Not-Stock CB350 Jetting on Stock Carbs

Good points, and I do have a query out to boretech as well.

xb33bsa said:

if you were a tooth off on the timing chain sprocket then in order for you to get the ignition timing on the marks the points backing plate would be pretty far out of rotation from where it normally has sat in the past.

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I did not buy the degreeable cam sprocket, assuming that stock setup would be close enough for my purposes. Unfortunately, the points backing plate isn't a good indicator because the dowel pin that goes through the cam is missing from the new cam. I tried to line it up as close as I could...

I figure if it's a whole tooth off it would be running even worse. I'm very nearly sure that I got it right, though.... Anyone have any experience on how it would run a tooth off?

yeah i think it would run terrible a tooth off might even allow a valve to kiss a piston.. i'm trying to roll this around in my brain but would probaly be down on compression as well ? it gives me a headache trying to think that one thru

I can also confirm that both valves are closed at the TDC mark on the generator, and I think that would be wrong if it were a tooth off, too.

berck said:

I can also confirm that both valves are closed at the TDC mark on the generator, and I think that would be wrong if it were a tooth off, too.

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no that wont verify , the valves are fully closed for in the range of 1/3 of a full crankshaft rotation no matter what grind the cam is

i am going to bet you did get it on the marks the thing is a performance cam that is streetable is really not a huge change from stock ,but you gotta remember the engine is an extremly precise device and the cam timing is probably number one thing that really should be right on the numbers
'especially if increased performance is the desire... mfg tolerances at honda, head and base gasket thickness's different from what was used on the spec engine,can all ad up to an error from what was originally intended like mr know it all in dirtbike magazine used to say "you should have known"

Most performance cams bleed off compression at lower rpm to promote cylinder filling at high rpm. That being said, I would check your cranking compression with the carbs in place and with them removed. A high difference will confirm they are a restriction. Also what are you running for air filters? Don't say Emgo pods....


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Stock air filters.

Don't have a compression gauge. Guess having one would wouldn't hurt.

redwillissuperman said:

I would check your cranking compression with the carbs in place and with them removed. A high difference will confirm they are a restriction.


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This is not necessarily so. Max psi can be reached through a very large restriction, it just takes more revolutions to fill the cylinder. However with MECHANICAL slide carbs only, when held wide open the cylinders should fill just as FAST to max psi as they do with open intakes. While this isn't a tell all on restriction due to obvious things like intake charging while in operation, if you notice the psi taking noticeably more revolutions to max out with the carbs installed as opposed to open intakes, then you SURELY need bigger carbs. RPM will only make it worse.

Remember that CV carb slides don't lift from throttle input, they lift from pressure differences created by a running engine. The cylinders will fill a bit slower with the carbs installed regardless of throttle position. This is one of the reasons that most older CV carbs don't provide the off idle response or overall performance that mechanical slides can. Some modern CV carbs are however responsive enough to be preferred by even some drag racers.

Something like this, maybe?

redwillissuperman said:

I would check [the difference between how long it takes to reach max]cranking compression with the carbs in place and with them removed. A high difference will confirm they are a restriction.

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berck said:

Unfortunately, the points backing plate isn't a good indicator because the dowel pin that goes through the cam is missing from the new cam.

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Get the pin.

Just as a point of reference on a bike with nothing in common with this one..... My GT750 makes roughly twice the power it did stock and uses stock but jetted CV carbs. I can on;y get it to do around 107mph in the 1/4 because I launch so slow but it runs nicely on the street with about 90 RWHP now.

I realize that it's a totally different animal, so no comparisons can be made but my experience with CV carbs is that they flow roughly 85% as well as a mechanical slide carb of the same size. On a GT they are 40mm (at the outlet) and have some pretty sever flow restrictions but they can be made to work.

DOHC has different experience to mine and so we draw different conclusions.

teazer said:

Just as a point of reference on a bike with nothing in common with this one..... My GT750 makes roughly twice the power it did stock and uses stock but jetted CV carbs. I can on;y get it to do around 107mph in the 1/4 because I launch so slow but it runs nicely on the street with about 90 RWHP now.

I realize that it's a totally different animal, so no comparisons can be made but my experience with CV carbs is that they flow roughly 85% as well as a mechanical slide carb of the same size. On a GT they are 40mm (at the outlet) and have some pretty sever flow restrictions but they can be made to work.

DOHC has different experience to mine and so we draw different conclusions.

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Different experience as in knowing how and where to measure a carb? Ugghhh. Maybe you mean different experience as in paid to have it done or got paid to do it.

Not so surprisingly we said the same thing about cv's flowing less than mechanical slides of the same size. Wonder who said it first.

Nobody said you can't make extra hp with CV carbs. In fact I said exactly the opposite in a previous post.

I have also not said once that the size of his carbs are causing his problem. (Although contrary to the post 2 below this one it certainly could be a large part of the problem) Only said that diagnosing carbs that are too small is a waste, and that using stock carbs with all his mods is simply, well, kinda lame and doesn't make a bit of sense to me.

I think something people are forgetting to account for is not only max allowable air but the amount of air given at any RPM in respect to throttle position for each size of carb.

barnett468 said:

Get the pin.

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thats right ! i should have said this already relying on just the friction of the m6 bolt thread torque is spooky and for sure you will be out on a lovely ride far from cell service when it slips and leaves ya starnded, goddam that murphy !!
i lost mine during some service work and just fashioned one out of some oxy welding, rod once in place it is captured and a drop of loctite sealed the deal for me,in theory it is not under any shear, just a locater

berck said:

I'm using a #35 slow jet, #68 primary jet, and #100 secondary. I cannot find anything smaller online.

Admittedly, I'm at 9,200 feet, which is only going to make things worse. But with a stock setup things were happy.

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what size was in it when it ran good before the repairs?

your carb size is not causing the prob.

unless you got high compression pistons, your compression is stupidly low . . increasing compression to around 160 - 170 psi per cylinder will help in several areas . . you need a very high quality gauge to check compression with otherwise you might as well not even do it.

if your bike starts easily without the choke and idles well without warming up, i would go down 2 sizes on the pilot.

buy jets here . . might try a 62 and a 90

http://www.jetsrus.com/FAQs/FAQ_identify_that_jet.htm#KEIHIN


if your bike idles decent and revs ok up to 4,000 rpm then it is highly unlikely that the cam timing is causing the prob.

if its really hard for it to get to 4k the cams could be way retarded . . if if gets there really quick they could be way advanced.
.

I think something people are forgetting to account for is not only max allowable air but the amount of air given at any RPM in respect to throttle position for each size of carb.

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I understand the logic of that, but I don't believe that it's the issue. As you know, gas velocity through a CV carb is a function of how much air the engine is pulling in and that changes the slide position. If the engine is pulling in much more air than before the slides will be higher than before and up to about 3/4 throttle that's really not a restriction.

In reality a rider will tend to use a little more throttle if the engine is intake restricted, but that is not the issue here.

I agree with Barnett that the OP should start by replacing his low speed jets and see what difference that makes.

Make me wonder if the needles are in the slides correctly or if they are way too high. Something is wrong here.

Once again, the carbs were functioning perfectly. The slides are rising, and they worked great before the rebuild. The jets before the rebuild are teh same as they are now: 35, 68 and 100.

Thanks Barnett for the link of where to buy jets. First place I've seen smaller than 68/100 primary/secondary jets. Sadly, still nothing smaller than a #35 slow jet, which is what I'm already running. This makes it seem possible to tune the stock carb, but the question is now: should I? I just wish I knew how big a limitation the stock carbs are. I'm 100% willing to grant I can get more power with a set of VM30s. But how much? 1%? I don't care about 1%. 5%? I probably care about %. 10%? If it's 10%, I wouldn't hesitate. 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. If I want crazy speed, I've got a CB1100 for that.

Bore Tech is worried about my stock exhaust as well, which you guys haven't mentioned yet. He suggests that I remove the mufflers and see if that makes any difference before deciding how to proceed. Since I just bought brand new stock replica mufflers, I hate to toss them, but I can probably resell them for what I paid. I'm sure the neighbors will love me for trying this. I guess the idea is to find out if they're a significant restriction.

berck said:

Once again, the carbs were functioning perfectly. The slides are rising, and they worked great before the rebuild. The jets before the rebuild are teh same as they are now: 35, 68 and 100.

Thanks Barnett for the link of where to buy jets. First place I've seen smaller than 68/100 primary/secondary jets. Sadly, still nothing smaller than a #35 slow jet, which is what I'm already running. This makes it seem possible to tune the stock carb, but the question is now: should I? I just wish I knew how big a limitation the stock carbs are. I'm 100% willing to grant I can get more power with a set of VM30s. But how much? 1%? I don't care about 1%. 5%? I probably care about %. 10%? If it's 10%, I wouldn't hesitate. 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. If I want crazy speed, I've got a CB1100 for that.

Bore Tech is worried about my stock exhaust as well, which you guys haven't mentioned yet. He suggests that I remove the mufflers and see if that makes any difference before deciding how to proceed. Since I just bought brand new stock replica mufflers, I hate to toss them, but I can probably resell them for what I paid. I'm sure the neighbors will love me for trying this. I guess the idea is to find out if they're a significant restriction.

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Excellent thought on the exhaust as far as your current running issues are concerned. I usually try to mention that component but it did slip my mind.

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does it even idle?

you're welcome for the link . . im glad it was helpful.


JETTING AND CARB SIZE

i would buy real honda jets only.

if it were me, i would just try the smaller jets and see what happens instead of just giving up completely on the carbs and starting over and having to do the exact same thing with those unless you enjoy abuse.

yes jets can be soldered and re-drilled but its very hard to do but this can not be done to a pilot jet.

if you cant find a smaller pilot, you could possibly change the pilot air jet if its removable.

unless you are a professional test rider, you may not notice a difference in power between your carbs and ones that are slightly bigger

berck said:

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.

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COMPRESSION

i would be for more concerned about what is likely around 125 psi of cranking compression.

i'm guessing that redoing the top end with high compression pistons if you dont have those is not what you want to do but it is the right thing to do . . everything else will simply be a band aid to try and compensate for the low compression if yours is low.

if your compression is low, and you increase it, your carbs will be LEANER with the same jets because it will increase the engines volumetric efficiency.

it will also increase throttle response and power buy a noticeable amount.

compression is king.


IGNITION TIMING

You will also likely benefit from increasing the timing slightly from stock.


SPARK PLUGS

I would run 1 step HOTTER than stock . . this will help reduce the carbon build up.


EXHAUST

Your stock exhaust is NOT the biggest rob you have . . the stock ex works fine at sea level . the higher you go, the less air that goes thru your engine . . the less air that goes in, the less air that comes out which means the the stock exhaust is actually LESS restrictive at higher elevations . . even though you modified your engine, there is no way th exhaust is causing a significant prob if it is in fact causing oe at all . . that being said, by all means, remove the baffles and try it that way.

no testing = no results


ELEVATION

This is really your biggest prob, and its nearly impossible to get anything that is not fuel injected and computer controlled to run right or have any power at all at 9,200 feet.
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were the new repop mufflers ever run on the stock motor or just with this one / i think it is even possible that a non oem muffler assembly has more resrtiction than even the oem ones, who knows how the tested them before production ?
that said it wont run optimum with shorty header pipes but it sure is a pretty quick and simple test to perform,sans muffs

No, I didn't run the new mufflers before the rebuild. I've still got the old ones (rusting internally, one of them lost an internal baffle), and could try them. The new ones certainly look like the old ones, and this design is hardly complicated. That said, the bike is now significantly louder, which I'd attributed to the cam, but maybe it's the exhaust.

With a #35 slow jet, and the float levels set to the stock 26mm, it only idled with the fuel screw all the way in, but did idle fine when warm at about 1200rpm (on the unreliable stock tach). Given that I couldn't find a smaller slow jet, I lowered the float level to 30mm which should make all 3 circuits leaner. Now it idles just fine as long as the fuel screw is at least 1/2 turn out. Further out seems to make no difference, and all the way in kills it now. So, yes, it idles. In neutral, if I open the throttle all way it bogs its way up to about 8,000rpm. It gets to 8,000rpm in neutral a bit faster if I use half throttle. Under load, it won't run very well at 2,000rpm at all, any throttle position. if I get it up 4,000rpm and work the clutch and keep it there, I can ride it between 4,000-5,000 rpm at half throttle.

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

Timing: 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).

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.

Elevation: I agree it's not helping things out, and that I'm never going to make more than about 75% of the total power available at sea level. I disagree that it's not possible to carbureted vehicles run up here. It's not ideal, but it's perfectly possible to tune vehicles to run up here. I put over 10,000 miles on this bike between 6,500-9,200ft with no problem and no changes other than a smaller secondary main jet (#100 instead of #105). I've also got a 1962 F100 that runs just fine with stock jetting up here. I'm I'll get more power when I jet it, but somehow in the 10 years I've lived here I haven't gotten around to it yet. Also no problem making other bikes, chain saws, and various other things with carburetors work. They all run rich unless tuned, and they're all down on power, but they work.

maybe see-ins how you got plenty of fuel, lash on a blue tank and squeak a lil dry shot nitrous in there
ha !
if the pipes are loud you would think they proly flow ok ,i dunno,boretech suggested and the process of elimination dictates trying it with more open pipes just to see if any change to the better happens.its a pretty easy thing to do no ?
once that test is performed you can check it off the list for now,who knows you stll may find you need freer flowing ex for top end power
good thing you dint leave like a rag stuck in the pipes, not that i would know how that could happen :-[ :-\ :