I'm kinda new the fourm world... So hang with me.

I have a 1983 XLT bronco that has a 351W which seems to be underpowered. It runs, drives, and idles fine, but acts like it's pulling a really heavy trailer. Hills can be challenging, as I need to use the pedal to keep up speed. It wants to downshift way to easily when I add just a little bit of pedal. Tranny seems to be fine however. I could be wrong.
I adjusted the timing to like 13ish degrees, because the timing was way to retarded. Still didn't fix the power issue. BUT I found that at higher rpm's ( 2800 or so), the engine starts to buck and shake. I then unplugged the vacuum advance from the distributor, and found that the bucking and shaking goes away, and the engine runs "normal". Still feels way to underpowered and powerless.

The engine has a edlebrock carb, edlebrock intake, and a MSD ignition module. The bronco is sitting on 33x12.5 tires.
The reason why I ask is because I also own a 1984 F150 with a 351w as well, and it has all the power in the world!! It's all stock minus the dual glasspacks and 33x12.5 tires! The f150 351w is like a little rocket compared to the 351w bronco! The F150 is so happy, and has no problem what so ever going up hills, or pulling a load, etc.... Forget it in my bronco....

I'm stumped. One would think that the bronco would smoke the f150, but no! I have been searching for weeks, and have not found an answer as to what could be the case.

 

Almost sure the cam is retarded... It's either cam chain stretch or it was indexed wrong...

But, let's do the basics first. What does the compression test say? What does the timing light show?

 

isnt 13* to far advanced for a carbed setup? if you are having better performance with the vac advance unhooked you either have a problem there or with the vacum line itself.

the bronco does weight more than your F150 as well. and we dont know how truly tired that motor is.

 

I also agree with the above in that the bucking was too much advance ... But a completely lazy motor with two different timing scenarios is still a matter for investigation...

 

Are they the same 351w?

There was the regular 351w and the H.O. (high output) version.

The normal setup had 140hp and 280tq whereas the H.O. had 210hp and 305tq.

That's a significant difference. I know you said you have an Edelbrock intake/carb, but I believe there were other changes, such as the cam, that contributed to the difference between the two. I'm wondering if your '84 is the H.O. and the '83 is the regular version.

 

Thanks for the replies everyone!

When I got the Bronco, the timing was really retarded. It still lacked power (at idle, the 10* line was straight up on the balancer when I shot it with my light). I unplugged the vacuum advanced, and moved the default 10* line just left of the timing pointer. I timed it to match the F150, due to the fact my F150 has more power than the bronco. Did I do something wrong?

I unplugged the vacuum advance on my F150, and the distributor makes a quick "slap" like something is releasing due to lack of vacuum. The engine drops in rpm, and stutters.
I do the same thing to the bronco, and nothing "slaps", and the engine does not drop in rpm. Do I have a bad distributor? I blew into the vacuum line, and there is nothing blocking it. Not as much as the F150, but I have vacuum pressure.

I didn't know there were two different types of 351w's. How do I check the numbers?
IF it is the smaller engine, is there a way to rebuild it with more power to match the H.O version?

I'll try the compression test.

 

The "slap" sound you're hearing is the vacuum advance. The vacuum from the engine will pull on the canister, which will move a small arm that advances your timing. As the vacuum lets go, the arm will snap back, which is what you're hearing.

The best way to test it is to put a timing light on the motor and then apply suction to the other end of the vacuum line (leaving it connected to the distributor). Your timing should advance as more vacuum is applied. Generally, up to 10 degrees or so. If not, your advance canister may be bad and needs to be replaced (not necessarily the whole distributor).

Double check that the harmonic balancer hasn't slipped. The outer ring is held on with vulcanized rubber, and after the years, that'll lose its grip, allowing it to slip. The result is that the timing marks are no longer correct. Easiest way to diagnose this is to pull the #1 spark plug and visually verify that the piston is at Top Dead Center. Then, confirm that the timing mark is pointed directly at 0 degrees on the balancer. If it's off by a large margin, you more than likely need a new balancer.

As for how to tell the difference between an H.O. and a normal version, I'm not sure. But, I know they have a different cam (which you can swap in), four barrel carb, and possibly some head differences (not sure on that one, can anyone confirm?)

 

Also how big is that edelbrock carb and what intake are you running on it as well. its all about running the right combo of parts to compliment each other.

i agree with what is also mentioned above about the vacuum advance. it does sound like its not work correctly.
I also agree about the motor, 84 was the year of the 351w HO which had a bump in cam and other items.

 

All good points. These W motors are known to toss harmonic dampners. Sometimes the whole front pulley will come loose

Test the vacuum advance. You can get a mighty-vac or just suck on the hose. It should change RPM and you should see movement on the timing with a light. If not, time for a replacement vacuum advance ...

But these motors also have a bad tendency to not oil the timing chain enough, so they end up with a ton of timing chain stretch... Bingo, instant retarded timing and worse, retarded cam ... So the motor will feel sluggish down low. Add this to a bad vacuum advance and it will feel totally doggy ...

There are a lot of simple ways to enhance the motor. But, before any enhancements, you need to get the basics in order.

 

If you pull the distributor cap and try to turn the rotor back and forth you can get an idea of how much chain stretch their is. You shouldnt be able to move it very much at all.

 

Thanks for the replies!

I finally had time to check a few things like people suggested. Compression came to about 90psi on all cylinders. I tried rotating the dis. rotor, and it moves a little, but not much. I think it moves only because of the spring action, otherwise I don't think it moves.

The harmonic balancer has not moved. I tried the drinking straw trick/TDC trick, and found the balancer marks dead-on.

The carb is a edelbrock 4016 600cfm.

I shot the light gun again, and found my f150 total timing is 30-35*, while the bronco goes nuts and shoots way past 30*. Like 50+*. I'm thinking the vacuum advance is not working.

As for the H.O or standard 351, I think its a standard 351. Not the HO version.

According to what I can see, my issue is a tired, standard 140hp 351 (according to the specs), with bad vacuum advance. Would I be correct?

 

Low compression can sometimes be helped by freeing the rings. I've had good luck spraying Tri-Flow into each cylinder through the plug hole (goodly shot), rotating the motor with the starter (but not starting, pull the rotor), letting it sit overnight, and firing it up. Have sometimes gained 30 psi ... If Tri-Flow does not do it, Break-Free gun solvent sometimes does... Point being that with a tired motor, you can't really hurt anything and sometimes it works well

IF the compression comes up (?), I'd switch to Delo 400 or Delvac oil (15W-40) because of the higher detergency in the add pack and the diesel additives to suspend carbon (soot) in the oil. It will clean stuff gently over time and the motor will run better

You don't have enough leverage with the rotor to move the cam. You need to mark the rotor position and move the motor (crank) with a wrench and watch the rotor. If the crank moves, but the rotor does not, you have timing chain slop ...

 

90psi definitely isn't great. However, did you do the test on a cold engine? Or did you heat it up really well first by driving around? (Should do the latter).

It sounds like your vacuum advance is working, but it just overly generous with the timing it gives. If it isn't pinging while you drive, it's not advancing too much. That said, a new vacuum advance isn't super expensive.



A quick test you can do on the compression is to put some oil into each cylinder before you test it (like a tablespoon or so) and then redo the test. If the compression goes up, you have bad rings. If it doesn't change at all, you have valve sealing problems. Again, be sure the engine is good and hot.

 

i like what Brocluno said, I also run Delo in my motor even with a recent rebuild. it just keeps the motor clean.

throw a new vac advance on and see what happens if nothing else you will have a spare.