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Is there a diagram that shows how the vacuum tubes are configured from the petcock to the carb vacuum ports. On my bike there is a T between the #1 carb and the petcock vacuum port the #2 carb (assume right side). There is very little distance between the two ports and I am having difficulty with those connections making them secure and leak proof
So the vacuum from just one carb is activating the fuel flow (vacuum operated diaphragm)? I totally rebuilt my Maxim and I’m pretty sure both carbs were connected via a T. That said the distance between the left carb, T and the petcock is only about 1 1/4”. Barely enough to hook them up once the tank is put back. Do both of your carb to engine boots have tubes for the vacuum?
«So the vacuum from just one carb is activating the fuel flow (vacuum operated diaphragm)?» Yes; the right carb vacuum tube is plugged. But you make me think that it could be the reason why the engine does not want to go over 6,500 rpm! I will check that!
Thinking about that, I don't think that it is wise to have both carbs connected to the petcock as it means that the vacuum created by the intake pulse on one carb will influence the other one and some of the vacuum will be lost through that tube/secondary carb; that pulse is supposed to affect precisely one carb - the diaphragm circuit is a closed one. So I am going to stick to the original setup (one carb connected to the petcock only)
I don’t really see why it would need two vacuum sources either. But that’s the way mine was. I did manage to find a diagram for the Maxim and it seems to show just one carb vacuum line too. I wish the tubes were identified “vacuum” and “fuel” with their sizes.
A possibility with my bike was sometime in the past the right side carb boot was damaged by the previous owner. Probably in trying to heat up the boot to get it off. With this damage they would never have been able to get the bike to run right because of a vacuum leak. Perhaps also they T’d if off thinking that would help the vacuum on the petcock. Fortunately for me is I found another boot. The bike actually runs really well.
This is a picture I took during original disassembly. This is the left hand carb showing a T coming off the carb holder (aka boot). One leg of the T is inserted over the brass tube on the carb#1 boot. The other leg is a small vinyl line with a 90 that goes to the right carb#2, ….and it’s brass tube. Look at image showing the rubber tube with the larger diameter. That is the fuel line from the petcock to the carb. The smaller diameter tube close to the fuel line is the vacuum line to the T and the leg of the T that goes to the right side carb#2. What puzzles me is that you would have to have the petcock and T lined up perfectly when setting down the fuel tank (petcock). And there’s less than 1/2” of play. Look at the vacuum tube. The 1/4” indent shown on the vacuum tube plus the 1/4” at the carb tube means you have like 1/2” to play with. No engineer would have designed it that way.
So I took out the T then plugged Carb1 (left side carb) ran a new vacuum line with two 90 degree boots. One 90 for the petcock inlet and one 90 for carb 2 Ran the bike and low and behold no difference. The vacuum petcock is still doing it job.
Also I decided to sync my carbs with my DYI manometer. I had bench synced the carbs when I put them back together and was really close in the sync. But once I used this manometer to fine tune, the bike is definitely smoother. This is manometer DYI version 2. The first U tube only kept sucking AT fluid. V2 is much better and makes sucking fluid impossible. One 1/4 turn of the sync screw made the difference between balanced and unbalanced.
On my 78, only the right side carb was connected to the vacuum petcock and the left side had a vacuum cap. Having both connected would probably mess with the vacuum pulse signals in the carbs unless you could find and use one of those old school one way vacuum valves. Like a mechanical diode of sorts.
Funny thing is, both carbs were connected via a T. You can see that in the one of the pictures above. The vacuum fuel cock did work with both carbs hooked up. But I thought it was a bit hoaky the way the vacuum lines were hooked up to the fuel value vacuum port. But the good news is I did cap off one carb now and there is one line from the right hand carb and the vacuum petcock. And it still does what it’s supposed to.
