302 SBF stumble

[cgarb]

Recently sold my Mustang to a friend of mine, its a fox body, carb'd 302 street strip type deal. 3 months ago when I got it finished up and sold it, it ran ok. Now it has developed a stumble/hesitation issue. It does it with 2 different carbs on it, the 650dp that was always on it, and a 600dp that we borrowed to try. If you get the engine up to operating temp and whack the throttle open you can hear a gurgle come up through the intake...like valve noise or something. then it will clear out and rev clean. Same thing will happen under load when you drive it...put it in 2nd gear and nail it about 20mph roll, same issue. Timing idles at 25 deg and totals out at 36 by about 4000rpm. What types of issues should I be looking for to get this fixed. It seems to be getting progressively worse, could it be a wiped exhaust lobe?

[Geoff2]

Stumble fix: use a vac secondary carb.

[user-30257]

Recently sold my Mustang to a friend of mine, its a fox body, carb'd 302 street strip type deal. 3 months ago when I got it finished up and sold it, it ran ok. Now it has developed a stumble/hesitation issue. It does it with 2 different carbs on it, the 650dp that was always on it, and a 600dp that we borrowed to try. If you get the engine up to operating temp and whack the throttle open you can hear a gurgle come up through the intake...like valve noise or something. then it will clear out and rev clean. Same thing will happen under load when you drive it...put it in 2nd gear and nail it about 20mph roll, same issue. Timing idles at 25 deg and totals out at 36 by about 4000rpm. What types of issues should I be looking for to get this fixed. It seems to be getting progressively worse, could it be a wiped exhaust lobe?

Throw a vacuum guage on it. But first thing I would do, is check the valve lashsee of they are adjusted right.

Basic troubleshooting should be a good start, verify timing, vacuum leaks, valve adjustment, rvp of fuel etc

[427dart]

I would check the intake manifold bolt torque first and make sure they are at 18 ft. lbs. minimum.

[DCal]

A good old fashion compression test would hurt either, that will usually separate mechanical problems from spark and fuel issues.

[statsystems]

Stumble fix: use a vac secondary carb.

Why? So it will lose power?

Look at the accelerator pump circuit first. Diaphragms go bad. You don't know what your buddy or his buddies do when you ain't there.

It can't be that hard to fix.

[BigBro74]

I have had the exact problem on this exact car with similar setup(25 years ago so i am remembering back away...), and I really feel that (especially if it is a stick car) that on a 302 with a DP carb it just has almost no signal when you whack the throttle open hard. I had a single plane intake. for a long time I ran an 850,,because i owned it,, and it worked okay, but I knweeww it wasnt right because thats what all the rags told me.... so i saved my hard earned and bought a new 650 dp carb. iT WAS WAY worse!! so i studied as well as i could on it back then and it got better,, not perfect but better. the 850 had no PV in the secondaries and a little 25 squirter (mabye squirters) in it. so i set the 650 up that way and it helped--- very LONG pump shot that way, not as rich as soon. plugged the secondary PV and jetted up secondary accordingly (6 numbers if i remember...). the two carbs had different types of secondary linkage so they opened at a different rate, but i am talkin jabbing it wide open. Some vacuum advance off of the manifold might help too.... people will dispute- but the advance at low throttle position keeps things cleaner and it will respond quicker when necessary.... better cruise mileage too
Jason

BTW this car got 23 MPG on the interstate with both carbs...

[cgarb]

This car has been in storage for years before I sold it...this is my official guess. The valves seem noisier than I remember, when I switched to a mechanical cam some years ago I put a set of good used valvesprings in the engine. I think a couple of springs got weak and the kissed the Pistons, now with leaky intake valves it won't take the fuel and air with the compression forcing it backwards in the intake. I haven't had a chance to fully investigate yet, that will be this Sunday. I will let everyone know what is going on. I'm going to check the valvesprings on the engine with my spring checker and do a compression test. As long as it didn't drop a valve yet, it shouldn't be that terrible to fix if it's just a couple bent valves and weak springs.

[BigBro74]

One other thing crossed my mind- it has gotten a lot hotter since three months ago- my Cars response always seemed worse when it was hot and humid. very well could have sagged a spring- and “warped” a valve a bit...- good luck

[cgarb]

Its not just the stumble, the car is overall down on power too. Its a good 2 to 3 tenths slow and down 4 mph in the 1/4...that's what make me susupect the valvesprings.

[Geoff2]

Stat,
No it won't 'lose' power with a correctly sized vac sec carb. A 302 engine doesn't need, nor can use, 650 cfm of carb airflow when the throttle gets slammed open at 1500 rpm...

[statsystems]

Stat,
No it won't 'lose' power with a correctly sized vac sec carb. A 302 engine doesn't need, nor can use, 650 cfm of carb airflow when the throttle gets slammed open at 1500 rpm...

Ok. Thanks for the tip.

Except your air consumption math has been proven wrong. For decades. I'll keep throwing every VS Holley I come across in the garbage. Unless I can find a fool that wants to part with his money.

[novadude]

Except your air consumption math has been proven wrong.

Explain please. In my way of thinking, there will not be a strong booster signal with all four throttle plates slamming open at low RPM on a small CI engine. Weak booster signal = poor mixture quality. You can crutch it with pump shot, but that's still not going to give you a nice atomized mixture like you'd have with high velocity through the booster. I'd like to better understand WHY a mechanical secondary carb is better in all applications. Always happy to learn.

[Zmechanic]

very LONG pump shot that way, not as rich as soon.

I've solved one this way too. Switching to a 50 CC primary pump, and a brown pump cam. Before, it was a very fast ramp cam, had almost no shot left after the primaries were open very far. I sat down and thought about it and compared cam profiles. Switched it to a profile that gave it a nearly linear progressive shot throughout the opening, including when secondaries came in. Switched secondary to be a quick burst shot right as they snapped open.

No PV in either side, jetted accordingly. Locked timing. But without the changes, a full throttle snap would stumble every time. Bad news for a footbrake car.

[BigBro74]

Well, and on the same 85 mustang car, I tried using a 600 vacuum sec carb- which drove very well- but despite changing from lite to heavy and back again on secondary springs- it would not e.t. - as well as the double pumpers at the track- like said mine was a five speed stick, I never did try a 750 vac sec carb on that car..... sold it in 05..... it was fun to drive but I never got it to go very fast- it was my daily for 5-6 years never stranded me, and ran 13.1 on my highway tires back in the 90’s. Now mini Vans go that quick lol—-