HarleyTechTalk

Long story, will boil it down:

I have an FXR with Star Racing 126 TC engine.  I use an D&S D carb breathed on by XXX Carbs.  Due to weather and work, it sat for over 2.5 years and needed some work after.  Mostly electrical and that is not in my skill set.  I took it to a shop and they said it was not running correctly, so they rejetted it.  Before it sat, the engine ran like a charm at all throttle openings and RPMs.  Gas mileage sucked, but you don't have an engine like that for gas mileage.  I'm talking about better mileage from a 2010 Dodge Charger.  But it was smooth and controllable at all speeds.  But not so controllable when its tail was twisted, though.

Now it runs as if it has a vacuum leak, but neither the shop nor I could find one.  At idle I sprayed starting fluid everywhere I could find, including the vacuum fittings on the manifold and carb, and around the throttle shaft.  No leak that I could find at all.

To be more precise, it runs fine at idle, heavy throttle, and low RPM low throttle below about 2200.  Part throttle above about 2200-2300, it starts breaking down, rough running, ragged firing, etc.   All the classic symptoms of vacuum leak.

I have a DTT ignition with settings at 2/2 for initial and advance curve.  Ran fine this way before, and agrees with DTT recommendations.

I tried opening the idle screw and the enrichener while it was misfiring, but no change at all.  Didn't expect it to be better, but hope lives.  This happens at about 10-15% throttle opening above idle opening.  Open the throttle more and it takes off very hard.  Some of that is the accelerator pump shot, but I'm thinking about maybe the transfer slots are gummed up.  Shouldn't be; they were soaked in carb cleaner, but possible.

The jetting from the shop is 100 main, 34 intermediate, and idle screw about 1.5 turns out.  The jetting from XXX Carbs was 88/33.  The main jet feels pretty rich to me, but it is not supposed to work at those speeds.  I have an AFR meter but do not trust it because when the mix is in the correct range according to the meter, the engine barely runs, and has done this since the engine was running correctly.

One thought about jets:  XXX Carbs used S&S jets.  The shop did not, or at least they look a lot different from what I -know- a stock S&S jet looks like.  No telling what the actual flow is compared with S&S.  I put genuine S&S parts back in, 92/33, but no change.  Didn't expect it because I doubt the main has much effect down this low.  Hoped there would be a slight difference with the intermediate jet, but could tell none at all.

I ordered some S&S jets from Dennis Kirk, a 62 and a 72 to see if the air bleed makes a difference.  Not here yet, so no answer to this question.

Only thing I could think of was the air bleed on the outside of the carb is 84.  Seems it should be somewhere about 60 or so.  Not sure if there is an idle/intermediate air bleed on this one or not.  Can't find anything about this.

Anyway, I'm out of ideas on this one.  So, two part question:  Does the jetting sound in the ballpark, and can anyone get me on the path to make it run at part throttle?

Sorry to blather on so long.

Thanks in advance.

Two things. 

One get an AFR gauge that works, and also make sure the wide band sensor is in the exhaust stream well enough to sample. That may be the problem there since you said it was always suspect.

Two if it ran fine before they changed jets, put the same jets back in and see how it runs.

Did any of the electric repairs happen to ignition system? 

Maybe VOES has been activated?

You sure you have an accelerator pump on a S&S SUPER D carb?  you mentioned the accel pump shot.

He may have one of the third party float bowls with a Bendix style pump.

Most of the work the shop did was electrical.  The wiring harness was circling the drain, along with the turn signal module and no telling what else was either bad or would be pretty soon.

Hurricane Michael came through here with winds of about 155-160 MPH according to what the insurance company told a friend, so salt spray was everywhere.  The hard parts and the seat came out OK, but the ends of the wires and the connections were in pretty bad shape before the storm and even worse afterward.

The shop, and DTT, called it a MAP sensor, and it has one.  It's a VOES.  The bike was misfiring  when they test rode it, so they found the sensor was bad and replaced it. DTT advised them to use one and they said so in the instructions.  I'm not sure it needs it, so any advice will help.  Either way, it's misfiring.  I just don't want to burn a hole in a piston.

It does have an accelerator pump, just like the one pictures other than my throttle arm is brass.  Helps out a bunch on throttle response leaving a stoplight and on cold starts.  Probably not a good idea for racing.  Enrichment with the air flow at high RPM seems unpredictable.  All I read about was how a D could not be made to run on the street, but this one was docile as stock when ridden so as to not attract attention from the LEOs.

I did put the jetting back to stock and dropped the air bleed size about 10 sizes.  That was the only jet I had on hand right now.  No change.

Maybe it's time to crank up the air compressor pretty high and blow out the transfer slots.  Grasping at straws here, but got to try something.

S&S carbs are easy to tune. Check float level . Make sure the in richer is is seated.

If it runs decent up in the upper RPM, and in the very low RPM, but suffers in the middle, the main jet is probably too rich for the mid range but good for the top end. Aside from any other issues that the motor / bike has from sitting or other causes, I would get the AFR gauge working as it should. A lot will be shown as far as jetting when you can see the air / fuel ratio.

I used to do a lot of work with Don at XXX Carbs and have never seen or heard of him doing anything with a G carb.

I think everyone needs to sit tight until the OP posts some pictures of this D carb with an accelerator pump.

Your wish is my command.  Since you worked with Don, let me give my condolences.  He and I talked a fair bit about about this and he was a good man; intelligent and honest.  I sent the carb to him for reworking and it had the bowl with accelerator pump already installed.

Also, for the record, my first line in the original post:

"I have an FXR with Star Racing 126 TC engine.  I use an D&S D carb breathed on by XXX Carbs."

It seems that he did work on G carbs, though.  Memory fades, but I believe he offered some parts for them, like a venturi booster, and I think he did performance work, but would not bet on that.  This was about a year or so before he passed.  Wish he had trained someone so they could pick up his baton and run with it.

This is a D all the way and he did a spectacularly great job on it.  Only thing was that it was jetted rich.  No big issue there since he had to guess on the setup.  Does not take away from the quality of his work.

I had to downsize the pictures because I was allowed to only post <16K file size pictures.  There will be two pictures in two posts.

The other side of the carb.

I had a pumper kit on a B . 

So I see a Thunderjet, then the 88 main makes sense. I bet they also jacked around with the Thunderjet. They may have it all screwed up.

Quote from: Bullwinkle on December 31, 2023, 11:56:28 AMI had to downsize the pictures because I was allowed to only post <16K file size pictures.

interesting ......  a good reason to use an image hosting site

If your float is set too high it will do that.
If your bowl vent is blocked you'll never get it jetted right.
Those accelerator pumps can leak so as the throttle is opened it starts pulling fuel causing stuttering.
Close off the T Jet until you get the carb right then work on that.
Just a few thoughts 

Thanks, all for the help.  I don't upload pictures more than a time or two per year, but it might be a good idea to sign on with an image host.  Just in case.

Next day or two I'll work on this.  Take the carb off, make sure it is as clean as I can get it, then check float level, clear the vents, etc.  The jets I ordered arrived, so I will probably lean it out one step from how it came, since it was running rich before it sat.

The big uncertainty is how much tuning wiggle room there is before it burns a hole in a piston by going too lean.  Probably as rich as it is, there is a ways to go.  As it is, I don't have even a good baseline.

Quote from: Bullwinkle on January 01, 2024, 09:30:26 AMThanks, all for the help.  I don't upload pictures more than a time or two per year, but it might be a good idea to sign on with an image host.  Just in case.

Next day or two I'll work on this.  Take the carb off, make sure it is as clean as I can get it, then check float level, clear the vents, etc.  The jets I ordered arrived, so I will probably lean it out one step from how it came, since it was running rich before it sat.

The big uncertainty is how much tuning wiggle room there is before it burns a hole in a piston by going too lean.  Probably as rich as it is, there is a ways to go.  As it is, I don't have even a good baseline.

An onboard AFR with a realtime display is critical for tuning this carb.

Also, the knee jerk reaction is to think of the T-Jet as being needed for high rpm, big displacement, etc. Totally not true. Think of it more as a way to split the load of single main jet. The main jet simply does not have the bandwidth to cover the operating range from where the intermediate jet drops off and redline. Adding another fuel circuit allows the main jet to cover the lower portion of the band and the T-Jet to cover the upper rpm range of the bandwidth.

The main jet is easy to control where it comes in by changing the external air bleed for it. The air bleed for the T-Jet is a bit harder to do and requires enlarging or reducing the holes in the base of the T-Jet.

A good way to do this on the street is to isolate the various jets as they are tuned. For example, plugging the main and T-Jet while the intermediate jet is tuned allows the rider to see exactly where the int jet comes in at and where it runs out of fuel. Then, bringing in the main jet, starting with a huge jet and small air bleed will allow the rider to clearly identify the what the jet is doing and then align it. Thenmove on to the T-Jet.

Doing this correctly will require a lot of jets for the intermediate, main, Main A/B and Thunderjet. The idea of 'order a new jet' is silly. Unfortunately, most will cut corners on the AFR and jets.

Best of luck with your project.

Thanks for the tips about plugging the main and T-jet and then tune the intermediate.  I always tried to tune the main jet first, then work backward.  Maybe overthinking things, but I figured that the main circuit had more influence on the intermediate than the intermediate did on the main.

I did "order a jet."  Reason was that mostly I ran S&S Gs, most recently on a 111".  The jets for the G leave off well before the ones for the D start.   I think the biggest I ran on the G was about 78-80.  In order to tune, one must have tuning parts, so I ordered 96, 94, 1nd 92 main, plus 35 intermediate.  I had none of them, so if they are required, sooner or later I might need at least one of them.

The T-jet is another conversation because I have no idea of a starting point for it.  Maybe run it out through second and third gears and try the seat of the pants dyno to see what it likes.  At some point though, it still comes down to changing parts to see what works.

The AFR gauge is RB Racing.  Seems to be good quality, but I'm not so sure about it any more.  Might have to let the checkbook cool off before I order another.  Maybe do this the old way by reading plugs and listening to the sound.  A clean running engine has a whole different sound than one that is rich.

We'll see.

Quote from: Bullwinkle on January 01, 2024, 03:08:11 PMThanks for the tips about plugging the main and T-jet and then tune the intermediate.  I always tried to tune the main jet first, then work backward.  Maybe overthinking things, but I figured that the main circuit had more influence on the intermediate than the intermediate did on the main.

I did "order a jet."  Reason was that mostly I ran S&S Gs, most recently on a 111".  The jets for the G leave off well before the ones for the D start.   I think the biggest I ran on the G was about 78-80.  In order to tune, one must have tuning parts, so I ordered 96, 94, 1nd 92 main, plus 35 intermediate.  I had none of them, so if they are required, sooner or later I might need at least one of them.

The T-jet is another conversation because I have no idea of a starting point for it.  Maybe run it out through second and third gears and try the seat of the pants dyno to see what it likes.  At some point though, it still comes down to changing parts to see what works.

The AFR gauge is RB Racing.  Seems to be good quality, but I'm not so sure about it any more.  Might have to let the checkbook cool off before I order another.  Maybe do this the old way by reading plugs and listening to the sound.  A clean running engine has a whole different sound than one that is rich.

We'll see.

I really like RB Racing pipes and after reading what they say about AFR gauges and 02 sensors I purchased one. It used a narrow band sensor and was totally not suited to dial in a carb. Would probably be very accurate for a fuel injection system but the AFR on a typical carb cane all over the place. Hate to say it but you need an AFR that uses a wide band sensor.

Added: My red/white FXR is shown on the RB Racing website with their AFR gauge installed. I wrestled with it for a while, but was much easier after I switched to a wide band system. Eventually sold it to an annoying guy that I didn't like.

A follow-up to eliminate some variables.  I got tired of trying this, then that, you know the drill; the buy and try method of diagnosis.  So I took it to Docs in Minneola for them to wave their wand over it.  Doc even came in to assist and advise.

In short, it is definitely the carb.  The XXX modified carb was perfect other than poor gas mileage and running rich.  Smooth, power at all RPMs, good throttle response, the whole thing.  Until the bike sat.  So, Doc and Cory took that carb, and another D that I had, used the body of the other D and some of the parts of the XXX D and Frankencarbed one.  It is not as strong as the XXX S&S, but is pretty smooth and much better than it was.  But...

The power is noticeably down.  I wonder if maybe a Mikuni 48 or a one of those overbored Super G carbs might be the answer.

Before all this started, I dynoed this at a dealer, and at Thunder Beach, and both had it about 145 but AFR all over the map.  At least the rich end of the map.  It felt smooth, though.  The compression was about 10.5-10.8, and the rings weren't sealing.  This was with a stock G and a Mikuni 48, not a lot of difference.

We got Hillside Cycle to work on the heads and get a set of 11.5 pistons.  We ball honed the cylinders, installed the pistons & rings, and the XXX carb, and put it together.  It was noticeably stronger then, but ran very rich.  I did not dyno it, but felt like the difference between a stock Evo and one with a 27 cam, pipes, and carb work better, so maybe 10-12 HP. 

Back to today.  Doc and Cory tuned that thing so the AFR line is flat as a tabletop.  Honestly, it looks a lot like the line on a good FI bike.  Great job there, it just does not run as strong as the XXX carb for some reason.  Seems if the AFR is spot on, and there is enough air flow, then the power should be there.  The only apparent thing was that XXX had one of those bomb sight venturi boosters, and Doc did not think it necessary.  Otherwise, I can't account for the difference and a venturi booster should not make that big a difference.  If anything, it might cost a couple ponies but have better throttle response.

The Mikuni 48 flows almost as much as the D, so maybe spend some time trying to get it right.  In theory.

I talked with the folks at star racing when they built the engine, and they said that the longer body length of the D was as important as the air flow in making power.  Anyone know about this?  Maybe, like a carb spacer will sometimes add power.

Got to go.  Big weekend with my son and grandson, Saturday Daytona, Sunday Gatornationals.  Doesn't get any better.  Be back next week.

Thanks, all.