Compression too high.

Geoinvt · Apr 14, 2014

I've got a new 80cc (66cc) 2-cycle engine and jack shaft on a mountain bike. If I remove the plug I have no problem cranking the engine over. However, with the plug installed I can barely get the engine to move past the high compression spot. If I jump on the crank I can get the engine to turn maybe 120 deg. but not enough to switch from one foot to the other. Since I'm running a jack shaft I can't use the rear wheel to drive the engine. Does anyone have any tips for how I can get this thing started?

I'm going to temporarily disconnect the jack shaft from the engine and put the fixed sprocket, that the engine came with, on the rear wheel. This will allow me to use the bike to start the engine. It'll at least let me know that the engine will start. But, it won't fix my problem because it won't help me get the bike started when I run the jack shaft again. I also know I could put a thicker head gasket on but, I'm not sure how much this will really help me.

Has anyone else had this problem? Any suggestions? I'm not sure if these heads can be modified to use a decompression valve.

Thanks everyone - Geoinvt.

Theon · Apr 14, 2014

The trick to it is not trying to do a complete revolution of the pedals but rather a back /forth, so like up and down, focusing the down/start stroke to your kicking leg.
or like I did put a decomp valve in the head,
best solution though is the pullstart.
I was running 150 psi with the decomp valve,
But have dropped it to 125 Psi now.
Have a look at my Softtail.

Theon · Apr 14, 2014

And Welcome aboard.

maniac57 · Apr 14, 2014

Starting a shiftkit bike takes a certain touch. Think kickstarting a motorcycle rather than pedaling in a circle and you begin to get the motion.
Removing the freewheel for the initial startup might be a very good idea to save stress on the chains unless you can master the technique...you can re-install it after break-in when the engine should be quite a bit easier to fire.
I disagree with Theon on pullstarters. Every one I have had in my shop was troublesome and prone to breakage.

Theon · Apr 14, 2014

There is a technique to starting with a pull start,
If you want it to last.
I thought my pull start seemed well made, and have had no problems as yet.
I have been considering ordering a spare though, as it would also allow me to bench test motors.
A two stroke does not need to be yanked to start it.
It will usually take a couple of pulls on a cold motor to start it, but they only need to be long slow pulls, so that you don't stress the starting mech to a sudden stop when you run out of cord.
Don't shorten the cord if it breaks, and if it came with cheap cord, get some good stuff.
One of the best things about a shift kit is no peddle clutch starts and a pull cord.
Other wise they are a pain in the a**e.

mapbike · Apr 15, 2014

Either use a pull start or remove the head and add an extra head gasket, some of these engine used to come with two head gaskets, some performance will be lost but it might help with getting it started and when its broke in good and loosened up a bit you can remove one of the gaskets and see if its easier to start.

My personal option would be a pull start regardless but pedal starting is fine if it can be done.

Map
.wee.

Geoinvt · Apr 15, 2014

Really appeciate everyone's comments and suggestions. Last night I disconnected the jackshaft and temporarily installed the direct drive sprocket that came with the engine. I'll try to start it again tonight. If that doesn't work I'll give the second head gasket a try. I can't imagine being able to pull start this. I've worked on a lot of 2 strokes over the years but this one takes the compression record. Its so hard to crank that my original thought was it was siezed. Luckily, without the plug it cranks beautifully. Has anyone tried cranking one of these engines using an electric drill and socket?

2door · Apr 15, 2014

To use a drill/socket you'd have to spin the magneto side. Here's where you can get into trouble especially with high compression. The crankshaft where the magneto rotor nut threads on is only 8mm, about 5/16". It is known that trying to pry off the rotor incorrectly can actually damage the crank at the threaded portion. I've seen them bent and I've seen the threads strip from people overtightening the nut.

If you're having problems pedal starting the engine the compression might be too much for those threads. Easy does it.

Tom

rogergendron1 · Apr 17, 2014

I here ya bro i dont have a pull start though but even with the clutch pefrectly adjusted and tightend i have to be pedeling real fast to start it or my rear tire just drags lol and i have to let it out slowly so it grabs and slipps a little then catchez if i just dump it the tire qill drag....

I am running a dax f80 (70cc) with a full port and polish and a decked jug with a 70 cc puch hihi head and a thin .4mm copper gasket... She is just over 13:1 static compression chamber volum is only just over 5cc ... Definatly hard to start and i would get rid of the pull starter or get a fred head wirh a decoom valve if you need to keep it for free wheel or auto clutch reasons . A 5.6 fred head with a decomp valve is a good chunk of change though. But you will be at just over 12:1 and it wil be easy to pull start

Geoinvt · Apr 18, 2014

Well, she's up and running. Temporarily disconnecting the jack shaft and using the direct drive sprocket to get her started worked nicely. Once I get the engine tuned in I'll go back to he jack shaft.

I have a few questions maybe I can get somebodys input on. I can now start the engine but, getting it started does require getting up some speed by riding down hill. I haven't been able to get it started by just pedaling and, I'm a long way from being one of those people that just picks up the back end and gives it a kick.

I'm please it starts and runs but it still needs some work. The engine won't idle, but that's probably just a matter of adjusting the idle speed. What I'm questioning though is what happens when it has to pull a load. Lets say I'm riding up a long gradual hill. At 1/2 throttle it seems to run pretty well. If, however, I then suddenly hit the throttle it'll just bog down. At that point I have to go back to 1/2 throttle before it dies. I'm thinking that it boggs down because it's starving for fuel. I think I can confirm this because restarting the engine, even after its been run for 10 minutes or so, and should be getting warm, usually requires using the choke. Assuming I'm right, and it is a lack of fuel, would I now move the carb's mixture needle up or down. I'm guessing up, which would mean moving the clip one notch closer to the needle's tip. Correct?

How sensitive are these engines to the ambient temperature? Here in VT were still just a little above freezing, say 35-40 deg.F.

I'm having a lot of fun with this motorized bicycle stuff, and happy I found this forum. Your suggestions are always appreciated. Hopefully I'll be able to help one of you as well.

Best - Geo in vt

CTripps · Apr 18, 2014

The engine itself might be sensitive due to the aluminum expanding or contracting if it's too hot or cold.. since the fasteners are steel, they'll change at a different rate.

The other factor is the air at that temperature. I was thinging about it some time back, and ended up posting this thread. Basically, I was wondering why my motor ran differently first thing in the morning (really peppy and ready to go) vs. on the way home from work in the afternoon (happy to go, pretty predicatable output).

Basically, when it's close to freezing you're pulling in more air.. It may not sound like much, 70 or so CC's instead of 66, but if you're trying to tune your carb it has the potential to throw you off a bit. Likewise if you're trying to do it when it's a staggeringly hot day, you're actually drawing less air so you end up running rich.

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Compression too high.

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Theon

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