Author Topic: Rebuilt B25 wont start what am I doing wrong  (Read 424 times)

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Trebor

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Rebuilt B25 wont start what am I doing wrong
« on: 13 May, 2022, 10:52:35 »
Hi all

Need some advice, I have rebuilt a B25 engine but cant get the thing started, I have carried out the following work:-

New HT coil, new rect/reg, new plug and cap, new capacitor and condenser and alternator checked ok and spark is good. Carb has been cleaned and checked including the pilot jet bushing for the mixture screw so all seems good there, all settings set to the manual. Timing has been set at fully advance to the timing mark on the rotor. Points gapped to 0.015 thou. Good frame and engine earth tested as ok.

I have fitted what I think is a standard cam with the single line timing mark and I know that some come with the "V" and the "I marks, have I got the wrong cam??? Even if the cam was wrong I would still expect the engine to run??

The engine has new piston and has excellent compression.

I'm trying to start it on E5 petrol, anyone had any issues with this fuel?

When trying to start the engine I get a single fire of the engine but it wont run.

Any advice would be appreciated as I'm getting desperate!!!


chaz

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #1 on: 13 May, 2022, 13:03:09 »
sounds like the B40 was, I had the ignition 180 degrees out. dont forget timing with full advance unless you remove the possibility of a fault by fitting electronic. ticks over ok but wont rev, so next job is to play with ignition unit position.

cdsdorset

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #2 on: 13 May, 2022, 13:30:57 »
Sometimes a rebuilt engine just won't start up the first time. Have you tried giving it a bump start.

spaceman

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #3 on: 13 May, 2022, 13:57:06 »
As said, it's easy to get the ignition timing 180 degs out. Double check by turning engine to TDC with both valves closed and check a few degrees before that points are about to open - note that points cam normally rotates CCW in the opposite direction to crankshaft!


Trebor

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #4 on: 13 May, 2022, 17:46:38 »
Hi everyone, thanks for the replies.

I have checked I am on the right stroke and that the push rods are not inverted. Ignition has been set fully advanced on the rotor mark. Spark is nice and bright.

Taken the carb off again to see if there are any issues there???

Thanks Rob

chaz

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #5 on: 13 May, 2022, 20:21:21 »
easy start, if it runs on easy start then its a carb problem. is there a gasket or o ring on flange face, damaged or missing that may not be causing vacuum to suck fuel through carb.
many dont like it but it sorts a problem and you are not going to run it on a daily commute.

chrisb

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #6 on: 13 May, 2022, 21:46:21 »
This caught me out, but my B31 ignition lever worked the "other way round" than expected.  I'd accidentally timed it at full retard - and had the same symptoms.  One "fire" then nothing.   Hope that helps.

spaceman

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #7 on: 14 May, 2022, 08:24:34 »
I vaguely remember having problems with the rotor timing marks on a B25 and would recommend confirming the timing is properly set using a slightly different method. At fully advance, the timing should be 37 deg BTDC or -8.7 mm in terms of piston position. In the past, I've attached a timing disc to the rotor using magnets so that I could check the timing accurately without having to remove the chaincase cover. However, the simplest way to double check the ignition timing is using a pencil or equivalent inserted into the spark plug hole on which you can mark TDC and the correct (8.7 mm) timing mark. It doesn't need to be completely accurate in order to verify that the rotor timing marks appear to be correct.

If the timing is spot on, I would enlist a bit of help in order to try to bump start the bike with one person doing the pushing. With a bit of luck, this might get the engine going.

MGI

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #8 on: 14 May, 2022, 10:37:43 »
Easy start is a good way to get an engine fired up and often once it gets going on it, it then becomes easier to start. I always laugh when people tell me that easy start is bad for an engine and that the engine becomes addicted to it, as if the engine was human!

Trebor

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #9 on: 14 May, 2022, 11:41:47 »
Hi all

Thanks for the suggestions, I have checked for 37 degree before TDC and use a depth rod to check the 8.7mm measurement, the interesting thing is the 8.7mm puts the rotor mark some way out. With the rotor mark aligned the distance before TDC is closer to 6mm.

I do have a crank alignment plunger that fits into the crank so I am going to re-time by this method, I don't think any manufacturing tolerance difference should stop the engine running though???

Going to try bump starting as not tried this yet.

Thanks again

Rob

chaz

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #10 on: 14 May, 2022, 13:21:49 »
careful, bump starting... years ago I bump started fathers Honda C70... dragged me along the road .

Easy start, sorted my problem, the B40 wouldnt start on it so I knew it must have been timing/ignition.
the only time I dont keep using it is on 2 strokes, as it strips away the oil on the cylinder walls. one or two squirts max.

have you done a compression test?

spaceman

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #11 on: 14 May, 2022, 13:35:23 »
6 mm BTDC corresponds to 30.5 degs and, since it is the fully retarded setting that matters when you try to start the engine, reducing this from 7 degs to 0.5 deg BTDC (with 15 deg auto advance), might not help! If your B25 has a crankshaft timing plug then that should provide the most accurate way of setting the timing. However, given the fact that it is the retarded timing that matters when starting, it might be a good idea to also check what that is just in case the auto retard mechanism is mis-behaving. If you can tell me the piston position, I can translate it into an angle.

In terms of bump starting, as suggested, it's best to do it with you sitting on the bike to maintain control with someone else giving you a push. Last time I did it in this way, I found a nice down hill section of road to make it easier.

Dean Southall

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #12 on: 14 May, 2022, 15:32:26 »
Have you tried turning it off and back on again?
BSA: turning ordinary men into mechanics since 1910

Trebor

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #13 on: 14 May, 2022, 16:25:00 »
Hi all

Thanks for the warnings about bump starting, the last casualty from bump starting was when my wife was doing the pushing the bike suddenly started and she fell flat on her face. Probably need to ask my son to come over and do the pushing!!!

Mike Farmer

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Re: Rebuilt B25 wont start what am I doing wrong
« Reply #14 on: 15 May, 2022, 12:19:32 »
 :) :) :)

Have you tried splashing some petrol down the carb. If it fires at all your timing aint 180 degs out.

Mike 8) 8) 8)