Morgan Carbtune

[dan7629]

Has anyone used the Morgan Carbtune to balance vintage twins? Results?

[schrader7032]

It would appear that the Carbtune needs some kind of vacuum takeoff in order to measure the amount of vacuum being created so that the carbs can be balanced. That would be fine on the /5-on Bing carbs. The TwinMax does the same thing...however, it is a differential pressure gage and the object there is to measure zero difference between the two carbs.

On the /2, there is no vacuum takeoff unless you decide to drill the carb body and install one. Instead, I've used these devices, one for each carb:

http://www.dealparts.com/mm5/merchant.m ... COMPONENTS

Made a big difference in the idle and performance of the engine once I got things balanced.

[dan7629]

Thanks for the lead. I have several more modern bikes including a /5 that I could use the 'Carbtune' on.

I am not familiar with these devices you mention and could not find how they work on their website. Since I will need two of them, can you give me a brief review of their operation?

Any help would be appreciated.

Thanks,
Dan

[schrader7032]

Dan -

I bought two of the items from the link that I provided. No other adapters were needed for my R69S. Basically, I removed the air tubes from the filter to the carb inlet. With the rubber collars provided, the metering device slips over the inlet to each carb. Before I do any of this, I ride the bike for 15-20 miles. Once the meters are fitted, I restart the bike and then go through the process of setting mixture, idle speed, and cable tension. Each meter has a small scale with a floating needle, representing the vacuum or air flow...I don't know which...doesn't really matter. But looking at the needle on the scale, I adjust each carb to have the same scale reading.

As I said, the bike idles and runs smoother down the road now. I had tried the shorting technique and/or listening to get the same sound side to side, but obviously I was off somewhat.

[dan7629]

Thank you for the info.

Will the adjusters work on more modern bikes also?

By the way, I also have a '69S I bought in need of a lot of work. The engine runs, the numbers match, but someone put a /5 fender and rim on it. I might be asking you for info when I start to work on it.

Thanks again,
Dan

[schrader7032]

I would think that will work on other bikes. The only issue will be the diameter of the carb opening vs the adapters that come with the meters. I've never thought about that with my /7...the carbs have vacuum takeoff ports and I use my TwinMax to balance them.

As long as you can create a good seal, it should work fine.

[Bruce Frey]

You can make a carb balancing manometer for a few dollars with parts from the hardware store. Basically it is 20-30' of clear poly tubing using ATF or 2 stroke oil (in case it gets sucked into a carb). The tube is doubled back on itself in the middle to make a "U" ±3-4' high. The "U" should be mounted vertically on a stand. Enough liquid is used to fill the "U" about 3/4 full. The free ends are attached to the carb ports. Shut off valves are advised.

When I have some extra time I plan to give this a try.

Bruce

[schrader7032]

Bruce -

There are instructions for making such a manometer on the Airheads website...one can probably google this and find other examples.

These will work great on carbs that have vacuum takeoffs. The original post was regarding "vintage twins" which I take to be pre 1970 bikes. In that case, those Bings have no vacuum takeoff ports. Hence the resorting to other metering devices or the tried-n-true shorting method.

[dan7629]

What should be a good compression psi reading with the engine warmed up? What is the range from poor to excellent? I can't seem to find this info?

[schrader7032]

I read on the Mono forum that someone had 12.4bar on an R25 w/ 6.5:1 pistons. Hmmm...that converts to about 180 psi...seems a bit high!

In a similar discussion, 8.3 atmospheres were mentioned which is about 120psi...seems better.

Edit: OK, I tried something, but it may just be coincidental. For my R100/7, the average normal compression pressure I found listed in my Haynes was 134 psi. For grins, the CR for the bike is 9:0 to 1. With the piston at bottom dead center, the pressure within the combustion chamber is nominally atmospheric or 14.7 psi. Mulitplying by the compression ratio of 9, I get 132.3...pretty close to the average above.

So, for the R26, with a CR of 7.5 to 1, I get about 110 psi. The Haynes manual listed a swing from 123 (poor) to 145 (good) for the /7. That's about 8%. So, the range could be nearly 120 to 100 for the R26.

A crude approach...might provide some general numbers??