Thread: tuning and timing questions???

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Did you note Skids72 comment about setting the timing with the vacuum advance disconnected?

If you don't you'll end up with retarded timing.

With the cam you have and assuming the timing is retarded 11" vacuum is about right.

If . . . the engine is idling at 600 rpm or so and you increase rpm to about 1500 the vacuum gauge will indicate 17"-19" of vacuum.

I'm asuming your vacuum gauge is connected to full time manifold vacuum.
If it's connected to ported vacuum you'll get readings that are not correct.

Originally Posted by C9x

Did you note Skids72 comment about setting the timing with the vacuum advance disconnected?

If you don't you'll end up with retarded timing.

With the cam you have and assuming the timing is retarded 11" vacuum is about right.

If . . . the engine is idling at 600 rpm or so and you increase rpm to about 1500 the vacuum gauge will indicate 17"-19" of vacuum.

I'm asuming your vacuum gauge is connected to full time manifold vacuum.
If it's connected to ported vacuum you'll get readings that are not correct.

yes, i did all timing with the advanced disconnected. and that is good to know that the 11" of vaccum is alright, less to worry about then.

well thank you all for your help, finally got the purple people eater running good!!

Originally Posted by tim_bo16

yes, i did all timing with the advanced disconnected. and that is good to know that the 11" of vaccum is alright, less to worry about then.

well thank you all for your help, finally got the purple people eater running good!!

Keep in mind that if you don't pick up to 17"-19" vacuum at 1500-2000 rpm with no load on the engine things still aren't correct.
If it does, then everything is probably ok and you just need to pursue the timing the engine wants.

If you can't get the vacuum above 15"-16" you may have the cam a tooth off.

glad to hear you made some progress...

Is your hesitation problem better? another thing you could look at that could cause your hesitation would be vacuum secondaries opening too quickly... this can be adjusted by changing a spring I believe... I haven't actually worked on a vacuum secondary holley (I've always had double pumpers) but I understand that this can sometimes be a problem and is not hard to fix... hopefully others will have some details on this if you're still chasing this problem.

Cam sounds like a pretty good match to your combo...

Have you managed to get the timing advance up? I think you get a few more degrees up there you will be in good shape... Get to 36* and see how it goes... beyond that I think you could get some more power with a little more advance especially if you run premium but be careful and make sure you don't get any ping... then you went too far.

-Chris

C9X - alright well awsome, i will deffinetally check the vaccum at 1500-2000 rpm and check that, and if i cant get the vaccum should i try turning the cap as listed on the first page by skids?? but i was also thinking, if i would advance it more, the idle is going to raise up more as well wont it? im wondering this because i already have the curb idle screw almost all the way out as it is now.

and as for the hesitation problem... yes and no. it is a LOT more responsive and i can really feel the power curve but with that being a plus, it still cant turn the tires over on dry pavement. and trying to do so it still pops and farts. but like i said, on the plus side, i dont think ive ever seen my truck run as good as it does above the hesitation.

and as for the vaccum advance deal skids was talking about, that accually was my first thought just reading up on the manuals and holley troubleshooting crap. but after talking to holley tech, he said i would need the bigger .35 squirter instead of the .32. so after doing that, i believe that DID help a little bit, but i still have some messing around to do.

but all in all, i will keep you guys informed as soon as i mess with it a bit!

Don't go to the trouble of pulling the timing cover until you get the other stuff squared away.
Right, now, a cam off a tooth is a possibility, but do the easy stuff first.

If you're familiar with cams etc., you can check to see if it's installed properly by finding true top dead center and using a degree wheel and dial indicator.
A subject for another time, but in case every thing else checks out....

Like Skids said, make sure your accellerator pump adjustment is correct.

Looking from here, I don't think the carb is your problem.
More than likely a timing issue.

There's a lot of confusion about how vacuum advance works and where it comes in at.

The basics are, mechanical - or centrifugal as some call it - responds to engine rpm only.

Vacuum advance responds to load - or vacuum level which is lower when the engine is under load.

Ported vacuum operates similar to manifold vacuum, but ported is at zero at idle, 10" or so in the mid-range under light load and back to zero at full throttle.
Vacuum advance does not stack up the advance as rpm comes up whether selected to ported or vacuum unless the engine is running with no load and above the idle rpm.
It can help to think of a vacuum advance cannister as a vacuum retard device.

Double check the plug wire firing order.
An easy mistake to make and double checking is a good idea.

So if everything is correct, you nail the throttle, the vacuum advance drops out and you're running on mechanical advance only as the engine rpm comes up and you're 'all-in' at 2200-2800 rpm or wherever in the curve you set up the 'all-in' point.

Basically, you don't want a drag racing advance curve in a street driven car.

Looking from here, it looks very much like the distributor is a tooth off.
Get the engine up on TDC - Top Dead Center - on the firing cycle with both intake and exhaust valves closed.

Install the distributor so the engine will fire at that point.
Make sure you haven't set things up so distributor rotation is limited.

Disconnect and plug the vacuum advance line at the distributor so there's no vacuum leakage from the manifold.
A golf tee makes a great plug.

Start the engine, set the initial timing with a light and if everything is working right it should run ok.

Install the vacuum advance line and you should see or hear the engine rpm increase a couple hundred rpm at idle.

Don't sweat it if you run the engine up to 1500-2000 rpm with no load and see the advance as high as 45-50 degrees BTDC.
Doesn't hurt and in fact is what you want.
The extra timing advance is required to give the lean idle mixture time to burn.

Do you have a PCV setup on the engine?
If a PCV valve hangs up it can create a major hesitation coming off idle.

Just in case....

Good advice, C9x!

But I don't know where the cam timing/degreeing came from ... all I suggested above was juggling his plug wires around to give tim_bo16 more room to adjust the mechanical advance due to the interference he's having now with the vac can on the dist.

I agree you don't want to go messing around with cam timing at this point and probably want to focus on getting the ignition timing nailed down...

Thanks, C9x for helping out here. You have much more experience with this than I. BTW, tim_b016, I'm fairly new to this stuff too and a great deal of what I've learned the last few years came from the wise(guys ) on this forum...

-Chris

alright, well i just checked it again, and yes, at the 650 -750 rpm, it does have about 11 inches of vaccum, with about 9-10 * of timing (vaccum advance unhooked) checking for the "all in" timing, it does quit moving about 2300 rpm, and there and anywhere past that, its about 23-24* timing.

the vaccum at 1500 - 2000 rpm is about 17-18" vaccum

so were you saying that if my "all in" timing comes in at the 2300 mark, then is that too soon?

and as far as the pcv valve, i believe i have that, just the biggest vaccum port hooked up to the brake booster. the lines are not kinked or squished either.. so would that be my next step is to move the distributer a tooth ?

2300rpm for all in timing should be fine... yes either pull the distributor and move it a tooth to give you more room to adjust (can be tricky to get the oil pump drive shaft properly aligned with the dist when you do this... you'll need a BIG flat screwdriver or an oil pump priming tool to get it aligned right)... or as I suggested before, shift the connections of your plug wires on the dist and realign to the new #1 terminal...

good luck,

-chris

Tim is correct that 2300 rpm all-in is workable.
Thing to look out for is ping at part throttle, it probably won't happen, but it can so be aware.


The 17"-18" of vacuum at 1500-2000 rpm shows that the cam is probably installed correctly.

Keep in mind that every 1000' of altitude vacuum levels at idle will drop 1".
Looks like you're probably at the 1000' altitude mark assuming your engine is in good shape.

well i kinda got ahead of myself in waiting for a reply from you guys on the distributer turn, so i just went ahead and did it. re-timed everything, 12" vac at idle, still the 18" at 1500-2000 rpm. base timing was about 12, and all in was right at 32*.

took it out on the highway, listened for ping and all that and it turned out good!! and yep, my low end is back and no hesitation!! friggin amazing. hah

but yeah i really wanna thank you 2 guys for all ur help and input. just hope i can keep learning more as my problems will ALWAYS keep coming.

one thing, what is ur guys' imput on av-gas.. i believe my source is 100 low lead but i'd have to check back on that. i know i would have to mess with the timing again but thats not too much of a hassle now. we got a cruise comin up tomorrow and i was just wantin something more to have fun with. ??

Unless you're running a fairly high static compression ratio, you won't gain anything by running high octane.

High octane is all about slowing down the burn rate which helps to prevent detonation.

If your engine will run on 87 octane - and it might, skip the high price stuff.


The 462" Buick engine in my 32 is 9.0/1 compression ratio and I run 87 octane in it year round with no probs.
Not even in the summer heat waves here in the Dez.
104 degrees F today.


There are a few things you can do if you have an overly high compression ratio and none of them require the crank & pistons to be removed.

alright well i know that its not all that high compression. but my guess is somewhere close to 9.5ish.. just judging by the 76cc heads that i used to have and with the 64's that i have on there now.

but yep i was just curious on what the 100 would do. but screw it. yeah i know that it has run off of 87 once before. oh well... begining to save up for a new small block build.. either 406, or 383. i got a 350 and a 400 crank that i gotta get checked out and a guy owes me a 400 so if i can find a 509 casting 400, then thats the way ill go. streetability i dont mind, just simply because if im gonna eat the gas, i wanna go fast. so im gonna try for 375ish-400ish wheel hp. any suggestions?

The Chevy guys need to come on board here, but what looks good is a 383 with under 10/1 CR, 600 CFM Holley vacuum secondary or similar in an Edelbrock.

Take a look at Edelbrocks cam/intake manifold combos.
Dyno proven to work well together.

Considering the price of gas today, you'll probably want to opt for a mild cam, somewhere around 265 degrees advertised duration.

Try to find someone who's running a similar combination and see what they think about it as well as anything they would have done differently.

Experience counts for a lot and simply parroting things they've read in a magazine . . . I don't have to explain that....