Cam, Timing, and Vacuum Questions

[curtish57]

Hey all, 

The cam in my engine is here: https://www.performanceparts.com/elgin_engine_camshaft_e_930_p_perfomance_501_527_hydraulic_flat_tappet_1965_2000_chevy_396_454_bbc

I seem to be having an issue with low vacuum at idle, like less than 5. My engine is brand new, I broke it in over the summer (or broke the cam in at least I have yet to take it on a long drive) and I set the timing at idle to 9 degrees. 

I have no idea where my cam is as far as performance but I understand it should affect what my timing is and vacuum level at idle. Does anyone have the knowledge to decode where I should be with timing and vacuum? 

I should add it idles nice and revs great, pulling vacuum when on throttle, but it stubbles off idle and smells very rich. I've got a Holley brawler 750 with vacuum secondaries but I haven't gone crazy with adjusting anything there. 

My real fear is it's late valve timing, from what I've read this is a real pain to fix so I want to eliminate other potential causes of the low vac at idle. 

[DragCat]

Quick question, per the specs on the link, did you replace the valve springs and lifters ?

Requires Aftermarket Valve Springs:

Yes - 110 lbs Seat, 280 lbs Open

[curtish57]

56 minutes ago, DragCat said:

Quick question, per the specs on the link, did you replace the valve springs and lifters ?

Requires Aftermarket Valve Springs:

Yes - 110 lbs Seat, 280 lbs Open

Yes, according to the invoice upgraded valve springs were installed to match the cam. I bought the engine from a guy who already had it built. It doesn't say what valve springs specifically, it just says upgraded valve springs for camshaft. 

[DragCat]

2 hours ago, curtish57 said:

Yes, according to the invoice upgraded valve springs were installed to match the cam. I bought the engine from a guy who already had it built. It doesn't say what valve springs specifically, it just says upgraded valve springs for camshaft. 

Ok, what about the lifters? 

I'm NOT an engine guy, just looking at the specs from the link. 

Now when I installed my crate, I wasn't getting the results I had anticipated at the track. I tuned it per the build sheet instructions. I took it to a friend who adjusted the timing and changed the jets in the carb. Now I can't speak about  track performance, but drivability was a big difference.

Vacuum is a whole different conversation. Some members have added vacuum pumps to their Monte's. Vacuum is super important, just like grounds. Ive bee told, and have no reason to doubt, that upgraded engines/manifolds don't produce the same when running a stock vacuum line configuration. Just a suggestion, you can pick up a relatively cheap vacuum tester at HF especially when they're on sale. But from your comment I think you've got that covered. 

As far as "smelling rich" sounds like carb adjustment.  Holleys are normally pretty good outta the box with minimum adjustments in my experience. 

I'm sure someone with more experience will chime in with better comments 🤣

 

[72 Monte Carlo]

My Big Block camshaft is a little bigger than the one you have in your engine, but what I did was unhook the vacuum advance to the distributor and plugged it. Then with the car running at a steady 3k rpm, I set my mechanical distributor timing to 34 degrees advance. Then I reset the idle speed back down to 750-800 rpm. I think my static timing at idle without any vacuum advance is around 11-12 degrees now.

This ensures that you will have all your mechanical timing in at 3k rpm. 

[420ponies]

That size of cam, you should have plenty of vacuum (duration at .050"). Are you running the vacuum advance on the "full" port under the front fuel bowl or on the side port? Side port is partial vacuum off idle. I think I'd recheck the timing also.

7 hours ago, 72 Monte Carlo said:

My Big Block camshaft is a little bigger than the one you have in your engine, but what I did was unhook the vacuum advance to the distributor and plugged it. Then with the car running at a steady 3k rpm, I set my mechanical distributor timing to 34 degrees advance. Then I reset the idle speed back down to 750-800 rpm. I think my static timing at idle without any vacuum advance is around 11-12 degrees now.

This ensures that you will have all your mechanical timing in at 3k rpm. 

That should get you back to square one on your timing. You need a timing light with adjustable timing knob on the back so when you rev your motor(3000 rpm's),you can adjust back to the zero mark on the crankshaft balancer. once your back to zero on the balancer the timing light will show you your degrees of timing (34-36 degrees). Plug your vacuum advance back in and at idle you can see with the light what it is with the vacuum hooked up. As far as the carb, I had one of these(sbc) . I just changed out the jets to 72's, and was much better. Remember to adjust the idle mixture screws when changing out jets. I think they are up around 76 stock? You need the vacuum gauge(on the main port under fuel bowl), to get the best vacuum adjustment with your idle mixture screws.Youtube has plenty of videos on how to adjust properly. Find one with some credit though, like engine masters, etc., no fly by nighters! Hope this helps you out. That's why we're here.

[Jason72]

Read this thread.https://www.firstgenmc.com/forums/index.php?/topic/32967-383-crate-swap-into-my-72/page/4/#comment-281975

It starts out talking about radiator caps but eventually gets to your situation. 
I give my procedure for setting up timing and carb. 
 

[curtish57]

Awesome, thank you guys for the replies. I am going to get after it again this weekend or later this week. Also I think my floats are adjusted way to high causing the rich situation primarily, because even screwing the idle mixture screws almost all the way in the car barely reacts, not starving for fuel like it should. I've got the Holley with the sight glass and apparently they want fuel just visible below or near the bottom of it and mine is just the opposite, it's above the sight glass so I'll adjust those to a more reasonable level and see where I'm at. 

[420ponies]

There lies the problem.😊 .If you need to know how to set those look up engine masters on YouTube. Very detailed instructions on how to set those.

Edited December 31, 2024 by 420ponies
Added content

[Robnhood]

20 hours ago, DragCat said:

Vacuum is a whole different conversation. Some members have added vacuum pumps to their Monte's. Vacuum is super important, just like grounds. Ive bee told, and have no reason to doubt, that upgraded engines/manifolds don't produce the same when running a stock vacuum line configuration. Just a suggestion, you can pick up a relatively cheap vacuum tester at HF especially when they're on sale. But from your comment I think you've got that covered. 

I had a pretty stock Chevrolet 350, I was in the process of converting over to Holley Sniper Fuel Injector.  It wasn't running the best, and I was able to find a major vacuum leak on the Brake booster.  Once I replaced that vacuum host, the engine ran much better.  From what I have heard you can also use carb clean to help you find vacuum leaks.