28 hp carb on a 20hp motor?

[racefan2]

How will a 1991 Evinrude 20 hp motor work with a 1987 28HP carb on it. They look thike they will bolt up exactly and the only big difference I can see is the carb opening (drew a blank for the part name) size . How much difference will it make in performance by changing just the carb besides the amount of gas it will drink?<br /><br />What I really meen is if I get a 28 HP carb what will I have to do besides bolting it on to get the motor to perform good besides adjusting the lean rich mixture screw?

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

Where is this post. I cannot see it in the forum I had to search to find the post? It finely showed up with the reply. Go figure. I still have the above question.

 

[Chinewalker]

Re: 28 hp carb on a 20hp motor?

The 20hp intake manifold had a restrictor cast into it, and as such it will only draw so much fuel. Putting a larger carb on the motor will make it run like garbage, UNLESS you swap the intake manifold, too...<br /><br />Might be some other tuning differences elsewhere in the motor, too. I am pretty sure the 28 used the "old school" lower unit that had exhaust exit above the prop. The exhaust scavenging characteristics of that lower were different than the through-hub lower end of the 20 (which shared basic chassis with the 30/35hp models). Not sure which was better/worse, but something to keep in mind - more to making power than just dumping fuel in. What goes in, must come out...<br /><br />- Scott

 

[DHPMARINE]

Re: 28 hp carb on a 20hp motor?

Motors are just like pumps.What you pump in has to get out.If you had a clogged exhaust on your stock 20,power is lost,because it can't get out.If you pump too much in (I think the 28hp has a larger venturi) It can't get out,and power is lost.<br /><br />DHP

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

Thanks guys for the answers.

 

[G DANE]

Re: 28 hp carb on a 20hp motor?

Sorry to disagree, but this actually is one of the cases where it does a difference. Changing carb will almost bump it to 25 HP. The 20 has a slightly different cylinder head and carb with restrictor, otherwise identical, same block, exhaust, mid sedtion and lower. I would swap that carb, if I had one. Running at same speeds, I dont think you will notice any change in fuel consumption. Look at this: http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=28;t=027979#000004 http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=28;t=026713#000005 or do a search, the subject has been up more than once.

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

I may just give it a try and see what it will do. If it don/t run good I can just remove it and put the stock carb back on.

 

[G DANE]

Re: 28 hp carb on a 20hp motor?

Post back with results, would be nice to get it confirmed.

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

Well I finely got the time to install the 28HP carb(I had a week of fishing planned and did not want to tamper till the trip was over)I installed it today ond run it in a 45 gallon trash can so it had a load on the prop. It seems to run fine but it got late so I will have to wait till this weekend to test it on the water as it is suppose to storm here tomarrow now it is rain all weekend. Have to wait till front passes

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

Well I finely got it in the water today. It runs on the 28 HP carb on the 20 hp motor. I have a small sneece while the motor is in gear at low RPM every so offen and sneeceing or poping on first startup. till warm. It run better for sure. The best I could get with the 20 hp carb was 24MPH with maybe 2 gallon of gas in the tank. Tonight I have 7 gallon of gas same place river in the same direction and it run 27MPH and it had more but had trouble with porpoising at 27 MPH and I had trim left before it started and I could only get 5000 RPM the exact RPM with the 20 HP carb. Should it not have run at 5500 RPM with the 28 hp carb? I have a 10 x 13 Alluminum prop on the boat and it is a 16 foot semi V aluminum full decked bass boat maybe 700 boat and motor weight.<br /> I beleive I will have to move 1 of my batterys to the front of the boat to help balance the weight a bit. That will be next. I also removed the SE300 fin I have and it just porpoised more and run no faster.

 

[G DANE]

Re: 28 hp carb on a 20hp motor?

Good to hear. The sneezing is caused by lean low speed mixture. Try to turn the low speed needle out 1/8 or 1/4 turn and see if it helps. It can however be that carb needs to have low speed circuit cleaned, that takes that you get it off, disassemble it, remove gaskets needle and floater and soak it overnight in carb cleaner, blow compressed air thru all orifices, including the small ones over throttle butterfly, assemble it again. Gently turn low speed needle in to seated ( no force ) and back out 1 1/2 turn. Start boat and run it in forward gear, pulling boat unrestricted. Turn needle in 1/8 turn at a time, giving motor 15 secs to respond to new mixture. When it starts to sneeze or miss, back out 1/8 turn again. Thats the best setting. Sneezing can be a leaking carb gasket too, but you know if they were fine and tight. What the porpoising concerns, try to move trim pin on motor bracket one hole in, or out, and test. Usually porpoising occurs when bow is heavy, like a pendol, or motor trim position is creating too much bow lift.

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

G DANE thank you for the tips and I am going to redo everything and use your advice on carb setup.<br /> I beleive the carb roller sinc is wrong as the roller is touching the cam and I read in the service it shoul only touch what in gear and mine is touching in netural and changed the wrong thing for idle as I moved the cam on the carb which messed up the roller but it was not right from the start so I will have to readjust and start all over. I did find out one thing I was not getting full WOT as the butterfly was only opening maybe 1/4 inch short of full flat open.<br /> I have tilt and trim on the boat and a 5 inch setback also as I have a PT35 on the boat. I am 1 inch above the bottom of the hull but have room for 1 or more inches of room to move it. higher.<br /><br />There is no weight in the bow so I beleive a bit of weight there will give me more trim power to get more out of it.

 

[G DANE]

Re: 28 hp carb on a 20hp motor?

Do a little trial and error on weight distribution. Sync'n Linck on that motor is basicly that throttle starts to open exactly when roller is in center on the two marks on cam. It is so important, that manual advises to make a tool from a crocodile clip with a piece of wire in it. Place that on empty end of throttle shaft pointing upwards.( throttle shaft movement amplifier ! ) Watch exactly when the first movement starts. Too early opening is equal to low rpm sneeze and if worse "shooting".There is a little pin thru throttleshaft on empty side, should rest against casting at full throttle. WOT throttle is then adjusted by moving the metal stop on the rod that goes from vertical throttle shaft, connecting the gear below cowling to stator plate link. The rod runs like under statorplate, on port side of motor looking from behind.This rod takes over throttle opening, you could say, when cam wont advance it more.The metal stop should be placed exactly to provide WOT. If advancing too much, you can break the little nylon clip on roller bracket. WOT max timing advance is adjusted by turning the little rubber stop screw in starter bracket that stops timer plate rotation. You can set it by hooking a timing gun up to a spark plug lead and setting spark at 4 deg less that required, at starter motor crancking RPM's. The RPMS will make system provede the last 4 deg's of advance when motor runs at higher RPMS. Set 1, carb to roller link, 2, WOT advance, 3, throttle advance rod.<br />I really like those motors, I had one on a 16' semi V dighny, did 28 mph with 2 grown persons, tank and gear. You will get there too.

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

Right not my throttle roller is in netrual and the roller is centered in the marks and touching the cam. It moves almost out of the two marks when throttle is moved just a bit. <br /><br />What is a crocodile clip? Is it a paperclip? <br /><br />If this is true about the roller just tuching in the center of the two marks that would meen my timeing is off a bit as it is sitting dead center of the marks right now in netrual.

 

[G DANE]

Re: 28 hp carb on a 20hp motor?

You will do this a lot easier with a mnaual with photos to guide you. Lots of public libraries have og can get clymer generic manuals for you motor. They will guide you thru the process. When motor is iddling, roller shouldnt touch cam at all. First throttle is given only by increasing spark advance, when stator plate turns. The the roller bracket starts opening throttle butterfly, when stator plate is turnes so much that roller is right between marks. Even more turn af stator plate, and the little rod pushes bracket away from roller.

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

Man I tinkered a bit with the cam roller adjustment or the Sync'n Linck as you call it correctly and have it adjusted to where it is just away from the cam but not I get a sneeze at 1/4 throttle and a mis or wants to stall. I have neighbors that are not all that happy with the motor running for hours off and on so I am just going to take it into the shop where they did all my work on my 9.9. They still only charge 35.00 per hour but they do great work and the motors always run great when it comes back from them as they have found problems quickly and do not part out trial and error style (toss a part on and see if it fixes it and keep replacing till they hit the problem and you get a huge bill)<br />GDANE thank you for the help though and will reply back as soon as I get the boat back but running out of time as it is getting cold fast and will have to get it put away here in a few weeks.

 

[racefan2]

Re: 28 hp carb on a 20hp motor?

I have not forgot but I have not gotten the boat back yet. It has been there 10 days and they still have not even looked at it so I will now have to put it away for the season as the snow is flying now so I am quite upset at this moment. I will keep you all posted

 

[rodbolt]

Re: 28 hp carb on a 20hp motor?

we tried to tell ya it would not work correctly. the 28 had a split gearcase and was an above the prop model and has a different exhaust tuner pipe than the 20. the 25/35 has a much larger opening than the 20 as well. the only way your gonna get anything is to replace the intake or machine it out then remove the powerhead and swap out the exhaust tuners and start playing with props. the split cases on the 28's and the 80-83 25's had a different gear ratio than the through hub EX models as well.<br /> your very lucky that shop will touch it at all. I would not.

 

[G DANE]

Re: 28 hp carb on a 20hp motor?

Rodbolt - Being a newbie, I dont want to start arguing with you old dogs, I know these swaps are experiments, but I did that swap once sucessfully, and if you search old threads, I think John from Illonois did it too, managed to make it turn 700 rmps more on test wheel, but had to richen up the intermediate circuit a bit, to remove an hisitating acceleration issue. I havnt had that. The 1991 20 HP is thru hub, as you can see on the photo of his boat.

 

[rodbolt]

Re: 28 hp carb on a 20hp motor?

yes the 91 20 is. the 28 was not, they did share a lot of block components, but the 20 has a very small exhaust tube compared to the 25/35. exhaust expansion tuning is as critical as carburation,induction and port timing on a two stroke. more so than on a four stroke. gearcase ratios ands propeller designs will come into play as well. I used to play a lot with the little motors myself.<br /> at 75 an hour I dont like to experiment unless the customer is fully aware that its an experiment and agrees to pay the costs wheter it works or not.