Gl1200 heads

[Skooltecher]

Hello, new here.

Meet “Doc”

I’m 50 years old and never really rode motorcycles since I snuck my older brother’s bike out of the basement when I was a teen age boy. I am an auto mechanic and have a 30 year history in that field. I recently started teaching auto mechanics at the local high school so I thought “what a great way to spend the summer off, touring on my dream bike from the 80’s”. So I bought a 1984 interstate (fixer) to ride this summer. It has a really poor, stumble and occasionally an intake backfire. Has little to no acceleration coming off idle but then takes off at 2500 ish rpm.

Concern- stumble when first throttled

Cause- vacuum gauge bounces erratically at idle on all intake tubes. Indicating burned valve or timing.

Correction- inspected timing belts, ok. Inspected cam and valve movement, ok. No compression test performed. (so far) new plugs, wires. New carb kit installed by me. Adjusted carbs by ear. It runs a bunch better but still slow coming off idle.

Additional observation. There Are pieces missing from under the carburetor. From pictures I’ve seen there are metal lines coming off each head and running to a piece attached to the bottom of the carb. Both the tubes and carburetor piece are missing. Previous owner blocked off exhaust holes in head with a freeze plugs and reinstalled intake tube with a spacer to hold freeze plug in place.

[ekvh]

A fair amount have removed the tubes you refer to which just dumped additional air into the exhaust to pass smog tests. Sounds like you have plugged idle jets. They provide the majority of the fuel off idle to 1/4 throttle.

Valves are seldom burned on these. The 1200 has shims which could be needed on yours. The manual covers it in detail and calls for a special tool to check them.

A compression check will tell you a lot.

[Aussie81Interstate]

Initial symptoms as described of erratic vacuum at idle - definitely needs a proper carb sync - not just by ear (not doubting your years of experience either), before doing the above- I would strip the carbs (no need to break them apart)- do one at a time and clean every passage 3 or 4 times -there will be build up in the passages and the jets, get them super clean and you will be amazed at how good it will run then after the sync is performed.


BUT - do yourself a favour and change the timing belts before you go any further..

good luck

[Skooltecher]

Thank’s. The carb kit I used was from eBay. I replaced all the brass and cleaned the passages thoroughly. The one obvious issue was one of the membranes had a tear on the vacuum slide. I repaired it with some rubber patch and it seemed to work.

Membrane

I’m questioning myself on the idle passage. There was a small tapered screw accessable from the outside of the carb. It has an x (or cross) pattern on the head and a small spring on the tapered end. Is that the idle passage?

[Aussie81Interstate]

Air mixture screw adjustment - they have a very fine point - and often get damaged when they are initially - bottomed out to get a base setting then wound back out a certain number of turns.. the point can be damaged with over vigorous bottoming out the screw , and this could be a cause of the issue you are having. There is a spring and usually also a small washer in there as well.

That diaphragm needs to be replaced (hold them up to the light - you might find some really fine holes in them) - if you have patched it - and is acceptable it may be ok - but if it is not supple enough it will affect the running of the bike. Best off to find another diaphragm if possible.

cheers