Carb Theory & Practice

[E350]

I am thinking about larger displacement engines and their carbs.

(Maybe a Big Bore JM781 for me of a 760 based big bore.)

Question: When do carb diameters need to be increased to handle the larger suction and larger fuel needs of a larger displacement cylinder/crankcase engine?

Given the Venturi principle, to keep the air flow moving fast, it seems that a smallest diameter carb which is capable of delivering the maximum amount of fuel needed for a particular size engine would be best.

And it seems that putting a carb which has a larger diameter than oem diameter carb on an oem sized engine would actually reduce air speed, and thus reduce atomization and thus reduce engine performance.

Also even if you do increase the cylinder size over oem, why wouldn't you just take your existing smaller diameter carb and increase the fuel jet size?

Isn't it true that for the same engine vacuum, the smaller diameter tube (i.e., carb would have the fastest airflow to the crankcase to the ports to the cylinder to be ignited, no?

Why did Yamaha engineers decide that a 61x 701 could best pull air/fuel from a single SBN44?

But each cylinder on a 62t 701 would best pull air/fuel from two SBN 38's each with their own dedicated cylinder?

How would putting two SBN44's or SBN46's on a oem stock 62t affect performance?

 

[waxhead]

If you put a bigger engine below the same size carb then you will find the air speed through it increases. this creates a higher pressure drop in the venturi and can some times make so much of a difference you have to jet down.
A two stroke likes all its air/fuel and it wants it now. SO the bigger the carb the more hit and low end you get, the smaller carbs can give you smoother power but for freeride you want the hit.
The went for the single carb as it was easy/ cheap and gave the power required

 

[E350]

Ok, as a quick for instance.
My 1994 FX-1 with the stock 61x and SBN44 runs perfectly.
I have the FX-1 Factory Pipe with the dimple for the FX-1 which I bought from Flash-FX quite a while ago?
I have your (Wax') head pipe.
I don't want to change anything on my stock SBN44, so I want to buy another carb for the pipe set up.
What carb should I buy?

 

[Sparkplug]

In layman's terms. The bigger the hole, the less restrictive it is.

I want someone to explain, in logical terms, why the trend for low popoff started. Why did yamaha run a factory popoff of 65 psi, on their 1200cc non pv engine, if it hinders fuel delivery? What does lowering the atmospheric pressure required to open the needle, do to allow more fuel into a engine? Can someone explain.

 

[Quinc]

In layman's terms. The bigger the hole, the less restrictive it is.

I want someone to explain, in logical terms, why the trend for low popoff started. Why did yamaha run a factory popoff of 65 psi, on their 1200cc non pv engine, if it hinders fuel delivery? What does lowering the atmospheric pressure required to open the needle, do to allow more fuel into a engine? Can someone explain.

I cant answer that, but oddly enough a brand new 46mm mikuni carb comes with 150m 120P only 14psi pop off. Be interesting to know what they come pre-jetted for.

 

[WFO Speedracer]

I can answer some of it, it has to do with restriction, the more restriction you have the higher the popoff needs to be, so small carbs with OEM intakes and flame arrestors high restriction , high popoff pressure, so now you bolt on a low restriction flame arrestor and it no longer runs right , this is when you have to lower the popoff to match the restriction.

Also restriction is not just limited to the intake system, case in point Kawasaki 650SX ,early models came with smaller opening on the exhaust manifold combined with no bridge on the ex manifold, 1 7/8" exhaust hoses and 28mm carbs, later models came with 2" hoses, larger opening on exhaust manifolds as well as a diveder and 38 mm carbs , the system needs to be matched to work properly.

I worked on two GP800'S where someone had removed the choke plates and shafts, neither of them would plane out or ever get on the pipe and go.

On the first one I left the primer system as it was and I took another flame arrestor screen and overlapped it with the first on, the screens are stainless steel with perforated holes, the holes on the two plates were a mismatch maybe blocking off a third of the hole, the ski ran perfectly after that, on the other ski I installed a long 60mm bolt where the choke shaft used to be, this boat also ran great , n o jetting changes or needle position changes were needed.

The small amout of restriction change by removing the choke system on both of these skis created major tuning and running issues.

 

[kevbo]

They were trying to make the part throttle mixture as lean as possible for emissions. It takes an overly rich pilot and popoff for the best throttle response, thats why we make the engines fourcycle a lot at low throttle..

 

[kevbo]

I can answer some of it, it has to do wit restriction, the more restriction you have the higher the popoff needs to be, so small carbs with OEM intakes and flame arrestors high restriction , high popoff pressure, so now you bolt on a low restriction flame arrestor and it no longer runs right , this is when you have to lower the popoff to match the restriction.

I worked on two GP800'S where someone had removed the choke plates and shafts, neither of them would plane out or ever get on the pipe and go.

On the first one I left the primer system as it was and I took another flame arrestor screen and overlapped it with the first on, the screens are stainless steel with perforated holes, the holes on the two plates were a mismatch maybe blocking off a third of the hole, the ski ran perfectly after that, on the other ski I installed a long 60mm bolt where the choke shaft used to be, this boat also ran great , n o jetting changes or needle position changes were needed.

The small amout of restriction change by removing the choke system on both of these skis created major tuning and running issues.

Mikuni I-body carbs were designed specifically to run leaner. Its easy to overdo it by freeing up airflow above them.

 

[DylanS]

In layman's terms. The bigger the hole, the less restrictive it is.

I want someone to explain, in logical terms, why the trend for low popoff started. Why did yamaha run a factory popoff of 65 psi, on their 1200cc non pv engine, if it hinders fuel delivery? What does lowering the atmospheric pressure required to open the needle, do to allow more fuel into a engine? Can someone explain.

I showed my girlfriend this post because for some reason when I tried to explain this to her I’m “immature” and “sleeping on the couch”

If I had to take a stab at it I would assume it has to do with “freestyle” setup big cc motors ripping massive amounts of air through the carbs right off idle non stop.
At some point air velocity replaces manifold pressure and pop off pressure has zero to do with fuel delivery.
Mikuni says the regulator portion affects the 1/4 and down throttle range but how much does that range change on a big motor with a big carb(s)?
With a big carb and a big engine with very little if any restriction via the arrestor I’d imagine manifold pressures are very very low so people started setting their pop off very low to get enough fuel into their engine for that moment before air velocity takes over.
Of course If the pressure is too high you’ll have a lean condition momentarily or in some cases so severe it can kill the engine. So big engine need lot of fuel fast with low pressures ——> low pop off pressure.
I think a lot of guys run pop off so low they’re drowning the engine and don’t even know it.
I’ve never run lower than 19 personally
Could be wrong, just my take, I’ve been wrong before !

 

[WFO Speedracer]

They were trying to make the part throttle mixture as lean as possible for emissions. It takes an overly rich pilot and popoff for the best throttle response, thats why we make the engines fourcycle a lot at low throttle..

Exactly designed for emissions not performance , this is why most skis will run much better with a Mikuni aftermarket carb than a stock Mikuni carb, the circuitry on the aftermarket carb is not hindered by emissions laws. They also put accel pumps on a lot of the I bodies to be able to lean it out at idle, again for emissions. Seadoo even went as far as dropping one cylinder at idle on the RFI skis, it dropped to one cylinder at idle, the net time you went back to idle it dropped the other cylinder, they also fooled aroujndwith this on Mercury outboard, some of their multi cylinder two strokes didn"t' have an idle circuit on two of the carbs .

 

[WFO Speedracer]

Mikuni I-body carbs were designed specifically to run leaner. Its easy to overdo it by freeing up airflow above them.

Yep I body carbs are some of the worst to modify, it is also one of the main causes of failure on 720 Seadoo engines running a Factory pipe, everyone wants to blame top end seizures on the pipe, it's the carbs actually causing the problems.

In the case of the 650SX you have so many people with aftermarket parts syndrome, they just swap intakes and carbs without thinking about the exhaust manifold and exhaust pipe restriction, then they wonder why their skis isn't as fast as their buddies ski that has the right combo of parts on it.

 

[brapperdoodle]

This is a great thread! I have been working on carburetors of all kinds for 20+ years and I am fascinated by how simple they are, but how amazingly sensitive they are.
Ive read countless articles and books on carbs but I still learn something new all the time. I really do love to learn and there are some very intelligent people on this forum. Some real ding-dongs too haha

 

[WFO Speedracer]

Really the only way to make an I body carb good is to do the Jetworks mod on it, that's a pretty extensive mod from a stock emissions carb standpoint.

 

[kevbo]

Really the only way to make an I body carb good is to do the Jetworks mod on it, that's a pretty extensive mod from a stock emissions carb standpoint.

Im thinking that a 2mm larger throttle plate and "maybe" an additional pilot hole would make the I-body perfect. I may do it for myself but the whole aftermarket carb infrastructure is built around SBNs. I still like Keihin CDKs.

 

[WFO Speedracer]

Im thinking that a 2k on are stock, larger throttle plate and "maybe" an additional pilot hole would make the I-body perfect. I may do it for myself but the whole aftermarket carb infrastructure is built around SBNs.

Luckily most of the skis I work on are stock, as I get older my motivation to work on everyone else mistakes is less and less. If I am working on modified stuff it is probably mine and I am the one who modded it.

 

[kevbo]

Ok, as a quick for instance.
My 1994 FX-1 with the stock 61x and SBN44 runs perfectly.
I have the FX-1 Factory Pipe with the dimple for the FX-1 which I bought from Flash-FX quite a while ago?
I have your (Wax') head pipe.
I don't want to change anything on my stock SBN44, so I want to buy another carb for the pipe set up.
What carb should I buy?

On a stock pump FX-1, I would install a 62T cylinder and get the rpm up. A single 44 is fine on the 61X case, It provides a lot of feedback while you are learning how everything works.

 

[waxhead]

I would also stick with the stock 44, its as good as the stock manifold.

 

[Sparkplug]

Weird. The tech at yamaha said they did it specifically to prevent fuel dribbling in the carb because the engine had crazy vibration at idle. 1200 non pv. It's pretty well documented. And yanah came out with a specific fuel plate for the mikuni carb for the problem. Didn't mention emissions at all.

Tim couldn't tell me why my movi dribbled fuel at idle. Round. And Round. And Round. And Round. Was told 19-20psi popoff. Went to 37. That was like 80-100 hours ago.

Strokers have a ton of signal on the engine. When you hit 7k and let off, the signal is still trying to open that needle. 19 psi is less than half of what mikuni engineered the carb for.

I'll continue to listen to them. But appreciate the conjecture.

 

[DylanS]

Weird. The tech at yamaha said they did it specifically to prevent fuel dribbling in the carb because the engine had crazy vibration at idle. 1200 non pv. It's pretty well documented. And yanah came out with a specific fuel plate for the mikuni carb for the problem. Didn't mention emissions at all.

Tim couldn't tell me why my movi dribbled fuel at idle. Round. And Round. And Round. And Round. Was told 19-20psi popoff. Went to 37. That was like 80-100 hours ago.

Strokers have a ton of signal on the engine. When you hit 7k and let off, the signal is still trying to open that needle. 19 psi is less than half of what mikuni engineered the carb for.

I'll continue to listen to them. But appreciate the conjecture.

You run 37 on what setup?

 

[stefan]

Weird. The tech at yamaha said they did it specifically to prevent fuel dribbling in the carb because the engine had crazy vibration at idle. 1200 non pv. It's pretty well documented. And yanah came out with a specific fuel plate for the mikuni carb for the problem. Didn't mention emissions at all.

Tim couldn't tell me why my movi dribbled fuel at idle. Round. And Round. And Round. And Round. Was told 19-20psi popoff. Went to 37. That was like 80-100 hours ago.

Strokers have a ton of signal on the engine. When you hit 7k and let off, the signal is still trying to open that needle. 19 psi is less than half of what mikuni engineered the carb for.

I'll continue to listen to them. But appreciate the conjecture.

Have you tried too double up on the siphon valve ? that took care of my dribbled on a pair of FS 49s, Novi has a great right up on checking return fuel pressure that might help with dribbled too.