Maxi-bore feed back wanted

For those of you who like to fiddle around with OEM VM carburetors.....I happened upon these items. They do make sense because they help shape the venturi properly. As for DO THEY WORK? I can't speak with authority because I have not tried them. U.F.O.

On a small motor CRS29's definitely work great. You do need to order extra jets though because they always seem to come jetted to rich on the Pilot and starter jet circuits. Depending on the engine you may also want different needles. Those item add to the cost.

The same holds true with the RS34 Mikuni carburetors with regard to pilot jets and needles except in the opposite direction. It's not uncommon to need richer needles and / or pilot jets. Additional costs as well.

Last summer I worked on a lot of old bikes. Honda, Yam, Kaw & Suzuki. They were vintage 80's and 90's models, many were cruisers. They all had one thing in common. They wanted more fuel on the pilot circuits and they were all STOCK motors. Some had aftermarket exhaust systems.

I've taken brand spanking new Honda ATV's and immediately added larger pilot jets to get them to run properly. The pump gas has changed and what used to work for jetting needs to be changed. I've noticed this for the last couple of years especially here in the eastern part of the USA.

A truly good carburetor does more than flow more air. It also delivers a correct fuel curve. One is no good without the other. I'd be curious as to what jet changes are made to a stock set of carbs for the O Bore or Maxi Bore treatment.... ie. are the needle jet and needle being swapped out in favor of say O-6 /5DL31 units? The later the model of VM carb on the bike, the leaner they went with jetting from the factory. I don't see how you can treat every bored carburetor the same if you don't swap out the midrange circuit jetting

One last thing.... the bit about adding 140CFM to the engine should be take with a grain of salt. The carburetor is the first restriction to flow. The valve is the last. What happens up stream is one thing. What happens at the final restriction is affected by the first and any other restrictions in between....

I agree on the gas being different. It seems to me that it has changed over time.
Pump fuel is different today than it was 5, 10, 20, 30 and 40 years ago.


.

Thank you to everyone for feed back. I have opted out of the Maxi-bore and spent money on other parts. I'decided to scale back on the project so I can get the bike back on the road. Have decided to buy a different head and do some porting polishing ect. With that I will do a 1075 kit and CR specials, all in do time.

LarryC wrote: For those of you who like to fiddle around with OEM VM carburetors.....I happened upon these items. They do make sense because they help shape the venturi properly. As for DO THEY WORK? I can't speak with authority because I have not tried them. U.F.O.

On a small motor CRS29's definitely work great. You do need to order extra jets though because they always seem to come jetted to rich on the Pilot and starter jet circuits. Depending on the engine you may also want different needles. Those item add to the cost.

The same holds true with the RS34 Mikuni carburetors with regard to pilot jets and needles except in the opposite direction. It's not uncommon to need richer needles and / or pilot jets. Additional costs as well.

Last summer I worked on a lot of old bikes. Honda, Yam, Kaw & Suzuki. They were vintage 80's and 90's models, many were cruisers. They all had one thing in common. They wanted more fuel on the pilot circuits and they were all STOCK motors. Some had aftermarket exhaust systems.

I've taken brand spanking new Honda ATV's and immediately added larger pilot jets to get them to run properly. The pump gas has changed and what used to work for jetting needs to be changed. I've noticed this for the last couple of years especially here in the eastern part of the USA.

A truly good carburetor does more than flow more air. It also delivers a correct fuel curve. One is no good without the other. I'd be curious as to what jet changes are made to a stock set of carbs for the O Bore or Maxi Bore treatment.... ie. are the needle jet and needle being swapped out in favor of say O-6 /5DL31 units? The later the model of VM carb on the bike, the leaner they went with jetting from the factory. I don't see how you can treat every bored carburetor the same if you don't swap out the midrange circuit jetting

One last thing.... the bit about adding 140CFM to the engine should be take with a grain of salt. The carburetor is the first restriction to flow. The valve is the last. What happens up stream is one thing. What happens at the final restriction is affected by the first and any other restrictions in between....

Nebr_Rex wrote: I agree on the gas being different. It seems to me that it has changed over time.
Pump fuel is different today than it was 5, 10, 20, 30 and 40 years ago.
.

I don't know if it's just the moonshine they're adding to it or if there are other chemicals involved as well but if I put jetting in a carb that I used in the 90's...the plug won't color up and the bike is cold blooded as hell. I think it's safe to assume there's not more oxygen in the air than there was 20 years ago so it's got to be the fuel :silly:

LarryC wrote:

Nebr_Rex wrote: I agree on the gas being different. It seems to me that it has changed over time.
Pump fuel is different today than it was 5, 10, 20, 30 and 40 years ago.
.

I don't know if it's just the moonshine they're adding to it or if there are other chemicals involved as well but if I put jetting in a carb that I used in the 90's...the plug won't color up and the bike is cold blooded as hell. I think it's safe to assume there's not more oxygen in the air than there was 20 years ago so it's got to be the fuel :silly:

I think it's a little of both.

.

I'm running VM29's on my stock with a pipe GPz750 motor and on the dyno with 117.5 main's
A/F was going in the 15's. Now have 120's but haven't been back to the dyno
but I think it may want more jet still.