newbie engine question

[doug22]

Hi All,

Picked up a '68 C-10 project. I've done a couple of motorcycles before, but this is my first SBC. It has a bit of an odd setup - 307 block, with a 327 4 bbl intake. It was without the carb when I purchased it. I bought a rebuilt, year correct, Quadrajet for a 327 from Carbs Unlimited. I put it on, but the truck won't run.

I'm getting good fuel to the carb. I cleaned the fuel lines, POR-15'd the tank, replaced the sending unit, and can see good 'spurts' at a clear inline filter.

I did a quick and dirty set of the timing using the marks on the block and I'm getting good spark from the HEI. Though, since it is an oddball with the 307/327 (not to mention the HEI), I'm worried the cam might be different. Not sure how to proceed if that's the case.

I've hooked up the PCV and distributor vacuum advance at the carb. The other two vacuum ports I've capped. At the back of the carb there is a large threaded opening I'm not familiar with...

I've adjusted the two idle mix screws at the front of the carb to 1-1/2 turns out. I haven't messed with the floats yet; assuming they are roughly okay since the carb was purchased as 'rebuilt'.

If I hold the primary choke plate closed, I can get it to run for a few seconds, but it quickly dies if I hit the throttle and let the plate open. It sounds okay and I think it's firing on all cylinders as each header pipe is warm.

I'm worried there are crankcase gases coming out of the filler neck. Not burning oil; seems like mostly exhaust.

Not sure where to go next. Waiting for the battery to charge back up now

Any ideas? Thanks much!

[mmiddle]

Welcome....
First, generally speaking, an SBC is an SBC is an SBC. A 307 with a 327 intake will make no difference. The only difference in SBC intakes is the earlier versions had the oil fill tube in the intake verses the valve cover other than EGR and choke types.
The HEI is a smart up grade that will work perfect with any SBC.

I sounds like your problem is the vacuum hole at the back of the carb. Usually, if you have factory power brakes this is where it gets its source. If you are not using it from a brake booster then plug it off. You will need 1/4 NPT pipe plug.

One question... This intake, is it at square bore (Holley) or a Spread bore (Q-jet) intake? I assume you have a rebuild Q-jet carb.

[doug22]

Thanks mmiddle. I'll get a 1/4" plug. There is just the master cylinder; no booster.

Are there any other vacuum ports I've missed? I'm using the PCV and distributor advance ports and have plugged the other two.

Yes, it is a Quadrajet, using a 3919803 GM5 spreadbore intake.

[Longhorn 70]

Hmmm...If you have a lot of pressure coming out of the crankcase, the FIRST thing I would do is a compression/leak-down test. You'll find out where the gasses are leaking from the cylinders that way. Could save a lot of time..


WELCOME to the board.

[mmiddle]

There might be a vacuum port in the intake between the carb and distributer, generally this was a later 70-80 smog port. If yours is an older intake it likely will not have this port. This port if there will also be a 1/4 pipe plug.

[doug22]

I plugged the brake booster port. It is acting a little differently now - still won't stay running but is quicker to spit fuel out the top of the carb and I touched the headers and they seemed hotter than before. Timing?

I do have a compression gauge, Longhorn, and I'll test that next. Out of daylight now...

[mmiddle]

I fit is spitting up it sounds like your timing is off.
Let establish base-line timing.
remove #1 plug (lefthand front) and rotate engine onto compression stroke then align timing mark on balancer to "0" on tab.
Remove Dizzy cap, the rotor should be pointing to #1 cylinder. if it is not then remove Dizzy and use a long flathead screw driver to rotate oil pump drive shaft to align with #1 cylinder. Reinstall Dizzy with rotor pointing to #1. ***Remember that the gear is helical so you will need to drop it in slightly retarded for it to engage the oil pulp drive shaft.*** Also make sure the vac advance in on the right hand side pointing towards #2 cylinder***
Once you have that, Test align the Dizzy cap. The electrical "knob" should be about 90deg. to the engine.
Rewire the engine. #1 is the first terminal after the "knob" going clock-wise looking down on it.
Firing order is 18426572. Left side of engine is #1,3,5 &7 Right side is #2,4,6 &8.
Good luck. It should start and run. Then retime to 4-12deg advanced at idle with vac-adv. unplugged. Stock timing is usually 4deg. Most guys run between 6-8 deg.

[doug22]

Running well now - looks like it was simply timing. I'm not sure what I missed the first time through, but went through everything again and she fires right up. Haven't tested compression or put her on a timing light, but she runs pretty strong for a tired old 307. Thanks!

[hugger6933]

Welcome to the board, even the old and tired 307's will run real nice with a proper tune, they aren't well known for power but they are decent transportation and I think that is what the General had in mind when he came out with it. Jim