Technical Questions - Slide Drilling/Vac Ports

10724 Views 31 Replies 12 Participants Last post by smithman, Jul 30, 2018

Aduro

Discussion Starter · Jul 13, 2010

Figured I'd dump a couple questions into the same thread as they dont really warrant a new thread each.
Anyone else with little niggling questions feel free to hijack.

*NOVEL AHEAD*

First up, drilling carby slides,

*flame disclaimer: I have no intention of trying this half-assed to drag moar powah from my little twofiddy, i read a write up somewhere*

What effect does this have? pros, cons? Read a writeup about doing it to an RVF, my mate has an RVF and keeps asking how i can modify it to make it better seeing as hes to lazy/retarded to do it himself.
Theoretically Fattys tuning guide of a high flow filter and larger jets/needle washers could be applied to any bike with carburetors?

Second:
I can only imagine how out of sync my carbs are atm as they haven't had a balance in ages but theyve been fucked with a fair bit, so Ive looked up a bunch of write ups and I'm just going to maguyver up a Manometer from some vinyl tubing and two stroke oil, but i cant work the VACUUM PORTS out. Stud mentioned there may not be four tubes and I'd have to fit some.
So far All Ive found is these two tubes (circled green) can anyone confirm if these are the vacuum ports? If so how the hell do i get four readings from these? (yeah im a tad stupid)

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Cheers guys.

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1 - 20 of 32 Posts

GSXR750K4

· #2 · Jul 13, 2010

RE: Questions.

Don't know about CBR carbs.. But Alot of peeps have drilled out the throat on 12mm pocket bike carbs to 14mm.. Personally, I just purchased the proper 14mm carbs..

As long as the throat was not drilled out more than the width of the slide it worked well..

Aduro

Discussion Starter · #3 · Jul 13, 2010

RE: Questions.

The write up i read was about drilling the actual slide to improve mid range response, the actual diameter of the throats stayed the same i think.

GSXR750K4

· #4 · Jul 13, 2010

RE: Questions.

Drill the slide?? As in holes through it??

:huh:

sir.b

· #5 · Jul 13, 2010

RE: Questions.

Aduro said:
Theoretically Fattys tuning guide of a high flow filter and larger jets/needle washers could be applied to any bike with carburetors?

If you increase the amount of air an engine is able to breathe in, and compensate for this air with the right amount of additional petrol, you get more power. That's a constant on all fuel burning engines.

Justin.

Matt

· #6 · Jul 13, 2010

RE: Questions.

you also need to get the exhaust out more quicklier

zane

· #7 · Jul 13, 2010

RE: Questions.

if u think the 2 breather tubes are the vac ports get someone else to do work on bike. the vac ports are not on the carbs they're on the cylinder head. download the manual and read it!!!

and about drilling slides, this isnt a half arsed method, this is good old tuning of carbs, changing jets, needles, and springs are all cheap ways of getting more out of the carbs.

first get the basics done. have ur bike serviced with new plugs and have carbs balanced. so ur starting off with the bike running at a 100%

sir.b

· #8 · Jul 13, 2010

RE: Questions.

Matt said:
you also need to get the exhaust out more quicklier

Will help if you do, but no, it's not necessary.

The engine uses power to overcome the air intake restriction, and it also uses power to overcome the exhaust restriction, but these two losses are independent. Power can be gained by improving the flow of either or both.

Justin.

Matt

· #9 · Jul 13, 2010

RE: Questions.

i was told that on the cbr (by stasi, from rb racing) that rejetting without new pipes is a waste of time...

Stavros

· #10 · Jul 13, 2010

RE: Questions.

GSXR750K4 said:
Drill the slide?? As in holes through it??

:huh:

There are small holes in the bottom of the slide next to the needle. Drilling them out will affect the movement of the slide and therefore the response time of the carbs.

Yes, you drill a hole in the slide, but the drill goes in parallel to the needle, not down the throat of the carburettor.

I think RC162 solved some of his tuning issues by enlarging the existing holes in his slides, or possibly even drilling new ones. His case was fairly special, though. He was running open carbs without any kind of filtration or airbox, and he didn't even have the velocity stacks fitted because they would have fouled on his frame rails.

Tromac

· #11 · Jul 13, 2010

RE: Questions.

sir.b said:
Matt said:

you also need to get the exhaust out more quicklier

Click to expand...

Will help if you do, but no, it's not necessary.

The engine uses power to overcome the air intake restriction, and it also uses power to overcome the exhaust restriction, but these two losses are independent. Power can be gained by improving the flow of either or both.

Justin.

If you aim for power, then it would be silly not to reduce the increased back pressure, due to needing to force a larger volume of fluid out of the same orifice, as it's going to take more power to push that exhaust away.

Personally, while one would probably still get away with a power increase, I would see it as a waste of time as I'm a fan of things being done properly rather than almost literally half arsed.

sir.b

· #12 · Jul 13, 2010

RE: Questions.

Matt said:
i was told that on the cbr (by stasi, from rb racing) that rejetting without new pipes is a waste of time...

We weren't talking about a cbr, but the point stands for the cbr as well. Perhaps Stasi was saying you may as well change the pipes at the same time as getting it rejeted if you're going to the trouble? I dunno. Anyway, if you significantly derestrict the air intake, you'll lean out, and will need to rejet, the guys who have run open air setups have proven that significant air intake mods require rejeting..

Justin.[hr]

Tromac said:
If you aim for power, then it would be silly not to reduce the increased back pressure, due to needing to force a larger volume of fluid out of the same orifice, as it's going to take more power to push that exhaust away.

I think we're all in agreement that exhaust mods increase power. Matt made it sound like exhaust mods were necessary to see benefit from modding the intake, which isn't the case.

Personally, while one would probably still get away with a power increase, I would see it as a waste of time as I'm a fan of things being done properly rather than almost literally half arsed.

I made a cold air intake for my TT Soarer. It cost a couple of bucks in parts, it fed cooler denser air to the airbox, and it didn't attract attention from police or the EPA. They run a little rich, so no re-tuning necessary. I don't see why you have to spend half the value of the bike on a full system just because you want to do some cheap work on the intake for a pony or two

Justin.

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richo

· #13 · Jul 13, 2010

RE: Questions.

sir.b said:
Matt said:

i was told that on the cbr (by stasi, from rb racing) that rejetting without new pipes is a waste of time...

Click to expand...

We weren't talking about a cbr, but the point stands for the cbr as well. Perhaps Stasi was saying you may as well change the pipes at the same time as getting it rejeted if you're going to the trouble? I dunno. Anyway, if you significantly derestrict the air intake, you'll lean out, and will need to rejet, the guys who have run open air setups have proven that significant air intake mods require rejeting..

Justin.

What he's saying aligns perfectly with what you said Justin..

If you allow more air into the system or make it easier to clear the exhaust you need to compensate for this added flow with more fuel.

If you add more fuel without freeing up intake or exhaust, you're wasting your time.

Aduro

Discussion Starter · #14 · Jul 13, 2010

RE: Questions.

zane said:
if u think the 2 breather tubes are the vac ports get someone else to do work on bike. the vac ports are not on the carbs they're on the cylinder head. download the manual and read it!!!

and about drilling slides, this isnt a half arsed method, this is good old tuning of carbs, changing jets, needles, and springs are all cheap ways of getting more out of the carbs.

first get the basics done. have ur bike serviced with new plugs and have carbs balanced. so ur starting off with the bike running at a 100%

Done a shitload on the bike already, fork oil and seals +20c mod about 7000km ago, steering head bearings round the same time. (At the service day a few months ago was shown how to do all this)
Chain and sprockets have less than 500km on them

Plugs and oil are less than 2000km old. Same goes for air and oil filters. Cleaned the carbys and replaced the manifold rubbers around the same time, aswell as a brake bleed, all done myself with no issues, about as good as its going to get atm apart from a carby balance which is why i was asking about the vacuum ports in order to make myself a budget manometer so don't go calling me a total noob.

As i understand airfilter and rejetting is relatively inexpensive compared to a whole new exhaust system, and shes already got a straight through can on her.

Would it be worth the trouble drilling the slides on an RVF? having ridden it the V4 doesn't seem as responsive as the I4 which seems to be much more free revving and climb much faster even in neutral.
Mind you I'd still love an RVF due to the extra power, much longer gearing and amazing corner feel.

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sir.b

· #15 · Jul 13, 2010

RE: Questions.

richo said:
What he's saying aligns perfectly with what you said Justin..

If you allow more air into the system or make it easier to clear the exhaust you need to compensate for this added flow with more fuel.

If you add more fuel without freeing up intake or exhaust, you're wasting your time.

What the? Read it again Richo. He's saying no point rejetting unless you do the exhaust, ie, air intake mods never need a rejet.

Justin.

Matt

· #16 · Jul 13, 2010

RE: Questions.

from my experience i have found that an increased diameter pipe on a cbr seriously drops low and midrange power on a cbr250rr. i am lead to beleive that this is due to their not being enough back pressure on the system. (what that means, i'm yet to find out to my satisfaction)

what i would really love to do is find out the stock power to a rejetted and then rejetted and new exhaust is. but that would require three dyno runs, which would be money better spent on other go faster bits for the bike.

Johnnie5

· #17 · Jul 13, 2010

RE: Questions.

Aduro said:
Would it be worth the trouble drilling the slides on an RVF? having ridden it the V4 doesn't seem as responsive as the I4 which seems to be much more free revving and climb much faster even in neutral.
Mind you I'd still love an RVF due to the extra power, much longer gearing and amazing corner feel.

www.400greybike.cc

specific instructions re slides on the 400

iirc 2.5mm drill and some washers, makes a huge difference along with the right size jets

richo

· #18 · Jul 13, 2010

RE: Questions.

sir.b said:
richo said:

What he's saying aligns perfectly with what you said Justin..

If you allow more air into the system or make it easier to clear the exhaust you need to compensate for this added flow with more fuel.

If you add more fuel without freeing up intake or exhaust, you're wasting your time.

Click to expand...

What the? Read it again Richo. He's saying no point rejetting unless you do the exhaust, ie, air intake mods never need a rejet.

Justin.

That's not how I read it. To be fair, with a restrictive exhaust you're unlikely to get signifiant gains from induction mods with anything less than forced induction.

Exhaust and intake are symbiotic, big improvements in one won't work without improvements in the other. And to keep fuel/air constant you need to only add fuel when you get more air.

I don't know if I'd go as far as to say "useless", but I would agree that unless you're doing something pretty radical to the airbox (and keep in mind that there's a point where you can't flow more just by opening it up more..) the stock jetting would be fine. The stock cibby zorst is going to hold up the flow pretty bad.

Tromac

· #19 · Jul 13, 2010

RE: Questions.

Matt said:
from my experience i have found that an increased diameter pipe on a cbr seriously drops low and midrange power on a cbr250rr. i am lead to beleive that this is due to their not being enough back pressure on the system. (what that means, i'm yet to find out to my satisfaction)

Needing "back pressure" on a naturally aspirated four stroke is a myth. With a turbo some pressure is needed for the turbine to spool up, and with a two stroke with an expansion chamber engineered correctly there will be a negative pressure wave created such that it will suck the air into the cylinder at the right moment (almost a form of forced induction), but a normal engine does not require back pressure. The pistons will still move up and down, the valves will still open and close and the crank will still turn. Pressure puts strain on the engine and takes away power from turning the crank.

People probably confuse "needing back pressure" with tuned length exhaust setups, which work very similar to scavenging two strokes. This is where the engineers take advantage of the pressure waves created in the exhaust system to extract the exhaust gasses and increase efficiency.

Increasing the diameter of a pipe with a flowing fluid will decrease the fluids velocity and increase pressure. Conversely, decreasing the diameter will increase the fluids velocity and lower the pressure. Hence, a balance is needed and while not immediately intuitive, choosing too large a pipe will create too much pressure and cause inefficiency due to the extra strain on the engine.

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Matt

· #20 · Jul 13, 2010

RE: Questions.

thats a new one.

how then do you explain the loss of power?

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