Big bore stroker problems.

[Johnny 5]

The 62mm 60.8 (3mm crank) engine I built back in the summer of 2012 runs good but doesn't like constant high rpm. It pulls off the line great climbs hills without bogging down but has a vibration all the time. I know the bigger bore and stroke will cause the vibration, but after a long full speed run it feels like it has a lot of back pressure. I am running the fmf universal muffler that is for two strokes with the stock head pipe on the cylinder to a custom home made header pipe to add 8 inches to the header pipe to reduce some of the crazy loud noise. I made it curve to the right then back to the muffler like the hammer head exhaust I had seen. It works great on my 155 buggy and the 183cc buggy also just wondering if I should make a larger inside diameter header pipe to get more exhaust out faster.

The stock pipe I/d is about 18.5mm and the head port is 22.5mm. I am thinking of making a larger head pipe out of a Honda seat frame tubing I got from a body shop car. The tubing is mandrel bent already and free!.

The other issue with the performance is the timing chain is advanced due to the cylinder spacer rising the head up about 2mm causing the cam to rotate forward a bit. I had to take off .015 of a inch off the head to get the cam to lay in the head without stretching the chain. I used 400 grit wet paper on the granite counter top for about 2 1/2 hours to get it the head clearance since nobody local wanted to mill my cylinder head. I rotated the head while sanding it in figure 8s and measuring from time to time. I finished it with 600 grit for a nice finish.

I might go to heavier rollers to get the rpms down at cruise. I am running the motorio dr2 with 18mm rollers 10.5/ 12.5. It takes off like a monster but revs out too fast. Next trip to the desert I am going to try 11/13 rollers to get some of the rpm down or 12/14. The motor 183cc will tach out at 7700rpm and my 155cc goes to 8800rpm and is tapped out.

Just want to know if anybody has any ideas for the 183 buggy. The muffler did help on the heat issue and the clattering sound while going in loose sand washes for extended time.

[Masteryota]

I would start with the head pipe size. That seems to be a pretty drastic step down from the port size, plus you added length, which adds back pressure. An easy experiment would be to try a length of pipe about the same size as the port, and see if it bogs at all. The smaller diameter has an positive effect on low end torque, but prohibits free breathing for the upper RPM range. If you decide to make a new pipe, try to make as few bends as possible, and use wide radius bends if you need to plumb around things. I like CKAU's pipe with the 2 90's and a 45 to put the pipe above the CVT cover, but that may not work for you.

As for the timing, it may be advanced too much and causing issues, but if you are not getting any 'pinging' from the engine under load, you might be ok. Aggressive timing is good for heavily modded engines, but you might need to look into a longer chain. Perhaps Tom will chime in here with words of wisdom.

[Johnny 5]

I tried a 92 link chain and it was too long and the 90 link stock chain was not allowing the cam to sit in the head flush. So I saw a youtube video on sandpaper milling on a flat surface. It worked so I could get some slack back in the chain and my piston to valve clearance is good.
Compared to my 155cc red buggy with the same cam, port job, exhaust system it runs at full throttle a lot smoother and pulls all the way to the max rpm until it hits 8500/8800 rpms. The blue 183cc buggy at 3/4 throttle or more for a extended run down the main dirt road were we go ridding feels like it is laboring to maintain, but on short burst on the tight trails it runs great from 1/4 to full throttle and back to 1/4 and full throttle.

It reminds me of the tunnel ram with two 750 vacuum carbs I ran on my 355 in my 68 chevelle back in the early 90s. I wanted two 390cfm carbs but my friend talked me into the bigger carbs. I could get it to idle and at full throttle it would run like crazy but at part throttle it would surge due to the weak vacuum singnal. After a month of trying to get it to work I put a single 750 on a wiend team g manifold. It did look very cool with the carbs sticking out of the hood.

Back to the subject, as for jetting I am running a 127 main 34 pilot and at a full throttle run at 50 yards ignition off the plug is dark brown and clean on the other side of the plug. The jetting feels good and as of right now I am thinking the orange coil may be the problem. I have read that the stock coils are in some cases better, maybe it cant keep up with the spark or the cdi may be bad? I am running a stock cdi in the 183cc because the orange advanced cdi dosnt work as good.

It runs good just could run better due to all the money spent.

[Masteryota]

Like I said before, I would start by uncorking the exhaust. The coil and CDI, IMO, would give you a misfire and break up at higher speeds. It sounds like that isn't your issue. Your concern is very close to many vehicles I see with clogged cats. Customers don't see the immediate damage a bad O2 or converter has till a year or so later, when their cat has partially melted and now they can't pass grandma on her way to church.

[ckau]

I agree with master. That motor is choking down. I don't 'know what head port sizes you have but you need a head with 30mm ports. That motor is an air pump. you got to get all the air you can in and out as efficiently as possible. Toss that 2-stoke muffler. The flow design isn't optimal for a 4-stroke. A straight through is what you want. I've messed around with a gazillion different configurations. I found, (with 30mm ports, the one that works best for me) is a pipe 1" dia, with a 1" to 1" 1/4 step up to match a 1"1/4 dia, 16" long straight through muffler, Total length of pipe from head flange to muffler mounting point is 28".
This configuration doesn't do much if any, for low end torque but it doesn't seem to take away either. I compensate with cvt and gearing changes to compensate. It does allow my motor to breath better on the upper end, 6,000+ rpms. I can get some extra top rpms with this set up. I'll experience valve float before the motor loads up.
I wrap the lead pipe with header wrap. I don't know if this increase the hp as claimed, if so that's a good thing but It does reduced radiant heat to keep things cooler. you can touch the pipe without blistering your skin. It keeps the whole system quieter by muffling harmonic vibrations in the pipe. Most all heat and noise exits the muffler. I picked up a SuperTrapp universal for 150cc range motors a while back and I'm waiting for some warmer weather to try it out. I'm hoping the SuperTrapp will allow some more fine tuning to the exhaust.