1977 R75/7 Started today after 14 years. One cylinder not working

[Gurstang]

After many months of rebuilding I started my R75/7 today.

The left cylinder seems to work great. The right one not so much. I have to pull up on the throttle cable at the carb, to probably about 1/2 throttle before it will fire.

I have rebuilt both carbs. I did not replace the rubber seal on the main throttle shaft. I did blast all passages with compressed air many times. Verified they were all clear.
I installed Dyna III electronic ignition. I am still getting the timing just right, it is about 3 degrees advanced right now.
New spark plugs.
I tested the spark at the cable with a timing light.

I have the carbs set at the basic settings from the Haynes manual. Throttle stop screws 1 full turn clockwise after they contact throttle stop plate. Mixture screws are 1.5 turns counter clockwise from all the way in.

I had this issue when checking the cylinder.
http://forum.boxerworks.com/viewtopic.php?f=1&t=10299


Not sure what could cause this. Float setting? Low compression from the rust spot?

[Seth]

so start off with the basics.
compression check. Valve adjustment.

Also, I believe the Dyna has separate pickups for each cylinder. When you check the timing, do you get a single image, or is it split.
One cylinder might be "3 degrees" out while the other could be much more (not even showing up in the window).

If you think it could be carb related, try swapping them, left to right. Not that you'd want to run it that way on the street because the cables would be on the outside, but it will tell you if the same cylinder runs, it's not the carbs.

[Gurstang]

so start off with the basics.
compression check. Valve adjustment.

Also, I believe the Dyna has separate pickups for each cylinder. When you check the timing, do you get a single image, or is it split.
One cylinder might be "3 degrees" out while the other could be much more (not even showing up in the window).

If you think it could be carb related, try swapping them, left to right. Not that you'd want to run it that way on the street because the cables would be on the outside, but it will tell you if the same cylinder runs, it's not the carbs.

Yea, I was considering swapping the carbs as a test.

The timing looked ok, but maybe im missing it and just seeing the S on the good cylinder every other, it could super far retarded that Im not seeing anything and not realizing it.

I also am thinking maybe the carb piston is stuck upwards? They moved freely when I rebuilt them 6 months ago, but maybe not anymore.

[melville]

Check for a plugged pilot (idle) jet on the right carburetor. It's a tiny passage and it doesn't take much to clog it. Take the jet out to do so. Drop the bowl and use a flat blade screwdriver.

Also check the valve clearance. If too tight, they may not seal at low revs but might be adequate at higher revs.

The idle jet check might take 5 minutes. Valve clearance check 15 minutes if you've done it a few times.

[Gurstang]

Check for a plugged pilot (idle) jet on the right carburetor. It's a tiny passage and it doesn't take much to clog it. Take the jet out to do so. Drop the bowl and use a flat blade screwdriver.

Also check the valve clearance. If too tight, they may not seal at low revs but might be adequate at higher revs.

The idle jet check might take 5 minutes. Valve clearance check 15 minutes if you've done it a few times.

Should the idle jet have a passage that runs the entire length of the jet?

[Wobbly]

I agree with all the suggestions above.... also...

► Add new metal-core plug wires and new NGK 5K Ohm spark plug caps to the list. The OEM plug caps do go bad and are really hard to diagnose. It's like $5 for 2 caps, so it's really NOT worth the cost of putting it off.

► Is your fuel over 6 weeks old ? Your location is not known to me, but the ethanol laced fuel in the USA goes bad really quickly.

[Gurstang]

swapped sides with the carbs and it is definitely the carb.

I took the idle jet and idle adjustment screw out and made sure they were clean. Blew air into all the passages.

I figured out that I had the choke valves turned 180 degrees wrong. Fixed that. Chokes have an affect now.

I still have the problem though.

Does the idle jet get its fuel from the main jet? or is it the long tube going down the corner of the carb?

[SteveD]

Does the idle jet get its fuel from the main jet? or is it the long tube going down the corner of the carb?

Blowing air thru the passages may not be enough. Try squirting carb cleaner thru, then put some fishing line thru the holes to loosen any crud, then spray thru again.

The long tube in the corner with a jet in the bottom of it is part of the cold start circuit. If that jet blocks it will cause poor cold starting.

[Wobbly]

Carb cleaner will remove any varnish, but will not work on corrosion. Corrosion can be removed by dipping a wooden toothpick into thinner and twirling it in the jet, but it may also increase the orifice size. Therefore, the best practice is to simply replace the #45 slow speed jet, needle, and needle jet all with brand new parts. It's simply not that expensive for the excellent results you'll get.

Physical cleaning of the main jet makes no difference becasue slight extra richness at top end generally delivers better performance. Physical cleaning of the enriching jet in the bottom corner of the float bowl makes no difference becasue the choke simply isn't turned ON that long.


If you are in North America you can finish the fuel system cleaning with a product called StarTron. 1oz in every fuel fill-up until the bottle is used up will go a long way toward cleaning and restoring remaining parts of the fuel system.... the tank, petcocks, T-fittings, etc. That will add a lot of insurance to the project.

[Gurstang]

The main passage through the idle jet was blocked.

That passage is so tiny, I can't see light shine through it even on the working jet.

Thanks for the helpful suggestions.