Dead Spot @ 1500-1800 RPM (1 Viewer)

[CruisinMT]

1bbl D-40 Aisin Carb, Stock Dizzy Vac Advance, New Fuel Filter, Clean Air Filter, NEW RV cam, adjusted valves, 20-22 inches of vacuum.

I've been having a problem when feathering the gas in town at about 1500 -1800 RPM my engine sputters, but as soon as I get to about 2000 rpm it has power again. It's very strange that it is only in that range. anything above or below that RPM range it accelerates fine. I can rev through the dead spot. Anyone else have this kind of problem?

 

[JohnnyC]

have you rebuilt the carb?

have you had the dissy apart to see if the advance weights are stuck?

what color are the plugs? black with sutt or wet or clean?

see link:
Verrill: Plug Color Chart

 

[FJ40Jim]

At that speed the carb is transitioning off the top of the slow circuit and into the main circuit.
Look for a restriction in the idle circuit, or a problem w/ the air side of the primary circuit.

 

[CruisinMT]

Thanks Jim can you put that into simple terms for me maybe a drawing or picture of what you're talking about. I know very little about carbs. Also do you have a picture of the correct needle for my D-40. I've noticed needles in other carbs are tapered but mine is straight and flat at the tip is that correct, it looks correct from my haynes but I don't know how accurate the pics are in there.

 

[CruisinMT]

anyone have info or translate what Jim told me?

 

[JohnnyC]

look for dirt

 

[rsbcruiser]

anyone have info or translate what Jim told me?

I'll take a shot, MT is sayin that your carb goes through a change in the way it works around those RPMs. Thus, if it aint a workin right, problems are most likely to occur about those RPMs'. Your carb is actually 2 carbs in one with one carb working all of the time and the second one starting to work about the rpms you are talking. And yes I have had this problem, you can actually tell what it is by driving your rig and paying attention to the way it behaves. For example, if you accelerate and the motor stumbles then surges as rpm rises then it is too much gas. if as you accelerate, it hesitates until rpm goes up and then starts running normal then maybe lean, too little gas, secondary carb jet could be clogged or something causing the carb to not give it enough gas. Sorry for the long explanation. Hope it helps.

 

[CruisinMT]

as i slowly increase RPM's it begins to stumble then surges up and down. This is when I hold it steady in that RPM range.

 

[rsbcruiser]

as i slowly increase RPM's it begins to stumble then surges up and down. This is when I hold it steady in that RPM range.

Oh dude, sorry! did it again, looked at your info, it is a 1bbl ! Not even sure if they have 2 circuits. Anyways, ya that kinda sounds like rich state to me. Hey I know! Check your spark plugs, if they are black all of a sudden then ya, carbs dumping fuel.

 

[FJ40Jim]

Oh dude, sorry! did it again, looked at your info, it is a 1bbl ! Not even sure if they have 2 circuits.

No RSB, you're pretty much right. There is a slow circuit that works all the time, then there is a main circuit that comes on above a certain throttle opening. If the carb doesn't get into the main circuit soon enough, or if the slow circuit maxes out fuel flow before it's time for the main to start working, then there is that sag/surge in between.

Check the FSM for the cross-section view of how the carb works. when it's disassembled, look at the two circuits mentioned in my first post.

 

[CruisinMT]

i cant find a FSM that coincides with my d-40.

 

[CruisinMT]

are you talking about the main air bleeder, and the main and slow jets?

 

[CruisinMT]

i adjusted the step up rod and it idles even better now. I still have a slight hesitation though. I'll take the entire carb off tonight and try to blow out all the jets again, hopefully i can figure this out. I'd like to do a trail ride this weekend sometime.

UPDATE: I have to pull the choke about 1/8 out and it runs perfect acceleration and everything no more dead spot. Looks to me like I need to adjust my carb a little more any tips?