jnf

Can't seem to overcome a slight stumble at cruise on a 300hp with the 351 cam. To date I have rebuilt carb (adjusted floats); new plugs, points, wires, condenser; established optimum total timing which translated to 12 degrees at about 900 rpm; replaced vacumn can and compression tested all cylinders with excellent results. Vacumn is reasonably steady at around 16-1/2 at 900rpm. This is a Carter carb and my next step was going to be experimenting with jets of which I know nothing about. Any ideas or suggestions much appreciated.

MasterDave

Don't know what your cam is offhand, but your vacuum seems OK and you have everything seemingly in order. I do not like Carter carbs (sorry), but I have dealt with problems like you're having. You should go get a set of springs for your carbs needles. These springs require a certain vacuum to enable the needles to release and give more fuel at a given throttle setting. If you install a lighter/heavier set of springs, the carb would give more fuel at a lower/higher throttle setting. Make sense? Hope so, anyways it always seems like it's the transition from a steady foot to slight throttle pressure that causes the stumble. A performance shop would be able to give you better advice depending on what is in there now. Jetting alone will cure the problem, but might cause others, like plug fouling. Lars is your man here also, he helped me through my probs. Have fun.

SWCDuke

I assume you mean the "151" cam from the 327/350. What year engine is it?

Vetterodder

You might be getting too much advance at cruise and it would result in surging and stumbling. Disconnect and plug the vacuum advance and take it for a drive to see if it helps. You could check your gross advance, total plus vacuum with a timing light by leaving the vacuum connected and reving it up. If it's much over 50º, you'll probably want a vacuum canister with less advance.

plaidside

Do you have a WCFB or an AFB?

magicv8

If it's the correct Carter for a 300hp then it's not the jets. It may be the accelerator pump - which is adjustable. It may be the vacuum advance. I have an adjustable vacuum advance and it makes stumble removal easy. Accel markets them. Make sure the vacuum advance has a bushing to fill the gap in the breaker plate hole.

wombvette

There are a lot of possibilities but I can think of two things you might want to check. Check to see if you have the proper vacuum advance. The 151 cam requires a very fast acting can. The AFB either carter or Edlebrock has power valves that are prone to stick. They are under the little teardrop sheet metal plates. Take them out and clean with ultrafine sandpaper also make sure that the idle screws are balanced well.

jnf

Gentlemen, thanks for the information. To clarify, this is a 327, 300hp with the cam from the 350hp, AFB, not WFB carb - sorry. Total timing with advance is 58 degrees. I have adjusted idle screws as best as possible. Can I assume that a heavier needle spring would offset a problem with too much advance at cruise? Does the nature of the stumbling - steady stumble at around 1500 rpm in second and third gears - suggest a problem with too little fuel? Would a heavier spring make this problem worse? Also, it was suggested that the 151 cam requires a fast acting advance can. Would tinkering with the springs contradict this? I will clean power valves as suggested. I have already adjusted accelerator pump according to specification. Maybe the idea of an adjustable can is the best way to go if in fact the can is the cause.

jnf

Sorry Duke. Missed your question. Engine is 1965.

SWCDuke

I don't think your problem has anything to do with the ignition curve. The 327/350 was
set up with the same vacuum can as the 365/375 engines - 16 crank degrees at 8".
This will insure that the plunger is pulled all the way at idle so the advance is constant, which
helps idle stability. As far as the centrifugal is concerned you want to bring it in as
quickly as possible, the limiting factor being detonation. Total initial plus centrifugal should
be in the range of 36-38 degrees. Vacuum advance of 16 degrees would give a total
of 52-54 above the end of the centrifugal curve at light load. My '63 340 is set up this
way and I never had any trouble with low load stumbling. It always amazed my how well
the engine ran. Even with the Duntov cam and a 3.08 rear it would pull smoothly from
1000 revs in top gear and never stumbled or hesitated at low revs and load. Idle an low speed/load
operating charactieristics are superior to the FI system IMO. You mentioned your total advance-
intial, centrifugal, plus vacuum is 58 degrees. This seems a bit high, but you can just
back off the initial 6-8 degrees to see if it makes the problem go away. I doubt if it
will, but it's worth checking.


In '63 the 300 w/MT and 340 used the exact same AFB. The calibration was changed a bit in
the next two years, but the '65 3721SB should work with the 151 cam. A lean surge/stumbling condition at steady low load that
you describe could be caused by too low a float level. (The accelerator pump and power valve springs are not an issue under these conditions.) I know you said you set them,
but you might want to double check that they are set IAW the shop manual procedure and
specification. The low load stumbling you describe is in the transition range from the
off idle system to the main jets, and float level is critical to this transition.

The nominal idle mixture setting is 1.5 turns out from the seat, and
I'd recommend an idle speed of 750-800, but if you want to keep it at 900 that's okay.

Play with the idle screws turning each an equal amount out (richer) or in (leaner) until you achieve the
the highest vacuum/idle speed. Unless you live at high altitude you'll probably find
something in the range of 1.5 to 1.75 turns out the best setting.

If the floats check out, I would suspect dirt and/or deposits in the idle/off idle system The idle/off idle system contains several small passages for fuel and air including
the air bleeds in the primary venturi cluster. Something as simple as spraying some
Gumout on these tiny air bleeds might clean then up if the dirt is just on the surface.

When you rebuilt the carb, did you soak everything in carb cleaner to clean out all the dirt
and deposits? I suspect this is a fairly simple problem that is related to some minor
adjustment, and you never want to rule out the possiblity of an assembly error such as leaving out
a small part or gasket.

Duke




Duke

d1_bradley

Not too familiar with the carb, but does it have a "check valve" under the acc pump? Would look like a small ball bearing?

jnf

Duke, so you think I should figure out where these extra parts go? Just kidding, of course. Although I just recently discovered where the o-ring goes on the choke mechanism. I have thought about going in and replacing this as I suppose there could be a small vacumn leak there.

More importantly, given what you have pointed out, I think I will pull the carb apart and soak it again. In retrospect, the small passages could have used a little more attention. In the process, I can re-check the float setting.

The vacumn reading at 750-800 rpm is jumpy which is why I'm at 900. Even then, I've been having a hard time establishing the optimum idle mixture as the vacumn reading seems about the same in a rather broad range of idle screw settings. Hard to tell with the jumpy needle on the guage.

Thanks to you and others for the suggestions from my previous post. I will continue to fiddle and will post any improvements when realized.

jnf

d1_bradley, don't know about check valve. Will take a look.

magicv8

The check valve under the accel pump nozzles is usually a pointed metal piece with a "triangular cross section that vaguely resembles the needle valves. It is referred to as a "ball & weight discharge chech valve" at times - so a ball valve is not impossible. It's purpose is to prevent fuel siphoning at high flow rates. I agree with backing off the initial a few degrees and/or installing a variable vacuum advance.

SWCDuke

A vacuum leak (such as a missing O-ring between the choke housing and main body) could definitely be the culprit. Tearing the carb down and soaking it
in carb cleaner is a good idea too, to make sure all the tiny passages in the idle/off idle system are clean. Be sure to soak the venturi clusters as they contain the main air bleeds.

Once an AFB is clean and set up properly it should provide many years of reliable service. In my experience it's a very reliable (and simple) carb. The first problem that develops is usually (dirt) in the choke system, so removing the choke housing for cleaning every few years is a good preventative maintenance practice, and you can remove and clean the choke housing without removing the carb. Just be sure you install it with a fresh O-ring to prevent a vaccum leak!

Duke