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View Full Version : Any 2.5HP Mods to increase power?


rgvkid
12-30-2008, 12:10 AM
Im picking up a Harbor Freight 2.5HP engine and i would like to see if i can try and get a little more out of it. Do you have any links or advice on mod tips and degoverning?

Any tips would be greatly appreciated.

Russ2251
12-30-2008, 12:32 AM
Assuming it is new, any mods now will absolutely void warranty.
Does it have a straight or tapered shaft?

ZnsaneRyder
12-30-2008, 02:11 AM
I have one of these new 2.5HP motors too, it's a 5/8 straight keyed shaft.

Only power mods I've done to mine, is completely remove the air cover, and replace it with some expanded-metal mesh to hold the foam filter in. Just use the two thumbscrews that normally hold the cover on to hold the mesh on. I haven't got to use it with a load yet, so I'm not sure how much power difference, but it revs much more freely with the mesh. That's the easy part.

Now the HARD part:
These 2.5's have a strange, non-kart friendly throttle setup, so it's better to not use any of it and just go straight to the carb butterfly.
Remove the throttle arm and mounting completely, it won't be needed anymore. Have to remove the gas tank to do that. Also remove the governor arm from the shaft that comes from the crankcase. Use a piece of metal to make a new throttle arm on the plastic bracket, and a piece of wire to move the butterfly on the carb. Reuse the small spring as a return spring. A custom throttle for sure.

Have fun with the new engine!

Russ2251
12-30-2008, 02:19 AM
so I'm not sure how much power difference, but it revs much more freely with the mesh.Can you prove this?
If there is a difference, then there was/is a defect.

brendonv
12-30-2008, 03:17 AM
im building a go kart atm with a 2.5hp briggs, look at my thread in the go kart section. I will have finished it in like 2 weeks so keep looking to see the results. With my 5.5:1 ratio with 10" wheels im looking at around 35km/h top speed. Although the governor is removed so i may hit 40km/h if i push it. 35-40km/h is fast enough off road anyway.

oscaryu1
12-30-2008, 03:31 PM
im building a go kart atm with a 2.5hp briggs, look at my thread in the go kart section. I will have finished it in like 2 weeks so keep looking to see the results. With my 5.5:1 ratio with 10" wheels im looking at around 35km/h top speed. Although the governor is removed so i may hit 40km/h if i push it. 35-40km/h is fast enough off road anyway.

You'll notice virtually no difference with the governor on or off. Sure, it'll free rev crazily, but there won't be that much of a difference. I had a 2HP, with a header, filter, 3HP carburetor, 3.5HP cam, ect, and it was still much slower than my electric toys.

scar
12-30-2008, 07:16 PM
You'll notice virtually no difference with the governor on or off. Sure, it'll free rev crazily, but there won't be that much of a difference. I had a 2HP, with a header, filter, 3HP carburetor, 3.5HP cam, ect, and it was still much slower than my electric toys.

insert joke here ___HAHA!!______

ZnsaneRyder
01-01-2009, 01:56 PM
Can you prove this?
If there is a difference, then there was/is a defect.

The defect was the crappy-designed air cover with a rectangle vent measuring about 1 inch x 1/4 inch. You can put your hand near the breathing vent and the engine would rev up slower, it's THAT bad.:ack2: You can also feel the air rushing into the vent with your hand, which to me shows the restriction. Go to Harbor Freight to see for yourself and check out the 2.5HP engine. When you see the stock air inlet, you would laugh.

I don't have an audio or video clip to prove it, but I showed my brother the difference, and it was plain as day.

For comparison, the Honda-Clone 6.5Hp engine's airbox is not near as bad as the pure chinese designed 2.5HP's airbox.

JerryAssburger
01-02-2009, 07:15 PM
ZnsaneRyder...
Hopefully you can post pix of this motor. I guess the up-side to all of this is that you're finding room for improvement with that motor.
Is it one of the leaned-over OHV types, or is it the cloned version of the Lifan 90cc flatty (itself a clone of a Honda motor?) If it's a clone of the Lifan 2,8hp motor, they were allowing a governed rpm of 4000 on my son's DoodleBug minibike... stock.

JerryAssburger
01-06-2009, 12:10 AM
Wow. Went to Harbor Freight to see the motor firsthand... tiny little bugger! Its 79cc displacement, and it is one of the OHV types (not a copy of the Lifan 152f like I earlier ASSumed.)
Would probably be a great project motor for those places that the 6.5hp just WON'T fit. It is definitely smaller, physically!


They're awfully new to do an exploratory on yet, but applying the same tried and true techniques to the little guys should yield 4 - 4.5 hp without any expensive parts.
And they're small enough to fit into some pretty wild projects!

A gas powered True Big Wheel, anyone????

I like karts
01-06-2009, 02:20 PM
If you're willing to take it apart and void the warranty, What I'd do is get a good air filter, get a bigger carb or take out the choke in the stock one (Hey, a hand works well as a choke too, ya know :P), Make or somehow acquire a stepped header that fits it, and I might do a little smoothing and de-burring of the ports. All of this can be done for very little cost, and if you tune it right, this may get you to around 4hp.

Kenny_McCormic
01-06-2009, 04:43 PM
Just a header/straight pipe will get .5hp-2hp depending on the motor.

I like karts
01-06-2009, 05:45 PM
^ a straight (stepped header) gives a proven gain of .5 hp. That's on a briggs flathead though, so I wouldn't be surprised if that only gave you around .2-.3 hp on this little engine

KingCobra
01-06-2009, 07:55 PM
A nitros bottle :D

oscaryu1
01-06-2009, 08:46 PM
A nitros bottle :D

They DO sell 49cc nitrous kits. $40 on eBay. I read that they work great with 196cc GX200's (4cycle). No doubt on these it'll have even more sense of a "boost".

jr dragster Tyler
01-07-2009, 03:36 PM
Nitrous is pretty hard stuff on crank's and crank bearing's.

KingCobra
01-07-2009, 06:27 PM
After what I have seen NITROS do to engines of all types, I won't go near them.

jr dragster Tyler
01-07-2009, 06:42 PM
If properly set up properly it will last a long time. Most of the time it isnt setup properly and all I takes is one fuel fogger to small and you have a nitrous backfire ripping off the plastic airbox.

ZnsaneRyder
01-09-2009, 06:30 PM
Wow. Went to Harbor Freight to see the motor firsthand... tiny little bugger! Its 79cc displacement, and it is one of the OHV types .............. tried and true techniques to the little guys should yield 4 - 4.5 hp without any expensive parts..\

4HP+ would be nice, as I wasn't expecting that much.

This engine is NOT a clone at all, but it's own model and size. It looks to be designed after the GX200 clones, but really shrinked down to 79cc, because the Valve cover is huge, and so is the pull-starter. The ports are short and wide on these, but look like porting sure would help.

I also like how the stock header on these unbolts from the muffler, so you can attach your own exhaust to it.

The carb is not very big, and I think that's where most of the restriction is, so it's only 2.5HP. I wonder if a GX200 carb would fit. Getting 4HP out of something this small, and still having the low-end torque, and no-oil mixing of a 4 stroke would be great.

I also notice, this 79cc engine is more tolerant to high revs and backfires less without the governor than a GX200, and floats the valves less, but they still float some when the RPMs max out. I imagine some springs could be found for these engines.

ZnsaneRyder
01-09-2009, 06:52 PM
ZnsaneRyder...
Hopefully you can post pix of this motor.

I didn't take pics of my 2.5 engine, but here's the HF page with a picture. On my engine, I removed the muffler cover, and removed the airbox cover, and it looks a lot better.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=97964

oscaryu1
01-10-2009, 09:25 AM
If properly set up properly it will last a long time. Most of the time it isnt setup properly and all I takes is one fuel fogger to small and you have a nitrous backfire ripping off the plastic airbox.

Lol, these are CO2 size'd bottles with pretty much direct injection into a hole drilled in the air filter LMAO :D

ZnsaneRyder
01-21-2009, 12:42 AM
Me and my bro have been doing more work with the soon to be bicycle trailer with the 2.5HP engine. Gotta weld the 48T sprocket & hub together to get it rolling.

So far the engine fits perfectly, and is does not vibrate very much when running while mounted to the trailer! MUCH better than a shaky 6.5! This is a GREAT engine to put in small spaces where you don't want a 6.5HP engine.

I wonder how rgvkid is doing with his project?

JerryAssburger
01-22-2009, 06:30 PM
2.5 hp on a big wheel! or one of those Barby Jeeps! or on the one of those standup scooters with the 10" rims!