Race / Performance carbs

[Scubaproh20]

I was looking at my 40073A carb and its dissapointingly under engineered. The jet is bulky and unsophisticated and the flow dynamics look goofy with the auto choke. I'm thinking about the obvious like trimming the 1/16th'' extra thread off the mounting screws for the throttle and choke plates for (improved flow) i know its minimal and not much to be done,

is there a supplier of racing carbs or reeds ? I'm sure there is more power to be had by removing the inefficiencies in fuel dispursion Slide carbs ? any neat tricks that will work with the timing advance setup intermingled with my throttle?

 

[levi_tsk]

Re: Race / Performance carbs

i wouldnt mess with those screws.... theyre bradded on the end and if you go filing em down you run the risk of taking too much and then the screw can come loose and go into the engine and youve got a mess... the rest of the gang on here will tell you this - you can squeeze a little extra hp out of an engine by modding it but if you REALLY want more hp - buy a bigger motor

 

[Scubaproh20]

Re: Race / Performance carbs

My boat is tiny, bigger motor means more weight aft , I know i have 5 posts here but i'm not new to the internal combustion engine. Anybody ever rigged slide float carbs or mikuni type carbs or carbon fiber reeds or otherwise improoved reeds to a marine engine? 70's tech is basic to say the least (But reliable). My 15 evinrude was a 9.9 with an improved carb, intake and exhaust. Modern 700cc 2 strokes make 120hp @7500rpm no reason a 716cc shoudn't make 60 @4500rpm

 

[bob johnson]

Re: Race / Performance carbs

there is an aftermarket reed available that is said to help performance.....most of the mods probably come from exhaust porting mods.. and larger carbs, maybe shaving heads a little


playing with timing....you give up reliability when you are on the edge of making the motor go faster!!

I am sure the rpm limits has to do with the weight of the components and the strength of the rods and bearings... not apples and apples when you talk about a whole other motor!!!!!!!!!!

just because it is the same size in CCs doesnt mean it is even close to the same DESIGN...flow characteristics are everyting...

you could add 10% more hp and not go 2 mph faster!!! its a boat!! not very efficient

good luck

bob

 

[Scubaproh20]

Re: Race / Performance carbs

Bob you're right, its way too much work to get er done right. Even puting a different carb and getting it to work with the timing system would be a nightmare. I think I am going to gather up parts, a piston port snowmobile engine probably an arctic cat or polaris jet ski that was wrecked and build a jet boat.

 

[garzilla]

Re: Race / Performance carbs

Most two smokers (performance) rely pretty heavily on a tuned exhaust too no?

Pretty bad arse to have a wicked stinger exhaust system coming out of your tiller!

 

[tx1961whaler]

Re: Race / Performance carbs

Your particular motor was not designed at all to be a high performance motor. It was designed as a workhorse boat motor and was made to be dead-arse reliable. That why so many of them are still around after 50+ years. If they were high-strung screamers, chances are they would be in the scrap heap by now.

 

[Scubaproh20]

Re: Race / Performance carbs

Your particular motor was not designed at all to be a high performance motor. It was designed as a workhorse boat motor and was made to be dead-arse reliable. That why so many of them are still around after 50+ years. If they were high-strung screamers, chances are they would be in the scrap heap by now.

This is most certainly true, if your snowmobile stops its a long walk if your boat dies its to the bottom , if you're on big/ moving water.

Back to my original pondering, the engineering is good as far as reliability, but even the carb is basic (only a pulse a jet looks more simple ), the 35 and 28 is the same as the 40 i think just de-tuned versions

1 : Who makes those reeds and reed plates that are supposed to be better? I'd like to check them out


i cant do cdi on a motor like this but we can admit a tbi (throttle Body Injection) gets you less bang than a good carb or mpfi or direct high pressure injection now used on modern cars

so a better carburetor that delivers the same amount of air >(just more effectively) and fuel just mixed better should give a good gain, and not increasing airflow by a larger bore intake exhaust the power band wouldn't be moved to higher rpm with a loss of torque stressing the engines design

Obviously this won't make it a rocket not my goal, but it could however make it better even 5hp would be big for my 450lb boat.

 

[bob johnson]

Re: Race / Performance carbs

Boyesen reeds.

look them up....

bob

 

[metriccrescentwrench]

Re: Race / Performance carbs

One of the guys at ScreamAndFly did reed tests awhile back. For top end power it really wasn't the reed design but rather a cut reed cage that made a difference (less restrictive airflow). In addition to Boysen reeds, Chris Carson Marine reeds have a good reputation in the high performance outboard market.

 

[Chinewalker]

Re: Race / Performance carbs

Your old 40 is also a crossflow motor with a HEAVY flywheel. Spinning it up to make more power can result in busted crankshaft. Newer motors of same cubes are loop charged, which helps explain the power difference on a cube by cube comparison...

 

[Scubaproh20]

Re: Race / Performance carbs

Your old 40 is also a crossflow motor with a HEAVY flywheel. Spinning it up to make more power can result in busted crankshaft. Newer motors of same cubes are loop charged, which helps explain the power difference on a cube by cube comparison...

Reed engines arent as good as piston port or a looped engines, and 2 strokes don't get as aspirate 100% or expirate exhaust 100% because they lack the specal intake exhaust stroke a 4 cycle does. but I'm not going to ever rev this engine I just want to alleviate inefficiencies (the parasitic losses )

 

[bob johnson]

Re: Race / Performance carbs

Reed engines arent as good as piston port or a looped engines, and 2 strokes don't get as aspirate 100% or expirate exhaust 100% because they lack the specal intake exhaust stroke a 4 cycle does. but I'm not going to ever rev this engine I just want to alleviate inefficiencies (the parasitic losses )

I am not understanding that. I have a LOOP CHARGED engine and it HAS REEDS!!

bob

 

[Scubaproh20]

Re: Race / Performance carbs

It's loop charged to help increase the exchange of air fuel and exhaust as effeciently as possible in the cylinder. looping is an engineered air flow across the intake and exhaust sides. not having looping and being a reed especially a star shaped reed as opposed to the better angled reed pack is the least effecient I'm sure i'm singin to the choir lol

Reeds = ok

reed pack=ok+1

Reeds with lookping=really good

piston port=really good at high rpm

rotary valve=really good

 

[Scubaproh20]

Re: Race / Performance carbs

Darn neither makes reeds for my engine, but I must Thank you Lt. Johnson and Ensign for your insight and the possible future resources.

 

[bob johnson]

Re: Race / Performance carbs

Darn neither makes reeds for my engine, but I must Thank you Lt. Johnson and Ensign for your insight and the possible future resources.

LT COMMANDER !!!!!!!!!!!!!!!!!!!!

dont demote me!!!!!!!!!!!!!!!!!!!!

please


ha ha

good luck

bob

 

[Scubaproh20]

Re: Race / Performance carbs

I actually have a longshaft on a short transom and it is slow to plane and doesnt do it well when i have 3 ppl with fuel and gear, so I ordered the short driveshaft and water tubes to remove the 5'' extension, I'm hoping it drasticaly improves top speed and perfromance. I know its calculators but my 1.75-1 gear ratio 40 hp slip and a 14 pitch prop with 4500rpm should do 30 i'm getting 18 mph with my 8 pitch over revved, and 15mph with my 14pitch under revved, carb and ignition up to spec with the book , I'm thinking the torque on the transom from the LS and the extra drag is being a huge effect

 

[ONERCBOATER]

Re: Race / Performance carbs

sounds like you would do well with an 11 or 12 pitch prop.... or try venting your 14 and see if you can get far enough up the RPM range to make enough power to fly The long shaft to short shaft should help alot.... it will also help to adjust height of motor when you get your short shaft on... by adjusting up until you start losing the speed gains you will see or until you cavitate too much in turns. The comment about shaving the heads for the johnyrudes is correct I wound up doing it to my 1983 15hp in order to true up the surfaces due to a long ignored headgasket leak by the previous owner. it is definitely faster than the average 15hp... but not worth it unless you have a reason to do it.... when i have to go into it next time for something i am going to see what i can do on the reed cage end of things... and if i ever have to split the case for any reason in future i will port a bit and polish......
that said... i agree with the minimal gains do to the design of the engine they were not meant to be speed demons just dependable long lived workhorse machines.... (think 300 straight 6 ford or 292 straight 6 chevy type performance) unfortunately when i have an engine that works... i tend to want to mess with it amd make it work better, unfortunately they are not piston ported not rotary ported... and as such the gains are minimal at high RPM.
oh CDI is definitely possible... I considered it seriously when looking at the mercury engine that i built a new wiring harness for... but the switchbox worked and that was the way she was built to start with so i left well enough alone. The CDI units out there I was considering are intended for the RC industry.... would likely have to mess about with the timing curve to get it right... but I am sure you could get it working if that was really your desire.... likely more trouble than worth if you already have a working engine though.

Sean

 

[Scubaproh20]

Re: Race / Performance carbs

Its not trimmed where its a barge plowing through the water and it doesn't hop or bounce, when its short shaft it will no doubt be adjusted for trim and the 14 pitch is quick with me alone I just dont think the 16' fiberglass whaler with a rude 60 should be tripple my speed, though with the same number of passengers remember my boat is a 14' rowboat with a 40 and a tin top. the whaler had to be over a ton dry ,with me near 1100lbs fully loaded. something doesn't add up

 

[tx1961whaler]

Re: Race / Performance carbs

18 mph is too slow. My 13' whaler should be about the same as your 14' tinny and I'm nearly twice your speed by myself.