Hey guys, I recently installed a 390 in my 63 and was taking it for a test drive. As I was leaving town I was heading up a steep grade doing about 60mph and not long into going up hill the truck started to jurk and slowed way down. When I dropped down to 3rd it seemed to run fine. everything on the engine is new including the mechanical fuel pump. Any ideas what to check first?

 

Does it idle OK? Only high speed miss or performance problem? One question, are you running points or some kind of electronic ignition?

 

Yes it idles ok, a little high right now but I still need to adjust the carb a little. It only happens at that high speed going uphill. Kinda like it is running out of fuel. I am running a form of electronic ignition.

 

Easy checks
1. Ignition - Check timing. Also check spark plugs for condition, e.g., excess carbon and other types of fouling.
2. Fuel - Check filters, including air filters. If you suspect contamination, it may be worth it to install an inline filter either before the carburetor, or one before the fuel pump.

Harder checks
1. Carburetor - Check for internal contamination that may be plugging the jets. Also ensure it is properly adjusted, with no intake leaks to alter the mixture.
2. Rear axle ratio - If it was changed out for a higher ratio to obtain better fuel mileage, this may be the cause. A four speed typically would have a 3.50 to a 4.11 or so.
3. Timing chain - Timing chains stretch and camshaft gears lose nylon teeth over time. This causes the camshaft timing to retard, sometimes enough to affect engine performance. The best way to check this is to check for timing chain slop.

Edit: Also check compression. A worn out engine will usually - but not always - give other symptoms as well.

 

Carb/fuel checks would make sense, check for adequate fuel pump volume and pressure, filter clogged, proper float level, damaged/clogged power valve, etc. May be a bit unusual for a new fuel pump to be bad, not unheard of though either. Also check for bad fuel hose/air leaks, it does sound like a fuel delivery issue at high demand.

 

I was thinking fuel delivery problem before I finished reading your op..

 

Or vacuum leak.
May be small during idle (hi vac) but too much under WOT (already low vacuum)
Check advance diaphragm, booster etc too

 

Since member's have covered carb/fuel adjustment, so if fuel lever and timing is correct then curious if the truck has an auto trans. I am not familiar with auto trans but wonder if issue can be related to automatic transmission linkage adjustment, perhaps a 'kickdown' or 'downshift' throttle adjustment???

Had an issue shortly after tuneup on the 65 with the 390 where it was fine on level ground but not enough power to blow a candle out going up a mild incline hill, found timing was off a few degree's. Anyhow, food for thought.

 

I know this is an old issue, but in this case I would check to make sure the distributor is advancing correctly.