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So I got my b50mx project done but it is running really hot and I've been told that it is a timing issue. The bike starts fairly easily once you give the carb a good tickle. I have an Electrex ignition which I set exactly as the instuctions that come with it indicate. And to my knowledge the valve timing is set correctly. This is my first time working on a fourstroke engine so I don't know for sure if I have everything right! I am looking for a shop or an individual in Southern California who go through and check to make sure all is correct and rectify if not. If anyone has suggestions as to where I could go in Socal I would appreciate it!
Here is the bike running.

Last edited by johnu; 06/16/21 5:57 pm.
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Aye, it must be said, that does look to be running a tad on the warm side!

I would be looking to check the mixture rather than the timing. try running a bit richer.

In my dimming memory the MX used to have a very satisfying cough (actually a bloody big BANG) on the over run. I have my overbored B44 (with a small "peanut" muffler) set this way. It does not run hot plus you get a nice gout of flame coming out the exhaust as well!

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Definitely check your timing with a strobe light.

Retarded timing will make your pipe glow red pretty quickly and performance will be flat. Advanced timing clue is kick back when starting.

And don't run it like that for any length of time.

Avoid much idling, verify oil flow, get your timing set, and then go out and ride it to load the rings to begin seating them.

These engines are pretty simple. Get yourself a timing light and check that first.

Last edited by Stuart Kirk; 03/18/21 9:19 pm. Reason: More info
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Many years ago, I had a similar problem albeit on an old Triumph T120 which seized fresh pistons on a pretty short test ride. I measured everything, swapped pistons and double checked carbs and timing and seized it again. It was late in the day and putting the bike in the dark garage, I could see a slight glow in the head pipes. Long story short, the alternator rotor had shifted and partially sheared the key so that the timing mark was off. A degree wheel found it and it was repaired. The third time was the charm as they say and the bike ran well after that.

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And before you start, verify the timing line on the stator with a piston stop & timing disc.
Lucas Stators all had the same mountings but there was different positions for the timing line.
Even brand new ones have been known to be wrong.


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Remember that he has fitted the Electrex ignition system. If he has fitted it "exactly as the instructions", timing is unlikely to be the issue. He no longer has a Lucas Stator or an alternator, broken key or not!
It is possible that he could have installed it incorrectly and therefore gotten the timing wrong, but if it was THAT far our would it have even started? mind you the same could be said of the mixture! Lets face it the B44 and B50 were lazy machines, any excuse to not start and they would take it. and any excuse for the B50MX to break your bloody leg if you were unwary!

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HOT

Sounds lean (backfiring & popping)

How did you set the ignition timing?

How thick is the pipe tubing?


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There's a saying among ICE (internal combustion engine) repairmen. "90% of carburetor trouble is ignition.) The basic idea is to be sure the ignition system is working proper before going on to solve carb problems.

I just had a look at the Electrex install instructions and note there is a basic problem. Their timing marks "move" when you rotate the stator to adjust the timing. A fixed, non moving timing mark is needed to accurately verify timing. (Like the original timing pointer in the primary case.)

Also, the rotor is not keyed to the shaft but secured by the grip of a tapered collet. Tapers and collets usually hold just fine if torqued properly but the possibility of slippage is real. It would most likely slip towards being retarded.

So, first check the rotor position relative to TDC as per Electrex instructions and correct if needed. Once properly located, consider pinning it.

Next, arrange a fixed timing pointer and mark the rotor for 34 deg BTDC.

Then, check timing with a timing light between 3000- 4000 rpm and adjust as needed.

Finally, ride it to see what jetting work it needs. (Popping on decel is usually an air leak at the head or a lean idle setting or plugged idle jet.)

Hope this helps.

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Originally Posted by C.B.S
HOT

Sounds lean (backfiring & popping)

How did you set the ignition timing?

How thick is the pipe tubing?

Thanks for the reply!
I have a 290 main jet and 19 pilot
I set the timing exactly as the Electrex instructions as I understood them. Find tdc then align the timing marks highlighted by blue dots. I checked it a couple of times to make sure it is right.
Header pipe is stock.

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Originally Posted by Stuart Kirk
There's a saying among ICE (internal combustion engine) repairmen. "90% of carburetor trouble is ignition.) The basic idea is to be sure the ignition system is working proper before going on to solve carb problems.

I just had a look at the Electrex install instructions and note there is a basic problem. Their timing marks "move" when you rotate the stator to adjust the timing. A fixed, non moving timing mark is needed to accurately verify timing. (Like the original timing pointer in the primary case.)

Also, the rotor is not keyed to the shaft but secured by the grip of a tapered collet. Tapers and collets usually hold just fine if torqued properly but the possibility of slippage is real. It would most likely slip towards being retarded.

So, first check the rotor position relative to TDC as per Electrex instructions and correct if needed. Once properly located, consider pinning it.

Next, arrange a fixed timing pointer and mark the rotor for 34 deg BTDC.

Then, check timing with a timing light between 3000- 4000 rpm and adjust as needed.

Finally, ride it to see what jetting work it needs. (Popping on decel is usually an air leak at the head or a lean idle setting or plugged idle jet.)

Hope this helps.

Hi Stuart thanks for the reply!
My understanding with setting the Electrex timing is that you find tdc then just line up the marks highlighted by the blue dots. Re-check at tdc and if still aligned then it should be good to go. Is this correct or am I misunderstanding something. There is also a red dot and mark on the stator that is 34* but as far as I can tell this has no bearing on the setting as explained by electrex.
I will purchase a timing light to check it as you explained. I've never used one but I can probably figure it out.

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Here are a couple of pics. One how I have it set and one of the Electrex instructions. My question is if I check it now with a light should the red dot (34* btdc) line up with the timing mark on the rotor?
[Linked Image]
[Linked Image]

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yes, as long as the RPMs are above the advance curve. in other words, at idle the rotor mark will be between the two dots, as you increase RPM the rotor mark will move towards the red dot. if it's set properly at the top of the curve the dot and mark will align and not move as RPM increases. you don't want the rotor mark to go past the red dot. if it does, or you can't get full advance then move the plate until it's right. for instance, on the Boyer the first setup only gets close enough to start the engine. the final setting is done by adjusting the plate. sometimes the initial setting is good enough. even when set right you should be on the watch for detonation. depending on your compression ratio and fuel available you may need to back off a couple degrees

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Hi,
As I understand it, you guys are on the right track as far as the Electrex instructions are concerned. BUT, all the timing marks they ask us to go by are in relation to the ignition system itself, not the engine.

That Is why my earlier post mentioned a non moving timing pointer fixed to the engine so you can check the ignition timing relative to the engine. You see, one problem is that the rotor could slip on the crank and the engine running would suffer but the timing would still appear to be correct.

Unfortunately, after thinking about it a bit more, this also appears to be true with the fixed timing pointer. The fact that that rotor that carries timing marks is not keyed to the crank leaves the door wide open for hard to find timing problems. The fixed pointer wouldn't even tell you if the rotor had slipped. However it would be helpful when adjusting timing by moving the stator.

(After you check it with the light, it would be a good idea to recheck the accuracy of your TDC setting.)

Last edited by Stuart Kirk; 04/08/21 9:31 pm. Reason: Thought of something else.
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what does customer service from Electrolux say ? ( this is the rhetorical question )

Last edited by quinten; 04/09/21 10:18 am.
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As with most things it will probably be a combination of factors.
Cannot help but notice that the exhaust looks like it has a slight leak at the head, sucking in air on the over run??
I see you have a nice new carb, I checked on the jet sizes on the Burlen web site ..... they quote a main jet size of 250 (not 290) and a needle position of 3.
Hard to see in the vid but the carb to head joint looks a little funky? do you have a good seal there?

Everyone, or at least most, seems to be focusing on timing, it just doesn't sound that bad to me, but hey, other than the last couple of years my knowledge and "feel" is 45 years out of date.

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Thanks Dave. No air leak at the exhaust to barrel joint, carb I had to put an extra spacer in there so that the stud doesn't foul the tickler. I changed the 250 main to a 290. Will check for air leak at the carb manifold.

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So you adjust the timing based on the rotor's relationship with that adjustable backing plate. But how do you ensure that adjustable backing plate is in the correct position relative to the engine?

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I think the possiblity of slippage of the rotor on the crank to be very small, assuming it has been assembled and torqued correctly.

Most importantly is that the TDC position was located accurately in the first place, using a piston stop and degree wheel.
With the stator fixed in its mid-slots position, the rotor should now be tightened to the crank with the blue marks aligned. Great care must be taken to ensure the crank doesn’t move during this tightening, which is very hard to do.
I would be tempted to tap the rotor onto the collet to provide initial lock, before tightening the nut (as I do with a magneto pinion).
BTW your stator isn’t in the mid-slots position, why not?

I would certainly want to double check throughout the process with an indicator pointer (as SK suggested) to ensure the blue marks were still aligned at true TDC after tightening. Only then can you trust the blue marks and the red advance mark.

IMHO these are only initial “get started” marks, to be checked by strobe. All EI’s have variation in components, so variation of precise advance, which is what makes strobing necessary.

If you have set the assembly accurately at TDC, then the alignment of the blue marks does mean TDC.
So when you strobe it at 4000 rpm, the rotor blue mark should align with the stator red mark.
If it doesn’t, note the degrees of mis-alignment and adjust the stator accordingly. Keep a note and sketch of exactly what you have done so you can roughly return.

As SK has said, a proper timing pointer is ideal for this, but that is easier said than done with your setup. A bent plate pointer could be made, that mounts to 2 cover screws.

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Ok, this is bugging me!
Questions ...... if the MOTOR is running this hot why isn't the bugger siezed? could it be that the motor itself is just fine??

If you zoom in on the exhaust something is blowing all that smoke when ever you rev up, and sucking it back on the over run. If you look at the exhaust the hot part is a good 6 to 8 inches down the pipe, whys that?

Is there a possibility that this is actually a slightly over rich (he fitted a larger jet) mixture burning in the exhaust with sucked in air??? like an afterburner!

just aimless musings .......

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It may well seize if you take it for a 20 mile thrash, who knows?

To my eyes the pipe looks excessively hot from the head to round the bend.

A rich mixture won’t cause that, but retarded timing will, as has already been said.

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Originally Posted by Dave Martin
Ok, this is bugging me!...... If you look at the exhaust the hot part is a good 6 to 8 inches down the pipe, whys that?

That's from retarded timing. Retarded ignition timing starts the burn too late so the still burning mixture gets farther down the exhaust pipe and heats it up red.

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Nah, sorry I don't buy that. If it was THAT retarded it wouldn't run, and the hottest part would still be by the head. This clearly isn't, it is much redder further down the pipe.

Of course a rich mixture would not cause a hot exhaust (it would actually be a lot cooler) ON ITS OWN. but if you added more oxygen through an obviously leaking header .......... I only suggested a rich mix as he has changed the jet .... in actuality any mix is likely to combust in the head pipe if you add air.

This issue, as I said before, is going to be a combination of a load of things. Timing is possibly one factor, mixture is going to be another, leaking head pipe yet another.

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Originally Posted by Dave Martin
Nah, sorry I don't buy that. If it was THAT retarded it wouldn't run, and the hottest part would still be by the head..
Well actually, it would run, but it would be kind of flat and dead and heavy sounding with weak top end.

It was the late sixties and I was 16 when I first saw the evidence first hand. I was rebuilding my friend's distributor B40 and had just fired it up in the driveway when it was getting dark.. After just a few minutes running, the pipe was glowing red hot. I got scared and shut it off to look things over but found nothing obvious. Now remember, it was a distributor motor so changing the timing was easy and I hadn't really set it proper to begin with. I was 16 after all. So we fired it up again and I rotated the distributor this way and that to where it sounded best and the pipe didn't get red hot any more. You can't get much more backyard than that.

Now, to reinforce my little theory, early catalytic converter equipped cars specifically retarded the idle timing to send more heat down the exhaust pipe to keep the cat warm so it could clean up idle emissions. And, these early cars could actually catch fire from an overheated cat, so they often had a overheat warning light to try to keep the car from actually burning down.

So, I submit, that retarded timing can indeed turn a head pipe red hot.

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I'm sure if you had a cast iron barrel and an alloy piston, it would have seized by now..


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I know a guy who used the Electrex STK-010 in a number of his BSA unit single MX bikes. He reports that with his B50 it runs best when set up closer to 35.5 degrees BTDC using the Electrex. Something to consider.

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