Author Topic: 1991 cb750 nighthawk brat  (Read 5291 times)

Offline Coopacoopacoopa

  • Posts: 72
Re: 1991 cb750 nighthawk brat
« Reply #20 on: Nov 29, 2017, 11:58:20 »
Liking those rear sets. Great build!

Offline djmaynard

  • Posts: 207
Re: 1991 cb750 nighthawk brat
« Reply #21 on: Dec 02, 2017, 23:42:41 »
Iím digging the engine start button and that sound....... eager for morning to come to play in the garage again.

Sent from my iPhone using DO THE TON

Offline harley497

  • Posts: 18
Re: 1991 cb750 nighthawk brat
« Reply #22 on: Dec 08, 2017, 16:08:58 »
Some highlights as assembly is going on..

udnerseat electrics tray, diy welded aluminum

custom harness and connectors..

stop/running light/blinkers location, weld-in nut

front end status so far..

digging the motogadget's dial!!!


'airbox'. well sort of.. btw very tight frame on cb750s from nineties, even pods dont fit well.. soon will see how this will work..

Offline harley497

  • Posts: 18
Re: 1991 cb750 nighthawk brat
« Reply #23 on: Dec 09, 2017, 16:28:38 »
custom downpipe bought by chance from friend of mine, just welded in sensor port..

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...

Offline harley497

  • Posts: 18
Re: 1991 cb750 nighthawk brat
« Reply #24 on: Jan 20, 2018, 17:46:15 »
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..

« Last Edit: Jan 20, 2018, 17:58:14 by harley497 »

Offline brad black

  • Posts: 149
Re: 1991 cb750 nighthawk brat
« Reply #25 on: Jan 21, 2018, 05:16:20 »
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.

Offline harley497

  • Posts: 18
Re: 1991 cb750 nighthawk brat
« Reply #26 on: Jan 21, 2018, 16:04:28 »
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

Offline harley497

  • Posts: 18
Re: 1991 cb750 nighthawk brat
« Reply #27 on: Jan 22, 2018, 15:57:39 »
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..

« Last Edit: Jan 22, 2018, 16:02:58 by harley497 »

Offline harley497

  • Posts: 18
Re: 1991 cb750 nighthawk brat
« Reply #28 on: Mar 17, 2018, 07:56:27 »
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!

Offline The Jimbonaut

  • *
  • Posts: 1273
    • Lost Apostle
Re: 1991 cb750 nighthawk brat
« Reply #29 on: Mar 17, 2018, 12:10:01 »
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.
"I'm telling you Donnie, nuthin' but nuthin' but right"