Bog when Engine is Warm

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I measured the total timing which is 36 degrees...so 20 degree advance (16 initial). Do you have a part number and a recommendation as to what springs I need to use to back this down to maybe a 16-18 degree advance? This is the standard MP distributor. Thanks!

springs will not change the total only the RPM that the 36 total occurs. A heavier spring or springs makes the 20 deg slope to occur (start/end) at a higher RPM at any point in the slope (& less advance=less pinging). (1) see (& post) what springs are in there now (2) with your dialback & tach note the RPM that the adv stop advancing (that's when the slots are maxed). Here's a chart for a start tho the crane springs mentioned are no longer available but Ace hardware springs work just need to experiment with a slightly heavier one till you get just under the pinging point at WOT up thru the gears & the slope is linear whether you change one or both springs

The advance doesn't stop until the engine is in the 2500 RPM range from what I saw. Wouldn't it be better to shorten the slots to just limit the timing advance?

I am not proficient with a welder so doesn't somebody make a plate that does this? I seem to remember somebody had a plate with various slots to limit the advance to 18/16/etc increments. EDIT: This is what I am talking about.

Any ideas? Does it work?

Posted By: GODSCOUNTRY340

Re: Bog when Engine is Warm - 09/06/14 08:25 PM

At idle, your accelerator pump arm should have NO play in it. It must start moving the instant the throttle starts moving. Look down the carb with a flashlight to confirm there is actually fuel being squirted into the carb, you can do this with the engine turned off. That 8.5 power valve may need to be replaced with a 9.5 or even a 10.5. A vacuum gauge will help here or just try the 10.5 to see if it helps. If the squirter is too big it can be confirmed with someone watching the exhaust for a puff of black smoke when you "blip" the throttle.

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/06/14 09:47 PM

Alright. I went and inspected the adjustment on the pump arm and it seemed to be adjusted right where the arm would start to get depressed instantly when moving the throttle.

Now this is something I have always wondered about but the accel pump arm has a little play in it. In other words, it can be pushed down a bit (maybe .030-.040) from its resting position before I 'feel' resistance like its pushing the diaphragm.

I had the throttle arm adjusted so it was holding the accel pump arm just 'south' of that resting position but maybe .020-.030 from where it began to feel resistance. There as no gap at all between the throttle lever arm and the accel pump arm. I went ahead and backed off the adjuster nut 1/2 a turn or so which puts more tension on that arm and brings it where it will start to feel that resistance as soon as the throttle is moved.

Does that sound right? Hopefully this makes sense...

So anybody care to explain why if this was misadjusted, why it would be crisp while the engine was cool and get a bog when warmed up? This particular circuit should operate identically hot or cold, right?

Thanks for the suggestions so far.

Posted By: BSB67

Re: Bog when Engine is Warm - 09/06/14 10:20 PM

Does the carb have a choke? If the choke is actuated before it is warmed up, you cannot compare.

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/06/14 11:44 PM

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Does the carb have a choke? If the choke is actuated before it is warmed up, you cannot compare.

Electric choke. It starts to come off after about 20-30 seconds.

Posted By: YO7_A66

Re: Bog when Engine is Warm - 09/07/14 01:00 AM

Your choke could be richening the idle/transfer circuit when the engine is warming up. Then when the engine is warm and the choke is open, the engine could be seeing a lean spot off idle and during tip in.

I always adjust my pump cam linkage when the engine is idling and with a flash light. I lean over the engine "just enough" to see the front squirter while shining the flash light on that squirter. Then I move the linkage and watch how soon the fuel starts to pump out. If the fuel does not pump out as soon as I move the throttle, then I readjust it and check it again. I move the nut about 1/8 of a turn until I see fuel.

What pump cam color do you have? Do you have a hollow squirter screw with the 42? If not, then you won't see much of a difference in pump shot larger than a 37 squirter.
Note: A 42 is a large squirter. Maybe it is time for a pump cam swap.

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/07/14 04:54 AM

I will have to try your method of adjusting that accel pump arm.

I do have the hollow screw and run a pink cam. I always though the 42 seemed big... but it seemed to be what it wanted....

What else could be wrong that the large pump shot might be compensating for? I had the carb jetted on the dyno with wideband O2 sensors so I am pretty sure the jetting is right...

Any ideas?

Posted By: Cab_Burge

Re: Bog when Engine is Warm - 09/07/14 05:39 AM

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I will have to try your method of adjusting that accel pump arm.

I do have the hollow screw and run a pink cam. I always though the 42 seemed big... but it seemed to be what it wanted....

What else could be wrong that the large pump shot might be compensating for? I had the carb jetted on the dyno with wideband O2 sensors so I am pretty sure the jetting is right...

Any ideas?

Did the dyno have your exhaust system hooked up? Did you dyno tune it with teh air cleaner on and hot air from the radiator blowing over the carb air cleaners? If not you may need to retune it in the car

I get my motors close on the tune up on the the dyno wideban and then finish the tune up in the car, with or without my dual wideban hooked up, the butt dyno can be fairly accurate in the car

Posted By: BSB67

Re: Bog when Engine is Warm - 09/07/14 01:30 PM

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What else could be wrong that the large pump shot might be compensating for?

Really?

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I had the carb jetted on the dyno with wideband O2 sensors so I am pretty sure the jetting is right...

Jetting should be very close and likely not the problem IMO. However, there is no down side to stepping up the jets in the primary side at some point just to see, but I don't think you are there yet. Curious, do you know what changes they made in jetting at the dyno?

Is this the 870 Avenger carb?

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/07/14 02:11 PM

The dyno shop jetted it up a few sizes to 82 in front 88 rear. I don't remember what it was but I do recall it was lean before.

The carb is a Holley 4150 850cfm. It is not the 870 Avenger.

One of the issues I ran into with the carb was it had a really weird tune on it from the factory. Holley recommended it for my motor so I bought it. However, this carburetor is the same unit they put on the Chevy 502 crate motors so it wasn't configured like your standard Holley. Just weird things like 3.5PVs, super duper light vac secondary spring, etc...

Posted By: YO7_A66

Re: Bog when Engine is Warm - 09/07/14 02:40 PM

The PINK cam is the ""least"" aggressive off idle of the entire selection of pump cams!!!!!

I would suggest a white or red (very similar to each other and twice as aggressive off idle than the pink cam.) cam on position #1. If that helps but not all the way, then I would suggest the Orange pump cam which is more aggressive than the white or red cams and it also has more overall pump volume (which you may or may not need), But for testing purposes, I would leave the 42 in there and just swap out pump cams for now.
Note: you will need to shorten the pump arm screw to install the larger pump cams. Maybe 1/2 turn before insalling the white or red cam and then once the cam is in, readjust.

Posted By: BSB67

Re: Bog when Engine is Warm - 09/07/14 04:02 PM

Do you know how many turns out the idle mixture screws are?, and what the A/F ratio is at idle(from the dyno session)?

Better yet, do you have the A/F graphs?

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/07/14 05:17 PM

Idle mix screws are about 1.25 - 1.5 turns out from bottom. I have the graph but it only shows 3500 RPM+ so it might not be that useful here.

Another thing we may be grappling with is the tune was done with a marginal fuel pump which failed totally shortly afterwards. I also later found the ignition as marginal and had to go to a lower ballast resistor to get the coil output voltage back up. At the time of tune, the output was in the 10-15kV range...new ballast...in the 45kV range. No idea if this will affect anything here or not.

I have thought about experimenting with the cams but I didn't know enough to make an educated guess as to which one to try. I am appreciative of the specific suggestion on which color to try. My friend has the set of cams so I will see if I can steal his set to play with...

Thanks for the help so far....

Posted By: Supercuda

Re: Bog when Engine is Warm - 09/07/14 05:31 PM

Problem with most dyno tunes is that the tune WOT. Transition and driveability are not addressed other than to make sure it isn't totally horrible.

Your best best bet is to install a wideband O2 and data logger to tune it right. Otherwise your sorta guessing what needs to be done.

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/13/14 08:51 PM

I plan on breaking into this soon when I have some spare time. In the meanwhile, what do you guys run as far as cam/nozzle size on your BB strokers?

Mine is a 400>>499 relatively mild as far as cam goes. What size nozzle would you guys 'expect' to see on this kind of motor? I know a lot goes into what it will end up taking but is a 42 'off the charts'?

The only thing I have to compare to is my friend's SBC 350 which takes a 32 IIRC but much smaller motor....

Posted By: davenc

Re: Bog when Engine is Warm - 09/14/14 05:17 AM

On a dual-plane manifold, a 042 shooter seems quite large. As others have mentioned, the pink cam may not be the best for a street car since it is lazy in lift right off idle. I would suggest changing the pump cam first, and then come back down on the shooter. Every setup is slightly different. Try an orange cam to start. I suspect the shooter can come down into the range of 031 with the right cam but change one thing at a time. In my opinion, the cam is the more important of the two on a street car.

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/14/14 05:20 PM

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On a dual-plane manifold, a 042 shooter seems quite large. As others have mentioned, the pink cam may not be the best for a street car since it is lazy in lift right off idle. I would suggest changing the pump cam first, and then come back down on the shooter. Every setup is slightly different. Try an orange cam to start. I suspect the shooter can come down into the range of 031 with the right cam but change one thing at a time. In my opinion, the cam is the more important of the two on a street car.

FWIW I have the MP single plane manifold, not a dual plane.

I played around a little today with the car (more trying to test for vapor lock) but got it good and hot and tried to note this issue. It seems the bog only occurs if the throttle is blipped pretty fast and over 50% or so. Just giving it some moderate pedal or a blip under half throttle seems fine...no bog.

If the bog occurs at quick throttle opening over 50%, this means I need a cam that gives more fuel in that range (>50%)?

Either way, I will get my buddy's set of cams and try the red or orange and see what happens. I have a 32/38/42 squirters here I can try later if that helps the problem.

Thanks so much guys!

Posted By: davenc

Re: Bog when Engine is Warm - 09/15/14 12:25 AM

Was the "blip" with no load on the motor, or while the vehicle is in motion? If it stumbles or bogs with no load, then the setup is farther off.

You are correct about perhaps needing more cam lift above half-throttle. Note though that a smaller squirter will extend the duration of the pump shot. I still feel it is best to get the right cam first, and then fine tune with the squirter.

The chart you posted is helpful but does not show the effect of the two cam positions. If your motor is fine just off idle, you can try your current cam in the other position so that the lift occurs at higher throttle position degrees.

Posted By: cjskotni

Re: Bog when Engine is Warm - 09/15/14 12:45 AM

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Was the "blip" with no load on the motor, or while the vehicle is in motion?

I did the blip with the car rolling and/or at a standstill in gear so under a load. The engine doesn't stumble at all when it's out of gear so not that far off!

Posted By: cjskotni

Re: Bog when Engine is Warm - 02/23/15 10:05 PM

Ok guys,

I know it's been awhile since I posted this but I finally have the restoration part of the car done so now I have more time to circle back on these little mechanical gremlins.

I got the Holley tuning kit and swapped in the red cam (from the pink there) and went to a 37 squirter (from 42).

I had to shorten up the accel pump lever adjustment screw a lot...like 7-8 turns to get it where it was just resting on the pump lever with the throttle closed. I did the 15 thousandths feeler gauge as well on open throttle to make sure it didn't over extend the arm and all is good there.

Whoa what a difference! I scared myself a time or two when I blipped the throttle. Now I can break the tires loose at 40 mph when I blip it!

Of course now I notice that when I have it under WOT for more than 2-3 seconds, it is pinging a little bit so I guess it's time to roll the timing back a degree or two. Man that stroker can pull now!

Thanks guys!

Posted By: RapidRobert

Re: Bog when Engine is Warm - 02/23/15 10:45 PM

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what a difference! I scared myself a time or two when I blipped the throttle. Now I can break the tires loose at 40 mph when I blip it!

Of course now I notice that when I have it under WOT for more than 2-3 seconds, it is pinging a little bit so I guess it's time to roll the timing back a degree or two. Man that stroker can pull now!

Thanks guys!

You sound pumped! On the timing you might go to a slightly heavier spring for either of the two springs in there (I'm assuming 2 light ones) until the pinging stops under WOT, since if your initial and total are correct as is you would not want to compromise them for the pinging

Posted By: RapidRobert

Re: Bog when Engine is Warm - 03/08/15 05:49 PM

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Wouldn't it be better to shorten the slots to just limit the timing advance? Any ideas? Does it work?

(1) no (2) of course! (3) yes I hear the FBO plate works OK but 36 total is fine but it coming in too fast. 35 is a tried & true total for a std sb but strokers do have alightly different tiotal requirements but 36 ain't a dealbreaker. I am a fan of experimentation & wouldn't see an issue with trying the plate and or going to a heavier spring & seeing which one of the two works the best for your app. I'm sure the plate is cheap. EDIT Do you have a BB or SB? If BB I would stay at 36 & go to a heavier spring

Posted By: cjskotni

Re: Bog when Engine is Warm - 03/08/15 07:46 PM

It's a B stroker 400>>499.

Posted By: Cab_Burge

Re: Bog when Engine is Warm - 03/08/15 08:12 PM

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I had to shorten up the accel pump lever adjustment screw a lot...like 7-8 turns to get it where it was just resting on the pump lever with the throttle closed. I did the 15 thousandths feeler gauge as well on open throttle to make sure it didn't over extend the arm and all is good there.

Whoa what a difference! I scared myself a time or two when I blipped the throttle. Now I can break the tires loose at 40 mph when I blip it!

Of course now I notice that when I have it under WOT for more than 2-3 seconds, it is pinging a little bit so I guess it's time to roll the timing back a degree or two. Man that stroker can pull now!

Thanks guys!

Two suggestions, try a 32 or 35 squirter next,get the smalest one in it that won't bog with the accelerator pump arm set up properly

At the same time back the timing up to 34 degrees total with the motor at or above 4000 RPM

Don't change the amount of mechanical advance

, just the initial

If it still pings try a colder plug by one heat range lower

BTW, what brand and heat range, plug number, are you running now? Now that you have opened the door to more power we need to maximise it so you can really get pump up, scare the poop out of you

Mopowered

Posted By: cjskotni

Re: Bog when Engine is Warm - 03/08/15 08:22 PM

With the dizzy advancing 20*, this would mean only 14* of intial timing. On the dyno, we determined that the engine made the most power with 18* initial but of course pinged when you opened it up.

I don't think rolling back to 14* is enough timing?

I am running Champion RC12YC.

EDIT: I am going to try Autolite 3923 which is one heat step down from these Champions. I am guessing with the 10.4:1 compression this might not be a bad idea.

Posted By: RapidRobert

Re: Bog when Engine is Warm - 03/08/15 10:16 PM

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I am guessing with the 10.4:1 compression this might not be a bad idea.

You need more octane, if you build a monster you gotta feed it want it wants (needs). You're timing numbers ain't out of line but your SCR (& therefore DCR) is too high for your (current) choice of fuel.