Couple 350x questions

[RIDE-RED 250r]

My brother is a member here but forgot his sign-in info. He has an '86 350x we are scratching our heads on and after searching haven't found any real conclusive info.

1st question: K&N, DG full exhaust on it with a 10.25:1 piston. It has a very annoying backfire/crackling when decelerating. Exhaust sealing rings are new and do not leak. Carb is clean and synchronized, valve timing is correct. Also have tried a known working CDI with no results. The only thing that has seemed to improve it a little bit is going 1 step leaner on the pilot jet. That doesn't seem right to me, I would think it would want to be a little richer if not left stock on the pilot jet.. Anybody have any pointers on this particular issue? He is currently running stock pilot jet and #142 main. He ordered a few more pilot jets yesterday and will be trying different jetting when they arrive.

Second question: What is the benefit of the dual valve carb found on the 350x, 250sx, 400ex and so on? We were wondering if a 250r carb would work on the X... Basically, I don't see why it wouldn't since other smaller 4-strokes ran smaller conventional single valve carbs. Mainly just curious on that...

Thanks.

[jb2wheels]

Shots in the dark here:
- Guess #1 = lean on pilot or clogged causing the popping/backfire.
- Guess #2 - valves out of adjustment causing popping
- Guess #3 - "dual valve" because the carb is so big the dual valve minimizes bogging and gives the chance for the thumb thottle to fully open the big slide with limited thumb travel

[RIDE-RED 250r]

Like I said, the carb is squeaky clean.

Valve adjustments are as specified in the service manual.

The carb isn't all that big, 31mm if I recall correctly. What I mean by dual valve is that it has a slide as well as a butterfly that work simultaneously when the throttle is pulled..

[Yamaha_Rules69]

The problem with the poping during decelation is a jetting problem. It worries me when you say the carb is syrcronized, because that means 2 or more carbs all in tune (opening all the way to wide open, and closing at the same time) it has nothing to do with a single carb. I believe that means you are running rich on the main jet. Check your plug at wide open, and see if it is a little black. If so, go down a size, and see if it improves. If you need more info, you could do a search on here for "plug chop" or on google. Good luck!

[Dirtcrasher]

All I had to do was mess with my 86X air screw 1/4 turn at a time and the deceleration pop went away.

The pilot may be a bit small for a K&N also..........

[atc007]

If you can come up with a 400EX carb,,that should land you real close,,and 15- 25 years newer.

[RIDE-RED 250r]

The problem with the poping during decelation is a jetting problem. It worries me when you say the carb is syrcronized, because that means 2 or more carbs all in tune (opening all the way to wide open, and closing at the same time) it has nothing to do with a single carb. I believe that means you are running rich on the main jet. Check your plug at wide open, and see if it is a little black. If so, go down a size, and see if it improves. If you need more info, you could do a search on here for "plug chop" or on google. Good luck!

On a dual valve carb like the 350x, 250sx, 300ex, 400ex have, the butterfly valve and slide are mechanically linked and there is an adjustable tang for setting the lash between the 2 at zero lash. That's what I mean about synchronizing the carb. Sorry for any confusion I caused...

We were thinking we could get it dialed in better with more jetting experimentation. It runs strong all the way through the throttle range except for that popping....

From what I have seen when I searched this topic, it seems to be a common issue with a piped and K&Ned 350x.. But it seems that no sure fire cure was reported.

We will try some more jetting adjustments and I will post back what happens.

Thanks for the replies guys.

[Mosh]

I have to agree with DC. Those carbs are notoriously lean from the factory on the pilot which will cause closed throttle decel fuel starve. Try bumping up to a 45 slow jet. This will also help them start easier when cold too.