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Discussion Starter #1 (Edited)
Recently switched exhausts (Monster to Empire) & its spitting back through the carb, backfires through the exhaust, it does both simultaneously only on startup once the engine is warm. Jetting is 45 pilot & 2 1/4 turns out, I wouldn't think that would be lean, it doesn't backfire on de acceleration. I cleaned the carb thoroughly, blew it out with compressed air. New copper crush gasket when I put the new pipe on, no leaks in the intake, just adjusted all the valves recently when I installed the CRF cam, timing is dead on. I even tried another 450 carb and it still does it. This thing has been driving me nuts lately lol, any ideas on whats causing this?
 

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Discussion Starter #3
Go up to a 48 pilot
I'll give it a shot. It's just weird, In the three years I've owned the quad 2 of them with the Monster exhaust on it's never done this even with a 42 pilot. Soon as I throw the Empire on bam it started with a little thump (bloompff sound with a flame) out the exhaust when I shut the engine off. Didn't have but maybe 10-15 minuets run time with the new pipe before I decided to throw the crf cam in. Once that was done I took it out for a ride, it pulled hard & had no backfires whatsoever, only on start up once it's warm.
 

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Discussion Starter #5
New or used pipe?
Brand spankin' new man. Ordered a 48 pilot though I don't think that's it. There isn't enough difference between those pipes besides the shape of the can so I don't see that being it. Also ordered the boot that goes from the carb to head just to rule that out as a possibility.
 

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Discussion Starter #6
New or used pipe?
Hey duke, going back to when I installed the crf cam tell me if this makes sense to you. I had the decomp set at around 65 psi (when engine was cold), it would start fine cold, but once it was normal riding temps it would struggle to crank over. I checked the decomp when it was warm and it was well above 65 psi, it would've been higher but it struggled to turn over once it spiked at 65 psi. So I ended up setting it while the engine was warm to 50 psi, seemed to fix that issue. In my head I was thinking well maybe because thermal expansion is taking place and causing the compression to shoot up? Idk lol, just wondered if some how all that might be related to the issue i'm having now.
 

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The bloommp fart you describe and no start to me is a lack of cranking compression, usually happens when the valves are tight or seal bad. 48 pilot is way too big on any 06+ unbored carb.
Before EFI I was running between 40 and 42/ 155
 

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Discussion Starter #8
The bloommp fart you describe and no start to me is a lack of cranking compression, usually happens when the valves are tight or seal bad. 48 pilot is way too big on any 06+ unbored carb.
Before EFI I was running between 40 and 42/ 155
Yeah, I agree on the pilot part. It never has done that bloomp sound till threw the new pipe on, valves are all in spec. I'm gonna do a leak down test here soon. The quad literally has not been rode but 30 min since November of last year, it ran beautifully then, still runs the same now it's just that sound & backfire. These issues started when I decided to do a cam & switch exhausts.
 

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Just had an issue with a Yam engine I have built for a friend, there was a little shaving under the seat of an intake valve, I had to remove the head to find this and scratched it off with my nail, maybe something fell down in the port while changing the exhaust. Most likely the cam change, but if your valve lash/timing procedures are solid and that you are sure not to chew a valve, you could always pull start it to clean it off.

Also a bad timed cam could bend a valve that would then do this, let's hope not lol

A leakdown test will be gold, this is how I knew it was the intake valve on the Yam, where the air escapes tells you the story
 

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Discussion Starter #10 (Edited)
Just had an issue with a Yam engine I have built for a friend, there was a little shaving under the seat of an intake valve, I had to remove the head to find this and scratched it off with my nail, maybe something fell down in the port while changing the exhaust. Most likely the cam change, but if your valve lash/timing procedures are solid and that you are sure not to chew a valve, you could always pull start it to clean it off.

Also a bad timed cam could bend a valve that would then do this, let's hope not lol

A leakdown test will be gold, this is how I knew it was the intake valve on the Yam, where the air escapes tells you the story

I guess there's always that chance that something could have gotten in there. I'm pretty confident I did everything by the book for the cam swap, I think I read ghott's write-ups on here a million times before doing it, everything went smooth. I had the slide plate in the carb upside down that's what gave me the most problems during all this lol. I'll do the leak down later this week, report back haha.
 

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Discussion Starter #12
What are you setting your valves at? Also, was thinking of this a little, did you look at the o ring on the a/f needle?
Both intakes are at .006 & the exhausts are at .011, .012. The o-ring & washer are still on the a/f screw, the o-ring appears not to be damaged in any way.
 

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Discussion Starter #14
You get to do the leak down test? You know what shims you are at on the valves?
I have not, I haven' been able to get my hands on a leak down tester. I believe the intakes are at 1.60-1.65 & exhausts are 2.00-2.05, the right exhaust valve clearance is a tad loose at .012-.013, I went over everything again this past weekend, even threw the stock cam back in again lol & it still has the same issues. I posted the issues I'm having in a 450r Facebook group & nobody has any solid clue besides the obvious stuff that would cause this: Jetting,Timing, Leaks etc.

I did notice something this weekend when throwing the stock cam back in. I took notes of what the valve clearance was with the stock cam before I installed the crf cam & I had wrote down that the right exhaust valve had a clearance of .010. Now when I put the stocker back in with the old shims it now has a clearance of .012-.013. Maybe I didn't measure it correctly the first time? Idk at this point lol, I'm just trying to think of anything that could be causing this.
 

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The cams could have different clearances so that is possible. Do you still have the old exhaust? I would try it as well just to see. I had the same thoughts as far as jetting or an air leak in the intake or exhaust track but it seems like you are covering it. This stuff is hard to diagnose without being there with hands on.
 

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Discussion Starter #16
The cams could have different clearances so that is possible. Do you still have the old exhaust? I would try it as well just to see. I had the same thoughts as far as jetting or an air leak in the intake or exhaust track but it seems like you are covering it. This stuff is hard to diagnose without being there with hands on.
I sold the old pipe, but I did throw the exhaust it had on it when I bought it (stock header & HMF slip-on) which does leak at the can inlet. It didn't do the bloompff sound on shut down, maybe because the slip-on has more back pressure? It still had the spitting & backfire on start up once it was warm though. I just don't get it & its starting to get annoying lol. I'm gonna go to o'reillys and see if I can rent a leak down tester, I just don't see it being a problem with a valve or valve seat.
 

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Discussion Starter #17
I took a video of the sound it makes when starting once warm, seems to not be backfiring through the exhaust anymore. Still makes the bloompff sound, pop/cough through the carb though. The sound is muffled in the video, much more noticeable in person. https://youtu.be/9YMyc6l_9HM
 

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Discussion Starter #19 (Edited)
You can get a compression tester and check what pressure you are cranking at, may just be a decompressor issue, is it adjusted, throw a feeler gauge in the re should be 0,021-0,023
I have messed with the decomp during the cam swap. I had it set at 65 psi, but as I've stated before I took it out & rode it, when I shut it off it had a hard time cranking over, that's when I noticed the backfire issues (bike was never fully warmed up till this point). I checked the cranking compression while it was warm & it was well above 65 psi, struggled to turn over otherwise the reading would've been higher. I ended up setting it while the engine was warm to 50 psi. I do have a feeling that it may be a decomp issue just because of what I just told you. I'm probably wrong though haha.

I just wanna rule a couple things out here & confirmation from you guys would help me. If it was an air leak the backfire/carb spitting would be present all the time, right? Not just on startup & wouldn't matter if the engine was hot or cold. Same with if there was an exhaust leak. The issue only occurs once the engine is warm. I've played with the cam timing during me troubleshooting this issue and a tooth off looks completely not right, so I know my timing is good all corresponding marks are DEAD on. The only other thing I can think of is the Mid-Pipe on the Empire is a slip fit, I can feel/see no exhaust leak there but it may be a possibility?
 
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