Slight stumble at mid-throttle.

[Tim Ayers]

Hi All:

Trying to figure out a the cause of a slight stumble at mid-throttle. Here are the details.
'28 AV8. 292 c.i. flathead, Potvin 3/8th cam, Edelbrock Slingshot with two 97's. .043" jets and #67 power valve. Fuel pump is a Charlie NY converted early OHV Cadillac that is high volume, but correct 2.75 pressure. Ignition is a Petronix unit.

I had NOS Stellings air cleaners on it. They are the wire mess element type. I thought they may be choking it out, but it does the same with open stacks.

Carbs are adjusted correctly and timing is set correctly as well. When I get on it, between approx. 2,500-3,750 rpm, the motor will slightly stumble and cough like it's lean, but once to the floor corrects itself and run really strong. The car spins the tires in 1st & 2nd at will.

It only does this when getting on it in 2nd gear.

Another detail that I'm not sure could be contributing is I haven't installed the thermostats yet. The engine runs around 160-170 when fully warmed up.

Running straight water, new Walker rad and Skip's pumps.

Looking at plugs, they actually look like it's the mixture is good. I haven't pulled over and pulled the plugs right after it happens. Maybe that should be my next step.

I'm thinking it's not getting enough fuel.

Otherwise, the car runs super strong and idles great.

I have a pair of 48's I was thinking of putting on to see if that helps. I may fool with the advance a little as well.

Any thoughts welcomed.

[34fivewindow]

Tim, i would try .048 jets.

[deuce lover]

Try a vacuum gauge and see what the readings are.

[Charlie ny]

Tim,
Try a little bit of choke and see if that helps. One thing for sure....Strombergs
do not like to be cold.
Since this is above 2500 the power valves may enter into the equation.
Keep us posted.....I know you will !
Charlie ny

[Tim Ayers]

Originally Posted by Charlie ny

Tim,
Try a little bit of choke and see if that helps. One thing for sure....Strombergs
do not like to be cold.
Since this is above 2500 the power valves may enter into the equation.
Keep us posted.....I know you will !
Charlie ny

So besides possible carb adjustments and/or jetting, I'm starting to think it is also cold/engine heat related.

My son and I took it for a long ride. As we went along, it got better. Temp never went over 160 though. I need to get those thermostats in and check it again. Will continue to report progress on figuring this out.

[Tim Ayers]

Sponsored Links (Register now to hide all advertisements)

Update: I installed thermostats and that has helped, but not eliminated it. Motor was only running at 150-160 before. Now, it is closer to 180. After speaking with Uncle Max, going to try bigger jets. I’ll report back once installed.

[Bill OH]

I have a 221 with Max 1 cam, dual 97s with 45 main jets and 71 pv - runs great. elevation is about 600 ft as I suspect you are close to that. I previously had 43 mains and 65 pvs - stumbled. Put in 45 mains and ran 65 pvs - bogged - toooo much fuel. I would suggest 45 mains and 71 pvs, but you have enough displacement for Stromberg 48s with 48 mains - do not know what to tell you with regard to pvs. On my 97s, I cut the pv area by 50% to arrive at the 71s. I found the size of the pv and did some math - r x r x 3.1416 (pie). thus the 71 is half of the area of the 65 - a shot in the dark. It works very well. I am running 160 deg stats.

[Tim Ayers]

Originally Posted by Bill OH

I have a 221 with Max 1 cam, dual 97s with 45 main jets and 71 pv - runs great. elevation is about 600 ft as I suspect you are close to that. I previously had 43 mains and 65 pvs - stumbled. Put in 45 mains and ran 65 pvs - bogged - toooo much fuel. I would suggest 45 mains and 71 pvs, but you have enough displacement for Stromberg 48s with 48 mains - do not know what to tell you with regard to pvs. On my 97s, I cut the pv area by 50% to arrive at the 71s. I found the size of the pv and did some math - r x r x 3.1416 (pie). thus the 71 is half of the area of the 65 - a shot in the dark. It works very well. I am running 160 deg stats.

Excellent, Bill. I'm running 160 NOS Robert Shaw barrel type stats as well. They are in the upper hose near the rad neck.

I have a cut down fan and had to change the generator since mine went bad. I took approx. 2" off each blade. I left the fan off for the shake down run. Temp went close to 200 at idle. I'm surprised that the small fan does such much for cooling.

I have 45 & 46 jets coming. It all part of the process I guess. I also have a pair of 48's with 97 bases and tops that I'm going to send out for rebuilds as well.

[Bill OH]

I see that you have 67 pvs in your 97s. How about putting the 67 pvs in your 48 carbs with 48 mains? Try with the accelerator linkage disconnected and that will tell you if the mains are okay, then connect the accelerator linkage and see what happens at WOT - bogs - too rich or goes like a scalded dog - mixture just right. This how I checked mine and arrived at the conclusion that the 65 pvs were tooooo big. Need a motor up to temp, otherwise with accelerator pumps disconnected the motor will hesitate. Don’t you want tor place the thermostats near the motor so they open sooner?

[Tim Ayers]

Originally Posted by Bill OH

I see that you have 67 pvs in your 97s. How about putting the 67 pvs in your 48 carbs with 48 mains? Try with the accelerator linkage disconnected and that will tell you if the mains are okay, then connect the accelerator linkage and see what happens at WOT - bogs - too rich or goes like a scalded dog - mixture just right. This how I checked mine and arrived at the conclusion that the 65 pvs were tooooo big. Need a motor up to temp, otherwise with accelerator pumps disconnected the motor will hesitate. Don’t you want tor place the thermostats near the motor so they open sooner?

For right now, it's the only place the t-stats fit. They are pretty long. I wanted to use a "modern" stat, but I needed to machine the necks of my heads for them to fit. I didn't realize that until the motor was already assembled.

I need to take the hoses off and will try to get them closer to the heads and report back.

I read that same procedure for checking jets & PV's on Stromberg's site, so thanks for confirming. It seemed a little odd, but the more I think about it, it makes sense.

[Bill OH]

Tim: I will be interested in knowing the carbs, jets and pvs that you settle on and placement of the thermostat. Please report back- it will be appreciated. Bill

[Tim Ayers]

Originally Posted by Bill OH

Tim: I will be interested in knowing the carbs, jets and pvs that you settle on and placement of the thermostat. Please report back- it will be appreciated. Bill

Will do, Bill. For sure.

[Tim Ayers]

Well, found out what was causing the stumble. I ordered new, larger jets. While taking out the old ones, I noticed they were about clogged closed. I'm surprised it ran as well as it did. New .046 (up from .043) and it runs like a new car.

What is odd, the car only has about 300 or so miles since fresh. Not sure what could have clogged when up so quickly. I'm going to add so Seafoam to the tank in case there is more gunk in there somewhere.

I did no other changes. Carbs still have 67 PVs. I am going to get a pair of 48's redone and try them later on.

I've attached two pictures of this old hot rod.

[Bill OH]

Thanks for the report!!! 48 Strombergs will give more air flow over the 97s. I think moving to 48 Strombergs with 67pvs would be a good starting point - your displacement can use 48 Strombergs. My engine is 221 and 97s are big enough. As stated before, I would try 48 mains and 67pvs - see how it responds. I will be interested in the results. I think I would use larger air cleaners - get as much air into the carbs as possible I have Edmonds Aluminum cleaners on my 221. You might want to consider Techron made by Chevron for a cleaner.

Bill

[Tim Ayers]

Originally Posted by Bill OH

Thanks for the report!!! 48 Strombergs will give more air flow over the 97s. I think moving to 48 Strombergs with 67pvs would be a good starting point - your displacement can use 48 Strombergs. My engine is 221 and 97s are big enough. As stated before, I would try 48 mains and 67pvs - see how it responds. I will be interested in the results I think I would use larger air cleaners - get as much air into the carbs as possible I have Edmonds Aluminum cleaners on my 221. You might want to consider Techron made by Chevron for a cleaner.

Bill

Hi Bill:

Thanks for all of your replies. The air cleaners are NOS Stellings with that metal mesh element. It made no different if I ran those or old time "stacks" (which have no element at all).

I'll use Techron. Good suggestion.

[Bill OH]

Tim: I am interested in your results with the 48 Strombergs.

Bill

[petehoovie]

[QUOTE=Tim Ayers - I've attached two pictures of this old hot rod.[/QUOTE]