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Discussion Starter #1
I just recently changed the coolant in my 1987 vs700 intruder..

It's no longer overheating but now I have a backfire problem..

Sometimes backfires at idle, also sputters off the line.

I did an eargauge setting of the airfuel mix screws with the ears and it's no longer croaking off the line but the occasional backfire is still there..

When it happens it's coming out of both exhausts at the same time which is making it hard for me to narrow down which cylinder might be firing rich or lean.

Carbs were recently cleaned.. I believe this is purely to do with the air fuel mixture screws..


Ace Tuner
2,936 Posts
Those bikes have a bunch of cables that connect the two carbs together. (Some VS models have NINE cables total)!
It's a real pain to get the idle and off idle sync set correctly. You have to use the cable adjusters to set the off idle/high speed sync.
It takes a guy that understands how they work and will take the time to get it right.

Sorry, not the question you have in mind. My guess is you don't have an exhaust gas analyzer. So........
Assuming the synchronous was set correctly when the carb clean was done, here is a way to set the fuel mixture screws.

FYI. If the synchronous is off this will not work.

Warm the bike up, install new or clean spark plugs. (NGK)
If you had a wind tunnel for cooling you could let it idle long enough to 'color in' the plugs and make adjustments from reading the plugs.

No wind tunnel? You'll have to ride it to keep it from overheating.

Go out for a ride, never use more than 1/8 throttle if at all possible. (That way you are running on the pilot circuit, the part you're adjusting).
First time out go about 2~3 miles if that much. Let the bike idle as much as you can but avoiding overheating.
Take a look at the plugs.
Dark on the insulator = Rich. (Turn fuel mixture screw IN about 1/4 turn or more if plug is way dark).
No deposits at all on the center electrode, ground electrode or insulator = Possibly lean, run it longer and check again. (No adjust yet).

Keep this process up till you get a slight amount of color (tan) on part of the insulator near the center electrode.
The center electrode should be gray on about half of it and darker on the other side of it.
The ground electrode should be gray (or light) in color all the way near the bend where it connects to the threaded part of the plug. (+/- some).

You'll need to install fresh plugs along the way to verify results.

When you feel like you have it correct check the plugs after 20~30 miles to make fine adjustments.
It's time consuming but can be done if you stick with it.

All of the above is assuming everything is in stock form and in good condition.
If the bike has an aftermarket exhaust system it probably needs a jet kit that will need to be tuned from idle to near top speed.
Also if the motor is burning oil, carb tuning is all but impossible.

Honda teaches an idle drop method, doesn't work very well.
Yamaha will tell you it takes an exhaust gas analyzer, but "reading the plugs" is the most accurate way to tune a carburetor.

NOTE: The latest multi cylinder motors run much cleaner than these V twins.
Tuning those is a hole different ball game.

Well, I spent toooo much time on this. Hope this helps somebody. :biggrin:

Premium Member
5,643 Posts
First, the idle rpm should be 1000-1100rpm, any lower and you damage the engine from too little oil flow.

Next, there is a cable from the rear carb to the front, which allows you to set the idle rpm on both at the same time from the knob on the rear carb. It is adjustable in length at the front carb, so that both carbs have the same vacuum, thus airflow, at idle. You adjust it with a set of matched vacuum gauges connected to the vacuum ports with a pair of 5mm thread adapters. The adjuster is next to the frame and under the fuel tank, so this is best done with the tank removed and either hung with a long fuel line to the pump, or replaced with a small aux tank. I've done both, and much prefer the aux tank. One thing more - the throttle cable should have lots of slack for this adjustment, so that it doesn't interfere with the idle cable.

Third, once the vacuum is matched at idle, you reduce the slack in the throttle cable, then slowly advance the throttle at the grip, watching for changes in vacuum. If the vacuum rises faster on one, the throttle cable is too tight on the other. The adjustment on the throttle cable is also at the front carb, and you either loosen or tighten it until the vacuum tracks fairly well as you change the throttle. Once done, set the slack in the cable until you get about 1/8" free rotation in the grip before the throttle revs the engine.

Once you have the sync done, you should then adjust the pilot (idle F/A) screws. Depending on the market your bike was made for, these should be set between 1 and 2 turns out, usually a bit more on the front. This assumes you still have the stock air boxes and filters; if not, you are probably lean throughout the range, and need to look into adjusting the jets, too.

BTW, if you put your location data where we can see it, you are likely to be seen by a fellow Intruder owner who lives close enough to give you a hand - they made a LOT of them, and the 1400 has the same adjustments.
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