Carberator Hesitation

[Cleve]

Well we have my MK4 roadster up and running after 2 years. Have gone on a few short trips to break her in. We have a 302 blueprint engine with a performance cam and a Holley 600 carburetor.
We have a problem at the stop light. When the light changes I ease down on the throttle and sometimes the engine hesitates as I am moving the car into the traffic. I have been able to stop it from shutting off by babying the throttle. We have a brand new 600 CFM Holly from Blueprint. I think my problem might be an over active carburetor fuel pump. I think it might be pumping excessive fuel into the intake manifold before the butterfly opens enough to allow the proper amount of air fuel mixture. I know that adjusting the carburetor fuel pump procedure is setting the set screw barely touching the control arm for the fuel pump. I have watched a lot of you tube carburetor adjustment videos and one guy actually adjusted the fuel pump a little different. He used a .030 shim to adjust the distance between the adjust screw and the fuel pump control arm. I was thinking this might give the butterfly a chance to open a little more before the fuel in pumped into the intake manifold. I am not a carburetor man but this seems to make some sense. Tell me if I am wrong and if so tell me another way I could fix this hesitation problem . Any help would be appreciated because this could be a serious problem when you are maneuvering in traffic.
Thanks very much
Cleve

[Jeff Kleiner]

Sounds like it’s not getting a good accelerator pump shot.

Jeff

[cv2065]

Agree with Jeff. Check your accelerator pump. You could also check your float levels and idle mixture.

[StangRacer]

If your pump arm is adjusted correctly, you should see fuel come out of the squirter the instant the butterfly moves. If this is the case, try going up one size on the primary squirter.

[Super Dave]

It seems like Holley carbs always need bigger accel pump squirter

[CraigS]

Besides the accel pump adjustment the idle circuit can have a major effect on engine running up to maybe 1800 rpm. Are you familiar w/ adjusting the idle mix screws to lean best idle? As you move the screws 1/4 turn at a time you will notice that as you screw them out (richer) (1 screw at a time) you get a slow reaction in idle smoothness. But as you screw them in, you will suddenly get a very noticeable decrease in speed and increase in roughness. So get to that drop off point, and then slowly screw back out 1/8 turn at a time until the engine smooths out. That is your 'lean best idle'. Do this for each idle mix screw. There may be 2 or 4 screws depending on carb model. See how it drives now. But remember I said it can have an effect to maybe 1800? That is controlled by the idle mix jet which may or may not be replaceable depending on carb model. To test the rest of the idle system, after setting to lean best idle, turn all mix screws out 1/4 turn and drive it. Your idle may not be the best now but this test will let you know if the overall idle system is tuned properly for your engine. Turn them out another 1/4 and drive it. Maybe also a 3rd 1/4 turn. This technique was explained to me by a Quickfuel tech line employee and really helped my carb. Fortunately my carb had a replaceable idle jet. BTW I won't say that the .030" accel pump adjustment won't work but I am somewhat doubtful. That carb may have had too large a squirter so delaying it helped but I think that is an anomaly. The accel pump squirt is there for a reason. As the throttle plates open there is a massive increase in air flow. Air flow is what sucks fuel out of the jets. But that increase in fuel flow is delayed a bit behind the increase in air flow. So the pump shot is there to make up for the delay. Keep trying, keep watching youtube, carb tuning can be a lot of fun. BTW make notes of everything you do and it's result as it is surprising how easy it is to forget since it can often be a few days or a week between messing w/ your car. Now, did I turn those screws out 1/4 turn last week or was it in? Ahhh???

[wbulk]

I do complete jetting on Holley types a lot with a wide band O2 sensor. I know that carb very well. First what is your altitude. I believe that carb comes with a .028 squirter. The .028 will work but at sea level the .030 can work better. Often I see that is the wrong accelerator pump cam. The pink is too small. White, red and orange are slightly bigger in that order. On that carb white or red usually fixes that. It's a trial and error and sometimes you have to use the second screw hole with the white cam. I think you are lean not rich. Always check your floats first. The fuel pressure should be max at 6.5 lbs. at sea level and lower, all the way down to 5.5 at 5,000 feet. Send me a PM if you want to go into the extensive detail of improving that carb so we can discuss on the phone . It can really work good.
Real quick, one other thing to look at, under the squirter there is usually a weight or a ball. It can get stuck from the ethanol in the fuel if it sets for awhile. Make sure it squirts.

[Mike.Bray]

Just needs a little tuning.

345594239_5968457576614265_1227595185626544093_n_A.jpg

[rich grsc]

Finally someone that know tuning

[F500guy]

My experience with the carb that shipped on my BPE-Needs complete go thru for settings. BPE seems to just bolt on out of the box and it is probably in need of full calibration. Also review your timing, BPE sets it pretty conservative so there may be some needed adjustments there.

[Redstang69]

I second what Wbulk recommends. I've been fighting a pretty much identical issue on my mustang (302 with Holley 600). I'm betting you're too lean, not too rich. I've checked accelerator pump linkage adjustment and adjusted the idle mixture screws. What has helped the most was changing the pump cam and I also drained all the gas out and put fresh in. It still has a slight hesitation when you step on it or when pulling out but those two things definitely helped.
I also checked the vacuum advance and it's working. I might bump up a squirter size but I hate the thought of trying to get the one fished out and new one put in without dropping it down the intake. Any suggestions there Wbulk?

[F500guy]

I made big changes to nozzles and on my 750 dp on the BPE 427. I pulled the carb every time I made adjustments, I was afraid of dropping something! According to the Carb book (David Vizard) 30 cc pump should be good til about 800 hp. Hollow screws should not be needed until about 37/38 pump nozzle. Here is my adjustments so far:

Current Carb Settings.jpg

[F500guy]

1 more point set up point about new carb, make sure you have the primary transfer slot not to far uncovered. Lots of internet info on that. Requires carb to be removed

[Cleve]

Up date on my 600 cfm carburetor problem
I want to thank everybody for your comments on my problem . After watching many carburetor adjustment videos that pertain to my hesitation and the slight stall at take off I came to the conclusion that the accelerator pump was pumping more than enough fuel. So I adjusted the accelerator pump with a .010 shim to hesitate the pumping action. I tried several shims ranging from .015 to .005. The car runs the best on .010. Once in a while a slight hesitation but a whole lot better than with the standard setting. I believe another slight adjustment would be slightly opening the butterfly on the secondary air intake, but not enough to have it draw fuel. I finally got an appointment to have the car turned up using a dyno after waiting 5 weeks for the appointment .

[Avalanche325]

Transfer slot is pretty much step 1.

[AC Bill]

I recall back in the mid 2000's a Holley carb on Al Biex's FFR was doing the same thing. He told me it was fuel in the bowls being affected by G force under acceleration, causing fuel starvation. It would start to accelerate, but if he stayed on it, it would start to miss and stutter, then as it slowed, the carb would pick up fuel again. A lighter touch on the throttle pedal when starting off prevented this. He blamed Holley carbs for the phenomena.

[cv2065]

Are you on timed or manifold vacuum? Seems like many crate engine builders like BP, Roush, etc., default to timed.