WOT High RPM Miss / Studder

[IQ52]

"I will take your recommendation and swap them down for the +4% or factory jets?"

Just saying it is something to think about.

Have you adjusted the rev limiter? Pertronix III's come factory set at 5500

Not knowing about Pertronix I had kept silent. You should check into this. Maybe first?

 

[lewtot184]

I think the ground strap on the plug shows too much timing. what plugs are those? the projected tip seems to be long,....? my experience with the 800 avs is that they are too rich OOTB. I also would use the timed port for the vaccum advance and not manifold. also check to see how early the advance is coming in on the vacuum unit and if it's adjustable. the only good way to check for cam wear is take the tappets out and inspect them.

 

[threewood]

I set the Pertronix is set at 6200 rpms. I got the same stutter when I swapped in points.

I think the ground strap on the plug shows too much timing. what plugs are those? the projected tip seems to be long,....? my experience with the 800 avs is that they are too rich OOTB. I also would use the timed port for the vacuum advance and not manifold. also check to see how early the advance is coming in on the vacuum unit and if it's adjustable. the only good way to check for cam wear is take the tappets out and inspect them.

The plugs are NGK ZFR6F-11. How do I check when the vacuum advance comes in? What size allen wrench to adjust? It is a factory unit so I think it should be adjustable.

 

[727]

I think its a 3/32" allen wrench.

 

[RT K_line]

It sure sounds like valve float. You mentioned fast ramps or something, may be causing instability at that rpm. not an expert but fuel/ignition good and no issue bellow that rpm points to valve float or rev limiter if equipped. I had a similar Hi rpm miss at the track before I installed my tach. I started to shift at 5500 and never heard it again. my limiter is a 6000 so I figured that was it.

 

[lewtot184]

I set the Pertronix is set at 6200 rpms. I got the same stutter when I swapped in points.



The plugs are NGK ZFR6F-11. How do I check when the vacuum advance comes in? What size allen wrench to adjust? It is a factory unit so I think it should be adjustable.

use a 3/32 allen . turn the screw clockwise to bring the advance in sooner and counter clockwise later. I also use a vacuum tester/gun thingy to see were the vacuum canister opens and how much vacuum when fully open. there should be a number on the canister arm to tell you how many degrees are in it. I think you'd be fine with ported vacuum. also, it's my understanding that those fast ramp cams really drop off in power past the power peak. you may have several small things going on at once. personally, I don't think that most of these wedges make power above 6000rpm. they may show a cute number on an engine dyno but once all the belts and accessories/mufflers, etc are hooked it's a different ball game.

 

[threewood]

use a 3/32 allen . turn the screw clockwise to bring the advance in sooner and counter clockwise later. I also use a vacuum tester/gun thingy to see were the vacuum canister opens and how much vacuum when fully open. there should be a number on the canister arm to tell you how many degrees are in it. I think you'd be fine with ported vacuum. also, it's my understanding that those fast ramp cams really drop off in power past the power peak. you may have several small things going on at once. personally, I don't think that most of these wedges make power above 6000rpm. they may show a cute number on an engine dyno but once all the belts and accessories/mufflers, etc are hooked it's a different ball game.

Thanks! The canister is marked 9.5. I have a vacuum pump with gauge to see it it is accurate and what hg it comes in and when it maxes out.

I talked to Hughes several months ago and they confirmed that the cam is good to 5000 rpms for peak power so it makes since that it tails off. I have a 5400 governor but with the TransGo II shift kit the auto shift happens around 5800. I'm going to adjust the kickdown to lower that down to 5400 rpm.

 

[threewood]

It sure sounds like valve float. You mentioned fast ramps or something, may be causing instability at that rpm. not an expert but fuel/ignition good and no issue bellow that rpm points to valve float or rev limiter if equipped. I had a similar Hi rpm miss at the track before I installed my tach. I started to shift at 5500 and never heard it again. my limiter is a 6000 so I figured that was it.

Thanks. At this point shifting sooner sounds like the way I need to go. It's a street car so automatic valve body. If adjusting the kickdown doesn't do it I'll swap in a lower shifting governor.

 

[lewtot184]

I don't think you need over 34-36 degrees total timing; especially with pump gas. the 38 degree and up is more relevant to the old non-quench iron heads. I would look at jetting once you get the timing thing figured out.

 

[Bigal75]

What heat range are those plugs? 11 or 6?

 

[threewood]

What heat range are those plugs? 11 or 6?

6 is the heat range. I also have a set of NGK in the 5 heat range that I swapped out for the cooler plug for the summer.

 

[threewood]

I don't think you need over 34-36 degrees total timing; especially with pump gas. the 38 degree and up is more relevant to the old non-quench iron heads. I would look at jetting once you get the timing thing figured out.

I have aluminum heads, compression is 10.4:1 and the squish is .040". I can try 37-38 by welding up the slots a bit more.

 

[lewtot184]

I have aluminum heads, compression is 10.4:1 and the squish is .040". I can try 37-38 by welding up the slots a bit more.

I don't think you need 38 degrees with a tight quench and a plug with a fairly long projected tip. limit the total to 36 degrees. more timing is an indicator of poor combustion chamber efficiency and I don't think you have that.

 

[threewood]

I don't think you need 38 degrees with a tight quench and a plug with a fairly long projected tip. limit the total to 36 degrees. more timing is an indicator of poor combustion chamber efficiency and I don't think you have that.

Ok, I'll set total at 36 degrees. Thanks again.

 

[threewood]

Re-welded the slots to .340" which should get me 6x2=12 mechanical, plus the 25 degrees it is set at, which I can back off a degree to get to 36 degrees.

I also loosened the kickdown cable an 1/8" to see if it will shift earlier. Testing is tomorrow, it's too late now lol

 

[lewtot184]

.0154" equals 1 distributor degree. I install the advance plate and measure the distance between the centrifugal weight's pin and the open area of the slot using number drills. shoot for .139". this will give you 9 degrees in the distributor and then use 18 initial for a 36 total. I've also found that limiting the total advance with the vacuum advance to around 50 degrees works nicely. sometimes getting into to the 55-60 range can make cruising a little erratic. once this is sorted out go after the carb. I think you can end up with a nice performing and driving combo.

 

[threewood]

.0154" equals 1 distributor degree. I install the advance plate and measure the distance between the centrifugal weight's pin and the open area of the slot using number drills. shoot for .139". this will give you 9 degrees in the distributor and then use 18 initial for a 36 total. I've also found that limiting the total advance with the vacuum advance to around 50 degrees works nicely. sometimes getting into to the 55-60 range can make cruising a little erratic. once this is sorted out go after the carb. I think you can end up with a nice performing and driving combo.

Ok, I checked my timing this morning and it is at 26 initial and the total (at around 2200 rpms) is 36 degrees at operating temp, idles at 800 rpms. I also placed the vacuum advance on ported. I took it out and had no issues with a stutter over 5k. With the KD adjustment it shifted automatically at 5500 rpms. At 26 initial I am pulling roughly 12hg and if I retard it to 20 the vacuum drops to 9hg. I have always adjusted timing with highest vacuum and then verify with the light, backing off if it kicks back on the starter. I don't think the motor would idle very well at 18 degrees initial, plus I would definitely have to change out the step up springs in the carb. Is 26 too high initial?

I got my slot numbers from this article...

 

[lewtot184]

sure your not getting some centrifugal advance at idle? 26 initial hasn't been starter friendly in my experience; unless your releasing a lot of cylinder pressure cranking. anyhow, if it starts good and drives good give it a whirl were it's at.

 

[threewood]

sure your not getting some centrifugal advance at idle? 26 initial hasn't been starter friendly in my experience; unless your releasing a lot of cylinder pressure cranking. anyhow, if it starts good and drives good give it a whirl were it's at.

Well, it seems steady at 800 rpms and I checked it a few times after reving it up. I am more concerned with the total. I still need to check vacuum advance to see how much I am getting.
Thanks for the help!

 

[lewtot184]

what I do on the vacuum advance is cut a tee into the hose and attach a vacuum gauge; then go for drive. I look at maximum vacuum at cruise and then adjust the vacuum unit to where all the advance is pulled in at, or close, to that reading. you'd be surprised at the vacuum that the ported connection can pull.