1966 GMC 305 V6 Service

[Andice]

Rufton: I am impressed by the work you've done on your 305 V6 engine! Do you happen to have a part number for the oil filler cap? I could use a new one and yours looks just like what I need.

[DouglasRaymond]

Interesting, not sure I would get it right!

[Rufton]

Thanks. Maybe you can find a match at swap meet, ebay, etc.

[Rufton]

Strange road trip 305E V6 HWY milage.

1. 9.3 mpg (about 60 mph I-75)
2. 17.1 mpg (about 70 mph I-75)
3. 16.7 mpg (about 70 mph I-75 return trip)
4. 9.5 mpg (about 60 mph I-75 return trip)

Mileage calibrated against map quest.
Triple checked full tank when I got the 17.1 mpg; even saw gas in tank through filler neck.
Thought I was home free until the last 9.5 mpg reading.
I suspect the carb or distributor is misbehaving under 70 mph.
Maybe I'll advance timing a bit more than stock and verify timing advance is functioning. I'm a little suspicious of carb power valve function.

[Andice]

Rufton: Thanks for the detailed images of the oil filler cap. The cap in my 305E has two thinner mounting tabs on the underside. It kind if resembles the tabs on a breather/oil filler cap. I will consider trying a cap like yours. Can't explain your spotty fuel consumption, but mine is in the shop right now getting a proper tune since it has been backfiring a little. I may have damaged the power valve in my Holley carb

[Rufton]

I've got a spare Stromberg. When time allows I hope to rebuild it for comparison. I hope to install an UEGO O2 AFR gauge because it takes away a lot of guess work. I'm willing to consider holley as well. Don't think Stromberg power valve is affected by back fire but it's design does look primitive. Not sure how to test power valve function; suppose one could wire open/shut to see how it affects performance.

[Rufton]

The Stromberg power valve actuation rod ( looks like nail ). Moves up and down w/ vacuum to stroke spring loaded power valve (looks like jet w/ wire in center). Don't know if hangs up.

The old Stromberg suffered plenty of back fires through the years but power valve still functioned on the bench.

[Rufton]

Distributor seems to be behaving. I wanted to dial in a couple more degrees but advance already looks good relative to some other engines. Not sure if I ever hit mechanical advance stop; didn't wan't to spin it faster cause not confident in tach accuracy (indicates low compared to car tach).

7-31-2013 66 GMC 305E V6 timing data:

Timing (vacuum advance connected, cold engine):
6* 490 RPM
24* 1000 rpm
42* 1500 rpm
45* 2000 rpm
48* 2500 rpm

Timing (vacuum advance connected, warm engine):

9* 600 rpm
16* 1000 rpm
16* 1000 rpm
42* 1500 rpm
46* 2000 rpm
49* 2500 rpm
55* 3000 rpm

Timing (vacuum advance plugged, warm engine):

9* 600 rpm
16* 1000 rpm
25* 1500 rpm
29* 2000 rpm
33* 2500 rpm
38* 3000 rpm

Dwell (warm engine):

31 dwell 620 rpm
30 to 31 dwell from 620 to 2500 rpm.

Timed vacuum port Stromberg WW:

<2” Hg 620 to 900 rpm
4” Hg 950 rpm
6.5” Hg 1000 rpm
13.5” Hg 1200 rpm
15.5” Hg 1500 rpm
18.5” Hg 2000 rpm
18.5” Hg 2500 rpm

[TommyDuncan]

Originally Posted by Andice

Rufton: Thanks for the detailed images of the oil filler cap. The cap in my 305E has two thinner mounting tabs on the underside. It kind if resembles the tabs on a breather/oil filler cap. I will consider trying a cap like yours. Can't explain your spotty fuel consumption, but mine is in the shop right now getting a proper tune since it has been backfiring a little. I may have damaged the power valve in my Holley carb

I have a couple spare caps. I never compared them but they appear the same. When I get home I will compare them. PM me your address and I'll send you one.
Posted via Mobile Device

[duallyjams]

Originally Posted by Rufton

Strange road trip 305E V6 HWY milage.

1. 9.3 mpg (about 60 mph I-75)
2. 17.1 mpg (about 70 mph I-75)
3. 16.7 mpg (about 70 mph I-75 return trip)
4. 9.5 mpg (about 60 mph I-75 return trip)

Mileage calibrated against map quest.
Triple checked full tank when I got the 17.1 mpg; even saw gas in tank through filler neck.
Thought I was home free until the last 9.5 mpg reading.
I suspect the carb or distributor is misbehaving under 70 mph.
Maybe I'll advance timing a bit more than stock and verify timing advance is functioning. I'm a little suspicious of carb power valve function.

what's the rpm on the engine when your running 60-70 mph

[TommyDuncan]

Originally Posted by duallyjams

what's the rpm on the engine when your running 60-70 mph

I did the math and 60mph should be about 2150rpm. 70 should be pushing 2500rpm.
Posted via Mobile Device

[Andice]

I must have been smoking crack because my oil filler cap is just like Rufton's and likely the same as all other oil filler caps for this era GM vehicle. Sorry for the stupid posts and thanks for the offer T Duncan.

[awbrown]

rufton , what port do you have the vacuum on now ?? From what you are telling us it looks like it is on the ported vacuum , it should be on manifold vacuum .. all the vehicles I have had got much better milage on manifold vacuum than ported...

[performo]

Hello, Does anyone know the factory weight of the GMC 305 v-6?

Thanks in advance.

[TommyDuncan]

Originally Posted by performo

Hello, Does anyone know the factory weight of the GMC 305 v-6?

Thanks in advance.

the number I keep hearing is 800 lbs and judging by how the two I have on stands feel when I try to move them I believe it!
Posted via Mobile Device

[performo]

That works for me. I have been scouring the internet and can not find the weight specs. Thanks Tommy.

[TommyDuncan]

Originally Posted by performo

Hello, Does anyone know the factory weight of the GMC 305 v-6?

Thanks in advance.

I just stole this from Jolly's site:

The "GMC Replacement Engine Catalogs" lists the following dry weights:
305A = 738 lbs.
305B = 771 lbs.
305C = 840 lbs.
305E = 795 lbs.
351 = 860 lbs.
401 = 880 lbs.
478 = 925 lbs.
702 = 1485 lbs.
637 = 1219 lbs.
Posted via Mobile Device

[performo]

Great weights reference, Thank you

[3/4ton Randy]

Rufton, I would like to thank you for all the hard work, pictures, and detailed information you have put into this thread. I recently purchased a '66 with the 305 V6, and I have been a little worried about how to take care of it. You have saved us so much time and probably wasted effort. You are great!

[raylube]

X 2. Do you have a list of
part numbers and vendors?

[b.sharp]

I have just purchase a 1ton with one of these engines. I have no idea what exact motor it is. Is there a way to tell one from another. I know it's the V6 but do not know which one. It there anything major to check before trying to turn these engines over? Truck is junk and for parts but hoping to save the motor for a '68 GMC. Great thread!!

[MusicMan70]

Look here BSharp;

[b.sharp]

I will check it out this weekend. Truck is at my in-laws. Thank you for the information.

[TJ's Chevy]

One sweet 305 E! Doing fantastic work!

[Tx Firefighter]

This threads sets the bar.

Thank you for taking the time to stop and photograph the various steps along the way. A picture really is worth 1000 words.

That said, I'll go back to lurking and learning.