Backfiring through carb.

[swill777]

Hey guys, been away for a WHILE! But I'm back now....

So, my port side motor on my 1998 Sportster 1800, which is a 717 with the Mikuni BN-40i-38 carb, recently stopped idling correctly. This is what happened in order...

Last Nov. I took both carbs off, and opened the side with the fuel filter, cleaned everything, and put them both back together. This fixed the "surging" problem I had at full-throttle.
Boat ran good til about 2 weeks ago, other than this. Sometimes when starting, I would have to let the port engine (the problem one) idle for around 1-2mins, before eventually ramping up the throttle, or it would just sputter out and stall. I knew there was some kinda issue, but, eventually when it got going, it would work normally and I could always get it started.
So, 2 weeks ago I was out for a few hours and shut the boat off a few times, for a period of an hour or so, and got it started back up no problem. The last time, I started it, ran about 7 miles at 3/4 throttle (both motors) then shut it off for about 30 mins. This time, I could not get the engine to start, period, it would idle for a sec or 2, then just stall out. I tried, and tried, no dice. Brought the boat back in.
When I got home I noticed that the throttle cable on the port carb/motor, was loose all the way.. I guess the screws came loose over time... I remember reading that the idle adjustment screw works off of the throttle cable tension somewhere.. is this true???
Anyway, I adjusted the tension so it was similar to the other carb, with just a little slack. I also noticed that one of the strands in the cable broke, I guess from the friction of being loosened all the way. Should I be very concerned with this, or can I just cut the one strand back and use it for a while?
Anyway, so the cable tension is good, and I tried adjusting the idle speed, and also re-adjusted the low-speed screw to about 1-1/4 to 1-1/2 turns out. With the flame arrestor off, it seems that it backfires through the top of the carb, I can even see the spark. I checked and the accelerator pump seems to be working, and it shoots a little bit of fuel into the carb when u pump the throttle.. although, I suspect it doesn't work as good as the other carb. The other carb seems to shoot a hard, solid, stream of fuel.. where this one, kinda just sputs some out? is there something wrong with my accelerator pump?
Last night I was able to fiddle with it, and after backfiring a few times, and messing with the mix screw, I got it to run with the garden hose attached. Once it got up and running, it idled right around 3000rpm, and sounded fine and ran normal. I let it sit for a few mins, hit the starter, and it fired back up, no problem??
WTF is going on??? I had it in the water on saturday, after getting is started in the driveway with the hose, and I 100% could not get it started. I am thinking about maybe getting a full re-build kit for the carb, but I don't want to waste my money if this isn't going to fix it. I assume the accelerator pump is part of the carb, and is that something that I rebuild with the rest of the carb? Or is it seperate?
Let me know if you guys have any ideas.. I checked the plugs and they looked oily, but it seems that it has always run rich like this since I got it.. I threw a new set in there and it fired with them, then started doing the same thing.
Do you guys have any ideas? I thought the accelerator pump only does something when you pump the throttle.. so if the throttle was closed all the way, how could the accelerator pump effect the idle and starting of the motor?
This is super annoying cause the other engine runs great and starts right up. I really wanna get this thing to the point where I can count on it.

** just to clarify.. i am only having a problem with the port side motor/carb, and I did take off and clean both external clear fuel filters **

 

[swill777]

carb diagram

You guys probably don't need it, but here is a diagram of the carb from Mikuni and a fuel flow explanation. For the low-speed idle circuit, I just don't see what could be causing a problem other than the fuel-filter, check valves, or needle/seat not opening correctly to let more fuel through. It certainly seems that the carb is starved for fuel when starting...

 

[swill777]

So I got it to idle last night with the hose, backfired a few times, but then smoothed out. Brought it out on the water shortly after. Hard to start, but eventually it got going. Both engines running at the same time the "bad carb/engine" was pushing 6600rpm or so, while the other was doing 7000rpm. Both engines used to run at 7000rpm solid. So I brought the boat to a stop and tried just using the "bad" carb/engine alone. Only hits 5000rpm, and sounds like its hurting... the power just isn't there. I'm almost sure at this point that it's just not getting enough fuel, period. Throughout the RPM range in general. Drove it for 5 mins then shut both engines off. Then I checked the throttle cable to make sure it was pulling the throttle arm (and plate) all the way open, and it was. So the lack in RPM wasn't due to the throttle cable tension or adjustment. Guess what? Now I can't start the engine, LOL. I'm not far from the dock, and I can run on the other engine to get in, but MAN, this sucks. So I try to adjust the idle mixture screw all the way back in, then reset to 1 1/4 turns out, no difference, try 1 1/2 turns out, no difference, sputtering, backfiring, etc. What's funny is when it was running, it sounded ok... no sign of a timing probelm or anything like that. Plus I never messed with the timing on the motor before and it ran perfect, then all of a sudden started this crap. I really am starting to think something failed in the carb, or some crap clogged it up. I do have black fuel lines, but I don't know when they were replaced. Anyways, unless someone has a better idea, I am going to order the full rebuild kit for the carb, including accel pump. I also changed the plugs out again to brand new ones gapped perfectly (BR8ES). Even if its just crud in the carb, and not a failed component, I don't want to take any chances. I'm also picking up a compression tester today since I have never checked the compression of my engines. I'm sure its not perfect, but seriously both of these have been running perfect for the last 6 months, with no slow lack of performance. This is like someone threw a switch for the most part. I'll let you guys know what I find out.. I'm hoping I can get the carb off on Wed night and start tearing it apart.

 

[timmyboy76]

Seems that your carb is work'n, just the motor aint burn'n the fuel. Did you disconnect the rectifier, see if that worked. Cut the plug wires back?, check to see if spark at plugs are good?

 

[swill777]

Hey man, I should also add.
So when I was out on the water last night, and the boat was rockin back and forth, I could hear the carb "breathing", like a vacuum in and out "sucking" kinda of sound. Is this normal? Or do I have something that's not sealing right in the carb, or a vacuum leak?

No, I didn't disconnect the rectifier.. Where is it and how do I do that? I am pretty sure I am getting spark since when the engine does crank up, it runs... I just trimmed the wires back in nov, but I can do it again.
Is there an easy way to check to make sure I am getting a good spark (without getting shocked)?

 

[timmyboy76]

get'n good spark, hold wire w/ plug in it, and crank motor...blue spaark=great, orange/red=no great.with "tang" on block (hold w/ thick towel)
Rectifier, since we have same boat, is mounted inside engine compartment, on backside of "firewall", little yellow 2in box, should be 2 of them.
If "suck'n air, when running, then that motor would scream/bounce of rev limiter at 7k.

 

[swill777]

So it is okay that when my boat was rocking back and forth in the water that this carb was making a sucking sound, like in and out, when the boat was rocking??? I didn't know if that was normal.

I will check the spark then and when I disconnect the rectifier, which wires do I disconnect??

And hey, did you ever get your MPEM fuses deal sorted out?

 

[timmyboy76]

So it is okay that when my boat was rocking back and forth in the water that this carb was making a sucking sound, like in and out, when the boat was rocking??? I didn't know if that was normal.

I will check the spark then and when I disconnect the rectifier, which wires do I disconnect??

And hey, did you ever get your MPEM fuses deal sorted out?

I think, the suck'n sound, was the impellor move'n, being in water, and caus'n the motor to slightly turn, thus, creat'n a vacuum thru carbs, like pull'n in air...?
MPEM, yeah, guess it were wires for radio..etc, member "yelah", got back to me, mentioning, besides red cable,there were 2-red wires also...:cheers:

 

[swill777]

Ok cool.

I'm going to check the spark tomorrow, along with the compression on the engine.
As far as the rectifier, do I just disconnect one of the wires? If so, from what side?
Do they unscrew or something? You don't need to cut them, right?

If stuff checks out, I think I'm gonna take the good carb off the other motor and try it on this one.. Just to get that completely out of the way... I still think it's getting starved 4 fuel somehow.

 

[timmyboy76]

follow the red/yellow wires to a plug, then unplug it, but make sure battery is fully charged/good, since disconnect'n it, wont put charge on battery from stator assy..DONT CUT ANYTHING...:rofl:
Ifproblem still exist, and ompression ggod, spark good, then carb/s are need some attention. Might make a carb tester, for air leak test/pop-off test for needle/seat assy. You'll find it in "search" section or JPX sticky, along w/ air psi, for leakdown test, believe its 4psi for 10-15 sec...

 

[swill777]

Hell yeah man, I already have a similar rig for testing pressure in plumbing/natural gas pipes. I just gotta thread on a 1/4 brass barb fitting to 1/4" hose and I am ready 2 go! Mine has a tire type stem at one end, where you can pump in air slowly with a compressor. When/if I adapt the think up I'll snap a pic of it.
I'tl be too late by the time I get home tonight, But I'll check all that stuff tomorrow. Later!!

 

[Albysure]

What's the deal? Anything work out...I have same issue just no back fireing through carb.