First build: Mongoose + HT66

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Dr. Dognuts

New Member
First build was so easy, after I gave up on my attempt at an oversized tube Giant frame, and switched to standard tube cr-mo frame.

I've always liked Mongoose BMX bikes, and my first MTB was a mongoose switchback. I was so happy to find a Mongoose for sale for $50 at a second hand shop. The wheels were so true it was like the bike had never been ridden over a gutter, let alone belting down dirt tracks.

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Just testing out the fit.

The build took about a week, but I was enjoying the process, and didn't want to rush it.

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And now the fun of running the motor in. I've got about 80 kms on it now, and the motor is loosening up. The awful whine it first made is quieting down, the top speed is crawling up, and I am getting used to pulling in the clutch for slow turns and stopping.

I had a few little problems with fuel leaks, but otherwise, the bike is a treat to ride, very little vibration, reasonably quite, and a lot of fun.
 
Thanks, yes, I can see the addictive part, I'm already looking around for a cruiser frame to motorise.

This build needs a bit of a tidy up now. Clean up the electric wires and shorten the clutch cable a bit. I knew it would be fun to ride, but I'm making any excuse to take the bike out for a burn.
 
Good looking bike. Those Mongoose Switchbacks are one of my favorite bikes to motorize. I currently have two of them like yours. One has an 18" frame and the other one has the 16" frame. Both bikes are a pleasure to ride. I sold the Mongoose brand in my shop for many years and I have nothing but good things to say about the older models. Well built and dependable.
 
Interesting Bikesnfire, my first switchback had a 17.5" frame and this one is 19.5", makes me wonder if all the half inch frames were sent to Australia? And BTW, what is the difference between a switchback and a sycamore? The only difference I can see is the switchback has more rake/trail in the forks. Running gear seems the same. Or maybe the shifters, s/b had thumb shift, and syc has gripshift?

On to my questions! I am curious to the state of my sparkplugs, and the richness of my fuel. I'm using Penrite Hi-Per 2 stroke oil at 24:1 for the running in. Do my plugs look normal for this stage of the run in?

SDC10955.JPGThis is my plugs, with the needle clip set to 2nd from the top, riding at sealevel. On the left, is the kit plug, possibly Champion, but labled as GreatMaster 4118c, thats got about 80 kms on it, and pic is taken after two half hour rides, with about 8% WOT. On the right is a NGK B5HS, my current plug, same needle sets, after only 18kms of use. I assume it is better to be oily than lean at this stage, but just wanted to check if they seemed ok.

Problem 2 is oil creeping up the sparkplug thread. Sorry for the twisted necks if you look, but the forum wont accept the rotated pic. The oil slick comes from the plug hole, and not the head bolts.
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I dont have a torque wrench, but both plugs seemed to be tightened pretty snug, and I have tightend enough plugs to know whats up, but im also scared to give them more than an eigth of a turn over tight, because I dont want to pay for the screwup. Any ideas about this, or is it just the normal leaks.
 
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my first bike leaked a little oil from the plug. i used teflon tape on the top threads and sealed it up.

an old mechanic trick i've used to seal oil drain plugs was to wrap dental floss around the threads. never tried it on one of these, though.

your plug looks a little oily. might wanna move the clip up a notch or change the mixture. just keep checking the plug. you want it dry, with a nice cocoa-tan colour.
 
I'd go ahead and switch to 32:1 oil/fuel mix and make sure you get an air-cooled motor oil. The oil needs to be able to withstand the higher temps of an air cooled motor. Oils formulated for water cooled motors don't have the additives that hot running motors need.

Going to 32:1 should get rid of the black oily build-up on the plug. If after changing it's still oily go 1 range hotter on your plug. Many run too cold of plug thinking "larger number is better" but you can easily get too cold of a plug to function properly.
 
OK, I have two ideas how to dry up my plug..both from well respected guys here..

DaveC says lean out the juice(with dedicated air cooled motor oil), and bairdco says put some thread tape on the plug.

OK, I have re-written this too many times, so thankyou both DaveC and bairdco..thanks heaps for taking an interest in my project,(and previous contributers), and I am going to lean out the fuel to 32:1, and do something about the thread leak...dental floss sound like a great idea, it will surely get burnt up and pased thru the exhaust if it comes loose.

Just wondering, has anybody used hi-temp gasket seal in the spark plug thread?
 
My first motor leaked a little oil at the plug as well. What I did was over tighten the plug once and that carved the seat out on the aluminum head. I was a little leery tho that I might crack or strip the threads on the cast metal head but got away with it. After that I just tightened it normally.

The Teflon tape will work good but you will want it at the seat part of the treads only. The high heat silicon may work as well same story tho you don't want it in the motor.
 
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From what I see in the pics, I'd run a little leaner mixture.....32:1 is fine. Your sparkplug #2 the NGK looks like it needs to be snugged down a bit more as the washer doesn't look compressed at all and that's quite an oily mess you got goin on. Something is not seated properly in order to have that much seepage.

The only thing I ever use on sparkplug threads is anti-seize. The reason is self explanitory. You don't want to over tighten your sparkplug, but you want it plenty snug. A plus side of these 2-strokes is a replacement head is dirt cheap if you should happen to strip the threads.

And you are running as hot of a plug as you should for this engine...NGK 5 is hot!

dnut
 
Thanks everybody for your ideas.

The sparkplug wasn't tightened enough, which caused the seepage, and not burning the gas well. I managed to get almost half a turn more into it with no damage, and the leaking stopped, and the plug is dry now. I just got back from a 30km ride, and the plug is looking much better, dry, but maybe a little white, hopefully the new leaner fuel batch will bring that to a nice tan colour.
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I need to make up a new batch of fuel, so I will make that at 32:1.

Also I'll get the NGK6 heat range plug, and keep an eye on that. Tinker with the needle clip to fine tune.
 
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Congrats on the build looks nice.Once you build one you are going to have to build more. As for the issues that you are having I think they have summed it up pretty good nothing I can add. My opinion would be to use the telfon tape for the threads and for the oily plug you are just gonna have to play with the needle setting and carb after it breaks in a little more and you settle on an oil and ratio.
 
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