Vacuum Issues - Post Cam Swap

[Geezer77GMC]

You can check your cam timing without teardown, google "degree a cam". Really a good idea to do this during assembly to be sure the timing set is correctly indexed between crank & cam.

[OldsFan]

Why not just plan for doing the LS engine swap with 4L80E transmission? I joke.

Hmm...  well, I am actually thinking of doing that swap at some point with my daughter. She's really interested but not old enough yet. I figured a cam swap in the 305 would be a "fun and quick" project.

Really wondering if I pulled a bonehead move and set the valve timing in one of the retarded keyways. I set the timing set 4 different times during the cam swap. I remember pulling the sprocket and pressing it back on at some point - but not sure if it was after that picture was taken.

I've never run a cam at anything other than zero, but if it were 4* retarded, would that cause a drop in vacuum around 4-5"? That's one of those things that depends on the rest of the engine, so no one is going to be sure for my specific scenario, right?

Anyhow, I won't have a ton of time to mess with it today, but I'll reset the timing completely and get things to read ok with my other Q-jet.

[OldsFan]

You mentioned lean at idle, tip your choke blade slightly towards closed. If the idle improves you are a bit lean.

I put a glove over it and it speeds up a bit until it hits about 13:1 on the A/F gauge.

6 turns out is not uncommon with an 85 quad assuming that’s what’s on there. Aftermarket carbs are a crap shoot but you said it was fine before the cam change . These later carbs have fine pitched threads on the mix screws which require more turns.

Yep - original carb for this engine, 17085226 is the number. I counted 12 turns on the screws before they fell out, but figured the last two turns are probably not usable. So I am at 60% turned out, I guess. Regardless, yeah, there's still some space to go.

You can check your cam timing without teardown, google "degree a cam". Really a good idea to do this during assembly to be sure the timing set is correctly indexed between crank & cam.

I have hydraulic lifters so when I tried to degree the cam going in, I was only getting a ballpark because the lifters kept bleeding down. To really be exact, I'd need solid lifters and that would mean pulling the intake and whatnot - probably more work than just getting to the timing gear. I guess I could pull a valve cover and just do a rough check. If I were 4* off, maybe it'd show up despite lifter bleed down.

[TexasRed]

I would not pull the carburetor just yet.

[OldsFan]

Hey TexasRed,
I didn't see your message and already swapped the carb to my backup.  Nothing changed at all, other than a minor fuel leak at the inlet on the backup Q-jet. Running at 1700 rpm with the choke on, it is pulling about 17" Hg of vacuum. I had to shut it down before I could do much with it because of the fuel leak.

I did go through all of my pictures, though, and I am now fairly confident that I installed the timing set correctly.  I found one picture in particular with the timing set at zero and wet RTV waiting for me to install the cover. Less than 10 minutes later, I took a picture of the cover installed.  I never pulled it apart because I would have had to clean the RTV down - which I didn't do.

The vac is steady... just way lower than expected. I am starting to run out of things to check.

Could I have wiped cam lobes? It wouldn't run steady and smooth in that case, though, right?

Could it be the piston rings got damaged during the cam swap somehow? I don't think so, though, because it still shows a huge jump in vacuum (5-7") if I run it to 2500 and snap the throttle shut. Also, it wouldn't run smoothly, in that case, unless every ring got damaged the same amount?

Thanks.

[OldsFan]

Am I just overthinking this?  Could it be that this cam is shorter duration / lower lift than the original and just will never develop the 19" the original did?

[TexasRed]

It probably won't see 19". The 305 is fairly sensitive to cams. It should be higher though, upto 17 or 18".

Did you ever reset the idle mixture on either carburetor?

Did you set the timing as VileZambonie suggested?

[OldsFan]

I did adjust the idle mixture on the normal carb. If you watch the video, it is around 14:1 on the A/F gauge at idle. It leans out to 16:1 when I run it up and hold at 2K rpm, but I think that might actually be expected because there's no load. Obviously under load, I would not want it to be that lean.

I was going to set the timing as VileZambonie suggested while running the backup carb, but with the gasoline leaking on to the manifold I just shut it down. Already so dark, I figured I will have to deal with it tomorrow.

So, for tomorrow, I am thinking:
* fix the fuel leak on backup carb connection (probably just not tight enough)
* set the idle mixture via vac and A/F gauge on the backup carb
* set the timing as VileZ mentioned
* possibly get annoyed enough to tear off the front of the motor and physically check that timing sprocket

One other question: could the brand new timing set somehow have skipped a tooth after I buttoned everything up?  Only ever heard of old, worn out chains skipping... but maybe?

Another note: I did remove the EGR valve during the cam swap. It wasn't working (no stumble when manually opening it at idle) and when I removed it, it was plugged solid. I put an EGR elimination plate on there and it isn't leaking vac (tested it a lot). Any chance it has something to do with it, though? I had to remove a ton of those cruddy plastic vac lines when I removed it.

And once again - Thanks for all the suggestions.  Despite the frustrations, I am learning a lot with this project.

[VileZambonie]

Yes, you are overthinking this. Set the timing, make the adjustments and verify vacuum advance is at full vacuum. Adjust carburetor and see how it performs.

[OldsFan]

Thanks for the reality check, I am probably overthinking it.

After warming it up, I set the timing to 34* at 3500 RPM (I'll edge it towards 36 once I can actually drive the truck) and tuned the idle mixture screws with a vac gauge.  It was showing very rich on the air fuel ratio gauge so then I leaned it just a hair (1/4 turn or so on each screw) and left it that way.

Here's the video of where I left things for the moment. Vac is definitely steady and higher than it had been with the first carb. It improved some after the engine warmed up (rings warming up maybe?).
https://vimeo.com/496459181

The vac gauge in the truck seems to show a bit higher than the one I have attached to the engine for testing.

Unless you guys give me a reason not to, I am going to pull off the external gauges, put the hood back on and take it for a drive to test things out a bit.