Author Topic: The elusive surge  (Read 4370 times)

Offline Alexrayz

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The elusive surge
« on: January 09, 2022, 07:24:43 PM »
Hey all, hope you're doing good!

I have made significant progress with my bike the last few weeks I'd like to especially thank Alankelly1 for his most precious help and Nurse Julie for the O-rings and the mounting advice that made the job of replacing the old ones so much easier!.

So: I have stripped the carbs, cleaned and blown every jet screws and passages possible. I have swapped the idle jets I had from an overhaul kit with nice keihin originals, thanks Alan!
The floats have been set to 21mm and checked for holes. The carbs have been bench synced. O rings were replaced and put in place with red rubber grease to avoid shearing. The slides snap back nicely. The mixture screws have been set to 1 1/2 turns out.
After installing the carbs back I have fiddled with the idle stop screw to get to 1200rpm when warm and off the choke. I then set the fast idle screw to 0.2mm clearance.
I get a respectably stable idle at 1200 rpm when warm. Startup isnt too rough.
The problem I have is jsut that the damn thing surges, it can be when operating the throttle or just when left alone for a while. In which case only fiddling with the idle stop screw will make it come down. I then have to fiddle again to bring it back to 1200rpm.
I have removed the throttle cables to make sure they were not messing with anything and indeed no change in behavior. |

Overall it seems like a lean condition problem. But when I turn the mixture screws by a 1/4 turn in to enrich the mix it stutters. My guess would turn to an air leak but doing the "squirt brake cleaner on suspect spots"method to no avail. I tried the inlet boots, fore and aft; the top of the carbs on the gaskets, the depression ports on the boots and the spark plugs just in case and nothing.

Do you guys have any leads or ideas what might cause this?

So close!

Take care
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Offline Nurse Julie

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Re: The elusive surge
« Reply #1 on: January 09, 2022, 09:00:16 PM »
What spark plugs have you fitted Alex?
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Offline daz-j

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Re: The elusive surge
« Reply #2 on: January 10, 2022, 12:04:45 PM »
Hi Alex, apologies for butting in with an extra question but you beat me to it, I was going to ask advice for very similar issues - in my case the engine starts easily but isn't stable when idling - the revs can randomly increase up to ~3-4000, or decrease to the point of stalling, without any throttle input.  Hopefully we can learn together and crack the problems!

I've taken the carbs off a few times now (the joys!) to clean and recheck things - all appears to be ok to the best of my knowledge.  When refitting I've rechecked valve clearances, timing, pilot air screws, vacuum leaks, carb balance - again all seem ok but the issue persists.

So the carbs are off again - I'm slightly suspicious of seals I fitted previously so have just ordered a kit from Julie to replace them.  General advice welcome, but my main question for the forum at this point is what are the worthwhile checks for me to do whilst the carbs are stripped and on the bench please?

Offline McCabe-Thiele (Ted)

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Re: The elusive surge
« Reply #3 on: January 10, 2022, 12:56:14 PM »
FWIW - this a long shot as I do not know at what point the advance & retard mechanism trips in. I would check the mechanism is nice and free plus check the timing dynamically rather than statically.
Likewise as NJ has noted are you on the right plugs & caps &  leads.
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Offline K2-K6

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Re: The elusive surge
« Reply #4 on: January 10, 2022, 12:57:49 PM »
Small observation, have you bench synced the carbs as detailed in "tricks and tips" section of forum ?

There's simple but very influential information in there that sets the carb slides in relation to the throttle lift mechanism to allow correct balanced movement to work without the relationship being compromised.

It's easy to miss this potential conflict if the carbs have been dismantled and rebuilt.

Offline Alexrayz

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Re: The elusive surge
« Reply #5 on: January 10, 2022, 04:39:07 PM »
Hey Julie, finally got time to search for it, I knew it was buried somewhere in my mailbox.

The plugs are NGK D8EA and the caps fitted on it are XD05F and VD05F. The leads are originals, same as the coils.

Ted: I did do that, I have a daytona electronic ignition trigger, there is an LED that turns on to show when the sparks will happen and I'd say it is right on the dot. It was a static test though.
You'd recommend getting a stroboscopic light?

K2-K6 I have bench synced them yup. Some lad at work advised me to do a running sync to make sure everything is tip top on that front. Even with a bench sync you can get inbalances that throw off the whole thing.

Daz-J did you check float heights?

Thanks for your help guys, means a lot!

Take care,

Alex
If it aint broken don't fix it

Offline allankelly1

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The elusive surge
« Reply #6 on: January 10, 2022, 05:01:23 PM »
Question for the group rather than Alex but may help

With the original coils there are two HT leads but can you lose one HT output but the other HT output be OK?

I.e 1 coil does cylinders 1 and 4 and the other does 2 and 3 so can 1 coil intermittently feed a spark to say cylinder 1 but the spark to cylinder 4 is consistent as on the wiring diagram it look like all plugs “go to earth” unlike my RD where both HT leads make up 1 circuit so if you disconnect 1 cylinder you lose everything as you have broken the single complete HT circuit

Hope that makes sense

Just that an inconsistent spark on 1 cylinder will imitate this surge as when all 4 are running the revs will be X but if you lose a cylinder the revs will drop to Y as the other 3 cylinders have to carry the “dead” cylinder and then when it decides to run again the revs will rise

Just a question for the experts as had this issue on a old 125T where a dodgy CDi was dropping in and out so it would run on two then drop to one cylinder for a short period of time with a big drop in revs to where it would die if at tick over and then it would decide to work again with again the rise in revs back to correct idle


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« Last Edit: January 10, 2022, 05:16:26 PM by allankelly1 »

Offline Alexrayz

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Re: The elusive surge
« Reply #7 on: January 10, 2022, 05:43:45 PM »
Hey Alan,

I guess that could be possible with old cables. As it gets old with many manipulations and service the copper wires can fray and cut which could lead to intermittent connections.

I have never encountered the case in motorcycles or cars or else but I have in houses and offices with old electric installations.

Soo maybe?
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Offline allankelly1

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The elusive surge
« Reply #8 on: January 10, 2022, 05:52:18 PM »
Hey Alan,

I guess that could be possible with old cables. As it gets old with many manipulations and service the copper wires can fray and cut which could lead to intermittent connections.

I have never encountered the case in motorcycles or cars or else but I have in houses and offices with old electric installations.

Soo maybe?
Hi Alex

Yes that why I asked the question

When I did my Boyer system I bit the bullet as replaced the complete ignition system (As in theory the whole system was over 40 years old) So that as well as the magnet pick up system that replaced the points system, it also does away with the mechanical advance and replaces it with a “mapped” CDI (so you have an advance curve programmed in) and two new coils etc

But on the old Honda system I am not sure if both HT cable make up the HT circuit(Like my RD). Or as I suspect on the Honda,one end of the HT winding goes to earth but the other end is split to two HT leads, and then goes to earth via both spark plugs, hence one lead could be good and the other bad / damaged giving a intermittent spray to that cylinder.

Let’s see what the experts say

Best wishes Al


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« Last Edit: January 10, 2022, 06:23:59 PM by allankelly1 »

Offline 400 Cafe Racer

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Re: The elusive surge
« Reply #9 on: January 10, 2022, 06:22:27 PM »
Hey Alan,

I guess that could be possible with old cables. As it gets old with many manipulations and service the copper wires can fray and cut which could lead to intermittent connections.

I have never encountered the case in motorcycles or cars or else but I have in houses and offices with old electric installations.

Soo maybe?

Hi Allan and Alex,

To the best of my knowledge the spark performance on each coil provides for simultaneous sparking on both cylinders served by the coil - ie  1 + 4 for one coil and 2 + 3 for the other coil. This means in fact that the cyinder that is not on compresion stroke has the spark "wasted".

Unless there is a specific problem with one HT lead or cap, the spark performance should be the same.

My comments below assume that there are no carb or inlet manifold air leaks.

Having looked  at the issue you have got Alex, I encounted a similar situation during tuning my carbs after doing only a bench sync. I found that spending time afterwards with close attention on the vacuum guage balancing and mixture sync method, this unevenness and surge disappeared.  It only needs one of the cylinders not contributing equally to the tick over, and a little off with the mixture to cause this. When the off cylinder decides to clear a little and then "comes in and contributes", this causes a rise in engine revs for no apparent reason. You will be supprised when making adjustment with the vacuum guage, even - say 5 degrees - rotation of the balance screw for 1 cylinder will make a big difference to all the other cylinders.  Just keep repeating the sequence until they are all the same.

Hope this helps. ;) :)

Dennis

PS  Just thought I would add a small note to explain that for example in the same period (1970 - 1980 )that with a 6 cylinder car engine with 2 or 3 Carbs, to overcome a tendency for imbalance on idling the manufacturer would tend to put a small balance pipe connected across the manifolds so that all cylinders could suck Air/Fuel mixture, even though the individual Carbs may not be set spot-on.

In the case of the 400F all manifols are individual and have NO interconnecting balance pipe, so this makes the correct balancing and mixture for each Carb and Cylinder hyper critical to get best running and performance.
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Offline Alexrayz

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Re: The elusive surge
« Reply #10 on: January 10, 2022, 09:46:47 PM »
Hey Dennis, thanks for all that sweet sweet info, I think I will do the vacuum gauge sync next time working on the bike. Seems like it will clear up more questions and hopefully solve my issues.

I was actually wondering if there is a proper methodology for the mixture screw settings. I know that the look of the plugs is a telltale sign but I was wondering how precise it gets without requiring an IR exhaust gas probe.

Cheers!
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Offline K2-K6

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Re: The elusive surge
« Reply #11 on: January 10, 2022, 10:26:40 PM »
Certainly it's interesting to gain the contributing views already expressed, and hopefully add to the picture.

Generally these engine don't "surge" as such with a std setup but would more indicate something amiss that has not been quantified.

The reference to bench setup in the tips section is to very particularly include the routine contained in establishing the correct offset between carb slide position and overall idle speed control mechanism. I'd encourage anyone to fully apply that in rebuilding these carburettor to prevent effectively unseen errors while using.
Without that,  you can build in a mechanical conflict that becomes apparent as you try to set the idle rpm speed with the adjuster as it's possible to force a shift in balance between the 4 carbs by doing this. It shouldn't be there, obviously,  but completing that routine will remove that potential.

Carb balance is not in reality set for idle speed. That may seem contradictory, but it's primarily to set parity of slide height for wider open settings with idle speed smoothness a side effect resulting from this adjustment. 

Honda gave in their manuals at the time a routine to then fine tune each individual cylinder's speed at idle to trim that point of running,  once the parity of slide has been established, which is micro adjustment of the airscrew individually.

I'd,  at this point in how it's running, argue not to set the sync using gauges. If you've a problem on one cylinder that's not fully identified, then setting by using vac guages can put a compensating error into your carburettor balance that's not needed. Yes it may sound better at that point, but it's wrong with other mechanical errors needing attention first.
Pre requisites for vac synchronisation are that all other parameters are within adjustment before that is employed.

Bench synchronisation avoids the above as it's independent of engine condition and running problems residing elsewhere, also perfectly good enough to run the engine accurately.  Guages don't hold the upper hand here and it may help to diagnose faults by leaving accurately "bench" set as detailed.

Listing order of work, bench sync as tips post on here, then assess what you've got without further changes. It may influence the condition, but you can only check that one way.
Then look at idle air setup in manual and complete that to assess effects.

Offline K2-K6

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Re: The elusive surge
« Reply #12 on: January 10, 2022, 10:34:15 PM »
"Dennis

PS  Just thought I would add a small note to explain that for example in the same period (1970 - 1980 )that with a 6 cylinder car engine with 2 or 3 Carbs, to overcome a tendency for imbalance on idling the manufacturer would tend to put a small balance pipe connected across the manifolds so that all cylinders could suck Air/Fuel mixture, even though the individual Carbs may not be set spot-on."

That's generally used on SU, Solex type constant velocity carburettor and not essentially to promote mixture sharing but to give vacuum parity across the different intake tracts as it's the vacuum variation that would cause the fuel air metering slide to run at different heights in spite of each throttle butterfly being equal in position. 

These engine having a fixed relationship from one slide to another don't  manifest that variance, and so don't need it.

Offline McCabe-Thiele (Ted)

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Re: The elusive surge
« Reply #13 on: January 11, 2022, 01:07:44 AM »
Hey Julie, finally got time to search for it, I knew it was buried somewhere in my mailbox.

The plugs are NGK D8EA and the caps fitted on it are XD05F and VD05F. The leads are originals, same as the coils.

Ted: I did do that, I have a daytona electronic ignition trigger, there is an LED that turns on to show when the sparks will happen and I'd say it is right on the dot. It was a static test though.
You'd recommend getting a stroboscopic light?

K2-K6 I have bench synced them yup. Some lad at work advised me to do a running sync to make sure everything is tip top on that front. Even with a bench sync you can get inbalances that throw off the whole thing.

Daz-J did you check float heights?

Thanks for your help guys, means a lot!

Take care,

Alex


The bonus of a strobe check is you will see the timing fluctuating - that said it would not get you any further forward as a carb inbalance causing a surge would probably bring in the advance/retard mechanism thus showing timing fluctuation. Someone here will know at what r.p.m the advance trips in.
Sounds like you have it right statically with your method.

I did my timing statically initially and then dynamically - there was a very slight difference between the pairs of cylinders,  after adjusting 2 & 3 it made no obvious difference though I was not able to ride it on the road to check for what it felt like in use.





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Offline Bryanj

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Re: The elusive surge
« Reply #14 on: January 11, 2022, 01:37:41 AM »
Should be on full advance by 2500 rpm

 

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