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cruise surge/stumble

11K views 38 replies 11 participants last post by  1969chevynova 
#1 ·
other than the ****ty cruise surge and weak light throttle response she runs great but at cruise and i guess tip in it surges and stumbles anything over 2200 or so is smooth and clean, if i nail it were heading on a 12 second pass.

sbc 357 10:1, lunati 268/276, dart pro1 200cc heads, quick fuel hr 650 70 prim jet and 75 secondary rest of carb is stock.. should i bump up the primary jets or ifr's. the 4 corner idle is at 1/2 turn all corners i have both front and rear transition slots even squares idles at 750, hunt a little in gear. heres some pics of the plug during a 2000 rpm surgy cruise
 

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#4 · (Edited)
it is still running on idle circuit at that rpm, have you tried turning out idle screws a tad more to see if it improves, i have found that setting for best idle quality does not always work when driving, since you say it smooths out after 2200 is because it is getting into the main circuit, plugs look like it is a little lean or hot
 
#7 ·
When it transfers from idle to mains is a bit more complex. A function of vacuum and throttle position. It'll transfer over whenever there is enough airflow over the venturis for them to work right. Until then, vacuum works more to get it through the idle circuits. Stumbles are usually caused by a lean condition when the idle circuit is quittin and main ain't pickin up yet, a less than smooth transition.

Have you adjusted the idle screws at all?

I had one did same thing that was cured by a little more accel pump shot.

I agree the plugs look a little lean, assuming you reading them after cruising around and killing it without idling for long. Looks like it might could use a little more fuel at idle.

Disclaimer: I am no carb expert! Only know enough to usually get me out of a bind lol.
 
#10 ·
No the mains must be flowing and the engine vacuum has be lower than the rating of the PV for it to start flowing fuel.

To the OP, I believe the HR series carbs have removable air bleeds. Since it is the easiest thing to change I would go down 5 numbers on the IAB's and see what effect if any it has on the cruise surge.
 
#11 ·
I am running a quick fuel 650 double pumper on my 350 and I know the quick fuel carbs inside out and they are great carbs and from my experience the 4 corner idle can give the illusion that it is either too rich or too lean because they are so sensitive to adjustment. On the two corner idle usually a 1/4 of a turn is what has to be done in order to get a change with the idle mixture screws but on the 4 corner idle ones just even moving the idle mixture screw an 1/8 of a turn makes a big difference and I just recently put mine on and I started out at half a turn on all 4 screws and it took a little time to get them in to the sweet spot and to eliminate any issues to where it was too much or too little.

I do believe the idle air bleeds are .071 on all four and since this is on the transition circuit going into the main circuit you could go down to a .066 idle air bleed and I from research would only do the primary ones first and then see how it goes and if still not much of a change then I would proceed to the rear and go down to .066. I usually don't mess with air bleeds too much as I go more for changing out the idle feed restrictors in the metering blocks as just a .002 change makes a big difference unlike the air bleeds that you mostly have to go up or down .004 at a time to get a change in the calibration.

I think they come with a .034 ifr size on both blocks and they have .031 squirters and pink pump cams and I have a similar cam like yours currently at 219/227 @ 50 515/530 lift 112 lsa and its a voodoo hydraulic roller cam along with dart 180shp heads. I had to change my pink pump cams out to orange for the pump shot to get rid of a lean tip in. I would say your jetting seems pretty close according to your plugs and mine look similar to yours but with a little more tan color and I am running a hotter plug and it does not go down the ground strap in the middle like it is recommended to and it goes about 3/4 down which is a little too hot from how some folks want it in the middle but going down one plug colder then is to cold for my build and my plugs don't then burn enough so sometimes it can't get to the perfect plug.

Are you running ngk plugs by chance as from my experience ngk plugs work really well more so then other brands. As far as timing goes I have found that if our mechanical advance comes in to quickly it can cause some surging in lower rpm's and that happened to me once and no amount of adjusting my carb would get rid of it and then after trying different advance curves I got mine to where it will start to advance 2 degrees at 1200 rpm and then it is all in around 3000 rpm or a little more.

Make notes of every thing you did when you change something and try it out and see how they may effect or not effect things so like that you can go back and start over with a different adjustment. Those carbs are very sensitive to just minor changes and its nice once you get the sweet spot to where it goes smoothly between each circuit. Also are you running an automatic transmission or a manual and what are you rear gear ratio numbers and if running an automatic then what stall speed convertor are you running as that also plays a part in things as well. I at one time many years ago had a pretty stout cam of the comp 292 h which specs 244/244 @50 and i first ran a 2500 plus convertor and it did the surging and bucking type deal and then once I got a bigger stall of 3000 plus then I no longer had that issue anymore so several factors can come into play
 
#14 ·
There is an excellent chance that the Air Gap is the dilemma. A quick way to check is to drop a gear and cruise at 2800+ rpms (3500 is OK) and see f the problem is still present. If it isn't, then chuck that Air Gap.

The Air Gap is too cold causing the fuel in fuel/air vapor to drop out and form fuel droplets on the intake manifold. Once the air speed picks up, the fuel droplets evaporate and get into the cylinder. If you were running an AFR gauge you would see the following at light throttle tip-in : steady, lean (tip-in), rich (as the droplets evaporate), and then steady (as the air flow picks up.) Result - lean stumble.

Perhaps you do have a carb/tuning issue, but what I described in the previous paragraph is the Air Gap dilemma.

My two cents.
 
#19 ·
I once used the air gap intake and after a short time I did not like it and instead took it off and sold it and got me a weiand 8150 which is the same as the edelbrock performer rpm intake. Your stall is good for your cam that you have and I run 3.42 gears in my rear end and a turbo 350 and it runs fine. As far as the plugs goes the 3924 is a cold plug for the autolite brand as the next up heat range would be the 3925 and then the hottest 3926. When I ran autolite brand plugs for my aluminum heads I rant he 3926 and a gap of about .040 and a regular hei distributor.

I am running ngk brand plugs at the moment and they are the cross reference to the 3926. Your stuff seems to be matched pretty well and it may be a slight issues running the airgap intake. I know on my quick fuel carb until it gets warm it kind of runs a little picky at idle and off idle even if the engine is up to almost 180 degrees and once warm after about 5 minutes of driving it mellows out fine and no major issues.

If you have some extra idle feed restrictor blanks you could take a set and go up a .02 size in the primary blocks and see how it does and if it works then there is the problem but if not then you can always go back to the smaller size as they are changeable. I know the 4 corner idle carbs are a little bit pickier to tune wise compared to a 2 corner as just even the smallest turn of the screws on all 4 can either be to much or too little.

On mine 1/2 a turn out is ok at idle but on take off it will stumble a little then pick up then going up to 3/4 makes for to much fuel flow at that point so I am at about an 1/8 of a turn before 3/4 out and just moving it 1/16 of a turn either way can make it run fine or a slight stumble on the transition circuit at low speeds.

I have mine all equal but I have read that some engines depending on the specs and if the cam is not too agressive don't do well with the 4 corner setup and sometimes they will close the secondary idle mixture screws all the way in and back them out just a 1/8 of a turn and then start off with a 1 1/4 turn on the primary screws and adjust them only to get better results but that is a trial and error thing there.

On my previous holley 600 vacuum secondary with the 2 corner idle setup it was not as picky for adjusting wise compared to the 4 corner style. For me it seems the 4 corner idle setup is somewhat better for cams with a good amount of overlap compared to a lower level cam with lesser duration and overlap.
 
#21 ·
I think this carb may be a little fubar'd. I had the same model that ran great before this one but I stripped one of the squirter holes on it. So I found the carb I'm running now on craigslist really cheap, guy said he couldn't get it to work right..
It ended up being all kinds of modified, different linkage, different bleeds, huge jets, etc.. So anyway I put it all back to stock like my old one was and it has just never ran as good as the original..
I think I'm going to bump up to a 750 any way and let someone else deal with this carb.. My old hr650 was almost flawless.
 
#22 ·
ok i installed a holley 4779 750 double pumper.. she as crisp as ever on the primaries, and no surging.. But it does have a huge bog when i dead nail it.. haven't had a chance to mess with it set up all stock with Primary main jets 71, Secondary main jets 80, Power valve 6.5, Primary Nozzle 28, Secondary Nozzle 31. seem like it needs more pumpshot on the secondaries.. it will run crisp and snappy as ever at half throttle or under.
 
#24 ·
I run a quick fuel 650 double pumper on my 350 build and I have rear 3.42 gears along with a 350 turbo transmission and a 2000-2200 stall and it is in a 96 s10 so my rpms pick up really quick which is what a double pumper likes when going wide open throttle and out of the box mine had .031 shooters front and rear and like you I had a bog when punching it and it would sometimes quit which showed me it was too lean on the pump shot before I did anything with the rear setting I got rid of the pink pump cam which has a longer travel curve to it compared to other pump cams which will give a bigger quicker shot but less travel.

I ended up with a orange pump cam on the front and kept the .031 shooter and the went to the rear. I usually unhook the secondary throttle blades so for certain I have the right setup for primary shot. Then I went to the secondary side and I knew it was too lean of a shot and the pink cam was not enough initial shot so I changed it out to a orange pump cam as well. I doubt you would need a 50cc secondary pump shot as I have yet to see any street performance build needing that much of a shot unless the vehicle is not really good with a double pumper such as being really heavy and very slow to get the rpms up which would make it lug.

The orange pump can is in the middle of all the pump cams available and I would recommend you buy a pump cam assortment kit and maybe a set of .031 and .035 shotter sizes to play around with and I am sure you would be able to get the bog gone but the very first thing what is your timing curve cause if its not the correct curve and if it comes in to late or to fast then it will also cause issues as everything needs to be balanced as best as possible. On the pump cams on the number one position it will instantly give a shot as on the number two position it will have a slight delay on will go a little further in the throttle travel compared to number one.

On shooters the bigger the number the bigger the shot is instantly but will have less throttle travel compared to a small number which would have less of a shot and more of a shot across throttle travel.
 
#29 ·
I run a quick fuel 650 double pumper on my 350 build and I have rear 3.42 gears along with a 350 turbo transmission and a 2000-2200 stall and it is in a 96 s10 so my rpms pick up really quick which is what a double pumper likes when going wide open throttle and out of the box mine had .031 shooters front and rear and like you I had a bog when punching it and it would sometimes quit which showed me it was too lean on the pump shot before I did anything with the rear setting I got rid of the pink pump cam which has a longer travel curve to it compared to other pump cams which will give a bigger quicker shot but less travel.

I ended up with a orange pump cam on the front and kept the .031 shooter and the went to the rear. I usually unhook the secondary throttle blades so for certain I have the right setup for primary shot. Then I went to the secondary side and I knew it was too lean of a shot and the pink cam was not enough initial shot so I changed it out to a orange pump cam as well. I doubt you would need a 50cc secondary pump shot as I have yet to see any street performance build needing that much of a shot unless the vehicle is not really good with a double pumper such as being really heavy and very slow to get the rpms up which would make it lug.

The orange pump can is in the middle of all the pump cams available and I would recommend you buy a pump cam assortment kit and maybe a set of .031 and .035 shotter sizes to play around with and I am sure you would be able to get the bog gone but the very first thing what is your timing curve cause if its not the correct curve and if it comes in to late or to fast then it will cause issues as everything needs to be balanced as best as possible. On the pump cams on the number one position it will instantly give a shot as on the number two position it will have a slight delay on will go a little further in the throttle travel compared to number one.

On shooters the bigger the number the bigger the shot is instantly but will have less throttle travel compared to a small number which would have less of a shot and more of a shot across throttle travel.

you are right on i got the bog gone she runs almost like injection when fully warmed... i know have orange cam in front with stock 28 shooter and blue in the rear with a 35 shooter. i tried the 35 shooter on pink cam and it was better but still cut out when stabbed.
I think i may ditch the air gap seems that the car really needs to be warmed up well especially when below 50 degrees outside...

i just built my buddy a cheap mild motor 357 camel humps 292 cam .040 quench 10:1 and that thing runs so good didn't have to do much. the holley 650 dp ran almost perfect out of the box.. but i'm thinking his regular non air gap rpm manifold is the ticket. his runs good hot cold and in between!
 
#25 ·
Forget my questions about your timing I forgot that I read them already but everything the pump circuit will need tuning and the way I do it is first start with a different pump cam cover and see hot it does and if it seems better then I go to a bigger shooter size and see how it does and with time it becomes pretty easy to tell on what part you need to change on either the pump cam or shooter. Its always good to jot down notes so like that you can always go back and change it back if it seems to much. Minor changes at a time is what is needed on tuning that part and experience over time can make it a lot easier to get rid of bogs without having too much of a pump shot or the incorrect shooter and making a band aid type fix.
 
#27 ·
I have her running killer right now with the 750. i believe the body on the 650 was f
messed up..
i ended up with orange cam up front and blue in rear 28 squirter front 35 rear. shes crisp and doesnt stumble or bog at all. cruises clean and smooth, has more low end response than ever. my first quick fuel hr 650 was good like this.
timing is at 36 should i pull a little back. seems to lose a little power when i do.. maybe the plugs look that way because i took picture when it was cruise surging at 1800 i have a e curve with 10 degree vacuum advance 21 initial and 36 all in by 2500 or 3000 would have to pull cap. shut down pulled plug and took picture.


I have lots of carburetor parts jets squirters pump cams gaskets etc.
 
#30 ·
I once had the air gap intake and in colder weather here in Ohio during the late fall months before I store my truck for the winter and during early morning drives to work my truck with th air gap intake did not go too well and even after a fifteen minute drive down the highway when I had to check a few things my carb was still somewhat cold which did not allow it to run fully like it should.

I ditched the air gap and went with a normal weiand high rise dual plane intake which helped quite a bit for better fuel atomization and also cleaner plugs and less rough ideling when warming up. For all out racing its fine but for any normal street use I feel the extra air going under it does not help but makes it worse. Carburetors don't like cold very well. Once mine is warmed up it makes a world of difference.
 
#34 ·
I like the weiand over the edelbrock just for the fact that when putting in a vacuum fitting on the rear of the intake and running a holley carb you don't run into any issue of clearance problems. When used the edelbrock rpm when hooking up my brake booster I had to use a spacer to raise my carburetor up enough to fit a fitting to hook it up but on my weiand the area on the intake is made further back and allows me plenty of clearance without not needing any fancy small pipe plug fittings to hook up my brake booster while also allowing my double pumper secondary accelerator pump to not have any issues for clearance as well. As far as power goes either one will be pretty much the same and you would not notice any difference.
 
#35 ·
Don't get a dual plane intake with the notch cut out as that defeats the purpose of a dual plane and it allows somewhat more power on the top end but not much but it also takes some from the bottom end as well allowing for both sides to share the signal going through it along with the carb and I use the weiand 8150 which has been excellent for me on two different builds.
 
#37 ·
figured out the recipe.. 1 Holley 750 dp with 71 primary jets, 79 secondary jets, 6.5 pv, 31 primary shooter and orange cam on hole 1, and 35 secondary shooter with blue cam on 1. she's seemless still might be a hair rich but i can stab it from and idle in gear and it don't hesitate one bit now and cruises smooth as can be at any rpm. thanks for the help guys.
 
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