Flat spot R80

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Andymarksonline
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Flat spot R80

Postby Andymarksonline » Thu Sep 14, 2017 6:52 pm

Our '83 R80 RT has a bad flat spot around 3k. Starts fine and after 3k is sweet but it's quite bad between 3k and 4K.

It's had a top end rebuild and we've had the carbs apart new floats and seals in them. The valves have been adjusted according to the manual.

The bean can was removed but has been put back exactly where it was before (photographs taken judiciously!). New spark plugs gapped and fitted too.

We've tried the mixture adjustment at various settings but there seems to be no difference (only when idle).

Any ideas (I'm aware this is a shot in the dark lol!).

Thanks
Andy.


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Re: Flat spot R80

Postby windmill john » Thu Sep 14, 2017 7:41 pm

Is the flat spot at a certain throttle opening?

I am going to hazard a guess that the throttle is at zero to a quarter on. This will point to carb settings, cleanliness.

Points or electronic ignition?
Worth checking timing statically and dynamically.
Putting the can back on where you think it was is not really accurate enough. It'll run, but...
If timing is genuinely correct, get in those carbs again, or at least make that you set them up after a twenty mile run.

83... I guess electronic. Still, it's an Airhead... carbs... :smile:
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Flat spot R80

Postby Andymarksonline » Thu Sep 14, 2017 8:04 pm

Is the flat spot at a certain throttle opening?

I am going to hazard a guess that the throttle is at zero to a quarter on. This will point to carb settings, cleanliness.

Points or electronic ignition?
Worth checking timing statically and dynamically.
Putting the can back on where you think it was is not really accurate enough. It'll run, but...
If timing is genuinely correct, get in those carbs again, or at least make that you set them up after a twenty mile run.

83... I guess electronic. Still, it's an Airhead... carbs... :smile:
Thanks, yeah in my mind it points to fuel issue. Only rode it round the block the other day and it was flat spotting. The front wheel is now off as it's getting new rubber but even at a standstill and revving it you can hear it's not right.

Funny thing is the carbs are very clean (had a full drop down and clean with carb cleaner and a blow out with the airline). Was diligent with stripping and re assembling too as I've encountered problem with another r80 due to mistakes pitting them back together.

Plugs are very black and sooty which points to it running rich but we've tried a multitude of mixture screw settings hasn't made a difference!

Tempted to swap the carbs from the other one that's running great to see but I'll have to get permission for that as it's not mine

Regarding timing, I've seen so many posts and you tube videos about it but none seem to cover this bike. The Haynes and clymer manuals don't cover it either (they cover up to 1980 which is helpful though they are for this bike!). I know the principals of timing and electronics in general but never had experience doing it!

It's all a learning curve... before this year I'd not touched a carb since my teens with the tzr125!!!

Andy.


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Re: Flat spot R80

Postby boxerman » Thu Sep 14, 2017 8:07 pm

Check you put the carb needles back to the correct slot when you rebuilt.
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Re: Flat spot R80

Postby andyb » Thu Sep 14, 2017 9:22 pm

My R80R had a flatspot at 4000 rpm due to a T piece exhaust 2:1 connector...but not as exterme asyou are describing.
Sounds like either the carbs, carbs or carbs.....did you clean out the small tube from the air intake end of the carbs to the emulsifier below the main jet?
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Last edited by andyb on Sun Oct 01, 2017 8:52 pm, edited 1 time in total.
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Re: Flat spot R80

Postby Andymarksonline » Thu Sep 14, 2017 9:30 pm

Thanks guys.

We didn't touch the needles so can't be that. I think we'll have to strip them down and blow them out with carb cleaner again.

Andy.


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Re: Flat spot R80

Postby SteveD » Fri Sep 15, 2017 9:31 am

Check timing, then valves. Then ensure the tiny holes in the head side of the carb throat are clear. They're from the pilot jet and the mixture jet.
Sooty plugs sounds rich but they're best assessed after a good plug chop. Maybe screw in the mixture screw an 1/8th of a turn.
Go for a good 30 minute ride and get it well warmed up after doing that. If it still does it, go another 1/8th on the way home.
Cheers, Steve.
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Re: Flat spot R80

Postby IanB » Wed Sep 20, 2017 5:52 pm

Also - consider the ignition advance/retard. 3000 is about where it goes on to full advance, and if it's dry or sticky, or if there's dirt in there, it might be misbehaving. It is not unusual for there to be a slight flat spot around this engine speed on many airheads, but it should not cause you any actual trouble. Just drive through it between 30 and 40 MPH in 3rd gear.
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Re: Flat spot R80

Postby barryh » Wed Sep 20, 2017 7:05 pm

If you haven't already tried it, as a experiment raise the needles one notch and see if that fixes the flat spot.

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Re: Flat spot R80

Postby CharlieVictor » Thu Sep 21, 2017 8:05 pm

I would first check that both your throttle cables run freely. I had a similar issue on my R100S just off-idle, and it was the left carb cable that was stuck somewhere. I redid the routing and everything was fine afterwards.
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Re: Flat spot R80

Postby Andymarksonline » Sun Oct 01, 2017 6:06 pm

Well it’s been a while but haven’t been able to get into the workshop for a few weeks.

After stripping the carbs again and rebuilding them cleaning everything it still ran with a flat spot!

We did the valves first thing this morning they were a little tight but not too bad.

Luckily we have another r80 exactly the same that runs fine to compare to,so in the end we swapped the carbs over with the donor Bike and guess what... no flat spot but the donor Bike now has one!

Should note that we noticed the needle was in a different position (higher) in the carbs on the donor Bike so we changed ours to match. Also checked the jet sizes and they match. Presume the different needle positions is due to the aftermarket exhaust which is the same on both bikes.

God knows where we go from here... maybe a new set of jets though they looked fine I k ow it’s difficult to tell by eye.

Andy.


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Re: Flat spot R80

Postby windmill john » Sun Oct 01, 2017 6:33 pm

You could swap all the jets to test for carb obstruction
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Flat spot R80

Postby Andymarksonline » Sun Oct 01, 2017 6:44 pm

You could swap all the jets to test for carb obstruction
That was what we were thinking of just before we decided to give it up for the day!



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Re: Flat spot R80

Postby Andymarksonline » Sat Oct 07, 2017 12:49 pm

So before swapping any of the jets from the carbs that work fine I thought I’d completely strip and clean them. Did this morning cleaned out all the passages with some soft wire and thoroughly cleaned everything with carb cleaner blown through all passages and jets.

Reassembled and run the Bike, same flat spot!!!

Next job is the swapping of parts from working carbs to bad ones, something I was trying to avoid!


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Re: Flat spot R80

Postby Andymarksonline » Thu Oct 12, 2017 7:59 pm

Well managed to get into the workshop today and have a tinker. After a bit of messing around swapping carb parts from the donor Bike narrowed it down to the main jets. New ones on order from motorworks.

Andy.


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Re: Flat spot R80

Postby windmill john » Fri Oct 13, 2017 7:11 am

Slightly off topic.. ish.
I dropped my floats slightly to cure a high tick over, floats were new so riding higher.
I now have power restriction over the whole range.
It is not excessive, just noticeable. A little like someone holding the back of your bike when setting off.
It’s amazing how much a slight change in float height affects fuel delivery. I understand the principle, still amazing though.
So, not a flat spot, unless you mean at every speed!
This weekend I am adjusting the floats a little, damn the tick over :grin:
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Flat spot R80

Postby Jaythro » Fri Oct 13, 2017 9:54 am

Why not sort the tick over?

I had an issue with one which turned out the owner had fitted later carbs because he thought they were better

There is a big difference between earlier carbs @ 1 to 2 turns out versus the later mono at 1/2 turn out on the airscrew

That's mostly all it took

Do you know of the wasted spark technique, to set her up John?
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Re: Flat spot R80

Postby windmill john » Fri Oct 13, 2017 6:37 pm

I do have R80 carbs fitted to my R65, but this is due to a buggered float valve seat.
I had no problem with my tickover until I rebuilt the carbs with new diaphragms and floats.
I’ve rebuilt Bing carbs many times, never had this issue, tickover at 1500 rpm.
I don’t recall using these diaphragms before, came from Motobins, a little stiffer than my usuals.
The only way I could get the tickover down was by lowering the floats.
I could try replacing the diaphragms, just don’t want to waste that money.
Yes Jaythro, I use the wasted spark method.

The number of times I tried to correct this, drove me crazy!!!
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Flat spot R80

Postby barryh » Fri Oct 13, 2017 6:37 pm


It’s amazing how much a slight change in float height affects fuel delivery. I understand the principle, still amazing though.
I too found new floats to run lean when set parallel to the extent that it was really affecting power output so I initially set them slightly above parallel to get the engine running better. After a year I dropped them back down to parallel again and the bike ran fine which seems to show they had already absorbed some fuel.

All this got me a little paranoid about float level and the fact that even new floats set parallel isn't necessarily right. All the usual methods of setting them don't measure the actual operational fuel level and I was wishing we had that level specified by Bing.

After my last set of new floats had been in for a year I decided to create a reference point by installing a U tube to measure the real levels. I already knew the fuel level would be quite high and due to the carb inclination it would be all but lapping over the front edge of the bowl. I measured the level centrally at the position of the main jet and found that to be 6.5 mm below the flange. If you sight that along horizontally the fuel level is indeed about level with the front edge of the bowl. The whole point of the exercise was to be able to reset the level accurately if and when the floats get heavier still.

Having had a previous bad experience with old floats being of unequal weight and that having a big impact on the mixture I was also paranoid about both carbs having the same level. I was almost surprised that the U tube measurement showed that they were exactly the same.
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Re: Flat spot R80

Postby windmill john » Fri Oct 13, 2017 8:24 pm

The great thing about the Bings is that you can be quite far out with settings and still have a very rideable bike.
Besides my power reduction, I have a slight throttle imbalance which is annoying me more than the power reduction. Enough annoyance to get it resolved this weekend.
For Bing newbies, with the throttle imbalance, it’s easy to jump to the conclusion it is throttle cables, but due to this being at zero to 1/8th throttle, it can very easily be mixture screw or even idle jet.
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Flat spot R80

Postby Andymarksonline » Fri Oct 13, 2017 9:17 pm


It’s amazing how much a slight change in float height affects fuel delivery. I understand the principle, still amazing though.
I too found new floats to run lean when set parallel to the extent that it was really affecting power output so I initially set them slightly above parallel to get the engine running better. After a year I dropped them back down to parallel again and the bike ran fine which seems to show they had already absorbed some fuel.

All this got me a little paranoid about float level and the fact that even new floats set parallel isn't necessarily right. All the usual methods of setting them don't measure the actual operational fuel level and I was wishing we had that level specified by Bing.

After my last set of new floats had been in for a year I decided to create a reference point by installing a U tube to measure the real levels. I already knew the fuel level would be quite high and due to the carb inclination it would be all but lapping over the front edge of the bowl. I measured the level centrally at the position of the main jet and found that to be 6.5 mm below the flange. If you sight that along horizontally the fuel level is indeed about level with the front edge of the bowl. The whole point of the exercise was to be able to reset the level accurately if and when the floats get heavier still.

Having had a previous bad experience with old floats being of unequal weight and that having a big impact on the mixture I was also paranoid about both carbs having the same level. I was almost surprised that the U tube measurement showed that they were exactly the same.
20170928_161859A.jpg
Now that’s clever!


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Re: Flat spot R80

Postby Jaythro » Fri Oct 13, 2017 9:26 pm

The only way I could get the tickover down was by lowering the floats.
Should work in principal What numbers are your carbs and I can check the holy grail for you

Also why not lift the needle one stop or stick a couple of small washers below it to lift it a mill ???
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Re: Flat spot R80

Postby andyb » Fri Oct 13, 2017 9:29 pm

In a static situation I can understand the readings from the sight tubes but have the bike bouncing over some bumps, braking or accelerating and the fuel level in the carb bowls will not be very level! Also consider sideways forces if the bike has a chair.
As JW says, Bings are thankfully pretty tolerant. Best is to make sure that both carbs are the same and go from there.
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Re: Flat spot R80

Postby windmill john » Fri Oct 13, 2017 9:47 pm

Thanks Jaythro, raising the needles means replacing the diaphragms; maybe later when they are due to be changed.

I’ll check the numbers tomorrow, but luckily I have the holy grail Bing book :grin:
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch 1978-80 R65 - bit of a Shetland pony; frisky and naughty.
Friedrich 1993 R80RT - a Percheron, but can still mix it with the rest.

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Re: Flat spot R80

Postby boxerman » Fri Oct 13, 2017 9:52 pm

Thanks Jaythro, raising the needles means replacing the diaphragms;
Not on any airhead I've ever worked on..
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