TheSamba.com :: View topic - Again questions about 40hp switching to big bore 1385cc

Sun Mar 16, 2025 4:01 am

Thinking about upgrade a stock 40hp engine from 1961 with a set 1385 cc big bore from AA, I studied all the relevant threads I could find on thesamba.
But there are still questions remaining, so I hope to find some answers here.
The bore ist the only thing I want to change, along with reworked stock cylinder heads fitted with short studs.

1) in result, will the engine work louder than the stock 40hp?
2) would I need to shim the cylinders to correct the compression, or should I be happy about the higher compression? Will the engine block hold it without case saver?
3) what would you recommend for jetting the stock 28 PICT (without index)?
This question was covered several times and of course, there was no definitive answer as all the engines have different set ups in detail. Of course, there is no way around visting a dyno test and do a proper tuning. I would like to handle this question more theoretically, because I need to buy some jets for preparing the tuning need a tip in what direction I might need them.
The answers I found are going in two directions:

- Glutamodo always states he had to use a smaller main jet in context with the altitude where he lives.
- All other people take one or two number higher

The following chart (comparison between different engines with the same 28 pict carb) leads me to think of using smaller jets instead of larger ones.

The jetting for the 1500 is too much of course. The 1385 keeps the smaller stroke. As assumption, I might need a 120 or even 117,5 main jet, enlarging the air correction, g50 pilot jet (together with a custom made pilot air jet as there isn't one to buy), a smaller jet for the power fuel system and increasing the amount of acceleration squirt (what was changed by factory at later carb models and is easy to do).
I wonder, why most people like to take just a larger main jet.

Sun Mar 16, 2025 8:17 am

Moved here for better responses.

Sun Mar 16, 2025 8:20 am

louder, probably not, compression ratio needs to be measured at time of build by measuring head volume and calculation, this should be done on any engine. you select ratio in part based on grade gas you will use. you can tune the jets without need for a dyno,

just go for it, set the ratio for your gas grade, break in the motor with jets you think are good, even stock, they try other jets once broke in to see what you like do not over think it

Sun Mar 16, 2025 10:26 am

Opinions vary, but frankly, if you retain everything else stock there is no idea in bumping the displacement to 1385 cc unless you want more torque for lumping around town in 4rth gear at 1500 rpm. If you want to exploit the extra displacement you need to make the engine able to breathe.
For instance you can get the intake cut so that it will accept a 13/1500 intake manifold and bump the carb to a 30 mm.
Or, go with a dual carb system like the old Riechert or Okrasa.

Sun Mar 16, 2025 1:05 pm

Based on a 1385 I built a couple years ago:
1) Not louder. In fact, the one complaint from the owner was it was too quiet with the stock muffler tips.
2) Shimming may or may not be needed, depends on your desired setup. I ended up with 8.0:1 compression ratio using a 0.040" deck, stock cam, and stock round boss heads. No issues running cheap 87 octane gas here in West Texas. Reused the original case with no case savers, no signs of stud pulling.
3) I replaced the worn out 28PICT-1 with a newer 30PICT-1. The only jetting change needed out of the box was to bump the main up to a 125. IIRC, idle was a G55 and air correction a 125Y or 125Z.

Mon Mar 17, 2025 12:44 pm

Thank you all very much for your answers.
I like to keep the original, stock look (in detail) and also top speed, but obtain some little extra torque for acceleration and ability uphills. Used 40hp since years as daily driver and was generally happy with it.

I'm not good in maths and not a physicist, this is why I need help. The cheapest gas here in Germany has 95 octane. Will this open the possibillity to avoid shimming? I've heared that these p and c sets will create a 8,1:1 compression ratio?

Sun May 11, 2025 10:57 pm

pastellgreen wrote: Thank you all very much for your answers.
I like to keep the original, stock look (in detail) and also top speed, but obtain some little extra torque for acceleration and ability uphills. Used 40hp since years as daily driver and was generally happy with it.

I'm not good in maths and not a physicist, this is why I need help. The cheapest gas here in Germany has 95 octane. Will this open the possibillity to avoid shimming? I've heared that these p and c sets will create a 8,1:1 compression ratio?

No need to shim for lower compression. Thousands of engines running fine without.
You only loose hp & torque with 83mm big bore & standard heads if you lower compression.
I will recommend to use TDE or CSP Highflow tailpipes for better breathing.