Ironhead Backfire

[Iron Mike]

Just to throw this out there, perhaps it has already been done, but it is tough to correct a fuel problem on an engine in a poor state of tune.
Fuel mileage seemed to have been the main reason for the carb change. A few things need to be verified prior to r/r a carb (in my opinion) for a fuel mileage issue.
Verify timing
Clean air filter
Correctly adjusted valves on a worthy top end
Tire pressure

Tough to tune any carb without proper timing and compression. Your electric starter could be masking issues you would have addressed long before, had this been your kicker bike.

My $.02

[Ferrous Head]

Fuel mileage seemed to have been the main reason for the carb change.

I must have missed that one. I didn't see anywhere where he stated poor fuel milage was the reason for the switch.

And when I asked the question "what's the goal here ? why the swap" didn't see a response.

But it would make sense. Increasing fuel mileage by 10% would mean the new carb will pay for itself in under 30 years. (Based on average IH usage now by those of us in advanced years)

Sorry if I offended anyone older than me.

[01rk]

In post 36 I said 20-25mpg
I have not checked valve adjustment or timing, as I stated it starts and runs good with the S&S, there has been motor work done but I don't know what. I cleaned the air filter, the tire pressure is correct.
I don't plan on any hundred mile trips or hot rodding, just want it to be dependable and get better mileage.

[trappnman]

what was posted- was that the mileage was 20-25 with the S & S- so he changed carbs...and now has a backfire.

going from 20- 25 to at least 35...is a bit more than 10%. just not having to stop at a gas station every 60 miles would be a benefit thats priceless.....imo, of course.

[JB in NC]

Originally Posted by 01rk

In post 36 I said 20-25mpg
I have not checked valve adjustment or timing, as I stated it starts and runs good with the S&S, there has been motor work done but I don't know what. I cleaned the air filter, the tire pressure is correct.
I don't plan on any hundred mile trips or hot rodding, just want it to be dependable and get better mileage.

As has been pointed out above, a decent tune-up including a valve adjustment and setting the points and timing, and I might add, servicing the advance unit, should be a part of your approach. 20 - 25 mpg is ridiculous and those other aspects of a comprehensive tune-up may be having an effect. A little work up front should give you many miles of pleasant riding.

Jim

[kitabel]

1. find out what low speed jet you have in the S&S
2. replace it with the next smaller size
3. done

[sungod]

Originally Posted by 01rk

In post 36 I said 20-25mpg
I have not checked valve adjustment or timing, as I stated it starts and runs good with the S&S, there has been motor work done but I don't know what. I cleaned the air filter, the tire pressure is correct.
I don't plan on any hundred mile trips or hot rodding, just want it to be dependable and get better mileage.

01rk
Before you go riding again can you mark your throttle with tape at 1/4, 1/2, 3/4 ? This is super important to find the carb circuit causing the problem. Rpm does not pinpoint the circuit, we need throttle position instead. We need that feedback to help you. Engine warm no enrichment.

[Ferrous Head]

OK. Missed that post entirely.

My fault. I did finally ask the right question in post 31 (what is the reason for the carb change). But had you put that information in your first post I suspect this whole threa would have gone differently.

Your engine needs what it needs. By that I mean it needs an A/F ratio of about 14.7 to 1 through the entire rev range. The adjustable circuits on the carb are there to help you achieve that.

A badly tuned Mikuni will perform no better than a badly tuned S&S or a badly tuned Amal or a badly tuned Holley.

The Mikuni does have more moving parts to fiddle with so you may be able to "dial it in" slightly better than the early S&S. But it also means you can get it horribly wrong and chase your tail trying to fix it.

Dialing a carb in isn't a one shot process. You go through all the circuits getting them as close as you can and then you do that process again. And maybe even again.

Because if you make a change to one circuit it will most likely affect one or more other circuits. This is why I so often suggest doing it on a dyno.

I can't see where you specify which S&S you have. Maybe "B", maybe "E", maybe even a "G" ?

The early S&S B's actually have 4 circuits but because it's a fixed air bleed nearly everyone ignores it. That can be a mistake as the fixed size is too small by default. This air bleed determines where the transition from inter to high speed jets occurs.

This means the carb will generally be too rich around 3 grand. Doesn't matter so much if you ride around at 6 grand all the time. But at 3 grand the engine will not make as much power as it should and your gas mileage will suck.

If your poor old S&S is dialed in correctly you'll get close to 50 MPG from the bike. You' also get around 50 MPG from a correctly tuned Mikuni or Keihin. or Del'Orto or Bing or what have you.

Of course that;s a little dependent on how you ride as well.

[73HEAD]

Originally Posted by kitabel

Have a nice day

thanks brother, you have a nice day too.

[Ferrous Head]

Sorry, forgot to tell you how to fix the fixed air bleed. Simply drill and tap that hole and use a main jet there. Then, dial in the carb properly.
You have to be dead lucky to get your A/F graph where it should be by "seat of the pants".
Especially the rich area in the intermediate range. You will also err on the side of too lean when you use the "seat of the pants" method. and that's simply because the engine will make slightly more power in a "lean" condition. Not meaning so lean the engine stumbles but not at 14.7. It will make more power at 13 and below but the engine will run too hot.

The later E's have this mod from the factory.

Of course, all of this is just my opinion and I could be wrong.