carb sync- verdict is in

[kyle]

the verdict is in on carb sync rpm's... i usually set mine at fast rpm 2000-2500 and it always ran great and got good mpg's 39-43. so after reading some threads about doing it at 1000 rpm i tried that and well my mpg's kinda sucked, 35-38 was all i could get.

 

so i went back to hi rpm sync and went back to 42mpg on the first tank. i figure 90% of my riding is hiway at 65 mph so why not set carbs to that.. i will admit the throttle response was a bit better at 1000 rpm but top end and fuel mileage suffered. anywho for what its worth those are my results. all thats on 89 octane.

[kevin-vic-b.c.]

Interesting... I had my carbs adjusted over the winter and I know for a fact that they got adjusted at or near idel speed and yes my fuel milage totaly sucks now.

 

I now get about 20% less out of a tank than before.... the bike responds and drives well but the fuel milage is crappy.... next adjust will be at higher rpm I guess.

 

[RandyR]

makes me wonder if the carb jets are right?

[Big_Iz]

Understand that this is my own interpretation and anyone is open to disagree.

 

Carb sync, particularly on a 4 cylinder motor, should be done at above idle -- let's say 2000+ RPM (in the very low end of the cruise range at least). Why? Let's look at what the carb sync really is -- it is the adjustment of the mechanical linkage controlling how much the throttle blades are open on each carb to match it up to the primary (the carb directly controlled by the throttle cables). Going into midrange and upper RPM, each cylinder's power is governed by the main jet (air/fuel mix) and throttle blade opening. While you can adjust the jet size, they are typically fixed per carb, leaving you only the throttle blade to adjust.

 

At idle, I like to adjust the sync via the idle mix screw. Assuming the throttle blades are already synced, the idle sync becomes more of a fine-tuning of the air/fuel mix via the mixture screws. I'm not always able to get the idle mix to get the sync exactly close, but it usually gets better than having all 4 carbs with the exact same mix-screw setting. I don't go far with adjusting the screw, trying to keep the screws within +/- 1/2 turn of each other.

 

Anyway, that's how I go about it. Haven't had a chance to kick it off on the RSV yet, but that's on my to-do list.

[twigg]

Understand that this is my own interpretation and anyone is open to disagree.

 

Carb sync, particularly on a 4 cylinder motor, should be done at above idle -- let's say 2000+ RPM (in the very low end of the cruise range at least). Why? Let's look at what the carb sync really is -- it is the adjustment of the mechanical linkage controlling how much the throttle blades are open on each carb to match it up to the primary (the carb directly controlled by the throttle cables). Going into midrange and upper RPM, each cylinder's power is governed by the main jet (air/fuel mix) and throttle blade opening. While you can adjust the jet size, they are typically fixed per carb, leaving you only the throttle blade to adjust.

 

At idle, I like to adjust the sync via the idle mix screw. Assuming the throttle blades are already synced, the idle sync becomes more of a fine-tuning of the air/fuel mix via the mixture screws. I'm not always able to get the idle mix to get the sync exactly close, but it usually gets better than having all 4 carbs with the exact same mix-screw setting. I don't go far with adjusting the screw, trying to keep the screws within +/- 1/2 turn of each other.

 

Anyway, that's how I go about it. Haven't had a chance to kick it off on the RSV yet, but that's on my to-do list.

 

Some sync carbs in the cruising rpm because that is where they want them the smoothest. It's a reasonable position to take.

 

There is a problem though. Carbs do not maintain perfect sync right across the range of throttle opening. The reason for syncing at idle is simply because rough running at idle can cause bigger issues than a tiny imbalance on the highway.

 

As for the OP's problem ... That sounds like an issue in the Pilot circuits.

[Big_Iz]

Agreed on the pilot circuits

[BuddyRich]

Hmm, Always synced mine at idle and never got over 34 to 38. Makes sense though as the bike is at idle far less than its at 2, 3, and 4k. Gonna have to try this and see what happens.

[RandyR]

http://www.mikuni.com/pdf/hs40_manual.pdf

 

A mikuni manual that talks about carb tuning. Not our carb model, but the theory should be the same.

[Leland]

I agree that it sounds logical to sync nearer the cruising RPM. I have done it both ways, but mine seems to run smoother when syncing near idle. I can always tell when I need a sync as I get a vibration in my right hand grip. Can't really say that I notice a difference in gas mileage.

[1 Canuck]

It'll be nice when Yamaha finally decides to put fuel injection on their/this touring model. Ahhh...no more carbs.

[kyle]

I know what ya mean about the vibration in the right handlebar. i noticed also if you touch the gas tank while cruising it almost has no buzz feel to it, when synced at idle it buzzed pretty good. who knows whats right and whats wrong......

[Mel]

I have never done a carb sync on one of these V-4s, BUT in the past I've done lots of other 4 cylinder motorcycle engines.

 

Make sure the engine is completely warmed up. First, sync at idle, adjusting throttle opening on each carb using the adjustment screw(s) so that all four would draw the same vacuum, then adjust the idle mixture screws, and then the idle throttle screws again if the mixture screws changed any of the vacuum readings at idle, making sure the idle speed is correct.

 

THEN I would lock the throttle (or have an assistant hold it) at a steady RPM, about 2,500 to 3,000 and adjust the individual carb cables to balance the vacuum readings at high speed.

 

Two totally seperate adjustments to address two different circumstances on the carburetors. This is the only way I would know how to make it right for both idle and higher speeds. It seems to me that adjusting only one or the other is only doing 1/2 the job. Just my .

[kyle]

hey mel, where are the idle mixture screws on this bike ?? i looked up under the tank and did not see alot to adjust.

to be clear, i started the sync at 1,000 rpm had it dead on and then kicked it up to about2,300 and the final adjustmant was fairly small but made a huge difference.

[MidnightSpook]

I should probably give tuning my carbs a shot. I priced having it done here in south florida and they want almost $400.

 

Which tuning tool would be the best one to buy?

[CaptainJoe]

"Which tuning tool would be the best one to buy? "

 

 

Here we go again!...

 

 

 

 

Morgan carbtune.... hard to beat for the price but due to homeland security expect a 2 month wait...

[twigg]

I have never done a carb sync on one of these V-4s, BUT in the past I've done lots of other 4 cylinder motorcycle engines.

 

Make sure the engine is completely warmed up. First, sync at idle, adjusting throttle opening on each carb using the adjustment screw(s) so that all four would draw the same vacuum, then adjust the idle mixture screws, and then the idle throttle screws again if the mixture screws changed any of the vacuum readings at idle, making sure the idle speed is correct.

 

THEN I would lock the throttle (or have an assistant hold it) at a steady RPM, about 2,500 to 3,000 and adjust the individual carb cables to balance the vacuum readings at high speed.

 

Two totally seperate adjustments to address two different circumstances on the carburetors. This is the only way I would know how to make it right for both idle and higher speeds. It seems to me that adjusting only one or the other is only doing 1/2 the job. Just my .

 

Now you have me confused

 

WHen you sync carbs you generally do it at idle. If you then change those settings at a higher rev range, they will be out of sync at idle .... There aren't TWO settings to do this.

 

Guys .... Some of you are over thinking this. It's simple. Sync the carbs according to the book, Adjust the idle mixture, preferably with an exhaust gas analyser, then sync again.

 

It's not hard.

 

If, and only if you understand fully what you are doing, then by all means sync higher up the rev range!

 

If syncing carbs at idle leads to significantly different gas mileage that syncing at 4000 rpm, then you have a carburation problem to fix

[jemorrisonjr]

Just got mine in about 3 weeks

[Mel]

hey mel, where are the idle mixture screws on this bike ?? i looked up under the tank and did not see alot to adjust.

to be clear, i started the sync at 1,000 rpm had it dead on and then kicked it up to about2,300 and the final adjustmant was fairly small but made a huge difference.

 

Like I said, Ive never done a synch on one of these V-4s. What I do know is that for emission control, the EPA may have mandated that the mixture screws be sealed after pre-set at the factory (maybe no adjustment at all?). My heaviest experience was before some of the current restrictions, working on Mikuni and Kehin carbs. I was merely presenting what I feel to be a proper carb balancing procedure, do the idle and high speed both, not just one of them.

 

One of the other guys that are involved with the Maintenance Days, etc. should be able to answer this one. Lots of them have been involved in complete carb rebuilds and have a thorough knowledge of these carbs. Sorry I couldn't help more. Good luck.

[Mel]

Now you have me confused

 

WHen you sync carbs you generally do it at idle. If you then change those settings at a higher rev range, they will be out of sync at idle .... There aren't TWO settings to do this.

 

Guys .... Some of you are over thinking this. It's simple. Sync the carbs according to the book, Adjust the idle mixture, preferably with an exhaust gas analyser, then sync again.

 

It's not hard.

 

If, and only if you understand fully what you are doing, then by all means sync higher up the rev range!

 

If syncing carbs at idle leads to significantly different gas mileage that syncing at 4000 rpm, then you have a carburation problem to fix

 

In my past experience, there were stop screws to adjust the idle speed on each carb. At this time the throttle cables are slack, doing nothing to control the carburetor.

 

When you twist the throttle this action picks up the cables, and they can (will) become stretched or otherwise individually mis-adjusted, picking one or more at different times. This causes the vacuum readings to be different from one carb to another. You then adjust the individual cables to synchronize (balance) the vacuum reading at high speed. When you return it to idle, your first (idle) adjustment wiil be unaffected by the high speed adjustment. Don't how to otherwise state how it's done.

[twigg]

In my past experience, there were stop screws to adjust the idle speed on each carb. At this time the throttle cables are slack, doing nothing to control the carburetor.

 

When you twist the throttle this action picks up the cables, and they can (will) become stretched or otherwise individually mis-adjusted, picking one or more at different times. This causes the vacuum readings to be different from one carb to another. You then adjust the individual cables to synchronize (balance) the vacuum reading at high speed. When you return it to idle, your first (idle) adjustment wiil be unaffected by the high speed adjustment. Don't how to otherwise state how it's done.

 

It doesn't work like this.

 

The throttle cables are a simply Push/Pull, effectively it's just one cable. Adjust the cables to the book spec before you touch the carbs, and they will have no such variable effect. Those cables really will not stretch much, the tension on them is tiny. In any event, it's the cable adjustment that takes care of that.

 

You are mixing up two different things and thinking that there is a link between them when there isn't.

 

The carbs are all effectively balanced to one reference carb whose vacuum isn't adjustable.

 

So you set the left bank, then the right bank, then the left to right. Those settings are independent of any cable issues.

 

The reason that the vacuum varies at different rpms is simply due to the dynamics of the system. You are dealing with 4 carbs, and even if they were identical, the airflow to each isn't.

 

So they will vary, a bit, throughout the rev range and we can only sync them at one spot.

 

The debate is about "which spot", and if everything is in good order the differences really should be marginal.

[twigg]

hey mel, where are the idle mixture screws on this bike ?? i looked up under the tank and did not see alot to adjust.

to be clear, i started the sync at 1,000 rpm had it dead on and then kicked it up to about2,300 and the final adjustmant was fairly small but made a huge difference.

 

The idle mixture screws are directly below each diaphragm cover about 1/2 below the edge of the cover.

 

They may be covered by a tamperproof plug, just dig it out.

[V7Goose]

In my past experience, there were stop screws to adjust the idle speed on each carb. At this time the throttle cables are slack, doing nothing to control the carburetor.

 

When you twist the throttle this action picks up the cables, and they can (will) become stretched or otherwise individually mis-adjusted, picking one or more at different times. This causes the vacuum readings to be different from one carb to another. You then adjust the individual cables to synchronize (balance) the vacuum reading at high speed. When you return it to idle, your first (idle) adjustment wiil be unaffected by the high speed adjustment. Don't how to otherwise state how it's done.

Just to make it clear to other readers of this thread, nothing in this quote applies at all to a 2nd gen .

 

On our bikes, all four carbs have the linkages mechanically connected and activated by a single cable. The throttle cable is correctly adjusted when the cables at the carb have NO slack in them. Sync should be done at idle.

 

But I do believe that the synch should be double-checked at a higher RPM - somewhere around 3-4K. If the sync goes off at that RPM, then there is still something wrong. I have found that absolutely minor adjustments at idle can make significant changes at speed, so often this problem can be resolved simply by redoing the synch at both low and high RPM with extra care. And remember, you MUST blip the throttle after each time you even touch one of the adjustment screws - even slight pressure from the screwdriver will change the reading, so a quick blip is needed to let them all reset to the natural state.

 

If repeated careful re-synching does not get the carbs to remain in sync at all speeds, then something else is still wrong. Often this can be caused by one or more jets being partially plugged - this is almost a certainty if your engine is not pulling over 10" Hg at 1,000 RPM. Other causes for the vacuum to be off at speed include plugged air passages in the carbs, pinholes in a diaphragm or bad O-rings under the caps. These are CV carbs, so the position of the slides (which is what is affecting the vacuum readings along with the butterfly valves) is not controlled directly by any cable or throttle linkage, but by the pressure differential of the air on both sides of the diaphragms.

Goose

Edited May 10, 2011 by V7Goose

[twigg]

What Goose said .... Oh ...

 

Don't forget those tiny o-rings under the idle mixture screws. If they are bad it can play havoc with the idle speed ... You'll never get it right.

[Mel]

If these carbs have positive open, positive close, so be it. I was referring to spring returns. There still should be seperate adjustment for idle speed and high speed. Moderators can delete all my stuff if it will help un-confuse the issue. I hereby subtract my plus !!!

[fwbpastor]

"Which tuning tool would be the best one to buy? "

 

 

Here we go again!...

 

 

 

 

Morgan carbtune.... hard to beat for the price but due to homeland security expect a 2 month wait...

 

I got mine in less than 2 week. I took the advice of everyone and ordered the Morgan Carbtune and am very pleased,