Engine problems

automart · Feb 18, 2007

Need some help here.

I have a rebuilt stock 351C 4v, except for small cam. It is back firing when accelerating, I have moved the timing around and tried everything there. I had the MSD distributor and removed it, installed the stock dist and it is still doing the same thing. I've changed the stock intake to an aluminum Torker....didn't help. The 70 Mach I has a factory tach that is working correctly, I still unhooked all the wiring and just hot wired it thinking I may have a short somewhere in the tach wiring. It's still missing and back firing.

There is a real strong gas smell and black smoke when it is idling. I had the motor professionally rebuilt, heads worked, everything. The only thing I can think of now is that it may have a broken valve spring. Anyone else have any ideas. It's really discouraging!

jikelly · Feb 18, 2007

Could be a carb problem. At least the strong gas smell at idle and black smoke make me think that. Carb could be flooding your engine.

Do you have spark at each cylinder?

Is there any problem with your coil or coil wiring?

351w_swap · Feb 18, 2007

have you tuned the carb? I may be wrong but it sounds like your getting too much fuel. If you haven't tuned the carb correctly try moving the adjustment screws or replacing the jets. If you think you have a broken valve spring it isn't that hard to check, just pull the valve covers and inspect everything. It is also possible that you have a valve that is seating poorly or an overtightened rocker. Try the first two suggestions first then try pulling the rockers, checking the pushrods while your at it and put the rockers back on correctly.

automart · Feb 18, 2007

Thanks,

This is a real pain, I have changed all the wires when I change the distributor, the carb was new, It's a Holley 750, it did sit for 3 years as the car was being restored (it's still not finished)

I have tried the adj screws, that is why I thought it maybe the intake as there is not much difference in the engine. There are no vacuum leaks. The holley has 72's in the front which should be fine. I really doubt it's vavle spring but I'm just lost as to what it maybe. Years ago I raced a 302 in a Maverick so I'm not completely stupid, but this 351C makes me think I am.

jikelly · Feb 18, 2007

automart said:
Years ago I raced a 302 in a Maverick so I'm not completely stupid, but this 351C makes me think I am.

Yeah my 73 stang sure does its part to keep me humble.

Once I dropped in a new 302 and the stang started and ran long enough for me to go to the gas station. On the way back it started backfiring like crazy, wouldn't accelerate, and I barely got it to crawl the three blocks back home.

I fooled with it a week trying to figure out what the heck was wrong, changed wires, distributor caps, plugs, my coil, put my 2bl carb back on and singed my eyebrows when the car hiccuped and shot flames out of it. Finally I got so fed up I pushed my car about 4 blocks to a mechanic that had a shop not to far from the house. Boy was I wishing I could put my car on a diet. The mechanic at the shop was real nice and he gave me a ride back home once he saw I could barely stand up, and promised to look at the stang the next day.

So I was at work when he called and told me it was ready to pick up and running great. I felt like a real dumb a%^ when he told me it took him 5 minutes to get it going. I'd turned the distributer 180 degrees out when I installed it, so it was just a timing issue. The old guy got a real kick out of my rookie mistake, and said the hour he spent laughing was payment enough for working on my car.

automart · Feb 18, 2007

Well, I haven't really looked at that. I have drove the car around the neighbor hood (I live in the country) Sometimes it would do ok, still smelled gas and eventually would not run very well. I would retard the timing which seem to help, then the next day it wouldn't start unless I advanced the timing......I'll check the 180 thing, hopefully that will be the problem.

chemeng · Feb 18, 2007

I blew a power valve in a Holley a couple years back and it did something similar. It would run Ok for awhile, then the carb would fill with gas, backfire then dead as doornail unitil the gas evaporated.

jikelly · Feb 18, 2007

At TDC your distributor should be pointing at the #1 cylinder. From there it's just a matter of the right firing order.

jcode68 · Feb 18, 2007

If you were 180 out it wouldn't run at all, just turn over with lots of backfiring and flames out of the carb (ask me how I know). Since yours is running, just poorly - are you sure you have the correct firing order for the plug wires? If 2 cylinders are off, then you would be flooding on those cylinders and then backfiring accordingly. I have read about some cams that change the stock firing order to improve HP. Just something to rule out before you go further throwing new parts at this problem.

automart · Feb 18, 2007

I just went to the cold garage and checked the TDC, I don't think there is a marker on the t/c cover to show where the TDC is. I did see the TDC on the harmonic bal but nothing to gauge it with. I got the TDC up to where I thought it should be, pulled out the #1 plug and the piston is at the top, darn it! I did here a rocker arm noise on the right side so I pulled the valve cover off, checked all the rocker arms as they are screwed all the way down. I didn't notice any valve spring problems on that side. What else could it be? The plug I pulled was dark where it is running rich. Although my carb is new and rebuilt now, I'm not 100% sure that the jets in front are 72's. I guess I'll pull the carb off.....again and tear the bowls off and see.

automart · Feb 18, 2007

Firing order is correct....believe me, but stock correct. I didn't see anywhere on the cam info that the timing might be changed. It's a comp cam. I'll go to the web site and check that.

Thanks

D.Hearne · Feb 18, 2007

If it's just been rebuilt with a new cam, you very well could have a wiped lobe or more. Were any of the rockers loose? And just as jcode68 posted, if it were 180* off, it wouldn't run. Period. Check the firing order again 13726548. There should be a bolted on timing pointer on the front of the timing cover. The Cleveland pointer is stamped steel.

automart · Feb 18, 2007

I really, REALLY appreciate everyone's help. The motor probably doesn't even have an hour running time on it. I hope it's not the cam. I checked the compititon cams web site, didn't see nothing on a different firing order. I'm really stumped on this one. I think I'll start it and pull wired off individually to see which cylinders are firing the same. If not, I'll pack it up and take it somewhere. It's a little embarrassing to not be able to figure this out...

Thanks again!

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