2.5 HP HF Performance Tips / Tricks

I've asked before, too. seems to be ne territory for us, but I imagine that the go kart guys have figured some things out.
SB

Well, you can do anything you want to them if you are patient and have the right shop! I see the carb as the first drawback, not tuneable. I would use something sporty like a mikuni or dellorto. The piston could be switched to billet, but good luck finding one that fits. Good oil can make a huge difference in performance. Use a high zinc formula synthetic! Upgrade the plug wire and plug, that's all I can think of. Port the exhaust and intake, polish the valves.

Well chain, any carb is compatible with enough tinkering and patience. I don't know of any that mount without a lot of help

The biggest bummer about em. Purty perfect as is. Hope I am wrong. Spent a whole bunch of cash looking.

http://motorbicycling.com/f37/hf-engine-tips-tricks-19256.html

I bought one of these yesterday and have already stripped the excess junk off of it. Despite not having run it yet, I won't be looking for any performance mods whatsoever. This is way more than enough motor for the street, IMHO!

But I'll be watching!

Here are a few modifications that have proved to add power.

Installing a tuned exhaust system. We suggest the exhaust manifold be cut and turned forward to allow the pipe to be re-routed away from the rider's leg, and also allow for a smooth bend in the pipe to enhance flow.

We also found the intake air system to be very restrictive, and installing a "K & N " style breather helps.

There is very little that can be done internally to increase performance as the motor is basically tuned to the max. The head can't be milled to increase compression as the valves & piston are already very close.

Of course removing the governor will allow the motor to exceed the limited RPM range, and it is doubtfull than the added range will harm the motor, unless the carburetor size is vastly increased.

The tuned exhaust, and modified intake breather causes the motor to run on the "lean" side, indicating a possible RPM increase via larger jetting [main jet].

I am currently using one of the HF motors to test the prototype 2-speed drive system, and hopefully I will find time to adapt a slightly larger carburetor in the process.


Have fun,

I've wondered about that stock air filter. For such a big box of a thing it has very few air intake holes. I've had times riding along when my pant leg got up against it and the engine loses power very quickly. After reading your post I removed the plastic cover, left the foam in place and held it together with a couple rubber bands while taking a test ride. Immediate and noticeable power gain. I had made a tuna can version and removed it when I discovered my leg running into it. I may make a smaller version from a regular size can to see how that does. My big one is in Minnesota, so I can't switch back to it until I get home. I was thinking that I may try cutting a "lid" for the stock air box out of electroplate tin, flanging the edges with the burring machine and punching numerous holes in the tin lid. Nothing to lose it seeing how that works. The advantage in that is the stock air box lies on the same plane as the side of the engine and won't run into my leg.
I'll be real interested in what carburetor you come up with. How is that 2 speed with the HF? What size rear sprocket are you using... 56? I have a 65 tooth from a Schwinn exercise bike on mine and am thinking something in between would be good, more like 60 if there is such a one. If I can get more power out of the engine then the 56 might be OK. Looking forward to that two speed being in production... hint, hint.
SB

Hi Silverbear,

A friend of mine gave me a vintage Worksman tandem, and I have been considering using 2 motors with the 2 speed. Just not sure if I want to use 2 HF 79 CC or 2 HS 49 CC motors.

I am thinking if it will survive 2 motors and my "need for speed", it should do well on the average single engine motorbike.

One other consideration on adding power to the HF would be in the flywheel weight area. As you already know we [EZM] moved the spinning clutch to the rear of the drive and run it at a reduced speed, thus reducing the "flywheel" action and allowing the motor to spin up much faster. If the clutch is attached directly to the crankshaft, some provision should be employed to vastly reduce the spinning weight and reduce the additional flywheel action.

Have fun,

Good link chain!

Thanks big time Chaiman. Uber cool

Yes, thanks for the link. If I understand correctly from reading the information there that if we increase the air flow into the engine then it sounds like we may well be creating a lean condition requiring more fuel. So we'd need to either change the carburetor to a larger one or use a bigger jet. If anybody does this jet modification to the carb. please post pictures so those of us who have never attempted such a thing will have the courage and knowhow to proceed with successful results. This sounds like the way to go for an inexpensive increase in horsepower. I want to try it, but you go first...
SB

chainmaker said:

If you go to that site, and on the right side there are links to tutorials for some of the mods.

Click to expand...

I saw that and read some. I still find the tutorial for the drilling out of the main jet unclear. I think it assumes you already know something about this. For example, does that jet get removed from the carburetor, then the hole it fits in gets drilled out to the next drill size? Then you would need that next sized jet wouldn't you? And since the jet has a slot for a screwdriver, then it sounds like something is threaded. If so, then wouldn't you need to tap new threads so the larger jet could screw in (if it does screw in) and after test runs and checking the condition of the spark plug to see if it is still lean you're supposed to go to the next size drill bit and enlarge it a bit more. Or I don't understand. So you keep getting more jets until Goldilocks decides everything is just right? Where do you get jets and how much do they cost? If jets are not real cheap and you have to go through four or five of them to hit things right, then it sounds like you might just as well have ordered the larger carburetor for which there is no adapter to make it fit the HF engine, but sometime or other there will be an adapter in a kit to make it work and if you want it right now then you have to make your own... out of what? See what I mean? Someone familiar with re-jetting carbs understands what they are saying (and doesn't need the tutorial in the first place), but a neophyte is still clueless. Or maybe you don't remove the jet, but drill out the jet itself. I like things written for dummies eager to learn with illustrations or photos to make things crystal clear.
And I was looking at that replacement foam filter if you choose to use the stock air box (for which I'm making an electroplate tin cover today)... it is about 4 by 4 3/4 inches and costs $8.00, not counting shipping. Supposed to be a better piece of foam resulting in more power. Come on. Somebody must know what kind of foam this is, where you can buy it reasonably priced and maybe locally. What about a scrubber pad or scuff pad... whatever you call it. This isn't rocket science... something that lets air in and keeps crap out is an air filter.
We need to know how to do this on the cheap and get pretty good results and also what is possible more ideally if you have the bucks. I'd just like some more power, but don't plan on racing or need to come in first. if i can do something for a few bucks and some labor, awesome!
SB

I suspect you can get that foam from the same people who supply the Greyhound manufacturer, and by getting a container worth at a time it works out about a penny a foot... a great savings for a life time of foam. You could be the foam king. Even sleep on your riches. I think I'm going to go the poor boy route and pick up a 3M scuff pad, but first I'll try things with my spiffy new metal cover for the stock box to see if it is improved with many more sources of air. .3 bazzillion holes should do the trick. Will post a photo later with a report on possibly spectacular results! Stay tuned... don't move that dial!
SB

Hi everyone,

I will gladly explain a few items on carburetor jetting [15 years of professional motocycling racing].

The main jet must be removed from the carburetor [use a good screwdriver that fits the main jet slot or you can damage the jet]. Next you must own a set of number drills [#1 is largest and #70 is smallest]. Drilling a jet is never as good as the original, but will work. The reason a drilled jet isn't as good as an original is because the drill will not leave a perfect surface [drill marks & scratches]inside the jet. If you drill the jet too big, then it can be soldered close and drilled again.

AFG is one of EZM authorized dealers and has a lot of experence on similar motors to the HF 79 CC, however he told me "not much can be done on the inside of the motor".

The best and least expensive way to add power is to improve the incoming air flow and the out-going exhaust flow.

When the carburetor indicates a lean condition after the air and exhaust modifications, it simply means the motor is now breathing better [moving more air]and additinal fuel must be added. Although I have played with larger carburetors, re-jetting the original is by far the most cost effective option.

Currently the tuned flex pipe is the only tested option, however the tuned pipe and header may very well produce more power, but must be DYNO tested to work out the details on the smaller HF 79 CC motor.

Have fun,