Ported Vacuum? backfire/studder when OFF throttle?!?

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Nov 5, 2003
Shippensburg PA
OK, tracking down what I *thought* was a fuel supply issue, but after new rubber lines and filter I realized it isn't a fuel supply issue...

When under load it runs great with good pulling power.
When you let off the gas to maintain speed or slow down it sputters, backfires and bucks. (in gear)

New FJ-60 Distributor that I know is good.
1974 F n' 1/2 engine that's been built and has way hi compression (12-1 maybe? Builder shaved a bit more than I wanted :D)
when ported vacuum lines to vacuum advance on distributor are DISCONNECTED, the problem goes away.
timing was tried at the ball, as well as an extra 6 degrees. better at the extra 6 deg, but still has the problem.

I'm running w/o the vacuum advance now w/o any problems. However, Engine is sluggish on acceleration (not as much advance)

The carb was done by Jim C and came back with what I *thought* was ported vacuum and a red cap over it. pics are below:

#1 Do I have the vacuum hooked up right? the only other vacuum I have is under the drivers side of the carb and is hooked to throttle linkage stuff?

#2 Ideas as to why it's doing this? Seems like it's got too much advance for the throttle position and is misfiring?



Vacuum appears correct, although where the port enters the throttle bore is more important than its location outside. If Jim says it is "ported" vacuum, I would take his word.

Describe the running problem in more detail with more details about the operating circumstances. what kind of "backfiring"? Out the carb? rumbling in the manifold? Blow your muffler off? When and how does it stutter?

I would tune it up to specs first so you don't have too many variables to consider at one time.
X2 What Pin said above.

Problem occurs on lift throttle... Reducing throttle from 75% to 25%? Or worst when reducing throttle to 0%?

Which of the two fittings on the dissy canister are connected to vac?
of course I believe Jim! :D I'm just second guessing myself because of this problem. I think he *ports* all the carbs he does (that need it of course)

OK, I had both vacuum diaphragms connected with a T. I have not tried ONLY one or the other. Just disconnected both and capped the carb.

Backfiring is a hi pitched POP that I'm pretty sure is from the carb. Sometimes one every second or two, getting worse to rapid fire and bad bucking.
Studdering is kind of a bucking motion of the vehicle, throwing you around in the seat . When this happens it feels a LOT like fuel starvation.

I'll try to describe this....
problem occurs after driving for a minute or two. Under medium to hard acceleration and throttle it runs fine w/o any misses or backfires. (hence I don't think it can be a fuel supply issue) When you then let off the throttle to merely maintain current speed after a bit it starts backfiring and bucking, progressively getting worse until you get on the throttle again.

if you fully leave off the throttle the problem seems worse but is hard to tell as the overall problem is not consistent. I think sometimes it's loaded up more (fuel) and worse?

Problem seemed worse when timing was set to the ball (factory) and I think was a little better when advanced an extra 6 deg (craftsman light with adjustable advance)
Also, moving advance up an extra 6 sped the idle up very noticeably. (shaved head w/ hi compression. Not sure what cam but not OEM and *mild*. Not an RV)

I know spark is good as I just went through all this in another post where I thought my magnetic pickup was bad, but it was actually the coil. Coil is not new but is a used FJ-60 coil (w/ igniter disconnected) and MSD off road ignition.

And like I said, the problem disappears when you disconnect and plug the vacuum. The engine has noticeably less power under acceleration that way though :crybaby: Plus, vacuum advance is the main reason I went the FJ60 distributor route. Obviously it likes to accelerate better with the vacuum.
If the problem really disappears when you disconnect the vacuum advance line, then I suspect that you have a timing related issue. I don't see how it could be anything else. It would be too advanced if it runs better with the vacuum advance disconnected (unless you had it connected to the vacuum retard line, which is the inside one with the piece of tube on it in your picture). If you have it connected to the retard, then it is too retarded.
wait.... are you talking about the distributor? I thought BOTH diaphragms on the FJ-60 distributors were advance?!? :confused:
Maybe they are. I'm not that familiar with them. Anyway if the problem disappears with the vacuum line disconnected, then it is an advance problem.
was just tinkering again. Confirmed that it runs fine w/ the vacuum disconnected. Runs like poo with it connected.

Also found that the backfiring and bucking can happen under light to medium acceleration throttle as well. usually OK with foot off the throttle (which I thought was an issue before)

Backfiring is mainy in the carb but there is some in the muffler as well.

The whole thing is just too inconsistent to pin down.

checked the vacuum w/ engine running (suck test) and they both advanced. When sucking on the hose, the timing ball moved down or counterclockwise. This was for both. Is it a TOO much advance problem?
I'll have to try running it with one or the other diaphragm attached and see what happens...
Confirmed that it runs fine w/ the vacuum disconnected. Runs like poo with it connected.

The whole thing is just too inconsistent to pin down.


This sounds inconsistent.

The amount of advance is critical and you can have too much. The amount changes with engine speed, load, temperature, air density etc. In modern EFI engines, this is under electronic control. It is also under electro-mechanical control in cruisers, but people seem to want to remove this control and think it is going to run better.
Whoa! Do not run both vac advance fittings simultaneously. :rolleyes:

The inner (main) vac fitting will give 19-20 degrees crankshaft advance.
The outer (HAC) fitting will give 6-7* crank advance.

The recurved dissy has a max mechanical advance of 20* crank.

if all the advance came in at once (20 + 7 + 20) and the base timing is 10* for good off idle performance, then the spark could happen as early as 57* BTDC!:eek:

See the avatar pic? Thats an engine that was run on the freeway for several hours in that condition, until the detonation finally broke up the rod bearings.

A fresh, hi-comp engine, set the base timing around 10, and connect the vac advance hose to the outer advance fitting ONLY.

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