View Full Version : Off-Idle Stumble and Air Screw Question

03-20-2012, 11:15 AM
Sorry I don't have a picture at the moment to show this, but I recently pulled the carb from my blast to check the jetting the previous owner was running.

Specs: 2000 Blast with Vance and Hines Exhaust, K&N in an unmodded airbox, new airbox gasket, new superboot.

The pilot/slow jet is a 45 as reccomended for mods. The main jet didnt have any numbers but I assumed it was changed due to screwdriver marks on it and the changed pilot jet.

When I first bought the bike, the module died so I replaced it with a new one and the LED is lit when the timing mark is in the timing window.

I've had hard starting issues with the bike in the cold last year but it would run fine after getting it started the first time. It starts now (temp ~70F) and will have normal idle, then ramps up to a higher idle and finally backs down to a normal pace (sounds like the AE is working okay?).

When I had the carb off, I wanted to make sure that the air screw plug was removed and that the screw was set at the reccomended 2.5 turns out. From my reading, I believe the plug is between the bowl and the intake side of the carb.

Here's the strange thing. Where my idle screw plug is supposed to be is a large brass thumbscrew which I can adjust. I don't know if this is factory or added by the previous user or is even correct.

I'd like to make sure that this is the idle screw before I go on figuring out why I have my stumble/lack of power from idle to about 1/3-1/2 throttle. The bike will cough if you quickly apply up to a 1/4 of throttle and, when riding, feels choked up until about 1/2 throttle.

If anyone has any comments, suggestions, etc, I am all ears. This is going to be my gf's commuter to work this year to save on gas and parking. I need it to be as reliable and as predictable in performance as possible.


03-20-2012, 04:33 PM
Module died? What do you mean? What module? The ignition?

Also, the brass screw is not factory, but is better than factory as it makes adjusting the idle screw easier. However, I would unscrew it all the way and make sure all the parts are there.

Should look like this:


Should also have a washer, spring and o-ring (so remove it with care). Owners are often careless and just pull it out and put the new one in and don't realize that the washer and o-ring fell out when they took it out. Make sure they are present. They look like:


It also sounds like you may have a tear in the boot - its the achilles heel of the blast. If it has even a minuscule tear in it the bike runs poorly, if even at all. It also makes the bike run lean and could blow the engine. Test it at idle by spraying WD-40 on it and see if the idle changes, and look for bubbles. If I were you, I would just replace it. They should be swapped out to a stronger support anyways - good thing to invest in dans superboot:


A sign of boot failure is exactly what you describe, idle increases without cause, loss of power, sluggish etc. First take care of those things, then if it is still an issue, then we can talk about jetting, shimming the needle, A/F screw etc. But first rule out the obvious things.

03-20-2012, 05:30 PM
Thanks Pierce.

That picture is exactly what I have on the bottom of my carb. I'll have to pull it out and check for all the parts. I figured someone put something like that in but I never came across some on the forums saying they used one.

To the module, it is the ignition module, the one on the right side of the engine under the cover. Once I replaced it, I static timed the engine via the timing plug and the LED on the module.

To be honest, I don't think its the boot. It has Dan's Superboot on it and that is on a few months old. When I put that on, I also replaced the venturi and the airbox-to-carb gasket. I did the WD-40/ether/carb cleaner test after replacing the boot and at no time does the engine race. The idle situation is only when it first starts. I wanted to describe what the startup of the bike sounds like so you know what the AE is doing at startup.

Now that my concern over the idle screw is put a bit to rest .... my main question would be can my sluggish/low power on the bottom end be a symptom of too lean of a pilot jet and/or the need to shim the needle. I didn't want to dig into the diaphragm if I didn't have to but it looks like I'm going that way.


03-20-2012, 07:54 PM
Got it - if idle only increases at first start you are correct, thats just the auto enricher.

Shimming the needle helps with low end hesitation. Its actually incredibly easy, just make sure to put everything back exactly as it was. Not to say that you wouldn't, but have traced many problems back to owners or po's that mistakenly re-installed the carb stuff. Blasts like to run on the rich side. Might want to try a 48, you seem to be describing lean symptoms. Any popcorn in the exhaust on decel.?

Set air fuel 2.5 turns out from soft bottom and go from there. Most end up somewhere around 2 3/4 out. Don't go much over 3 or it will fall out.

03-21-2012, 01:44 PM
I don't have any popcorn on the decel. The bike runs surprising well over the rest of the powerband.

The bike isn't sptting and sputtering through the low end like its loading up or is too rich. I'll have to pull the top of the carb off and see if its shimmed.

The only thing I got a chance to do last night was turn the air fuel screw out a bike more and it seemed like the flat spot was less pronounced (but still there).

I really wouldn't mind tuning this so much if 1) the V&H wasn't sooo loud, 2) I live in an apartment complex and 3) I live by a police station. Blastin' around the parking garage will surely piss some people off.

03-21-2012, 09:09 PM
Spitting and sputtering in 1st? So no backfiring, but its jerky in low speeds? Did it always ride like this? Are you certain the timing was done correctly? It will run poorly if it is not. Replaced with stock ignition?


To tune the idle screw I would just perform the drop idle test and let it be. I would also pull the main jet and verify what it is. It should run smoothly in all gears, so something is not right. I would also check that all of the crab parts are accounted for and in place. Check for shim, and I suspect that something was done to carb that is causing this.

captain chaos
03-22-2012, 04:32 AM
Personally, I'd check and adjust the needle height. 1/4-1/2 throttle fueling is done more by the needle than the pilot. Also, if you have to go out more than 2.5 turns on the pioot screw that's a good indication that you probably need the next size up on your pilot jet.

Remember, "reccommended" setting don't work for everyone. EVERYTHING affects carb tuning, altitude, barometric pressure, temperature, ect, ect, ect.

here is the best carb tuning guide I've found. I've tuned several carbs following this and it works perfectly.
....crap, dead link.
This one is the same, just ignore the "rpm ranges"
The original one I posted was for "Low RPM (under 7000) engines it's exactly the same but the "rpm ranges" are just adjusted to lower levels. All the same practices apply.

03-22-2012, 10:56 AM
Factory is set at 2.5 turns out - so don't go any less! Believe me - the blast likes to run on the rich side and factory is set a bit lean.

If you really want to tune the carb just get a wideband. I spent years chasing ghosts with carbs. Screw all the reading and follow the numbers. What I fiddled with for years took me a few days to get set with my wideband sensor. Takes the guesswork out of it.

03-22-2012, 01:31 PM

I believe is said it "isn't spitting and sputter through the low end like its loading up or is too rich" above. Its not backfiring and doesn't sound/feel like a timing issue. Its not jerky, just generally lacks power in the lower part of the RPM range (almost feels like a two stroke).

I'll have to do the drop idle test. I know I'm at around 3-3.5 turns out now and its better than it was before. Home depot didn't have any brass washers last night so I need to get them from somewhere else before I tear the carb back off of it.

03-22-2012, 07:52 PM
Use some fingernail polish to mark the closed position of the AF screw. Then unscrew exactly 2.5 turns out. Perform drop idle from there. Ace hardware has the washers.

Rich and lean symptoms are deceptively alike, even for those that have a great deal of experience tuning. Unless you run it through a dyno its all subjective. Just be careful - really careful - as I have seen many blasts run their machines into the ground really quickly because of this. It certainly doesn't help that out bikes are air cooled so run hot as is (liquid cooled is far superior), but a single carb is the Blasts only lifeline and every small change effects everything else.

I strongly recommend these as a good guide:

Stock - 45/170

Exhaust change only - 45/175

Exhaust and intake - 48/175 or 180

reset idle mixture screw to 2 1/2 turns from full in or for best idle speed. If best idle is more than 3 turns out, switch to a 48 pilot jet and re-adjust idle screw for optimum idle

Once all this is done and you feel you are tuned close to perfect, then shell out for a dyno - our if you have an wideband tune away. If you use a wideband the blast likes to be on the rich side. 12's 13's when you pull the throttle. What would drown most of my bikes the blast seems to respond well too.