Road Trip Performance Issues

[Zilverado]

So just got back from vacation which included a short road trip to Minneapolis – we left the Twin Cities the morning of the day the 35W collapsed – very creepy feeling indeed. Hope all Minnesota board members and their families are OK.

The trip, with the truck, resulted in some questions…

To start with, about an hour before leaving home and heading to the campground for the first night, I noticed a huge fuel leak. My fuel pump was leaking bad – not with the truck running, only while resting. So I picked up a new pump ($30) on the way out of town and replaced it the next morning at the campground. Gotta love these older trucks…

A couple of days later we left to head to the Twin Cities for a couple of days – an 8 hr drive from Southern Manitoba. Just the truck, no trailer. Heading south down I29, into a fairly stiff head wind, the truck had serious trouble maintaining the 75mph speed. It would start to hesitate (jerk) as if it was being starved of fuel. Driving in OD was not even an option, as the truck again could not maintain those speeds. We had to slow down and maintain around 65 to 70mph in order to keep everything smooth. Once we turned south-east down I94 from Fargo to Minneapolis, things were a lot better. 70mph was doable – even in OD.

All was well on the return trip as well, until the day we actually left the campground and headed back home again, pulling the trailer (a small pop-up tent trailer). Into another strong headwind, the truck again had similar hesitation issues even maintaining 60mph.

So, having never experienced this before, my question is - - was this a fuel delivery issue, or is the 305 just under-powered? If it is fuel delivery, could the new fuel pump be an issue or is it a carb/float thing? I know a 350 (or bigger) would likely not have these problems, but like I say, this is new for the 305 as well.

I know I’ve read where guys have installed after-market inline electric fuel pumps. Is this a possible solution?

Any thoughts?

[Pyrotechnic]

Has it been able to maintain high speeds before? What RPM are you running in overdrive ?

[Zilverado]

I have never noticed any issues before, although our speed limits here in Canada are slightly slower than our American neighbors. This was also the first time I drove the truck at these speeds with a fiberglass topper (cap) on the back.

RPM wise, in OD, the engine would be be at around 2000 rpm at 70 or 75mph. In drive, this would increase to about 3000.

At our local speeds (around 65mph), it does around 1600 in OD, and does well - except for this last trip.

[Pyrotechnic]

I have a feeling you are lugging your motor.

I have a 77 350 with a Quadrajet with a Turbo 350 and stock stall speed converter and 3.08 gear. If I go easy on the throttle and transmission runs through the gears and lands in 3rd imediately, It would land under 2000 RPM. Accelerating to 50-65 MPH in 3rd gear at that RPM was awful. God forbid I get on a hill, I'd hear the secondaries starting to open. I'd have to downshift it to get it back in the RPM range. Once I hit around 2300 RPM, I can feel the power coming back on and accelerating more is not tough to do.

For normal conditions your truck may have run fine, but I think the headwinds and trailer towing may have pushed it to the limits and caused the motor to lug.

The power band for these motors for best fuel mileage and power is 2000-3000 RPM roughly. I'd say as high as 2300-3200 for my truck. I think you were seriously lugging your motor and you might get even better gas mileage with gearing that will put your motor in it's powerband. I know the general belief is the lower the RPM, the better the gas mileage, but with engine in question, that is not true.

I would run no less than a 3.42 gear with a 700R4 behind a stock 350/305.

One thing you could do is drive around with a vacuum gauge on your dash. Get up to 65 MPH with it in third gear, note the vacuum reading, then shift into OD and see if the vacuum reading goes lower. If it does go lower then you are working the motor harder.

[Zilverado]

Originally Posted by Pyrotechnic

...One thing you could do is drive around with a vacuum gauge on your dash. Get up to 65 MPH with it in third gear, note the vacuum reading, then shift into OD and see if the vacuum reading goes lower. If it does go lower then you are working the motor harder.

Interesting run-down, thanks. I definitely would like to know the optimum operating range for this engine/tranny/rear-end setup.

Okay, so this may be a stupid question, but where would I connect a vacuum gauge if I was to install one? Would I tee into the big vacuum line coming off the back (base) of the carb, or is there a simpler place?

[Pyrotechnic]

Any place where there is manifold vacuum. On a Q-Jet I believe the ones at the base of the carb are manifold vacuum. If you connect the gauge, start it up, and you are reading 18-20 inches vacuum on the gauge at idle, then you tee'd into the right place.

[Ivan D.]

yeah, for the vacuum gauge look for a manifold vacuum source, I had to T-ee into the line for the vacuum modulator of the tranny cause I only have one small vacuum port on the carb and that I use for dizzy advance. 2000rpms sounds about right for powerband, I get decent fuel economy there, but when I go down to 1500-1600rpms in 2nd I can cruise all day long, doesn't work in 3rd gear tho - guess it's all gearing issues.

[Srubydo1986]

just a thought. didnt you rebuild the carb at some piont i think maybe your agjestments might be off . for 65-70 it might be fine but it might be getting starved. It just a thought also check the fuel line going to the pump might have kinked them a bit.(done it before)

or maybe you canadain truck cant stand the american air.

[Zilverado]

Originally Posted by Srubydo1986

just a thought. didnt you rebuild the carb at some piont i think maybe your agjestments might be off . for 65-70 it might be fine but it might be getting starved. It just a thought also check the fuel line going to the pump might have kinked them a bit.(done it before)...

You're right, I did rebuild the carb earlier this spring. I baically left the settings as they were - just cleaned it and replaced the basic parts. The only thing I was kinda curious about was the float setting, I basically just took the new float out of the box, did a "visual" comparison to the old and installed it. I had the carb dialled in at a shop a couple of weeks ago and it seems to run great, but I was curious if the rebuild could have effected something. Unfortunately, I have never driven the truck for extended periods at the higher Interstate speeds before, so I really have no benchmark of before and after the rebuild.

As for the fuel lines, I'll have to take a look, but I think everything went back together fairly cleanly.

Originally Posted by Srubydo1986

...or maybe you canadain truck cant stand the american air.

Yeah, it's used to the clean air we have up here.

[Zilverado]

Just a couple of additional thoughts...

I did notice a fair amount of "pinging" on this trip - especially under load. I have never really noticed this before. I do run 87 octane fuel, so this could be part of it, but should I look at my timing? Would this cause a high-speed hesistation?

Also, how much of an effect does a truck topper have on drag on the truck? This was also a new thing this trip.

[Russell]

Pinging could be caused by going lean as well. I'd be hooking up that vaccum guage, and also checking my timing with a good light