Does 9:1 sound about right?

 

At 9:1 you're to high IMO for 87 octane and need to be careful for a utilitarian truck.
Here are a number of threads to read for background.

 

I would suspect that carb is well past it’s useful life. Choked carbs running are not good! Rebuilding a bad carb is not worth the money sometimes, I think you will find the new carb will be a savior. You don’t need to tighten it down so hard, maybe a 1/2 turn past tight on the spacer.

good luck, I suspect you will be fine with a new carb !

 

Thank you! I hope so. At first, I had it tightened down where I thought it was appropriate. Since dealing with this vacuum leak, I've tightened it down as much as I could without stripping the "posts". It helped slightly but a whole new set of gaskets is definitely what it needs.

I will post here first thing after I get the new carb up and running to let everyone know how it goes. Until then, miscellaneous tasks will keep me busy.

 

Thanks! I'll give it a look.

I will also browse the forum for the answer to this question but I'll ask it here in case anyone reads this thread later on. Which would be the better combo, lower compression and more timing advance (say 12*-14*) or or higher compression and lower timing advance (6*-8*)? I assume they are at least partially connected because of octane knock, though I am still somewhat new to non computer controlled engines.

 

After reading a handful of threads, it looks like 8.5 - 9.1 will run 87 octane depending on timing advance. I knew from previous projects that aluminum heads are at less risk of detonation but they are way out of budget due to the amount of body work the truck needs to be used as a daily driver (new floors and a spare bed to cut patch pieces from). The gas station a few blocks away has 89 octane for a dollar or so more per tank. I think I can live with the extra $80-$100 per year that would accumulate over the 87 octane. I still wouldn't go above 9.1 just to be safe so I can avoid 91-93 octane for the principle of it.

As for more timing advance vs more compression, I'm still not sure...

 

I can't find a straight answer on the timing advance vs more compression. Clearly a balance of the 2 is more desirable than sacrificing one for the other. If I want to use 87 and not chance it, 8.5:1 seems to be enough compression for what I'm wanting to do. We'll see how gas prices are looking while I work on getting it road worthy. If they stabilize, I may build the engine to 8.9:1 and hope I can still run around 12 degrees advance. Worst case with that setup is, to alternate 87 and 89 on fuel ups if it does ping a little. I do think 87 and 8.9:1 is generally considered safe though. I can see where 9:1 would begin to be a little iffy.

How soon I rebuild it depends on how well it runs as is right now.

 

Also take off the egr valve and check that it's not stuck open. If it's full of carbon just buy a new one they are cheap. They can create huge undetectable vacuum leaks
also verify your vacuum advance on the distributor works. I sucked on the tube to mine and air went right through it! Didn't move the dizzy at all. Another leak! The little rubber diaohram in the vacuum advance canister was shot, replaced the canister. Another hidden leak can can happen there

 

Is there any way of testing the egr other than blowing/sucking on the vacuum port? When I do that, no air passes in either direction. It does make the noise you'd expect as the diaphragm shifts, but other than that, I don't notice any other signs of a leak. Could it be stuck closed and if so, would that cause any problem? If the egr is the problem, I will likely just delete the whole setup to simplify things and deal with recurving the distributor, drilling the carb jets, and advancing the cam timing later to pre egr specs.

As for the vacuum advance, when I was setting the timing, the rotor was pointed 1 cylinder off at number 8. When I pulled it out to adjust it, I noticed the clip for the vacuum advance was missing. After I got everything lined up, I bought a new clip, installed it, and tested the vacuum advance by sucking on the hose. It moved the plate and held the vacuum. I capped off the vacuum retard on the distributor. Regardless, I have had it disconnected and plugged with a golf tee because I read you disable it to set the initial timing. The way it was "idling" at 1400 rpm would've been high enough to send a vacuum to the ported vacuum port that the vacuum advance would've been connected to. Since I already had the timing at 14 degrees instead of the 12 I was aiming for, if I combined that with the vacuum advance, there might have been some pinging or hard starting.

The egr is also capped off at the moment. According to the diagram from 1974, the egr connects to a vacuum tree by the thermostat on the intake. It still has the vacuum delay valve like the diagram. I think in 1975 or 1976, they moved the egr to ported vacuum (I might be wrong about that, but I've read that egr systems are supposed to work while cruising, not idling). When I get the new carb, I'll test out the egr on one of the ported vacuum nipples.

Just in case anyone was going to ask, the pcv seems to be working. When I first started looking for a vacuum leak, that was one of my first suspects. Oddly, it was all hooked up like the '74 diagram says (to the top of a vacuum tree at the back left of the intake) but when I disconnected it to plug it with my finger while running, the vacuum nipple was capped off so no vacuum was getting to the line going to the pcv. I pulled the cap off and took out the pcv to inspect/clean it. It rattles like it should and just to be safe, I sprayed it inside and out with carb cleaner. Everything with the pcv is hooked back up and has been since before I got the idle down to 750 rpm and during so I have ruled that out as one of the problems.

Thanks for the advice!

 

The compression ratio you run is going to be affected by the chamber design. And do not get hung up on the ignition advance you can run remember fuel-air ratio also plays into this. As you raise compression advance is going to be reduced regardless of the fuel quality or chamber design. Ideally, you want to opt for the maximum compression you can get with out preigntion on the fuel you plan on using. Things you need to consider, chamber design, cylinder head material (Aluminium heads will be able take a full point more compression on the same fuel and chamber design than cast iron heads) Piston design (flat tops will tolerate more compression than dished will) , clearance volume of the cylinder, Fuel air ratio you plan and running are you going to run for fuel economy or fat for power.

9.0:1 on 87 octanes is fine for basically all OHV Ford cylinder head designs just the open chamber 335 and 385 heads "may" (not all combinations) need the total timing dialled back just a bit. But the loss in power or in efficiency by dialling back the timing will not come anywhere near to offsetting the power and economy gains of the higher compression ratio.

A Cleveland with modern Aluminium heads and built properly can run all day long at 10.0:1 on a good quality 87 octane. I run a set of D0VE heads on a 460 at 10.2:1 on 87 octane all the time with no issues at all (Other than the occasional run-on when I have been pushing it when the camper is loaded), as long as it is good quality fuel I have no issues with preignition.

When building an engine opt for as much compression as you can practically stand More compression equals more power and higher efficiency.

 

Do not forget that camshaft selection will play into dynamic compression as well. All depends on what you want out of the engine.

 

Good catch missed that ...

 

It’s easy to get caught up in building too much engine for the application. More and bigger isn’t always better. I’m in the process of planning a 351C build for a street Mustang, with open 2V heads. I really want to bump the compression up to help with the open heads, but need to be able to run pump gas, which is limited to 87-89 around here. Shooting for around 9.5:1 with a longer duration cam to help bleed off cylinder pressure and make it work. But have to keep in mind an engine for a 2800lb car and a 4800lb truck have different needs.

Keep in mind that factory motors from this era were set up for economy and emissions, not power. A well thought out build, while keeping in mind drivability, will be much better than factory power levels. Gobs of power is great, but there’s always a trade off.

 

Thanks! I appreciate it.

I have been doing some homework on engine builds. As much as I'd love to use aluminum heads and run near 10:1 compression on 87, I just don't think I could afford a build like that. I'm more of a multiple cheap projects kind of guy rather than the one perfect weekend driver guy. I'm still thinking 8.9:1 would be enough of an improvement over the 8:1 I think the '74 390s came with. Stock heads, maybe new headers (it has an old dual exhaust already), a 4bbl intake and carb, paired with the new cam, pistons, and a few miscellaneous upgrades should be pretty affordable and give me a truck engine that will haul a car, get survivable gas mileage, and still be fun to drive. I don't have any hp/torque numbers that I'm targeting yet but I'm looking for an overall well rounded driver, not a drag truck. Before I make up my mind on a rebuild, I'll start a new thread and way options with you all. I will continue reading until then. Thanks again!

 

I'm still very early into the planning phase but I've been looking at the Lunati Voodoo 250/256. I know that seems a little mild, but I like the idea of having the power right off idle as I do a lot of hauling in the bed and occasional towing. I don't think I've mentioned it, but the current truck I use is a family members 2007 honda ridgeline, so even as is, I think this truck is/will be stronger. (Noticeably I hope)