What would cause this surging?

[1wild&crazyguy]

square the pv's and jets, but jet up 2 sizes.
After the shooter tuning, thats all can say from here...

 

[crackedback]

I might isolate the carb/intake tract and see if that helps. If the surge goes away, then hook up stuff one at a time and see if the surge comes back.

What's the A/F meter say when it does the surging?

 

[magnumdust]

I might isolate the carb/intake tract and see if that helps. If the surge goes away, then hook up stuff one at a time and see if the surge comes back.

What's the A/F meter say when it does the surging?

Well the surging from my first post went away. I guess maybe i was that off with my idle screws and it caused the engine to act a little funny.


So I tried to get some video with my go-pro today and well: :banghead:
[ame="http://youtu.be/gQqxoBwt1U0"]302 Found[/ame]

I couldnt seem to find a good mounting point and it "looked" like the a/f would by visible from here. I tried working with the video editor but no real luck on clarifying things.

So here's the short version:
1st gear WOT: it went as high as 15.1, but stayed around the high 14s
2nd gear basically the same
3rd gear was down to 14.3(ish) for most of the time.

Only went to 4k between gears

 

[crackedback]

It VERY lean if it's in the 14's at WOT.

WOT shouldn't be over about low 13's at worst and likely closer to mid 12's for best power.

Try running it up, WOT in 2nd and 3rd gear and see where the A/F are running. If they don't come down into the 12's, you need a bunch more jet in it. Could be a fuel delivery issue, bad float level... lots of potential issues.

Turn the idle mix screws out a smidge more. See if that helps the cruise A/F little. It's likely gonna need more jet than what it currently has if it's the 4010 style.

 

[1wild&crazyguy]

how much compression does this motor have?

Rob, remember when u ran a wide band and was chasing lean numbers like this?
If i remember right, it didnt really clack either, but it was detonation, or am i wrong?

 

[magnumdust]

It VERY lean if it's in the 14's at WOT.

WOT shouldn't be over about low 13's at worst and likely closer to mid 12's for best power.

Try running it up, WOT in 2nd and 3rd gear and see where the A/F are running. If they don't come down into the 12's, you need a bunch more jet in it. Could be a fuel delivery issue, bad float level... lots of potential issues.

Turn the idle mix screws out a smidge more. See if that helps the cruise A/F little. It's likely gonna need more jet than what it currently has if it's the 4010 style.

9.5 compression(stock magnum compression). Its running 87 octane and i havent heard any pings/knocking in it.

 

[OldmanRick]

Just to refresh guys, he started out with the idle A/F in the 12.0/13.0 range. Running 7 lbs. pressure with a electric pump, which i consider the max it could handle. I recommended dropping the floats a hair and that brought the idle numbers back in line. I'm thinking too that it needs to be loaded at a higher rpm to get a true WOT reading. Although it's not that easy out on the road. Might help to get the secondaries open a little sooner? I was unable to find what spring this carb came with.

 

[crackedback]

Detonation will make the meter read a bit wacky.

I think this is the spec sheet
[ame]http://static.summitracing.com/global/images/instructions/summit%20carburetor%20spec%20sheet%20m08750vs.pdf[/ame]

 

[1wild&crazyguy]

a/f readings should always be under load, as for detonation, its called inaudible detonation, you dont realize it cause you dont feel or hear it.

how much cranking compression does it have?

under cruise it should not be 14+. under wot it should be 12's, take it on a freeway on ramp and stomp on it, and if to no avail try the weakest sec spring u can get...after that, either a bunk carb or ...back to cranking compression test again, what does it have?

 

[OldmanRick]

Thanks Rob. The silver/plain spring is rated around 2200 to 8100 r's. Probably only open a quarter way or so at 4K. The little bigger secd. jet will help, but you got them open enough to really get the fuel flowing. You may be on to something wild with the detonation? I agree, with 87 octane it may be borderline.

 

[BillGrissom]

Surging can be due to several causes, both of which lead to a "positive feedback" run-away condition.

One is a too lean idle mixture. As the engine speeds up, it gets a better mixure which it likes, and runs faster, ... My 69 Dart was like that for years. Besides surging, it idled poorly in "D", slowing way down and sometime dying. Finally got a carb that made it run perfect. It wasn't just an idle mixture adjustment, something was wrong in the idle metering block of the 1st 3 carbs (Holley 1920).

Another is that the distributor curve is off. As speed increases, it get advanced spark too early which it likes and runs faster, ... I kind of recall that one purpose of the vacuum advance is to establish a "negative feedback" region around idle so the engine speed is stabilized. Read the Holley Commander 950 manual (search web). It discusses setting a spark timing schedule for a stable idle. I expect they designed similar into mechanical distributors, but few people today understand exactly what the designers did.

 

[magnumdust]

Well i'll have something a little more concrete on wednesday. I ordered a usb-serial adapter for my wideband and then i can datalog the a/f ratios and do another drive around. That way instead of trying to get my video to be in the right spot, i can just post a picture of the datalog

 

[crackedback]

Try raising the float level. As long as fuel isn't dripping out the boosters, you should be OK.

As little as 1/8 turn can make a pretty decent change in A/F ratios.