Ironhead Backfire

[jeffro57]

Originally Posted by 01rk

What ever the stock jets are is what is in there, new vm38.
I am 80 years old I don't plan on racing anywhere just want it to run ok

put the S & S back on, follow the guide and ur probs are over

[joe s]

I would say use the Mik, it's a great carb once you get it dialed in. The difficulty is there are a lot of variables, as sungod pointed out. You don't need a dyno, but you do need a methodical approach to tuning. S&S has that in their instructions, and so does Mikuni.

[kitabel]

Have a nice day

[Ferrous Head]

VM38 is the model and size, and has nothing to do with a particular engine, state of tune, etc. It's been used on engines as small as 125cc and as large as 84".

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.

It's not "Simple as a rock", all of the overlapping circuits are already built in. There are only 2 jets, why is that better?

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 ?

[Doc308]

If the backfire occurs when you are less than 1/4 throttle open, you need to adjust with the idle screw. If it occurs between 1/4 and about 3/4 throttle , you adjust the clip on the piston
‘s needle valve. If it occurs from 3/4 to wot, you replace the main jet , located inside of the float float bowl behind the drain plug.Ihave used the 38 mm VM on all of my my old bikes since the 1970’s and it is my favorite carb. They are incredibly smooth and easy to adjust. I had problems with one Mikuni I bought on EBay years ago and found that a po fooled around with the jets. The best out of the box Mikuni kit you can buy is from the Sudco Company. I’ve had the occasion to buy from them a number of times and , most of the time, the carbs were set dead-on right out of the box, aside from idle speed.

[sungod]

Originally Posted by 01rk

I put a VM38 on the '74 XLCH I bought and now it backfires through carb when I get on the throttle, if I put the enrichner on a little it's not as bad, turning the air adjustment screw does not help. It starts and idles fine. Does not backfire with the S&S that was on it when it bought it.

Skipping the rants and getting back to topic here.
As JB in NC stated eliminating the pop can be as simple as moving the needle clip down to richen the mix.That procedure will take less than 15 minutes and no expense. Last point make sure there is no slack in the throttle cable.

[jeffro57]

never heard of a throttle cable causing backfire

[rejeanprimeau]

Originally Posted by jeffro57

never heard of a throttle cable causing backfire

That cable control the carburetor, first to be checked.

[doodah man]

Define “…when I get on the throttle…”. If you are quickly/sharply snapping the throttle open that might be the whole problem - there is no accelerator pump to richen the lean spot created from opening the throttle like that. The super e has a pump on it so you don’t really get the lean backfire from that carb. Or maybe an intake o-ring leaking by unmetered air is another possibility to consider. “If I put the enrichener on it’s not as bad” - I say check for intake air leak next.

[Doc308]

Originally Posted by doodah man

Define “…when I get on the throttle…”. If you are quickly/sharply snapping the throttle open that might be the whole problem - there is no accelerator pump to richen the lean spot created from opening the throttle like that. The super e has a pump on it so you don’t really get the lean backfire from that carb. Or maybe an intake o-ring leaking by unmetered air is another possibility to consider. “If I put the enrichener on it’s not as bad” - I say check for intake air leak next.

The Mikuni dos have a great snappy throttle response without an accelerator pump. It is not normal for a Mikuni to backfire or miss if it's i dialed in properly. If there is an issue with quickly turning the throttle, it can be adjusted out.