Help me with jetting??

[ishoelaceee]

Ok so I have a 1977 cb750k, slow project I have been working on finally got all the wiring sorted out and got the bike running. I know that I have some jetting to do but I'm not sure what exactly jet the carbs too. I'm running a Vance Hines 4into1 open header and individual pod filters. I know there is a lot of experienced CB owners on here hopefully someone can help me out.

 

[dirtdigger]

Nobody can tell you what you need for jetting. Every engine will need something different, to many things affect jetting, engine size, engine condition, intake setup, exhaust, altitude, temperature, fuel type and quality, etc. You need to get it running and see how it runs at different rpms and how the plugs look then go from there.

You will probably want to add a little onto that header pipe, be giving up some lower end and mid power having it that short and the scavenging of the 4 into 1 will be lost if you leave it like it is.

 

[ishoelaceee]

Nobody can tell you what you need for jetting. Every engine will need something different, to many things affect jetting, engine size, engine condition, intake setup, exhaust, altitude, temperature, fuel type and quality, etc. You need to get it running and see how it runs at different rpms and how the plugs look then go from there.

You will probably want to add a little onto that header pipe, be giving up some lower end and mid power having it that short and the scavenging of the 4 into 1 will be lost if you leave it like it is.

Bike will not run off the choke. And if reved it hangs up a bit. It had a 115 main jet which I'm sure is making it run lean. I was thinking about running a 125 main and 2 turns out on the fuel screws. But I'm 800 above sea level with temps from 70-99 degs and going to be running 93 octane if that helps at all.

 

[dirtdigger]

If it wont idle off the choke then you have plugged idle passages, non of your mods will cause a large enough difference in jetting that it shouldn'e at least try to idle. Hanging rpms can be from lean/vacuum leaks, carb sync off, binding cables, ignition advancer sticking/hanging up.

 

[amc49]

X2 to all that. Are those carbs the OEM ones? Then research what the OEM jet was to be sure the 115 is at least correct for that motor, I for one do not know. Even with those parts the idle jetting and settings should be virtually the same as OEM unless very lean to begin with. If the 115 correct for original then 125 might be fine but plug checks in order to make sure. Forget somebody telling you the exact jets to use, it just doesn't happen. Motor condition and even the spark plugs you use will affect it.

For sure that header if stopping right there is going to muck up things, they really need that tailpipe. I've drag raced with and without them just like that and 1/4 second lost with no pipe past the collector, the bike was a dog run like that. The fastest 10 hp. loss you ever saw. Looks like a sidewinder drag pipe missing the meg on the end, the meg is half the power of the pipe and much of the torque. DON'T jet with it like that unless you are going to ride it like that, pipe added and/or muffler and all your work down the toilet. All header advantages are tossed the second the pipe length is butchered like that, the tuning is destroyed.

If carbs not the OEM ones then like said, you are in no-man's land. You have appointed yourself as the fuel systems engineer, pretty much can't teach that over the 'net. If the carbs not set up specifically for that motor type then the airbleeds, needles, needlejets will be off a mile and no jetting on the planet may be able to get past that.

 

[dirtdigger]

115 should be stock size. These carbs are fairly lean on the idle and off idle and they have an accel pump to compensate but you will have to verify it works, a lot of times they dont because the diaphragm is bad or the check balls are stuck. I can just about guarantee you will need to do some needle shimming, have dealt with these carbs before and the mid range is kind of lean on a stock setup, then throw pods and an inefficient exhaust on and they tend to want more fuel to make them work. Jetsrus.com has the shim washers and the jets you will need, they ship fast and are cheaper then oem. you will have to run it and see and jet from there. But you need to get those carbs cleaned out and figure out why it wont idle before you do ANY jetting.

 

[madmtnmotors]

Jetting is best done on a dyno...

 

[ishoelaceee]

115 should be stock size. These carbs are fairly lean on the idle and off idle and they have an accel pump to compensate but you will have to verify it works, a lot of times they dont because the diaphragm is bad or the check balls are stuck. I can just about guarantee you will need to do some needle shimming, have dealt with these carbs before and the mid range is kind of lean on a stock setup, then throw pods and an inefficient exhaust on and they tend to want more fuel to make them work. Jetsrus.com has the shim washers and the jets you will need, they ship fast and are cheaper then oem. you will have to run it and see and jet from there. But you need to get those carbs cleaned out and figure out why it wont idle before you do ANY jetting.

Carbs at super clean inside I took them apart and was surprised about how clean they were I blew all the ports out with brake clean and they are all cleared out for sure. Been working on getting the battery box sorted out and the wiring tucked, so that I can get one more thing done. But should have that done this week and then I'll be able to try and get the bike warmed up and see if I can the idle sorted I know that the exhaust isn't the most efficient thing but kind of goes with look of the bike as dumb as that might sound.

 

[amc49]

Yours and do as you will but recognize your 'look' costs you a pretty good chunk of power there. Some are OK with that but tuning that out may be harder as well, you have a sharp, hard, VERY short sonic impulse there that is almost impossible to jet for. You have basically taken a good header and turned it into 4 separate straight pipes which NEVER run as well. The header relies on that length after the collector to create a wide rpm range pulse that makes power everywhere, from low-mid and all the way up. The inertia flowing down the further tube is what pulls hard and LONG on the cylinder to scavenge it, you are killing that. The length also times to overlap, shorter length can have you sucking WAY too much fuel out exhaust to then run lean in effect while you jet up too rich. Read what I say lower down about burning valves. You may not have accurate plug readings.............I have seen some really wonky 'high-perf' (LOL, think the twin one barrels some of the chopper guys go to on these to dump 4 carbs) setups demand richer jetting just to get somewhere close to the power a stocker makes, a sure indicator you have something wrong with your major setup. It points to an engine not pulling as hard on jet to demand bigger, ergo, engine is at fault in some way power wise.

X2 the dirtdigger notes. Silly to tune mains with no decent idle, part of the main jet fuel is what is in idle, if you change idle you are affecting mainjet amount to a certain extent.

As well, get too lean there and a short pipe like that can under certain circumstances warp/burn valves. Full pipe length stops that. It may be though that the collector saves you there by mixing all the impulses enough to not backflow the cool air that does it.

If carbs are stock for the motor then messed up or leaks at intake manifolds if it won't idle, doesn't matter how 'clean' they are.........literally. Something missed there. Have you run the throttles way open just to keep it running? Then work on tuning while dropping the idle speed. Need a good sync there or additional problems. You can bench sync very close using simple small drill bits. Close enough for sure to run acceptably to tune it further out.

Jetting on a dyno.............LOL........while 100% true I have done hundreds without one. Modern wideband O2 sensors make it even more possible. I used to jet 2X4 tunnel ram drag car engines, fun there..........oddball crap like the 368 inch pro stock AMC engines we used to race at almost 800 hp @ 9000 rpm. My younger brother used to drive a pro Camaro as well in heads up match racing, 720 cu.in. fat block GM at high 7's and over 200mph 1/4 mile. Three freakin' stages of nitrous, jetting for that was a BITCH. One mistake and an instant $40,000 worth of melted slag. It held the world record for fastest IHRA pro stocker in the world there for a little while in the late '80s.

 

[madmtnmotors]

Jetting on a dyno.............LOL...

I fail to see the humor.

While an "experienced" tuner may have a more refined "seat of the pants" dyno, I get the impression that does not apply here. The intake and exhaust setup here can quickly lead to burnt valves as you have indicated and even holes in the pistons. Sure, you can get a modified bike to run without any A/F ratio measurements and call it good, but how will you know it's running as well as it possibly can? As far as comparing the jetting of a 720 cu.in. big block to the jetting of our 45 cu.in. bruisers, is kind of apples to oranges. Volumetrically we're talking the capacity of just over one half of one cylinder of a 720 cu.in. big block, so even small jetting changes to these small engines has a much greater impact. I am not questioning your tuning abilities, your advice is genuine and spot on, and all we can do is try to steer others towards the light.

 

[ishoelaceee]

Yours and do as you will but recognize your 'look' costs you a pretty good chunk of power there. Some are OK with that but tuning that out may be harder as well, you have a sharp, hard, VERY short sonic impulse there that is almost impossible to jet for. You have basically taken a good header and turned it into 4 separate straight pipes which NEVER run as well. The header relies on that length after the collector to create a wide rpm range pulse that makes power everywhere, from low-mid and all the way up. The inertia flowing down the further tube is what pulls hard and LONG on the cylinder to scavenge it, you are killing that. The length also times to overlap, shorter length can have you sucking WAY too much fuel out exhaust to then run lean in effect while you jet up too rich. Read what I say lower down about burning valves. You may not have accurate plug readings.............I have seen some really wonky 'high-perf' (LOL, think the twin one barrels some of the chopper guys go to on these to dump 4 carbs) setups demand richer jetting just to get somewhere close to the power a stocker makes, a sure indicator you have something wrong with your major setup. It points to an engine not pulling as hard on jet to demand bigger, ergo, engine is at fault in some way power wise.

X2 the dirtdigger notes. Silly to tune mains with no decent idle, part of the main jet fuel is what is in idle, if you change idle you are affecting mainjet amount to a certain extent.

As well, get too lean there and a short pipe like that can under certain circumstances warp/burn valves. Full pipe length stops that. It may be though that the collector saves you there by mixing all the impulses enough to not backflow the cool air that does it.

If carbs are stock for the motor then messed up or leaks at intake manifolds if it won't idle, doesn't matter how 'clean' they are.........literally. Something missed there. Have you run the throttles way open just to keep it running? Then work on tuning while dropping the idle speed. Need a good sync there or additional problems. You can bench sync very close using simple small drill bits. Close enough for sure to run acceptably to tune it further out.

Jetting on a dyno.............LOL........while 100% true I have done hundreds without one. Modern wideband O2 sensors make it even more possible. I used to jet 2X4 tunnel ram drag car engines, fun there..........oddball crap like the 368 inch pro stock AMC engines we used to race at almost 800 hp @ 9000 rpm. My younger brother used to drive a pro Camaro as well in heads up match racing, 720 cu.in. fat block GM at high 7's and over 200mph 1/4 mile. Three freakin' stages of nitrous, jetting for that was a BITCH. One mistake and an instant $40,000 worth of melted slag. It held the world record for fastest IHRA pro stocker in the world there for a little while in the late '80s.

I have the rest of the sidewinder pipe. Sounds like it might be worth while putting it on and trying to make it look decent. Would simple extending the pipe out a foot or two help it any? On one hand I like the look of just the header but on the other hand I don't want to sacrifice too much power and have the bike run like shit. Is there any chance you live around Ohio lol I would just bring the bike to you. I work at a Toyota dealership as a tech I know fuel injection and know nothing about carbs wasn't raised around them.

 

[dirtdigger]

foot long collector would help greatly and would be a good place to start. I have run 1 foot to a little longer and worked very well for me on my big bore motors.

 

[amc49]

X2. ANY collector length there will help. No difference in carbs and EFI other than one squirts the fuel in there for you. Apples and oranges like someone said. I like apples AND oranges. If the rest of pipe is a megaphone then it will help it for sure but be a bunch louder, the meg does what they do, amplify sound. It amplifies pull inside the header too. Why all modern 4-1s pretty much come with canister mufflers rather than the old school meg, of course they don't tell you about the power lost going to that 'trick' canister. All the racing places want quieter exhaust now.

'I fail to see the humor.'

Sorry, really...........I tend to ridicule dynos, they can be tailored to make almost any reading you want. Not reliable at all. You can't strap one on your back to take it to the track either. Sooner or later the machines have to run in the real tangible world and readjusted for that. FYI, often dyno jetting can have you melting motor in that real world, the dyno load is too short to truly reflect the heat rise that occurs over longer use say a roadrace. You often end up bumping fuel up in actual use. Or you better.

'Volumetrically we're talking the capacity of just over one half of one cylinder of a 720 cu.in. big block, so even small jetting changes to these small engines has a much greater impact.'

Uh, and why would that be so? VE applies pretty much the same physical laws to either one. By the definition of small I at least mean in RELATION to the engine size being worked with, they have no idea whatsoever of how big or small they are. A 10% jet change is the same on a 1 cubic inch as a 100 cubic inch. There is only one physical rule that applies to size alone, being the one that guarantees that a bigger motor cannot make as much power as a smaller. The hp. per cu.in. thing, Actual VE can never (well hardly ever) be higher on the bigger motor, it's less. Other than that they are the same. You must have close to stoichiometric on either size to get it to run well.

Engines are nothing more than air pumps, who cares about size? On the big engine mentioned above trying to achieve lean best hp. on a given day for best time could have you frying the motor with only two numbers in jet change. The nitrous makes that MUCH worse. They are that touchy if looking for every last bit. Look at the top fuelers, if on the last runs to the top you aren't melting the motor in the traps you are not running with the top dogs, they all do it. That's intentional. Lean best hp. setting melts engines on fuelers, simply too much heat for the technology of the parts. We melted one every now and then as well if big money was on that run.

Absolutely no insult intended or implied here, let's get that clear, I just tend to prick brains. My full-hearted apologies if that offends. I've been lucky enough to mess with enough different types and sizes of motor to begin to run them all together as the same, they are NOT but then again they ARE. The true apples and oranges there.

'While an "experienced" tuner may have a more refined "seat of the pants" dyno, I get the impression that does not apply here.'

100% true and like most postings I have drifted.

 

[madmtnmotors]

I like apples AND oranges.

As do I!

I tend to ridicule dynos, they can be tailored to make almost any reading you want. Not reliable at all.

They will not accommodate changing atmospheric conditions at the track either, but I find it hard to believe that any but the most experienced tuners can accurately measure A/F ratios better than a tailpipe sniffer.

You can't strap one on your back to take it to the track either.

I am working on doing just that, since:

"Modern wideband O2 sensors make it even more possible"

Absolutely no insult intended or implied here, let's get that clear

None taken.

I just tend to prick brains

As do I!

like most postings I have drifted

As have I.

Extending the exhaust, with either pipe or muffler is a step in the right direction. You can also improve VE/drivability by addressing the deficiencies in the intake tract with the use of velocity stacks instead of pod filters. Of course, velocity stacks don't provide much in the way of filtration, which is another argument for just using the stock airbox (velocity stacks built in!). Even if you have to use the stock airbox just to get it running right, then you can start making changes one at a time. Changing too many things at once makes it difficult to know what went wrong.

Edit: I see that the stock airbox does not appear to be an option based on the geometry of the chopper frame you are using.

 

[ishoelaceee]

As do I!




They will not accommodate changing atmospheric conditions at the track either, but I find it hard to believe that any but the most experienced tuners can accurately measure A/F ratios better than a tailpipe sniffer.




I am working on doing just that, since:









None taken.





As do I!





As have I.

Extending the exhaust, with either pipe or muffler is a step in the right direction. You can also improve VE/drivability by addressing the deficiencies in the intake tract with the use of velocity stacks instead of pod filters. Of course, velocity stacks don't provide much in the way of filtration, which is another argument for just using the stock airbox (velocity stacks built in!). Even if you have to use the stock airbox just to get it running right, then you can start making changes one at a time. Changing too many things at once makes it difficult to know what went wrong.

Edit: I see that the stock airbox does not appear to be an option based on the geometry of the chopper frame you are using.

I was actually looking into velocity stacks. I found some that were super expensive and some that were not. Do you know of a place to get some decent stacks at a decent price. I think the pod filters look like shit and like the look of stacks.

 

[dirtdigger]

If you had the round top carbs I have a set of heavy aluminum velocity stacks but they wont fit the later keyhole carbs as they are larger then the earlier ones. Problem with velocity stacks is you have zero filtration. Have seen some cheap filters that snap onto the velocity stacks but they are not near as good as filtering as pods or even the stock air box.

 

[madmtnmotors]

I was actually looking into velocity stacks. I found some that were super expensive and some that were not. Do you know of a place to get some decent stacks at a decent price. I think the pod filters look like shit and like the look of stacks.

From $63 to $77 for a set of four:


http://www.steeldragonperformance.com/products_2.html

 

[amc49]

Hope that battery inside box will be vented outside it, a spark from solenoid could ignite hydrogen trapped inside from charging battery. I'd be drilling a hole or two in bottom of box for water drainage, no way is that box going to stay 100% waterproof if driven in a hard storm.

 

[kanjoh paul]

foot long collector would help greatly and would be a good place to start. I have run 1 foot to a little longer and worked very well for me on my big bore motors.

I have a problem with my new Honda. I bought it just two weeks ago and I have been in the garage for three days now. It idle very well but doesn't sound good and can't carry me up the hill. I dont know where else to get help from.