Topic: Popping at low revs/decel, plug fouling

[zabzob]

Everything on my bike was great. Last spring, I got rid of my rusty headers going into a 4 into 2 exhaust and replaced it with a Delkevic stainless 4 into 1. As per title, since then I've got popping at low revs and on deceleration. I have also got some plug fouling happening on cylinder 2. Last summer, it got so fouled (looks carbon fouled to me) that cylinder 2 stopped firing, and the popping stopped, so it seems isolated to that cylinder.

Question is: popping on decel and at low revs seems like exhaust leak, but could that also foul the plug? As I said I'm pretty sure it's carbon fouled, hoping not oil. Rebuilt the head probably less than 5000 miles ago and bored out the cylinders 0.5mm and had new rings in less than 1000 ago.

[newday777]

Is the popping out of the end of the exhaust? Popping back through aftermarket 4into1 systems is typical to hear compared to stock exhaust because of less baffling in the mufflers.
Or is it popping back through the carbs/air cleaner?
Are you running the stock air cleaner or pod filters?
Are the carbs spitting wet gas back into the air cleaner rubbers so they are wet?(check after a ride) (This will cause the plug fouling and is a sign of plugged internal carb feed ports)

[zabzob]

Stock air cleaner, stock jets. Sounds like it's popping out of the exhaust. I'll check the air cleaner side rubbers after the next ride.

The exhaust could be more poppy than stock, but weird that it's only cylinder 2 and stopped after that one fouled out.

[scottly]

Question is: popping on decel and at low revs seems like exhaust leak, but could that also foul the plug? As I said I'm pretty sure it's carbon fouled, hoping not oil.

It's more likely the rich condition that is fouling the plug on that cylinder is causing the popping. Un-burnt fuel can build up in the exhaust, then be ignited from the hot exhaust gas on a subsequent exhaust stroke. The popping stops once the plug gets so fouled the cylinder quits firing.   

[HondaMan]

Question is: popping on decel and at low revs seems like exhaust leak, but could that also foul the plug? As I said I'm pretty sure it's carbon fouled, hoping not oil.

It's more likely the rich condition that is fouling the plug on that cylinder is causing the popping. Un-burnt fuel can build up in the exhaust, then be ignited from the hot exhaust gas on a subsequent exhaust stroke. The popping stops once the plug gets so fouled the cylinder quits firing.   

+1 what he ^^ said, plus this: if the gasket for the exhaust pipe at the head isn't well sealed, it can make a pipe 'pop' on decel, too.

[zabzob]

Thanks all. If it's popping at low revs and decel, what circuits would I be looking at? Does anybody have a map of the fuel/air flow for each circuit?

[scottly]

Generally speaking, up to 1/4 throttle is dominated by the idle/pilot jet, 1/4 to 3/4 by the slide needle, and full throttle by the main jet. In your case, since the problem is only on the one cylinder, I would start by checking the fuel level in the float bowl in that carb with the "clear tube method. Also, if the holes in the side of the emulsion tube are plugged it can cause a rich mixture.

[newday777]

Do you have Hondaman's book on restoring these 750s? He goes into good detail and pictures of cleaning the internal ports with a fine wire(high E guitar string wire works great).

[zabzob]

Sounds like it's time to buy Hondaman's book! Order is in.

I rechecked a few times with a float level tool. I'll have to do the clear tube method.

I'll have to take it out again to make sure I'm right about how much throttle triggers the popping. I wonder if it's more slide needle. I should double check the clip position when the carbs are off too.

[newday777]

Sounds like it's time to buy Hondaman's book! Order is in.

Did you order it through the site Sponsors section and get the 15% off?

http://forums.sohc4.net/index.php/topic,65293.0.html

[Ozzybud]

Fix the popping issue first before yarding off the carburetors
Since you added different exhaust is when this problem arose. My experience is that popping is caused by an exhaust leak at the head. It sucks in fresh oxygen upon deceleration and ignites any umburned fuel. A rich condition will definitely make it worse. Check the fitment of the header to the head. Sometimes the rear brackets need to be slotted a bit to allow the headers to seat on aftermarket units.

[scottly]

I wonder if it's more slide needle. I should double check the clip position when the carbs are off too.

The slide needle should be in the same position as the other three carbs? Do the clear tube test before pulling the carbs. Are you sure the plug fouling started after changing the exhaust, or could it have been going on before?

[MisterKnife]

Also be sure your exhaust is sealed properly at the head. That will create popping as well

[TideWater2]

Can the popping cause any harm?

Mine has a fairly even robust popping across all cylinders. It was fitted with NOS pipes in 2009 that were not ran after install, they had zero carbon inside when I bought the bike.  The pipe to the flange appears to be sealed as there is no visible signs of leaking. That would leave to gasket behind the flange suspect. Removing the flange bolts can be a challenge, correct?