Engine building questions

M

MaxSpeed

New Member
#1
#1
I would like to put my stock 2.8 HP DB engine on the operating table to see if I can make it perform tons better. I know about the engine swaps, but I don't want to take the easy way out (not just yet). I am mechanically inclined and able to do most work (except for machine work) myself. This would be a great training tool for me to get acclimated with small engines. However, I need some answers to the following questions first:

1. How much material would be safe to shave off of a stock 2.8 hp Doodlebug head and engine block in order to bump up compression?

2. Will I need to shave the valve stems the same amount as the head and engine block combined in order to avoid valve to piston contact?

3. Would shrouding the valves be of any benefit?

4. Would I need to shave any material off of the valve cover.

5. Should I add 18 lb springs and remove the governor?

6. Can anyone recommend a billet or steel flywheel that is compatible with the stock engine?

7. Would the stock rod withstand 5000 rpm without failure, if not, where can I get a decent rod?

8. What size diameter tubing should I use to make header pipe that would compliment these mods?

9. Are the valve springs interchangeable from engine to engine?

10. What other parts (carburetor, air filter kit, etc.) would I need to purchase to tackle this job and build a reliable engine that can safely rev to 5000 rpm?

Thanks in advance for everyone's help.
 
Strigoi

Strigoi

#2
#2
no one makes aftermarket performance parts for that engine (like rods and flywheels). there's a reason you don't see the smaller hp engines modded.

i think you'd be better off spending the time/money modding a 5hp flathead or a 6.5 clone.
 
125ccCrazy

125ccCrazy

Well-Known Member
#3
#3
Strig is correct, no billet parts or even any good performance parts available for the 2.8 other than Promod's billet jackshaft set up..

11/16" tube would be sufficient for a header

a 16mm mikuni should work for an aftermarket carb but you'll have to fab up your own intake..

the engine is a flathead so there are no valve to piston issues, only valve to head clearance

.030 would be safe to mill off the head

rounding off any sharp edges within reason in the cumbustion area will help with flow and reduce hot spots

The valve cover is on the back of the block so it won't have any clearance issues..
 
M

MaxSpeed

New Member
#4
#4
125ccCrazy said:
Strig is correct, no billet parts or even any good performance parts available for the 2.8 other than Promod's billet jackshaft set up..

11/16" tube would be sufficient for a header

a 16mm mikuni should work for an aftermarket carb but you'll have to fab up your own intake..

the engine is a flathead so there are no valve to piston issues, only valve to head clearance

.030 would be safe to mill off the head

rounding off any sharp edges within reason in the cumbustion area will help with flow and reduce hot spots

The valve cover is on the back of the block so it won't have any clearance issues..
Click to expand...
What is your opinion on removing the governor? Is there a substantial risk of the flywheel letting go? I only plan on going WOT for short periods, maybe 15-20 seconds at a time. What kind of RPM do you think the motor will see with your recommended suggestions? How about advancing the timing? Is the method of advancing timing the same no matter what type of engine?
 
125ccCrazy

125ccCrazy

Well-Known Member
#5
#5
MaxSpeed said:
What is your opinion on removing the governor? Is there a substantial risk of the flywheel letting go? I only plan on going WOT for short periods, maybe 15-20 seconds at a time. What kind of RPM do you think the motor will see with your recommended suggestions? How about advancing the timing? Is the method of advancing timing the same no matter what type of engine?
Click to expand...
The flywheel should be ok unless you wind it to tight or free rev it high, most problems with flywheels are that they are not seated correctly, the coil clearance is too tight, loose coil and it drops down on the flywheel, cracks, severly out of balance or extreme rpms, in most cases the rod will let go before the flywheel..... As far as max rpms, thats hard to say, I don't think 4500 to maybe 5000 isn't within reason..... The timing can be advanced with a custom offset key which is the usual method on these type of engines unless you can oblong the holes in the coil to move it for advancement.... Govenor removal is not a bad thing if common sense is used while riding... These engines are cheaply built and won't handle as much abuse the older briggs and tecs would handle without a govenor... Like anything else when modifying is involved there is always a risk of damage and the longevity is reduced....
 
M

MaxSpeed

New Member
#6
#6
125ccCrazy said:
The flywheel should be ok unless you wind it to tight or free rev it high, most problems with flywheels are that they are not seated correctly, the coil clearance is too tight, loose coil and it drops down on the flywheel, cracks, severly out of balance or extreme rpms, in most cases the rod will let go before the flywheel.....
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Do you have any recommendations on how to make sure the flywheel is seated correctly (to include torque specs for mounting nut)? How much timing advance would be recommended and where is the best place to buy a pre-made timing advance key?
 
M

MaxSpeed

New Member
#7
#7
Strigoi said:
no one makes aftermarket performance parts for that engine (like rods and flywheels). there's a reason you don't see the smaller hp engines modded.

i think you'd be better off spending the time/money modding a 5hp flathead or a 6.5 clone.
Click to expand...
When I was a kid I had a 5hp Tecumseh powered go kart. I constantly got beat in races by a kid with a 3.5 hp Tecumseh go kart. He would never tell me why he was able to beat me. I studied his engine relentlessly until I found it... (he had a paperclip on the throttle linkage to override the governor). I performed this mod to my engine and blew the muffler right off. Needless to say, the 3.5 hp go kart had won his last race with me. Strigoi, I know you're right about the old saying, "There's no replacement for displacement," but it sure would be nice to make this little engine perform beyond everyone's wildest dreams. I say it's worth a shot. If it doesn't pan out then I'll go with the 6.5hp swap.
 
Strigoi

Strigoi

#8
#8
MaxSpeed said:
Strigoi, I know you're right about the old saying, "There's no replacement for displacement," but it sure would be nice to make this little engine perform beyond everyone's wildest dreams. I say it's worth a shot. If it doesn't pan out then I'll go with the 6.5hp swap.
Click to expand...
i'd rather not spend time/money on something that might work (and getting minimal gains if it does) than just go with something proven. i also like having some power and starting with a 2.8 isn't even an option for me.
 
M

MaxSpeed

New Member
#9
#9
Strigoi said:
i'd rather not spend time/money on something that might work (and getting minimal gains if it does) than just go with something proven. i also like having some power and starting with a 2.8 isn't even an option for me.
Click to expand...
Ok Strigoi, you've talked me into it. I will scrap all plans to modify the stock 2.8 and step up to a 6.5 hp engine. I want to be sure and buy the right engine though. Will you please take a look at the 6.5 hp engine in the link below and tell me if this is the Honda/Clone that everyone is buying from HF and having excellent results with?

- Harbor Freight Tools - Quality Tools at the Lowest Prices
 
M

MaxSpeed

New Member
#10
#10
If I am going to go up to the 6.5 hp engine I might as well do things right the first time. I plan on using my mini bike as a pit bike and to run the 1/8 mile occasionally (maybe more frequently if I can talk my friends into racing mini bikes). Would the items listed in this bolt on performance kit be a good great addition to the HF 6.5 HP engine? Is a billet flywheel and new piston necessary for extra reliability? I don't want to buy parts that I won't need, so I need everyone's help in putting together a good package that won't "break the bank" as I also drag race a motorcycle, which is an expensive hobby in and of itself.

affordablegokarts: Bolt-On Kit 6
 
Strigoi

Strigoi

#11
#11
that's the engine that everyone uses and that kit will work. as long as you aren't keeping it wound out for long (more than a few seconds) the flywheel should be fine. i'd get some 18lb valve springs ($8 for two from AGK) and a header for sure. i'm running that same cam and carb and have been happy with them.

the flat top piston isn't necessary but you'll get a bit more compression with it. they're fairly cheap and everything will be apart anyways since you're doing the rod. i see it as one of those "might as well do it since everything is apart" type things. 8° keys are also cheap and easy to install when doing these mods too.

you'll be happier with a setup like this which will probably put you in the 9-10hp range instead of the maybe 3.5-4hp that you would have gotten from the DB engine.

you'll also need a 3/4" clutch and a way to mount the engine. i'd recommend getting the plate from cjtmini, but if you're mechanically inclined (which you seem to be) mounting it yourself shouldn't be too hard. get a vacuum cap to plug the pulse fitting in the intake since you won't be running a fuel pump.

i'd try running the engine stock until you get used to the power. it will make it a bit more fun once you install the mods. if you're used to the 2.8, this will be a huge difference to begin with.
 
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M

MaxSpeed

New Member
#13
#13
Strigoi said:
you'll also need a 3/4" clutch and a way to mount the engine. i'd recommend getting the plate from cjtmini, but if you're mechanically inclined (which you seem to be) mounting it yourself shouldn't be too hard. get a vacuum cap to plug the pulse fitting in the intake since you won't be running a fuel pump.
Click to expand...
Who makes the best clutch for high hp applications? In regards to the fuel pump, would it be better to run one or not?
 
Strigoi

Strigoi

#14
#14
just get a regular Max Torque clutch. i've used one on mine for quite awhile and it's held up just fine. if you're running the stock tank with a Mikuni you might want to consider it. it's gravity feed so it just depends on how well it works. if you're having problems with fuel getting to the carb then get a fuel pump.
 
M

MaxSpeed

New Member
#15
#15
Strigoi,

What about replacing the crank while I'm inside of the engine? I saw one of your posts with photos of a clone and Honda crank. The Honda crank appears to be more robust and smoother looking.
 
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Strigoi

Strigoi

#16
#16
the clone crank will be fine with those mods. if you come across a cheap Honda one then go for it. i've had one sitting around for quite awhile but just haven't bothered to swap them yet. i'd get a billet flywheel before worrying about the crank.

if you do come across a crank, make sure it's for a GX200. the GX160 one will fit but it has a shorter stroke and won't work in the engine. that's one of the main differences between the GX160 and GX200 engines.
 
klapool

klapool

New Member
#17
#17
so I purchased my HF 6.5 and now I am waiting to build a work bench in my basement to tear into it.

If I wanted to do a mild build on this 6.5, from what I've read here, Removing the governor and installing the 18 pound springs would do. Is this a fair assessment?

I would like to up the HP some and have a project to do with my 10 year old through the winter but I have no intentions of maxing out the hp or reving the engine to 10,500!!

I just want to tear into the 6.5 and up the hp and rpm's some but not go crazy.

Any suggestions?
 
novaracer

novaracer

New Member
#20
#20
you will need to rejet it to. it i had the head off i would clean up the short side radius in the ports and trow a 8deg advance key in there.
 
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