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engine compression B33

Hi all, does anyone have an idea what a healthy compression reading should be on a B33 ?
Thanks

email (option): skywarrior1940@aol.com

Re: engine compression B33

I'm guessing you have a problem to be asking this question....What is it?.....Ian

email (option): ian@wright52.plus.com

Re: engine compression B33

That should be 6.8:1

John

email (option): john.kater@chello.nl

Re: engine compression B33

I think he's talking about lbs/sq. inch...Ian

email (option): ian@wright52.plus.com

Re: engine compression B33

Isnt it usually about 80 - 100 psi on a low compression engine ?

email (option): richardpurkiss@hotmail.com

Re: engine compression B33

Mines pretty worn (About 10.000 miles since i last put a set of rings in it, Smokes a bit’) showing about 125 psi on the compression tester,
Regards Tom Green,

email (option): rustytomm20@hotmail.com

Re: engine compression B33

Ian Wright
I think he's talking about lbs/sq. inch...Ian
6,8 bar is 98 PSI. Everybody happy 🤗

email (option): john.kater@chello.nl

Re: engine compression B33

Helpful info, thanks guys. Yes i was looking for the PSI compression reading. I'm having trouble starting it after a rebuild with modifications! I'm running a standard crank with a short rod, shortened barrel, forged 89 mm piston, goldstar cams, jap clutch to name but a few mods. I'm getting only 80 psi but i figure that's reasonable and should improve when the rings/bore bed in, if only i can get the thing to fire up, I'm thinking it might be an idea to tow it or fit a starter motor!

email (option): skywarrior1940@aol.com

Re: engine compression B33

If your using a standard compression tester make sure you kick the bike over with the throttle wide open...You'll get a low reading if there's not much to compress....

Whatever you get it will improve a bit as the rings bed in...However, I wouldn't worry about it too much...It's not something I ever check myself...If the compression feels good on the kick start, which it should after an engine rebuild, it will be OK...
If the compression feels very low then that's another story and you'll be looking at potential faults....

The bike should start without any problem if everything is set up OK...Again, if it doesn't then you'll have to start checking settings and fuel supply...Ian

email (option): ian@wright52.plus.com

Re: engine compression B33

Thanks for the advise, I'm just in the process of treble checking everything !
The bike is fitted with a Amal 932, which is giving me some concern as its bore size is smaller then the correct carb the Amal 289G and with larger cubic capacity I'm wondering if this could be my problem?.. thoughts anyone?

email (option): skywarrior1940@aol.com

Re: engine compression B33

I would think the "smaller" carb would actually make it easier to start (as long as it's working, per Ian).

email (option): btcoop77069@aol.com

Re: engine compression B33

['The bike is fitted with a Amal 932, which is giving me some concern as its bore size is smaller then the correct carb...']

Depending on the year a B33 would have an 1 1/8" carb...That equates to 28.5mm...So your 32mm carb is bigger than standard, not smaller...

You quote a 289 carb as the standard one, so a separate float chamber type... Fuel metering and jet sizes on these are different to the 932 concentric you have fitted....For jet size compatibility you should use the jet sizes listed for the Monobloc carb fitted to later models...

However, as the choke size of your carb is larger (32mm) than standard it would be best to start with the mixture set up slightly rich and then weaken it as/if required rather than risk running with a weak mixture...The slide cutaway would be the same whichever carb you have fitted..namely a 3 1/2 cut away...Ian

email (option): ian@wright52.plus.com

Re: engine compression B33

Thanks for all the info, I stand corrected, so my carb is bigger than the standard fitment.
I spent a few hours on it earlier today, still no joy, nothing! even tried some fuel down the plug hole, nout happening despite a good spark and timing checked. Now looking at valve timing to see if there's a problem there.
thanks for your help

email (option): skywarrior1940@aol.com

Re: engine compression B33

Still no joy at all attempts at starting, I've looked at valve timing.
I'm using inlet cam 65-2438 & exhaust 65-2436 so different timings from standard set up, by my readings using a degree wheel show the exhaust opens at 55d BBDC & closes at 35d ATDC The inlet opens at 30d BTDC and closes at 65d ABDC, both valves closed at TDC compression stroke, Im surprised the exhaust valve opens before BBDC, any ideas?

email (option): skywarrior1940@aol.com

Re: engine compression B33

Those cams precede the Gold Star cams 65-2448 and 65-2450 (touring cams)...They share the same valve timing figures, the difference being the 'silencing ramps' used on the later version...(more gradual opening)...They were listed as 'sports cams' for B Series engines but are actually fairly 'mild'...Personally, I always use a pair of 65-2446 Scrambles cams for tuned B33 motors...

Don't forget you have to set the tappets to the correct clearance before checking timing...If the cams are marked 'dot' and' 'dash' in the usual BSA fashion and you have the marks correctly aligned it is highly unlikely there is a problem there...

I'd bet a pound to a penny your problem lies elsewhere but it's always worth checking...Ian

email (option): ian@wright52.plus.com

Re: engine compression B33

Hi Ian, thanks for the advise, if or when I get this thing up and running i will look into those 2446 scrambles cams you mention. Yes the cams are installed dot to dot, dash to dash and are marked IN & EX accordingly. So i now know the valve timing isn't an issue, it narrows it a bit i suppose. I'm sure the ignition timing is set correctly at 11.5 BTDC Compression stroke, both valves closed. I'm using a depth gauge to measure, and I have it set so that pushing the mag into full advance releases thin paper grasped by the points. So assuming that's correct? I need to move towards a carb fault, im using a Amal 932, i might swap it out for a Mukuni and see how i get on.
Thanks for your help

email (option): skywarrior1940@aol.com

Re: engine compression B33

Just a thought, could a carburetor spacer/extension be the fault, vapour lock or something similar?

email (option): skywarrior1940@aol.com

Re: engine compression B33

I doubt the carb spacer is the cause of your problem.....Also, the 932 you are using should be fine for the bike subject to checking it is set up correctly and that it doesn't have any other problem...Introducing a Mikuni that is not set up for the bike may just be adding another problem...Random changes made without any basis in observation are unlikely to produce a result in my experience...

Usually starting problems relate to basic settings (valve timing, ignition timing, carb settings, contact breaker setting etc.) electrical faults, carburettor faults or fuel supply issues....

I suggest you read the article I wrote for the 'technical section' of the forum on 'ignition timing'...This covers the theory behind timing the spark and the practice of setting ignition timing...

You must be absolutely sure you have followed the correct method and that the resulting setting is accurate....

Finding a fault is just a process of elimination in practice if you are unable to diagnose the fault from the symptoms, working through a list of potential problems until you hit the right one...So you must be sure that whatever aspect you look at it has been definitively resolved...You can then ignore it and move on to other possible causes of the problem.... Ian

email (option): ian@wright52.plus.com

Re: engine compression B33

Ignition timing for the B33 should be set at 3/8" BTDC with the adv/rtd lever set in the fully advanced position before setting...Ian

email (option): ian@wright52.plus.com

Re: engine compression B33

Ian, thanks for that information regarding the timing, i will adjust it accordingly. to be honest i think ive almost come to the end of the road trying in rectifying the problem, its had me stumped and i never thought i would be beaten on anything, but a year and a wrecked knee cap later its never as much shown the slightest glimpse of life!

email (option): skywarrior1940@aol.com

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