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jjjaffo 11-25-2018 10:06 AM

street avenger issues
 
My 408 big block chevy is 9.67 to 1 with factory big oval port 101cc chamber heads. I have a 270 magnum cam in it and a weiand stealth dual plane intake plus super comp. headers and 2 1/2 inch exhaust. I have a Holley 770 street avenger carb on it. It had a terrible stumble when you crack the throttle at most any speed. I mostly fixed this with a bigger pump shooter. However at speed if I just tip into the throttle it immediately starts to misfire unless a fair amount of throttle is given. I have put at least a hundred miles on the new plugs but can't get a reading on them. They look as new as the day they were installed. I have 70 jets in the primaries. Could it be too lean? I don't have an A/F guage at this time. I have about 13" of vacuum at idle and 18" at speed. I tried an 8.5 powervalve and it gave too much fuel too early. The 6.5 seems to work the best. I checked timing and for vacuum leaks and all seems well. I tried ported vacuum and found manifold vacuum gave the best results for the spark. Does anyone have a jetting recommendation?

BogiesAnnex1 11-25-2018 11:36 AM

I'd change the primary jets to 75's and observe what goes on. Reading plugs is difficult except in gross terms without a lot of experience an A/F gauge reduces this to simple.

You might be picking up the secondaries to easily, if this is not adjustable then pick up a spring kit and play with some stiffer springs to slow the opening time.

Bogie

ericnova72 11-25-2018 02:30 PM

70 jet is on the lean side for a carb of that size...the common 750 Vacuum Secondary's come with 72's, and original first 750 Double Pumper was 76's , then every revision after the original was 70 or 71....but the Doubles have a much richer idle and transition circuit.

You may want to read this thread, it pertains to all Avenger carbs and not just the 670 mentioned in the title.....I'd want to do this modifications rather than richen main jetting and possibly negatively affect fuel mileage.

At typical cruise rpms, you are running on the idle and transition circuit and not really the mains unless you are geared pretty deep, 3.73-3.90 or more....so that's why the "lean Avenger idle/transition circuit" thread may be of importance to you.

https://www.hotrodders.com/forum/hol...ix-507033.html

jjjaffo 11-25-2018 05:58 PM

Quote:

Originally Posted by BogiesAnnex1 (Post 4637877)
I'd change the primary jets to 75's and observe what goes on. Reading plugs is difficult except in gross terms without a lot of experience an A/F gauge reduces this to simple.

You might be picking up the secondaries to easily, if this is not adjustable then pick up a spring kit and play with some stiffer springs to slow the opening time.

Bogie

I have been dabbling with different springs and the problem there is usually the secondaries not opening soon enough. Still working on it. I'll try a richer jet size.

jjjaffo 11-25-2018 06:05 PM

Quote:

Originally Posted by ericnova72 (Post 4637945)
70 jet is on the lean side for a carb of that size...the common 750 Vacuum Secondary's come with 72's, and original first 750 Double Pumper was 76's , then every revision after the original was 70 or 71....but the Doubles have a much richer idle and transition circuit.

You may want to read this thread, it pertains to all Avenger carbs and not just the 670 mentioned in the title.....I'd want to do this modifications rather than richen main jetting and possibly negatively affect fuel mileage.

At typical cruise rpms, you are running on the idle and transition circuit and not really the mains unless you are geared pretty deep, 3.73-3.90 or more....so that's why the "lean Avenger idle/transition circuit" thread may be of importance to you.

https://www.hotrodders.com/forum/hol...ix-507033.html

I read your article and it seems to make some sense. I'm really not sure if I'm still on the transition circuit or not at speed. At 60 with a 3:42 gear, 27 inch tall tire and 2400 stall convertor I'm at about 3 grand on the tach. I don't really know when it gets on the primary circuit. Any help for me?

ericnova72 11-25-2018 07:00 PM

Quote:

Originally Posted by jjjaffo (Post 4638015)
I read your article and it seems to make some sense. I'm really not sure if I'm still on the transition circuit or not at speed. At 60 with a 3:42 gear, 27 inch tall tire and 2400 stall convertor I'm at about 3 grand on the tach. I don't really know when it gets on the primary circuit. Any help for me?

there isn't really a hard and fast rule on the rpm that the engine will be on the mains, it is more of a throttle opening amount and engine demand issue, but you are in that range of switching from one to the other.

I'd try the jump to 75 jets, if it doesn't seem to help much then you know you're not up on the mains yet and need to be working on the idle/transition circuit.

TommyK 11-26-2018 07:17 AM

It sounds like the transition slot is running out of fuel before the mains are fully flowing. Before you pull the carb apart you could try restricting the idle air bleeds with some small gauge wire and see if it improves. This will richen up the idle/transition. If it helps you know the problem is the ifr/iab relationship.

If you end up pulling the carb take a look at the t-slot exposure. If you have a lot of slot showing below the throttle blades the effective range of the transition circuit is limited.

55 Tony 11-26-2018 08:15 AM

Quote:

Originally Posted by jjjaffo (Post 4637865)
It had a terrible stumble when you crack the throttle at most any speed. I mostly fixed this with a bigger pump shooter. However at speed if I just tip into the throttle it immediately starts to misfire unless a fair amount of throttle is given.


You changed to a bigger squirter, but have you tried changing the plastic cam that actuates it? The bigger squirter will give it more volume but shorter duration squirt. Have you watched the squirter when opening the throttle by hand at about the time it has the problem to make sure it is pumping fuel with the slightest movement of the throttle? I think there will be two holes in your cam, you can remove the screw and turn it to the different hole. Also if still not happy with that part, buy a kit of cams and install a more aggressive one. Be sure the adjustment on the accelerator pump is properly adjusted also. You should get a squirt of fuel throughout the throttle opening range, and with a fast opening of the throttle, the squirt should last a little longer than the throttle movement.



I saw a mention of the secondaries opening too slow, yes they pretty much suck with any of the springs they give with it. I have a fix for that but get the primaries working right first.

eric32 11-26-2018 10:53 AM

I recently did some research on the vacuum secondary pod and noticed some options on tuning with them and some simple modifications that is easy to do and I can post some information later on but you want to figure out your primary side of things first. As stated up the jetting some but also if you have a chance is your cam the roller version or the flat tappet?

I have not had my hands on a holley 770 street avenger in many years so I don't know right off hand on what the air bleed and idle feed restrictor sizes are but I can maybe guess here that its very close to the 3310 750 vacuum secondary carb.

What is your timing curve as that can affect things. I once had a issue like yours and no amount of carb changes would help and I found out that I did not have my distributor set back right after changing out a new rotor and cap on it. I was off on my timing by a small amount that was enough to give me tuning fits.

What color pump cam is currently on your carb and what size shooter? After figuring out your primary side of things I can round up my research and photos and post here or maybe in the 670 street avenger post on the secondary side of things. Also is your street avenger the 2 corner idle or 4 corner idle and of so how far out are your idle mixture screws?

jjjaffo 11-26-2018 07:55 PM

I've actually heard of the small wire trick before. Thank you for mentioning it to me as I'd forgotten all about it. As far as my T-slot position it's not quite a square but seems to be in line with what I've read about.

jjjaffo 11-26-2018 08:01 PM

Quote:

Originally Posted by TommyK (Post 4638091)
It sounds like the transition slot is running out of fuel before the mains are fully flowing. Before you pull the carb apart you could try restricting the idle air bleeds with some small gauge wire and see if it improves. This will richen up the idle/transition. If it helps you know the problem is the ifr/iab relationship.

If you end up pulling the carb take a look at the t-slot exposure. If you have a lot of slot showing below the throttle blades the effective range of the transition circuit is limited.

I had actually forgotten about the wire trick. Thank you for bringing that to my attention. I will do that before any jet changes. Also my T-slot is almost but not quite square which should be pretty close to where it has to be.

jjjaffo 11-26-2018 08:20 PM

Quote:

Originally Posted by 55 Tony (Post 4638099)
You changed to a bigger squirter, but have you tried changing the plastic cam that actuates it? The bigger squirter will give it more volume but shorter duration squirt. Have you watched the squirter when opening the throttle by hand at about the time it has the problem to make sure it is pumping fuel with the slightest movement of the throttle? I think there will be two holes in your cam, you can remove the screw and turn it to the different hole. Also if still not happy with that part, buy a kit of cams and install a more aggressive one. Be sure the adjustment on the accelerator pump is properly adjusted also. You should get a squirt of fuel throughout the throttle opening range, and with a fast opening of the throttle, the squirt should last a little longer than the throttle movement.



I saw a mention of the secondaries opening too slow, yes they pretty much suck with any of the springs they give with it. I have a fix for that but get the primaries working right first.

I actually was going to order a cam kit this week. Right now it has the red cam on it and it seems to deliver a pretty healthy squirt. The more I read you guy's posts the more I wonder about the Idle/transition circuits. With low speed driving up to highway speed if you ease into the throttle it stumbles, but if you continue to gently push the throttle down it will come out of it and start putting you back in the seat. Could this difference be the carb coming on to the main circuit? I know the secondaries are not opening yet at this point because I'm just not that far into the throttle yet.

jjjaffo 11-26-2018 08:36 PM

Quote:

Originally Posted by eric32 (Post 4638135)
I recently did some research on the vacuum secondary pod and noticed some options on tuning with them and some simple modifications that is easy to do and I can post some information later on but you want to figure out your primary side of things first. As stated up the jetting some but also if you have a chance is your cam the roller version or the flat tappet?

I have not had my hands on a holley 770 street avenger in many years so I don't know right off hand on what the air bleed and idle feed restrictor sizes are but I can maybe guess here that its very close to the 3310 750 vacuum secondary carb.

What is your timing curve as that can affect things. I once had a issue like yours and no amount of carb changes would help and I found out that I did not have my distributor set back right after changing out a new rotor and cap on it. I was off on my timing by a small amount that was enough to give me tuning fits.

What color pump cam is currently on your carb and what size shooter? After figuring out your primary side of things I can round up my research and photos and post here or maybe in the 670 street avenger post on the secondary side of things. Also is your street avenger the 2 corner idle or 4 corner idle and of so how far out are your idle mixture screws?

Okay, the cam is a flat tappet hydraulic with 270* advertised and 224* at 0.050". The pump shooter is a 31 and I have the red cam in it. I have 13" of vacuum at idle (800rpm) and 18-19" at highway speed. My engine seems to like a lot of timing as there is almost 18* at Idle with vacuum advance unhooked. Total is around 36* at about 4000rpms. I bought a spring kit to kick the timing in sooner but have not installed it yet. It's a Pertronix flame thrower distributor and it gets a full 12 volts. It never misses a beat clear past 6 grand when you floor it, but the advance is a bit lazy. Any thoughts?

ericnova72 11-27-2018 04:23 AM

Quote:

Originally Posted by jjjaffo (Post 4638267)
I actually was going to order a cam kit this week. Right now it has the red cam on it and it seems to deliver a pretty healthy squirt. The more I read you guy's posts the more I wonder about the Idle/transition circuits. With low speed driving up to highway speed if you ease into the throttle it stumbles, but if you continue to gently push the throttle down it will come out of it and start putting you back in the seat. Could this difference be the carb coming on to the main circuit? I know the secondaries are not opening yet at this point because I'm just not that far into the throttle yet.

Sure sounds like you are onto the right track.

The red cam is one of the smaller cams if I remember correctly.. Will likely work it's best on the second hole in the cam, second hole in the linkage plate.

TommyK 11-27-2018 06:15 AM

Try a blue cam.


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