After exhaust mod...cutting out at top-end.

[gone_fishin]

my inner-tube also "wasn't there" before i even installed the engine, but i think the small-outlet endcap does make a difference...it may not flow as freely, but i can feel and hear the difference that a lil bit of back pressure makes.

after this learning experience, i'll just look at my gutted-but-capped muffler as a very-untuned chamber.

 

[uncle_punk13]

Thanks Blaze, I was about to go into that, but you got it covered. That's the the thing with two strokes. Mechanical they are simple, very few parts, and more bang from yer engine per stroke. The down side is that the theory and why they work the way they work is so complex it gives me a headache.
Four strokes have more moving parts but the theory that makes 'em go is simpler...

 

[fastboy9]

Sorry to bump such an old thread, but I'm getting the exact same problem. Its really annoying . I'm running an expansion chamber pipe, and yes I know that it isnt tuned perfectly for the engine. But is there any other way I can eliminate this bogging. Ghost0 if your listening did you get this problem? I know you have the same pipe as me but you have modded the ports right? So I was wondering if I do some port grinding I can solve this problem.

What do you guys think?

 

[wayde]

any chance people that at the top end you're draining you fuel bowl and starving the engine for fuel and when you let off or reduce the throttle it has time to refill that what was happening to me at one point so i took the carb apart and ever so slightly bent the float forks to allow a little more fuel in.. just a thought

 

[Ghost0]

I think you are all on the right track. If you have completely cut the baffle out of your stock pipe or have a tuned pipe as fastboy9 does, you are now working with a tuned pipe that is out of tune. Even the stock pipe with all the baffles removed will act as a tuned pipe, as long as it still has the end on. Your symptom is that they have good low end torque but are falling flat at high RPM. You have a few choices, first let's clarify that the "70cc" motors are ported for torque and the "50cc" motors are ported to rev a little higher. With that said you will never get a "70cc" to rev that high which is probably good so you won't tear it apart. You can re-port it, raise the exhaust port and lower the intake port, that will help. But the easiest thing to do is shorten your pipe. The shorter your pipe the more power you will produce in the higher rpm range. The longer the pipe the more power you will produce in the lower rpm range. To clarify, the distance between the exhaust port in the cylinder and the beginning of your expansion chamber is the critical dimension. Just keep shortening until you get what you are looking for. My pipe caused me the same problems until I shortened mine. It could go a little shorter but I like how it runs with the gear ratios that I have. If I went to a lower set of gears I would probably want to shorten my pipe up a bit.

Wayde does make a valid point and I would look into his suggestion. On the same note if your fuel tank seals well you may be creating a vacuum in the tank also causing a similar situation. You can alleviate this by drilling a very small hole in the top of your gas cap.

 

[fastboy9]

Thanks for the help guys, I'll try shortening my header pipe even more. Then I'll raise the exhaust port a bit and make it match the manifold. And also lower the intake port slightly and match the intake manifold.

I'll post the results as soon as I can.

Fastboy

 

[fastboy9]

Ok then I read my spark plug and it was a slight bit rich. This was one notch away from the leanest setting. So I put it on the leanest setting. The day before I raised the exhaust port by about 2mm.

Problem solved! There is now no bogging at all, the bike goes all the way to 36mph smooth as anything! The bike is now running the best it has ever run. I'm off on a ride in the morning now to have some fun! I think the thing that really solved the problem was leaning the mixture, as the bogging sounded as if it was getting too much fuel.

Fastboy

 

[Ghost0]

I am finding the same thing. I already went to the leanest setting and I think I can go leaner so I cut another groove in the needle so I can try one notch leaner. Off for a ride to check it out. Will report back later.

 

[motor puzzle]

Could this help some of you?

Saw these tips in the ZBOX site and thought it might help somebody with the issues described in this thread. My apologies if it's completely unrelated...

#3 Improve fuel flow in the Carby.

Pull apart your carby and inside the float chamber you will see the fuel needle. It isn't obvious but the small hex nut at the tip with the tiny needle hole unscrews. I replaced this jet with a Dellorto PHVB jet of size 76 which is larger than the stock. This increases the fuel flow at full throttle but has less effect at lower revs. I always felt that the 70cc was starved of fuel at full throttle. This will fix this. I have not tried larger jets but the 76 seems about right.

Use this link to buy replacement jets: http://www.ozminimoto.com.au:80/productdetail_right.asp?id=49&catid=72

#4 improve air flow through the carby.

By far the most noticeable Mod. Simply pull apart your carby removing all screws and valves. You are left with main body of the carby, we want to enlarge the main inlet cylinder which can be done using a round metal file and sandpaper wrapped around the round file rod. I enlarged the main throat of the carby by under 1mm all the way around and expereinced an amazing improvement in throttle response and engine smoothness. This Mod is so easy and works so well. Finish with sandpaper and wash all filings of thoughrougly. Magic!

 

[BoltsMissing]

Can anyone suggest if the HT's mismatched shape of exhaust port and the standard muffler/gasket is actually a mistake and no one has bothered to correct when the engines are packed, or do we shape the gaskets and the exhaust header to match the exhaust port ?