carb jetting

« **on:** June 07, 2008, 09:37:09 PM »

I have a Holley 750 DP (brand new) with 73 in the primary and 81 in the sec. Engine is 468 olds 10:1 comp 280H cam pocket ported heads. Pulls about 10 in hg at 900 rpm with a 6.5 pv. I was wondering if the jets seemed to be about the right size. The engine is on a run in stand and it seems to be rich. Any suggestions?

?

Thanks,
Cole

« **Reply #1 on:** June 07, 2008, 10:32:49 PM »

You won't know for sure until you get it on the water under load. I always start off with stock jetting than make small adjustments until its right.

« **Reply #2 on:** June 09, 2008, 07:18:57 AM »

thanks.

Another question,
When the engine is at idle and I stab the throttle, it wants to backfire. But if I roll the throttle, then it is fine. could this be an accelerator pump issue?

« **Reply #3 on:** June 09, 2008, 12:42:34 PM »

Could be fuel or it could be ignition related.

What distributor and has the advance been adjusted (springs/weights changed)?

I don't have any experiance with an engine on a run in stand.

« **Reply #4 on:** June 09, 2008, 01:40:30 PM »

Thanks,
It has a MSD ready to run marine distributor / MSD 8.5 mm wires / 6 al (not hooked up right now) I put the lightest springs in it that I could thinking that I would want it to advance fast being in a jet boat, but the it was basically adavancing at almost idle an running like sh*t. So, I put the heaviest springs back in it (the dist. came with them in it) for now.

I called Holley and they told me that the power valve is too big. They explained to me that a 6.5 pv starts to open @ 13 in hg and that it was pulling too much fuel at idle. He sayed that this would also cause the back fire issue. I orderd a 4.5 (as per their suggestion) and will go from there.

« **Reply #5 on:** June 09, 2008, 02:11:58 PM »

Ok, remind me to not call holley tech.

The 4.5 PV is the better PV for you overall, but it isn't going to cure your problem.

Your squirter is too small most likely, or your pump cam is on the wrong position or not big enough. If you have slop on the acc pump arm that will cause it, as will a missing check valve / needle in the squirter passage.

You should havesome pump shot the instant you move the throttle plates, any lag will cause you to have issues.

« **Reply #6 on:** June 09, 2008, 03:53:39 PM »

Quote from: speedymopars on June 09, 2008, 02:11:58 PM

Ok, remind me to not call holley tech.

The 4.5 PV is the better PV for you overall, but it isn't going to cure your problem.

Your squirter is too small most likely, or your pump cam is on the wrong position or not big enough. If you have slop on the acc pump arm that will cause it, as will a missing check valve / needle in the squirter passage.

You should havesome pump shot the instant you move the throttle plates, any lag will cause you to have issues.

That is what I have been hearing from people and I thought that I would call Holley and get their input and they said what I posted above. But it is making some sence that the pv would cause it to seem rich. Also, not getting a quick spray and distribution of fuel when stabbing the throttle would cause the falling on its face and back firing. I guess the first thing I can do is check the clearence on the acc pump arm. Holley tech web page says .015. Does that sound correct?

Thanks

« **Reply #7 on:** June 09, 2008, 04:27:01 PM »

yes - but that is .015 from the open throttle position - that is so you don't bottom the pump diaphram.

Any backfiring or dying when you whack the throttle is 99% of the time acc pump related. The other thing to check is if the carb is leaking fuel via the PV port (blown PV). With the carb full of fuel, without the vaccum of the engine running, there should be no fuel dripping from the bottom of the carb.

« **Reply #8 on:** June 09, 2008, 06:16:53 PM »

Quote from: speedymopars on June 09, 2008, 04:27:01 PM

yes - but that is .015 from the open throttle position - that is so you don't bottom the pump diaphram.

Any backfiring or dying when you whack the throttle is 99% of the time acc pump related. The other thing to check is if the carb is leaking fuel via the PV port (blown PV). With the carb full of fuel, without the vaccum of the engine running, there should be no fuel dripping from the bottom of the carb.

Thanks,
So, I looked at the squirters and I have a 28 in the front and a 31 in the back. At what increments should I step up the squirter sizes?

I was thinking about putting the 31 in the front and ordering a 36 for the back. Is that a good start? Or should I go in smaller or bigger increments??? Also, do you think that I am wasting time changing the primary squirter?

« **Reply #9 on:** June 10, 2008, 11:23:03 AM »

Correction:
I was thinking about going to a 31 in the fron and a 35 in the rear.

« **Reply #10 on:** June 11, 2008, 02:56:23 PM »

You may just flip them first to see if that takes care of the problem (or at least makes it better before you order that parts). You may find 28s in the rear work fine.

31's or 35's in the front is a good start. I usually just walk up the sizes (starting with primaries) until it's right. Once the primaries are diled, then I start playing with the secondaries until it's right. Anything above 37's you will also have to change the screw to a hollow one.

« **Reply #11 on:** June 11, 2008, 03:12:49 PM »

Thanks, I will start there.

« **Reply #12 on:** June 23, 2008, 03:46:13 PM »

Went to the lake yesterday and I am still having some issues.

From idle, if I hammer it, it will pop a little bit and fall on its face.

From crusing speed (2800 or so), if I hammer it, it has a pretty good hesitation, then it pulls hard.

Took a plug reading and it is lean

Just to refresh:
The carb (750 dp) came with 6.5 pv, 28 primary squirter 31 in the rear, 73 primaries and 81 secondaries, 30 cc acc pumps.

What I have done to try to solve previous problems:
put a 4.5 pv in
put the 31 squirter in the front and a 35 in the rear
put 70s in the primary

I was planning on putting some 72s in the primary and making a cam change from white to pink then taking another plug reading (I am being told by Holley to get the plugs to look good and then work on the poping / hesitation with squirter size and cam changes)

Any other suggestions as what to do?

Thanks,
Cole

« **Reply #13 on:** June 23, 2008, 04:00:10 PM »

Quote from: KonaCole on June 23, 2008, 03:46:13 PM

Went to the lake yesterday and I am still having some issues.

From idle, if I hammer it, it will pop a little bit and fall on its face.

From crusing speed (2800 or so), if I hammer it, it has a pretty good hesitation, then it pulls hard.

Took a plug reading and it is lean

Just to refresh:
The carb (750 dp) came with 6.5 pv, 28 primary squirter 31 in the rear, 73 primaries and 81 secondaries, 30 cc acc pumps.

What I have done to try to solve previous problems:
put a 4.5 pv in
put the 31 squirter in the front and a 35 in the rear
put 70s in the primary

I was planning on putting some 72s in the primary and making a cam change from white to pink then taking another plug reading (I am being told by Holley to get the plugs to look good and then work on the poping / hesitation with squirter size and cam changes)

Any other suggestions as what to do?

Thanks,
Cole

Swap the squirters. 35 in the front, 31 in the rear. 71 or 72 jets in the front.

Pink cam on #1

« **Reply #14 on:** June 23, 2008, 05:17:01 PM »

Quote

Swap the squirters. 35 in the front, 31 in the rear. 71 or 72 jets in the front.

Pink cam on #1

X2, need more initial fuel injected at throttle pick-up, also check the "slack" or accel. pump clearances of the arm.

« **Reply #15 on:** June 24, 2008, 12:44:58 AM »

Quote from: GT Jets on June 23, 2008, 05:17:01 PM

X2, need more initial fuel injected at throttle pick-up, also check the "slack" or accel. pump clearances of the arm.

Also, might try going back to a 6.5 power valve. That should help your hesitation when you hammer it at 2800 RPM.

« **Reply #16 on:** June 24, 2008, 07:00:31 AM »

Thanks,
We'll give it a try

KonaCole

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