High Speed stutter

[steve hawkins]

When I ride my R65 cafe racer at sustained highish speed, say cruising at aprox 75mph, if I then try and accelerate, still in 5th, the engine bogs down a bit and stutters/coughs.  If I grab another handfull i.e. to open the throttle further, it eventually (after not more than a couple of secs) clears and onwards we go.

If I accelerate directly beyond this speed we do not have an issue, i.e. we move past the critical point.

It sounds carb related or perhaps the single tap?

The Cafe Racer has at least 100,000 miles on it (probably 104-105) and so do the carbs.  They we recently rebuilt with new seals etc, so I not think I have an issue which will be solved with a carb rebuild, as such.  I am more thinking along the lines of jetting or perhaps, since the carbs are a bit ong in the tooth, perhaps the slide is sticking (100,000 miles cruising at aprox the same throttle setting?)

Any thoughts?

[Jon_P]

yep i would agree on carbs. so where are you at on the throttle at 75mph? sounds like you are hitting your main jet when you wick the throttle. funny every one messes with the main jets but they dont really hit till your at that 3/4 and up range. and this sounds like your mains or fuel supply.

now before the rebuild you did not have this problem? so everything was working like it should before? wonder if when you wick the throttle it is starving for fuel? like the floats dont open the needle valve fast enough?




btw steve see the cafe project for sale? sharp.

[Bob_Roller]

Were the rubber diaphragms in the carbs replaced, it sounds a lot like the problem when one of mine cracked. Have you checked the metal screen filter that is attached to the fuel petcock for possible clogging?

[Justin B.]

Maybe raise the needles a notch or fatten up the idle mixture screws a half turn?  I would think a diaphragm issue would cause it to "hit a wall" and not go any further and a fuel supply issue wouldn't "catch up" and take off after a couple of seconds.  Maybe a vacuum passage that supplys vacuum to suck up the slides might be restricted?  Maybe the bike is trying to keep you from getting a speeding ticket?

[thrang]

Doubt if its a diaphram problem as when mine gave up the ghost the bike would just stop accelerating at around 70mph. On top of what Justin's just posted, I'd add are the carbs balanced, and when were they last cleaned? Although its unlikely I'd also check the tank breather as it could be as simple as a bit of crap blocking the hole and slowing the fuel feed.

[nhmaf]

It does sound a bit like the fuel starvation problem - I'd certainly check the screens
in the fuel tank/petcock assembly, and if you have an inline fuel filter (which I
don't think that you do) I'd also pitch that and see if the problem persists.
If so, it might be a float sticking a bit or possibly a needle with a bump or rough spot
on it that impedes its movement, since the last 1/4 of throttle travel is much less
often used.
 :-/

[steve hawkins]

Yes, we all seem to be in more or less agreement.  Carbs/jetting/mixture wrt main jet or fuel supply.  Chucked the inline filter ages ago...All I have is a newish petcock with a standard screen

The problem has been there for a long time and the carb 'rebuild' including new diaphrams has not affected it.  I have been living with it as I normally accelerate through it, but I loaned the bike out to my brother the other day and he stumbled across the hesitation.  Its more a clearing of the throat, before it starts to accelerate.

So I feel I need to give fixing it another crack.

I have the bike set up with standard settings as the engine is more or less standard even if it is a mixture of pre and post 81 parts - post 81 heads on a pre 81 engine - lightened pre 81 flywheel - but still nowhere near as light as a post 81 flywheel.  Even with the different pipes and the loss of the airbox.....I am running individual filters directly on each carb.  Looking at the plugs, they seem to be the right colour, (mid/dark brown) on a run.  The hesitation was there before I changed the pipes or removed the airbox.

A session on the rolling road would probably find the issue quickest, but I have not currently got any money to throw at it.  So I have to check the cheapest things first.  I wonder if my brother would let me borrow is R100 GS tank with a dual feed?

Cheers

Steve H

[Jon_P]

i forgot that you had air pods on it. that is one of the things that does happen with pods. i have never had them on the r65 but i have had them on others and have had weird things like hearing it go a little lean as a large truck pass by even had on a 4 cylinder where the outers go rich and the inners go lean. pods can be thicky. my gut tells me its the biggest part, you are letting the engine get all the air it wants in and which is a free flow in, the only thing is you are still using a stock exhaust which limits exit gases so it does it a wall to say till it evens outs. noe i have a unrestricted exhaust on mine, you dont want that. it is to loud. my wife said it loouder than most harleys around here.

[steve hawkins]

It is not a stock exhaust, but it is stock headers.  I have some rather loud BSA style 'silencers'  which gives my wife plenty of warning to kick the boyfriend out when I am coming home!

That said, all they have a simple spoon baffle inside and I know they are limiting the outright top speed a smidge.  That is not a problem as such, as I rarely drive at 100mph on my local roads, as if I was caught, it would be an instant driving ban for a year, followed by loss of job, loss of house, loss of...well, you get the picture.

This issue existed before I put the pods on, and I think also before I put the silencers on.  But I must have a swap around and see if I can clear it.

Steve H

[steve hawkins]

Well,

I seem to have got rid of the stutter.  I put in a 150 main jet and lifted the needle - i.e. the clip is now in the lowest posistion.

This is since I am running post 81 heads on a 79 bike with flat top Bings.  The UK settings for the jets for a post 81 bike are 145 main jet and 3rd from the top for the clip on the needle.

However, this has highlighted that I have definately got a top end restriction.  My top speed is only 90 mph in 5th on the old bike computer - loss of a full 10 to 15 mph, this is with the throttle held fully open, and I checked to ensure that there was no more available movement on the carbs.  

If I recall, in a CV carb, it is the airflow through the carb that determines how high the needle is lifted, due to differential of air pressure either side of the diaphram.  Therefore if the airflow through the carb is not high enough, due to some restriction upstream or downstream, then the fuel being pulled into the carb will also be restricted, and that will prevent the engine form reving out.  I was at full throttle in 4th and could not get it beyond 85, and I did not feel that is was redlining - not that I have a tacho fitted.  It just would not give any more!  I couldnt hear the valves bouncing!

I will have to have a look at those short 'silencers'.  The spoon baffle  (a small dished disc of metal with holes drilled in it) at the front end of the silencer, if I remember rightly, would seem the likely cause as it effectively restricts the airflow across the point that is the similar diameter as the headers (accounting for the dish shape of the baffle).  I will have to get my drill out, if I can reach the baffle, and make the holes bigger.  

If not I will have to punch the baffle out, but that will make an already loud bike louder - probably too loud, even for me.  As that is all there is to the inside of the silencers - a single spoon baffle.

If all else fails, I might be refitting the standard looking silencers, which are louder an free flowing than stock....or look into something else...

Comments?

Cheers

Steve H

[Jon_P]

supertrapps mini cans like the ones i put on. you can tune the louder or quiter to your needs, or in this case more flow or less flow out of the exhaust.

i thought it sounds like a restriction but with the pods and exhaust mods i wouldnt have guess that it was with you current set up. the supertrapps are not cheap but work nice though you will most likely have to make a adapter like i did to get them to fit the stock pipes.

[Jon_P]

well i am in some of the same boat your in steve. i didnt know it but i have hit the wall at around 70 mph. i have some new mains on order to see what it will do. the only thing that buggs me is thrang commit about his giving up the ghost and all it would do was 70 mph cuz i'm close to that?

i didnt order 150 mains like you i only went up two and then order two above those so i will get 144 & 146 but will keep heading up if it helps.

[thrang]

It was bloody embarrassing Jon. The Bimbo was struggling that hard my missus on her Suzi GS450 was able to out accelerate me on the up hill sections going to Stainmoor summit on the A66. One set of expensive diaphragms later (from Mills BMW in Sunderland-ouch!) replaced in a field at the Axeman rally and the Bimbo was pulling like a train again.

[nhmaf]

In most cases involving those types of pods in my limited experience I've found it difficult to tune things.
I'm not sure if it may be related to the engine running more or less lean at different RPM ranges
than can often be adjusted out with a basic jet swap, or that there may be a bit of bernoulli effect
occurring (air isn't being rammed into the carbs at high speed, and perhaps the high velocity wind traveling
in the opposite direction past the pods (like in the earlier mentioned "truck" example) is making it hard to
maintain the proper velocity/vacuum in the carb for full fuel atomization at those higher throttle openings.
As weird as the snorkel airbox is, it would seem that it is designed to help maintain some small positive
airbox pressure due to the front facing inlets.

I'm not an aeronautical engineer, just an electrical one, so my hypothesis in this topic may be completely misdirected.

I assume that you've probably got rearsets on your cafe, but if not (or if you still have your pillion footpegs still on, maybe
try moving your feet back to the rear pegs to see if the change in airflow around your legs/feet have any impact - good or bad ??

[Ed Miller]

The symptom of a too-rich main jet is a rhythmic stuttering called 8-stroking.  My only experience with this is with my Triumph, which already had free-flowing separate intakes and K & N air filters, since replaced since I don't enjoy rebuilding my motor THAT much.

I put TT pipes on my Triumph, which lack the stock cross over and are larger in diameter than my stock pipes.  I did not change carb jetting at first, but experimented to see what I needed.  The bike 8 stroked at full throttle, even if I eased up on it, so I found smaller mains.  I ended up going to .170 mains, from .190 stock mains, before the stuttering went away and it ran clean on full throttle.

Yes, the bike was slower, not to mention that it made my ears bleed.  The cornering clearance was great.  I have since refitted a stock exhaust with one cross over, stock main jets, and if I don't rush it, I can pull full throttle with smooth running.  I don't know the top speed but it's faster than my friend's R75/5.  I think the reason I can't just yank it to full throttle is that it is maybe still a tad rich, but I'm comfortable with that since I welcome the extra cooling, if I'm going to ride like that very long.  I never have, though.  

The symptom of too small mains is the bike will pull stronger at slightly under full throttle than it does at full.  The trouble with these rules of thumb, and not just the ones I've mentioned here, is that our R65s have CV carbs, so that the throttle position is not directly related to the carb circuit.  But if you think you are definitely running on the main jets, and it may be 8 stroking, consider trying some smaller mains to see what difference that makes, not just assuming that you need larger ones for your better breathing motor.  Anybody want some .200 Amal main jets?