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Discussion Starter #1
Okay, I didn’t know if there was a chat or anything for this but I’ve got a SBC 400 with a Holley truck avenger 770, full msd ignition, mechanical fuel pump, And when I rev it in neutral passed half throttle it cuts out and if I hold it there long enough the engine dies, and under load it does the same thing, And the secondary’s don’t open, I put a new power valve, adjusted the floats, cleaned the jets, lighter secondary spring, adjusted the air/fuel, blew out the vacuum for the secondary, and nothing has changed, I’m not sure the heck it could be, I’m thinking the mechanical fuel pump and I just want to find out before I go to deep
 

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A fuel pressure test check would confirm if it was fuel delivery.
If it is fuel delivery don't just assume it is the pump, it could also be something other than the pump, such as a crimped line, a deteriorated and interior shrunk hose, a plugged fuel sock on the end of the in-tank pick-up, a pinhole in a line or hose on the suction side of the fuel pump causing it to suck air, a mostly plugged fuel filter..

Have you tried revving it until is shuts off, then immediately checking fuel level in the fuel bowls?? This will confirm the engine is running out of fuel..

I ask all these qestions because your issue also has a high probability of being ignition problems. If the carb has fuel in the bowls when it dies, it's not a fuel delivery problem.

What exact ignition box or distributor are you using, and how old are the components??

On another note, the vacuum secondary's should never open when the engine is revved in nuetral, no matter how hard or how long you try.....vacuum secondary's should only open when the engine is at WOT and has a load on it, there is no load in nuetral or park.
 

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Discussion Starter #3
Okay I’ll check the bowls after I do that, and yes I know the secondary’s won’t open In neutral, I was saying it bogs down and dies in neutral and under load it does the same thing as well as the secondary’s don’t open under load, I never thought about the ignition system, The distributor is a just MSD Chevy V8 pro billet, and the ignition box is a MSD 6AL digital with rev limiter, the parts were new in 2012-2013(I think) bought the intire swap for my truck with all the papers,
 

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Do check the things that was mentioned by ericnova72 and before you go changing anything else on the carb what you need to do is start from scratch and make a checklist and check simple things first then move on to more bigger things. The first thing is to make sure your timing is correct and right for your build. What is your cam specs and the rest of the engine combo?

Timing is very critical for a carb tune as if your timing is off you end up with a bandaid type of a tune and it covers things up the wrong way and ends up with a horrible tuned carb. Also is your vehicle for offroad use more so then anything else or a street rod type deal? The truck avenger carbs are more for off road use then normal street driving and cruising. There fuel calibration is way different then a normal standard 770 street avenger carb.

The vacuum like stated only opens up under engine load and this occurs when your cruising and give it a big enough of shot to hold it open from half to wide open throttle and the air flow going through the primary barrels after it makes up enough velocity which in effect there is a tiny hole on the passenger side venturi barrel that the air goes through an inner tunnel in the carb that goes to the vacuum pod that will then pull open the secondary butterfly's.

The power valve will only open when the vehicle is under load as well similar to the vacuum secondary. With a 6.5 power valve it will open when you give it enough throttle to allow the vacuum to go below 6.5 and thus the power valve will open allowing extra fuel to keep the carb from going way lean unde the heavier load being put on the engine.

There can be many things to make it quit like that. It could be something as simple as needing to re set your timing correct if that is the case or your carb might not be adjusted right or is to lean on the fuel circuit setup.
 

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Discussion Starter #5
Yea it’s a 88 Toyota landcruiser, I put the SBC 400 and a 4 speed manual, I’m building it to be able to spin all 4 35” tires with no problem, and thank you all for the great help, I fixed the issue, I took the primary metering block off and figured out one of the passages was blocked so I blow all the ports out then soak it in a heated bio-solvent tank, and now it’s running great
 
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