Holley carb problem

[BEB]

Mr. Fixit:

The problem is that I've already tried that.

If I adjust the spring loaded adjustment screw so that the cam engages the accel. pump instantly, then it means that there is absolutely NO gap at all at wide open throttle.

Using a Green cam there is a delay but using a Black cam it is instant, so logically I should use a Black cam? However the bog/stumble actually seems worse with a Black cam - it doesn't add up?

According to Holley (and most fellow Cobra owners on this website) it should be cured just by fitting a bigger pump nozzle but this doesn't work either?

I'm at the stage where I'm pulling my hair out.

Think I'll set the damn car alight and buy a "sensible" car with the insurance money !!!

[Mr.Fixit]

Get the cam to touch the spring arm by advancing the cam, not by lowering the spring arm. That can certainly be done with the green cam, and not have it bottom out the pump arm.

[BEB]

What do you mean by "advancing the cam"?

Do you mean fitting it in position 2 or something else?

Keep perservering with me as you sound like you can help me. I really do appreciate your help.

Thanks...

[niles]

BEB.
The black cam is the "smallest" ---reread! If it stumbling; it's most likely lean. Your observation on movement verses cams is wrong. Forget about the .015 at WOT. Adjust the cam down until it moves/pumps upon throttle movement. Also everytime you change the idle stop adjustment(idle speed) you must readjust the cam spring. One half turns on the spring adjust nut will take you from stumble to right on to stumble!! Put a orange or green cam in and adjust like several have told you. quit messing with multiple variables and get the basics down.

gn

[Mr.Fixit]

Originally posted by BEB


What do you mean by "advancing the cam"?

Do you mean fitting it in position 2 or something else?

Keep perservering with me as you sound like you can help me. I really do appreciate your help.

Thanks...

Plastic cam has two holes, right?
Throttle shaft arm has two holes, right?
Use the right holes so the cam is allready moved forward enough so that as soon as the throttle moves, the cam is moving the accell pump. Then check for bottoming out of the pump.

You have checked that the accel pump is squirting, right? the red rubber umbrella may be torn, or the check ball may be stuck, depending on which style of check valve yours has. Also, make sure the hole in the bowl gasket is aligned with the accell pump orifice. If the accell pump squirrts when it is depressed, these are OK, no need to disassemble.

[BEB]

OK Guys,

I will fit the Green cam in no.2 position and adjust it so that the pump arm moves immediately when opening the throttle !

The accel. pump system is definitely working as gas shoots out of both pipes fine.

I think it is just a question of "timing" the shot correctly?

[bullseyeprec]

I didnt find a huge difference in cam or squirter size. I could make the engine starve or hesitate but foun that it can be made to work with a number of different cams/squirters. I did have best luck in number 2 pos. since I idle at 950rpm. what kind of vacuum do you have??? Mine idles at between 14-15" at idle. Thats max vacuum at idle. my 750 dbl pumper had primary idle mixture scrwes in the primary side out about 1 1/2 turns and secondary ones out about 1 turn each. Maybe your problem is that the power valve isnt getting a low enough signal from your gradual acceleration to work it. bigger power valve number opens faster (works with less vacuum) please post your idle vacuum and check your power valve number. It would be best to know the vacuum reading when the car seem to stumble but thats difficult unless you have about 20 feet of vacuum hose. Any thought guys?? maybe power valve is his problem? I had similiar issue and everyone said its float level or accel pump nozzle/ cam so I adjusted floats till I was blue in the face and eventually removed the accel pump cam so I had NO accel pump cam and I stil had hesitation.....powervalve fixed my problems. When I first got car it had a little off idle hesitation and accel pump cam/squirter did fix that but at 2000rpm maybe its his power valve???

[Steve Dickey]

BEB,
I just re-read through this thread. Sorry you are still having problems. By now I'm sure you have adjusted your accelerator pump correctly, 0.015" at WOT and moves as soon as the throttle opens. Do yourself a favor and richen up your mains a size or two. As Mr Fix-it previously stated if you try and run it too close to 14.7:1 you will have lean spots. When I was jetting my carbs with the A/F meter and approached 14.7/1 I experienced the same stumble you describe. Richened things up a bit and it all went away. As for your power valve that is not your problem. It's number depends on your engine. Measure your vacuum at idle and get a valve that opens 2-3 inches of mercury under that. That should put you in the ballpark. Later you can drive around with a vacuum gauge and size the valve 2 or so inches under a cruise or light acceleration. You don't want the valve opening till you put your foot in it a bit. Good luck and keep us up to date.
Steve

[A Snake]

I'm not the carb expert I wish I were, but after reading the entire thread, I have to ask BEB and the helpful group a couple of questions. It may be that the carb is being tinkered when it really wasn't the problem at all.

MrFixit, you can jump all over me if you want.

1. Has any consideration been given to the possibility that the carb is too big for the application? Run the displacement X rpm @ 80% Volumetric efficiency. Go to the Holley site and check the numbers to see what should be on the engine. Without looking, I'm guessing a 650 would be more than plenty. Remember, you can use more carb if it has vacuum secondaries. Mechanical carbs are pretty fussy that way.

2. That mild cam makes a ton of vacuum at idle, and up into the 2500-3000rpm range so there should not be any problem with vacuum signal at all.

3. Playing with the accelerator pump cams is fun if you're working on a race car. As the carbs come out of the box, they are pretty well dialed in for a wide variety of applications other than all out racing. Again, check carb size.

4. Last, I would strongly suggest taking the distributor out and having it set up on a distributor machine. Playing with springs is asking for troubles such as burnt valves and broken rings. It doesn't matter what a book says about a certain spring doing a certain thing. The acid test is how it actually performs in YOUR distributor. You should have about 38 degrees of total advance and DEPENDING on compression, 10- 12 degrees initial at idle, assuming the cam is degree'd straight up. This all assumes a comp. ration of between 9 and 10:1.

Add all of this together and you may find that all of the monkey fiddling wasn't needed at all.

Hope this adds some food for thought.

Al

[BEB]

WOW - a wealth of info Guys. Thanks.

Comments:

It will happily idle at 800 rpm with 15/16 Hg vacuum.

I have a 10.5 Power Valve in at the moment, but Holley suggest I go back to a 6.5?

As I've already said; I cannot get both .015" gap at WOT and the pump to work as soon as the throttle opens which is why I thought that maybe the Green cam was wrong and a different colour/profile would be better?

I have got a new Speed Demon 650 vac. sec. carb which I've tried but which had a hesitation further up the rev range which even spending 2 days on a chassis dyno couldn't fix. Went back to the Street Avenger 770 and that particular problem went away. In theory I would be better with a 650 over a 770, but in reality the engine prefers the bigger carb?

It is a new MSD (non vac. adv.) distributor set at 14 degrees at idle and 34 degrees total at 4500rpm and 10:1 CR.

I think that covers all of your points?

[Mr.Fixit]

carb CFM size shouldn't matter a whole lot. The difference between a 650 and a 750 will not be noticable.

What main jets are you running? have you tried going up a jet size on the primaries, the acell pump can only do so much.

you will not get a Holley to run perfect at an A:F ratio of 14:1 go for 12:1

[BEB]

Currently running 72's in the primaries - I've got a spare set of 74's so maybe I should try them?

Nobody has yet mentioned drilling the restrictors in the idle transition circuit. Is that something I should consider rather than going bigger on the primary jets?

[Steve Dickey]

No
Steve

[A Snake]

Agree with Steve and Fixit. Do NOT drill anything. Try the larger jetting. Would suggest trying 73's first. Always better to go in small increments.

When you do this BEB, go back to the out of the box set-up at the same time. That way you will only have one thing you've changed instead of several. Then if it gets better, but not fixed, try the 74's. I suspect the jet change will take care of it.

As to AF, Fixit is absolutely right. 14.7 is way way too lean for a Holley. 12 to 12.5 is about right. Holley carbs have never been known for mileage and that's why. They just don't work right leaned out too much.

Al

[orionlearn]

Simon, I am late to this thread, so I may be offering some ideas that have been said already.

My first thoughts are that your 770 maybe too large. Your cubes, a guess at your VE being a max of 105% and a 5500 rpm high end would yield a need for a 683 cfm carb. The lean bogg at the lowend would indicate a big gulp of air. This combined with a a2f of 14 would create a lean bogg.

I agree with others that you want to be at 12.5:1 to help richen up the mixture. Also, 32 or 35 shooters should help, but I have noticed a similar problem when using the cast shooter vs. to holley shooters with extended brass inserts.

All said, you won't hurt the engine at 14.7:1 or below. With the price of fuel in the UK, I would live with the slight bogg!

Best Regards,

Bob