New Motor on the Invader

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Mammoth

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I like looking at all the pics on this forum so I figured I would add some from my collection.
This 5 horse Tecumseh engine came with the Invader when I first bought it. I messed with it a little bit to get some more hp but it still wasn't enough.


Purchased the 420cc and got to work. Noram clutch


Took off the governor so defiantly had to add an aluminum flywheel. Timed it at 30*


Ground 2 angles on the valves. 45* for the seating surface and then 30* to help the gas move past.
Chucked the exhaust valve in the drill press and sanded the 90* corner off the head. This allows the gas to pass by more efficiently. We always gain 2 to 3 percent when this one move is done. Right here.


Welded up the squish band and then milled down the head to get 13-1 compression.


Ported it. Pics don't do it justice. Should have taken them before the valves were installed.


Ordered one of the headers off the web. Did the math to figure out how long the header needed to be on a 420cc engine that is going to run around 6500 rpm and came up with 33 inches. The header was far short of that so I added some length. Its at 36 inches now. Once I get a tachometer on it I will trim it to proper length.


Made an intake tube for a 40mm Dellorto and installed it with the biggest air filter I could find.


 

Mammoth

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Went out to the airport and picked up some 100LL aviation gas. I like the blue colored gas.


Pulled on the recoil starter and felt how strong the compression really was. Figured I would just have to man up on this. It fired on the 3rd pull and ran strong, but the jets were too big. The carb did come off an ATK605. It sounded very loud and angry.
Put smaller jets in it and gave it another pull and the recoil starter handle shattered.
Made a simple one out of aluminum.


I couldn't just pull it right through the compression stroke it would come to a hault no matter how hard I pulled, so I would ease into the compression and then pull. It ripped the handle out of my hands one of the pulls making my hands hurt.
Got it running and started adjusting the carb. The idle was high and couldn't get it down in time before the clutch started smoking so I shut it off.
Took the chain off the clutch to solve that.
Gave it another couple pulls and it took a deep bite out of my finger.
I won't be pulling on anything for at least another week.


To counter the compression problem I ordered one of those compression release switches. If I have time next weekend I will install it.
Really wanted to ride it this weekend, oh well.



.
 

anderkart

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I saw that you raised compression, (so I'd expect it to pull harder than stock) but did you also defeat/remove the compression release feature your engine originally had?

Because if not, I remember reading the compression release cant function unless the valves are adjusted within specs, and I was thinking that might possibly be part of your problem here...
 

Poboy kartman

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I saw that you raised compression, (so I'd expect it to pull harder than stock) but did you also defeat/remove the compression release feature your engine originally had?

Because if not, I remember reading the compression release cant function unless the valves are adjusted within specs, and I was thinking that might possibly be part of your problem here...

It crossed my mind as well. And maybe a wee bit of too much advanced timing?

EDIT: And suck it up man! You got two hands....JK...
 

Mammoth

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My engine tuner friend that is helping me build it called and asked if it had a compression release? He was thinking the same thing.
I had pulled the side cover off for a minute, but hadn't noticed if it does. Is there one on all the pred 420's?
I will see if it pulls easy when the switch is off.
 

Mammoth

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Little off topic but trying out this new light. Has its own rechargable battery pack and is rated at 7500 luminums. Haven't used it driving yet only while sitting in the yard, turning the steering wheel and making engine noises.:eek:


 

OzFab

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Took off the governor so defiantly had to add an aluminum flywheel.

What gave you that idea?

My engine tuner friend that is helping me build it called and asked me if it had a compression release he was thinking the same thing. I had pulled the side cover off for a minute but hadn't noticed if it does. Is there one on all the pred 420's?

The compression release is attached to the stock camshaft...
 

chancer

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What or where did you get your theory on needing that length on the exhaust? I have not seen that before on a 4 stroke.
 

Mammoth

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I took off the stock flywheel to reduce the rotating mass so it will get to the peak horsepower RPM faster.

What I've been taught about 4 stroke headers is higher rpm motors need shorter pipes and lower rpm motors need longer pipes basically.
My engine tuner friend named Craig has been working on this question for a long time.
I just bought a book he recommended to help get a better understanding on header design.

There is also a certain length the intake has to be that's why mine is so long. The equation is 88000 devided by the engines rpm.
When i asked him about that equasion he smiled and said I don't know how they ever figured out that formula, but it works.
 

Mammoth

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Finally had a chance to work on it today.
Pulled out the camshaft with the mechanical compression release.


Got rid of that extra weight.


Here's the new compression release switch.


Put the head on the mill



Switch installed



Assembled the engine again. Tested out the switch and it works great, couldn't be more pleased with how it functions.
Gave the engine two pulls with no fire so I flipped the choke lever and it fired right up and idled smooth.
It had a stutter when the throttle was engaged, but once it got passed that it roared strongly.
Think the slide needle needs to be raised up one notch.

I always try to respect the neighbors by not making too much noise late in the day, but since i was excited to have it all built I fired it up at about 6:45pm. When I shut it down to adjust the needle the neighbor was at the fence saying whats going on over there.
He said he was sitting on the couch when he herd the noise so he came over to check out what we were up to.
He said it sounded good.:cornut:

Tomorrow I will work on tuning the carb more and then take it for a test ride:wai:
 

Mammoth

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Had two chances to work on tuning the engine. The first time it was raining all day and had to call it off. The second time it was still raining, but it let up for about 45 minutes which was just enough time.

I had raised the slide needle up to the last notch, but it still had a flat spot on the low end so I replaced it with a bigger needle and that cleared it up.

Tuned it in well enough to do a test ride. The tires were wet and the asphalt was wet because it was still sprinkling rain, this made it real tough to ride.
Just bumping the throttle would cause the tires to peel out.
When trying to give it gas going around turns the front end would just wash out.

The carb slide was sticking right at the bottom which pointed out another problem. The breaks are definitely not strong enough to slow this kart down.
I thought this might be a problem before, but I wanted to wait for the first test ride to see.

A new disc break is on order and should be here shortly.
I also made up a new throttle cable and polished the carb slide.

I started to shoot a video while tuning the carb, but after about 10 seconds my stupid smart phone ran out of battery.
 

Mammoth

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Well this motor is officially dead.
For the first time in a week it stopped raining long enough to take it for a break in lap around the block and it seized.
Just pulled the cylinder head off and the bore looks good. Pulled the oil plug and blackish grey oil came out.
Looks like the rod seized on the crank.
Pulling off the side cover right now to take a look.
Man it was running real good too. Lots of power. I dident even get a chance to take it over half throttle.
 

J_Walker

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ahh man, u a lucky SOB. all those tools to play with and do with parts as you please...

I am as curious to the exhaust header thing too... The super marine spitfire engines had a 2000RPM redline [GOBS OF TORQUE] and like maybe a foots worth of header. wouldn't they added more header if it took back on power?.. [this is the only good example I had for a low RPM motor that's tried and tested. ]
 

Mammoth

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Yep the rod has lots of friction on the crank. I never touched the rod so it was still to factory settings. The oil level was good so that makes me question how tight the rod bolts were torqued.

Here is the book that craig told me to read. I haven't had much time to look at it yet, but I looked at the exhaust section and must have learned 10 things right off the bat. I will take the book to craig and have him point out key info pertaining to these motors.


Craig works on aircraft engines in his shop and I get to take a look at them. He always points out how miserable the engines really are.
He had a cylinder head on the flow bench the other day, and said in all his years this is the worst he had ever seen.
 

Mammoth

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Took it all apart and didn't really see anything that stood out to be the culprit. Bolts didn't feel too tight. Maybe a piece of crud plugged the oil hole. Can't say for sure.
Glad I payed the extra $80 for the 2 year warrantee.

I brought the book to Craig and had him walk me through calculateing the length and size of pipes.
Here is the equations we used.
The little scribbling I wrote next to "RPM" says your peak horse power rpm which is somewhere around 1000 to 1500 rpm less than your peak rpm.
We just put 5000 there.





In the end we came up with 34.4" inches long with 1-3/8 tubing.

Then I asked him about the Spitfire engine and well that conversation lasted an hour. If your interested in those motors you should have been there. He really dropped a ton of info an me. I couldn't even tell you everything he said.

Some of the things he mentioned about the short exhaust was they didn't even start utilizing tuned exhaust until the 50's.
When they built those pipes they said that they produce 200lbs of thrust helping the plane forward, but test were never done to prove that.

Craig said the exhaust comes out at a 45* angle so if you want to use the thrust it would have to point 90* degrees straight back.

He said if you made a proper header it would have a minimum 10% more power and then you could still aim it backwards to utilize the thrust.

He also mentioned the motor is v shaped and the cowling is round so there is room under the cowling to fit the pipes in that space.

When that motor was designed it was state of the art, but we have learned a lot more about engines in the last 70 years.
 

Mammoth

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Back in action



Took 2 hours and 20 minutes to switch over all the parts and get everything diled in.

One way to make a motor faster is to take off all the weight you can. Here is what it weighed right out of the box. 75.2lbs


And this is how much it weighs with all my parts put on it. 56.6lbs


Got a good ride in today. Broke it in by slowly raising the load over a half hour period until I was at about 75% load capacity.
I have to say this motor is quite impressive. Definitely the fastest invader I've ever ridden.
Normally you could just hammer down the throttle and go for it, but this one you really have to pay attention to what your gas foot is doing. It pulls pretty hard.

I noticed the valve keepers were turning a little bit on the last motor so after this last ride I pulled the cover off to take a look, and this is what I saw. The valve keepers had turned 90*. These are after market stiffer springs too.


Last week I had ordered the wedge type keepers because this was making me nervous, but they hadn't come in yet. I will wait for them before I take the next ride.
My brakes haven't come in yet either, that order didn't go through very smooth.
 

Denny

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Valve and keeper rotation is a good thing. It helps the seats wear evenly for better seal as opposed to hammering all the time in one place. Drag racing for 35 years off and on has taught me one thing you can not predict what length the primary pipe and diameter should be. the engine will like what it likes. Too large of a diameter pipe will slow down the exhaust gas velocity also so will a too long of a pipe. Multi cylinder engines will like one thing, usually a longer primary before the collector than just a single cylinder. The best way to figure out you're primary pipe length would be to make 1, 3ft. long pipe and paint a stripe on it with regular old Rusoleum and cut it off about 4 inches past where the paint burned off after a short high speed ride. :thumbsup:

Denny
 

Mammoth

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On the old motor the keeper was actually starting to slide over lining up the valve stem with the release hole. It was half way there when I tore down the motor. Just one push with my finger and it popped apart. This is what made me nervous.

I was studying the keepers today and noticed the flat part in the exhaust valve keeper had to be pointed down so the rocker arm could clear.
After the ride when they turned 90* the keeper was touching the arm half way between the mount bolt and the push dimple. But maybe thats just normal, I don't know. Seems weird to me.
 
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