Carb butterfly valve wont open fully. Help please

[MystyBlue2]

Hi folks i have my mercury 40hp 2 cyl 2 stroke finally mounted and control cables fully linked and correctly adjusted. I THINK....

Problem im having is after following my manual for link and sync and carb set back to factory settings, now when i engage forward gear and full throttle on the control box the butterfly valve is only opening about a quarter of the way, its fully closed at idle and at WOT its nowhere near horizontal.

If i remove throttle cable from throttle cam i can manually give it throttle until the butterfly is horizontal but not with the cables attached.

Ive started at the beginning again and followed manual procedure again incase i did it wrong but getting the same result, ive tried making the throttle cable a bit shorter at barrel side to see if it pulled the throttle cam a little more but to no avail.

The carb roller cam is correctly sitting at correct gap from throttle cam at idle and rubber throttle stopper is flush snug at the back end.

Ive played around with roller cam and i can either have the butterfly fully closed at idle and a quarter open at WOT, Or fully horizontal at WOT but wont fully close at idle.

I have read an article online that some manufacturers do this deliberately to "detune" carbs by restricting air flow aswell as fitting smaller fuel jets,

It seems like there needs to be another 2 inches of cable travel to keep opening the butterfly up but its fully extended with no more travel.

Does this sound like this 40hp could well be a 30hp or do you guys think its a issue on my behalf or cable/synchronisation problem.

Cheers folks

 

[BruceK]

I'd say you need a smaller bell crank. Is the command lever from the 40 for the 40 or from your old 25?

 

[MystyBlue2]

I'd say you need a smaller bell crank. Is the command lever from the 40 for the 40 or from your old 25?

Its a total different control box and cables altogether these cables fit under cowl whereas the 25 i had the cables attached at side of motor base outside the engine.

Dont mean to sound thick but what is a bell crank? Lol cheers

 

[BruceK]

This concept

 

[MystyBlue2]

Ah rite ok so like the throttle cam configuration and the throttle linkage with the threaded barrel that snaps onto the ball part of throttle cam?

I will take a look at this part tomorrow and will maybe order a new one if they are cheap enough,

Im also maybe thinking that its a throttle linkage/ broken throttle cable problem as if i disconnect the throttle cable at engine side i can move the throttle cam assembly by finger and achieve WOT with butterfly at 90 degrees,

Once i put the cable on the throttle linkage pin the range of cable at throttle cam area from idle to full throttle is about 2 inches and butterfly valve is only open a quarter of the way and seems like it needs a further 2 inches of travel to achieve a full WOT and 90 degrees.

Is this a possibility and could old worn cables have this much "slack" to cause this lack of travel issue? Or do you guys think im possibly looking in the wrong place?

Any other suggestions on what could cause this as i am completely stumped and racking my brains and cant find much info online?

Cheers

 

[BruceK]

I dont think it's a bellcrank on the carb, but rather one in the motor housing or the throttle controls themselves.

 

[MystyBlue2]

Okie dokie is there any way i could test to find out if its the mechanism in the control box without getting a new box? There are scary expensive lol

I have found the other crank for throttle that connects via the throttle link rod to the throttle cam £30 on ebay so il get that and try that.

Cheers BruceK

 

[BruceK]

Well before you rush out and buy remember the amount of offset and also the differential in radius between in and output dictates the difference in throw. So you will need to calculate the bell crank size required.

 

[BruceK]

If the crank is based on horns then the ratio is pretty easy to determine as it's just the radius differential. it does however become more complex if it's a circle where you can have offset and radius differentials

 

[MystyBlue2]

If the crank is based on horns then the ratio is pretty easy to determine as it's just the radius differential. it does however become more complex if it's a circle where you can have offset and radius differentials

Oh god!! Lol i thought they would all be the same hahaha ah well everyday is a school day and looks like im going to learn something new. Il delve a bit deeper on this before i buy then. Thanks for your help mate il keep you posted with my findings and results!

 

[BruceK]

I'm not suggesting you fabricate one or anything similar through a market search but that somewhere along the line between throttle lever and carb there is a mismatch. They have either been assembled incorrectly or are a mismatched pairing. However understanding the principal of bell crank throws will help you in solving which of the issues it is and how to go about remeding it.

i.e. you can turn any length of throw one distance into any length of distamce throw another simply by observing the ratios of radius length and offset from the perpendiculars.

 

[MystyBlue2]

I'm not suggesting you fabricate one or anything similar through a market search but that somewhere along the line between throttle lever and carb there is a mismatch. They have either been assembled incorrectly or are a mismatched pairing. However understanding the principal of bell crank throws will help you in solving which of the issues it is and how to go about remeding it.

i.e. you can turn any length of throw one distance into any length of distamce throw another simply by observing the ratios of radius length and offset from the perpendiculars.

Ive just measured my cable,

Dont know if this is the way to do it but this is what i did and found...

Put control box in forward and WOT,

Made a mark on the cable on the thread where the black plastic slider ends, Then put control box back into neutral and measured from the back of plastic slider to my mark amd found that total range of motion is 1 3/4 inch.

This was done with the cables removed from engine to ensure total free movement and when i repeated this procedure with cables linked up the range dropped to 1 1/2 inch!

If i remove the cable (engine side) and move the cam by hand from idle to WOT (butterfly fully open) the total range is 3inches.

Looking at replacement cables on ebay they say in item description that the cable has 3" range which is double what im getting.

I think this could be a cable issue, maybe wrong cables to begin with?

Is it possible to just replace just the throttle cable as gear cable is selecting gears just fine or would i have to replace the throttle and gear selector cable together?

Cheers

 

[Jamie Dundee]

Are you sure the butterflies are intended to fully open on your engine? If it is a down tuned motor fully opening the air intakes with smaller main jets might not do the engine a lot of good. My last outboard didn’t fully open the butterflies but did produce the rated power output.

 

[MystyBlue2]

Are you sure the butterflies are intended to fully open on your engine? If it is a down tuned motor fully opening the air intakes with smaller main jets might not do the engine a lot of good. My last outboard didn’t fully open the butterflies but did produce the rated power output.

I honestly dont know as the engine was already removed from boat and remote box was seperate and disconnected from motor when i viewed and purchased. I thought it was standard practice that carb butterflys should always be tuned to be fully open to achieve peak performance UNLESS it has been downgraded from factory.

My motor cowl has 40hp but could also just be a £10 sticker, the motor doesn't have any info on transom bracket. All i can go by is the engine serial which is 0G083344 and when typed into crowley marine or marine engine.com its listed as a 40hp not the down tuned 30 so i cant think of any reason as to why the carb would be restricted or butterfly would have part motion. Would have been alot easier if I'd seen it linked up on previous boat to see if this is normal but all i can go by is the generic rule of thumb that it should be perfectly horizontal no more no less and it's not.

I think im going to try a new cable for £30 if it dont work then back to square 1 but atleast i have a spare cable

 

[Jamie Dundee]

I actually tried manually opening the butterflies on my last outboard out on the water. Didn’t go any faster, just made more noise. Have you tried contacting Mercury or searching out an owner‘s forum? Might save you 30 quid.

 

[MystyBlue2]

I haven't tried to contact mercury as its in the states unless there is a uk centre i could contact...maybe, and my local mercury dealer wants a months worth of overtime to even take a look and wont disclose any info on this matter to me purely for his own financial gain.

I could possibly just wait and see how it performs on the water under load and maybe wire up a tach to get a better pic of what the engine is doing then go from there i suppose.

Got a long wait now though the snow has arrived lol. Cheers

 

[BruceK]

Are you sure the butterflies are intended to fully open on your engine? If it is a down tuned motor fully opening the air intakes with smaller main jets might not do the engine a lot of good. My last outboard didn’t fully open the butterflies but did produce the rated power output.

If he can fully open by fingertip direct to carb, and, the cable throw is meant to go 3 inch but is only going 1.5 inch. AND with 1.5 inch the throttle is half open.

Jeez guys. Join the dots.

 

[BruceK]

Dan your cables are worn. However before buying new lay them out from the throttle lever in a straight line. Fully extend them out and apply a bit of tension on the end. Fully retract. If you are getting a 3 inch throw then the cables are worn (sheaving, you are loosing on the curves) If you are not getting a 3 inch throw your gearshift is the issue.

Keep as many bends as possible out of the routing and the shallower the bend the better

 

[MystyBlue2]

Dan your cables are worn. However before buying new lay them out from the throttle lever in a straight line. Fully extend them out and apply a bit of tension on the end. Fully retract. If you are getting a 3 inch throw then the cables are worn (sheaving, you are loosing on the curves) If you are not getting a 3 inch throw your gearshift is the issue.

Keep as many bends as possible out of the routing and the shallower the bend the better

Brill i will do that ASAP there is only 1 loop in the route and its about 10" diameter and kept away from all other wires and fuel line to allow as much free movement as possible, it has a nice smooth curve from gunnel through the rubber gaiter into the well where it has another smooth curve where it goes under the cowl to be linked up. Cheers Bruicek i will give your way a go!

 

[MystyBlue2]

Dan your cables are worn. However before buying new lay them out from the throttle lever in a straight line. Fully extend them out and apply a bit of tension on the end. Fully retract. If you are getting a 3 inch throw then the cables are worn (sheaving, you are loosing on the curves) If you are not getting a 3 inch throw your gearshift is the issue.

Keep as many bends as possible out of the routing and the shallower the bend the better

Hey Bruce bit of an update...
I tried your way of laying cables out and had 3 inches of travel, So i borrowed a cable from a mate and had the same issue,

I thought this could be possible for both cables to be worn but chances would probs be slim so i put my cable back on and this time attached the cable at throttle at engine side and disconnected from control box and layed them out in a straight line to see if the lack of throw was in the box....

Well it was, the throttle pivot arm in control box had jumped out of place and altered all the angles of throttle and gear selection movement,

With 4hrs of shouting, swearing and losing my patience i finally managed to put all pivots into the right places and now the throttle pivot has the 3" of travel back. Plenty of grease on all connections and promising myself to NEVER take apart a box again and its all sorted.

When i got the box the saftey lanyard switch has been removed (ive wired up a dash mounted one now) but i suspect in them doing so the mechanism has be moved and has not been put back properly resulting in stiff and lack of movement in control box.

Thanks for all your help and hope this could help someone in the future with similar issues.