Backfiring through carb

[Woody1960]

PCM 251 1988 with only 215 hrs

Symptoms:
Runs great up to 2400 rpm
Backfiring a lot through carb above 2400 rpm under load
No backfires when in neutral, easily revs 4000 rpm then



Done:
New points
Timing adjusted to spec 9BTDC at 900 rpm (triple checked)
Carburetor cleaned, new gaskets
Fresh gas


What can it be?

[Rednucleus]

Triple check that your plug wires are in proper firing order

[Woody1960]

I will check the order of the wires as suggested. Cancbe that but it is odd it runs great up to 2400, and up to 4000 not under load.

[hal2814]

Have you looked at your spark plugs? If they look shiny and new you may have a leak in your exhaust manifold somewhere. Last time I got backfires the riser on one side of my 83 Ski was leaking into the cylinder chamber. It caused back pressure that fired back through the carb. That’s a scenario that would get worse under load. I’m lucky I didn’t bend a rod.

[Woody1960]

It has only 215 hours only in freshwater. Plugs look brown/ black not shiny. But it can be a leak.

What about the POWER VALVE in the carb? Would a bad such valve cause this?

[core-rider]

Usually timing, incorrect firing order, or valve lash can cause backfire through the intake. Is this a new issue after maintenance of some kind, or existing issue you're trying to diagnose? Unless tore the engine down, or did something to affect valve lash I'll assume that's not the case for now.

I would start by finding TDC of cylinder 1 and verifying the timing mark on the balancer... they can spin and make the marks inaccurate over time.

Triple-check firing order with plug wires. May also want to run the engine at night to see if you have any plug wires loosing spark to a manifold or something. You'll see a spark jumping gap in the case, but doubtful this is your issue.

The power valve is just for that initial stab of the throttle to assist acceleration without causing a lean condition. Just slowly adding throttle while accelerating while already under load it doesn't have as much to do with it. In your scenario I'd say it has no affect.

[BigRedSV211]

I have the same exact problem, let me know if covering the vents works for you.

[Woody1960]

Core -rider you have some good input there.

Still I am puzzled.


-Engine has never been a part. The PCM 351 1988 has hydraulic lifters right = self adjust


-I have adjusted the ignition to 0 BTDC, and 5BTDC, 10BTDC now back at about 9. Adjusted to the point of barely starting since too late. I will check the markings anyhow.
-Cables look good, cap looks good, points look goods.
- It runs great up to 2400 with load, and easily revs 4000 without load without any backfires
-It does not matter if I accelerate slow or fast, backfires at 2400 in each case.
-Thinking it was lean pops I have had someone accelerate while pumping back, and forth on the lever pushing in gas with the accelerator pump. I stod by the engine, and saw how gas was pushed in. Still the same problem.

An ignition problem would manifest itself even when the engine is in neutral right?. Shorts, jumps in the cables etcetera should have nothing to do with if the engine is under load or not. Detonations have to do with the load yes but I have no detonations. It runs smooth up to 2400 at each time.


To me it seems like a fuel problem since the engine's fuel need changes under load.


bigred are your symptoms EXACTLY the same?

[NautiqueTim]

You need to check to see if the timing advance is working.

[Woody1960]

It was a stupid mistake.

Cable 2 & 7 were swapped.

[Scooter G]

Good call guys, good find Woody1960. I was watching to see how many times firing order got called out. It's amazing what happens when a plug fires on an intake stroke.
If you've never gotten a firing order, out of order, you haven't changed enough plugs and wires, lol.

[Scooter G]

BigRedSV211, I think you wanted to be over here on MN Ryan's thread:

https://www.planetnautique.com/vb5/f...1-when-surfing

[Woody1960]

Well the good thing is I fixed other stuff. :

-I have a clean carb with the backfire protector installed. (Holleys before 1991 have no such thing so you can blow the power valve).
-New power valve
-New points
-Perfectly adjusted timing