Discussion on single carb conversion

[Venturezach]

Hello all, it was suggested to me to start a thread on the topic of the single carb setup. Thank you patch for the suggestion . So, to begin I will start from the beginning describing the issue I was having. I acquired my ‘83 venture xvz1200 via trade for a half-rotted boat. Being mechanically inclined, I’ve always loved wrenching on things and having projects going (good for keeping sanity intact too) . That being said, I knew I was a bit over my head when I discovered dry rot in the rear of the boat by the engine. Original plan was to pull engine and outdrive, gut and remove all dry rotted areas and rebuild the hull. But this was going to be very costly and at the time fate and my conscience decided it was time to hand off this project. I came across an ad on Craigslist where a guy was looking to sell or trade his motorcycle. I already had my M1 license and owned other motorcycles before, so I contacted the seller and he agreed on a straight across trade. We were both upfront and honest about what the issues were. I informed him about the dry rot, and he informed me that the bike did run, just not very well and he even trailered it to my house when we made the trade. So here I was with my new project and excited to get it road worthy. First thing I did was look everything over (repair receipts, condition of components ect.) and made a to do list. Being unfamiliar with the venture scoots, I started searching on the web for any information I could find and whalla! I found this site with as much information anyone can ask for and met some great folks on here too. Now for the problem which is the topic of this discussion. Bike was darn near impossible to start when cold (had to spray starting fluid in airbox opening every time) and lacked power with popping and stumbling. I ran sea foam a couple times through the tank, inspected diaphragms and peeked at carb internals without pulling off carbs which I know is not a full and thorough inspection. I replaced the spark plugs as well and tested ignition system using inline spark tester. Ignition system checked out good so I eliminated that as the source of the problem. I knew I was looking at rebuilding the carbs. Unfortunately parts can be hard to find for these old bikes and it wasn’t until recently that I had found a source for the parts necessary to rebuild the carbs. Another thought that I had was maybe the carbs were out of sync which is very likely. Anyhow, I knew a rebuild was necessary and was on the hunt for parts. I came across a vw manifold conversion kit with a solex pict 3 carburetor. The flow rate of this carb is 200cfm and the flow rate of the original carbs are 180cfm combined. 45cfm/carb. I knew it would work in theory, but I wanted to gain more information and see if anyone else had successfully done it. I found that some have attempted this conversion using their own custom made intake manifold and various 2 barrel carbs. I did not find anyone that used the vw carb and manifold that I found. It looked like a good solution to my problem, as it came with instructions and the number of the seller who offered installation support over the phone. I also thought that 1 carb was much easier to deal with than 4, so I decided to go for it and got it on order. When it arrived and I was installing it, I had to run a power and ground wire for the electric choke as per the instructions provided. It took about half a day getting everything tuned and dialed in. Once that was done, all that was left to do is put the covers back on and enjoy the ride! I’ve been running this setup for almost 3 years now and I am very happy with it. Worth every penny spent in my opinion. Please feel free to share any input or questions. Cheers!

[Patch]

Thanks Zach, I'm trying to lookup the venturi sizes the site I usually go to isn't showing it.

 

In the meantime could you spec the intake or adapter you chose?

 

Another question for you: Not sure if I was involved in your conversion thread so I'll ask this of you now; did you check compression and note it after 5 revs then for maximum compression?

[Venturezach]

I can’t find the specs on the intake manifold. It only came with installation instructions. I’ll try reaching out to the seller to see if I can get the information on the intake manifold. As for the compression, the numbers were between 170 and 175. I don’t remember exactly what they were being that I did the compression test about 3 or 4 years ago but I do remember the compression was within spec. If I do find the information on the intake manifold I’ll be sure to share it here.

[Patch]

Thanks I appreciate that.

 

Going to try and break down some work around's to efficiencies that I've used to sort mods that I played with..

It started when I was a young fellow building towers. Just took the theory and incorporated it..

 

So compression tests can be quick and simple or, you can turn them into a graph at a range of throttle plate angles to predict efficiency gains and losses. It takes time but its inexpensive and can be done by almost anyone.

 

So if the crank is the heart of the engine then feeding it must be the goal. We know that inertia can compensate in fine tuning masking minor misses...blabla

This also means that at low rpms we can better guess induction mod effects as we are essentially measuring off the weak driving force of the starter;) Not quite like comparison compression test but, same thinking.

 

Happiness to be achieved at pre-determined values such as in a touring bike like eg. getting up to cruising speed/rpm range. Lots of ways to achieve torque also, many ways to use it to just simply get to the brutish side of things call HP; but then that effect tank range!

 

Touring bikes pose a challenge as they have a greater range demand, move more mass over longer distances for extended periods of time. That's a tall order...

 

Now if in your case you have achieved a measure of success than better understanding of this sheds a lot of light.

 

I'll stop here: The venturi numbers may help shed this light as we know the carb has at least 2 but, are there more in the intake and or adapters? See crank angles and overlaps play a role in dynamic cfm requirements, see, we think of the cycles as individuals but in fact they have overlaps.... hmmhmm meaning that if you can say that as with a CV vacuum slide setup, hammer down 1/2 throttle with no lean/rich moments or bogs then there is more going on then accelerate compensation and jetting;) hmm WOT in a touring bike doesn't mean much often.

 

Let us know

thanks