Stromberg 81 Jets?

J

jeepguy1948

Well-known member
If anyone here is using a Stromberg 81? If so, I would like to know what jets and power valve you are using. I am currently using the “stock” jets (what was used on the V8 60) which are .035 Jets and a 71 PV. I feel that it is running lean and I don’t want to burn a valve or do other damage. I will be experimenting but would like to know what others are doing. The motor is otherwise stock except for a 5.5:1 head.
 
I tried an 81 in the 1990's and felt it wasn't strong enough to get me what I wanted out of my engine. I switched to a 97 and found the CFM was better suited to the large bore Model A engine. There are, of course, variables that may affect whether an 81 will work for you: cam profile, type of downdraft manifold, cylinder head type (5.5 in your case), size of the cylinder overbore up to 0.125". I found with the 81 I didn't need an electric fuel pump, but I did with the 97. The late 1931's with their direct fuel path to the carb seem less needy of a fuel pump than the earlier Model A's with the on/off valve beneath the gas tank.
Marshall
 
Marshall, the math says that the 81 is sufficient but barely. I want to get the car running and driving good with the 81 and try a 97 down the road (I have several on the shelf). At 200 CID the banger is a good sized motor but it operates at a very low RPM so that skews the calculations.
 
What do the plugs say? You should be OK even the smallest flathead was bigger than the A and operated at a higher RPM.
 
The V8-60 is 136 cubic inches. My question is not is the 81 big enough, the question is what jets others are using. The plugs suggest lean to me but I’m no expert. I doubt that I am the only person on this forum using an 81.
 
In Jim Brierley's book he says he uses 81s with the stock jetting - #35 main and 71 power valve - and he says it runs fine. If you have it on the S setting, try switching to W.
 
alexiskai is right. I have done some dyno work on an engine that I ran in my speedster. With two 81's and one 97 it put out 107 HP, with just two 81's it dropped to 106 HP, not much, with just one 97 it dropped to 100 HP. A surprisingly small amount of drop although on a hill climb it was quite noticeable. All carbs had stock jets, which I found to be the best. In your case I would first check the fuel level in the carb, if it is correct I would change only the main jets to .037's, leave the power valve as is, it only comes into play at wide open throttle.
 
Alex I have Jim’s book but must confess that I forgot that that information is in there.
I think that the 81 should provide better drivability (I won’t be doing any hill climbs) than a 97 but I have a 97 on the shelf to try out down the road.
 
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