Big cams and Bernoulli's principle (venturi jetting)

So I put a monster cam in my 250 thumper. The old one was worn to a nub.

The carb has a 155 air jet and a 130 main jet. With the 130 it bogs on the top end-3/4-4/4 throttle and hesitates a bit at 1/8 throttle, just after letting the clutch out. I tried a 140 air jet, no change. 135 jet and it was far worse. The plug seems a touch rich but being colour blind, tan and brown a like grey and black to me on a plug.

So. If what I recall from Bernoulli and his scruples, with less intake vacuum with a big cam and more air going into the motor creating MORE venturi vacuum - I'm sucking more fuel into the motor with the same displacement, so richer.

Lean it out a bit? I ask first only because I don't want to take it out for a rip and run it too lean.

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why dont you tell us exactly what type of thumper and carb it is first, or is "thumper" the name of the mfg?

sorry but i have no idea what you are talking about and you are trying to get to fancy with bertollis pizza principle.

the bigger the cam, the less vacuum you have at idle.

also, engines with big cams run better with carbs that have accelerator pumps . . they also occasionally need the ignition timing changed.

saying that it bogs means nothing because it could be rich and could be lean.

post photos of your plugs.

If it stutters/burbles even the tiniest bit when you accelerate, it is rich somewhere . . If it simply hesitates as if it is out of gas, it is lean.

Post a photo of the plug . . The plug on the right is shiny which means it is wet which means its oily . . It is possibly rich too but its hard to tell because of black deposits from the oil . . This engine should be repaired . . The middle plug is perfect . . The left plug is lean.

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This plug is bone dry so the black means it is rich . . If both the threads and the porcelain is black, i would start by reducing the main jet size first . . In the case of a plug that has extremely heavy deposits like this, I would go down 3 sizes on the main because just one will not be nearly enough and two will likely no be enough . . Four would be too much.

The less vacuum is only a low engine speed phenomenon, so it is irrelevant when driving, but can make idle tuning troublesome. Presumably the larger cam will result in larger intake volumes at the design max power rpm, resulting in higher air velocity across the needle jet opening, and draw more fuel. I suppose that would mean that you wouldn't have to fool with the jetting if it was good prior, but in the real world there are more factors to consider and the resulting complexity is a bit too much to work it out reliably "on paper" so a "cut and try" solution is the only answer - even on a dyno. Always err on the rich side and dial back. Looking at spark plugs for mixture is in the main only useful if doing plug chops at max output on new plugs. There isn't any color to observe, only a soot ring at the base of the insulator which is right hard to look at without cutting up the plug. I get the main jets right by looking at the plugs and leave them. Drivability gets fixed by fooling around with everything else. If you change the mains to improve drivability, you risk being too lean (or rich) for good WOT performance.

barnett468 said:

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This is probably one of the best spark plug color explanations I have seen... just sayin

barnett468 said:

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why dont you tell us exactly what type of thumper and carb it is first, or is "thumper" the name of the mfg?

sorry but i have no idea what you are talking about and you are trying to get to fancy with bertollis pizza principle.

the bigger the cam, the less vacuum you have at idle.

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You lost me after that. I'm concerned you don't know what a thumper is.

I'm asking about the venturi effect of faster velocities past the needle with a bigger cam. Same air volume essentially going across the venturi, but at a higher speed, drawing more fuel out. Therefore potentially needing a smaller jet.

A thumper is a single cylinder motorcycle.

cosworth said:

You lost me after that. I'm concerned you don't know what a thumper is.

I'm asking about the venturi effect of faster velocities past the needle with a bigger cam. Same air volume essentially going across the venturi, but at a higher speed, drawing more fuel out. Therefore potentially needing a smaller jet.

A thumper is a single cylinder motorcycle.

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It was a joke . . Some people that ask for help actually mention what bike and carb and general set up they have, ie My bike is a 1973 Honda 750 with Mikuni VM 32 mm carbs a header with no air box.

JOKE

Noun

Description:

1. Something said or done to provoke laughter or cause amusement, as a witticism, a short and amusing anecdote, or a prankish act:

He tells very funny jokes. She played a joke on him.

2. Something that is amusing or ridiculous, especially because of being ludicrously inadequate or a sham; a thing, situation, or person laughed at rather than taken seriously; farce:


Bertollis piza principal will NOT help you one single bit, therefore it is irrelevant . . If you want to get fancy, you can enter all your specs into a desk top dyno program, like compression, cam timing and lobe separation, port volume etc, plus, try to determine the amount of scavenging your particular exhaust pipe provides with your particular cam etc, and you will have at least remote idea of what the engine vacuum is at different rpms....or, you can forget all about trying to over think it and simply let the bikes performance and the spark plugs tell you what it needs just like everyone else does.

Basically, your bertollis question can not easily be answered because it is not exactly the same for every app . . The principle itself doesn't change but the variables change the results, but in general, you would need BIGGER jets with a bigger cam

Your timing curve also affects your jettting.

cosworth

if it was running good before and the jetting was right on

and the cam change is relatively linear then it would probably be easier to just reduce the jet needle one or 2 sizes

this will reduce the fuel flow across the spectrum and not require playing with all the jets

so if you have a p8 try a p6 ect or q2 go to a q0

if indeed its the increase in fuel flow due to air velocity this will reduce the fuel flow across teh entire range except pilot

cosworth said:

So I put a monster cam in my 250 thumper. The old one was worn to a nub.

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i would run a magnetic drain plug then.

cosworth said:

The carb has a 155 air jet and a 130 main jet. With the 130 it bogs on the top end-3/4-4/4 throttle and hesitates a bit at 1/8 throttle, just after letting the clutch out. I tried a 140 air jet, no change. 135 jet and it was far worse.

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dont change the air jets, put the old one back in.

what carb is it?

does it have a header?

how did it run before?

are the 155 air jet and 130 main jet the same ones you had prior to changing the cam?

cosworth said:

The plug seems a touch rich but being colour blind, tan and brown a like grey and black to me on a plug.

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was the plug new when you changed the cam?

how many hours on the plug?

post a photo of the plug.

cosworth said:

Lean it out a bit? I ask first only because I don't want to take it out for a rip and run it too lean.

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easy deal, with the previous jets reinstalled, go up on the main until it obviously stutters/burbles when you are cruising in third gear at around 3000 rpm and open the throttle all the way . . then go down on the main 3 sizes and try it again.

as was mentioned, jet the main first, once it is jetted to tun well doing the test i described, leave it alone and move on to raising and lowering the needle because it is free and easy to do.

tell us what happens when you raise and lower it . . we can help you from there.
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Your highly likely to get flow reversion at low rpm so may need a MUCH smaller pilot jet
Go UP on main jet then drop needle to lean out mid range.
You may also have to adjust float level 1~2mm higher than stock to lean out bottom end~mid range
It is very rarely possible to 'jet the main first' on a bike you can't get running well enough to USE main circuit and on a CV carb you won't get close to 'full throttle' until close to redline

PLAZACYCLEJOSH said:

This is probably one of the best spark plug color explanations I have seen... just sayin

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It would be a lot better if it worked with modern fuels :
A -plug that brown is running rich

I ordered some jets and they came in today. Smaller and smaller I went and I'm down to 125 and it runs better. But still horrible on the top end or at wot. With a 135 it barely goes over 70kph. The 130 jets flutters at 80 a bit and if I open it up to wot at 80 it bogs immediately. With the 125 jet it cane get up to 90kph then do the same.

I'll play with main air jet a bit more too. Stock it has a 120 main jet and a 140 air jet. It now has no air box, just a velocity stack, the exhaust header went from 1 inch to 1.5 inches and the came is a bit fatter than stock. It's amazing to see just how rich the bike runs now. I'll try the 157.5 air jet I have with the 125.

I'll have to order some smaller main jets and play with the float height as mentioned. Makes me realise just how worn the stock cam was that I had to compensate so much with main jets sizes before.

You are absolutely correct about Bernouli, but the question is whether the motor is actually pulling in more air to have an impact. Let's back up a bit. What the air jet does is to change the fuel slope and what that means is that fuel supply does not increase linearly with RPMs at a constant throttle opening. With a big air jet and everything else the same, fueling will be leaner at high revs than with a smaller air jet. Notice that all said at the same throttle opening.

We know that different jets are most effective at different throttle openings, but main jets do not flow the same amount of fuel at low revs as they do at high revs - that's Bernouli at work. But with a big air jet at higher revs, the motor is "sucking" in more air through the air jet than at low revs.

All that means usually is don't mess with the air jets until the mian jet is right in the ball park and then change air jets to fine tune wide open fueling at different revs.

Just off idle is all about the pilot jet and cutaway and air screw. You can take the main jet out and it will make very little difference at small throttle openings. Seriously.

Mobius is correct that you want to get bottom end right and then the main jet and then change needle and needle jet for driveability and PJ is right that most of that spark plug stuff doesn't apply to modern fuel. Barnett posted a good picture in a different thread with a plug chopped up to show where you read mixture - down at the bottom of the insulator.

Silly question but how did it run with the old worn down cam etc? Was it any better at the mid to top end or was it always weak? From what i remember of 250 singles back in the day, they never had much get up and go out of the box. Is yours much worse than it used to be or was it always that way?

Another thing - and this addresses some of the same issues that Barnett alluded to. If the cam is longer duration than the old one, it will open earlier and close later and if the motor doesn't need the extra duration, air will travel slower through the carb than before and not faster. You could argue that higher lift just slows gas velocity across the valve seat and not through the whole port, but more cam may actually lift too far and the motor may not be drawing in as much air at normal revs as it used to if the available time-area is more than it needs at the revs it is running.

It starts better than ever before and idles fantastic. Bottom end seems far more barky and the power seems the same. But the bike runs out of puff on the highway. The opposite effect of what I was going for.


Seriously though. The bike idles so loud now I'm worried about upsetting neighbours.

I have a 112.5 jet that I may try with the needle up a ring.

teazer said:

Another thing - and this addresses some of the same issues that Barnett alluded to. If the cam is longer duration than the old one, it will open earlier and close later and if the motor doesn't need the extra duration, air will travel slower through the carb than before and not faster. You could argue that higher lift just slows gas velocity across the valve seat and not through the whole port, but more cam may actually lift too far and the motor may not be drawing in as much air at normal revs as it used to if the available time-area is more than it needs at the revs it is running.

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yes.

cosworth, i wasn't dismissing your question, but it cant easily be answered because of all the different variables which also include dwell time of the piston at both ends of the stroke etc, that's why i said it was irrelevant . . you did what i suggested and had instant success, thats the most important thing.

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cosworth said:

...the bike runs out of puff on the highway. The opposite effect of what I was going for.

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This is NOT EVEN CLOSE to what you stated in your post that I pasted below . . You need to make clear, concise, accurate consistent statements, otherwise we may make wrong suggestions which won;t do you any good at all.

cosworth said:

I ordered some jets...Smaller I went... ..I'm down to 125 and it runs better. But still horrible on the top end or at wot. With a 135 it barely goes over 70kph. The 130 jets flutters at 80 a bit and if I open it up to wot at 80 it bogs immediately. With the 125 jet it cane get up to 90kph then do the same.

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cosworth said:

I have a 112.5 jet that I may try with the needle up a ring.

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Make ONE change at at time . . NEVER make more than one unless you are a jetting expert . . As I said, change the main FIRST, until it no longer burbles on top . . You can NOT jet the other ares until the main is properly jetted, it just doesn't work that way.

I would try a 115 instead of a 112.5 first if you have one..
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I have changed jets at 2.5 steps from 155 to 125. It's getting expensive. And with almost zero change. I know it's a drastic change but the baby steps I'm taking are getting nowhere. Stock is 120.

barnett468 said:

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This is NOT EVEN CLOSE to what you stated in your post that I pasted below . . You need to make clear, concise, accurate consistent statements, otherwise we may make wrong suggestions which won;t do you any good at all.





Make ONE change at at time . . NEVER make more than one unless you are a jetting expert . . As I said, change the main FIRST, until it no longer burbles on top . . You can NOT jet the other ares until the main is properly jetted, it just doesn't work that way.

I would try a 115 instead of a 112.5 first if you have one..
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Ok maybe something is being lost in translation but that is exactly what I typed. The bike has no go on the highway. The smaller jet, by a factor of 5 (120,125,130) , is making gains on this problem. The bottom end seems ok but wot and an speed past 80kph is non existent. Not sure how that is contradictory.

Smaller jets, more speed. More speed is what I want to solve since lower torque curve is not bothering me. My plan with this cam was to have better performance on the highway and climbing. Not bottom end.

What am I not being clear about? I'm thinking about a drastic jump in the main jet but compensating for the bottom end with a needle shift. The small steps I'm taking are not netting me much for 20 minutes of messing about and an $8 jet of which I now have a bag full. 8 jets purchased. $64. All in the wrong direction it appears.

A 112.5 is 3 steps (2.5 increments) from stock and I have been playing with a range of 8 steps with little change except once I went leaner it didn't studder at 70-80kph, it did it at 85/90. If I have a decent power curve and non lethal cylinder head temps with a 112.5 then I can baseline at 115 or 117.5 next. I've had it up to 140kph before so I'm not tuning to hit 110kph. Going from a 155 to a 130 made zero difference. But a small, very small gain was made going from 135 to 125.

Now in the time I have taken to type this I've run out of time for a few days to try it.

cosworth said:

Ok maybe something is being lost in translation but that is exactly what I typed. The bike has no go on the highway. The smaller jet, by a factor of 5 (120,125,130) , is making gains on this problem. The bottom end seems ok but wot and an speed past 80kph is non existent. Not sure how that is contradictory.

Smaller jets, more speed. More speed is what I want to solve since lower torque curve is not bothering me. My plan with this cam was to have better performance on the highway and climbing. Not bottom end.

What am I not being clear about?

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I suggest you read my post again.

Also, the term "better performance" means nothing without specific detail as to exactly what type of "better performance" you wanted.

In general, if you leave the gears the same but increase the cam size, it will accelerate SLOWER, not faster . . Yup, that's right.

If you wanted more power with the same gear ratio, you should have put a big bore kit on it.

I suggest you take a compression test.

FYI - I have not only jetted well over 500 vehicles, I also ran the R and D department for Kawasaki Motors and I was solely responsible for testing and determining the exact jetting spec for many vehicles.

I was also one of the top racers in the country and built my own bikes, so it would seem that I might just know exactly I'm talking about.

cosworth said:

A 112.5 is 3 steps (2.5 increments) from stock and I have been playing with a range of 8 steps with little change except once I went leaner it didn't studder at 70-80kph, it did it at 85/90. If I have a decent power curve and non lethal cylinder head temps with a 112.5 then I can baseline at 115 or 117.5 next. I've had it up to 140kph before so I'm not tuning to hit 110kph. Going from a 155 to a 130 made zero difference. But a small, very small gain was made going from 135 to 125.

I'm thinking about a drastic jump in the main jet but compensating for the bottom end with a needle shift. The small steps I'm taking are not netting me much for 20 minutes of messing about...

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I would say that an increase of 20 kph before the problem occurs is a noticeable increase and noticeable improvement, but since you are complaining that the results are too small for the effort involved, I suggest you sell the bike and buy a new one that won't trouble you with these types of problems, especially since it clearly sounds like you don't enjoy working on it.

the 112.5 may certainly work, and a 110 might even be better, but its easy to go to far, and since you were gaining 10 kph per every reduction of 5 on the jet size, it is logical to continue along that same path.

cosworth said:

Now in the time I have taken to type this I've run out of time for a few days to try it.

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This is an odd thing to say, because unless you have no fingers and had to type this on your cell phone with your nose, it couldn't have possibly have taken you as long as it would to change a jet and test your bike.

Also, implying that it is my fault that you allegedly ran out of time to work on your bike because YOU made the decision to reply to my post instead of working on your bike is simply ludicrous . . I did not ask, or force you, to reply to my post . . You made that decision all on your own.
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With a longer duration/higher lift cam you may need a significant 'bump' in compression.
Have you done a dynamic compression test with new cam fitted?
You may find cranking pressure is way below stock numbers
Modern four stroke MX bikes run a theoretical CR around 14.5~15:1 (or more on smaller motors)
Actual cranking pressure shows a CR closer to 10.0~11:1
BTW, you can't have 'more air' going into motor unless you increase displacement, you actually have less air staying in motor until rpm goes up (probably over 6~8,000 rpm)

I've taken this cam out. No amount of range on my carb would make it run. I tried as many possible combinations from 112.5 to 150 along with air jets and needle height. I'd reset to base setting then creep away rich or lean and it just wouldn't run.

This motor beyond oversquare. To fit a big cam on this motor, you have to match it with porting and larger valves.