Topic: Intake Manifold Modification Thoughts

[BlueShamu]

Hey folks...newbie to HeyMow and think I have stumbled across a gold mine of info!

I'm not in the same application as what the majority of you guys are....but I have to believe there is some crossover information that I can learn from this site. I run a 36 HP Vanguard on a Prodrive brand mud motor. I have been doing some  very basic "tweaks" to improve performance.

So, here's my first post and questions:

What is there to gain by opening up and clearing out the metal between each intake runner when using a stock carb? You know, where you are knife edging and rounding the corners at the carb spacer?

What could be potential pitfalls of getting rid of that metal and opening it up to where both barrels of the STOCK carb are feeding both runners at the same time instead of individually as designed?

[royalblu68f100]

It will have more power at higher rpms but will not work as good at lower rpms. At least this is my experience, they do something similar on banderleros [sp].

[BlueShamu]

I have tried it out...but, haven't run it under a load. It purrs at idle. Runs great at WOT. I have a really bad lag/stall from idle to WOT....if I try to gun it. If I bring it up slowly, I can get 1/4 throttle or so and then gun it with pretty much no issues.

What is "limiting" it at the lower RPM's?

I have put bigger mains (from a horizontal shaft) in the carb. Pilots are the same between vert and horiz engines. I'm thinking this is what I probably need to get bigger to get rid of the lag/stall? I've also adjusted the air/fuel mixture screws as much as the limiters allow...even broke the tabs off to get a little more; but, no luck. Then, broke the stems off completely trying to remove the plastic stops! Duh!

Should I focus on trying to drill the pilots a little bigger or try to adjust the A/F mixture screws more to get the better throttle response? Or, is this thinking outside the box not going to work?

[George Herrin]

GOOD LUCK WITH THE STUMBLE WITH THOSE MODS. tHOSE MODS ARE MADE FOR ENGINES WHICH RARLEY CRACK THE THROTTLE. MEANING THEY STAY IN THE HIGHER RPM RANGE RUNNING BASICALLY FLAT OUT. NO AMOUNT OF REJETTING WILL STOP THE LOW STUMBLE. BEEN THERE DONE THAT.

[BlueShamu]

Gotcha. Thanks for that advice. And yes, we do tend to keep these things on their rev limiters...at least the ones that have them! May not sound like much to you guys turning double this, but, I just went from 4350 coils to 4700. LOL I haven't loaded it up, yet with this manifold/jet adjustment...but, prior, I ran pretty much in the 4200'ish range. New coils were added in prep for a cam.

[BlueShamu]

I guess I should add that heads are decked 0.050", de-shrouded and ported. Exhaust is ported/polished. Manley SS valves. This gave me some modest initial gains.

Do you think it would be a better option to port the runners and leave them dedicated to each barrel?

[cycloneracer]

I've had 2 different twins this was done too.   Both had the bog.  Ran great on the top end but couldn't get rid of the bog.   Need some sort of accelerator pump. 

[BlueShamu]

There are a lot of guys in the mud motor world doing this mod with the Mikuni 42's and 45's with good results. They are just using a clamp on adapter to convert from the large single bore carb to the intake manifold.

[Cromwell C4]

I've done this on several v-twin racing engines with great success. It just takes some patience to get the jetting correct. And I have a new style two barrel carb that actually has a built in accelerator pump 

[Rocket Ron]

The mod's your'e doing work's well  with mukuni  flat slide with acc. pump 45mm.for the size  eng. your'e dealing with. On smaller eng. like mower race eng. flow bench say's yes. But on the track  you  will have less bottom end. Won't show any benafit until 7-8k Not much Tq. on  Dyno.. Without tq. your takeoff will suffer. And so will corner exit speed .Layman's term's no exit speed. No straight away speed Like George said been there done that.But Iv'e never raced mud puppie's Before either?    Ron.

[BlueShamu]

In reality, I probably desire more low end torque and power for load carrying and to turn the prop in thick mud/vegetation....and hope for an added bonus of gaining a mph or two on the top end.

[BlueShamu]

I really do appreciate all the input you guys are offering. Sure glad I stumbled across y'all!