Sync Or Not?

[arveejay]

So, I have cleaned, installed and sync'd my carbs. I need to adjust my air screws just a little. Will I need to sync my carbs again after I adjust the airscrews just a little. No problem if I do I just wondered if it was really neccessary? Never could spell that word.

[Jay B]

if the tool is handy you might as well. When I sync mine I get everything done at once so I dont have to mess with it.

[earlfor]

Re: Sync Or Not?

When you synchronize the carbs, you are setting the positions of the slides to balance vacum levels. I set the slide positions to give a good mid range mixture. Tailoring the high end mixtures is accomplished by slide needle position. The idle air screw works in conjunction with the fuel pilot needle to provide idle mixture and has nothing to do with intake vacum levels.

Earl

So, I have cleaned, installed and sync'd my carbs. I need to adjust my air screws just a little. Will I need to sync my carbs again after I adjust the airscrews just a little. No problem if I do I just wondered if it was really neccessary? Never could spell that word.

[arveejay]

I initially thought that the air screw adjustment should not effect the synchronization but I wasn't exactly sure. I do agree that I will check the sync anyway. I have a cheap mercury sync gauge and am not really sure how far up the scale I should set the levels. The factory gauge would be nice. I have my #4 running rich and 1-3 look fine.

[earlfor]

On my 750, I'm set to 23 inches of vacum at 2500 rpm. That gives
a midrange mixture that is noticably on the rich side. I'm at sea level. For a leaner mixture, you will want the slides raised slightly above that setting which will mean setting to a LOWER vacum.
(raising the slides results in a larger throughflow area and if volume remains constant, then velocity will decrease and vacum readings will be lower)

Earl

I initially thought that the air screw adjustment should not effect the synchronization but I wasn't exactly sure. I do agree that I will check the sync anyway. I have a cheap mercury sync gauge and am not really sure how far up the scale I should set the levels. The factory gauge would be nice. I have my #4 running rich and 1-3 look fine.

[arveejay]

So Earl, I checked my gauges and there is a scale from 2-56. If I'm understanding you correct I would keep adjusting till my rpm was at 2500 and all lines of mercury were at 23 in. vac.? I'm about 1000 ft above sea level. Let me know.

Bob.

[earlfor]

I dont think being 1000 ft above sea level is going to make any big difference. You are correct. Adjust all carbs to 23 in at 2500rpm.
The books usually show carbs 2 and 3 to be set to a couple inches less vacum than 1 and 4, but I have tried it both ways and can find no noticable difference in burn mixtures between the two ways, so I just set all four to the same vacum levels.

Earl

So Earl, I checked my gauges and there is a scale from 2-56. If I'm understanding you correct I would keep adjusting till my rpm was at 2500 and all lines of mercury were at 23 in. vac.? I'm about 1000 ft above sea level. Let me know.

Bob.

[earlfor]

If you have the carbs synchronized for vacum and 1 and 3 look fine but #4 plug is reading rich, the first thing I would check is the float height setting on #4 carb. If 1 and 3 are fine, then both the ignition and timing circuits are ok. Its a carb adjustment problem and if vacums are equal, then it has to be a fuel flow problem.
Float level too high will result in too much fuel draw.

Earl

I have my #4 running rich and 1-3 look fine.

[arveejay]

I checked the plugs today after I rode it yesterday and let it set. 1, 2 and 3 all looked whitish to gray and #4 was all black and sooty, I cleaned and gapped it, backed the #4 airscrew out 1/4 turn and took it for a ride. I'm getting a slight backfire in the exhaust at idle and back pressure and a slight hesitation that happens when I'm at a constant speed, any speed and slightly roll on the throttle. If I hit the gas moderate to hard it just launches off and runs great. When it cools I will check the plug again and now that I got an idea about the sync levels I will check them too.

[Anonymous]

I've got the standard Motion Pro mercury sync tool but I've never used it. To sync my 2-cylinder 400 do I simply plug off two of the lines or do I need a different tool?

Thanks, Steve

[arveejay]

Well I just found the problem. The intake tubes from the air box to the carbs came loose. After it was properly hooked back up and taken for a ride hesitation and backfire are both gone. It was sucking a bunch of air. Runs like new again. Will check the plugs when bike cools off again.

[earlfor]

Steve
Just hook up two of the lines. You dont need to do anything with the two lines you are not using.

Earl

I've got the standard Motion Pro mercury sync tool but I've never used it. To sync my 2-cylinder 400 do I simply plug off two of the lines or do I need a different tool?

Thanks, Steve