[455rebel]

Reman hei,
Total timing 33-35
Ignition timing 15 btdc
Vac can set to start after 8 vac
Vac can limit to about 10-12 degrees
One stock, one med spring on advance
All in by 2900-3000rpm.
Problem is I can't seem to get it to run proper with man or port vac.
Only difference is idle(faster on man vac)
When crushing at 1600-3000rpm on either vac port it slight stumbles.
Or hunts,or surges.
I have 2 carbs a reworked q jet and a Holley 800DP
It acts the same with either carb so I am pretty sure it is the advance?
Just don't know what to change?
Thanks
Ps the Holley will smoke the q jet at the track, but the q jet gets way better mpg and it still has a working choke...

[chiefbigb]

Try doing away with the vacume adv and bring the timing in by 2800 mechanicly.

[steve25]

When did this issue start, with the install of the re-man dizzy?
Can you put in enough initial to make the motor hard to crank,if not than the outer section of the balancer may have slipped.
Is the 800 a spreadbore, if not and you are using a adapter you may have a vacuum leak.

[Cliff R]

The quality control of the remanned HEI's is OK to pathetic. Most likely it just needs custom tuned for what you are doing, and positive stop welded in to keep it from adding timing at high rpm's. Using lighter than stock springs aggravate that deal even worse, and may be part of the problem.

My car runs 7.20's at 96mph in the 1/8th mile with a q-jet on a factory HO intake and stock HEI with a positive stop welded in, so the q-jet isn't holding you back at the track with 8.90 runs.

Most likely what you need here is carb and distributor tuning to get the most out of your combination.......Cliff

[steve25]

I see you have 400 heads on the 455 now, what casting number heads are these ?

[chuckies76ta]

Vac can set to start after 8 vac. When crushing at 1600-3000rpm on either vac port it slight stumbles. Or hunts,or surges.

So what vacuum is your engine making at idle? And if the vacuum can starts to add advance at 8" of vacuum! Hmmm, wouldn't that kinda cause surging as the vacuum drops below 8" while driving? Maybe I'm missing something.

Would you not want a vacuum can tailored to your engines vacuum.

Charles

[455rebel]

Vac is set at 8 so it won't adv while starting engine.
Makes about 15 vac in park, 12-13 in gear ideling @ 800 rpm.
Stumbles 1600-2000 rpm ( 3000 my bad)
Thanks

[steve25]

Both carbs may have a issue, the Q jet may have its power piston hanging up and the Holley may have its power valve doing the same!

Try this test out, un-hook the carbs accelerator pump and with the motor at temp and out of gear wing the PRIMARY throttle wide open fast!

The motor should rev but stumble a tad, if its pops back thru the carb then the fuel curve is lean and your timing is not the issue!

Also check by flipping each carb over that you do not have the idle set so high that the primary throttle blades are not at the end of the fuel transfer slot in each side of the throttle plate. If this is the case you will never get rid of the surge unless you get more idle air into the motor and then back down the the throttle opening!

[455rebel]

400 head 6X cut 20 thousands
73 main , 43 rods
I changed the power piston spring until off idle stumble was gone (when dialing the carb in)
Slight stumble with pump unhooked off idle , no pop
Throttle blades correct
I have been tuning on classic cars 30 plus years and this feels like timing.
Any other ideas
Thanks
Ps my home moded vac can is the only other thing I can think of?
Could I stil be pulling in too much advance with 12 degrees?
Pss it all started when I recently hooked the vac adv up.
I have drove the car almost a year with no vac hooked up.
It was dialed in with no vac, I got to thinking it might get more mpg and run cooler in traffic if I tuned the vac in.
Thanks

[chuckies76ta]

Disconnect the vacuum can, plug the vacuum line and check your timing, inital and total. Take it for a drive see how it runs..

Standard or Auto?

Charles

[455rebel]

Turbo 400 , 3.15 posi
Vac is unhooked , until I find something new to try.
Car idles well , cold and hot, runs well wot, accelerates well from any mph, secondaries come in just right ( played with spring adj a lot)
It just won't cruise right between 1600 - 2000 rpm.
It is not bad, most would leave it alone , I just want it perfect as possible.
Years ago I had a similar problem with one of my amc ( 1972 401 powered matador)
It was the vac can bringing in too much advance.
As stated earlier I moded my stock hei can to 12 degrees advance, just don't feel this is too much.
Thanks
Ps I might have to try a crane vac can?

[ta man]

Have you tried leaning or richening the idle mixture?Do you have any more dist springs..I usually find the heaviest springs from the Moroso kit work fine,maybe having a heavy spring and a medium spring is causing a timing fluctuation at your cruising rpm.Have you check the timing at your cruise rpm to see if its steady?

[chuckies76ta]

GM makes different cans that operate at different vacuums.. You need an early can that works with you engine vacuum.. Here's a good read.http://www.corvette-restoration.com/...c_Adv_Spec.pdf

Charles

[Sun Tuned]

Probably a issue with the pickup coil....

[steve25]

It's not too much vacuum advance, but it may not be a smooth shift in advance under load as its falling into and or out of advance in a big way right in that 400 or so rpm range, and that may very well be the root cause of the issue you have , so I would stuff in a new vac can at this point!
The only other thing I can think of that may be part of this responce issue would be that your timing chain is shot!
If you have a question about that, than pull all the plugs out, pop the dizzy cap off and grab the balancer and move it back and and forth, does the dizzy rotor lag behind a lot in regards to rocking the crank back and forth?

[455rebel]

Chain only has a couple thousand miles on it( rebuilt whole engine)
I am ordering the crane can today, will install when time allows and get back.
Thanks
Ps after can install and tune I will address the idle restrictions.

[455rebel]

Crane vac can came in , wrong can they got me one to hold vac for power brakes.
Recorded right one, should be in tue or we'd next week.

[455rebel]

Can installed, waiting on break in weather for test drive, snow and ice here in tn most of this week.

[Squidward]

How I did mine after getting the Crane unit was as follows:

I set my initial based on getting my total in (init + mech, NO vac) for 36. I had around 20 mech, so I made initial at 16, without any vac advance connected.

With vac advance disconnected and plugged, car at operating temp, I blocked the wheels and put the car in drive (auto). I rotated the distributor (initial setting) until I had optimal vacuum on my vacuum gauge. I noted this timing setting, which for my car was around 33-34* - the "sweet" spot. I set initial back at 16*, and shut the car off now.

At this point, I loosened the vacuum spring setting a large amount on the vac advance diaphragm with an allen wrench. CW or CCW, I can't recall - just read the directions. this ensured that max advance was achieved by the vac canister at a vacuum much less than what my engine was capable of pulling. I then adjusted the vacuum advance step limiter to allow only 18* advance at full vacuum. This takes a few tries, and requires popping off the disty cap each time to adjust. Maybe there is a thumbrule, like 2 degrees per step, but I found 18 by doing one click at a time.

What I had now was initial at 16, plus full manifold vacuum adding 18, equaling my idle "sweet spot" of 34. The only thing I had left to do was to set the rate at which the vacuum advance came in. I would unplug the vac advance, make an opposite adjustment (like 1 turn) from above on the allen wrench to tighten the spring pressure, and then reconnect vac advance and see if my timing had come off the 34*. If it didn't, I'd give the screw another turn and check it again. If it did come off 34*, then I would come back a half turn and call it good. I was shooting for the spring tension setting that would start dropping timing out at the threshold when I came off my max manifold vacuum.

This got me to the point where full manifold vacuum got me my sweet spot in gear at idle. I don't have start issues either, because the extra advance doesn't come in significantly until the engine starts and creates vacuum. And when I mash it to the floor the vacuum goes away and it is strictly a mechanical curve at play. Geoff from across the pond (Australia) sent me this method when I was struggling with my tune. I've been pleased with this technique.

There are probably other expert views, but I was the guy in your shoes a while back. This got me in the game.

My differences? Much smaller engine, 350P. I pull 20" at idle. My mechanical is all in fairly early, like 2400. My cam is relatively mild, and I am using a qjet. My car is relatively light (1st gen FB). Again, I am by no means an expert, but the info above helped me understand things better at the very least.

[455rebel]

Geoff and I had been posting in pm section and he gave me his procedure.
I am still tuning on the setup. Waiting on weather break for test drives.
Can't seem to post in pm now, something wrong with the site...
att Geoff-ak now 9 turns out,initial backed down from 16 to 14 btdc.
Starts cold and warm ok now in shop, need to drive and get it hot.
Ps try to send me another pm.
Pss cam specs later, have to find my card.
Thanks