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Old 02-26-2012, 11:56 PM   #1
Voltaire OP
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Carbs or Ignition

I'm hoping to get some ideas.....
My racebike is a 900 with a 336 and 36mm Dells running total loss electrics does not like to idle, dies if less than 2000 rpm
The Dells were a swap meet special and I have rekitted them and recently tried some new( er) slides....still no good.
I suspected it was an air leak or idle circuit issue....now not so sure.
The ignition is the base model Boyer....and after doing a bit of net trawling I'm wondering if it is not really up to the job.
Battery is a 32amp yeller Motobatt.
Once bike is running its ok, but it won't pull past 6500 in top.
Anyone had issues with these units? I gather they are pretty old tech and not that happy at low revs, any input appreciated....as I don't have an alternator mounting anything else could be a challenge.
cheers
John
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Old 02-27-2012, 02:10 AM   #2
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i tol ya bifor din i

Any engine with cam,intake and exhaust mods will need more advance at lower speeds. Before you go changing too much try advancing the ignition at idle - if your idle speed improves and increases then you are on the right track. There is just not enough manifold vacuum, and ignition needs to be advanced. I've run engines with silly cams on 38 deg advance - and 24 deg at idle. A hot cam in a BMW is about a Briggs & Stratton grind, so it's not like you dealing with 120 deg overlap....but it will have less vacuum than a standard cam.
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Old 02-27-2012, 10:58 AM   #3
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Geez guys, I came to watch the blood letting and you're debating lawn mower cams?? I'll go and sit outside Voltys' house with R80 idling just to annoy him....
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Old 02-27-2012, 11:15 AM   #4
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If I was you I'd put it all back to standard .
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Old 02-27-2012, 11:27 AM   #5
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Dell's should idle perfectly and pull well beyond that RPM. You have troubles at both ends of the rev range and maybe in between? I have put a sport cam and Dell's on three bikes and never had to readjust ignition timing off of stock. One of them was a 900. Since you have got troubles all over, you need to start with the basics. Leakdown, compression, and testing the ignition. I highly suspect that a lot of the 336's reputation of not idling well if at all is from the cam exacerbating a problem that was already causing idling issues before the cam was installed: Leaking valves.

supershaft screwed with this post 02-27-2012 at 11:34 AM
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Old 02-27-2012, 12:11 PM   #6
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Originally Posted by supershaft View Post
Dell's should idle perfectly and pull well beyond that RPM. You have troubles at both ends of the rev range and maybe in between? I have put a sport cam and Dell's on three bikes and never had to readjust ignition timing off of stock. One of them was a 900. Since you have got troubles all over, you need to start with the basics. Leakdown, compression, and testing the ignition. I highly suspect that a lot of the 336's reputation of not idling well if at all is from the cam exacerbating a problem that was already causing idling issues before the cam was installed: Leaking valves.
Bit of background. The engine has been completely rebuilt with new bearings and rings. Valve guides have been relined and a 3 angle valve job done.The motor has only run for a matter of hours.
The idle thing is more just annoying than anything as it should idle, once its running it pulls right into red line no worries. Its only on the very long back straight that it wont pull past 6500 in top.
I won a race on it against two BSA Gold Stars and a Dunstall Commando so I am reasonably happy with its performance.
Is certainly a different bike to ride than Maxheadrooms R90s, which seems to run a lot smoother.
You are correct with saying start with the basics. I intend to pull out the Boyer and stick points in to eliminate that.

I had issues with a Boyer 20 years ago in Germany on a /5 and sidecar.
( Pic in from back in NZ but you get the idea)

when it was below 0 degrees it was hard to start and needed the box warmed up....put up with this all the way back to London , shoveled away the snow fitted the points and sent Mr Boyer back his ignition.....which they replaced.
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Old 02-27-2012, 12:32 PM   #7
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Relined guides? I thought people no longer did that to cars let alone bikes? The fact that your engine was just assembled by I guess someone that relines guides is all the more reason to start with the basics. Valve jobs leak and rings don't seat and that is the tip of the iceberg.

6500 in top gear doesn't tell me much without knowing your FD gearing but I suspect it should be doing better. What is the last gear it will pull 7500 in? But before I ask that question I believe most all /6's should have the oil pickup galley to the pump enlarged? I would have titanium valve spring retainers in any race engine. I have them in my street engines! I couldn't rev my engines above 8000rpm without them. Even if you don't regularly rev your engine that high they are still good insurance for missing gears.
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Old 02-27-2012, 12:52 PM   #8
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Idle: I suspect you just need to fiddle with settings or different idle jets to get it to behave. 36mm will most likely be different from the known functioning 38mm Dells in R90s, but i would think your idle jet numbers should be close. I would start with mixture screws around 1.5 turns out and overly raised slides, get it warm and start adjusting mixture then speed, rinse, lather, repeat until it is happy. If your mixture screw bottoms and idle was still picking up, you need a leaner idle jet. Conversely if you screw out more than 3 turns or so.
It should behave well with stock ignition settings.. SS asked about lower gear behavior, that is needed info. You may be off on your main jet if you cannot pull through in top gear. Lean is mean, but too lean will not rev out, nor will too rich. If it is jetted right, you may just be up against physics. 38mm Dells and 44mm intakes will work well for your setup.
My R75 runs 36mm Mikunis, std timing and idles embarrassingly well, yours can too.
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Old 02-27-2012, 12:56 PM   #9
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Oh, Lose the fat battery! Good God man! If you have to keep the starter, get a Shorai.it will work all weekend. Better yet, lose the starter and get a 3A to run your ignition. Make your lazy friends push start you. It is the least they can do for the show you give them.
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Old 02-27-2012, 02:36 PM   #10
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Oh, Lose the fat battery! Good God man! If you have to keep the starter, get a Shorai.it will work all weekend. Better yet, lose the starter and get a 3A to run your ignition. Make your lazy friends push start you. It is the least they can do for the show you give them.
Looks like I better get my roller starter finished.
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Old 02-27-2012, 02:40 PM   #11
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I can't remember are you running dual plug heads?

I use an old Boyer analogue ignition running two Dyna brown coils to the twin plug head with 40mm Dells. It will idle happily at 800 rpm. I actually have it set so it doesn't idle when cold , but at full temp it chugs along at 800, just so I don't accidentally stall it on the grid.

It does sound a bit like an air leak, but if that were the case, I'd expect it to not want to drop back down after a blip of the throttle. That's usually a good sign of an air leak.

The one thing you might check is strobing the ignition. Despite what Boyer say, it keeps on advancing to 5000 so that;s where I strobe mine. It might upset the neighbours, but hey. With twin plugs I time it at 28 degree at 5000rpm.

As for the 6500 in top gear, it may just be over geared for the engine.

Here's a graph of speed vs RPM assuming a 130 rear tyre, but no matter what tyre you're running, it will be in the ball park. At a rough guess I'd expect to see maybe 120mph from a moderately tuned bike ?? Maybe??

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Old 02-27-2012, 11:18 PM   #12
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Cheers guys for your input ....I'll go and do some fettling.....
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Old 03-01-2012, 12:33 AM   #13
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I downloaded a Degree Disc, laminated it and fitted it to the crank with a pointer.
Ran the engine up to 3000 and 'z' appeared. Degree Disc read 30 degrees.
Advanced the pickup plate a bit more at 3000 'z' moved to top and Degree Disc read 34.
Higher revs result in more advance.....
Reading the Pazon vs Boyer article it describes the operation of the Boyer analogue quite clearly....bit of a dog by the look of it.
Sadly Pazon don't do one for BMW airheads.
FD is 3.0:1 and it pulls 4th really well, tacho is a bit vague so I'm wary of going into the red, but the back straight is soooooo long you have plenty of time to look at it. I must get a rev limiter off Ebay.

I managed to get it to idle at about 1500 with the timing advanced to 34 degrees and this gave 15 degrees.

SS, whats your spin on guide liners, they look pretty good to me, and once shown I did some VW heads too.

Next I'm going to try a set of points as I don't have a alternator to mount a crank fired unit on.

I'm going to get a Pazon for my Norton after reading about the Boyer
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Old 03-01-2012, 12:41 AM   #14
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I have no problem with the boyer. It sounds like the ignition is over advanced

Use your degree disc to make a mark on the flywheel ( a blob of white paint is all I use) at 28 degrees if you're running twin plugs, 31 degrees if not, and strobe it at 5000rpm.
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Old 03-01-2012, 01:28 AM   #15
Voltaire OP
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Originally Posted by pommie john View Post
I have no problem with the boyer. It sounds like the ignition is over advanced

Use your degree disc to make a mark on the flywheel ( a blob of white paint is all I use) at 28 degrees if you're running twin plugs, 31 degrees if not, and strobe it at 5000rpm.
Cheers, I'll try that tomorrow.
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