Not-Stock CB350 Jetting on Stock Carbs

Stock compression at sea level for a CB350 is 170 PSI.

Compression Test Altitude Compensation Factors
Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

170 x 0.773 = 131.7 PSI for 8500 FT elevation.

I don't have a chart for 9000 Ft, but it would be about 2 psi less....129PSI.

So your compression is about where someone would expect it to be for over 9000 Ft. Elevation. This is of course assuming it wqas done correctly with a proper gauge.

Mydlyfkryzis said:

So your compression is about where someone would expect it to be for over 9000 Ft. Elevation. This is of course assuming it wqas done correctly with a proper gauge.

Click to expand...

Except that I've not measured it at all....

Rags in the exhaust would do that - or paper towels to keep them clean - ask me how I know that.

What about the needles in the slides? Are you 100% sure they are correctly located?

Have you run it without air filters and now I think about it, what filters are you running on it?

Stock filters. Will try no filter when I try no mufflers.... Needles should be good, as they worked great before the rebuild, and there's no positioning with stock CV carbs.

teazer said:

Have you run it without air filters and now I think about it, what filters are you running on it?

Click to expand...

For the third time, he is running a stock filter setup.

berck said:

Except that I've not measured it at all....

Click to expand...

LOL. Yep.


These guys are good at that kind of research so get used to it. Just step back take a deep breath and soak in the knowledge from these highly respected dtt members. These guys are gods so they should have this handled easily, right xb?

Hehe. Gotcher tail yet? no? Keep on runnin round the table then.

Air.

You built an engine that pumps more than stock.

It wants to move more air.

Let it.

The boretech guy thinks there's not enough air moving as well. Thats why he suggested exhaust. Shoot, what does he know, right.

I'm glad someone noticed.... I figure you can't ask for advice and complain about what you get. I'll run some experiments to annoy the neighbors this weekend and let you guys know. I wish I had access to a dyno.

berck said:

I'm glad someone noticed.... I figure you can't ask for advice and complain about what you get. I'll run some experiments to annoy the neighbors this weekend and let you guys know. I wish I had access to a dyno.

Click to expand...

Change it, ride it, evaluate it. Change it, ride it, evaluate it. Change it, ride it, evaluate it.

If a change makes it better, do it more. If a change makes it worse, do it less more. Or wait, do it more less. No wait, nevermind.

Yes, just change shit, be it carb, electronic, mechanical, till it works right. Hands on is the only way to make progress.

Bottom line is, that's tuning, no matter who ya are. It's not the preferred specs that dictate how well an engine runs, it's how well an engine runs that dictate the preferred specs. Take your math and stuff it, when it runs the way you want then you'll know the numbers are right.

Yes tell us what happens during your tests and be unbiased. So many mechanically minded people tend to hyperfocus and that is the enemy of diagnostic procedures.

Doesn't matter who's right, just that the problem is found.

I'm DohcBikes, and I approve this message.

(p.s.) please somebody find the humor in that). Lucky Post #711.

im not convinced it is a rich condition just because the plugs are sooty they will get sooty if the mixturte is too lean to burn completely.a modified engine with a longer duration and more lift does not create a stronger vacum signal down at idle and coming up from there or will it be trying to pull more air thru than the stocker to contrary it should only start pulling more air for the hp that it can deliver once up on the cam and breathing properly ,hell some car motors that are "hot" need a godam belt driven vacum pump just to have power assist brakes 'cause they cant produce enough of it at low rpm's cause of the cam overlap . the only reason the engine is got more hp potential is 'cause the breathing at the intake, the drop in pressure that the piston going down creates is extended in time by the overlap which includes easrlier intake valve lift combined with later ex closing,and the fact that the exhaust flowing out is what is helping bring in the fresh charge not strictly the mechanical part and this overlap extended duration situation is not nearly as efficient as a stocker at lower rpm's hence why a hot cammed motor usually always looses some low torque

actually i kinda got that backwards, but im not gonna edit out my mistake cause i think all of this is good discusiion....but the idea and effect is cylinder filling for more hp the intake is actually helping the exhaust evacuate and leave behind a cleaner combustion chamber for a fresh charge the fluid collumn and inertia of the intake, charging in and that it has has some time to stack up at the intake valve when it is closed and storing that inertial energy in the form of some pressure all this only once on the cam's designed efficiency range, and its not happening efficiantly at lower rpms
and of course this is the purpose of a certain ideal length and shape of intake tract including the throttle and airhorn up untill it is fully exposed to atomsperic pressure

berck said:

Compression: more compression sounds grand, but I opted for stock compression pistons very deliberately. Pre-rebuild, I got detonation when hot under full load even with 91 octane gas, and seriously retarded timing. I'd been adding toluene on hot days to prevent this, which worked fine. After disassembly, it became obvious the detonation was likely because of the huge amounts of carbon caked on the top of the pistons.

Anyway, that said, there was no way I was going to go to 12:1 pistons. 12:1 pistons need race gas, and a bike that I can't fuel at the pump isn't very useful to me. Also, the cam I used is not a full-out race cam. Description: "TT style performance. Strong mid-range, added top-end."

Click to expand...

You came here for advice so it would be helpful if you listen to it . . Your comment shows that you are completely unfamiliar with how elevation affects compression . . First of all, 12:1 pistons probably do NOT need any more octane than 91 at nine thousand and two hundred feet

You loose around 4.7 to 5 psi of cylinder pressure for every 1000 foot increase in elevation, so the math is simply 9 x 5 = 45, so you subtract that number from whatever your compression would be at sea level and that is around what it will be at 9200 feet . .

A performance engine has around 170 psi and can run on 91 octane . . When an engine gets to 135, it is fairly weak . . When it gets to 115, it may or may not even run above idle.

I general, the bigger the cam, the lower the compression becomes because it closes the intake valve later.

berck said:

Bore-tech recommended the piston/cam combination for a more powerful street bike, and they have experience with them working together.

Click to expand...

If you told them you were at 9200 feet and they told you to use standard pistons, they are retards and obviously have zero experience with that combo or any other one working at that elevation.

If you told me you were at 9200 feet and wanted to buy stock pistons for it I would not have sold them to you.

berck said:

I'm perfectly willing to increasing the timing advance once I get things running, assuming this doesn't cause detonation on 91 octane fuel (the best we can get at the pump in Colorado).

Click to expand...

You wont know what any of the changes will do until you try them as DOHC said, however, in general, the lower your compression, the more timing you need.

berck said:

Spark plugs: I'm using the stock BG8ES for now. Hoping I get the carb dialed in to get rid of the soot on the plugs, but after it's running, I'm certainly willing to play with different plugs. I was using colder plugs before the rebuild to help with the detonation problem.

Click to expand...

How did the plugs look prior to the build?

It could have been a bit rich but still run ok.

Put in one step hotter ones now, not after you jet it . . If you get your compression back up to where it should be, go back to the stock heat range . . Also, keep in mind that at sea level, you are running in air that might be around 70 degrees . . My guess is that you are running in air that is maybe 50 and less at 9200 feet.

berck said:

I disagree that it's not possible to carbureted vehicles run up here.

Click to expand...

Read my post again . . i did NOT say it is impossibe to run a carbed engine at 9200 feet . . Lots of people do it every day.

berck said:

I put over 10,000 miles on this bike between 6,500-9,200ft with no problem

Click to expand...

You're quotes below contradict that statement.

berck said:

I want it to look stock, just have a little bit more power.

I basically wanted enough power out of this rebuild to climb the mountain pass on my commute faster than 55mph at redline in 3rd. Or go faster than 65mph when there's a 25mph headwind.

Click to expand...

It has a problem, otherwise you would not be trying to make it faster and your problem has always been low compression which you did absolutely nothing to fix, and in fact, you actually REDUCED your compression with your bigger than stock cam.

FACT - Your bike will always be a slow ass pig until you substantially increase your compression . . there are no ifs, ands or buts.

FACT - You will go a little faster if you install numerically higher gearing such as installing a front sprocket that is one tooth smaller than your current one . . If you want to make a smaller change, install a rear one that is 2 teeth bigger.

berck said:

Except that I've not measured it at all....

Click to expand...

You actually don't "need" to measure it because all it will tell you is that it is way f'n low which I already now, however, if you use a good gauge and prop the slides open when you turn it over, it will still be somewhat useful info.

Mydlyfkryzis said:

Stock compression at sea level for a CB350 is 170 PSI.

Compression Test Altitude Compensation Factors
Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

170 x 0.773 = 131.7 PSI for 8500 FT elevation.

I don't have a chart for 9000 Ft, but it would be about 2 psi less....129PSI.

So your compression is about where someone would expect it to be for over 9000 Ft. Elevation. This is of course assuming it wqas done correctly with a proper gauge.

Click to expand...

Now you have to account for his "bigger" than stock camshaft and calculate his dynamic compression with that info.

DohcBikes said:

I'm DohcBikes, and I approve this message.

(p.s.) please somebody find the humor in that). Lucky Post #711.

Click to expand...

Ok

Using my phone right now so I cant read all your thread, but check your timing with a lite if you haven't yet.

Some experimentation. Ran better without air cleaners. Since I believe it's running rich, makes sense. Maybe slightly better without mufflers, but that didn't seem to make much difference.

I bought some #62 (previously #68) primary jets and #80 (previously #100) secondaries.

#62 primary jets made things significantly better. With the air cleaners on, partial throttle operation is now much better. #80 secondaries on the other hand were just too lean. At full throttle, things got better with partial choke and #80 secondaries. With #62 primaries and #100 secondaries, it's maybe a little rich partial throttle, but goes crazy-rich full throttle. Most places don't seem to have anything between #80 and #98 secondary, but I did find an e-bayer with a #90 replica. I ordered that and we'll see how it goes.

Currently it revs to redline with partial throttle just fine... It's hard to get enough power to get it to do that under load, but with a slight downhill I can. If I work the clutch to keep it over 4k, it feels great until I open the throttle all the way.

I'm going to see how good I can get it with stock carbs, then evaluate whether or not I want to swap to VM30s, but at this point, I'm starting to think VM30s are in my future.

I'll also note that in the corner cases where it runs right, it sure feels like it's got more power than it used to... A couple of very quick runs up to redline in partial throttle, and it seems very happy up there.

that is saweet good progress
before you run it again re-torque the head, not hot, cold or slightly warm is ok
not recommended to go more than 5%-10% over spec especially if you used anti sieze or grease on the head nuts
dont forget to retourqu all the onthers as well rocker box ,cam journals etc
thanks for the report

Without stock air box the exhaust length becomes critical, longer than stock may be needed.
4mm on float height is too much, 3mm is usually max before you run into problems
Stock jetting should be fine for your altitude, fuel screw 1/2~3/4 turn is OK
Did you get 'good' pods or the cheapies that block carb passageways?

I think the cam is installed wrong or ground wrong. I'd first use a dial indicator through the spark plug hole to verify the piston timing to the degree wheel, then map the cam timing.

A giant pile of snow interrupted my tuning in November. We've had unusually warm weather lately this last week and I was able to make some progress. Sort of.

My stock carb had #35 slow, #68 primary and #105 secondary before the rebuild. Post-rebuild, I've gotten it to run best with a #35 slow (smaller would probably be better, but not available), #60 primary, and #90 secondary. I'm a bit surprised that it wants smaller jets after the rebuild, I guess the increased velocity causes that?

That said, it does not run well. I believe that #90 secondary is rich, but #80 secondary is lean.

In this configuration, it idles well, runs great up to about 1/4 throttle, but bogs down as the throttle is opened further. It runs wide open throttle until about 5,000rpm then hesitates and runs poorly until about 8,000rpm. At 8,000rpm it feels dead-on, and runs like a dream to redline. It seems rich in full throttle operation between 2,500-8,000 rpm. Decreasing the secondary main jet from #90 to #80 makes it run quite lean. Surges, pops, is generally unhappy and, well, lean. I'm a bit surprised in the huge change from apparently lean at 80, but rich at 90.

I've actually run everything available on the secondary. 80, 90, 100, 105, 115, 125, 135, 150. Initially, with weeks between orders from jetsrus, it wasn't obvious what I was dealing with . Yesterday, I ran everything. Starting from 150 going down to 90, it got better. Went from way rich, to just slightly rich. Going down to 80 made it obviously lean. Running the range has probably been good education for me, but quite time consuming and overall unfulfilling.

There are no jets available between 80 and 90 as far as I can tell. I did buy a .85mm drill bit to give that a shot, but I'm not very hopeful.

So, I've resigned myself to Mikunis. Which ones? Looks like the obvious choices are VM30 or VM32. VM30 preferred on stock bikes, VM32 on race bikes, and I've got something in between. At this point I'm more interested in easy of tuning correctly for my setup than peak power or anything else. Which would you guys suggest? I appreciate that I'm going to have to go through a lot of tuning to get it right, but if you could point me to a starting point, that would be most appreciated.

i will be running a pair of cb750 ko carbs they are about 28mm and alot better carb than a mikuni vm for a 4 stroke in my opinion
they are cable pull just like a vm and are just a bigger version of the sl350 24's