Thread: Less power after thumper exhaust

After crashing my CT70 last summer, I've had a small hole in the exhaust where the rear bracket is welded on. The bikes been running good recently even with a hole in the exhaust and bent rear shock. I just put on new level 3 shocks and the thumper exhaust from Trail Buddy. The shocks are great. The thumper exhaust sounds great, but I have less power now. Is this typical for a new muffler? Does it need to burn in before the power returns? Or was I just getting better performance out of the stock exhaust due to the lack of back pressure caused by the hole?

Go a little bigger on the main jet size. That will help as it now wants more fuel to go with the less restrictive exhaust/

Absolutely right -- You may also need to put on a high flow air cleaner .

Another good point, throw on a K&N on that carb and hang on, that baby is gonna run!

Airflow through your new exhaust is clearly different, not as "duh" as it might sound. You've very likely gone from a restricted pipe to one that's less restricted; but...there's another parameter, namely scavenging. While you might be leaving a bit of hp on the proverbial table with the existing air cleaner asm, doubtful that's the current issue. Best guess is that your weak running has resulted from a now leaner mixture and the fix is to richen the main metering circuit. First thing I'd try is dropping the jet needle clip to the lowest groove along with going slightly richer on the idle air bleed screw. That assumes that your carb has these adjustments; I've not worked on many post `75 carbs. If that improves things, then you might want to try going one size larger on the main jet. If you're looking for a few more mph, then I'd pop for a K&N or UNI air filter and the next one or two larger main jets.

No way to know if your current pipe is still a significant flow restriction. It should flow more freely than stock. Louder doesn't necessarily mean faster and hp = mph. Sometimes free-flowing pipes don't work as well as you might expect in a given rpm range or on a specific tune. Scavenging is the difference and it's a boring, longwinded, explanation most won't care about anyway. Get your jetting & ignition timing dialed-in and that's as good as the setup can be. Once that's been done, any changes in performance are the results of how the pipe scavenges and thus permanent.

Okay, so I don't have spare jets but I wanted to see if I could adjust the throttle needle position. When I pulled it out, I found that it only had one position(groove). I saw a number on it that read "198" or "19B". I'm not sure what carb I have on it. I've attached some pictures so hopefully some one can identify what I've got and make a recommendation on my next step. BTW, a couple of things you may notice in the pictures : 1. There is a lot of gas or oil residue on the carb.(possible seal leak?) 2. My air cleaner is not stock and it is squished against the frame. Any recommendations here would be appreciated as well. Maybe a new, better carb, outswept intake/K+N filter, etc? Oh, and can someone more computer savoy than me please copy and photoshop some arrows pointing to the adjustments on the carb that I should know about and explain what each of them does and how to adjust them.(I know it's a lot to ask, but it would really help me understand things better).

Looks like the original carb, an early emissions-era unit with no easy tuning potential. You could try placing an appropriately-sized washer between the jet needle carrier and the slide but no guarantee that this will even work; the needle has to seat properly inside the main jet well at idle. Might be possible to swap-in a jet needle & clip from an earlier model, you may need to change the slide, as well. It might also not be possible to mix & match these items across carburetors of different models. Plus, that sad, tortured, UNI filter looks like the foam is tearing away from the base in your second photo. I'd source another 16mm carb, from an earlier model CT70, along with an angled intake manifold to relocate the carb off the the LH side and actually allow the air filter to fit(!). If that filter really is torn, better replace that, too. These engines don't last very long if allowed to inhale sand & dirt.

i read this post earlier and must say..dude you seriously cannot run the bike like you are..racer is right about the air filter issue and the worthless piece of crap epa carb...you are simply running into a brick wall with that carb irregardless of your new muffler..hell it looks like it aint even much different then stock other then may be a freer flowing pipe..i dont even see much if any expansion chamber which like racer alluded to is beneficial to the scavening phase of the bike..i dunno 4 stroke thumper have basic same principle as 2 stroker in regards to the scavenging phase but in this case you have many issues to correct one being that carb which needs to be tossed for an older style that you actually tune...honda pipes are tuned to each bike and when they go you must start changing everything else in the circuit that revolves around the pipe....for this reason any honda i rebuild stock pipes are used...i just dont like messing with alterations (especially the EPA years) when it comes to hondas...just my take..god speed