no air leaks around carb - all manifolds and etc new and double checked... Float levels and carb sync is nearly ideal..

Noticed that cylinder compression is somewhat lower, around 120psi, but no difference across cylinders.. Its new piston rings, hope they just didnt sit yet..

Will see.. Some vids below..

https://youtu.be/SPqA29jVu1s

https://youtu.be/r0GVzUHN7tM

https://youtu.be/b7FglMFatfs
 
revving it in neutral is pretty meaningless. how it rides is the question. 110 to 120 main is pretty minor, certainly not going to make it puff black smoke. pull the air filter off and see if that changes how it rides. puffing black smoke sounds like there's something wrong carb wise or an air passage from the filter side blocked.

what does the ignition advance curve look like? if it has a sudden jump in advance (lots of the simple electronic ones do) then it will suddenly pick up rpm when it hits a certain rpm point. put a timing light on it.
 
on 91' cb750 ignition timing is fully electronic and non adjustable as per manual.. but sure will get the strobe and check the timing, thanks for the hint
 
Ok got another craze. Does the tank sit pretty??

Front part positioned higher than I expected and overall a bit uptwisted look.. Doesnt seem to follow seat line well.. Or I just need to take a timeout..)) But maybe lift rear part of the tank a bit..

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Last couple month was real fun. After first engine top end rebuild, compression was no more than 110psi and while engine was running good on idle, I also observed strange behavior like self revving from 2k to 4k and spitting/exhaust fire after 5k.

Given low compression I decided that I probably messed up with rebuild and went for disassembly again and this time gave cyl head and block to professional company who did the valve job, honing etc.

Only to find that compression is still on same level after second reassebly. This time I was hundred percent sure that all components and timing was right and I decided to try another compression meter.. and guess what, bloody compression meter was displaying wrong (lower) numbers!!)

So second top end overhaul only to find out that first time I did all right and that new and pretty expensive compressometer wasnt working))

Anyway its 150psi across all cylinders and engine behavior has something to do with carbs or igniton!
 
So many details on your bike I really like, but that tank is a standout. Absolutely gorgeous, love it! You could try shimming up the rear tank mount - that's what I did on my CB750 to give it a more horizontal line. Otherwise I guess you'd have to relocate and drop the two front tank mounts and that would be way more work. It would lower the front of the tank, but raising the back would be far easier.

Did I say I love that tank? So cool.
 
Yesterday my little sweet frankenstein saw public road for the first time. Temporary wooden seat is hardcore looking but surprisingly pretty ridable!!)

Tank is not in a final position.. Exhaust turned out to be way too loud, gonna put baffles or replace whole thing..

Idle and up to 3-4k runs pretty good on all gears, ie smooth acceleration and cruising moderate speed around 40mph.. In case of fast throttle advance and/or 5k+ rev becomes very rough, spitting back to carb, great difficulties with rev increase and acceleration.. AFR sensor shows rather lean during this.. Probably needle has to be shimmed up, will see, weather went real bad for next week..

Otherwise its a total joy to finally have a ride!)

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There is a 1991 bike like those for sale around me. This gives me some thoughts to how it could look.
 
Time flies very fast, a year passed and I hardly noticed! Had to rework/reweld forward tank mounts (on finished and painted frame%)) to bring tank closer to frame and more level.

Went through several iterations of seat and muffler too.. But seems to be pretty fine and close to a first really rideable setup. Got few more ideas but perhaps next winter.

Its still very cold so just did couple rounds not far from garage. Have to set up carbs now. Didn't really spent time on this yet besides some initial idle setting and intial sync.

Idles pretty stable though, but has some serious troubles accelerating. Spitting back to carbs and misfires even on mild acceleration.

I don't even recall which jets I used when assembled the carbs. That was just a guess given changes to intake/exhaust. Appretiate if you can give some advice:

-I got the wideband sensor and AFR shows 11-12 on idle which I guess is a bit rich.
-On acceleration from idle to 1/4 shows 10-11 which perhaps a sign for smaller pilot
-1/4-1/2 accels much smoother with AFR closer to 13
-didnt really open more as I'm still running brand new piston rings and cylinder hone..

It's just 10C now (I guess its 50F) and I intend to ride with at least 68F, not sure if I should even continue tuning or better to wait for warmer weather..

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https://youtu.be/QwfqzL5fXY0
 
I know its somehow not too much followers and commenters of this build but the bike came out real nice.

Figured out pretty good carb setup for my intake and exhaust configuration. Still some small adjustments to go before its fully done!

https://youtu.be/74DWekdXBwQ

https://youtu.be/AOVWyJbHvdY

https://youtu.be/H7-3bvcA5fI
 
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