Need jetting advice for 750C with 900 cams. Please, any advice is help!!!

[Dirtybird]

Before anyone says a thing...Yes I used the "SEARCH" function.

Any way. Earlier this year I bought a 1982 CB750C. The previous owner had rebuilt the engine and replaced the stock cams with CB900 cams. He had also added MAC 4 to 2 pipes which have zero baffles in the megaphones meaning basically they are really long and loud open headers. The last bit of this puzzle is he tried to get the CB750F carbs he had to work with pod filters (I know, not another one of those guys). Luckily I am not partial to pods and could live without the if it means having a faster bike. I have a CB750K air box that fits the bike but is missing a filter and the snorkel that attaches to the back by the battery. Shouldn't be a big deal. My only problem is I got the bike with RIDICULOUSLY dirty carbs. Probably the worst I have worked on but now they are all cleaned up, All gaskets and seals are good, and basically are new for the most part. I have a huge bag of jets he was trying to adapt before he gave up on the CV's and bought CR's that apartently were too big so I didn't purchase those.

My question is does anyone have any idea of a good starting point for jetting this jalopy of a bike? I know a lot of people seem to say that the cams don't require a jetting change and I plan to use the same open exhaust with a proper air box and filter. So I'm guessing (correct me if i'm wrong) I only need to jet it like it's a stock bike with open pipes. Also my carbs are bench synced and my floats are all set to 15mm. My sea level is roughly 600' too if that helps.

Please some one help me get this thing running so I can finally move onto the cosmetic stuff I should have been doing all summer!!!

 

[amc49]

First off 4 into 2 even if open pipe is not much better than OEM pipes. A 4 into one honest header will change tune but MAC pipes are not very good even the 4/1s. The CVs DO self adjust to a certain point to anything you do to motor but they also self adjust quicker than spit to any off running condition as well.

If the basic engine is not solid you will run worse jetted up even with airbox. You need airbox but the snorkel not important, a PAPER NOT K&N filter runs far better.

The cams really won't make much difference, try 75P-115S and if that's not close you got tune/engine condition issues. The carbs will utterly choke with much more than that. Don't bother trying to run with bare carbs, it will make you change things witlessly.

If somebody does not have the valves set to around .005" then too close valves will kill any carb work you do, seen it so many times I can't count. Compression check will show that right off the bat and silly to mess with carbs if engine is not solid enough, anything you do will still be messed up. Stock compression is 170 psi.

Engine with proper compression will run all the way up to 9500 rpm, not doing that says valves are off and leaking. The engines then don't rev out and 'Mr. Motorcycle' blames the carbs to screw them up next. Those CVs absolutely slow down opening with an engine off in compression, the engine 'suck' or draw is what opens them and often they are rich before you ever start to jet them due to engine being off.

 

[Dirtybird]

Thanks a bunch amc! I am pretty sure it is a carb issue at the moment seeing as how up until now I have been only trying to ride with pods and figured through my current research they are basically more of a cool factor instead of a feasible mod. The bike starts easy, free revs through the entire rpm range when it isnt under load, and there are no ticks, knocks, or rattles. It only runs poorly after about 4krpm when im actually riding it. This seems to be the same issue I have read whenever anyone has a problem with the pods. Im certain (and crossing my fingers) it's just carbs that were previously jetted way off by the last owner but I plan on checking valves, plugs, and compression over the winter.

I think I have those jets in the giant bag I received with the bike and will try them out in the next day or two. Any idea what the stock a/f screws should be set at? Also what placement the slide needles should be at? Sorry for all the questions. Just really want to enjoy the bike and move onto actually making it look good as well as perform decent. I know MAC pipes are at the lower end and honestly im not building a drag bike of any kind. Just like the way it sounds and glad I will be able to run the highway unlike my supermoto bike.

 

[amc49]

'...free revs through the entire rpm range when it isnt under load...'

Please understand that is absolutely worthless information, the engine is junk if you can't load it, when I mention revving all the way up that means with a load and in gears pulling hard.

The way engine revs when in neutral is pretty much no indicator of anything at all other than it runs and that's all.

The running after 4k issue can be pods or valves or both together. One thing nobody ever seems to consider is that the pods have to be oiled, the amount of oil being a big issue, most oil the filters too much, the engine then proceeds to draw off that extra oil until the filter media drops to the running state and the remainder oil stays in place by stiction. Problem being, you have just wet oil fouled the plugs doing that. They may pull back up alright but often they do not and the fouling then makes the engine go into the loaded staggers, it won't pull out for spit. Just like a car with K&N filter that is overoiled to suck off extra to foul the MAF sensor then they all want to change it when a simple cleaning fixes it if again the plugs are not fouled. I saw it all the time when I was in car parts. "I think, therefore I am my own worst enemy.'

I try not to overoil, patience is a virtue there, and I let filters sit for at least 24 hrs. on folded up paper towels to let any overage drain out of the media. Then they usually give much less trouble.

Pilot screws at 2-2 1/2, a bit richer than the EPA lean manual spec.

Nothing wrong with MAC and we run what we got, just that many think they have hi-perf pipes there and they really are not. A good header really wakes a motor up.

 

[Dirtybird]

Well I know I did not oil the pods (they were the cheapies anyway that came with it.) I have taken your advice and ordered a paper replacement filter for the air box. I am planning tonight to change my jets to the suggested settings you gave and try yo fit the box on. If the battery isn't dead from the cold weather (non insulated garage) I will try to fire it up minus the filter and take it for a spin to see if it's any better (I know it still won't be as good as with the filter but at least it will tell me if i'm close.) Thanks for the help and I will reply with an update once I do everything ;-)

 

[Dirtybird]

So I found 115 jets in my bag of bits from the last owner but no 75's. Decided to at least put in the 115 and buy some 75's at lunch tomorrow. Is it just me or is the 115 jet too long? Will this be okay to run with it being this length?

 

[Dirtybird]

So just a small update, The 115 jets I mentioned are just a smidge too long to the point they wouldn't let the bowl seat all the way against the carb body so I ordered new round ons like the 75 primary mains. Pitbike race season has been going hard this year so carbs still aren't in and of course it snowed tonight so no test rides but I have the carbs cleaned, I have the proper recommended jets in and the a/f screws are all at 2.5 turns out. Update coming soon on tune soon as this light snow goes away.

 

[amc49]

Dunno where you got those jets but they are NOT Honda jets. The stock airbox needs the stock jets, or P68, S100-102. The bigger jets are for running pods and header.

 

[Dirtybird]

Oh I guess it was just a miss understanding then. When you mentioned the P75, S115 I guess I figured you understood I was 100% planning on using the airbox and ditching the pods. I think I might have the new jets you have just mentioned. I guess it's a good thing my carbs aren't all the way installed yet. I will try to change those out then. Thanks