Ok, been working on the rebuild for awhile. Went with Engnbldr's 268 Street RV head, bored 20 over. New injectors. All went well at initial fire, smogged clean as a whistle, ran fine. Took it to have 2" pipe put after the stock manifold, to 2 1/4 from new cat to flowmaster, all stock routing. Now I have an idle stumble after warm up. Come to a stop light, idles fine. Sit for a few seconds and it stumbles like it wants to die (occasionally does) then slowly recovers to normal after a few seconds. Haven't noticed any other irregularities. Going to check my running temp to see that I'm not in closed loop, also going to test the AFM with an ohm meter. Any other suggestions?

 

26 views and nothing? Man, I hope I'm not "an original" with this problem...

 

Would maybe the timing have to be adjusted since the flow of the gases are slightly different?...o

 

I've heard of having to dig into and adjust the AFM after an intake or exhaust upgrade, but I dunno... very strange...

 

Does it always do it at a certain rpm? I had a stumble at 2k rpm a month or so ago. Turns out the wire going to my coolant temp sensor on the front (green plug) had broken. Once I fixed the wire it was fine. I guess there's a chance something could have got unplugged or maybe a wire is loose or broken after your install?

Rob

 

would'nt be an issue with backpressure would it?

 

That was my first thought after seeing the title. But with 2" to 21/4" and a cat and real muffler on there I can't see it being a backpressure problem unless there is an exhaust leak somewhere...

 

Since you have opened up the air flow through the engine it would probably be a good idea to adjust the AFM about two notches richer, your probably running a bit lean at idle.

 

Kinda what I was thinking with the AFM and the "opening up" of the motor. Wasn't sure if that would be accomplished by simply advancing the timing, which has been done already. If I were to richen the AFM, would I have to re-time it yet again?

There are no exhaust leaks.

Although there's a definite improvement with my setup over stock, it still feels a little unresponsive and anemic. Wasn't sure if I was/am expecting too much, especially with the slushbox behind it. Haven't experienced any overheating or detonation, but the whole cooling system is new as well (as far as the cooling goes).

 

+1
I am a hobbyist motorcycle builder and I have experienced the same thing. Everytime you increase the exhaust flow, you must add more fuel to the intake side. It's a balancing act between the two parts.

 

did you re-use your old O2 sensor/s

 

My thoughts about the mix exactly! My Honda dirtbike exhaust upgrade instructions specifically instruct you to NOT add the exhaust unless/until you increase the jetting accordingly! Knowing that jetting and EFI aren't the same, do you think the same idea applies here?

O2 sensors are new.

 

What RPM's are you running? Does it do it when the brake is pressed?

 

Thought about vacuum, but I don't see a difference with brake pedal, give or take. Running about 700-750 when warm.

 

I don't think you need to richen the mixture due to a larger exhaust. The O2 sensor should compensate for this.

HOWEVER, if this only occurs when the engine is cold, the O2 sensor is not a part of the equation. Thus the richening may make sense.

 

Increasing exhaust size should be compensated by the ecu as others have said. I would definately check electrical connections (including plug wires) and vacuum hoses. Those have been the ones to cause a stumble for me in the past.

Rob

 

Did you dial the AFM up after you put the cam in? http://www.lcengineering.com/technot...&catid=4&id=11 Your O2 sensor may have been able to compensate with new cam and stock exhuast. But now than your getting much better flow it may be more than the EFI can compensate for. Just brainstoming, you said all was normal till you put the header on right.

 

Correct. The exhaust upgrade coincided with the start of the problem.

 

intake manifold vacuum is probably down a little as a result of both the cam and exhaust and as such could affect anything relying on vacuum. VSV and dashpot actions may be delayed due to this as could egr valve operation.

also with reduced vacuum at idle, it won't be pulling as hard on the afm vane at idle hence the suggestions to adjust the afm a click or two richer, as is recommended with some other more aggressive camshafts.

 

Might also check your EGR, that may be what is opening at the wrong time or sticking open when it should be closed. That will cause the engine to stumble and/or die.