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Discussion Starter #1
hello i just picked up a Roper 16t that has an Onan motor in it and was wondering if anybody had some tips on how to get somemore power out of it with out putting thousands of $$$ in it. port and polish? change carbs? headers? any info would be great thanks.
 

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Deere 330 Killer
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johnnyby68 and searstractorfan and boomers_influence are the guys you wanna talk to.


Most guys shave the head, and take a dremel to the carb for starters. Some guys choose to bore them out. The 16hp Onans (BF/MS) came with a smaller than stock bore. So sometimes guys bore them out to standard Onan bore and get some HP there
 

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horse power is nice, but without traction, its wasted money. I would recommend you do a test pull to see what is needed, when i tested my wheel horse i found i had more power then traction.

basic hp is open the intake up with a high flow, and open up the exhaust and run straight pipe if rules allow, most do as far as I am aware
 

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Discussion Starter #5
im not worried about traction as i will be runing cut tires on the back. we are pulling a weight transfer sled, not a dead weight so i feel i need HP to get the sled moving off the line.

i had planed on port matching the carb/intake is there anyway to open up the jets in the carb to get more fuel threw it and running a header is a must in my opinion. i was also wondering if it helps to shave the head on a tractor like it does on a car, and if so how much do you take off?

how much can i bore the motor with out problems and what size piston do you guys recomend?

thanks
 

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trying to stay on the cheap here is what I would try:

check camshaft wear at the lifters lift/ration = cam lobe height

upgrade spark plug to one of them fancy pants types, they are more expensive, but you only need 1, and they do appear to make the engine run better (seat of pants test)

make sure valves are set right

replace fuel hoses, and double clamp every connection (small air leak robs massive hp)

make sure the gas cap vent works, make the fuel flow easy

play with the governor to get max rpm permitted by the rules
hp = (torque x rpm) / 5252

more rpm, more hp, within the basic constraints of the engine 10,000rpm = 0hp because you killed it


you can go all out, but I will leave that for the gas boys/girls to discus
 

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When putting open exhaust on this engine do not make the diameter too large or you could lose low end torque. Set the throttle for 4000 rpms and that should net about 4-5 more hp. I recomend synthetic engine oil and change the tranny oil to 30 wt to reduce drag. With cut tires and proper weight set up you will probably have a belt issue before the engine lugs too much. These tractors make great pullers but the true weak point is the belt. Let us know what mods you make and how it performs. Good luck and happy pulling!
 

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Discussion Starter #10
what would you recomend for an exhaust size? i heard these motors need more back pressure then most to keep low end power but, im not sure what size pipe to start with.
 

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We campaign 2 flathead twins, 1 Kohler and 1 Onan. We have experimented with several sizes. Our best results seem to be from 1 1/2". I personally like to tie them togather near the engine. It is simular to a balance pipe on a car header system. If the pipes are to large it changes the intake mixture and takes away some low end torque (critical if you have tires which require spin speed control when fully loaded or have a tightly packed track). The larger pipes make a deeper sound and may help on top end power but sacrifice some low end "grunt''.
 

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Discussion Starter #12
thanks i was planing on running the two pipes in to a 2 1/2" pipe going straight threw the hood.

any other tips for getting power outta this thing i was going to shave the head but heard i can only go .015 before the piston contacts the plug so im not sure if its worth the effort. and i plan to bore it out and run 18hp pistons. are you guys matching the carburator bore to the intake or boreing them both out? how big of a jet can i put in the carb with out problems.
 

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Deere 330 Killer
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run 1" pipes into a singular pipe using a union and then a 1 1/4" stack.
 

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When putting open exhaust on this engine do not make the diameter too large or you could lose low end torque. Set the throttle for 4000 rpms and that should net about 4-5 more hp. I recomend synthetic engine oil and change the tranny oil to 30 wt to reduce drag. With cut tires and proper weight set up you will probably have a belt issue before the engine lugs too much. These tractors make great pullers but the true weak point is the belt. Let us know what mods you make and how it performs. Good luck and happy pulling!
i've never had a problem with my sears onans slipping belts... i most generally hook my foot on the clutch pedal to put more tension on it... helps alot... dont do that i've noticed with my 12's that they'll start to slip some...

best way to get power from an onan idk really.... both cylinders into one exhaust pipe helps so i've heard never actually done it tho yet.... but all i've ever done is put a set of straight pipes and adjust the carb and points so it runs good and its pretty competitive... or was... havent pulled with it in a few yrs just got back into pulling....

best bet is keep you hook point as close to the rear of the tractor as you can. have it horizontal so the hook goes over top of your hook point... 26x12x12 tires and the rest is pretty much gearing and weight setup... cut tires help so i've heard i'm gunna find out next year probably
 

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Discussion Starter #17
theres lots of guys running belts around here (75% of the tractors) and seem to have good luck also. The main reason im looking for more power is il be pulling with the 18hp guys and i only have a 16hp
 

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theres lots of guys running belts around here (75% of the tractors) and seem to have good luck also. The main reason im looking for more power is il be pulling with the 18hp guys and i only have a 16hp
2 hp wont mean much trust me.... i've taken my stock 12 hp tractor and competed against the "stock" 12 cubs running 18hp or so and do fine i think.... its all in the setup air pressure etc.... not always about horse power it helps sometimes hurts in others
 

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Discussion Starter #19
i know 2hp is not a huge difference but the guys with 18hp are not "stock" so they allready have a jump on me. i would just like to make it as equal as possible.
 

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cheapest way to get hp is resurface the heads...(shave them 30 thousandths) do a valve job to make sure everything is tight... adjust your points with a timing light (take the rubber tube out of the flywheel shroud and there is timing marks in there) and make sure it runs good then see what happens... also make sure it runs like 3600 to 4000 rpms
 
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