Backfire - Carb and Pipes

[oowf]

I have several problems that I think are related.

I follow the 3 min, 3 mile warm up faithfully with my ride. (1998 FXSTS) Lately and it's happening more and more, I am getting backfires on acceleration (only after I close (push in) the enrichener), on deceleration and at times just in normal cruising.

The acceleration backfire is coming from the carb and rarely is heard in the pipes.

The deceleration is for sure in the pipes.

In normal cruising at times it is just like a major cough... almost like it's getting no fuel for a minute. I also get this sensation at times that I'm not firing on both cylinders.

Under major acceleration however, it doesn't backfire once I get to the power curve (don't know how else to explain that). At least if it does it's very rare.

Also including the acceleration backfire it is constant at take offs from stand still unless I up the revs prior to releasing the cluthc. (Sure don't like doing that!)

I have cleaned the plugs... then installed new ones day before yesterday gapped at 40.

I have read something about working with the carb but don't know how to do that. I believe the air/fuel mixing screw is sealed with an alluminum cap. Would like to know how to do this if it is practical.

My ride is a 1998 FXSTS with just over 11,000 miles. I bought it second hand in China (that's where I live and ride so dealers are hard to come by. To my knowledge there is only one in Beijing with a branch in Shanghai. I live far from both of those places.) so I am not sure if the carburetor is stock or not. Nor do I know how to tell. It does look like what is in my service manual though.

My pipes look normal but I know they have nothing inside. This ride is loud!

I have been wanting to check my timing thinking that may be part of the problem since the bike rides "hot". I feel it is much to hot at the end of a ride... but then I have read that could be caused by to lean a mixture at the carb. I have tried and tried to both find and/or make a sight glass for the timing hole but no luck so far.

I have also read a person can test for leaks with carburetor cleaner. Don't have any and sure don't know where to get any here. Is there something else a person can use?

I know this is rather lengthy but I would sure like to get this delima taken care of. I fear this backfiring is not doing much good to my ride!

Since buying this machine I have changed the fluids, adjusted the clutch, installed a new clutch cable I waited for one month to receive from the States, adjusted the primary chain to 3/4" play and loosest point, adjusted the hex bearing retainer and the fall away on the front wheel/fork, checked the oil pump with a clear flexiline to make sure oil was pumping into the tank and torqued all nuts and bolts to service manual specs. I think I get around fairly good with the maintenance as long as it's not major overhaul so if anyone can point me in the right direction with this back fine issue I would be most greatful!

Thank You and cheers!

 

[glider]

From what you describe, it sounds as if the carb is stock (unaltered) and too lean, the cause of the backfiring after the enricher is pushed in.

You can remove that plug over the idle mixture screw by drilling a small hole in it and if it doesn't come out on the drill bit, use a small screw in the hole to pull the plug out. This can be done on the bike by rotating the carb in the boot where it is mounted after removing the air cleaner parts that restrict doing this.
Richen the mixture on this screw by turning it counter clockwise to arrive at the highest and smoothest idle with the engine warmed up. You can seat this screw LIGHTLY and open it about 2 1/2 turns open and go from there. Always count when closing it down how many turns it was open for reference. You should be OK with just the idle mixture adjustment but if it doesn't eliminate the backfiring, then you may need to rejet the carb depending if you have changed the intake or exhaust from the stock pieces.
Also you can use WD40 to locate a leak in the intake.

Here's some posts to help you here.

Twin Cam CV Carb Rejet Suggestions - Harley Davidson Community

CV Carb Rejet Pics - Harley Davidson Community

Understanding /Tuning on a CV Carburetor - Harley Davidson Community

You may also want to drain the float bowl of any sediment or water as shown in this post...
Poor Performance With Carb Models - Harley Davidson Community

 

[TQuentin1]

As Glider says, getting to the idle-air mixture screw is no problem. On my carb, the plug was probably some sort of plastic resin that came out readily. Once out of the way, it gives you the ability to adjust things the way you used to on your old cars!!

If the carb looks like what you see in the service manual, it is very likely the OEM CV carb. This thing is very easy to work on. Depending on how the bike was treated before you got it, you might be ahead to order the standard rebuild kit (from J&P Cycles or similar) and a rejetting kit that will contain an assortment of jets and needles to give you some options. I also installed the "Yost" power tube in my CV carb that is supposed to atomize the gasoline better to ensure a better gas-air mixture.

The books talk about drilling out the port in the slider to make it more responsive, but be careful on that. Too big a hole, and you probably will not like the results.

While you have the carb off the bike and out of the way, replace all the rubber on the intake manifold so that you eliminate that as a source of your backfiring problem. I am not sure if the stuff to remount the carb comes with the carb rebuild kit, so a call to the tech service bunch at your supplier (J&P or where ever) to ask them about that wouldn't hurt. If not, make sure you get new stuff there.

I know it is harder to keep the old mule on her feet when you are weeks or even months away from getting stuff, so looking at the book and ordering stuff that you MIGHT need is very valuable. If you don't have one, you might also order the Factory parts manual for your bike. Takes the question out of dealing with the parts guys looking in the book (or more likely computer screen) at the same thing you are describing. I have had good luck ordering stuff from Latus HD when there is a need for actual Harley parts. Not sure if they would ship to China though.

Best of luck,

TQ

 

[dangerdan]

Could this just be a problem with bad gas.

From what I see on TV about China who knows how long gas has been sitting in the tank, whether its contaminated with water or even its octane level.

 

[glider]

Depending on how the bike was treated before you got it, you might be ahead to order the standard rebuild kit (from J&P Cycles or similar) and a rejetting kit that will contain an assortment of jets and needles to give you some options. I also installed the "Yost" power tube in my CV carb that is supposed to atomize the gasoline better to ensure a better gas-air mixture.

The books talk about drilling out the port in the slider to make it more responsive, but be careful on that. Too big a hole, and you probably will not like the results.

While you have the carb off the bike and out of the way, replace all the rubber on the intake manifold so that you eliminate that as a source of your backfiring problem. I am not sure if the stuff to remount the carb comes with the carb rebuild kit, so a call to the tech service bunch at your supplier (J&P or where ever) to ask them about that wouldn't hurt. If not, make sure you get new stuff there.


TQ

Much better to just buy the #45/46 low speed jet and the #180/185 jet than the whole kit. It's about $5-$10 compared to $70 or better for the kit. J&P cycles has the jets needed. As far as drilling the slide, stay away from that as it can cause a lean mixture under some instances in colder weather.

If you have the factory manual, it lists the stock size jets on the first page in the carb section.

 

[TQuentin1]

Glider is right about the jet kit. I bought the kit when I rebuilt my '91, and only used the #46 and #185. There ya go!!

TQ

 

[glider]

More than one way to skin a cat!:s

 

[oowf]

Thanks to Glider, TQuentin1 and dangerdan for your replys. Great stuff!

I have yet to find an outlet that is willing to ship to me here in China. If anyone knows of one that sells HD parts and/or after market that will ship internationally I would sure appreciate their name. I have looked extensively... filled out several on line orders just to be told they won't ship to me here in China.

My service manual is very clear on how to adjust the timing. I have the proper timing light but so much oil mist spews out without that sight glass it is impossible (along with my old eyes) to see what is going on. I'm currently trying to make on, by the way.

This afternoon I'm going to purge flush my bowl and adjust the mixture screw. Thanks to everyone who has offered information I found a lot of articles on how to do that...

Again, Thanks!

 

[oowf]

Well, today I flushed the carburetor bowl by backing out the purge screw and working it back and forth. The first time around I flushed through about two ounces of gas and it had a lot of minscule black objects like dirt. So I repeated the process about ten times with the last time dumping through about eight ounces of gas. Still dirty but not as bad.

I also removed the tank filter screen at the pit cock, blew it clean (didn't appear to be dirty in the first place) and put it back.

So... I ran out of time but decided to make sure my throttle and idle cables were adjusted correctly before getting into messing with the mixture screw. I couldn't do it.

According to the service manual I am supposed to loosen both cables to zero adjustment then make sure the handlebars were straight ahead and open the throttle wide open. Then lengthen the cable until the throttle cam just touches the cam stop.

I can't do that. I run out of thread on the cable adjuster before the cam gets to the cam stop. It's about 1/4" short of touching the cam stop.

Since I can't get that part set right I'm wondering if I'm doing things in the incorrect order here. Maybe I need to figure out why the cables don't adjust properly.

All the comments are surely appreciated. I will do everything suggested tomorrow but I am quite sure my pipes have nothing in them. Would that contribute to the popping on decel?

I guess this is the bain for buying a used bike...not knowing what is stock and what is not and what the last guy/gal did... although it is sure fun working at getting everything tuned just right.

This evening after I flushed the bowl and cleaned the pit cock screen I took her for a ride and it ran worse than it has before... actually it seemed the idle I had set about three days ago was way, way down now so I guess I need to adjust that tomorrow as well and I don't guess... I know I'm a little confused right now...

One stupid question. (I think I'm allowed one a day... <grin>) It seems to me it's going to be rather difficult to adjust the mixing screw with the bike running. Is there a special trick to that and also with the air box removed will I get a true tune?

Cheers and Thanks!

 

[glider]

No tricks to the adjustment other than the proper size screwdriver and let the bike warm up completely before starting on the adjustment. You may have to repeat the adjustment a few times till you get the feel of the highest and steadiest setting on the screw, usually in the area of 2 1/2 turns open. Careful not to seat the screw hard, just seat it lightly if at all.

Wit6h the throttle cable adjustment, it's important that the full open position is reached by the grip just a hair short of the carb being fully opened to avoid stress on the cable ends.