This carburettor thing...

[pmcburney]

I've been doing a lot of thinking over the holiday break, especially given the inclement weather here in Queensland (can't go for a ride unless I want to get saturated).


It occurred to me that perhaps part of the reason for the dreaded flat-spot has to do with the inherent poor design of the carb and port-valve arrangement.

As we all know, the carb has a mechanically-controlled primary circuit (via a cable), a vacuum-controlled secondary circuit and a separate mechanically-controlled port-valve, that must all be in perfect alignment and synchronisation lest the flat-spot rear its ugly head.

If there is the slightest misadjustment or issue with any of the disparate componentry, you have dramas (in fact you can have dramas regardless of how well everything is working and adjusted).

Take the secondary circuit butterfly for instance. Being vacuum-controlled, can anyone guarantee it will open perfectly in time and fast enough to work correctly with the port-valve?

You can adjust when the port-valve opens, but not the exact time and rate at which it opens in comparison to the secondary circuit butterfly for every condition and twist of the throttle.

If you (for example) give the throttle a monumental stab, there'd be no way the secondary (being vacuum operated) could keep up with the speed at which the port-valve will open.

There will very likely be a lag (and a lean condition flat-spot) because you've opened the port-valve well before the secondary butterfly.

And it seems the extra fuel added by the pump can't work all that well if it's splashing against the back of a closed butterfly valve?

In fact I'd hazard a guess that's the big surge we all get after the flat-spot as all that fuel gets dumped into the secondary intake when the secondary butterfly finally opens!

So...
Has anyone given any thought to modifying a standard carb so that:
a. the secondary circuit butterfly is mechanically controlled, and,
b. the port-valve is connected to that mechanical control so that it can only open as fast as, and at the same time as the secondary butterfly?

I don't reckon it would be difficult to add to or modify the linkages so that essentially, when you twist the throttle you open first the primary butterfly, then pull open (at the appropriate point) the secondary butterfly and the port-valve, i.e. attach the port-valve cable directly to the carb linkage, not the throttle/twist-grip.

Thoughts appreciated.


Cheers
Paul McB

[pmcburney]

No takers eh...?


I got busy in the workshop yesterday and I modified my carb so that the secondary butterfly is now mechanically connected and operated rather than vacuum controlled.

It was dead-easy - all that's required is the addition of a small flat-headed screw and nut fitted into the slot of the primary butterfly actuating lever to prevent the secondary butterfly linkage from sliding within that slot (of course it's much easier to do than describe).

I then set the port-valve to open precisely when the secondary butterfly starts to move - the two are now perfectly synchronized as the secondary butterfly has to move with the twist-grip as does the port-valve.

And guess what?

It made no bloody difference.

There is still a flat-spot. It isn't quite as prolonged, but it's still there.

I tried adjusting the opening times of each valve either way (secondary leading, secondary lagging) and there was no discernible difference.

I have now officially given up on the diabolical monstrosity Suzuki and Mikuni foisted upon RE5 owners.
I will now focus all my efforts to finding an alternative.

Even if I was to ever get it to work correctly, I cannot justify the ridiculously
finicky and labour-intensive setup procedure/s.

It should not be that hard to get a bike to run right.


Cheers
Paul McB

[wayne]

Indeed, Paul, I've thought about doing this many times.........of course thinking and doing are not one and the same. But I've certainly covered the same ground as an armchair RE5 mechanic.

I'm an aero modeller with large scale planes and helicopters so complex linkages are a way of life for us. But with my RE I was at a point of it ain't broke .............But I'm really interested to hear you tried it and very disappointed to hear that it did bugger all.

So what happened to your primary mixture experiment ? Something odd has happened with mine. As you may have read, I had it perfect, went leaner just to see (down to an 87.5 as mine had a 90 as stock)- no difference. Hesitation didn't come back. Went up to 92.5 and it ran just that little bit sweeter. Thought I was home and hosed.

BUT.......Steain was riding it the other day and giving it stick and noticed that it was reluctant to accelerate through 4,000 rpm. I tested it and he was right. This "other" problem was back. I had this years ago. A reluctance to accelerate through 4 to 5000. Below that fine, above that fine. If you backed the throttle off a little in that rpm band it would accelerate, or if you did it in a lower gear it would accelerate cleanly. But in top with high vacuum it would stutter and miss (this is very different to the "classic" hesitation and/or flat spot). Anyway, this symptom disappeared years ago. And when it came back from the musuem I specifically tested for it. Now those symptoms seem indicative of a rich mixture. Is this because I've put the 92.5 in ? Trouble is, when testing the 92.5 I spent most of my time between 2,500 and 4,000 looking for the hesitation or flat spot. You can ride an RE all day without exceeding 4,000 so I didn't notice if this occured the moment that I put the 92.5 in.

I haven't had the time to think too much about what circuit I'm running on at those RPM nor to test it as I'm spendin 80% of my time away with work (Bloody useless Rolls Royce 380's...we're picking up the slack). But I'll be going back out with my bag of jets and specifically testing to see whether upping the primay jet has actually caused this apparently too rich running 4 to 5.

Never an end to the fun of owning an RE5.

Wayne

[gokart]

IMHO there is a very easy way to know if your flat spot hesitation is caused by the vacuum controlled secondary circuit.
You only have to disconnect the hose from the valve. This prevent the valve to open the secondary main. Now you only drive with the primary main...
I think you 'll be able to feel that the result is absolutely the same for the flat spot ... except that the engine power is very low due to lack of secondary circuit...
If nothing has changed , don't hesitate to replace the 87.5 or 90 primary jet with a 95 or more.

Hope my english makes sense.

Regards
Alain

[gerryggg]

Some of you noted when i bought my RE last summer that it did not have the OEM carb. At the time I had no way of knowing how it ran with what appeared to be a flat slide Mikuni in lieu of the original. The individual I bought it from said he thought it had run for a short time with it on when he bought it 7 years ago. Having finished my bathroom project and survived the holidays, I have begun to tinker with this unit. Unfortunately I have to report that it could never have run with this carb in this condition. When I took it apart, not only was it full of water and gunk, it had no needle jet, jet holder or main jet! It doesn't look like they have been there for a long time.

[tom93gts]

Jan 1, 2011 3:35:47 GMT -5 wayne said:

BUT.......Steain was riding it the other day and giving it stick and noticed that it was reluctant to accelerate through 4,000 rpm. I tested it and he was right. This "other" problem was back. I had this years ago. A reluctance to accelerate through 4 to 5000. Below that fine, above that fine. If you backed the throttle off a little in that rpm band it would accelerate, or if you did it in a lower gear it would accelerate cleanly. But in top with high vacuum it would stutter and miss (this is very different to the "classic" hesitation and/or flat spot). Anyway, this symptom disappeared years ago. And when it came back from the musuem I specifically tested for it. Now those symptoms seem indicative of a rich mixture. Is this because I've put the 92.5 in ? Trouble is, when testing the 92.5 I spent most of my time between 2,500 and 4,000 looking for the hesitation or flat spot. You can ride an RE all day without exceeding 4,000 so I didn't notice if this occured the moment that I put the 92.5 in.

Wayne

I have always had this exact issue with mine! I have no idea what it is, all I know is it doesn't go away no matter what I do with cable adjustments. I wonder if it is somehow related to my new secondary vacuum issue...

[tom93gts]

Today the RE5 is back together and the 4k rpm problem is gone. The two problems I fixed when I had the carb off were:
1) The butterfly valve in the secondary port in the housing was slightly opened all the time, I adjusted the screw to allow it to fully close

2) the accelerator pump jet dripped fuel all the time. I had to pull top off carb and tighten the jet.

So it was one of these two things for me. I think the secondary throttle plate issue was the more likely culprit and it's an easy adjustment too.