Ironhead Backfire

[sungod]

Originally Posted by Ferrous Head

Anyone who believes that has never seriously looked at air flow requirements for an engine.
Port velocity is directly tied to the capacity of the engine and size of the venturi of the carburetor. A 38mm carb is simply not suited to a 123cc or a 1,376 cc engine. It will not work worth a fart on either of them.
A VM38 tells you the venturi size is 38mm. A VM28 may well work on a 125 but a 38 is not going to.



Technically, no. The VM series having 5 circuits as opposed to the S&S's 3 means you can "fine tune" the VM much closer too perfect than you can an S&S.

But will you ?

Because most people simply don't. They just want a carb that will run "acceptably". They don't like "sags" bogs", coughing" or "pops".

IF, you know what your doing and if you spend the time and money to dial in a Mikuni it will perform marginally better than an S&S. Maybe.

I would love to see someone make a VM38 work on a 125cc engine. Maybe at 48,000 rpm ?

Amazon sells a JFG Racing pwk (generic)38mm for 150cc cost is $36.

[01rk]

Did not mean to ignite a firestorm just wanted some suggestions on getting rid of the backfire. That being said, I will try some of the suggestions one at a time to see what works, starting with the air flow adjustment needle, right now it is out 7/8th of a turn. Next will be the adjusting the clip on the needle one notch at a time.
As I said I am 80yrs old so my reflexes are quite slow I roll on the throttle kind of gently, I really have not noticed at what rpms the backfire occurs, I will take note on my next ride.

[JB in NC]

A man much wiser than I once told me that if you are tuning something you are not familiar with, make big changes. That should show you if you are going in the right direction. (At the time I was teaching myself to tune carbureted nitro.) That's why I suggested that you move the needle clip two notches. You may trade one tuning issue for another, like going from too lean to too rich, but at least that will tell you if you are going in the right direction, or even adjusting the right thing. The fine-tuning comes later.

By the way, most advice given above is based on your use of the term "backfiring." I (and others, aparrently) take that to mean popping out of the carb, indicating a lean condition. If you mean by "backfiring" the motor popping in the pipes, that would indicate a rich condition, maybe accompanied by a vacuum leak. Better clarity of terms will always help when it comes to these discussions.

Best of luck with it. A 38mm round slide works well on that size motor.
Jim

[Iron Mike]

Just a statement.
Choke on Mikuni VM is on when pressed down (enrichener circuit open)
S&S enrichener is on when lifted up (with stock S&S cover and lever)

May not be the case here, but I have seen operators thinking the opposite.

[Ferrous Head]

Originally Posted by sungod

Amazon sells a JFG Racing pwk (generic)38mm for 150cc cost is $36.

Well, that would be perfect for my lawnmower. Get some SERIOUS HP out of that sucker.

[Doc308]

Originally Posted by 01rk

Did not mean to ignite a firestorm just wanted some suggestions on getting rid of the backfire. That being said, I will try some of the suggestions one at a time to see what works, starting with the air flow adjustment needle, right now it is out 7/8th of a turn. Next will be the adjusting the clip on the needle one notch at a time.
As I said I am 80yrs old so my reflexes are quite slow I roll on the throttle kind of gently, I really have not noticed at what rpms the backfire occurs, I will take note on my next ride.

Don't worry about the rpm range much in figuring this out. With this carb just try to take note of where about your throttle position is: 1/4 open, 1/4--3/4, or wide open. Usually the starting point for idle mixture tuning is about 1.5 turns out. Turning the idle mixture screw inward ( clockwise) richens the mixture (the opposite of many other brand carbs).

[kitabel]

Have a nice day

[kitabel]

Have a nice day

[kitabel]

Have a nice day

[Ferrous Head]

Originally Posted by 01rk

Did not mean to ignite a firestorm just wanted some suggestions on getting rid of the backfire. That being said, I will try some of the suggestions one at a time to see what works, starting with the air flow adjustment needle, right now it is out 7/8th of a turn. Next will be the adjusting the clip on the needle one notch at a time.
As I said I am 80yrs old so my reflexes are quite slow I roll on the throttle kind of gently, I really have not noticed at what rpms the backfire occurs, I will take note on my next ride.

Sorry mate, it's not really a "firestorm" (Isn't that a Honda ?) it's just the way we hold conversations on the forum.

You'll find most who answer you are just trying to help. Generally, with their own experiences as a guide.

If you truly want to start a firestorm, just mention the brand of oil you use.