swissarmychainsaw

I recently bought this truck as it is. It does not *quite run right. My main issue is that it has a "flat spot". When you are rolling and go from 1/8th to 1/4 throttle it sputters and bogs. I have to lift up off the throttle to get it to recover. If I *ease it from 1/8 to 1/4 its drive-able, but I want this to run right.

I'm new to LS engines, MSD boxes, Holley, AFR, data logging etc. but I'm willing to learn to get this thing running right.
The rig:
1976 GMC K15 (4x4) Suburban
I have purchased:
Holley Jet Kit.
Holley Accelerator Pump tuning kit 36-184
Wide Band AFR gauge/kit (not on yet)

Can you guys help me understand the process of systematically sorting through this issue?

I don't know what my timing is set at (or even how to find out. Is the MSD pill enough or do I need a Timing light to verify?)
Next I will install the AFR.

Thanks!
Nick

LLLosingit

iTrader: (4)

I haven't used the MSD but in the description it says it comes with 6 preprogrammed timing curves. I would do a little digging to see what one to use for a your engine. If it's stock they will likely have one that's pretty close.
Usually a dead spot when you step on it under load means it's either not getting enough fuel or it's getting to much from the accelerator pump. I do find it odd that you're having that problem with a brand new carb though.
Get your timing straightened out before you even think about touching the carb, Timing issues will cause similar issues.

dandd

Nick,

My efforts on a carbed ls is limited but I had a similar issue. The situation I had was a lean tip in, at a light cruise you give it a little throttle and the engine goes lean and stay there unless you back off or more throttle for the accelerator pump to engage. my solution was to reduce the idle air bleeds (and a whole lot more) so that more fuel would be available at the transition slot before the mains are engaged. I believe the carb you have does not have any adjust-ability to the idle fuel restrictors or the idle air bleeds. I ended up purchasing a quick fuel 670 so that I could adjust both the idle, high speed, emulsion and power valve circuits. The first step would be to get the air fuel ratio gauge install to ID the problem.

I have the msd 6010 and it allows me to transfer the current setup to my laptop. I take it the 6014 will do the same. Go to MSD web site and download the instructions.

Pop N Wood

iTrader: (1)

Hook the wide band up then use it to systematically go through each stage of the carb.

You start with the idle mixture, than the main jets to get WOT right. One that is right you tune the idle feed restrictors and air bleeds to get the AFR right with slow throttle movements, than last step adjust the accel pump.

In your case it sounds like you may need more pump shot. You can step that up a tad just till everything else is set up right.

New carb will need tuning right out of the box. Very rare not to.

It is possible and IMO desirable to drill out the various restrictors and air bleeds and replace them with set screws you drill to size yourself. Lots of wright ups on here how to do that. But that is down the road, start with the jets and accel pump for now.

LQ4BU

Just my two cents...when setting the idle mixture, I've found that after getting it close with the wideband, using a vacuum gauge will allow you to tweak it even more. I've always got a crisper, better quality idle that way.

TTur1996

If your not using a map sensor you will need to zero out the map on the box. I believe those pills all have map values. May not even be the carb. Just something easy to check before you dig into the carb.

Doug G

I'm also betting, like said above, it's in the tune. (MSD pill).

Go to MSD and search the box you're using to download the program for it so you can custom tune to your needs....no one is/was happy with the pills MSD supplied.

swissarmychainsaw

First, Here is a picture of the Suburban. Clean!



This week, I have:
Researched what Timing Curve I have in it currently.
MSD 6LS Curve 2:



I also got my wide-band AFR gauge installed and hooked up, here is a test and first start.
Youtube video


Me goosing the throttle and not understanding the numbers, Also - Cold so choke would be on:

MSD requires you to have a windows laptop to use their software (for custom curves and stuff); which I don't currently have.
I'm asking around to borrow one, etc.
--Nick

swissarmychainsaw

I took it out for a drive, and shot some questionable video with my young apprentice:

swissarmychainsaw

One thing I noticed today is that my fuel pressure is reading higher than I would expect.
Since I'm using a Carter 4594 - 5-9 PSI; I assume that my gauge should read between 5-9 PSI.

Instead I get the following, what do you make of that? It's reading about 15psi on a cold-ish start

G Atsma

Which fuel pump are you running? You might need a pressure regulator.

swissarmychainsaw

According to Summit's website it should not. I bought the truck with this on it, but the guy said this was the pump he was running.

Carter Universal Rotary Vane Electric Fuel Pumps P4594
These universal rotary vane electric fuel pumps from Carter operate using leaded and unleaded gasoline, gasohol, and fuel boosters. They have an internal pressure-regulating valve, and feature free-flow fuel delivery. These pumps have no points to burn out, and no shaft seals to leak or deteriorate. Easy to install, these universal fuel pumps come with a complete installation package and instructions.

G Atsma

Is the pressure adjustable on that pump? 15psi is too much for a carburetor, or at least it used to be. Too much pressure will lift the float valve off the seat.

TTur1996

That pill is what is messing with you. Go get a cheap laptop from the pawn shop. Prolly $50.00. You don't need anything fancy. Make sure you shut off the Map portion and zero the graph. Then on the timing graph. Get rid of all the dots but 2. Put the first dot on 14 degrees for cranking. Then go straight to 26 degrees at 200rpm and flat line it straight across from there. Go to the track and make a pass and watch your mph. Check a couple plugs to make sure your in the ball park. Next get your launch rpm where you want it. Then go to the jets up and down watching your mph. If it only goes up a tenth or so go to the next jet up so its not too lean. Safe. Then go to the timing. 26 degrees is a very safe starting point. Go up from there. No two combos will run exactly the same so don't listen to anyone saying it should do this or that. Your car will tell you MPH wise what it wants. Do the same with the timing. If the mph only goes up a tenth or so go back a degree. Keep it safe.

As for your fuel pump get a cheap holley regulator. 6.5 to 7 psi is all the pressure should be. I find it hard to believe that pump is putting out 15psi without flooding the motor out. Should be blowing smoke killing you with fumes. Turn the pump on with the motor off and look down the carb to see if you see anything dripping. If not I would check it with another gage. I used those pumps back in the day and never ran a regulator. But still checked and they ran right at 7psi. Now, I would never run any kind of pump without a regulator. But that's me. Best of Luck to You.

swissarmychainsaw

Ok, I'm going to get a laptop sorted out so I can start with the Timing.
Never done that though, so brace yourselves for questions!

I will also pick up a fuel regulator and a new gauge.

There are NO signs of flooding. No leaks, no gas smell, no black smoke, etc. So I'm inclined to suspect the gauge at this point, but $20 will prove it one way or another.

pdxmotorhead

I never fool with a Holley that I didn't install without double checking the power valve ,, little buggers can really screw up things if they have a hole in the diaphragm..
First thing I do with most carbed engines is add a regulator, mine always seemed to run better with one, and they are way less tweaky on float height and such.. YMMV..

swissarmychainsaw

I managed to scrounge a buddy's laptop. I followed @TTur1996 advice and set the timing as described.
It's remarkable, but that took the stumble out! I feel like it's just a start but the thing is so much more drive-able now (one test drive)
The weird thing is at idle, this shows nearly 40 degrees! What do you make of that?
(I loaded the software to the MSD box and "verified" it. Also, I could feel the difference!)

Thanks to everyone for weighing in so far, I really appreciate the help!
I have ordered parts, and they should be here by the end of the week!
Nick

TTur1996

Also shut off the idle control feature. That will cause the timing to wander, if on. It appears that the map is still adding timing. I have the 6012 box so its a little different. Maybe someone that has this box can chime in. But you need to turn off and zero the map feature. (Not sure if the idle control is causing that.) Stretching your timing out to 2000rpm can cause sluggish acceleration. Old school way was locking out the distributer so all your timing was immediate. Having it all in by 200rpm will or should give you crazy fast throttle response. Total timing will have effect on the throttle response as well. A while back I talked with Holley about the air bleeds. I was told not to touch them because they are set to each particular carb. And this was done on their flow machines so there is no way the average person can get it right by winging it, and that playing with them is usually masking another adjustment that wasn't quite right. I know there are guys that are looking for every last tenth. I like to play it safe. If you follow the procedure I already posted. You will find your sweet spot very quickly. Make sure you only make one adjustment at a time. Starting with the Launch Rpm, then Jets, then timing. Watch your mph. Check a couple plugs after your first pass just to make sure your not too lean or too fat. That carb should be fairly close right out of the box.

swissarmychainsaw

I'm not sure how to shut off the idle control, any idea there?

I zeroed out the MAP, and at idle (while hooked up to the computer) the timing no longer bounced around.

I also bumped up the curve to: 14 degrees at 0 rpm, then one dot at 1100 RPM 26 Degrees timing.

Throttle response is WAY better. Different animal. The only problem is that I'm now driving the Suburban around like an idiot with a giant grin!

I still have a lean condition when I give it part throttle and low RPMs (say in third gear, slow speed), but it's much much much better.
I'm having a blast, it's very rewarding to see improvements with small changes!
Maybe I can even get this thing to roast the tires!
:-)
Nick

swissarmychainsaw

I'm re-reading the thread and I realize not that I thought this said *2000* when it in fact says 200!

That's why I had it set the way I did!

Also, not even 24 hours later, a box has arrived from SUMMIT!
Nick