Working with dual 97's

gmanrides

gmanrides

Senior Member
I have been messing with a dual 97 set up for a 59AB. The engine is .030 over and I was told it has a 400JR cam. The carbs have .43 main jets and 69 PV's. When cold I hit the throttle one time and pull the choke (both are hooked up) and it will fire, just a little on the cold nature side. Once it warms up you can just hit the start and it will fire right up. I did the uni-sync on the carbs, adjusted idle jets to 3/4 out from seat. On initial throttle it acts lean and feels like a flat spot but once you crack it a few times the little flatty will rock. I installed new plugs NGK B4L to gauge how it is burning. On my road test about 12 miles it pulled good up hills, no sputtering or black smoke and no down shift required. However I feel that flat spot on initial throttle at take off but once you are rolling you can throttle it and the response is quick and sharp to the point it will scream. I pulled the plugs and they look just like I put them in no change in color but no soot either. I am just concerned about a lean condition. The 2 bolt crab is advanced to the last mark anything less and the engine response seems late. It idles best 700-800 rpm's. Any ideas or thoughts appreciated. Thanks Gary
 
I would say try .045, stock jets. Try the stock power valve also.
Stumbling as yours does is SOMETIMES an indication of a lean condition.
 
I agree it does seem a bit lean. I use a vacuum gauge to set the idle mixture. My screws are about 1&1/2 turn out. I believe the 3/4 turn setting is the basic setting for one Carb. Have about 15" of vacuum. After milling heads I had to go to .044 mains, I still have the 69 PVs.
 
Thanks guys, I'll try the idle screws first. The stock .045's with the stock 65 PV was too rich. When I throttled it black smoke puffed from the headers and carboned up the plugs on a short run. I can say I never felt the flat spot and it ran really well but definitely rich. I have plenty of jets (.041-.045) and PV's (65,67,69 and 71) to experiment with. Thanks again Gary
 
It can also be related to a temporary lean condition - due to the accel pump travel and when it hits the power valve. Have you tried the two different accel pump rod locations - to see if you see any difference? I've had situations where an engine ran quite differently when I used one or the other accel rod balls on the throttle shaft linkage.

I will say, that it is hard to read plugs with a lot of todays gas mixtures - just isn't like it used to be and frequently the porcelain will just stay pretty dang white. I'm going to put an AFR sensor bung into my headers - so I can get some better data as to what the carbs are really doing.
 
B&S, I have not swapped the rod, it is on the summer setting, I know the winter setting should richen it up and I think it lengthens the stroke of the pump. Could be worth a try and easy enough to do. Thanks Gary
 
The two balls on the end of the throttleshaft - "S" and "W" - determine when the powervalve comes to the party and how much beer to bring. More beer, quicker in the Winter...
Also, if you hammer it, the powervalve will start to open due to the hydraulic action of the accelerator pump and become fully open when the pin's depressed at the end of the stroke.
If you look closely, there's a very small checkvalve built in to the bottom of the accelerator pump; this works in concert with the main checkvalve for smooth operation. Especially when the pedal is pressed with less violence.
 
Terry, this particular intake has no vacuum port. There are two threaded pipe ports on the left side of mounting base that go down to the lifter valley. Thanks Gary
 
No vacuum port?? What distributor are you using? The carb style needs manifold vacuum for the advance vacuum port. It's not a big deal to add one just have to drill one of the intake runners, if necessary.
 
Yes sir, no vacuum port. I am running the 2 bolt crab. From what I have read there are a lot of folks who run it without vacuum. My understanding is it is a vacuum brake to prevent detonation/pinging which I have not experienced. If my interpretation is wrong I am always open to suggestions. Thanks Gary
 
Yes there are a lot of people doing a lot of things with their cars. Suggest you look into how the brake works mechanically. Back in 46-8 fuel formulas were not what they are today. Unless your nearing full race there is no need for high octane gas. Most of the shops that set the old distributors leave the brake setting about 1-1/2 to 2 turns in from full extended (Min. brake effect). With manifold vacuum high at idle and slightly off idle the brake should be nearly off the advance plate and you should get mechanical advance quickly. Without vacuum that is not going to happen (Max. brake effect). Send an PM to Bubba and ask his suggestion.
 
Thanks Terry, I have a spare crab that I need to send Bubba to go through. Just need to get it boxed and shipped. I'll ask him about it.
 
Well I did not get as much garage time as I was hoping for today but I was able to play with the flathead this afternoon. First off thanks for everyone's suggestions. I re-adjusted the idle jets with some improvement. I then relocated the pump rod and shazaam! I took my son along on a test run and set him back in the seat a few times. The only problem we had was when a rooster darted out in the road. I locked her up and still hit it with the right rear.
Terry I did play with the crab also with a vacuum pump and really couldn't see a difference. With that being said I am still going to drill and tap the intake for a port so everything is as it should be.
 
Good news - we always like to hear when something this motley crew pondered . . . actually made a positive difference. Would like to know what position you had the accel pump rod on before - and where it is now. My guess is that it is putting more fuel in - earlier.

The reason I'm asking is that I'd like to think about whether or not it was getting too much fuel and bogging . . . versus too little (going lean) and having to wait for the engine and associated carb flow to pick-up. In all my years messing with performance engines, I've found the most common problem is going lean (not enough accel pump squirt/volume at the right time). On modern engines and Holley 4-barrels, have spent a lot of time messing with accel pump nozzles, cams, pump-sizes, etc..

Thanks!
B&S
 
I moved the rods from S (Summer) to W (Winter) even though we generally do not have much of a winter. LOL, It needed more juice, I still need to take it for a longer drive to see how the plugs are burning. Not sure if I'll get a chance tomorrow, guess that depends on how long I stay at the swap meet in Nashville.
 
Update: First of all I would like to thank everyone for all the input I received. I had an extra crab that I sent off to Bubba and as everyone knows he does a fantastic job. Quick turn around too! I installed the crab and vacuum port for the vacuum brake. She fires right up, throttle response is quick and I couldn't be happier. Even though the other crab ran well it had to be advanced two marks to run well. The one Bubba did was dead nuts on I did not advance it at all. Now I am wondering if I really need to swap that 2.73 behind the T5? I might tear up something! :)
 
Thanks for circling back and updating us - nice to know that the engine is running well and you're a happy camper! Have Fun . . .

B&S
 
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