Single-carb conversion running poorly after change in air intake...

[TeeCat]

Hi, guys...

Some of you may recall that, just before winter, I had my single-carb conversion running extremely well. I had the jetting right, apparently, and was inducting air through a 4" hot rod-type paper element air cleaner mounted in an ABS plastic plenum. It worked beautifully until the ABS cracked due to mass, vibration, and possibly occasionally being bumped by my left leg while bracing to kick.

Thereafter, I went to the velocity stack that you can see in my gallery pics, largely for space considerations. It has a metal screen with a very unsubstantial piece of foam behind it, and I had planned on going to denser foam. However, this evening, I just wanted to get an idea of whether there would be any substantial change in the way the bike runs. There is.

The bike fires after the first hot kick and comes off the choke quickly, but it wanted to idle VERY high - upwards of 2 grand - until I backed off the idle speed screw. It idles choppily anyway, likely because of the unequal manifold plenum lengths, but tonight was really pretty bad... hunting and the like. As I pulled away from the curb to go around the neighborhood, the bike would stumble badly right off idle, and then sort of "catch up" with itself, and pull hard through 3rd (I never took it out on the secondary road). But consistently, right off idle, it would stumble, and then catch up. Also, on decel, it sounded terrible.

The plugs looked black, but I never really got the bike hot or up to sustained speed. All of the other symptoms, to my mind, suggest leanness.

So here's the question:

I only want to work one variable at a time. It seems logical to try to reduce incoming air by going to denser foam first, and then try again, right? Then, I guess the next step (I can only go so thick on the foam without choking the bike, I suppose) would be the air screw, correct? I'm wanting to see if I can get the air flow back to where the bike was happy by varying breathing (at the air source) instead of jumping right to the air screw or to a larger pilot, which I may not need to do, because the bike ran so well before the air intake broke and had to be re-thought.

Am I on the right track?

Thanks.

TC

 

[tonyc]

Tee Cat switch back to the other filtration system and see if it makes a difference. Tony C

 

[TeeCat]

Tony, I can't. The ABS custom plenum that I had made is broken. There is no way to mount the other air cleaner to the carb.

 

[Tomterrific]

I think it's probably a speck of dirt blocking the idle jet. Pull the mixture screw and stick the tiny tube from a can of spray carb cleaner in there and blast any dirt back into the float bowl. It takes only a minute and works most of the time.

Tom Graham

 

[TeeCat]

Tom, I had just reinstalled the carb after having taken it apart to clean it, thinking just that, because right after the ABS broke, it would not start or run without the choke. The cleaning evidently fixed that, but now I have this new problem. Plus, it takes throttle fine from idle; it's when you put it in gear and go to accelerate under load that it bogs off idle, and then "recovers".

It seems to me that I need to gradually restrict air intake and look for improvement to confirm my suspicion. Maybe increasing layers of cloth rubber banded around the stack, without manipulating any other variables? Then at least I'd know it's the intake.

 

[Stone]

would it be hard to re make the plastic part of your original intake? I would try that first before messing with it anything else. That way if it runs great you know it has to do with the differences in the air filters.

If its a PITA to re make that piece then I would try different foams as well as Tom's suggestion although you did say that you cleaned it. Also whats the weather like? big difference compared to when you last had it up and running ?

Im no carb expert just my

 

[TeeCat]

Stone, yeah, it was a real pain, unfortunately. And though it looked pretty t*ts, it was too heavy, subject to vibration, and was in the way, which is why it broke. Live and learn.

I'm assuming that the filtration is the problem because the carb has been cleaned (and that solved the no-start/no run without the choke problem), and the filtration configuration is the only change that has been made since the bike was running so great.

That said, I can either try to decrease the air coming into the stack by using denser foam than that crappy little disk that's in there now, or I can try the Mike's XS Performance pod that I bought for this carb and have not tried yet. Actually, that option might be the easiest thing to try first, just to see if the pod "richens" things up a bit and lessens (or eliminates) the stumble. I'm just wanting to use process of elimination on this, hence my focus on the air intake.

Again, I had to back WAAAAAAAAY off on that idle speed screw to get the idle down, and all of the symptoms seem to indicate a lean condition, I think. They basically go away once the bike is above off-idle, except for the sound as you roll off the throttle, which almost sounds like a dropped cylinder.

Friday looks good weather-wise, so it's back to the drawing board...

TC

 

[Tomterrific]

FWIW, I've cleaned a tank, cleaned the carbs, replaced all the fuel lines and installed those little cone shaped filters and still had dirt foul an idle jet. I actually was dumb enough to pull the carbs again and again. Then I got smart and just removed the mixture screw and blasted some carb cleaner through. All this before I resurected my old Yam, thankfully. Another thing I like is to use a clean paper towel below the drain plug of the float bowl to see if any specks come out. The last time I did this on a fellow's bike I found three tiny dark specks, one was blocking the idle jet. Learn from my lifetime of carb mistakes and blow some cleaner through the mixture screw hole. It is, literally, the easiest and most gratifing fix I do for people (when it works :^).



Tom

 

[TeeCat]

Tom, I'm going to try that first to see if it works. But I'm still inclined to think I have leaned things out too much with this stack, mainly because of the coincident change.

For the sake of discussion, do you think it's possible (especially with a single carb) to lean things out enough, just by reducing or removing filtration, to cause a really bad off-idle stumble like that?

(See, this drives me nuts 'cause I'll likely have to wait til at least Friday to try to sort it. Being obsessive is hell!)

TC

 

[OakBehringer]

Provided that your jets and passages are all clean...

Changing the air filter to a velocity is likely to have changed your mixture. You know this. Stop over-thinking it.

If you want to put more foam in it b/c you're scared that dirt is going to get in your motor, go for it, then work on jetting.

I wouldn't try to add foam just to make up for the change in mixture. Order a couple sized up jets and try 'em. You couldn't have it easier- you have one carb that's easy to reach

 

[TeeCat]

Provided that your jets and passages are all clean...

Changing the air filter to a velocity is likely to have changed your mixture. You know this. Stop over-thinking it.

If you want to put more foam in it b/c you're scared that dirt is going to get in your motor, go for it, then work on jetting.

I wouldn't try to add foam just to make up for the change in mixture. Order a couple sized up jets and try 'em. You couldn't have it easier- you have one carb that's easy to reach

I'd tend to agree, Oak, since the problem seemed to appear consistently and predictably last evening... right at that point where you just start to open the throttle under load, and the idle was as if the bike was not inclined to stall, but it wasn't stable either.

I think I have a pilot jet that's one size larger, just in case. But for now, I'll squirt that passage for good measure, and try the pod just out of curiosity, to see if there's an effect.

The main reason I wanted to go with thicker foam in the stack is because the little wafer that's in there is essentially useless, and I really don't want particulate matter in my motor.

TC

 

[pamcopete]

TeeCat,

Try it without any filter. Maybe your new filter setup is the problem.

 

[TeeCat]

Hi, Pete!

Say, wouldn't that produce the very leanest possible condition, though? Or is that the idea, for the sake of experimentation? Could the stack be introducing some sort of "venturi effect" that would be absent even with a wide-open carb bell? (Also, I was thinking that having only one carb might make the bike reeeeally sensitive to airflow... hmmmm...)

Blinded by science, dammit!

TC

 

[Stone]

I have the pod on the end of mine, havent gotten a chance to tune it but I would feel much better with that on there compared to a stack.

Then again your carb set up faces the opposite direction of mine...

 

[TeeCat]

I have the pod on the end of mine, havent gotten a chance to tune it but I would feel much better with that on there compared to a stack.

Yeah, man... you oughtta see that pathetic little wafer of foam in there now... good lord!

 

[5twins]

They don't call them "velocity" stacks for nothing.

 

[TeeCat]

They don't call them "velocity" stacks for nothing.

I think that little maxim might have bitten me in the a**... Guess we'll find out in the next day or so when time and weather permit me to work on it!

 

[TeeCat]

Update:

Okay... long story short... I don't think she liked that velocity stack.

It was mainly sunny (or at least dry) for longer than I had expected, so when I came home from work, I went out there to have a look at the situation. I really wanted to investigate the filtration variable before anything else so that, if there were a difference in behavior, I could pin down the cause. So, I pulled the stack off and, without touching anything else, installed the Mike's pod. As luck would have it, it fits nicely, though I really do wish it were blue or black.

Anyway, I took her out to the street and fired her up. She came off the choke quickly, which see seems to do with the single, but I noticed immediately that I had to dial way back in on the idle speed screw (more slide lift/opening) to compensate for having to back it off last night because of the high idle. I was easily able to set a seemingly stable 1200 or so rpm idle today. Good sign.

Once the bike was a little warm - not even fully warmed up - I dropped to first and pulled away from the curb. No discernible stumble. No discernible deceleration grumpiness. Well, this is good. The clouds were really rolling in, so I didn't take her out to the secondary road, but just up and down my street a few times, through third gear. I stopped, turned around, and started from a standstill several times, went through third with a good dose of throttle, and decelerated, all the while looking hard for off-idle stumble and deceleration grumpiness, but it never seemed to appear.

So, I'm very optimistic that the issue was a direct result of that stack leaning the mixture out to an unacceptable level. Weather permitting, maybe this weekend, I still need to take her out and go through all five, and get more of a feel for whether she's back to where she was with the first filtration system (that broke), but I'm feeling better about this. Just really don't like that red...

I'll keep updates, if needed, in this thread. Thanks again to all who assisted!

TC

 

[Tomterrific]

Glad it worked TC.

 

[TeeCat]

Hi, Tom!

Thanks so much for this additional info! As I read your post, though, I think we may have cross-posted; not really sure if you saw my update just above. Also, I may not have specified in this thread that I'm using a single Mik VM36 that's not a vacuum carb, and now that you mention it, that may be the reason for its apparent intolerance if the evidently lean condition that, based on this afternoon's improvement, seems to have been created by that stack.

Nevertheless, this is a learning experience and I really appreciate your input. My brain just shuts off when I get too many variables working, though!

More miles need to follow for confirmation,h but the idle seemed much more stable, and the stumble was not there today (nor the deceleration grumpiness), so I'm thinking this was cause and effect, and I'm optimistic.

TC