Help With Holley Carb Tuning

[nimbusnic]

I'm struggling with my new Holley Carb installed on the 110. I made the switch because I was tired of fooling with the unreliability of my old SUs and laggish accelerator response. On the advice of the guy at AB, I went with the Truck Avenger 470 cfm model with a JWR Offenhauser intake manifold and 2" Mr. Gasket dual port spacer. I have the rover 3.5, and some have claimed that this is slightly too much carb for this engine, which may be my problem. I think that the setup could be great if I can get it dialed in right.

Here are the symptoms:
1.) Can't get the engine over 2,900-3,000 rpm at speed/under load, meaning I can't go faster than 50mph (my Saudi spec 110 is geared pretty low btw). The truck used to do 80mph with the SUs with less noticeable engine struggling.
2.) The truck will not take off in second gear without wanting to stall. With the SUs, the truck would start from standstill and accelerate in 2nd. I very rarely used 1st unless I needed to go uphill.
2.) The carburetor "lean pops" when pressing the accelerator past halfway in fourth or fifth gear, also occurs sometimes if you try to take off from standstill in second gear.
3.) Very little power or acceleration in fourth or fifth gear, especially over 2,500rpm
4.) Engine idles fine but smells a little rich, and has a small amount of black fluid that comes out of the exhaust.

Sidenotes:
1.) I have a dual fuel tank setup, where only one fuel tank/pump is active at any given time, generating approximately 2psi. Both fuel pumps are new. The shop that performed my install blocked the fuel return line in order to generate an extra .5psi. No problem with vapor lock yet.
2.) The shop that did the install drilled out my pump tower nozzles to improve my 2nd and 3rd gear low speed performance. It was explained to me that this simply dumps more fuel more quickly when the accelerator pump is activated in low gear. Before they did this, the carb exhibited lean popping at low speed in these gears as well.
3.) If the truck is left with the ignition on but not running, with the rear tank activated, then the rear tank seems to be filling the tank under the passenger seat. It actually overflowed on me once. Is this normal operation of the twin tank setup? I'm fairly certain mine is OEM.

My best guesses:
1.) Inadequate fuel pressure... If I were to install an in-line booster pump (say the Holley "Red" 12-801-1), where in the system should I put it to avoid fouling up the twin tank system?
2.) Bad carburetor tuning... jet size, float level, secondary problems?
3.) Timing problem or problem with new mallory unilite distributor setup. Is there any adjustment needed to the vacuum advance on these systems to make them behave?

Any advice or expert diagnosis would be greatly appreciated.

[huck1974]

I'm struggling with my new Holley Carb installed on the 110. I made the switch because I was tired of fooling with the unreliability of my old SUs and laggish accelerator response. On the advice of the guy at AB, I went with the Truck Avenger 470 cfm model with a JWR Offenhauser intake manifold and 2" Mr. Gasket dual port spacer. I have the rover 3.5, and some have claimed that this is slightly too much carb for this engine, which may be my problem. I think that the setup could be great if I can get it dialed in right.

Here are the symptoms:
1.) Can't get the engine over 2,900-3,000 rpm at speed/under load, meaning I can't go faster than 50mph (my Saudi spec 110 is geared pretty low btw). The truck used to do 80mph with the SUs with less noticeable engine struggling.
2.) The truck will not take off in second gear without wanting to stall. With the SUs, the truck would start from standstill and accelerate in 2nd. I very rarely used 1st unless I needed to go uphill.
2.) The carburetor "lean pops" when pressing the accelerator past halfway in fourth or fifth gear, also occurs sometimes if you try to take off from standstill in second gear.
3.) Very little power or acceleration in fourth or fifth gear, especially over 2,500rpm
4.) Engine idles fine but smells a little rich, and has a small amount of black fluid that comes out of the exhaust.

Sidenotes:
1.) I have a dual fuel tank setup, where only one fuel tank/pump is active at any given time, generating approximately 2psi. Both fuel pumps are new. The shop that performed my install blocked the fuel return line in order to generate an extra .5psi. No problem with vapor lock yet.
2.) The shop that did the install drilled out my pump tower nozzles to improve my 2nd and 3rd gear low speed performance. It was explained to me that this simply dumps more fuel more quickly when the accelerator pump is activated in low gear. Before they did this, the carb exhibited lean popping at low speed in these gears as well.
3.) If the truck is left with the ignition on but not running, with the rear tank activated, then the rear tank seems to be filling the tank under the passenger seat. It actually overflowed on me once. Is this normal operation of the twin tank setup? I'm fairly certain mine is OEM.

My best guesses:
1.) Inadequate fuel pressure... If I were to install an in-line booster pump (say the Holley "Red" 12-801-1), where in the system should I put it to avoid fouling up the twin tank system?
2.) Bad carburetor tuning... jet size, float level, secondary problems?
3.) Timing problem or problem with new mallory unilite distributor setup. Is there any adjustment needed to the vacuum advance on these systems to make them behave?

Any advice or expert diagnosis would be greatly appreciated.I not real great with carbs either, but I had similar problems with the weber setup on my CJ7. I contacted them and they sent me different jets according to my climate/altitude and that fixed 80% of the problem. Then an old hot rod buddy helped my dial it in. Good luck!

[snuffer]

I used to run a Holley on a Camaro with similar problems. Mine needed larger main jets.

Also sounds like it could be ignition timing, (if the lean popping is the same as backfiring through the carb.

[nimbusnic]

Randy,

Thanks for that advice, and I think you're right that the jets are the main problem. I am using the 470cfm model, which is really used for 6 cylinder engines, but is supposed to work in this case since we are dealing with a low volume V8. The current jets are 57 for the primary and 57 for the secondary, which is as far as I can tell pretty small by Holley standards. The larger 670 cfm carb uses 68 for the primary and 89 for the secondary. What would be the next logical step up for my 470, and should I change both jets or just one? My gut tells me the secondaries need more, considering I am losing power at the top end when at speed.

I think another of my problems is low fuel pressure. My stock pumps only generate 2.5psi and .5 of that is bled back to the tanks through the selector solenoid. I am having the Holley Red fuel pump installed into the line just after the T-pipe (linking front and rear fuel tanks) and just before the fuel filter. It is supposed the generate 7psi, which should make the Holley happy, but I am waiting to see if it will wreak havoc on the twin tank system.

Ignition timing is definitely also a culprit, and yes I do mean that it has been backfiring through the carb. This happens at startup and when applying to much accelerator underway. The guy who installed the carb attached my dizzy's vacuum advance hose to the full manifold vacuum port on the carb, not the ignition spark vacuum port. He says that this is more correct for my application. Does that sound right?

Thanks again for the help.

[snuffer]

I would go up one size at a time on the jets, it doesn't take much to make a big change.

I'd try the the other vac port. It's been long ago that I had the carb , try one change at a time until you find the answer.

[GYM]

If yours is at all like the Holley's on my old Mopars, backfiring through the carb and lack luster performance can also be a bad power valve.

[nimbusnic]

I've heard a lot about Holley's coming from the factory with cracked secondaries but didn't know the power valves were susceptible. How do you check a power valve for proper operation?

[nimbusnic]

Nevermind my last question. Holley covers this on the installation DVD. If you can tighten each of the idle mixture screws all the way and the engine stalls, then your powervalve is not faulty.

Important questions before I proceed with hacking this carb:
1.) Should I try fiddling with my engine timing/vac advance before taking more dramatic steps to up-size jets?

***2.)Can anyone confirm that I do indeed need to hook up the vacuum advance of my Mallory dizzy to the "timed spark vacuum port" of the holley carb instead of the "full manifold vacuum port"? I assume the timed spark is correct, even though that would mean disagreeing with my local mechanic. You want more advance at higher rpm, right? He claims he has it set up so that the engine will not stall out when I slam the gas in second gear, using the reduced vac pressure on the "full manifold port" when the engine struggles to retard the timing and keep me going. Which ideology makes sense?

3.) What is a reasonable RPM and advance for ignition timing on a 3.5 V8? In the past I have idled well at about 1000rpm with 10 degrees of timing, but keep in mind it still runs like poop at higher speed under load. Is 800rpm with 6 degrees more ideal?

[LRover]

I used the timed port on my carb. You should be able to set the timing as per specs or a little advanced. Don't use the bypass lines back to the tank...I'll bet that's the problem. Sounds like it's starving for fuel. Watch your spark plug/tailpipe color...you may have to play with jet size some.

I hope that's some help...I have one of those Holleys but I have yet to install it because my current carb has worked better than I thought it would. I am currently running a 500 cfm Carter/Edelbrock carb on a 4bbl Buick intake installed on a 4.6 in my 110. Way better than the SUs. Engine idle is set at 600 but RPMs will go down to 300 in gear if need be without stalling.

[nimbusnic]

The fuel booster pump install helped me out tremendously and I now have an ideal 5.5 psi to the carb. I had a return line with a restrictor valve installed so that my rear tank would stop back feeding to the side tank and overflowing it.

Thanks for the advice on hooking the vacuum advance to the ported "timed spark" vacuum port. This fixed my carb's backfiring problem and gave me an additional 10mph in top speed. My truck will now hit 62mph, which is nowhere near what it used to do with the SUs (85mph), but it's a good start.

In order to get her to run nice, the timing was set to 13 degrees BTDC. What do you guys make of this? Do I have a combustion problem or is my timing mark just in the wrong place?

[cgalpin]

Not questioning why you switched, but can you tell me a bit about the unreliability of the SUs? What problems were you having? I'm hoping mine hold up, because I honestly don't have a clue how to tune them if they did give trouble. The only problem I have experienced is that (very infrequently) I'll get what feels like a lack of power at speed. If I jiggle the choke it goes away, so it hasn't been a big deal :)

[nimbusnic]

Charles,
My SUs were hard to keep in sync with each other, leading to rough idling and poor acceleration. I also had the same problem you described, but believe that was primarily due to low fuel pressure from a failing fuel pump. Oddly, my truck would cut out during rapid acceleration in colder temperatures but not when it was warm outside. Replacing the fuel pump in my rear tank helped this problem. I have since installed a booster pump and custom return line which works well with the twin tank system and is giving my Holley a healthy 5.5 psi. The straw that broke the camel's back with my SUs was when the passenger side carb busted through a seal and started over-fueling that side. This would have been an easy fix, but I decided instead to replace it with the Holley 470 from the truck avenger series, which is the setup recommended by AB. Having a carb with an accelerator pump has helped my low end acceleration (I can launch the truck in second gear).

My guess is that your SUs are fuel starved under power and are leaning out. Fooling with the choke is probably cutting off some of the air supply and bringing the mixture back into a usable ratio. You might consider installing a booster pump in-line prior to the fuel filter. Mine is mounted to the chassis behind the passenger side front wheel well, just past the T-valve connecting the two tanks and prior to the fuel filter. I hope this helps.

[cgalpin]

It's happened so seldom I am not sure if it's happened since I put in a new fuel pump, but I have already done that. I'll have to see.

thanks,
charles