The dreaded carb cough and interesting findings

[gearbanger]

Alrighty, I have an update, but still no change.

My James rubber lip gaskets and new flanges came in and I had some time before heading out to sea for a work trip so I installed both as well as new (gapped 0.040) spark plugs. I also checked my throttle plate position and before reinstallation, the plate was resting just before the transfer ports. I fired her up after and went on a little ride and noticed that my idle was hanging after warmup so I brought it down. On that first ride I had a few coughs while blipping the throttle at red lights. No change from before, still happened at the same frequency I’ve been experiencing. The following day i went on another ride and after she was good and warmed up i did the idle drop method to tune the IMS. I couldn’t get it to stumble when turning the mixture screw out, but got it to stumble past 1.5 turns in, so I just left it at 2 turns.

These rubber lip gaskets seal on two planes of the intake manifold, unlike the stock gaskets that only seal on one plane. I don’t own a propane bottle and I live in an apartment complex (not to keen on storing it in my studio apt) so I haven’t officially tried the propane leak test, but I don’t think an intake manifold leak is my ghost.

What I’m thinking and am going to try next…

Possible my ignition coil is sending a weak spark to the rear cylinder. I’ve checked ohms on the one that is currently installed and meets my service manual specs. Please note this is the 3 prong single fire 2006 ignition coil. I have a brand new spare that I bought a handful of weeks back and have not installed yet, so I’m going to give that a shot. If this is the issue, it would clean up the sooty rear plug.

Can a sooty rear plug also cause carb cough like I am experiencing?

Second guess is leaking exhaust gaskets. I also have a new set of those suckers and might throw those in with some copper exhaust gasket sealer and see if that solves any issues. If this is the case, it would cut out any incoming air from the front cylinder and richen the mix up, thus evening out the plug colors.

Third guess is my float bowl height. When I did a deep clean of my carb a few months ago, I set the float bowl to my manual’s specs with a protractor to get the correct angle. Maybe it’s set a little too high causing fuel to leak out through the jets and dribble into the rear cylinder? This is my least likely candidate since the carb is sharing two cylinders and I would think that both cylinders would suck in fuel evenly between each stroke…

Thoughts?

[gearbanger]

Bump while I’m away for work I can’t help but comb through every single post that might hold an answer to my issue. I’ve seen a few threads pointing to faulty coils and replacing with new OEM or dynatech coils solving issues. Fingers crossed!

[rokytnji]

Sooty plugs won't fire.

[wedge]

Originally Posted by gearbanger

I couldn’t get it to stumble when turning the mixture screw out, but got it to stumble past 1.5 turns in, so I just left it at 2 turns.

That is still indicating a problem. Yes, bad exhaust gaskets can cause it and you can usually hear them leaking if that is true. Also, once you get it adjusted with the IDM, you should always take a test ride and run it up a grade while feeling for that transfer point in the throttle over and over, and if it coughs or lags then turn the screw out another 1/8 turn and try again, then repeat if it coughs. I always need to open it up about 1/4 or so turn out more than initial setting.

[gearbanger]

Originally Posted by wedge

Also, once you get it adjusted with the IDM, you should always take a test ride and run it up a grade

Thanks for that recommendation, Wedge!

I have not heard nor felt any exhaust leaks and I have never heard a backfire out of the exhaust (which I understand can be a symptom of a bad exhaust gasket) but when I get back to my bike I plan on dropping a bit of oil in both spark plug holes to see if smoke evacuates from anywhere other than the exhaust pipes. If so, I have new gaskets sitting in my freezer and copper silicone gasket sealant on standby for the repair.

Since I have a new Twin power ignition coil on standby to swap out, I figured I’d grab a Dynatek DSPT-1 ignition module to have and try as well. I have found (far and few in between) a few posts regarding the 2006 SE ignition module causing intermittent problems similar to mine and resolution with a new module. I also reached out to Dynatek regarding my symptoms to see what their tech dept. Had to say, and the coil and ignition module was their recommendation as well, given the overly rich rear jug spark plug. Yes, I know, asking them questions is in line with asking a car dealership if I should buy a new car - of course they want to sell you one - but it was worth their 2 cents. And cheaper than the stealership doing half of the work and nothing to show for it

I should have that module by the time I return and do some swip swapping of parts - and possibly the oil evacuation test on my exhaust if no progress is made.

[gearbanger]

I've got an update for y'all!

I changed out my ignition coil, no luck. Still coughing through the carb.

Bought a Dynatek DSPT-1 and played with a handful of the pre-set ignition curves. Still coughed.

Went to a 46 pilot and got to almost 4 turns out and still coughing.

I bought a Kipa HSR 42 (mikuni Chinese knockoff) and rebuild kit on amazon. Rebuilt it to ensure good gaskets and o-rings were installed, threw in a 22.5 pilot and 160 main, 1 turn out on the mixture screw, installed her on the bike and HOLY SMOKES! Couldn't get her to cough by blipping the throttle while sitting in my parking space. NADA! The real test will be tomorrow's ride to work and to the hardware store - the AC Allen bolts were too long for the HSR adapter sleeve that goes over the carb - but man it feels like a new bike. Cost me $100 for the carb and although I've never got my hands on a true mikuni, the build on this Kipa knockoff is damn good. I did have to bore out the throttle cable receivers just a tad to get everything to fit together but no big deal. I'm sure I'll find my jetting to be a little off and I have to start over again with referencing on this form to educate myself with the right numbers people have had success with but all I can say is wow. After months of chasing this annoying ghost with the CV, I'm so happy its finally over with. I really wish I could have figured it out with the CV but I'll leave that to a project bike down the road that my spare CV will find its way onto.

Super impressed with the mikuni knockoff at the moment. I know it's overkill for my little 883 but if it ain't coughing, that's all that matters to me!

[sportsterpaul]

Congrats on your perseverance, happy for you!

[gearbanger]

Today I took her on a 50 mile run and wow. What a difference. From all the time I spent with the CV, I think the carb coughing I'd get while blipping the throttle is strictly a design limitation with the CV. The secondary restriction of the vacuum slide along with quick offset in mixture ratio when cracking the throttle most likely had something to do with it. Like I said in my previous post, I'll be back on the CV with a project bike in the future, but I can definitely see how the differences between the CV and HSR flat slide differ and aid in the resolution of my issue. The HSR (knockoff) absolutely slams. The response is crisper and I was blipping the throttle at every red light, just like the Harley rider depiction in South Park's "The F Word" episode. Not a single carb cough or stumble, and she was running like a bat out of hell the whole way.

For $100, I'm super happy and for anyone that wants to throw a few bucks at the cure to their carb cough, just give it a go. It's even an amazon prime item, so if it doesn't impress after giving it a try, just send it back.

https://www.amazon.com/dp/B07SH56R55...roduct_details

Before my test ride (after getting home from work) I finished off some mods to ensure a solid fitment.

The throttle cables needed to be rerouted to run on the left side of the bike before running between the cylinders to the carb. A side note, my throttle cables (at least) had just enough adjustment to take out slack in throttle play before being unscrewed entirely. A little modification may be necessary depending on your cable setup and available slack.

New A/C to carb adapter screws. Needed 1/2" 1/4-20 bolts instead of the 3/4" that come stock with the CV.

The final adjustment was to file off the first two (from the top) cylinder cooling fins as they kept the carb from seating fully into the carb boot. It was close on initial install, but figured shaving 1/4" of the corners of the fins would keep them from wearing away at the float bowl over time.

[DrMotorcycle]

Originally Posted by gearbanger

Today I took her on a 50 mile run and wow. What a difference. From all the time I spent with the CV, I think the carb coughing I'd get while blipping the throttle is strictly a design limitation with the CV. The secondary restriction of the vacuum slide along with quick offset in mixture ratio when cracking the throttle most likely had something to do with it. Like I said in my previous post, I'll be back on the CV with a project bike in the future, but I can definitely see how the differences between the CV and HSR flat slide differ and aid in the resolution of my issue. The HSR (knockoff) absolutely slams. The response is crisper and I was blipping the throttle at every red light, just like the Harley rider depiction in South Park's "The F Word" episode. Not a single carb cough or stumble, and she was running like a bat out of hell the whole way.

For $100, I'm super happy and for anyone that wants to throw a few bucks at the cure to their carb cough, just give it a go. It's even an amazon prime item, so if it doesn't impress after giving it a try, just send it back.

https://www.amazon.com/dp/B07SH56R55...roduct_details

Before my test ride (after getting home from work) I finished off some mods to ensure a solid fitment.

The throttle cables needed to be rerouted to run on the left side of the bike before running between the cylinders to the carb. A side note, my throttle cables (at least) had just enough adjustment to take out slack in throttle play before being unscrewed entirely. A little modification may be necessary depending on your cable setup and available slack.

New A/C to carb adapter screws. Needed 1/2" 1/4-20 bolts instead of the 3/4" that come stock with the CV.

The final adjustment was to file off the first two (from the top) cylinder cooling fins as they kept the carb from seating fully into the carb boot. It was close on initial install, but figured shaving 1/4" of the corners of the fins would keep them from wearing away at the float bowl over time.

Another one who's seen the light of flatslide carb's . You can make a decent case for CV's, and they have their place.....but nothing compares to that direct connection to the engine you get with a flatslide pumper.

As far as the coughing goes, too little ignition advance can contribute to that. When my '94 1200 was N/A, it had the stock Harley CV and 2-2 open pipes. I put in some plugs gapped at .040" like you did and that took care of 90% of my coughing, and the last 10% was adding the voes switch back in (previous owner deleted it).

That said, if it's cold enough, you might get a cough or two in the first couple of minutes; the fuel is "sticking" to the inside of the motor when cold much like rain does on a car hood. It causes a temporary lean condition, hence the coughing through the carb.