2005 Pops Hard on Deceleration!

[trevzor4242]

Just put a new to me motor in my 2005 after 2 years of it sitting. Woot!
I'm using my oem carbs, stock jets, and completely stock intake. This worked flawlessly on the last motor so I have confidence in them being synced, jetted right, etc. I simply cleaned them out and threw them on the new motor.

The bike ran flawlessly, smooth and had great power except that it backfires when coasting to a stop with the throttle closed (usually around 5k rpm), and the idle is a bit inconsistent.

Immediately I thought a vacuum leak was likely. I removed the carbs, meticulously cleaned them again, replaced all vacuum hoses. I did find a small puncture in a vacuum line leading to the coasting enricher. Bingo, I thought... But it still pops like before. I can see flames out the pipes. I checked the diaphragm in the coasting enricher and found no issues. Valves have been checked, idk what else to look at. Spraying brake parts cleaner around the carbs while running did not cause a surge lol.

TLDR:
-Used motor installed
-Popping on decel, weird idle
-Carbs cleaned multiple times
-valve clearance checked
-Faulty vacuum line to coasting enricher replaced (along with all other lines)
-Coasting enricher diaphragm inspected (no issues seen)


Any ideas what I could check? I think I've done my homework here and could use some expert advice.

For reference:
( https://faq.ninja250.org/wiki/How_a_...a_Closed_Mouth )

[Triple Jim]

Hi Trevor, welcome!

My guess is the decel valve is not working right. Let's see if @ducatiman will give you the whole story. He's the carb guy here we all use when we need a set restored to new condition.

[ducatiman]

TJ thank you for the introduction.

OP did you actually service the decel valve? Standard procedure within my full carb service, BTW. Compressed air with carb cleaner sprayed into the vertical hose fittings (with both valve removed from carb and diaphragm removed from valve) Note the cap has a tiny in/out opening as well, same deal.

Does diaphragm "stem bushing" allow float in/out effortlessly? Suggest to clean /polish any discoloration on the bushing end restored to a shiney state, also cleaning the associated opening as well.

3 hoses which attach to carb take-offs are tight? Note within my service, I've encountered many which demand replacement or trimming to correct them, thus restoring full vacuum status. The tiny "D" ring is present?

Thats about all I got for you.

[trevzor4242]

Awesome guys, thank you for the tips!

I did clean out the valve like you mentioned via compressed air. It seems to move just fine now (although was pretty hard to remove the first time).

All hoses on the carbs are tight, don't seem to be leaking. Spraying ether around the hoses while running didn't cause a jump in rpm either.

I toyed with the pilot screws a lot this weekend. They seem happiest at 2 - 1/4 turns out but I'm still enjoying a slight hanging idle. This week I plan to make a new carb syncing tool so I can dial that in and confirm it's not causing any of my idle instability. I haven't messed with it since swapping motors so can't hurt.

[ducatiman]

you've totally ceased the popping on decel ?

[trevzor4242]

From what I can tell, yes.
Carbs get synced today and if all goes well, I have a 3 hour ride on Wednesday with it.

Sensitive little engines these are!

[ducatiman]

grats on success

[trevzor4242]

Update: My problem was two fold - both a faulty vacuum line as well as a dislodged o ring on one pilot screw. I have solved both the backfiring and hanging idle problems.

To sum up:
The bike was backfiring on deceleration, so I cleaned the carbs and replaced a broken hose on the coasting enricher circuit. After reassembly and playing with the pilot screw positions I got it to stop backfiring but it was idling like crap. Removed the carbs again today and found that one of the pilot adjuster screw o rings was in there wonky. I must have done that last time I put it together and not realized it.

Hopefully this thread can help someone out down the line.

A few tips I have for anyone who is going to work on their carbs:
1) Mod your battery/airbox caddy to allow for easier carb removal (All I did was cut the battery section off)
2) If you have stiff rubber carb boots, try boiling yours to soften them up
3) Sync your carbs with a feeler gauge first because the sync screw isn't easily accessible with the tank on