Holley carb problem

BEB · #1 (**permalink**) 12-19-2003, 03:55 AM

Hi, I am from the UK and new to this site. I have a Cobra replica which isn't running quite right? Please can you help me.

I have a Street Avenger 770 on my SBC 409ci, Edelbrock Performer RPM intake, Dart Pro One aluminium heads, Comp Cams 268 hydraulic cam, MSD 6AL, Holley Powershot air filter. Manifold vacuum at idle approx. 15 Hg.

Jetting is 'out of the box' 72/75. 10.5 power valve. Green pump cam; no. 1 position. Std. plain vacuum spring.

The problem is that there is a slight hesitation on light throttle from idle to 2000 rpm. I have an air fuel ratio analyser fitted so I know for certain it is leaning off for a moment or two before it 'picks up' and the mixture goes to back to where it should be - i.e. around 14:1. The obvious thing to do is increase the pump nozzle, but I've tried that going from .025 to 028 to 031 to 035 - it seems to prefer .028 as going bigger doesn't seem to make any difference. I also tried raising the primary float level slightly, but that hasn't cured it either.

What should I try next? Should I be looking at richening up the idle transition circuit. Would it help adjusting the position of the secondary throttle plates. I'm reluctant to richen up the primary jets because they are already on the 'rich side' according to my air/fuel analyser. Should I try a lower opening power valve? But, surely that would make the problem worse? Would a different colour pump cam bring more fuel in sooner - if so, which colour?

Anyway, please help me as it is a mystery why I am finding it so hard to dial this carb in !

Thank you and best wishes,
Simon Winfield.

rdorman · #2 (**permalink**) 12-19-2003, 07:26 AM

One common issue that causes a lot of lower RPM part throttle issues is the relationship of the primary butterflies to the transfer slots at idle. You should have .040 to .060 of the transfer slots below the butterflys at idle. If you have more or less adjust the secondary opening until you are in that range.

I am STILL trying to get completely rid of the exact same issue you are talking about.

Rick

BEB · #3 (**permalink**) 12-19-2003, 07:43 AM

Hi Rick,

Does adjusting the transfer slot at idle to a bigger gap (or a smaller gap) richen things up a little?

Surely if I adjust the slot to make it a little richer and then adjust the secondary opening to compensate and get the correct idle speed then that should solve the problem?

Cheers,
Simon.

rdorman · #4 (**permalink**) 12-19-2003, 07:53 AM

I don't think I would look at it as if it makes it richer. As I understand it they are calibrated expecting a certain range of fuel from each of the systems, idle, transfer and main. If the plates are open to far at idle then you are getting more fuel from the transfer system and less from the idle system than was intended. Creating an imbalance. With my some what limited knowledge, that is the best I can explain it. On my 351 I had that issue. What first tipped me off was after adjusting the idle mixture for best idle, the idle screws where only about 1/2 a turn out. Normally a good starting position is 1 1/2 to 2 turns out. This indicated to me that I was getting extra fuel from some where. In extreme cases you may even see fuel coming out of the mains at idle!

If you can, replace the secondary set screw with some type that can be adjusted with the carb on the car. Set the primary butterflys where they should be and from there on out adjust your idle speed with the secondary butterflys. Honestly, I don't know why they don't just come like this.

Rick

Mr.Fixit · #5 (**permalink**) 12-19-2003, 09:17 AM

you probubly won't be very successfull tuning a Holley 4bbl for 14:1 A:F it will have to be richer than that much of the time to avoid lean spots, like the one you are experiencing now.

Hotfingrs · #6 (**permalink**) 12-19-2003, 10:13 AM

Another thing I would look into is the power valve....Holley's come stock with a 6.5 and I only use 2.5.....but 10.5??

niles · #7 (**permalink**) 12-19-2003, 11:39 AM

I have found that the accelerator pump screw is very sensitive.
If you have any slack in it at all you will have that throttle onset hesitation. Take out free play by adjusting the screw longer on the pump arm, ie, the pump lever should move as soon as the throttle moves. I have found that 1/2 turn will go from hesitation to perfect. Start with little to no play-test and then adjust the screw so that the cam operates soon by 1/2 turn at a time.

fixed mine

gn

BEB · #8 (**permalink**) 12-19-2003, 11:54 AM

Niles - I will try that. Maybe there is too much free play at the moment?

Hotfingrs - Holley have recommended I try a 6.5 PV with the 20Hg idle vacuum I have.

Mr Fixit - What sort of AF ratio should I be looking at in that case?

Rick - I shall check the transfer slots on both primary and secondary.

BIG thank you to all you Guys for your advice.

trularin · #9 (**permalink**) 12-19-2003, 12:29 PM

The Holley's I have seen have 10.5s.

I like the 3.5 for the power valve.

Have either of you two tried a differnt CAM on the accelerator pump? Run 50CC and make the cam come in faster. I think the kit has several different cams.

Hey, it's just a thought

rdorman · #10 (**permalink**) 12-19-2003, 12:33 PM

Personally I worked my way up from the 30cc pump and stock cam and 28 squirter all the way to the 50cc pump, biggest cam and a 50 squirter and still did not completely get rid of the hesitation. Think is might be something else?

The transfer slot relationship did make a big difference when set it correctly.

Rick

Hotfingrs · #11 (**permalink**) 12-19-2003, 01:23 PM

Rick....definately think you got something out of whack....squirter should not need to be any larger than low to mid 30's....i'm running 1050 dominator and only need 32 squirts...actually with the 50cc pump you only pick up 2cc of gas over the 30cc pump...pumps are rated by using 10 pumps...10 pumps will give you 30 or 50 cc....take the 20 difference and divide by 10 gives you 2cc per stroke.....to bad the weather isn't a little warmer...say 45 or 50 degrees more....i'd give you a hand....

Hotfingrs · #12 (**permalink**) 12-19-2003, 01:28 PM

another thing to think about.....if you nail it and it bogs then takes off....usually means to much gas.....if you nail it and it takes off for a moment then bogs and takes off..usually means jet are to small...then throw your distributor advance and timing into the mix......whole lot to diagnose on the net.....this is definately a hands on operation...

rdorman · #13 (**permalink**) 12-19-2003, 01:35 PM

My door is always open to some one who wants to turn some wrenches with me! I have a list longer than both arms of things I would like to do to the car this winter.

As for the carb, I think I am going to tear it down and go over it with a fine tooth comb, check EVERYTHING, put a kit in it and start the tuning process over. I know the jet set up is good, set them on the chassis dyno. Don't get me wrong, it is not horrible by any stretch, but it isn't perfect either. Now when I first bolted it on! it WAS horrible. Almost put my 1850 back on!

Rick

rdorman · #14 (**permalink**) 12-19-2003, 01:39 PM

Part of my problem is that I have not done one thing at a time so I have shot my self in the foot tuning wise. It does hesitate very breifly and then take off like a shot. We are not talking about a fall on your face type of bog here, just a slight hesitation. I don't see any sort of black puff from the pipes, though it is hard to look down and back when you stand on it!

I am off for the next nine days so I am sure I will fiddle with it a little weather permitting.

Rick

Mike_I · #15 (**permalink**) 12-19-2003, 01:42 PM

Just my O2 but I don't think that you went high enough when you were playing around with the pump nozzles. I just had the same problem as you and ended up going from a 25 to a 37 pump nozzle. Try going up from the 35 that you tried. Also, remember that if you end up going over a 37 you need to install the 50cc Accelerator pump.

Good Luck

BEB · #16 (**permalink**) 12-20-2003, 03:17 AM

Replying to Hotfingrs:

You say "if you nail it and it bogs then takes off....usually means too much gas.....if you nail it and it takes off for a moment then bogs and takes off..usually means jet are too small"

My problem is the former; it bogs and then takes off. However, because I have an air/fuel ratio analyser fitted in the form of a lambda sensor I know for sure that it is running lean momentarily so it can't mean it's too much gas?

I'm really confused now as everything points towards the accelerator pump system and yet I've tried pump nozzles up to .035" and different colour cams/cam position, etc. but still the stumble is there.

Help.....

CowtownCobra · #17 (**permalink**) 12-20-2003, 04:19 AM

What sort of distributor do you have? I have a Mallory Unilite and trying different springs has helped a similar problem for me.

BEB · #18 (**permalink**) 12-20-2003, 06:53 AM

I'm using a MSD distributor - vacuum advance.

I've tried the lighest springs,but I'm using the medium, blue springs at the moment. Do you think going to lighter or heavier springs will help and why?

CowtownCobra · #19 (**permalink**) 12-20-2003, 08:47 AM

Well, with vacuum advance I don't know. Mine is mechanical advance. I still have some of that stumble between 1500-2000 rpm, but being an old timer, I sort of accept it, after all, back in the 60's thats the way they all worked. For example, I purposely tuned mine for a lumpy idle. Oh, the memories...

BEB · #20 (**permalink**) 12-20-2003, 10:36 AM

Damn; I meant to say my MSD distributor is a NON vac. advance type. Would that info. affect your view on what springs to try?