Mysterious Change Of Engine Character & Power Loss After Carb Syncing?

You may be familiar with the story of my 4th Gen VFR; after it was stolen in Ireland and unceremoniously dumped into a nearby river, I managed to repatriate the bike back to Greece and have been working on her ever since to restore her to her previous glory.

I won't bore you with details but long story short is that she is running well but not as she did. When I first started the bike back up a few months ago she ran great! All the grunt and power delivery that I remembered was there so the motor at least seemed to be fine.

After sorting out the majority of things on the bike I needed to take out the rear axle to replace the brake disc which meant taking the bike to a shop to have it done. Whilst there the mechanic suggested we balance the carbs as she was idling rather roughly.
I didn't mind this idling but I thought it would be a good idea.

When I got the bike back the idle was rock solid. I don't remember her ever idling so steadily. I got her on the road though and immediately I noticed a problem with the power delivery. When I rolled on the throttle in the middle of the rev range the bike seemed to bog down instead of opening up.

It is hard to explain exactly what the bike was doing. There was a clear momentary hesitation and the acceleration seemed to "slow down". It is accelerating through the revs but not as it should and definitely not as it did before.

For as long as I can remember there was a nice step to the power around the 4K mark, where the bike would kick in and go through the revs cleanly. Now this power step was replaced with the exact opposite effect which was a rather unsettling feeling.
Opening up the taps I was expecting a power surge and instead I was getting bogginess, a bit like when you are doing though a deep puddle of water which you can feel slowing you down momentarily.

I took the bike back the the guy who admitted that he had also noticed that change of character and that the bike became milder from when I first took it to him.
However he swore that he never touched anything but the idle screws and never even took off the fuel tank.

When you are opening up the throttle just a tad (maybe a quarter?) the bike goes through the rev range nicely accelerating mildly as you would expect.
If you repeat this process with more throttle the bike revs up to around 4K nicely but then it stumbles momentarily and the level of acceleration is actually reduced. The engine note also changes at that point sounding like it's doing a lot of work when it's obviously not.

In all the bike seems to lack power. I'll take the bike to a dyno but there is nobody around here willing to work on a carbed bike which leaves me in a bit of a fix.

FWIW I'm not going to take the bike back to that mechanic. The issue shouldn't be related to the carb syncing and he already took a shot at it with no results, so I need to get it sorted on my own.

What could it be? I've been reading the forums on this issue but I haven't come up with any good ideas.

What bothers me is that the bike seemed to be running great when I first fired her up after all those months getting repared and only became like this after that fateful visit to the mechanic to sync the carbs.

If it's any help the carbs were replaced by some Irish friends after the river rescue. The new ones were sourced from a newer '97 bike with far lower mileage and were inspected before going in and found to be ok. (They were NOT taken apart though)

The bike now has new plugs, all the contacts were inspected, cleaned and where needed completely changed. The r/r is replaced with one from a 2012 Yammaha R1 and all the related wiring has been suitably upgraded.
I also replaced the generator which recently packed up and the chain/sprockets have about 6-7K in them.
The airfilter is all cleaned up (it has a K&N jobbie) and the airbox and related hoses seem to be fine without cracks or kinks in them.

Recent update: I took the bike for a short day trip and I noticed that on some uphills that I was tackling in 1st gear the hesitation around the 4K mark was quite a bit more pronounced even with less throttle.

Any thoughts how to proceed? If you need pics I'll be happy to oblige.

I would go through the carb sync process myself to ensure it was done correctly. Just because he had it idling well doesn't mean that the correct process was followed.

Fair enough, but I am a bit unwilling to pin this all to the syncing process.

Could a bad carb syncing have such a pronounced effect on the engine and the way it runs?

Besides (though I could very well be mistaken on this) I thought that when you get on the throttle the carbs' main jets take over and the idle setting have little relevance.

Traveller

Fair enough, but I am a bit unwilling to pin this all to the syncing process.

Could a bad carb syncing have such a pronounced effect on the engine and the way it runs?

Besides (though I could very well be mistaken on this) I thought that when you get on the throttle the carbs' main jets take over and the idle setting have little relevance.

Until you check that the syncing process was done correctly you will not know for sure that was the cause or not.

As for the carbs, they run as per the picture below. And this page is a good read. http://www.dansmc.com/carbs2.htm

[image: carb_jet_usage1a.jpg]

To me it sounds like the pilot screws are a bit too tight. I've not sync a VFR, but all of these type of carbs have a base pilot screw setting. Something like 1 1/2 turns from lightly seated or so. Perhaps someone else can chime in and tell us the base pilot setting.

A couple of things piqued my curiosity. And FWIW, I am no guru, and these are not absolute by any stretch.

As I understand it, the bogging issue you describe seems inconsistent with a carb scync issue. A carb scync and pilot circuits play a part in the lower 1/4 throttle, where your bike runs the best.

Your new carbs . . . I don't know how you would deem carbs okay WITHOUT taking them apart. I've only gone through maybe 75-100 carbs, and there are many people here with more experience than me, but I don't see how you'd find a clogged jet or incorrect needle setting without taking them apart.

I know this doesn't help you very much.

I'd start by verifying the carb scync. If it's off, I really wouldn't try and fix it, but it would be helpful to know a starting point. And then do a good carb cleaning.

Things I have found that caused a bike to run like crap, but still ran: A main jet that was lying in the bowl of a Duc 900SS with 600 miles, and a CM400 with the pilot and main jets switched in BOTH carbs. Yes, the jets looked identical except for the jet size.

I would re-sync the carbs, I have had similar issues before and correctly syncing the carbs fixed it. With the right tools it is a simple job.

The carbs have a vacuum port that should be sealed again after having hooked up the tubes to the sync dial.

I had my carbs cleaned after 90,000km by someone else and afterwards she ran like shit.... Turned out that he damaged the rubber vacuum connector for the Scotoiler, so a) the Scotoiler was not functionng properly but also B) the carb was sucking "false air".

So double check that these vacuum port are fully sealed...

NO ! Carb synch isn't that influential at above 1/4 throttle, so i think it's something else. Maybe a carb clean would help.

Have you had a look at the sparkplugs ??

All suggestions are greatly appreciated!

blackdot

Until you check that the syncing process was done correctly you will not know for sure that was the cause or not.

To me it sounds like the pilot screws are a bit too tight. I've not sync a VFR, but all of these type of carbs have a base pilot screw setting. Something like 1 1/2 turns from lightly seated or so. Perhaps someone else can chime in and tell us the base pilot setting.

True, which is why I'll go through the process again to make sure it was done properly.

Cheers for the link; it's a great read indeed.

I have both the Honda service shop manual and the Haynes one so I'll try and go over the process again.

mk2davis

A couple of things piqued my curiosity. And FWIW, I am no guru, and these are not absolute by any stretch.

As I understand it, the bogging issue you describe seems inconsistent with a carb scync issue. A carb scync and pilot circuits play a part in the lower 1/4 throttle, where your bike runs the best.

Your new carbs . . . I don't know how you would deem carbs okay WITHOUT taking them apart. I've only gone through maybe 75-100 carbs, and there are many people here with more experience than me, but I don't see how you'd find a clogged jet or incorrect needle setting without taking them apart.

I know this doesn't help you very much.

I'd start by verifying the carb scync. If it's off, I really wouldn't try and fix it, but it would be helpful to know a starting point. And then do a good carb cleaning.

Things I have found that caused a bike to run like crap, but still ran: A main jet that was lying in the bowl of a Duc 900SS with 600 miles, and a CM400 with the pilot and main jets switched in BOTH carbs. Yes, the jets looked identical except for the jet size.

I absolutely agree; I have no way of telling what condition the carbs are at the moment. For sure 20 year old parts can benefit from some TLC even if they are low mileage.

Dutchy

The carbs have a vacuum port that should be sealed again after having hooked up the tubes to the sync dial.

I had my carbs cleaned after 90,000km by someone else and afterwards she ran like shit.... Turned out that he damaged the rubber vacuum connector for the Scotoiler, so a) the Scotoiler was not functionng properly but also B) the carb was sucking "false air".

So double check that these vacuum port are fully sealed...

That was the first thing I checked as it's easily fixed. The vacuum ports are sealed but I've ordered some new caps for good measure.

squirrelman

NO ! Carb synch isn't that influential at above 1/4 throttle, so i think it's something else. Maybe a carb clean would help.

Have you had a look at the sparkplugs ??

I thought the same about the influence of syncing on wider throttle openings, but I've been known to be mistaken quite often.

As for the sparkplugs they are brand new.

List of to-dos so far:

- Resync carbs

- Double check hoses and vacuum ports

- Open up carbs and clean them

If it turns out that the carbs aren't the culprit what could it be? Do you think that the fuel pump could be causing this?

Keep in mind that the bike was fully submerged for almost a day and water has gone through the pump and sundries.

V-FORE

I would re-sync the carbs, I have had similar issues before and correctly syncing the carbs fixed it. With the right tools it is a simple job.

If it turns out to be just a resyncing issue I'll be over the moon with joy! :)

I don't think it would be the fuel pump. There are some members that are running 3rd/4th gen bikes without a fuel pump at all, and the change is only apparent after sustained high speed runs. From what I have read.

With the little info we have, I'm leaning towards a rich condition over lean since she clears up with more air. But that is really just a shot in the dark.

Try spraying carb cleaner around the boots. Rhe added fuel should affect engine speed if there is a crack/leak.

Pull airbox top and check that all 4 carb slides seem to rise equally in response to throttle while engine is running.

Frankly, i'm AMAZED that the submarine bike ever ran again !!! :goofy:

squirrelman

Pull airbox top and check that all 4 carb slides seem to rise equally in response to throttle while engine is running.

Frankly, i'm AMAZED that the submarine bike ever ran again !!! :goofy:

Oh those VFRs are a lot tougher than one might think mate. :)

I'll get cracking this weekend. General consensus seems to be that the problem is carb related, am I right?

Long time since I've posted an update but I think I have good news.

After reading all the advice I finally decided to go ahead and do a new sync to the carbs. Turns out the front vs rear cylinders were completely out of sync. I brought them in line and the bike was immediately much better! She was pulling strongly and the hesitation seemed to have smoothed out significantly.
I could have left it like that but I decided to go the whole nine yards and tear the carbs apart for a full rebuild.

I started from the fuel bowls which apart from a couple of clogged drain passages they were quite clean.
As I had decided to clean the carbs anyway I proceeded to take out the jets and boy am I glad I did.
Although the jets et all were looking very clean I noticed on carb #3 that the main jet was COMPLETELY blocked by something that looked like a thin piece of paper.

[image: P4140005.jpg]

I mean look at that thing! Was it any wonder that the bike was bogging down when getting on the power? With a main jet so utterly blocked I am amazed that she was running at all.

Anyway, with the bottom part of the carbs now clean I turned my attention to the needles & slides. Again, I was thinking that there wouldn't be much need for any cleaning as the carbs were generally in great shape but I plowed on and here came surprise #2.

As the needles were spotless I just wiped them down and set the slide assemblies down on the bench next to the slides I had previously pulled from my old carbs, when I noticed something rather baffling.
The tips of the needles on the new carbs I had on the bike now were quite different to the needles I had on the old carbs.
(Remember that I had to replace my old carbs completely due to extensive water damage, so I got a very clean set on ebay which I was now using on the bike)
 

To be more specific the new needles' tips were very pointy, leading to a sharp V, whereas my old needles' tips were much more tapered. I removed both new and old needles from the slides and the mystery deepened: they had nothing in common!

The new needle:
[image: s-l225.jpg]

The old needle:
[image: a20791c12c07e5d7115b2f_m__73125.14129742]

You probably have guessed already but the needles I had in my old carbs were anything but stock. In fact these are the Dynojet needles that come with the Stage 1 kit for the VFR. All these years I was running the Dynojet setup and I had no idea!

I turned back to the bottom of the carbs; if I had the Dynojet needles I would have been running bigger main jets as well. And lo and behold: the old main jets were #126 vs #125 that came as stock in my VFR.

It may not sound like a big difference but as the previous German owner had gone to all the trouble to put on the Dynojet kit he must have set it up on a rolling road and decide that the #126s were optimal.

In any case I would test the theory soon. I cleaned up the Dynojet needles / jets and put everything back to the newer carbs, cleaning up and reseting everything as I went.

I put the carbs back to the bike and took her out for a spin. The change in power, delivery and feeling was frankly shocking! The bike had been totally transformed to a snarling sportsbike that had no qualms over laying down rubber on every occasion even on 3rd gear. (Our slippery city roads help here too!)

There was a bit of traffic so I didn't have the chance to let the bike stretch her legs but I managed to get the engine over 4K on 2nd gear and after gaining traction she actually did a power wheelie! Needless to say I came back home with a huge grin on my face. I'll know more soon about the carb overhaul after I finish changing the clutch pack but for now I am one happy customer. :)

While we're on the topic, has anyone pulled the clutch cover on their 750?
The reason why I'm asking is because as I pulled the cover, the spindle of the recution gears came out. It would have been dead simple to just put it back on if it wasn't for the fact that it also has a thick washer that looks like it could go either to the front of the gears or behind them.

The gears with spindle and washer is this bundle:

[image: !B8eWVg!!2k~$%28KGrHqF,!jcEzKkLyvEEBM3HR]

Honda's own schematic seems  to show that the washer goes behind the gears.

However on some pics it seems to be in the front
[image: DSC00006.jpg~original]

while on others it seems to be at the back:
[image: 20120919_094218_zps12ffd6a7.jpg]
 

The washer could easily go either ways; for some reason I am not 100% sure and I'd hate to have to find out the hard way that I made a mistake. Could you guys help out?

That washer goes behind, so between the gear and the engine block.

It is a bit of a fiddle so place something like a cloth underneath to stop it from rolling away. :-)