Longer intake runners vs stock bolted carb to head Q's

HollywoodMX

Coast to Coast
I put on new tm38's on my xv920, I made custom filters but I want to use the filters I bought but yjr carbs are almost touching the heads on both sides. I can make a 2" 45 degree elbow and add it to the intake track post carb just wondering the effects on fueling. Previously I had a cast 2 into 1 on it and with the fcr I found the velocity of the spray around bends to be an issue ( pooling of fuel) so I had to switch to a HSR which had less fuel velocity.

I'm my car racing days, extended intake runners also produced more hp, not sure that applies to bikes.
 

doc_rot

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Extended runners will do differnt things on different engines. typically longer intakes create more torque and less horsepower and a short intake the opposite, with so many variables at play the only real way to know it to dyno it. A hard bend in the intake runner will always be a difficult to overcome and probably more of a factor in the fuel pooling than the velovity. Moving the carb further from the intake valve can cause the fuel pooling issues as well. perhaps fuel injection? a throttle body is typically a little shorter than a carb and the injector will be close to the intake valve.

a little off topic - but what ever happened to your supercharged KZ?
 

HollywoodMX

Coast to Coast
Extended runners will do differnt things on different engines. typically longer intakes create more torque and less horsepower and a short intake the opposite, with so many variables at play the only real way to know it to dyno it. A hard bend in the intake runner will always be a difficult to overcome and probably more of a factor in the fuel pooling than the velovity. Moving the carb further from the intake valve can cause the fuel pooling issues as well. perhaps fuel injection? a throttle body is typically a little shorter than a carb and the injector will be close to the intake valve.

a little off topic - but what ever happened to your supercharged KZ?
Okay well they're 45° Bend which I don't think it's a harsh angle. The other two into ones used to have between 45 and 90° bends so maybe it's not that bad. See what happens.

The KZ I still have it it out of the three bikes would be the last one to finish my Cafe XV is quite close as well so once I'm done with the tracker which is pretty quick here I'm going to switch to the cafe and then get to the KZ.

I Custom Engineered a new intake manifold for more power the only problem now is with idle it is actually pooling at idle from not enough vacuum so I get surging idle. I bought a S&S 57 mm carb with four Mains which three are adjustable. I got it to run but the idle is not good, so I bought a atomizer that would hopefully try to atomize the fuel better but requires matching to install.

I started a company 5 years ago which stole my life and soul, now getting back to the bikes.

Ps the Cafe 1002 is around 350lbs with an estimated 90hp/80tq thus my intrest to finishing it next. I loved my busa but there is just something magical with low weight bikes with power in the city. The kz is not light lol. That super charger is heavy!
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irk miller

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Having an RD that's right at 300lbs and should dyno no less than mid-60s, I can fully attest to what that's like for urban riding. You keep a smile on your face.
 

doc_rot

Oh the usual... I bowl, I drive around...
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vacuum gets weaker as you open the throttle and should be strongest at idle. 57mm carb for both cylinders or each? either way it seems way too big for the displacement, if you want a smoother idle you likely need to down size your carb. What kind of carb is it? What size is the intake port? what does the transition from carb to intake look like? its not necessarily about the angle of the bend but how the bend is achieved. if there are any steps or odd transitions it will hurt laminar flow.
 
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irk miller

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Sounds like you lack velocity, so the carbs are reacting too slow, which is why the surging. If they're all pumpers, you're pooling for the same reason. Does feathering the throttle help compensate at the bottom end like an SBC with an oversized Holley?
 

HollywoodMX

Coast to Coast
Having an RD that's right at 300lbs and should dyno no less than mid-60s, I can fully attest to what that's like for urban riding. You keep a smile on your face.
Ya man so much fun! I have to weigh my tracker but it's light too.

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HollywoodMX

Coast to Coast
Hold up guys, I think I derailed the topic when I started talking about my supercharged kz after someone asked and we are blending the two conversations.

My kz is 1175cc, roots type super charger so the intake bolts to the SC. I'll take pics and post in the kz thread. I know someone who has the same kit and 43mm was holding the bike back. I think he was switching to dual s&s carbs but I can't remember the size but over 70mm combined.

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der_nanno

Faster!
That is quite a long intake for this engine and carb combo. It will work, because large displacement fourstrokes are stubborn beasts, i.e. they create a lot of vacuum at low rpms anyway.
Are those the 43 or 47mm OD TM38s? If they are the 43mm carbs, you could go with VM38-200 inlet rubbers and a wedge between head and inlet rubber, dramatically shortening the overall intake runner length.
 

gt alex

Been Around the Block
That is quite a long intake for this engine and carb combo. It will work, because large displacement fourstrokes are stubborn beasts, i.e. they create a lot of vacuum at low rpms anyway.
Are those the 43 or 47mm OD TM38s? If they are the 43mm carbs, you could go with VM38-200 inlet rubbers and a wedge between head and inlet rubber, dramatically shortening the overall intake runner length.
The volume from the valve to the mouth of the intake should be about the same volume of the cylinder its feeding. But that will work fine. I'm sure the length is OK but there are formulas to calculate length but at the end of the day it has to fit the space.
This is the last manifold I made and it dynoed well.
IMG_20210516_233411.jpg
1stdyno.jpg
 

HollywoodMX

Coast to Coast
Well with some "English" and trimming I was able to get the filters to fit but there are vaccum leaks both manifolds, the oring supplied for the intake adaptor are too small or poor quality. Im either going to make gaskets or see if I have a fatter oring.

This mikunioz kit is very far from plug and play.

Edit. I forgot to mention front cylinder was pooling fuel as I thought might happen with the long runner.

IMG_20220804_184406.jpg
 
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HollywoodMX

Coast to Coast
That is quite a long intake for this engine and carb combo. It will work, because large displacement fourstrokes are stubborn beasts, i.e. they create a lot of vacuum at low rpms anyway.
Are those the 43 or 47mm OD TM38s? If they are the 43mm carbs, you could go with VM38-200 inlet rubbers and a wedge between head and inlet rubber, dramatically shortening the overall intake runner length.
Ya the 43s. At that point I machine another fcr kit. I didn't want to do that again. But in hindsight thats that route I wish I did.

Mikinoz claimed the kit plug and play and I wanted a relaxed install for once. I didn't want to spend hours fudging around with my fcr kit but in the end I think the fudging time is the same and I have 2 fcrs extra too. The fcrs out perform the tm's too.
 

doc_rot

Oh the usual... I bowl, I drive around...
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thats short enough i wouldnt think it would cause problems. what do the transistions look like in the intake?
 

HollywoodMX

Coast to Coast
thats short enough i wouldnt think it would cause problems. what do the transistions look like in the intake?
When things are custom you never know with fueling I learned.

It was pooling gas in the front cylinder for my exact worry which is when a spray of fuel hits the bend. It's a flat slide carb thing.
 

HollywoodMX

Coast to Coast
Got it running good now. I had to make some gaskets and play more with the jetting and bowl heights. And I can't believe how sensative the fuel screw is on the tm's like one turn out wrong the bike runs like ass at idle like it's got a vaccum leak.
 

der_nanno

Faster!
Yes, the taper they have on the air-screws is stupidly steep... but it's the same problem on the VM38s, which is why I adjusted them with a wideband - just too heavy handed to make them work decently any other way.
 

HollywoodMX

Coast to Coast
Yes, the taper they have on the air-screws is stupidly steep... but it's the same problem on the VM38s, which is why I adjusted them with a wideband - just too heavy handed to make them work decently any other w
I was blown away coming from 2 stroke air jet tuning where you go from 0.5 to 1.5 turns and find the sweet spot somewhere in between. Even 1/2 a turn is a big deal on these carbs. I leaned it out a little too much and got stumble after, I fixed it with half a turn WTF! 1.5 turns on the fuel jet alone = vacuum leak symptoms, coughing/backfire, die on idle. That's not good considering temperature changes. Those changes should only be off idle throttle crispness and better idle.

Im going to do a kit review here on the tm's here when I can. I got it tight but I'd rather have spent the energy on the FCR's again. Even my bowl specs where off on the TM's factory, I followed the directions and it was worse. I got it good with the old fashioned blowing into the fuel hose trick with angles, which I made sure the angle were right with my FCR's that were set with measurements.

I probly could have got deltoros for the same price, they are a lot smaller body wise, not sure how they perform, I have heard good things. I doubt they come close to an FCR, I don't think anything does other than FI. I see these super built xv 'pintrest' builds and I laugh they have the god awful stock carbs. Some of these builds like 15k investment but the leave the shittiest of shit carbs (Hitachi). RIP those builds. Bike looks fantastic but the stock carbs are so bad a 1k build with any flat slides will leave you in the dust. And that's why I spend 20 hours fucking with this shit cause the power gains are worth it.
 

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