Author Topic: B31 Poor Performance  (Read 1901 times)

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martinh10

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Re: B31 Poor Performance
« Reply #15 on: 13 June, 2021, 22:26:03 »
Thanks Idie I'll give that a try.

Calum

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Re: B31 Poor Performance
« Reply #16 on: 14 June, 2021, 18:48:40 »
Hi Bess, Yes I've tried the needle either side of middle position which seems about right. The standard main jet is a 150. I've also tried a 160 and a 170 but reverted to the 150. At the moment I'm really struggling to see what it could possibly be.

Have you tried a smaller main? It might be too rich at wide open throttle... for any given speed, a larger bore will be drawing faster across the bridge and so more fuel. I wouldn't have thought a rebore of +.030" to have made much difference really but I know it can do. A friend had to mess about with his S1 Land driver carb jetting after only a .020" over bore (as a percentage it might be greater, 3.046" standard bore on a 2l) and it made a big difference.

I would have thought a mag would only get better with speed, and you'd get erratic missing etc not just like a speed limiting effect?

martinh10

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Re: B31 Poor Performance
« Reply #17 on: 14 June, 2021, 19:57:01 »
Hi Calum, I agree with you on the magneto - I now don't believe that's the problem. Also it doesn't appear to be running rich - I think the 150 main jet is right. Lying in bed this morning, wondering whether to bother getting up I developed another theory as to what the problem may be. I'm busy tomorrow so I'll try it out on Wednesday. Wish me luck.

tdc

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Re: B31 Poor Performance
« Reply #18 on: 16 June, 2021, 17:19:38 »
My1957 BB33 hc. returned from America would die when the mag got hot .so rewound commutator .new valve springs and exhaust valve. Gold star exhaust and silencer and it performs very well indeed .I don't know if anything has been done to the bottom end but the new exhaust has made it very responsive . Good luck .

martinh10

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Re: B31 Poor Performance
« Reply #19 on: 16 June, 2021, 17:34:07 »
More disappointment! I had convinced myself (with the help of John Robinson’s “Motorcycle Tuning: Four-Stroke”) that the oil could only be getting so dirty so soon due to exhaust gas being drawn into the cylinder with the fresh gas on the intake stroke, resulting in a significant decrease in volumetric efficiency.
Given that the valve and ignition timing was known to be correct I thought the only other obvious contributory factor was the exhaust system. The B31’s I had in my youth were remarkably insensitive to all sorts of abuse of the exhaust system but I thought the high level system I had fitted to this one (see photo) may be the problem.
I still have and refitted the tatty standard low level exhaust, which at a total length of about 61” is 9” longer than the high level. I have been out on the bike today and regret to report no change in the dismal performance.
I think I’m going to throw a sheet over it for a while and get on with my RD400 restoration.


Calum

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Re: B31 Poor Performance
« Reply #20 on: 18 June, 2021, 13:20:27 »
I'm running the same pipe as you (an Armours one) and I have to say it seems to go quite well indeed. In fact the bike kooks nearly identical!
I'm currently waiting for my mag to return from overhaul as it was not behaving at all when hot (ruled everything else out)

martinh10

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Re: B31 Poor Performance
« Reply #21 on: 18 June, 2021, 13:49:49 »
Cheers Calum, hope you're sorted when you get the mag back.

martinh10

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Re: B31 Poor Performance
« Reply #22 on: 24 June, 2021, 13:51:57 »
I’ve had to get back to the B31 - this lack of performance is really bugging me, especially after all the money it’s had spent on it (way more than it’s now worth). I’ve taken the head and barrel off again and again, after less than 50 miles, the piston crown has thick carbon deposits on the spark plug side/middle but nothing on the opposite side (see photo). The scratches are where I have been poking a screwdriver through the plug hole before lifting the head and the loose bits on the farside are where I have scraped some carbon off.
To me it looks like incomplete/poor fuel burn in the combustion chamber but I just can’t see any reason for that. As stated previously everything is bog standard with a new 276 carb, a really strong spark from a newly reconditioned mag, new rebore, piston and rings, new valves, springs and guides. Valve and ignition timing has been rechecked and is correct.
Any thoughts anyone?

Pete C

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Re: B31 Poor Performance
« Reply #23 on: 24 June, 2021, 17:23:19 »
just out of interest, what sparkplug are you using?

Gadorey

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Re: B31 Poor Performance
« Reply #24 on: 24 June, 2021, 17:59:26 »
You mention you have a new carb (276). But what size is it?
Also, you’ve made sure the decompressor is not touching the exhaust valve while the engine is running?
Maybe a valve sticking open when running, or catching causing a delay in closing?

Running out of ideas here, but very interested in what you find. 

Gui

Gadorey

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Re: B31 Poor Performance
« Reply #25 on: 24 June, 2021, 18:12:25 »
Just to make sure everything is correct with the carb.

For the B31 it should be a 1” carb, 150 main jet, nr 4 slider, std pilot jet .106.
Can you confirm this is what you have?

And I know it’s probably a stupid question, but…
You are sure it is a B31 engine with 350cc? And not a B31 bottom end where the previous owner put a B33 top end on it to get the 500cc?  Because that would require a 1 1/8” 276 carb.

Gui

DAVE BRADY

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Re: B31 Poor Performance
« Reply #26 on: 24 June, 2021, 18:24:26 »
Hi,

I am not a B31 expert but to me it looks like it could also be an oil problem.  The bulk of the carbon is opposite the inlet indicating that the inducted mixture is pushing any oil present in the head away and keeping that part of the piston cleaner.  It is also a bit cleaner near to the plug where the combustion should be most efficient.  Was the engine smoking much and is the exhaust valve a  good fit in the guide?

Dave.

martinh10

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Re: B31 Poor Performance
« Reply #27 on: 24 June, 2021, 19:09:26 »
OK, several replies in one here.
I'm using NGK B6HS plugs Pete which I believe are the equivalent to the originally specified champion L10S. I bought two at the same time and have tried them both. I keep thinking of buying another to try but have not got round to it. I'm past caring if I look foolish at the end of this because it was a duff spark plug. If you suggest another plug let me know.
The carb is brand new, made specifically for a B31 with all the bits you specify Gadorey. I've had it in bits and everything is spot on. It's definitely a B31. I had it rebored to 71mm +.030. Decompressor is clear of exhaust valve and valves/guides are now OK. I did have a sticking exhaust valve initially but that has been sorted. I'm keeping my eye on your sloper issue.
I agree with your assessment Dave but I'm damned if I can see how any oil is getting in there. I've lapped and blued the head to the barrel. I've made plug gauges to ensure the new valve guides are the right size and I've now got the right rocker oil feed banjo bolts fitted (1/64th dia. hole). I will try another plug but will be amazed if that fixes it.

Gadorey

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Re: B31 Poor Performance
« Reply #28 on: 24 June, 2021, 19:24:38 »
The plug you’re using is fine. Even if it was too hot or cold, it would just improve the combustion but not explain the lack of performance like you are having. Even if oil is getting in there it also wouldn’t explain such a lack of performance.

Most things have been covered:
Good spark from magneto
Proper ignition timing
Good and clear fuel flow
Correct valve timing
Valves closing completely
Correct valve timing
Correct carb set up
Correct spark plug and gap
Exhaust is not blocked

Only two other things I can think of
Wrong cams
Slider not opening all the way when you twist the throttle fully open

Is there any change when you’re pulling at constant speed and play with the choke lever?

No more ideas at the moment…
Gui

Bess

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Re: B31 Poor Performance
« Reply #29 on: 24 June, 2021, 19:53:44 »
Hi,
     Sorry to hear you are still having issues. You are covering much of the usual solutions, as Dave mentioned it looks like an oil issue, are the rings staggered? If so have you tried Shell V power which is 99 RON to see what difference it makes?

Best wishes...