Crappy throttle response and detonation...Save me!!

[Frankenstang65]

Ok...so here's the deal. I've been having a lot of trouble trying to get my car to idle/run right for the past few days. It starts up pretty well, and idles ok (around 700 rpm or so), but whenever I punch the throttle it stumbles like it's not getting enough gas. It'll rev ok to about 1500 or so, then I hear a loud "GASP" from the carb and then it chokes out. If I richen the air/fuel mixture it seems rev up a lot better, but only when I get it so rich that black smoke is pouring out of the tailpipes.
A friend of mine suggested that the timing might be off. I retarded the distributer a little (I have to do it by ear because my motor is missing the pointer) and it seemed to help things slightly. Advancing the timing didn't help at all. The only problem (after retarding) is that now it detonates like crazy even at 1/4-1/2 throttle on flat ground. I checked the vaccuum advance to see if that was the problem, but everything there seems to be working ok.
Is there something simple I'm over looking? This thing has got me pretty frustrated. Also, do any of you know where the timing pointer is supposed to be on an early (69-73) 351W? Maybe if i get that figured out, some of my other problems will go away.

Any advice/insight would be greatly appreciated...thanks in advance!

[Fostang]

Well first you need to make sure the timing is correct. Can't tune a car without knowing the timing. Has it ever run correctly or did all of a sudden start doing it someday. I don't think that it's because its running lean. When is the last time you rebuilt the carburator. Maybe you got some bad gas somewhere and its time to rebuild it. Check the fuel pressure. Hows the fuel filter? Sounds like there is a problem with the secondary. Also 700 rpm seems a little high. Check it out and let us know.

[mustangman65_79]

When timming by ear, I know cause I have to do it too. Take the distributer and turn it till u find a point where the motor is at it's fastest idle. Retard the distributer just a little, then check it by reving up the motor only a little. About 1500rpm, and pull off the vaccuum tube, and it should idle down some, put it back on to check still more. Keep moving the distributer till it has the greatest RPM ajustment between off and on(vaccuum). But remember not to move too far from the point u found earlier;(the distributer and turn it till u find a point where the motor is at it's fastest idle. Retard the distributer just a little, ).

This is how I do it, and my car runs a lot better now. U don;t want your car retared so much that it sounds really good, and it wants to die. But not too advanced that it's where the point of breaking(the fine point discriped above.)


I hope I helped some what. I'ts hard to explain w/o doing it myself.

If this doesn't help your prob, plz repost.

[Stang5]

I would deffinetly start with cleaning the Carb. It is very simple and could help alot. I was having some trouble the other day with hessitation and couln't fix it with the air/fuel misture. So I cleaned out the carb, and it fixed everything...give it a try.
DM

[70_Nitrous_Eate]

Some other things to check (cause I don't think your timing is the problem)


I'm leaning to a fuel problem. Namely, engine isn't getting enough of it...

Check the following items in this order.....
Fuel Filter (just put in a new one)
Fuel pump (you should be able to read at least 4PSI at full throttle)
Power valve in carb (not opening)
Plugged jet in carb.

I once had a small rock (piece of sand) that plugged one of my jets on my old 2barrel 78 stang carb. When the secondary barrel would open, the car would stumble and die. That one took me a while.. car would drive fine until I put my foot into it a little!

[Frankenstang65]

Thanks for the replies!

The car has sat for the last few days (been too busy with x-mas stuff to mess with it) and now it won't even start. When I crank it for a few seconds and pump the accelerator the engine will putt once or twice but not actually run on its own. Then it will just crank and crank untill I stop and wait for a few minutes and do it all over again.

I'm almost positive that it's a fuel issue given this most recent development.

I'll pull the carb and give it a good cleaning per stang5's advice. Hopefully that will do the trick.

70_Nitrous_Eate, I changed the fuel filter ( the other one looked to be fully functional though). I'm 99% sure that the fuel pump is not the problem (I checked it a couple on months ago, and all seemed to be normal). How do I check to see if the power valve is working properly? And I'll find out if it is a jet problem when I clean it in the next couple of days.


mustangman65_79, thanks for the tuning advice. I'll give it a try as soon as I get her up and running again.

If anyone has any other ideas please let me know..I'll keep you posted

[thunderbolt]

OK here is my take. You get decent throttle resoponse from idle to 1500 rpm. Then out of go. If you richen the mixture it gets better, but you get carbon in the exhaust. How are you richening the mixture, mixture screws or jets. It sounds to me as if you need bigger jets in the carb. From idle to 1500 you are dumping raw fuel into the carb via the accelerator pumps. The fact that you have good throttle response here means your engine wants fuel. About 1500 you are probably out of accelerator pump shot and running out of fuel. Then it runs better with richer mixture. If you do this by adjusting screws you are probably getting more gas in but it is not mixing well with the incomming air. Bigger jets will let more fuel in and mix it properly. Possibly it could be a low float that is running out of fuel in the carb bowl? Don't know about this one, but sounds like your motor wants more gas to me.
Hope it is a dirty carb though. Had a similar problem and a good cleaning fixed it. But if that dosen't do it I would go after fuel delivery.

[70_Nitrous_Eate]

OK, to answer questions:

Powervalves are cheap, just spend the $5 and put a new one in instead of fooling with the old one.

Here's a few more things to try now that the car won't even start...

First thing I'd do is crank it for a while and see if you can get anywhere. If she still won't start, pour about 1 tblspoon of gas down each of your 2 primary barrels. Now try to crank it over again... if it fires up nicely for a second then dies or runs really rough, then it's definately a fuel problem.

IF you've determined it's a fuel problem, then I'd try the following first... Jack up the back end of the car, then disconnect the fuel line at the pump (the line that goes from tank to pump). You should see a healthy stream of gas come out. If not, you've got a blockage in the line or tank.

If you do get a healthy supply of fuel, then your problem is obviously the pump or carb. Fuel pumps can die suddenly, I still think it's a good idea to measure your fuel presure. If you can't get the car started to check the fuel pressure then lemme know and I'll teach you the super-ghetto, very unprofessional way of checking the pump

[Frankenstang65]

I tried the tblspoon of gas down the primaries trick and it went putt, putt, putt, and pushed some foul smelling (grayish) smoke out of the tailpipes. But it still wouldn't run on it's own. I'm gonna pull the carb tonight, I'll change the power valve and do some cleaning, hopefully that will take car of it.

I jacked the back end up and disconnected the fuel line, and the gas came out in a stream. So from this point on I'll assume that the problem is either in the pump or carb.

Since I can't get it running, I may have to resort to 70_Nitrous_Eate's "super-ghetto very unproffesional" way of checking fuel pump pressure...lol. Let me know how to do that when you get the chance.

Thunderbolt, thanks for the help. I was adjusting the screws when I talked about richening the mixture. It ran great for a long time without any problems, so I'm thinking that it is not a case of the motor needing more fuel via bigger jets. I'm leaning (optimistically) towars a dirty carb, too.

I'll keep you up-to-date on what happens next...thanks.

[thunderbolt]

If it ran fine for a while then I bet it is dirt or something in the carb. When I said I had something similar, that is exactly what happened, the float wouldn't completely close and it was pouring gas over the bowl directly down the venturies. When it ran it sputtered, coughed, and blew lots of black smoke. Finally it just wouldn't run. After a rebuild and a good cleaning it was fine.

[Frankenstang65]

I took the carb off and gave it a good cleaning. I didn't notice anything out of the ordinary though. I changed the power valve and adjusted all the linkage before I finally started it up.

It ran really bad (rough, popping sounds) but I adjusted the distributor and let it idle for a while. As it warmed up it ran better and better and has run excellent ever since. The bad throttle response is no longer a probelm and once I got the timing a little closer to spec (gracias, mustangman65_79) the detonation went away as well.

Thanks for all the help, I really aprecaite it.

later

[mustangman65_79]

It sounds like a little of both problems. I glad we could all help out in u getting your car going again.