Carbs make more power than FI?

[mekilljoydammit]

I'm sure with modern materials you could make some pretty impressive buggy whips too.

[Truckedup]

I'm sure with modern materials you could make some pretty impressive buggy whips too.

A buggy whip is just an accelerator pedal....

[turbo2256b]

Truckedup, i follow you about the mcycle carbs, i remember them well from wrenching on the early Z-1 Kawez. Remember Dennis Dean/ Denco from Calif., but back to carbs, somebody has just recently come out with a supposedly revolutionary carb which reduces parts in the throat area, mabye somebody remembers it??

At one point was going to go with 6 Quicksilver carbs when they first came out on my 250 ford 6. Constant velocity is an advantage also they claimed they were automatically altitude compensated. thoughts about pikes peak entered my thoughts EFI wasnt really around back then.

[mekilljoydammit]

A semi-serious question... the carbs I know most are Weber one-choke-per-cylinder jobs, with swappable emulsion tubes to change the circuit, in addition to more normal stuff like main and air bleed, idle mixture stuff and accelerator pump stuff. Does tuning the emulsion circuit get to be where the money and expertise is on Holley style stuff? Or do open plenum manifolds kind of just neuter some of the need to mess with emulsion circuits, or is it more of a "you bolt this part number Holley on these sorts of engines and they all want a similar fuel delivery curve"?

[Brian P]

Bikes almost always have slide valve carburetors...Do anyone think they are capable of more top end power than a auto butterfly carburetor if both are tuned properly for the engine? Slide valve carbs do not have a fuel coming from a central location like the venturi cluster on an auto carb...The fuel comes from a single needle jet on the bottom wall of the throttle bore , At WOT there is literally no obstruction to air flow

I do bikes, I don't do cars other than drive them (sometimes hauling bikes).

Variable-venturi CV carbs are capable of functioning decently over a wider speed and load range than fixed-venturi automotive carbs. Fixed-venturi carbs are fighting a battle between having decent driveability but restrictive on top (if on the small side) or being unrestrictive but suffering from driveability issues because of insufficient vacuum signal (if on the big side). I know that all sorts of crutches were made to cover this up (vacuum secondaries, etc) but the motorcycle-type CV carb simply doesn't have this issue. Plenty of auto engines from back in the day were simply allowed to be strangled. I'm not talking about the high performance stuff, I'm talking regular daily drivers. Most Chrysler 225 Slant Sixes were strangled with a single-barrel carb. The modern 3.6 litre 6 cylinder (Pentastar) in my van is actually slightly smaller displacement but makes nearly three times the power of the strangled Slant Six.

Ford tried variable-venturi carburetors but screwed it up. I don't know the details of those carburetors, only that everybody hated them. Don't know why.

But with all that ... My roadrace bike (Yamaha FZR400) has a bank of four Mikuni BDS carburetors. I can't get rid of a part-throttle rich spot no matter what I've tried, although it's not much of an issue on the track because it's only at that RPM and throttle combination for any length of time on the cool-down lap. It appears that high revs and part throttle draw enough air through the main circuit to pull fuel from that circuit and the throttle is open far enough to draw all it can through the slow circuit, and the two together are too much. If I lean out the slow circuit it's too lean on takeoff from a stop. If I lean out the needle it's too lean somewhere else.

Fuel injection? No issue. You want to lean it out at 8000 rpm part throttle? Open up the laptop and change the map. No wrenches required. You can even data-log how the engine behaves on track, and all the serious race teams do that.

[turbo2256b]

FORds variable Variable-venturi s worked great but had to be very accurately assembled and adjusted. Due to production tolerances they didnt do so well

[Truckedup]

Bikes almost always have slide valve carburetors...Do anyone think they are capable of more top end power than a auto butterfly carburetor if both are tuned properly for the engine? Slide valve carbs do not have a fuel coming from a central location like the venturi cluster on an auto carb...The fuel comes from a single needle jet on the bottom wall of the throttle bore , At WOT there is literally no obstruction to air flow

I do bikes, I don't do cars other than drive them (sometimes hauling bikes).

Variable-venturi CV carbs are capable of functioning decently over a wider speed and load range than fixed-venturi automotive carbs. Fixed-venturi carbs are fighting a battle between having decent driveability but restrictive on top (if on the small side) or being unrestrictive but suffering from driveability issues because of insufficient vacuum signal (if on the big side). I know that all sorts of crutches were made to cover this up (vacuum secondaries, etc) but the motorcycle-type CV carb simply doesn't have this issue. Plenty of auto engines from back in the day were simply allowed to be strangled. I'm not talking about the high performance stuff, I'm talking regular daily drivers. Most Chrysler 225 Slant Sixes were strangled with a single-barrel carb. The modern 3.6 litre 6 cylinder (Pentastar) in my van is actually slightly smaller displacement but makes nearly three times the power of the strangled Slant Six.

Ford tried variable-venturi carburetors but screwed it up. I don't know the details of those carburetors, only that everybody hated them. Don't know why.

But with all that ... My roadrace bike (Yamaha FZR400) has a bank of four Mikuni BDS carburetors. I can't get rid of a part-throttle rich spot no matter what I've tried, although it's not much of an issue on the track because it's only at that RPM and throttle combination for any length of time on the cool-down lap. It appears that high revs and part throttle draw enough air through the main circuit to pull fuel from that circuit and the throttle is open far enough to draw all it can through the slow circuit, and the two together are too much. If I lean out the slow circuit it's too lean on takeoff from a stop. If I lean out the needle it's too lean somewhere else.

Fuel injection? No issue. You want to lean it out at 8000 rpm part throttle? Open up the laptop and change the map. No wrenches required. You can even data-log how the engine behaves on track, and all the serious race teams do that.

The British like SU CV carbs for their sports cars..The Mikuni flatslides are not CV and are noted for instant throttle response.Some racers don't like them because the response may break the rear tire loose....My Ducati 900 two valve engine has two 38 MM Mikuni BDST, a down draft CV carb....Ducati guys hate them, mid range richness that doesn't want to resound to tuning.....I messed with pilot jets, needle position and air bleeds just like the other guys....Then I lowered the float level 2 MM..They all told me that's not the right way too do it.....The mid range rich cleared up, idle and fuel mileage improved, and the top end is stronger....
Guy who race carbureted Ducatis use Keihin FCR's, it's a non CV and everyone likes the better throttle response...What works on a big two cylinder may not work on a smaller 4 cylinder...............

You may get a smile out of this....My dinosaur 650 Triumph record holder with supposedly to large 34 Mikuni TM's responding to full open snap twists of the throttle.. A well tuned Triumph with stock 30 MM carbs would backfire through the carbs if you do this...

https://www.flickr.com/photos/75289626@ ... ed-public/

[turbo2256b]

Having adjustable/ jet able orfaces of air and fuel in all circuits works out the best. Most have no idea other than changing a main jet as to how to tune a carb. Main jet is a cruse jet most dot know that. Just changing the main jet might do well at the drags but street and and road race not so hot. Started doing this stuff back in the mid 60s with no AF meter.

[Truckedup]

Having adjustable/ jet able orfaces of air and fuel in all circuits works out the best. Most have no idea other than changing a main jet as to how to tune a carb. Main jet is a cruse jet most dot know that. Just changing the main jet might do well at the drags but street and and road race not so hot. Started doing this stuff back in the mid 60s with no AF meter.

In a vehicle turning 2000 rpm at 50 mph with high vacuum ,an auto carb may still be seeing a major influence from the idle transfer slots..A mechanical slide bike carburetor has a large overlap between the four fuel circuits..

[turbo2256b]

Having adjustable/ jet able orfaces of air and fuel in all circuits works out the best. Most have no idea other than changing a main jet as to how to tune a carb. Main jet is a cruse jet most dot know that. Just changing the main jet might do well at the drags but street and and road race not so hot. Started doing this stuff back in the mid 60s with no AF meter.

In a vehicle turning 2000 rpm at 50 mph with high vacuum ,an auto carb may still be seeing a major influence from the idle transfer slots..A mechanical slide bike carburetor has a large overlap between the four fuel circuits..

setting up a carb to run on the idle circuit at desired cruse speed usually gives the best economy

[peejay]

And just how many dead bodies do you see at busy intersections that have died from SMOG??
NONE!!

Given that more smog would make it harder to see those piles of dead bodies, that’s a difficult question to answer.

4000 in five days, 8000 more in the subsequent months, in London, 1952.

https://en.wikipedia.org/wiki/Great_Smog_of_London

The good old days weren't...

[AMXstocker1]

i thought this was an interesting setup anyone familiar with the Dambest E-jet?

[swampbuggy]

Interesting indeed, Mark H.

[David Redszus]

I can't get rid of a part-throttle rich spot no matter what I've tried, although it's not much of an issue on the track because it's only at that RPM and throttle combination for any length of time on the cool-down lap. It appears that high revs and part throttle draw enough air through the main circuit to pull fuel from that circuit and the throttle is open far enough to draw all it can through the slow circuit, and the two together are too much.

Two items to consider:
A fuel delivery system based on pressure differential is subject to superposition pressures due to pulsed air flow.
At certain engine speeds the inlet air pressure curves will stack to produce both an alternating high pressure and low pressure. High pressure causes a fuel flow reversal while low pressure causes excess fuel to be delivered.

Carbs deliver fuel based on the difference in pressure between the venturi and float bowl pressure which is open to the atmosphere. A partially closed throttle at high rpm will produce a very strong fuel signal.

Once upon a time we built a carburetor flow bench that measured mass air flow, air pressure and fuel signal pressure.
It was not easy to match the fuel signal to the air mass flow. But that was a dry system simulation; the presence of wet fuel would change the numbers more radically.

[Tuner]

My roadrace bike (Yamaha FZR400) has a bank of four Mikuni BDS carburetors.carburetors.

I can't get rid of a part-throttle rich spot no matter what I've tried,

although it's not much of an issue on the track because it's only at that RPM and throttle combination for any length of time on the cool-down lap. It appears that high revs and part throttle draw enough air through the main circuit to pull fuel from that circuit and the throttle is open far enough to draw all it can through the slow circuit, and the two together are too much. If I lean out the slow circuit it's too lean on takeoff from a stop. If I lean out the needle it's too lean somewhere else.

Have you tried more slide cutaway?

Click this and scroll down. http://www.iwt.com.au/mikunicarb.htm