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Discussion Starter #1
So I am tuning my CB750 with my vacuum gauges soon and I realize how crazy it is that you can't do this on a 350 twin. Or can't do it easily I guess. When I start my wife's bike cold I get hot exhaust from the left pipe pretty quickly and the right pipe exhaust stays pretty cold for a while. The right pipe is still touchably cool for a minute or two while running. I know the cylinder is firing OK because the bike will keep running when I pull the left plug's cable. And each throttle body will separately rev the engine. When it's been driven a few miles it seems to equalize more. But at times even when warm I get a lot of instability in idle speed - sometimes it wants to rev high and sometimes you need to give gas to keep it from stalling at stops.

I'm sure it's poorly tuned. I've tried all sorts of things, even used a infrared thermometer gun pointed at the jugs on each side to see if the warmed up bike had equal temperatures. I'd think being able to use vacuum gauges would make it a whole lot easier.

Anyone get vac gauges on a 350?
 

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Hey jayelwin, I decided the ports would be a good idea also, so I made some on my CL350. You can read about it HERE. Ironically, although I added the ports with the engine intact and in the bike, I never got around to synching the carbs with a gage. Not long after I made the ports, I decided the cam chain was noisy and parked the bike to avoid a catastrophe. I'm finally doing the new cam chain and tensioner job, and it would have been the ideal time to have drilled the ports while the head was off. Oh well!

Ray
 

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Discussion Starter #3
Thanks. I'm totally going to look into this.

I never checked but you think my CB750 taps are M5 0.8 also?

Also do you have any further away photos of where exactly to put the new holes?
 

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Thanks. I'm totally going to look into this.

I never checked but you think my CB750 taps are M5 0.8 also?

Also do you have any further away photos of where exactly to put the new holes?
I don't know the size of the taps on your CB750. If they are not permanently installed, it would be easy enough to check their thread size. Usually a hardware store or a bike shop will have a template card with threaded holes and studs of various sizes so that you can check the size of your threaded fasteners.

Some people drill and tap the holes and leave the vacuum taps permanently installed, placing caps over them. I'm too cheap to buy the tool since a couple friends already have one that I can borrow. Hence the machine screws used as plugs.

Here is a picture from further back to show the plug location. Sorry for the delay -- I just noticed your reply today. This shows the left side, but is typical of both sides.



Ray
 

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Discussion Starter #5
Thanks for the picture. Now I see.

Why couldn't you have just drilled in on the other side without all the special angle drill just where a hole could have simply been placed?


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Why couldn't you have just drilled in on the other side without all the special angle drill just where a hole could have simply been placed?
I'm not sure which other side you are talking about. Take a look at your bike with the engine installed in the frame, and tell me where you could use a drill to poke a hole in there.

My picture is a view looking upwards at the bottom of the inlet port on the cylinder head. There is no room to drill from the top downwards -- the hole would be too close to the gasket surface. You could drill up from the bottom in between the two inlet ports, but that would be even more awkward and with the carbs installed you would not have access to the vacuum taps. You can't drill straight in from the side, or you will encounter the bolt that attaches the boot to the head. So I am confused as to which other side you mean.

Also remember that the hole needs to be drilled perpendicular to the surface so that a plug like in the picture will be able to seal the hole when not in use.

Ray
 
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