Author Topic: CB750A Barn Build (Read 17053 times)

POPS 911 · « **Reply #50 on:** July 07, 2015, 05:32:51 AM »

TURBO : Get on the net and look up THUNDER PRODUCTS they bought the rights to DIAL-A-JET and have the kits to sell = story on how they work are on that site. One of the reasons I like the HONDA KEIHIN CARBS in that the late models 500-750-900cc CV carbs have a accelerator pump that squirts more fuel when cranked open [ the stock CB750A carbs have a enrichment system also ] and this bike will not run right with that system off those Matic CARBS ] I know that as I tinkered with taking that system off a stock set = poor results. And I thought I had a room full of carbs !!!!!! I do have a extra set of stock 750A matic carbs with all the hoses and valves that are like brand NEW....... back ups for my 14,000miles 1976 CB750A one owner that I own for years.
10,500 rpms shifting with a tach or shift light

? CM400A Hondamatic's tops out at 7000rpms and the ignition with a built in rev limiter shuts the engine down = coasting and rpms come back down the bike refires , that why the gear ratio is important on the 1320' to race the TURTLE.

TurboD · « **Reply #51 on:** July 07, 2015, 05:41:02 AM »

I understand that, keep in mind these are CV carbs. At 5,000 in the rpm range the slides are not open 100%, my guess 50-60% open. These bikes are slow reving as result of being geared to use only 2 speeds for the entire 1/4 mile, low gear is a long one.

My thinking is that it is lean as well, and it responds good to more jet everywhere except the mid range. Every time I add more jet the mid range issue gets worse, I jetted the first set of carbs up to the point where it will now barely even make it through this rpm range, it misses or stumbles so bad.

I have run this with and without pods both and it does not change either way.

Again I can lean it down and it will go through this range cleanly, but the bike is noticeably slower and flatter responding.

Bill/BentON Racing · « **Reply #52 on:** July 07, 2015, 06:46:07 AM »

We cut a couple coils off the stock springs. Check out how quick they do or don't move now, on bench in neutral or loaded, but saw ours didn't respond as quickly or go up up as we thought, me and Jim F. It worked, midrange burble gone, 60 ft, 1.9's, G'luck, Bill

TurboD · « **Reply #53 on:** July 07, 2015, 10:35:15 AM »

Thanks Bill. I was wondering that if I changed or altered the timing of the slide, if it would help during this area of the rpm. If (when) I can get it down into the 1.9 60 foot range, if should really run decent as it pulls hard on top.

Pops Thanks I will look them up. Yes these carbs do not have pumps in them, however my problem is not from the hit, they will pull good from 2,000 wide open. Its just that crazy 5-7k range that's killing me. Lol

Bill/BentON Racing · « **Reply #54 on:** July 07, 2015, 11:57:46 AM »

D, we also got an old Dyna jet kit and used what we could, Bill
Jim F. might remember more.

scottly · « **Reply #55 on:** July 07, 2015, 07:31:45 PM »

Quote from: TurboD on July 07, 2015, 05:41:02 AM

I understand that, keep in mind these are CV carbs. At 5,000 in the rpm range the slides are not open 100%, my guess 50-60% open.

Better to actually check it, rather than guess. My guess is that the slides are fully open, based on my recent experience dyno tuning a CV carb. Look into the carbs and watch the slides while revving the motor.

TurboD · « **Reply #56 on:** July 07, 2015, 09:48:21 PM »

Scottly The slides in CV carbs open up only as much as demand calls for. I know that these will and do open fully. When I said that I guess that they are 50% -60% open, I mean at that given rpm and load when the problem occurs which is exactly what I would expect. My thought is that for that brief time during this given load and rpm, the carbs are going rich, with most any other carbs simply lowering the needles with the C-clips 1 or 2 notches would lean and clean up during this rpm/load. But with these carbs I do not have that ability.

scottly · « **Reply #57 on:** July 07, 2015, 10:10:29 PM »

You're guessing. Check the slides at 5000 RPM in neutral; the CV carbs respond to air flow more than load. If the issue where due to a rich condition bad enough to cause missing, the plug chop would have shown sooty plugs. What I believe is happening is the slides are fully open before the air velocity is high enough to pull fuel up through the main jet, until 7000. Your carbs had some sort of air-box when stock, and without it a CV carb will react completely differently.

scottly · « **Reply #58 on:** July 07, 2015, 10:23:34 PM »

Read here about my CV tuning quest

:
http://ascot500.com/index.php?PHPSESSID=afad9jlocojf21i28vrko55md6&topic=269.0

TurboD · « **Reply #59 on:** July 08, 2015, 05:17:01 AM »

Scottly When I say that I am guessing, that is simply a matter of speech. In all actuality whether the opening of the slide during this rpm be 40% or 75%, that is completely unimportant as long as the engine makes good power. I was simply throwing out a number to help explain roughly where in the slide opening the problem is occurring.

I fully understand what you are saying. My first thought to tell someone with this problem would be, it is lean. The plugs have never looked dark or sooty nor has the engine ever smelled to be rich at any point, the problem is that when I increase the jetting the problem actually get worse, a lot worse, it had it richened to the point where the engine did not even want to rev and make it through this rpm range. The only time that It would run "cleanly" through this range at full throttle, I had jets smaller than stock, The engine felt completely flat and made no power. After quick test ride, my brand new Chrome pipe started to turn color at the tops. The engine clearly wants the larger jetting everywhere except 5-7K.

POPS 911 · « **Reply #60 on:** July 08, 2015, 05:33:18 AM »

Stock YAMAHA SECA XJ750R SLIDES SPRINGS = 89mm long Stock HONDA CB900F SS BOL D'OR SLIDE SPRINGS = 53mm long very thin and soft. Could a 36mm shorter spring make the carb react better

? Two different makes of carbs...... The Keihin CV'S I have got down the tune as the TURTLE runs fine thru LOW-DRIVE even with the DA changing from 9AM TO 11PM at the track. FACT: If the TURTLE 400CC + 2CYL- 3valve. runs fine with32mm or 34mm KEIHINS then why not a 750cc+ 4CYL. 2 valve be the same as TWO 400CC side by side for 800cc. the only difference is 400cc three valve to a 750cc two valve engine.

TurboD · « **Reply #61 on:** July 08, 2015, 05:40:02 AM »

Pops Yes A shorter spring could very well possibly help, I would think that a shorter or different rate spring would allow the slide to be higher or lower during this rpm range, changing the jetting as well. I am going to look into this, it is raining here today, and most likely will rain out the track tonight.

TurboD · « **Reply #62 on:** July 08, 2015, 11:50:55 AM »

It is wet and the track has cancelled, so no testing today.

Bill I cut 2 coils from the springs, the change in the static "feel" of the slides is very minimal but it most likely will not take much. Another interesting thing that I noticed while looking at and comparing these carbs to others, is that the butterflies open backwards from my other Honda and Kawasaki carbs. I would think this has somewhat of a different effect on the air flow passing across the lift holes in the slide.

POPS 911 · « **Reply #63 on:** July 10, 2015, 07:47:54 AM »

TUEBO : Did you check the float levels with carbs on the bike sitting straight up on the bench or swing arm lift

Seen a few riders that wonder why their bike was under powered running lean - my QUICK CHECK on the bike showed them 2 out 4 carbs were low fuel in the bowls..... Fast by Gas carbs have the clear bowls and you can see the level in the bowls. Years ago a company made clear bowl for the Mukuni's 29's and I have not seen any of those in years. Another lean =surging from a slow flow 35 year old pet cock and a pinched yellow fuel line = fix PINGEL......
Quick check for the HONDA white plastic FLOATS put all 4 in a bowl off distilled water and watch the floating as all should be the same. As these are not adjustable they have to all be the same - you cannot get these new at the HONDA order desk in MARYSVILLE, OH as they no longer make these for stock.

TurboD · « **Reply #64 on:** July 16, 2015, 05:15:10 AM »

It finally stopped raining to make it to track again. The bike did go a little quicker last night, it went a 13.89. The 60's and 1/8 mile times on average were quicker, but it just was not going the number one would expect to see in the 1/4. I have still not gotten on top of carbs just yet, I even installed one of my Fast O2 sensors on the bike to help with tuning. I am now starting to think that the fuel delivery system is compounding to tuning problems.

dragracer · « **Reply #65 on:** July 16, 2015, 07:23:13 AM »

Did you do anything with the converter on the bike? Ie; increase the stall??

TurboD · « **Reply #66 on:** July 16, 2015, 09:34:55 AM »

No Frank I have not, and it is killing the ET for sure. Its not helping with carb tuning either, having to go from basically a fast idle to 10,500 at WOT very slowly is asking a lot.

TurboD · « **Reply #67 on:** July 17, 2015, 02:14:58 PM »

I have decided to try some Keihin's after not having much luck with the Hitachi's. I have them on and a rough baseline guess as to jetting, the engine already seems to have a more stable idle. It looks like I am done this evening as it is raining again.

Bill/BentON Racing · « **Reply #68 on:** July 17, 2015, 07:27:20 PM »

D, keep at it, it took a long time to get it right, Bill

dragracer · « **Reply #69 on:** July 17, 2015, 09:22:15 PM »

Billy, what was the name of the company in Houston,Texas that did the stall increase on the last converter for Samauto??"

POPS 911 · « **Reply #70 on:** July 18, 2015, 06:54:53 AM »

TURBO: I had a carb compare day with carbs from KZ & J=MODEL CV'S 1977-1982 XJ750R YAMAHA -CV'S 1982 CB750 NIGHTHAWK 1983 44mm CV'S
STOCK CB750A 1976-78 The stock 750 Hondamatic carbs measure 10.5 INCHS from outside of number one to outside of number four and are 26.6mm inside the plug.
The only carbs close are the XJ750R SECA 1982 at 10.5 inches from outside number one to outside number four . NIGHTHAWK CB750 DOHC carbs were 12" and all the KZ & J MODEL slides or CV'S were 11 or better inch wide. If you can get those 750 DOHC CV carbs in those NEW SOFTER boots [ 1976 CB750 F-K-C ] if you can get those 750 DOHC CV carbs to fit in that stock Hondamatic head = you are the man. I only wish I could do that match up so it would be the same as my CM400cc Hondamatic race bike = which runs fine with the right size CV CARB'S which was a easy fix with 450A boots and the three size KEIHIN carbs by HONDA that fit onto that head = just push them in , jet, shim and pod w/ 2-1 open header.

POPS BK911

TurboD · « **Reply #71 on:** July 18, 2015, 07:11:48 AM »

Thanks POPS. The Nighthawk 650 32mm and 750 34mm are not that far off, with the new boots the 650 carbs (which is what I am using) go on pretty easy.

I rode the bike some this morning. I am somewhat close with both the idle and top end jetting, but the low and midrange are going dead rich with any sudden throttle opening and stumbling. I'm a little baffled as to if the slides are opening too slow or jumping open too quickly causing this, I can convince myself either way. I cannot look into the carbs going wide open down the road, so I'm trying to get it on the dyno Monday.

Here's a picture of the CB650SC Nighthawk carbs on the bike.

dragracer · « **Reply #72 on:** July 18, 2015, 07:29:53 AM »

Its too bad you can't mount a go pro camera in a position to record exactly what's going on as you ride the bike. Maybe you can borrow one from a member of your race team

TurboD · « **Reply #73 on:** July 18, 2015, 10:02:25 AM »

HaHa Frank, I have one and that thought already occurred to me.

TurboD · « **Reply #74 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

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Author Topic: CB750A Barn Build (Read 17053 times)