Adjusting fuel/air in Honda 24hp

[95Viper]

Started GX670 for first time since installing Stretch exhaust, removing governor and K/N filter. Fstg21 is on vacation so I cannot ask him. Idle was high so I lowered it a little. I will have the tach hooked back up soon so don't have exact rpms but it wouldn't let me shift because the high idle wasn't letting the clutch release the belt. Lowered and it shifted fine.

Anyway, my question is...at idle I am getting a lot of popping. Maybe every 1-3 seconds or so a pop out of exhaust. Random just idle, idle, pop, idle, pop pop, etc. Not like a loud bang or anything but enough. How would I know if my air/fuel mix is good? Maybe use hotter plugs? I am thinking that this must be unburned fuel.

I only lowered the idle screw under the air cleaner and held the filter over the intake while adjusting because Honda is so nice about not allowing you to adjust with air cleaner on. I did not mess with the two fine screw adjustments on the carb. Fstg21 and I removed those factory caps (heated off loctite) so they can be adjusted. They have a dab of silicone on them now so they haven't moved since we adjusted them months ago.

I can remove a plug and look at it but would it make more sense to take it for a 5 minute drive to see if they are black, grey or white? Basically how long do you need to let engine run before checking plugs?

[fstg21]

I'm back.

You may have a leak between the carb and the intake.( if you had it off)

Or you may have a leak at one of the header gaskets.

Did you do the valve springs or rockers yet?

If so, the valve adjustment may be out.

If none of the above, then it's probably a lean backfire.

Rev the engine a few times from idle, do you see any puffs of black smoke out the exhaust?


It should just puff as it transitions from idle circuit to the main jet. On the GX670 carb mixture screws, clockwise is richer, counter clockwise is leaner. The screws control the air bleeds. They primarily only function at idle.

Make very, very small adjustments. 1/8-1/4 turns

After the filter and the exhaust I'd expect a higher idle because you're getting more air in and out, it probably leaned you out too much at idle.

To check the plugs, warm up the engine, then swap plugs. Do 3-4 full throttle runs up and down the street. Shut down the engine at the end of the last run back to the house and coast up the driveway. Pull the plugs and check the color. Adding some cheap octane booster will make the color more visible. This will only tell you if the main jet is correct.

Did you put an advance key on the crank yet?

[95Viper]

For clarification, the popping is not like a firecracker but not pleasant cool harley type rumble either.

[95Viper]

Hey, welcome back. Didn't have carb off and didn't change springs or rockers yet. That is another story. I will fill you in later.

No puffs of smoke. So cut away the silicone dabs and adjust those two small screws?

If I adjust them and it goes away, how long do I run it before checking plugs? Are the OEM plugs good now with the changes that I made? Should I go to a hotter plug?

[fstg21]

Oem plugs are fine. You have to put in new plugs to use that method.
Warm it up on old plugs.

Swap in the new plugs then do 3-4 full throttle runs up and down the street. Shut down the engine at the end of the last run back to the house and coast up the driveway. Pull the plugs and check the color. Adding some cheap octane booster will make the color more visible.

Literally 5 -10 minutes of driving

The correct plug is BP5ES NGK Part number 7832. Most oreillys carry them for $2.50 each.

You can also run a 7333 BP6ES. One step colder. If you start getting detonation or little aluminum flecks on the ceramic.

If you plan to do alot of testing, buy a case of 12 from rockauto or clubplug and save some $

[fstg21]

If you adjust it and the sound goes away, leave it. You can't really hurt the engine with the air bleeds, because if you go too lean it will start stall and pop. Too rich, black plugs.

You may just need a fresh set of plugs.

[95Viper]

Alright...I am good and off to some testing.

I just noticed your comment above though. Did you put an advance key on the crank yet?

What is that all about? We never talked about that.

[fstg21]

You can advance the timing by using a milled key on the flywheel side.

You have to run premium fuel. It does give a nice torque boost.