Stock 302 running rough

[edmedlin]

My 302 has began running a bit on the rough side. It is basically a stock motor with a Edelbrock Torquer2 intake with EB 750cfm carb very mild cam. It doesn't use any oil, has great oil pressure and compression (110/ +-2 across all cyls ) It started acting like it was going flat when hitting passing gear then clearing up and running normal. It doesn't miss. It just feels flat. Like it is loaded up. Now it is like that all the time. It runs like it is at about 85% power. It starts good and all. I am going to do a tune up (plugs, rotor, cap and new fuel filter) and see if that might be an issue. Anyone have some other ideas???? Like I said, it all began only when I hit passing gear.

 

[Broncomt]

Maybe see powervalve on carb is leaking.

 

[hammer189]

Thats a really big carb for that engine. Summit hasa carb sizer for engines and a 302 is around 500cfm for a carb. Obviously not the problem, but if you buy something else keep that in mind.

 

[DirtDonk]

Running rough and being a little low on power could be a vacuum leak. It leans things out enough to lose power, especially with a higher rpm manifold and large carb being maybe a bit more finicky to minor issues like that?

It's a hard thing to diagnose of course, with so many variables, but that's where I'd look even before the tuneup. Mainly because it's so easy to do with a spray can of something. If the engine is newish and painted, you obviously don't want to use carb cleaner all over things, but if not, then carb cleaner spray is your friend.
Brake cleaner evaporates quicker, and "usually" before it does any harm to new paint. But use sparingly.
So something benign like WD40 or even water might be best for tracking down a small vacuum leak.
Or, in your case, a big vacuum leak if it's leaving your power flat.

How many miles on the motor? New cam too you said? Was it properly broken in (the cam, not the engine) and are you using oil with extra zinc additives in it?
If not, then even the cam can be suspect at this point.

Along with all the other things. Like carburetor issues and ignition issues and alignment of the planets issues and 50th anniversary issues. You know... Typical Bronco stuff.

Paul

 

[needabronco]

I'm going to say your carb is HUGE and is likely flooding your engine when passing, perhaps your float level is too high and maybe now it's stuck? I'd pull the carb off and give it a good once over. Check your plugs to see if your running rich. Vacuum leaks would be a very close second in this scenario.

After that its the usual stuff, timing, checking the cap/rotor and wires.

 

[edmedlin]

Thanks for the comments.... The engine has 35,000 miles in this configuration but the problem is new. I thought of the vacuum issue right away, especially a vacuum advance issue when it first started. The EB 750cfm is more of a street type carb and not unusual if adjusted properly for this application with this dual plane intake. Mileage and power is a great balance and throttle response has been very good. I am not the one who broke in the motor after the new rebuild, so whether it was broke in with high zinc oil or not, I really don't know. I would guess not. The PO did not seem to be all that knowledgeable in those things. Plugs are normal for the miles, but not heavy with deposits. I did change them. Rotor, cap and filter needed changing. It did help some. It still seems flat. I am strongly leaning toward a cam that has flattened out. The way it began, and got worse.... It just points toward it.

 

[hammer189]

Anything in the oil?

 

[77BRONKOTN]

Getcha a timing gun and see if its advancing.

 

[DirtDonk]

...The engine has 35,000 miles in this configuration but the problem is new..

Probably not an improper cam break-in then. Never seen one last more than 10k miles in those cases. And that one was unusual to say the least!
Most won't make it 5k

...I am not the one who broke in the motor after the new rebuild, so whether it was broke in with high zinc oil or not, I really don't know. I would guess not..[/QUOTE]

The break-in sounds legit, but not using high-zinc oils throughout it's life is turning out more and more to be a culprit with aftermarket cams.
Not that they're made out of different materials necessarily, but more likely they're more highly stressed than stock cams.

...I am strongly leaning toward a cam that has flattened out. The way it began, and got worse.... It just points toward it.[/QUOTE]

Be real sucky if it's the case, but it's easy to verify at least some of the issues without pulling the engine apart completely. Start by just taking the valve cover off and taking some measurements. If that doesn't pan out, you can take the intake off and visually inspect things.
But I'd only do a quick inspection at first and not rule out some of those other things yet.
Yet...

Good luck. Hope it's something more benign like the carb just needing some TLC from getting junk in it's trunk.

Paul

 

[edmedlin]

The oil is fine, no signs of any water or any nefarious issues like that. Vacuum advance seems to be working just fine. Timing is right on. Have advanced and retarded it some just to see if it would make any difference at all. None. Just made it harder to start like I would expect. Valve lash is next. Don't have specs.... going to need some help on that anyway. I am about at the end of my "one arm bandit" work anyway. I have a severe injury to my right arm and have to work one handed. I have a lot of rehab ahead and it will be some time, if ever, when I am back to normal. Me and an airbag had a collision that tried to separate my right shoulder and arm from my body over the winter and I am not over that yet.

 

[edmedlin]

This is one of those "smack yourself in the head and call yourself an idiot" moments. I had drove the Bronco to the race track for practice day to watch some friends practice their late models. It was a couple hundred mile round trip. I had bought gas at a station there. I use premium 91 octane, it was... but not my usual brand I get around here. Long story short.... I added some octane booster and Lucas top-end conditioner. It started running somewhat smoother. Filled with my usual fuel, added some more octane booster at that time (23 gal tank) and changed filter again. Voila! It is clearing up! I did the work this morning before that last post, but had not started the Bronco yet. Over the last few minutes I took it to the station and filled er up and brought it back home, changed the fuel filter again (just a precautionary measure but it looks fine). It runs smooth up to 70mph. That is about 2800rpms cruising. I kick it down and it is fine at 4000. I feel a bit of hesitation there, but it seems to keep getting better. I think I got bad gas. Damn! Can I say that here??? There are worse words I could, and would like to say.....

 

[broncnaz]

Tune up will probably help a lot. I just did a plug change on my 73 it was way over due and I could tell it needed it. Most 302s have non adjustable valves so your probably going to be out of luck there. Another factor is plain old age sooner or later engine wear is to much and you just dont make the power which also can require retuning the carb to get rid of flat spots.
But as others have said that carb is way big for a 302. If you were running 12,000 RPM is would be a good carb but your mistaken on it being a good combonation on that engine much less in a bronco application its good for the track not a heavy 4wd. Just because its marketed as a stret carb means nothing it the size that hurts performance. Swap to a 600 cfm you you'll notice a differance in driveability you wont lose power and mileage will probably go up as well. even better get a 500 cfm carb and you'll be suprised. The thing is heavy vehicle, small engine, big carb is not a good combonation. Will it work sure but there are way better combonations.


Ive built a few 460's for trucks and they ran way better with 600 cfm carbs than they did with the larger 750 or 850 carbs. Even on my bronco Ive run 600 cfm carbs for a long time went to a 500 cfm carb and it improved the driveability quite a bit I also picked up a little mileage over a 600 cfm.

 

[edmedlin]

I am going to change that. I am going to an EB 600cfm 4bbl. It should work great on the dual plane manifold. I use 500cfms for racing, so I do know how it works. I won 3 Nascar modified championships running 500cfm 2bbls with the same manifold on this Bronco when others were running 4bbls.... It isn't a matter of "too big" or anything like that. Jetting and other things come into play. I have a jet kit for this 750cfm and it is not flooding out the 302 by any means. It is not even running rich. If anything, it is a bit on the lean side right now. Probably more like a 600cfm or so leaned out a bit. As long as the engine is getting fuel when it wants it, you are ok. You don't want to burn pistons, but you don't want to foul plugs either. If I took the Holley 500cfm 2bbl off my race car and put it on my Bronco with the adapter, it would flood the hell out of it. It is built for a 358 Nascar motor. It is all semantics..all numbers.