Author Topic: CB750A Barn Build (Read 17050 times)

hotdog · « **Reply #75 on:** July 18, 2015, 04:47:59 PM »

Turbo, after ultrasonically cleaning my Honda Cbx750 34mm carbs (same as the u.s. n/hawk I think) I removed the choke slide which runs all the way L to R across the rack, and placed the main choke lifter (cable) to the very fay LH side so it could be lifted/operated by hand similar to the oem 750 carbs. Needs to be held in place if you want the choke to stay on for any time after starting - but got rid of the need of a cable setup......

Bill/BentON Racing · « **Reply #76 on:** July 18, 2015, 06:19:18 PM »

D, I saw that with ours, we did have a Dynajet kit, maybe drilled holes and cut springs. I can pull a spring and measure and shoot you a pic, hell can pull slide and check it too? Problem is ours is the Yamaha Seca carbs. They're 32's, they work well, well NOW they do! Lol. I've got a set of CBX 750's, 34's I'm trying next, Bill

Retro Rocket · « **Reply #77 on:** July 18, 2015, 06:41:39 PM »

Quote from: hotdog on July 18, 2015, 04:47:59 PM

Turbo, after ultrasonically cleaning my Honda Cbx750 34mm carbs (same as the u.s. n/hawk I think) I removed the choke slide which runs all the way L to R across the rack, and placed the main choke lifter (cable) to the very fay LH side so it could be lifted/operated by hand similar to the oem 750 carbs. Needs to be held in place if you want the choke to stay on for any time after starting - but got rid of the need of a cable setup......

Dale, I think the night hawk got completely different carbs to the Aussie CBX mate, the Aussie CBX had around 95 HP, the American CBX nighthawk only had around 65 HP, I'm pretty sure they had different carbs, cam and a few more changes...

Bill/BentON Racing · « **Reply #78 on:** July 18, 2015, 08:46:11 PM »

That's why I went with the Aussie CBX version, thanks Bear! K, Bill

scottly · « **Reply #79 on:** July 18, 2015, 10:03:26 PM »

Quote from: TurboD on July 18, 2015, 01:40:26 PM

Well I can never sit idle. I mounted my Gopro and here is what I got. During both tests I am working the throttle in effort to get the slides to open, finally when rpm increases they raise and the engine winds out. I'm not sure if I want to drill the slide holes larger or cut the springs, both would require replacing if not the problem.

https://youtu.be/oO5Rp1balmQ

When I finally looked down the throat of my FT carb, I noticed the slide was fully open at 3000-3500, so your problem is the opposite. When you shorten a spring by cutting coils, it reduces the preload, but increases the stiffness, so be aware of that. As far as drilling holes, I never got that deep into the system to figure out how to regulate the slide opening, but I would try restricting the air inlet at the top of the mouth of the carb and watch how the slide responds. Duct tape, your finger, whatever..

TurboD · « **Reply #80 on:** July 19, 2015, 06:27:50 AM »

In theory that is true. But any time a spring has a lot of compressed preload (meaning installed length is lot shorter than free open length), cutting coils will always lighten or soften the force it applies. So yes anytime you cut coils from the slide spring, the slides are easier to open.

I am zeroing in on a tune up now, I drilled the slides some and added main jet. My "seat of the pants" feel kept telling me it was lean. It is somewhat close now, I think at this point I may lower the needles slightly to lean the low mid range and part throttle.

POPS 911 · « **Reply #81 on:** July 20, 2015, 01:32:52 PM »

TURBO : Get on Thunder Dial A Jet web site = you need #DJ102 for those Seca Yamaha carbs at $239.95. The XJ750R carbs fit the Hondamatic small mounting snouts 33mm OS with new boot from a CB750F 1976 and with a small thin blade screw driver you can dial in all the fuel you need in about 4 minutes, as these carbs had the CALIF. CARB regulation that made all those years under jetted. Long ago a top tuner wrench spinner told me to shim the needles [ the ones with out grooved in needles ] but do not change the needles !!!!!!!!

POPS BK911

TurboD · « **Reply #82 on:** July 20, 2015, 01:58:43 PM »

Thanks POPS I have the Keihin's pretty close now, I need to slightly lean the midrange, either remove the one shim under the needles or reduce the main jet one size.

Bill/BentON Racing · « **Reply #83 on:** July 20, 2015, 08:10:06 PM »

D, I got a spare / backup set of Yamaha Seca 32mm's if you need them , Bill

POPS 911 · « **Reply #84 on:** July 21, 2015, 03:51:23 AM »

TURBO: Those 650 Honda Hawk carbs must be smaller then those SECA 750 carbs you have

?? Try this put SECA CARBS back on and wire in a two step off that rear brake stop light switch [ so when your foot is on rear brake pedal and your rolling the throttle wide open you can two step the motor not to pull you thru the lights ] you must be used to two stepping and not CREEPING ...... All the BUSA at NTR NOW A DAYZ HAVE TWO STEPS and nobody complains about no electronic rules in street ET as all have electric over air button or auto shift [ MPS a young man from Columbus Ohio, now Florida has all the tools to make your BUSA a fast ST/ET BIKE ] Dyna two step and those slides will be up !!!!!!!

POPS BK911
My TURTLE has a built in stall at 3500 rpms so I just pull in and lock the LEFT hand OEM parking rear brake lever and turn throttle all the way open, on that last yellow I throw out the rear brake lever like it has a clutch = .10 - .02 lights.

TurboD · « **Reply #85 on:** July 21, 2015, 07:18:38 AM »

Bill thanks for the offer, I have a couple sets of Seca carbs. For now I am working with the Keihin's, they seem to respond better to changes for me and I have plenty of jets for them.

POPS The 650 Nighthawk and my 750 Seca carbs are both 32mm. The converter is the next item I need to address, the low stall speed (around 2200) is KILLING the ET. My rear brake is operated from my left handlebar allowing me to hold the brake while giving some gas.

Bill/BentON Racing · « **Reply #86 on:** July 21, 2015, 12:01:41 PM »

Mr. Brittain in Texas has the answers. Did ours for 3500 rpm, it works, but brakes don't. ...yet! Have contact info, Bill

Jim F · « **Reply #87 on:** July 21, 2015, 02:59:08 PM »

This is what we found in our carbs after
A very lean number one cylinder
Had to drill, machine out the hole
Get the broken drill out, weld and re drill
Worked good after that

dragracer · « **Reply #88 on:** July 21, 2015, 03:41:31 PM »

Quote from: Jim F on July 21, 2015, 02:59:08 PM

This is what we found in our carbs after
A very lean number one cylinder
Had to drill, machine out the hole
Get the broken drill out, weld and re drill
Worked good after that

Jim F,

As I recall, You and Bill spent more time correcting the previous owner's screw up than you spent on actually tuning those fully rebuilt and repaired carbs in the end. When it was all said and done, you guys had changed a lot of things to get them working good and the bike making clean passes.

Bill/BentON Racing · « **Reply #89 on:** July 21, 2015, 07:28:23 PM »

YOU WOULD BE CORRECT! !!!

Jim is still to be commended! Bill

Jim F · « **Reply #90 on:** July 22, 2015, 10:49:47 AM »

a lot of this and a lot of that but it worked out good

worth the effort

J

POPS 911 · « **Reply #91 on:** July 22, 2015, 10:55:12 AM »

TURBO: Use that left hand rear brake light switch for that DYNA TWO STEP = when left hand rear brake is pulled in YOUR TWO STEPPING throw out the brake and go to the high side while screwing out the throttle [ 1/4 turn ] all the way open.

TurboD · « **Reply #92 on:** July 23, 2015, 05:44:37 AM »

I manage to edge the ET down a little more last night, I ran a 13.56. The Keihin's seem to work pretty good, and I still have some fine tuning to do. Now I think the fuel system is not keeping up as it is running out of gas through the traps, I knew this would be a problem sooner or later as the petcock is really small and limiting. I am looking into options to fix this.

POPS 911 · « **Reply #93 on:** July 23, 2015, 01:25:12 PM »

I find it strange that a 1981 CM400A HONDAMATIC has a 3500rpm stall converter in this motor as stock, and the 1978 CB750A HONDAMATIC only has a 2000rpm stall converter in the stock motor. Thanks TURBO for all your learning information as I have been waiting to see any 10 anything HONDAMATIC.

POPS BK911 NHRA

Bill/BentON Racing · « **Reply #94 on:** July 24, 2015, 01:48:33 PM »

2600-2900 stock stall speed on 750A. Up to @ 2200 rpms we could hold with left hand brake, more not possible without line lock etc.Seen 3500 on ours a time or two! K, Bill

hotdog · « **Reply #95 on:** July 24, 2015, 04:16:36 PM »

Have read some guys just run an inline fuel tap from a small motors shop (Briggs/honda etc) gravity feeds more than the stock. Good to see keihins go good.....

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POPS 911 · « **Reply #96 on:** July 24, 2015, 05:03:02 PM »

Why not a rear hydraulic brake hooked with the front hydraulic brake with a double banjo bolt

?? FRONT and rear brake on and a switched off the left hand brake lever two step set at 2800 rpms. Pull in brake on second top yellow bulb and screw the throttle [1/4 turn ] all the way open. All the grudge bikes run two steps in ST/ET listen to the sounds of RPM,s ups and downs................. select firing.

jweeks · « **Reply #97 on:** July 30, 2015, 07:08:45 PM »

Sam's carbs are from a GS750 that were reracked. I used a Dyna-jet stage 3 kit for the richer needles and air idle circuit restriction to prevent an over rich bottom end. The carbs had a broken drill bit in the air idle circuit for one of the carbs, unknown to me. I first offered to refund Sam's money when I heard of it. Jim F. did a fine job of getting it out of that passage and welding out the passage. The next mistake was cleaning the carbs which removed the idle circuit restriction "buttons". I believe that eventually the idle passage was drilled and tapped to allow a jet restriction for the idle circuit. A lot of forum advice on the tuning of the carbs didn't help. Some time on a chassis dyno gave direction for the main jet and clip placement on the needles. They are now a non issue on the bike. 32 mm CV carbs will run 11's all day long. That's enough for most Hondamatic drag racers. Yes, the 34 mm CV carbs with the cheap intake manifolds will fit onto a K head. Anyone who saw my bike at The Cup event in Virginia saw 34 mm Keihin's on a ported K head.
Neal Chance transmissions did my torque converter. The first effort was some internal machining to allow for increased clearances. I copied that for Sam's bike. The lack of power at the rpms the brake would hold kept the launches just under 3k. I've stated it repeatedly. A hand lever controlling the rear brake is wrong for drag racing a Hondamatic. I've tried it using a longer lever than was tried on Sam's bike. You can't create enough line pressure with your hand to hold the bike above 3 grand. A foot mounted master cylinder with a Biondo line lock will produce enough line pressure and hold it for higher rpm launches. On my 1080 cc motor the converter machining got me around 4,200 rpm. I wanted higher rpm launches than that so I went back to Neal Chance transmissions for what else you can do to this? They milled out a row of 16 cast blades in one side of the converter and welded in small aluminum sheet rectangles at a different angle at a substantial cost to me. This got me to 6,000 rpm launches with marginal brake hold. Power brake it too long and the rear wheel will start to creep.

jweeks · « **Reply #98 on:** July 30, 2015, 07:16:22 PM »

I hit the wrong button again.

If you want to have an ATM Hondamatic (which was Sam's objective at the start of his build) you have to use a system that will give you enough pressure to hold your bike on the starting line at wide open throttle for 2 seconds. The load of the motor will act as a rev limiter. You won't ever stall the converter/motor. Eventually I'll post some sequential time slips to show everyone just what i'm talking about. A hand held rear brake lever is a variable for your ET. Your launch rpms will vary and so will your ET's. You can be good enough to win some rounds, but not enough to have confidence in your dial-ins. Confidence in what the bike will run will always help your reaction times. I've done this over 25 years. I'm trying to share my knowledge.

kmb69 · « **Reply #99 on:** July 30, 2015, 08:02:40 PM »

Quote from: jweeks on July 30, 2015, 07:16:22 PM

.....I'm trying to share my knowledge.

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Author Topic: CB750A Barn Build (Read 17050 times)