Need Help 1981 Cb750 carb issues

[DangerZoneNYC]

Hello There everyone Im new to the forum I'm unsure if I'm posting this in the right section or not a head of time so if i need to make a correction please let me know and i will, otherwise here is my issue Me and my dad are building his 81 cb750 it ran great after cleaning the carbs and basic work, my dad wanted a brat styled cb so with his vision he wanted cut pipes and pod filters so with our research we found that when doing these two specific mods for the bike to run best after installed we would need to upgrade the jets so we upgraded to stage 3 jets followed everything in the DynaJet Kit, I believe were running 3-1/2 turns out air fuel mixture which at the time seemed to be a lot but we figured follow the book it came with they designed it. But now it will only crank with starter fluid when cold once bike is on it stays on and warmed up it will power on with no starter fluid but now we can't even get it to turn fully over if anyone could shed some insight on what tune setting we should be running with this set up and please any advice will be a great help! The booklet the jet kit comes with says aftermarket air filter and exhaust but i don't know if cut original pipes would be considered the same .

Thanks ahead of time

Brandon

 

[amc49]

(shaking head sadly)

Dyno Jet and their utterly crap kits......................you have envisioned a set up that absolutely runs like crap on that motor. Cutting OEM pipes is a good way to lose 10 horsepower. The pods another 10 so now making as much as a Honda 550 maybe.

If you read ANYTHING on the stage 3 setup it is ONLY for competition and with a header and non-stock engine. The kits don't work well at all on the early DOHC and commonly better without them at all. The stage 3 is for over 5000 rpm only running, ergo, RACE ONLY. Even then it is crap. The needle cut sucks and doesn't work on anything but maybe an 1100.

Can't help with engine not even turning over, could be running rich enough to have already damaged the motor. Or you have killed the battery, carb setup won't touch that.

There is NO tune setting for that setup as it is for a junk motor, of course you see that now eh?

Go back to 2-2 1/2 turns on the idle screws and stock size jets, THEY may be too big as you have killed the flow of the motor by removing pipe and the pods do the same. 75-115 on the jets with a header and if good plugs and ignition it will run better but the pipes the way you got it killed it. You need a close to full exhaust or a cheap 4 into 1 header and a stock airbox. If totally stock then 68-102 jets. Anything bigger WILL NOT WORK!!!! Use Honda jets too, the Dyno Jet parts are not the same size jet even with same number on it, they are bigger.

If you have never adjusted the valves, well, that's an issue all by itself. Nobody ever does and you also see how that works out. You are the typical user, the valves don't seat to leak, bike then runs like crap, then you work on carbs more until it won't even start. How most of them die.

Junk the Dyno Jet parts, they are garbage. I once knew someone who worked directly with the owner of the company and after MANY different custom tune setups the guy admitted that their kit did not work with the DOHC for spit. Those kits go on PLENTY of engines that then never run right, if you know what you are doing you can get all the parts for half off and never touch the kit.

When you go to pods the slides drop to not open as much, that makes the carbs go rich, then you richen more in error and then plugs so wet they foul to not start. The CV carbs have driven thousands before you crazy in the same way, they HATE pods. The slides actually open more slightly lean not rich. If valves not set then the slides drop again when the engine cannot pull as hard on the carbs due to the valves. You want the slides opening as fast and as quick as they can. Why Dyno jet drills them, not to increase power but actually just to get slide back to opening as much as it did before! What does that tell you???? The harder the engine sucks the faster the slides open!

Not trying to be mean but the truth of it all, I used to buy them like your condition for a pittance (stealing really) then rework back to stock to sell a bike that runs to 9500 rpm in every gear.

Rule #1 and locked in concrete--when you go to pods the engines flow LESS air, and then you jet them up to run worse. No way will that ever work. A stock airbox is the first part I go looking for to fix one running like yours. If it doesn't fit your idea of what you want in the bike, well, there will be trouble there. Oops, already there. The engines don't even like the stock airbox with like a K&N replacement filter in it, they run worse. OEM paper filter only.

Adjust the valves to .005", the bike runs longer and better.

Did anyone read the label on the pipes that said modification would make bike run worse? They weren't lying there. 4 separate pipes cut short loses a LOT of tuning there, it even can burn exhaust valves.

Take your own prescribed medicine, you said it ran great until you started messing it up, go backwards. Give up on the direction you are going now, if you don't you WILL be forced.

 

[SeattleRod]

Brandon, I am by no means an expert in the subject like amc49 is. He and many others on this forum and other forums have guided me through my project bike, so I'll share my experience with my 750.
I bought a 1980 750 custom last spring. It was in rough shape, barely running. I mean it'd sputter after multiple tries and then die. The previous owner had installed the Dyna Jet Stage 3 and had pods. After reading about the stage 3 and what entailed (needle change and enlarging the holes on the sliders), read that the general consensus was to stay away from pods or you'll be chasing your tail trying to get the pods to work with these carbs. The previous owner sold the bike because we could not get the bike to run.
Since the slider holes were enlarged, the carbs were junk now. I bought a used carb bank on eBay for $160, cleaned them and rebuilt them with Randakk's kit.
Then, I checked the valve clearances. I had to change 10 shims and one bent exhaust valve. Then I checked the timing. THEN, I synchronized the carbs.
I had the engine idling steady at 1200rpm. I rode the bike through the summer. Don't forget to add a inline fuel filter between the tank and the carbs, otherwise you'll be cleaning the carbs rather soon, like I did.
Moral of the story, stay away from pods on these carbs. The bike runs like sheeat, if it runs at all. They look cool but they don't work on these carbs. My two cents.

 

[amc49]

X2 on any extra fuel filter, with ethanol gas and steel tanks you have utter fits on these with superfine powder rust, it goes right through the OEM filters like they are not even there.

I'm not convinced that simply drilling the slides messes them up unless the job was botched. In my own testing the bigger the hole there the faster the slide lifts and that generally is a good thing. Others as well have dropped the Dyno parts to still use the same slides and reported they still worked fine.

Still, the carbs will ALWAYS be sensitive to pods, they remove the airbox and that is what makes the carbs all share the same pressure conditions in a confined space.

Think of it as this...............each slide lifts based on a suction impulse when the intake valve opens, refer to that very limited short space as a 'duh' in a longer stream of downtime when the piston is doing all the other things it does when not on intake, ergo, squeeze, power, and exhaust.

When you isolate the impulse by pod so no others share it, you now have a 'duh,---,---,---,duh' etc. going on every 180 degrees of crank rotation as the one sucks then the others do too but now isolated because of pods.

With airbox and the resultant joined space you instead get a 'dud, duh, duh, duh,' that EACH carb can feel in pressure drawdown to then build slide vacuum. Each carb feels its' fellows' vacuum impulses to then lift more, it's CRITICAL to making CV carbs work. The paper filter was picked to be a restrictor after a certain point and how the slides open up 100% of the way while the vacuum that lifts them has been dropping off (vacuum is always highest at idle). Even a K&N stock replacement filter in the airbox can mess these up. BTDT.

Pods no way in hell can be worked to get rid of that very bad issue. People tape them up to 'get more vacuum' and block them off with metal and other gimmicks but all that does is make the clean look of pods look like a dog bike. They'll also tell you it runs better when it doesn't at all, it's the human way.

The ones who fight them then go back to the OEM airbox...................THEY know. And will tell you in a second, but of course the corporations like DynoJet and K&N won't tell you spit there.

Like every other lie on the planet, it's about the money.

These carbs are far worse than rubber diaphragm CVs because the slide is much heavier, why they often have 3 big slide holes as compared to 1 smaller one on rubber carbs. The vacuum in slide comes from both pure pressure drop vacuum and Bernoulli effect vacuum too, caused by matter racing at speed past the holes. The engines even new only make maybe 10 inches HG. at idle so it has to be augmented by the passage under the slide. With 2 holes in back and 1 up front I have often wondered if there was any difference in flipping that around to make the 2 in front.

 

[Chris in Dorset England]

Wow! You don't mince your words, do you! I'm in the UK, already I have picked up your advice on valve clearances. I shall bare that in mind. thanks,