Hi Folks,

First post in this forum (but been on FTE for a few years).

I posted a question about this earlier in another forum, but got no response, so I thought I would try here.

I just had a 1968 302 truck engine rebuilt: .03 overbore, heads port matched and TAD bump removed, new valves and screw-in studs, Competition Cams roller tip rockers and FE268H-10 hydraulic flat tappet cam. The engine came from Ford with a cast iron 4 bbl intake and their carburetor. I'm using an Edlebrock F4B and a Holley 1850 600 cfm carburetor with vacuum secondary. I had to rebuild the carburetor because it had sat in someone's garage for about 20 years. It's a stock rebuild, and I made sure all the parts were cleaned of the gummy build ups, and all the little passages were clear.

When I assembled everything, and managed to get it to start, I found it running way too rich at idle. Plus, when I try to rev the engine, it stumbles, indicating maybe it's not getting enough acceleration enrichment. It also stumbles and pops out the exhaust if I slowly rev it up to about 2000 rpm and try to goose it. I'm not sure that that's not also from getting too MUCH gas from the accelerator pump.

Anyway, the spark plugs are badly fouled from running over rich, so I will clean them out before trying again. I will get into the carb again to make sure I assembled it correctly. Is there anything I should look for? I'm thinking about the only thing that can cause constant richness is the power valve.

 

Perhaps a ruptured power valve diaphragm, or no vacuum signal to it?
Clogged idle emulsion tube air vent?
What is your idle mixture screw adjustment set at? I.e. how many turns are the screws out?

 

Sorry for the late reply. For some reason, my browser is not highlighting topics with new posts. I posted this on the other forum that I first put up the question.

I found the problem taking apart the carburetor again. It was the power valve, but not due to a ruptured diaphragm. For some reason, the new gasket I used on it had two gaskets stuck together, and I did not notice this when I installed it. The two were not working, so they allowed gas to leak into the intake manifold, especially during idle, when the manifold vacuum was high. So one gasket was stuck to the valve, the other stuck to the metering block. I scraped off the one on the metering block, cleaned it, and put it back together. After cleaning off the carbon fouling the spark plugs, the thing started right up, and idled much better, though still with a slight vibration. No more nasty smoke.

But all is not well yet. The engine seems sluggish; it does not kick up fast when I goose the throttle. On a quick test drive around the block, it seems like it can not generate power to get out of its own way. Definitely, it is running weaker than before the rebuild, when it had the old Motorcraft 4 bbl carburetor and stock Ford cast iron intake and cam on it.

So I think the Holley 600 still needs some tuning. The question is, where do I go from here? Shift the pump cam position? Bigger main jets?

 

Well congratulations on finding the problem, that could have been a very easily overlooked problem.

As for general lack of power, I would guess the timing is way out after the rebuild. Or even the timing chain out a tooth, but this is less likely. Try rotating the distributor each way to see if you can get a sudden power increase. You don't need a timing light for initial tests, but keep an ear out for detonation.

 

what distributor are you running..... if you tell me its a point with points I will tell you to trash the points setup and go pertronix.... i know there will be some who disagree with me... but points suck plain and simple.... you also need to verify it works properly too.... because advance plays and important roll in how the engine will operate.... verify that as you increase the rpms from idle the timing increases.....it is probably a stocker with vac advance... and that might be bad as well.....

Next check for a vacuum leak somewhere something......

Then fire the thing over and set the timing... garden variety small block wants ten initial timing then advance....

Then look at the carb..... start with primary jet size.....

 

Thanks for the replies.

The engine builder installed the cam straight up, and I verified it before final assembly.

I'm using the same distributor that was on it before the rebuild, when the engine seemed to have run fine with the Ford carburetor (except for the oil burning and valve tapping). Yes, it's a stock point type unit with vacuum advance, but it also has a vacuum retard, which I'm not using. I verified that the advance worked with a vacuum pump, although I'm not sure what the curve is there; ie, how much advance for a given amount of vacuum. It's hard to tell if vacuum advance is working properly by reving the engine, since it also has centrifugal advance on it. I have another distributor I can try (also a stock point type with vacuum advance). I can look into a new electronic distributor, but I think those are kind of pricy.

Oh yes, once I fixed the carburetor problem, I did tune idle mixture with a vacuum gauge. I think I got the highest vacuum readings with the screws about 1 turn out. I also tried setting timing with it; starting from the base 10 BTDC, I tweaked it to get the best vacuum reading. I think it's around 14 BTDC right now, and I see about 15-16 inch Hg, though it's not as steady as I've seen on stock engines. So the idle is not the problem on the carburetor.

I'm wondering if the carburetor is being properly fed with fuel. I have a small transparent fuel filter right at the carburetor (and a bigger one at the tank), and for some reason it's not always full; it's only about half full with liquid that sputters out. I don't have a full tank of fuel, but I'm sure there is enough to cover the fuel pickup. So I need a way to check fuel flow and pressure of the original 40 year old mechanical fuel pump that it's still using. Maybe I can replace it with an electric pump.

 

you need a fuel pressure gage... but those glass filters are not always full of fuel..... especially if you are using a stock style fuel pump which driven by a sprocket on the cam.

If this is a stock rebuild with a stock profile cam that 600 CFM is probably too much carb. Stock 302s used a 350 CFM two barrel of a 450 CFM four barrel. A 600 is a monster compared to those carbs.

what you are saying about timing seems mean this thing will run fine. 14 may be a bit much initial, but that shouldnt cause a sputtering.

First I would replace the fuel filter at the tank, because something dumb like that can cause you to do hours of troubleshooting..... I just had to replace mine.... one day last week my bronco just wouldnt start and I had to rev the hell out of it to get it to run.... just had the feeling that the filter at the tank was bad.... replaced it and guess what... truck ran fine..... so replace that next....

The next thing I would do is order up a set of springs and a quick change cap for the vac advance diaphram cap and try a heavier spring for the secondaries.... if you have a stock cam they are probably coming in to quickly and flooding out your engine...

 

The fuel filter at the tank was a new installation; it's a huge metal can with a lot of capacity. I did this because there is still a lot of junk in the tank that kept showing up in the glass filter, despite 3 attempts to flush it out. Maybe that filter got loaded up with crud already; I can bypass it and see what shows up in the glass filter. I may have to pull out the tank and hose it out or something like that. I'll look for a fuel pressure gauge that can easily fit into the existing fuel lines.

The engine is using a Comp Cams FS268H-10; 219@.05, .456 lift, installed straight up. It also has an Edelbrock F4B intake. It's not too radical, but it is a little more than stock. Is 600 cfm too much for this combination?

 

if there is a lot of debris in the tank that filter could already be clogged...... the best thing to do is get another filter..... i like the clear plastic ones you get from napa that make visual inspection easier.... the reason why I say to replace that is it will take that out of the equation..... and its cheap....

600 CFM is not too much...... but you might have to adjust jetting and when the secondaries come in. Do you know what primary jets you are using? What color accelerator pump cam are you using? My gut instinct is that the secondaries are coming in to quickly..... and a heavier vac diaphram spring will fix that.

 

I'm skeptical that the problem is a clogged fuel line. When this happens, the engine runs okay then starts sputtering badly and missing, it doesn't merely lack power. Typically with a fuel starvation problem the vehicle will run normally at low speed. When the accelerator is floored, there will be excellent power until the carburetor fuel bowl is emptied, when the vehicle starts missing and bogging down.

 

well... I just had to replace my filter and all it did was run poorly at idle and was really hard to start.... I alway recommend doing that unless the filter is brand new..... all it can take to throw your fule supply for a loop sometimes is to run the tank down low, which i did about a week before my line was clogged....

 

You might go up one heat range on your plugs to see if that helps with the fouling. A Duraspark ignition is relatively cheap and will give you a hotter spark. You have a vacuum gauge, that also doubles as a presure gauge so you can check if the fuel pump is up to snuff.

 

Looks like I have a little from column A and a little from column B.

It idles and seems to run OK in neutral, with fuel pressure around 5 psi. So I took it for another drive around the block, but I couldn't monitor pressure with the engine under load, as in while driving. Aside from the same sluggish acceleration I had before, it started to buck and choke a couple blocks away. I managed to nurse it back home after letting it idle for a little bit. This definitely points to no fuel delivery, or clogging filter. So I have to clear that up first before going any further. Time to pull and wash out the tank...ack! Any suggestions on the best way to do this? Or what NOT to do?

 

before you go draining the tank and go through all that mess put a filter on it and see what happens... they are like five bucks..... if the new filter fixes the problem.... see how long it lasts...... before you drain the tank and go though all that...... it might last a year or more.... and if it does I wouldn't worry too much about flushing out the tank.....

 

Well, that's sort of why I installed that big can filter at the tank in the first place. It's been there for the duration of a few test runs, and it has cut down on the stuff getting to the glass filter up front. I'll try bypassing it and see what shows up again.