Strange little stumble in BBC

**N2GLOCK** · 07-06-2011, 09:32 PM

So my bbc has a strange little stumble that I'm having issues diagnosing. Base timing is set at 14 and total advance is 35 The stumble is present when I first take off and again just as the secondaries open. The carb is a Holley 750DP. I first started out with 74/80 jets and a #55 power valve. Now I'm at 77/81 jets and a #65 power valve. The problem is still there but it's about 75% better. I pulled some plugs today and it looks like the motor might be running a bit on the lean side. The plug color is like a light gray and nowhere near the white when I first started running the boat. The ignition system is all new and fuel pressure is within specs. Any ideas? I was thinking of maybe going back down to a #55 power valve and increasing the primary jets by one. Also, the timing at 14 has me kinda nervous since factory spec calls for a base of 10 If I bring the timing down from 14 to 12 my total advance ends up at 33 which is good but then the stumble off idle is more noticeable. I know that I should probably send the carb to get dialed in by an expert, but w/ a baby on the way I'm on a tighter budget. My current setup is as follows: Close to stock cam (I verified this by taking vacuum readings) square port cast heads, Victor Jr intake, Holley 750DP w/ shaved choke horn. The ignition system is dialed in. I completely rebuilt the dizzy and the external coil and MSD box are not that old.

**gn7** · 07-06-2011, 09:40 PM

double check you float levels

Is this a jet or what?
What ignition system is it.

**lbhsbz** · 07-06-2011, 09:49 PM

Sounds like the throttles start opening before the pumps start squirting. Pumps should squirt at the first sign of throttle movement.

**N2GLOCK** · 07-06-2011, 09:53 PM

Originally Posted by gn7

double check you float levels

Is this a jet or what?
What ignition system is it.

Triple checked the float levels. This is a jet boat. Dizzy is an MSD billett w/ external MSD coil. Taylor wires, 6AL box and R44T spark plugs.

**gn7** · 07-06-2011, 09:56 PM

I don't think thats his problem considering he can make better with some jetting, and it gets worse when backs down the timing.
The open plenu Victor Jr with a mild little cam isn't helping here.
I see now that it probably has a stock points dizzy with a MSD box.
I would lock out the advance and set the timing a 35* or at the very least, get the weakest springs I could get my hands on and install them. Locking out the dizzy would be easier and better

**lbhsbz** · 07-06-2011, 10:02 PM

Ok, why would the adv. Curve cause a stumble when it comes onto the secondaries? It wouldn't. It's going lean in the transitions....

**N2GLOCK** · 07-06-2011, 10:04 PM

Originally Posted by gn7

I don't think thats his problem considering he can make better with some jetting, and it gets worse when backs down the timing.
The open plenu Victor Jr with a mild little cam isn't helping here.
I see now that it probably has a stock points dizzy with a MSD box.
I would lock out the advance and set the timing a 35* or at the very least, get the weakest springs I could get my hands on and install them. Locking out the dizzy would be easier and better

I'm not running points nor would I ever do so. When I first got the boat the dizzy was full of rust. I tore it down and rebuilt it with all new parts from MSD. I installed the new p/u coil according to MSD's instructions to the "T" and I did install the softest springs in the kit. I'm all for locking out the advance but won't I get a hard start condition when the engine is hot?

**bville** · 07-06-2011, 11:11 PM

Check your accelerator diaphragm for holes and make sure its adjusted right.If it is you can try going the next size larger on the squirters.

**gn7** · 07-06-2011, 11:17 PM

Originally Posted by lbhsbz

Ok, why would the adv. Curve cause a stumble when it comes onto the secondaries? It wouldn't. It's going lean in the transitions....

I am willing to bet the timing curve is slower than cold molasses. It may not be only the timing curve. But with that manifold and cam arrangement, I would be giving it all the timing I could as fast as I could. Throw in the fact that the compression is probably down around 8.5, and this thing could use some timing. I would eliminate that and THEN start trying to find out if it needs anything else.

N2GLOCK, I don't think you'll have any trouble starting it. Good starter, and batery it will start right up.
Your engine will love you for it.

Double check you accelerater pumps and make sure they are working correctly and there is no gap between the accel's and the linkage lever

**Hass828** · 07-07-2011, 06:55 AM

Originally Posted by gn7

I am willing to bet the timing curve is slower than cold molasses. It may not be only the timing curve. But with that manifold and cam arrangement, I would be giving it all the timing I could as fast as I could. Throw in the fact that the compression is probably down around 8.5, and this thing could use some timing. I would eliminate that and THEN start trying to find out if it needs anything else.

N2GLOCK, I don't think you'll have any trouble starting it. Good starter, and batery it will start right up.
Your engine will love you for it.

Double check you accelerater pumps and make sure they are working correctly and there is no gap between the accel's and the linkage lever

X2

**FastRat** · 07-07-2011, 01:03 PM

Originally Posted by gn7

Double check you accelerater pumps and make sure they are working correctly and there is no gap between the accel's and the linkage lever

X2...it will stumble, if there not set rite!!

...just sayin' my .02 cents

**N2GLOCK** · 07-07-2011, 03:07 PM

Today I gave the carb a thorough once over and everything looks good. I checked the fuel for water and there was none. The crank pulley has not walked either. I had one of my techs here at work check to see if the squirters were working at the proper time and he said that it looked normal. When I changed the power valve, I gave the local carb shop my vacuum readings at idle and 2500 and according to him he gave me the correct power valve. I see that a couple of members mentioned that I'm probably having issues due to my current cam/intake combo. When I first bought the boat, I had a sneaking suspicion that the intake was probably gonna be an overkill for the cam that is currently in the motor. I'm fairly confident that I have the right advance spring/stop bushing combination in the dizzy so I'll leave that alone at this time. I've checked the advance w/ a light and it's doin it's job pretty quickly. Either today or tomorrow I'll increase the primary jet size by one and I may switch out the #65 pv to a #55 again. At this point, I probably shouldn't increase the base timing since my total advance is already on the high side of the spectrum. My quest for some more hp will lead me to the land of large camshafts this coming winter. Therefore, I'll be sure to get a cam that's gonna feel right at home with that intake. I'd be willing to experiment with locking out the dizzy as long as I can return it back to the way it was in case I don't like it. If anyone cares to explain how this is done, I'm all ears. Thanks to everyone who chimed in.

**gn7** · 07-07-2011, 03:29 PM

You can always lock out the dizzy and keep the same total. Maybe even get away with a touch less. Also, if it really bothers you to lock it out, you can go to the weakest springs, and the shortest bushing that allows the least amount ot advance. then set the timing for max advance, and the intial falls where it falls. So it might be 35 total, and 20 intial.
The manifold has some serious plenum size for the combo, compared to a dual plane. If you don't want to try the timing thing, maybe a change in the accel pumps if the problem is still there after this next test.

If the stumble is right off when you hit the throttle, try a set of shooters .002 larger. If the stumble is a little delayed after you hit the throttle, and differnt cam with more fuel shot.

**ol guy** · 07-07-2011, 03:49 PM

Originally Posted by N2GLOCK

Today I gave the carb a thorough once over and everything looks good. I checked the fuel for water and there was none. The crank pulley has not walked either. I had one of my techs here at work check to see if the squirters were working at the proper time and he said that it looked normal. When I changed the power valve, I gave the local carb shop my vacuum readings at idle and 2500 and according to him he gave me the correct power valve. I see that a couple of members mentioned that I'm probably having issues due to my current cam/intake combo. When I first bought the boat, I had a sneaking suspicion that the intake was probably gonna be an overkill for the cam that is currently in the motor. I'm fairly confident that I have the right advance spring/stop bushing combination in the dizzy so I'll leave that alone at this time. I've checked the advance w/ a light and it's doin it's job pretty quickly. Either today or tomorrow I'll increase the primary jet size by one and I may switch out the #65 pv to a #55 again. At this point, I probably shouldn't increase the base timing since my total advance is already on the high side of the spectrum. My quest for some more hp will lead me to the land of large camshafts this coming winter. Therefore, I'll be sure to get a cam that's gonna feel right at home with that intake. I'd be willing to experiment with locking out the dizzy as long as I can return it back to the way it was in case I don't like it. If anyone cares to explain how this is done, I'm all ears. Thanks to everyone who chimed in

Being that this a double pumper, my first question would be what size squirters? In your'e first post you stated it stumbled going into the secondaries. That would tell you it is not making a smooth transition from primary to secondary butterflies. simple fix is to add a better shot of feul to eliminate the lean out. You can tune around the open manifold with a tune on the carb to assist the transition. M

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Strange little stumble in BBC

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