Well, I installed and tried my China megabomb today. Also dropping to a 12.5pj and a 137.5 main jet (one step leaner on each) since I was getting dirty anyway. Not the one change at a time scientific method, but I was familiar with these jet settings from previous experiments, so essentially the major change was going to be the bomb.

It's definitely quieter, but with the megabomb in place my bike was coughing now and then at low speeds, and seemingly had its desire for high rpm curtailed. Mid range seemed strong, but I didn't think it was necessarily stronger. Rolling off the throttle from WOT didn't reveal anything wrong with the main jet.

I just couldn't see anywhere that this megabomb was adding to what I already had. In fact it felt slower.

So I swapped the 1.5" OD header back on, leaving everything else the same, and the bike was back to its old self. No low end hesitation and pulling hard everywhere and at WOT.

Though I read somewhere today where a guy had to go up quite a few mj sizes after installing the FMF version, I don't think the main jetting was off or at least that far off to matter.

My box of goodies including the a/f meter and O2 bungs should be here in a week or so. After I get the bungs welded on I'll be able to know more about what's up with my jetting and proceed accordingly.

As for now, I don't see where the bomb did anything of benefit except lowering the exhaust note

 

How well did the header match up to your slip-on ? The real megabomb will only match up correctly with the FMF slip-ons. If the slip-on has a much smaller o.d. to the headers' i.d. then the megabomb doesn't work as intended and power goes down.

 

This mega bomb is 1.375"OD engine to the bomb, then 1.5" after the bomb. My mid pipe was a very tight fit into the end of the bomb header. No clamp necessary.

Same smaller OD mid pipe that expands to larger that was used in the 1.5" header that was on it. So other than the bomb header the rest of the system was the same.

The difference in OD to the bomb could explain why is felt restricted. I'm tuned for the larger header. I'll play with jetting after my speedo drive magnet thing arrives. Hopefully in a week or two.


- I put a smaller OD pipe on my XR200 once and though it idled fine, it stalled when I tried to rev it and wouldn't start again until I'd put the larger header system back on. Then it fired up immediately and revved freely again.

 

I've spent at least an hour surfing the inet looking at dyno charts on various bikes using the original megabomb and little to zero hp gain seems to be the norm

 

Since your midpipe and, I guess, slipon are a good match to the header, you should find everything you need on my megabomb vs stock header dyno test thread. For instance, upper rpm trq/hp should be up with a loss down lower, AFR should go richer over the stock header...etc.ect..

 

I don't know if your results would be valid? I have the 36mm carb and drilled air box. You evidently went from stock header to the megabomb, where I already had a larger header on and went from there to the megabomb with its smaller header pipe. Apples to oranges possibly.

Fwiw the complete 2006 CRF250 dual exhaust system I have here uses a header pipe that initially starts out at 1.375" OD, then expands to a larger size. That's smaller than the 1.5" that was on my Kaw when I bought it. The CRF and megabomb pipes' smaller OD header sections are apprx the same length.

 

Yep,
yer right ..

 

While my mid pipe's forward reduced connecting section was a perfect/tight slip fit into the end of the megabomb, your asking about it reminded me that when I initially began using this midipipe and muffler designed for the XR200 it was about a half size smaller than a perfect fit into the 1.5" OD header that was on the bike when I bought it with. I could cinch the slotted part of the header's swedged end down with the clamp, but using a slightly too small mid pipe connection isn't the way to go overall.

I've been playing around with it and see that the swedged end of the header was never expanded quite large enough to accept a pipe that's the same size as the header's OD. Even after spending a lot of time straightening out the slotted sections that I'd clamped down on it's obvious the swedge just isn't large enough.

So I'll attend to that probably having to get the swedged end cut off and a 1.5" ID section welded on to the end of the header, then have a 1.5"OD section added to the forward section of the mid pipe (which will match the mid pipe's OD once the smaller connector is cut off) to make a seamkess transition from header to mid pipe.

It's hard to find ss tubing in this small town, so next trip to Cebu I'll just take the header and mid pipe with me to get it fixed quickly before I move on to Marcelino's cam mod.

Luckily my ship has come in, or is about to on Sunday-meaning my large box from the USA with a/f meter and other bits (BB331 and 216cc hi comp XR200 piston etc etc),will be available for pickup sometime between Mon-Wed

I can't imagine the novelty of watching my a/f ratio AND rpm at the same time as I will finally having a tach on the bike (TinyTach) ...tuning should soon then be much easier !


I think AFR did go richer with the megabomb, but lacking the a/f meter and the desire to seemingly go backwards with jetting I didn't jet down to see what was happening. The engine just sounded all bottled up in the megabomb.

Hopefully the smart fone app that lets me test between and settings I choose; like 25 kph to 50 kph etc, will prove reliable enough to test quickly using a one gear blast that's not so hard on the engine. I may go back and play with the megabomb at some point.

 

I recently purchased one of these header from eBay as well. The Seller is Just Off Road and the listing for the header itself specified "no modifications" so I figured maybe they'd made changes so that now the ID was correct. No such luck. I wound up taking it to the local Monro Muffler shop and the mechanics there did a nice job expanding the pipe so it now slip fits the exhaust. It cost me $20 and I gave the mechanic a $5 tip. So, consider that if you're going to use this header you'll need to find someone willing to work on it, take the chance it'll split and pay $25.

I was going to send it back but having to wait for the seller to respond, take the exhaust to the post office and deal with the hassle of working out the return made me take the chance.

 

Question is does it improve performance at all?