Stumble off idle -

[Grubby]

I just spent two hours of quality time with my Cobra.

Removed carb.
1. No indication of vaccum leak
2. Transfer slots were at about .028. So I backed off the idle screw to get them to .015.
3. The clearance under the accelerator pump bolt is .015
4. Confirmed float levels. Fuel just sloshes out when the car is shaking at idle.
5. Set the idle screws at 1.5 turns out.

Then I fired it up and warmed the engine.

Adjusted idle mixture screws with my vac gauge. Best vac was about 7 at 550 rpm idle. Screws are pretty close to 1 turn out. My vac gauge is old and beat up - it may not be accurate.

I do like the idle somewhat faster. Another 150 rpm would be nice, but that would open the transfer slots further.

The stumble was immediately off idle. That has moved. Off idle is now crisp.

Now if cruising at 1500 - 3000rpm and you get into the throttle pretty hard it stumbles. Moderate or less throttle and it pulls just fine. The stumble is a single, quick loss of power then it goes. The stumble decreases as you approach 3000 rpm. It is most noticeable at 1500.

I could not see any sign of black smoke under hard throttle. Primary jetting seems to be dead on. The side pipes and spark plugs are both a deep tan.

What are your suggestions? This does not seem to be the idle circuit.

John

[patrickt]

Try putting a stock, unmolested, 31 nozzle back in before you do anything else. BTW, I idle at 750 with just under 13" of vacuum. Before you get side tracked with stuff like when your power valve opens, etc. I would first go back to the original nozzle.

[blykins]

Originally Posted by Grubby

I just spent two hours of quality time with my Cobra.

Removed carb.
1. No indication of vaccum leak
2. Transfer slots were at about .028. So I backed off the idle screw to get them to .015.
3. The clearance under the accelerator pump bolt is .015
4. Confirmed float levels. Fuel just sloshes out when the car is shaking at idle.
5. Set the idle screws at 1.5 turns out.

Then I fired it up and warmed the engine.

Adjusted idle mixture screws with my vac gauge. Best vac was about 7 at 550 rpm idle. Screws are pretty close to 1 turn out. My vac gauge is old and beat up - it may not be accurate.

I do like the idle somewhat faster. Another 150 rpm would be nice, but that would open the transfer slots further.

The stumble was immediately off idle. That has moved. Off idle is now crisp.

Now if cruising at 1500 - 3000rpm and you get into the throttle pretty hard it stumbles. Moderate or less throttle and it pulls just fine. The stumble is a single, quick loss of power then it goes. The stumble decreases as you approach 3000 rpm. It is most noticeable at 1500.

I could not see any sign of black smoke under hard throttle. Primary jetting seems to be dead on. The side pipes and spark plugs are both a deep tan.

What are your suggestions? This does not seem to be the idle circuit.

John

Stumbles are lean conditions. I would add a bigger pump shot and also think about sucking the secondaries open slower.

If you want a higher idle, you can do it a couple of different ways. You can add a little more initial timing, which raises the idle speed. You can also richen the idle mixture a tad and jump it up.

[Gaz64]

Originally Posted by Grubby

I just spent two hours of quality time with my Cobra.

Removed carb.
1. No indication of vaccum leak
2. Transfer slots were at about .028. So I backed off the idle screw to get them to .015.
3. The clearance under the accelerator pump bolt is .015
4. Confirmed float levels. Fuel just sloshes out when the car is shaking at idle.
5. Set the idle screws at 1.5 turns out.

Then I fired it up and warmed the engine.

Adjusted idle mixture screws with my vac gauge. Best vac was about 7 at 550 rpm idle. Screws are pretty close to 1 turn out. My vac gauge is old and beat up - it may not be accurate.

I do like the idle somewhat faster. Another 150 rpm would be nice, but that would open the transfer slots further.

The stumble was immediately off idle. That has moved. Off idle is now crisp.

Now if cruising at 1500 - 3000rpm and you get into the throttle pretty hard it stumbles. Moderate or less throttle and it pulls just fine. The stumble is a single, quick loss of power then it goes. The stumble decreases as you approach 3000 rpm. It is most noticeable at 1500.

I could not see any sign of black smoke under hard throttle. Primary jetting seems to be dead on. The side pipes and spark plugs are both a deep tan.

What are your suggestions? This does not seem to be the idle circuit.

John

John,

In your point 3 above, " The clearance under the accelerator pump bolt is .015", please say you understand the .015 is NOT under the bolt as some think.
The .015 is ADDITIONAL travel of the pump lever measured at WOT. It ensures the pump diaphragm won't bottom out at WOT.

If you have ANY slack/clearance in the pump mechanism, you'll have a flatspot off idle.

[RET_COP]

Good point Gary, you should squirt gas at the slightest throttle movement. Bring the rpm's up in the 750 range, at least for better oiling. Do this by using the mostly primary throttle because you said you had .015 slot showing, more might help to get the transition from idle to main circuit. .015 t-slot showings under the secondaries is plenty. If you have to go more than an eighth of a turn on pri. to get 750-800 rpm, Stop, use a little secondary. "Balance"
Lou

[patrickt]

Originally Posted by Grubby

Best vac was about 7 at 550 rpm idle. Screws are pretty close to 1 turn out. My vac gauge is old and beat up - it may not be accurate.

John, I think you should really double check this with a new vacuum gauge before you start doing any more monkeying with your carb. With your build, if you're down in the single digits with your vacuum, at a normal idle, then I'm going to call "foul" and point to something like an intake manifold leak. You said you double checked your timing and adjusted your valves, those are two other typical causes for low vacuum.... Brent & Bobcat, you don't think his vac is a little low?

[Bobcat]

Patrick .... depending on the cam and idle speed , 7" of vacuum may not be too low . My 482 has a long cam ( 250 @ .050" ) and has about 7" to 8" at 1000 rpm . Having said that , if the vacuum is really that low , then the PV should be in the 3.5 " range ..... and a check for vacuum leaks should definitely be done .
Grubby , you said the timing was ok , but what is your initial and max ? I run 22 degrees initial and 35 degrees max . Ignition/timing should always be checked out before working on the carb .... eliminates one variable . Most carb/ignition people recommend something in the 14 to 18 degree range for initial advance .
550 rpm , for me , is way too low an idle ... I like between 900 to 1000 on my engine .
BTW , as Brent and others have pointed out .... only change one thing at a time or you may have one change cancel out the other . Yes , it`s time consuming , but it`s the best way .

[patrickt]

Originally Posted by Bobcat

Patrick .... depending on the cam and idle speed , 7" of vacuum may not be too low . My 482 has a long cam ( 250 @ .050" ) and has about 7" to 8" at 1000 rpm . Having said that , if the vacuum is really that low , then the PV should be in the 3.5 " range ..... and a check for vacuum leaks should definitely be done .

I vaguely remember his build specs -- and his cam is smaller than mine (mine is 245 @ .050, but with a 114 LSA) and I'm over 12" at idle. If he really is at 7", and I bet he still has the stock PV of 6.5, then it might be easing open and causing problemos. Brent's just spending too much time with that new bratty bundle of joy... he should be on here chasing the important things in life, like carbs.