New 393C running, Won't Rev Beyond ~3500 RPM

[barnett468]

I played around and ended up with 20 degrees of timing at idle set in the handheld and it starts easily when hot. Due to the insane sensitivity of the IAC it's either IAC of 0 and the computer retarding the timing to around 18 degrees with a nice idle, or IAC of 50-60 with full timing and a rougher idle. I went with IAC 0.

ok, unfortunately i have absolutely no idea what you are talking about.

1. you manually advanced timing to 20 degrees . the rpm increased and it still started easily when hot?

2. you connected the computer and it retarded tje timing 2 degrees to 18 and it idled at the same rpm?

3. you changed something and it idles like crap?

Timing is 20 at idle (18 in practice)' date='[/quote']
I is ether 18 or 20 . which is it?

Ok' date=' taking it for a quick ride to beat the weather, there was a marked improvement. A slight tumble when tipping into the throttle,[/quote']
It sounds like your accel pump is squirting way too much fuel . the accel pump is the last thing you should change . go back to the stock one and see if the stumble goes away.

but a quick half-throttle flashes the converter and results in tire spin. At 2/3-3/4 throttle it really screams, zero to 40 in about 2 seconds (or damn close to it), with 50 and the overdrive kicking in before I can shift to 3rd, all without going but a tick beyond 5000 RPM. Near or at WOT it rumbles and sputters a bit and won't rev much at all beyond 4500. No power and very sluggish acceleration.
So, what am I looking at when scratching WOT? More fuel?

put it in second and rev it until it stops revving and tell us how far it revs.

.

 

[barnett468]

post a photo of the two front plugs again

buy a 1" spacer.

what size are your header tubes?

I'll need a new air cleaner, too, then. Header primaries are 2", which are big enough to fit the exhaust ports.

The plugs look the same. Electrodes are bare metal with a slight dark tan powdering. Dark tan deposits stronger on the tips. This was after a good 20 minutes of idling followed by 15 minutes of driving with 12.5 AFR vs. 14.0 AFR from yesterday.

2" tubes are too big for your app.

your plugs now look like they should . based on the color, the jetting is very close.

as you can see by your markedly increased power now, there is not really a "best AFR" setting, so s i mentioned, it is impossible to tune using an afr gauge only.

.

 

[Casey72]

The idle air control has to be set manually by adjusting the throttle blades. The idle air control is touchy, to say the least, requiring a small amount of compromise. If there is too little air from the IAC (throttle blades are open too far by a hair) the computer will retard timing to make the target idle RPM happen. Thus, a setting of 20 degrees in the computer turns into 18 degrees when the engine is idling.

Alternately, too little throttle blade and the IAC pumps a bunch of air, harming idle quality, but not having to retard the timing set in the computer to keep idle RPM on target. Timing is 20 at idle in this case.

I've chosen the former option, so it's showing 18 degrees at idle.

Manual adjustment of timing at the distributor isn't possible for any driving. Initial timing must remain set at 15 BTDC for use of the fuel injection's timing control.

Accelerator pump is also a setting, not a physical change. Very easily done.

In 2nd gear the engine will rev to 4000-4500 in second at or near WOT, but will hardly continue to climb in RPM and produces no acceleration while heading toward 5000 RPM.

So, 18 degrees at idle and 2nd gear revs to around 4500 at WOT.

 

[barnett468]

need to know exactly how far it revs.

check to insure the throttle is opening all the way when the gas pedal is on the floor.

after you change the electronic pump cam setting see if the hesitation/stumble is better or worse.

change the wot afr to 9.

you can try the 1" spacer without the air cleaner.

 

[Casey72]

Will do. How far it revs depends a bit on throttle position. Nearly WOT revs higher to 4500. WOT is dead at 4000.

 

[barnett468]

Will do. How far it revs depends a bit on throttle position. Nearly WOT revs higher to 4500. WOT is dead at 4000.

ahh . the lot thickens .

what does your afr gauge say when you accelerate with 3/4 throttle?

what does it say when you accelerate with wot?

definitely set the wot afr to 9

leave cruise afr at 12.5

 

[Casey72]

Will do. How far it revs depends a bit on throttle position. Nearly WOT revs higher to 4500. WOT is dead at 4000.

ahh . the lot thickens .

what does your afr gauge say when you accelerate with 3/4 throttle?

what does it say when you accelerate with wot?

definitely set the wot afr to 9

leave cruise afr at 12.5

I'll look at AFR damn close tomorrow at the throttle. I didn't video the readings today. And WOT will be set to 9.

 

[barnett468]

Will do. How far it revs depends a bit on throttle position. Nearly WOT revs higher to 4500. WOT is dead at 4000.

ahh . the lot thickens .

what does your afr gauge say when you accelerate with 3/4 throttle?

what does it say when you accelerate with wot?

definitely set the wot afr to 9

leave cruise afr at 12.5

I'll look at AFR damn close tomorrow at the throttle. I didn't video the readings today. And WOT will be set to 9.

xlnt . yeah check afrs while driving before and after the change so you have some sort of number trail to follow vs performance etc.

 

[Casey72]

ahh . the lot thickens .

what does your afr gauge say when you accelerate with 3/4 throttle?

what does it say when you accelerate with wot?

definitely set the wot afr to 9

leave cruise afr at 12.5

I'll look at AFR damn close tomorrow at the throttle. I didn't video the readings today. And WOT will be set to 9.

xlnt . yeah check afrs while driving before and after the change so you have some sort of number trail to follow vs performance etc.

That's still the plan and I believe that the tuning so far has been necessary. That being said, my "extra set of hands" buddy says that a momentary bit of WOT gave him a similar problem just before the engine was swapped. It was just an instant at WOT before backing off and the engine was getting swapped out shortly, so it didn't seem like much of an issue until I bitched to him about it. Well, maybe it was significant and would have come up in conversation had he not left town prior to the engine swap. So, I'll tune some more and take it up with MSD tech support if I'm not successful.

I appreciate everyone's suggestions and assistance so far. I'll provide more information as I try things out.

 

[barnett468]

Ok, so it did the exact same thing with the previous, smaller and lower performance built engine but it just did not do it nearly as bad so you did not notice it yourself and he forgot to mention that he noticed it?

Yes, I think if there is no big improvement with 9.0 AFR at WOT, it is likely not going to help making it even richer and it would be time to look at something else.

 

[Casey72]

Ok, so it did the exact same thing with the previous, smaller and lower performance built engine but it just did not do it nearly as bad so you did not notice it yourself and he forgot to mention that he noticed it?

Yes, I think if there is no big improvement with 9.0 AFR at WOT, it is likely not going to help making it even richer and it would be time to look at something else.

I don't remember the last time I had it at WOT with the previous engine. If it's a WOT issue it wouldn't have shown up for me, at least not recently. I've been thinking on it all day and wondering about the TPS and fuel circuits. You're right, if 9.0 doesn't work, I don't think the problem will be tuning.

 

[Jeff73Mach1]

"Due to the insane sensitivity of the IAC it's either IAC of 0 and the computer retarding the timing to around 18 degrees with a nice idle, or IAC of 50-60 with full timing and a rougher idle. I went with IAC 0. "

Not sure, but with the cam specs you listed, I would expect that it SHOULD have a rough idle. Maybe you should take the IAC and set it to 50 and see how it goes.

 

[Don C]

Yes, I think if there is no big improvement with 9.0 AFR at WOT, it is likely not going to help making it even richer and it would be time to look at something else.

+1. I would start with the O2 sensor, bad, loose, or bad connection. The location sounds good.

I also agree with you that the 2" primaries are too big, more suited to wide open exhaust/racing/radical build.

 

[Casey72]

"Due to the insane sensitivity of the IAC it's either IAC of 0 and the computer retarding the timing to around 18 degrees with a nice idle, or IAC of 50-60 with full timing and a rougher idle. I went with IAC 0. "

Not sure, but with the cam specs you listed, I would expect that it SHOULD have a rough idle. Maybe you should take the IAC and set it to 50 and see how it goes.

It has a rough idle, but a "this thing will die at stoplights" idle with too little throttle blade dialed in.

 

[Casey72]

Yes, I think if there is no big improvement with 9.0 AFR at WOT, it is likely not going to help making it even richer and it would be time to look at something else.

+1. I would start with the O2 sensor, bad, loose, or bad connection. The location sounds good.

I also agree with you that the 2" primaries are too big, more suited to wide open exhaust/racing/radical build.

I've been looking for the system to throw me faults, which it hasn't, but I doubt that's necessary for there to be a problem.

As for the headers, I've read that you can't go too big and that smaller is better on the street. And that it isn't. In the end, the exhaust ports were 2.1" x 1.95", so I went with 2" primaries for the stroker, which I had seen recommended somewhere. I won't pretend to know the best answer for primary size at this point.

 

[Carolina_Mountain_Mustangs]

Hey I know noting about your system to help with that. The subject of exhaust brings back some thoughts. My old boss in engine shop built his own headers for each engine. Once it was decided what rpm range it would be running he used the same formulas to calculate header length and dia. that are used to figure dimensions for a pipe organ. He always told me that for the header to work the length was important so that the pulses of each firing would pull the exhaust out from the next. He always said that you could only tune a header for a frequency and that no header was right for all rpm. You had to pick something in the middle.

I know with small go cart engines too large of a dia. pipe will kill the engine. I do not think bigger is always better.

Just my thoughts as usual don't know enough to be dangerous.

 

[Casey72]

Hey I know noting about your system to help with that. The subject of exhaust brings back some thoughts. My old boss in engine shop built his own headers for each engine. Once it was decided what rpm range it would be running he used the same formulas to calculate header length and dia. that are used to figure dimensions for a pipe organ. He always told me that for the header to work the length was important so that the pulses of each firing would pull the exhaust out from the next. He always said that you could only tune a header for a frequency and that no header was right for all rpm. You had to pick something in the middle.

I know with small go cart engines too large of a dia. pipe will kill the engine. I do not think bigger is always better.

Just my thoughts as usual don't know enough to be dangerous.

I've heard something similar, but not the pipe organ part. I'm willing enough to jump into things to be dangerous. Actual knowledge...well, that's why I'm here.

 

[Casey72]

As it turns out, AFR only goes down to 10.0. It didnt matter. The problem persists. Won't rev beyond 4500 and there's no acceleration at WOT. Part throttle driving, even pretty deep into he pedal, is pretty awesome.

The numbers it's spitting out at WOT don't look bad at all. Everything is tracking, but the engine bogs and rumbles, won't accelerate and there's a strong smell of fuel when at WOT. I'm at the point of "no impact, no idea" with this issue.

The good news (lol) is that I know why the IAC has been so touchy as of late. It has gone out. It was perfect during idle today and I figured I'd really nailed it. After driving for a bit it pegged out and only came down a little one time. Since then, it pegs out as soon as it's driven. I'll be asking MSD about the issue now.

 

[Casey72]

I'm going to try a new coil for now, as previously recommended here, and procure a replacement IAC.

 

[barnett468]

It has a rough idle, but a "this thing will die at stoplights" idle with too little throttle blade dialed in.

I don't know exactly what you are trying to say here.