gtpro700

Alright I have a 350 with AFR heads and a hydraulic roller cam. The cam has a lobe seperation of 108 which I thought might be causing my problems possibly with dialing the carb in, but im not sure now. The motor wont idle consistently and when you rev it up it doesnt come right back to idle quick like it should. It also will idle sometimes fine at say 900 rpms then it will come back down from a rev and idle at 1000 rpms. Main problem is after you rev it it wants to stall a lot of the time if you dont play with the gas to keep it going. I intially thought my carb was messed up but I switched my carb onto my cousin car which is a similar setup and it worked great on his. The timing should be fine with about 36-37 total timing in it. So if the carb isnt a problem and timing is fine what else could it be? We checked vacuum when it wanted to idle at around 900 rpms and it was around 9-10 so im not sure if there is a vacuum leak or not because there was little fluctuation. It is a decent sized cam with a pretty agressive lobe seperation of 108 so I think 10 inches of vacuum should be about right?

That just leaves ignition which im running a mallory hei distributor with a MSD 6AL. Both are new so the only thign im thinking of is if the wire inside the distributor that you use to hook up the msd box to it is grounding out slightly or something. The signs also seem like it could be a vacuum problem but it seemed to read alright tho??? Anyway to determine if I have a vacumm leak some other way? Also I need to open my throttle blades a decent amoutn extra to keep the motor running. Im not sure if this will indicate any type of vacuum problem or not? ANY help is appreciated this one has me stumped... Thanks guys!

M1tch

My car has acted funny a few times and it was either timing too high or too low. Try turning your timing down to 32 degrees just to verify its not that. The idle fluctuation is normal after revving which will cause it to momentarily idle faster. Are you sure your idle screws and fuel pressure is correct? That is crucial, you might not be getting enough fuel to that big cam. Turn one of the idle mixture screws clockwise until it starts to stumble and then back it out until it smooths out and add another half turn or so maybe more. To verify try it in drive and in park to make sure there is no stumble. But it sounds like a carb issue.

1. check idle screws/ fuel pressure/ float bowl
2. turn timing down a few degrees/ check base timing
3. vaccum is low but you have an aggressive cam

What kind of carb and cam do you have? Do you have the right plugs? But it sounds like a carb/fuel issue to me..

305sbc

iTrader: (3)

I would plug any vacuum lines such as power-brakes and PCV for testing until you find the problem.
Unless you are running a locked-out full advance at idle, then your total timing shouldn't be a factor.
Your engine may respond better to more timing at idle though. If you have a vacuum advance distributor then this will be fairly easy. If you have vacuum advance, but it's malfunctioning, then it could be causing the problem.

gtpro700

Thanks for the help guys. I have already had the timing down around 32-34 degrees and it seems better or no different at 36 now so I dont think it is a timing issue. I have all the vacuum lines plugged right now as I try to figure the problem out as well. I dont think it is the carb because I put it on my cousins motor which is almost an identical build and it ran great. I also have checked the float level, idle mixture screws, and fuel pressure and they all seem fine. I do have a vacuum advance distributor and I was wondering how I could tell if it is malfunctioning? Carb is a 750 mighty demon and cam was custom built by cam motion. 238 lift @ .050 and .585 lift @ .050 Plugs I just changed and they are the ones AFR recommend for the heads. I am leaning towards a vacuum leak somehow because I spoke with the guy at cam motion and he said I should probably have atleast 12 inches of vacuum maybe even 12-15. So I could possibly have a small leak somwhere. I am goign to recheck the intake bolts and go from there right now. Any other advice is appreciated guys. Thanks again

305sbc

iTrader: (3)

You use a timing light to check the spark advance at idle with the vacuum line hooked up to the advance, then without the vacuum.

A common mistake is to not use an adjustable vacuum advance unit when running a large camshaft like that.
Another mistake is to hook the vacuum to a ported vacuum source rather than full manifold vacuum.

A normal place for spark advance at idle without the vacuum advance is around 10* BTDC, and Between 25* and 30* with the vacuum hooked up at idle.

Have you checked for fuel pressure at the carb or flooding?

gtpro700

To think about it I do think I noticed the vacuum advance is working because I went to check timing before I pulled the vacuum line and it was in the 20's I think so that shouldnt be it. The motor runs better with the vacuum line on the one towards the back of the motor out of the two in the demon which I believe is the full manifold vacuum. My cousin and I think it might be a vacuum leak at the intake due to the block possibly being decked. My cousin said it can throw the angle off ever so slightly where the intake sits and can cause a small vacuum leak. I havent had any problem with fuel pressure or flooding either. It is really leaning towards a vacuum leak unfortunately. The only thing that sucks is the car sometiems idles alright with the vacuum advance hooked up to the full manifold vacuum port so when you take the advance off to read vacuum at idle the car doesnt want to consistenly idle. Thanks again for the advice guys. Any other help is appreciated.

M1tch

is your vaccum guage bouncy? If so you might have over tightened valves, or the other way around

gtpro700

My cousin had the vacuum guage and I was the one tryign to tweak the carb to keep it running. So im not sure but I do think it was pretty steady but maybe just low. I dont think the valves are tight. I just got done adjusting them recently at 1/2 turn past zero lash like usual but it is somethign I might check if I cant come up with anythign else.

gtpro700

Alright so I really thought it was a vacuum leak and I decided to pull the intake and do the gaskets over. After I did the intake gaskets I fired it up and I am still having the problem. This means the most likely I dont have a intake gasket leak, the carb is fine because it worked perfect on my cousins car, and I just changed the plugs. The only thing really left is ignition. I am definitly leaning towards something to do with the distributor and its vacuum advance. It would make sense because its when you rev it it doesnt come back down quick like the advance is getting stuck or something. Because havign that extra advance would make the motor stay rev'd im guessing. It been crappy weather but I plan on look at it tomorrow if all goes as planned. I might mess with the springs and see if that is it but I dunno. Could I also oil or lube any parts in there on the advancign mechanism to make sure its not binding or something? Any other ideas are very welcome and appreciated because if its nothing with the distriburator im not sure what else it could be. Thanks guys!

M1tch

Do you have the stock weights/springs in the distributor? Plug up your vacuum advance and plug the port on the carb and see if it still acts up. The only time my car acted funny idling was when my timing was really retarded. It would chill at 900 then would you would rev it, it would stay at 1200 for like 3-4 seconds before coming back down. And the vacuum was low too

gtpro700

Alright I have been messign with the car and found out that giving it more timing somewhat seems to help. I locked out the timing and the motor will idle pretty good now but if you rev it it still wants to keep the revs up around 1500 or so and you have to play with the throttle to make it come back down. The carb isnt the issue because I tried it on my cousins motor (very similar build) and it worked just fine, fuel pressure is constant at 6 psi, timing is now locked out at 34-35 degrees, and I checked AFR at idle and it seemed about right at around 14:1 ish. Now I am really confused!! I have also just changed the intake gaskets just because I thought it might be a vacuum leak. Also all of the vacuum ports on the carb are plugged too. Only thing really left for me to try is to readjust the valves and hope that it might be an overtightened valve or something. I adjusted them with the method of gettign it on the comopression stroke of the #1 cylinder and doing the half of the valves and then rotatign the motor to the #6 compression stroke. Some people have said this way doesnt work with large cams is this true because then there has been peopel that says it is just fine. Hopefully I will have time this weekend to readjust the one cylinder at a time. The vacuum gauge was a little bouncy but im not sure what is acceptable. The cam has a decent lope to it so im not sure if that should effect it. It would bounce right around 11 inches of vacuum and probably touch 10-12 inches with it bouncing. Please let me know what you guys think it is. This is driving me absolutely crazy!!! lol Thanks guys!