New bike 85 Interstate runs poorly

[mainer4]

Hi
Just bought my wife's dream bike, a baby blue GL1200 with 40K miles on it and in excellent physical condition. Went for a ride on Mother's day and I was quite disappointed in the bike's performance. I have owned SOHC 750's for many years and my current ride is a beautiful '75 Supersport that could so leave this GL1200 in the dust. So I know the 1200 is not running right, I could hear occasional small backfiring at up to 45 MPH. Idles well once warmed up but just goosing the throttle it doesn't sound right and on the road trying to accelerate it acts like it is running on 3 cylinders and sounds thumpy. The harder you accelerate the thumpier it gets. Pulled the plugs and noted that front fight cylinder's plug looks varnished yellow on the electrode while the other three look normal tan color. Compression check confirms all 4 cylinders are within 5PSI. Close inspection of carbs indicates that a vacuum line is disconnected from the suspect cylinder. All 4 carb intake manifold clamps are very loose. Reconnecting the line and tightening the clamps makes no difference in how the machine runs. (New gas btw and cleaned all 4 plugs). Same backfire. Possible bad spark or lean carb condition I suppose. Played with the sync on that carb bank but no difference with +/- 1/2 turn. Exhaust pipe is hot though, so it does seem to be firing, but the pipe is not as hot as the other one on the same bank. I am thinking of swapping plug wires to see if that makes a difference since the plug swap made no difference to start with. I read in the manual that timing is not adjustable and the valves are hydraulic so assume no adjustment possible there either.
So this is what I have done so far:
1. Replaced plugs
2. Replaced wires with HONDA OEM
3. Replaced all O rings on the intake manifold and internal on all 4 carbs after removing them and checking all jets, needles, etc. Carbs were very clean with one slow jet slightly plugged.
4. Set air intake valves to California setting (was set to 49 state setting) since this is a California bike
5. Synced carbs
6. Replaced fuel pump after confirming it was bad (my fault I think).

The bike has more power and is much smoother on acceleration (actually accelerates now). 40MPG on the road with two riders.
All good, but still spits (small misfires) in the 1750 to 2700 rpm range. I was running 91 octane fuel. But switched to 87 on advice of Honda Mechanic - no difference at all. Only way to overcome it is to engage the enrichment valve (oh wait they call it a choke), then it vastly improves except the choke won't stay engaged.

I am thinking this bike is running too lean but am not very excited about taking everything apart again to try to resolve. Really, what were they thinking?? this is the biggest PITA to work on of all my Honda's - even my 1975 400F is easier!

I have read somewhere that there was a HONDA factory correction for drivability problems for these bikes that included a revised main jet and needle. Anyone know if my symptoms are what this is supposed to fix and what a part number might be or what to ask for. Maybe a Fall project...

[PanMan75]

Assuming all is back correctly, which seems to be if it is running much better, try setting the pilot screws back to initial spec and sync the carbs from there. I believe the California settings for an '85 is 3 turns out.

[mainer4]

Assuming all is back correctly, which seems to be if it is running much better, try setting the pilot screws back to initial spec and sync the carbs from there. I believe the California settings for an '85 is 3 turns out.

Thanks, actually the setting is 1.5 turns out.

[neoracer]

go back thru old threads in the carb section and you might find some more info for your problems

[bs175dths]

“Only way to overcome it is to engage the enrichment valve (oh wait they call it a choke), then it vastly improves except the choke won't stay engaged.”

The “choke” on this bike is not a restriction of air, but an increase of fuel, so it is an enricher.
IAC – When you had the carburetors off, did you check the performance of the throttle piston? If these don’t have a smooth transition while gliding up, the corresponding engine cylinder is not going to get the fuel it requires and the engine will sound like it is starving of fuel.
On my bike, when raising and lowering the throttle piston with my finger, I noticed resistance in both directions.
Once apart, I found each throttle piston and cylinder wall had some residue of some sort. I thoroughly cleaned the throttle piston and cylinder wall with gasoline until clean and smooth.
Make sure the piston’s diaphragm is seated properly in the body of the carburetor before closing the top of the carburetor, or the piston will not function as it should.

The carburetors on your ’75 SOHC 750 and 400F are different than the ones used on the GL1200.
The 750 and 400 throttle needles are directly connected to the throttle cable where the GL is operated by a difference of atmospheric pressure. There is air stored in the top of the carburetor (there is a small hole in the throttle needle piston) and as the air passing through the carburetor increases, the difference in air pressure causes the throttle piston to rise. As it does, the throttle needle is pulled out of the throttle tube and since the needle is tapered, as it rises, it effectively increases the area in which fuel can flow from the bowl. More gas, more air, more speed, raising the needle, making the area larger, more gas, more air, more speed and down the road we go.


So, those throttle needle pistons must glide up and snap shut, or engine performance will suffer.

I hope this helps.

BTW, the power plant in my picture above is a 75 SOHC CB750….., also a kick butt design.

One more item…… yeah, the GL is difficult to work on ANYTHING. It is part of ownership.

-Ride On

[PanMan75]

Assuming all is back correctly, which seems to be if it is running much better, try setting the pilot screws back to initial spec and sync the carbs from there. I believe the California settings for an '85 is 3 turns out.

Thanks, actually the setting is 1.5 turns out.

Going back through my Clymer I am seeing that the setting for an 85 interstate is 3 turns out, high altitude is 1. (Chapter 6 page 169) I used the Clymer setting when rebuilding my carbs this year and she only required minor adjustment when syncing.

[old gold]

Did you check the tiny filters behind the float valve on each carburetor? It does not take much to restrict the fuel flow through them. Don't ask how I know.

[bs175dths]

Did you check the tiny filters behind the float valve on each carburetor? It does not take much to restrict the fuel flow through them. Don't ask how I know.

I won't ask how you know but will ask, how long did it take before you looked there?

When I did my carburetor rebuild, I had my assembly out many times. So many times I can now extract them in RECORD time.

[old gold]

Did you check the tiny filters behind the float valve on each carburetor? It does not take much to restrict the fuel flow through them. Don't ask how I know.

I won't ask how you know but will ask, how long did it take before you looked there?

When I did my carburetor rebuild, I had my assembly out many times. So many times I can now extract them in RECORD time.

3. The real PITA was the little air valve on the top of the assembly that had to be removed before you could get the assembly and had to be off to get the carbs back in. Live and learn is a slow process sometimes.