Good day everyone,
Looking for some help. I have restored 79 CB650 in my profile pic.
I took it to local bike shop to have carbs synched new tires and other stuff done.
Carbs all cleaned up valves adjusted
I talked yesterday to the owner of shop and he said the bike runs good in shop but he test rode it and it loses power at 70mph. When he lets off throttle it comes back. This guy knows his stuff.
Any idea I can give him to help. He said it’s definitely a fuel issue
Thanks
A couple of things:
1. Check to see if the 2 vent tubes from the 2 inner carbs are still there. These are 3.5mm (4mm will do) hoses that go up and over the airbox to reach quiet, still air behind it. This provides the higher pressure the bowls need to push fuel up the jets. Since the 650 is a lean-burn engine, this is mighty important. Some of the 1979 CB650s only have 1 of these hoses instead of 2, with all the carbs 'jumpered' together with short pieces of similar hose to share the bowl pressure line.
2. If the carbs were just rebuilt, the new float valves will be MUCH stiffer-sprung than the OEM versions were. This makes the float bowls run low because the spring seat pressure is too stiff. This is common with all of these bikes today. Reset the bowl depth at LEAST 1mm deeper than stock: I usually set them closer to 2mm deeper, or else set one float at +1mm deeper and the other at +2mm deeper fluid levels. This imbalance then tends to slow the closure of the float valve(s) slightly, thus deepening the float bowls a little more. I do this (either both at 1mm deeper, or stagger the floats +1/+2 mm deeper) with ALL the carbs I rebuild for these bikes now, and it works well.
Part of this problem is due to modern gasolines. They burn far slower than the 1970s gas, and they are less dense to boot, often having ethanol added. This makes the floats ride higher, shutting off the fuel like they are trying to starve the engine. Use Regular grade instead of Premium, too, for better power at 6000 RPM ranges. It burns about as fast as the 1970s Premium used to, while modern premium burns like 1970s aviation gas used to: you don't need that in a moderate-compression engine like these.
