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Discussion Starter #1
Hi all,

Finally a warm enough day here for me to fire up the 77 bonnie for the first time in a few months and try out some of the repairs and tweaks I've done. After a few kicks, bike fired up, ran very rough, then died. I did a few quick tests and determined the left cylinder is not firing. I don't think it is electrical (good spark, swapped cables) or valve train (clearances correct), so that leaves me with fuel/carb. I tried flushing out the float bowls and putting in fresh fuel to no avail. The only thing that seems to get the left cylinder intermittently firing is if I give the bike lots of gas, or I hold down the tickler to flood the carb.

Two things I did recently that I think are relevant: change the throttle cable and remove the choke slides from both carbs. I noticed that the throttle cable has excessive slack in it: so much so that it jumps off the mounting point on the grip. So I may need to shorten the cable. I did check that both carbs were in synch by the twist grip when I installed it. Interestingly, the previous throttle cable was not in synch with the slides on each carb: there was about 1/8 inch difference between the left and right slide, so I'm wondering if that is relevant -- as in, did the previous owner compensate for a known issue. Or could this be because I removed the chokes (I have never once used them), which I know lots of people have done, in which case, do the carbs need to be adjusted?

Any suggestions as to how I should go about narrowing this down, so I don't end up chasing a red herring?

Thanks.
Neal.
 

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I assume you have blocked up the cable inlet into the carbs.
 

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Discussion Starter #3
Correct...I tapped a screw into the holes. It's probably an air-tight seal, whereas I don't think the choke cables were. Would that matter?
 

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Hi,
Have you cleaned out your pilot jets? Not sure how good your gas is where you are, could be the pilot jets have blocked with "old" fuel.
Have you checked that the slides and needles have gone back correctly? It is easy to catch the needle on the top of the jet and push it up into the slide, rather than it dropping down into the jets. Also, there is a ferrule that sits between the twist-grip body and the top of the upper throttle cable. If this falls out, you will have too much slack in the throttle cable.
I also have a '77 T140V and have also removed the choke slides. I sealed the holes in the carb tops with RTV. Never had an issue, and did not make any adjustments to carb.

All the best,
Andy
 

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Discussion Starter #5
Thanks Andy. Will double check the needle is seated properly. I did flush it with clean gas, but will give it a shot of carb cleaner too in the pilot jet as recommended. Will report back if that made any difference.
 

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Run some SeaFoam or other carb cleaner with every tankful to avoid having to clean the pilot circuit constantly.
 

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If mine runs on one cylinder at start up, it has been the two little holes in the carb just after the slide near the inlet. One fault i had with one cylinder running and then it picked up and ran on both, was a plug cap that had burnt out its resistor. It would spark most times at higher rpm, hence, it ran ok except at low speed. Check those caps. I now use caps without a resistor.
 

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Discussion Starter #8
Thanks for the additional suggestions.

Run some SeaFoam or other carb cleaner with every tankful to avoid having to clean the pilot circuit constantly.
Totally agree, I have been doing this all along, including the gas that has been in the bike for the winter storage. I should own stock in SeaFoam at this point.

If mine runs on one cylinder at start up, it has been the two little holes in the carb just after the slide near the inlet.
Will shoot some cleaner in there this afternoon when I have a chance to fiddle around with it. First I need to fix the slack throttle cable, although that shouldn't impact the idle issue that I'm having.

One fault i had with one cylinder running and then it picked up and ran on both, was a plug cap that had burnt out its resistor. It would spark most times at higher rpm, hence, it ran ok except at low speed. Check those caps. I now use caps without a resistor.
Interesting, I didn't know there was a resistor in the spark plug caps. I did swap the cables though and it didn't help, so perhaps not the issue right now. I did confirm I could get a spark even at low engine speeds just from me kicking over the bike.

Also, forgot to mention, I did a compression test on the cylinder and all looks well.

Will post an update later today with what I find.
 

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Not all caps have a resistor. If it does have a resistor it will be marked with ohms. You might also, start it in the dark and look at the coil tops for sparking if there is a crack.
 

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Hi Neal,
This is not 100% reality but generally electrical systems tend not to go wrong nearly as much as carbs when the bike has not been run for a while. Except for the battery, that can sulphate if left discharged-but if the battery is good then I would tend to look at the carbs first, especially now ethanol fuel is almost inescapable.
You also say that on full throttle it runs and with a tickled float bowl it runs; this would put me in mind to check the pilot jet first. On original Amal carbs it is not removable, on replacement premier (Amal) carbs it is.
There are plenty of posts on how to do this using a #78 drill bit (.016”) spun by hand, or a G string from a guitar shop (not victoria’s secrets). If you can find a post by @StuartMac on cleaning the jet, you will find very clear instructions.

From your description of the throttles, I am guessing that you have twin pull throttles.
It is important that you synchronise the cables from the fully closed position, not when the slides are resting on the idle adjustment screws, as these could be out of synch and will throw trying to set your cable synchronisation.
So either set your cables with the idle screws fully out, or, perfectly synchronise your idle screws before setting the cables-either way this should be a good enough base set up to have a reasonably nice running engine.

AsSearch for this thread might help with the setup procedure
'70 bonne 750 idling on one cylinder

If Cleaning and setting the carbs does not work, then move on to the ignition side.



regards
Peg.
 

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Discussion Starter #11
Thanks Peg. I gave the carbs a shot of carb cleaner to no avail. Need to get a #78 drill bit to try to clean the pilot jet as suggested. I'm pretty sure I have the slides going in synch now as I fixed the slack throttle cable as recommended. If, after cleaning the pilot jet, I'm still having this issue, I may try to swap the carbs to see if it is definitely the carb or ignition related.

btw, Rancid as in the band? They came through Philly a few years back and got to see them opening for dropkick murphys. They still sound good.
 

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When you clean the pilot circuit recognize that it must be clean from the pilot jet or orifice all the way to the tiny hole in the venturi just beyond the slide. Spray cleaner into the pilot jet and it should shoot out of that hole, not sputter, or it’s still clogged. A sew needle or safety pin can be used to unclog the hole.
 

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Discussion Starter #13
So today I cleaned the pilot jet circuit, reset the position of the air screws, and adjusted the slides as recommended here: Bushmans Carb Tuning Secrets . The #78 drill bit really did not meet much resistance going into the jet, so I suspect any residue was pretty minimal. I probed the jet several times to ensure it went completely through then shot it with carb cleaner to flush it out.

Sadly did not make a difference. As before, bike starts up, then sputters out after 5-10 seconds or so. The only way I can keep it running is with the throttle at ~1/4 open or more. I adjusted the air screws to pretty extreme positions but still didn't make a difference -- it either starts/dies or doesn't start at all.

I'm beginning to question my assumptions about what's wrong. I'm assuming the one cylinder isn't firing based on two signs: 1) the exhaust gas is cold coming out of the one side muffler, and 2) when I pull the plug on the presumed not firing cylinder, it doesn't make a difference (but when I pull it on the other side it dies instantly. I was thinking perhaps I have bad gas, but I've never had any issue with this gas station before (it's good quality name brand premium fuel) and I used SeaFoam in the gas immediately after I bought it as I usually do. Maybe it's time to move on from away from the carbs to other diagnostics.

How can I tell if the ignition circuit is working okay? I tried swapping new plugs, and swapping spark plug cables, but that did not make any difference. Compression in both cylinders is good. I adjusted the timing a few years back and don't have any reason to suspect it would all of the sudden fall out of timing.

As always, appreciate your thoughts!
 

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Hi Neal
You do not say if youve the original points or electronic ignition, this will make a huge difference in how you approach diagnosing an ignition fault.
Regards
Peg.
 

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Hi Neal,
This is good news, the Boyer will fire both coils together, so a lot of components are eliminated from the search.

Firstly, your idea to swap the carbs temporarily is brilliant, it means certainty that the carbs are, or are not the cause of your woes. If you can do this, it is a small amount of work for a huge amount of components to eliminate.

You have already done a compression check that you feel is OK, and valve clearances do not close up when you are not riding.
The ignition trigger and Boyer E.I. Amplifier run both coils together and fire both spark plugs together. So I think we can place these to the back of the queue for investigation.
You have swapped, plugs and HT leads completely without the misfire moving to the other cylinder.

the next items in the line are the coils, if your ht leads are long enough it is easy to just swap the leads into the opposite coil, so the left coil fires the right plug and the right coil fires the left plug. If the leads are too short, then you will have to physically swap the coils in their holders.

I think that if I was you, I would swap the HT leads first-because it takes seconds to do.
If the fault did not immediately swap to the other cylinder, then I would swap the carbs.

Good Luck
regards
 

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Discussion Starter #17
Hi all,

After swapping the carbs today, the same cylinder still is not firing. This rules out the carbs as the most likely culprit. As a silver lining I guess I'm glad the choke removal/new throttle cable was not the underlying issue.

This leaves me thinking it's some sort of ignition issue. I did swap the cables at the spark plug ends previously, which made no difference. Would swapping the cables at the coil end show anything different? Perhaps I am misunderstanding what to do. I was able to produce a spark on both plugs by manually kicking. I'll search around on the forum for ways to troubleshoot the coils, but any suggestions are welcome!

Thanks!
Neal.
 

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Hi Neal,
This is proving a difficult fault to pinpoint.
There are 3 areas that could make it run on one cylinder, with the added information that the engine fire on both cylinders at higher throttle openings.

A) Carbs/fuel.
B) Ignition.
C) Mechanical.

A) The carbs
These have been cleaned and adjusted and importantly been swapped. The bike neither ran on two cylinders and the fault did not swap to the other side. This appears to eliminate a carburettor as a faulty item.

The only other item in the fuel delivery would be an air leak on the inlet tract. This might fit with the running at higher throttle openings. When you open the throttle wide the vacuum in the manifold drops, this will stop sucking air in through any air leak and the carb should function again.
If the tickover screws were a long way out (one very high and one very low), the engine might run on one cylinder only until the throttle is opened.

B) The Ignition.
The Boyer Electronic ignition low tension (12v) circuit fires both coils together, this immediately discounts the low boyer and low voltage wiring because if there was a fault here both cylinders would stop firing.
The High Tension side (spark) of the ignition circuit has components that are unique to each cylinder, if one of these components was faulty then the engine would only run on one cylinder.
Coils
HT leads
Spark plug caps
Spark Plugs

you have swapped the cap end of the ht leads on to the other cylinder without the fault swapping to the other side, this effectively tested the coil ,HT lead, and Spark plug cap all at once. This leaves the Spark Plugs, didyou try swapping these?
The HT circuit is most stressed under load and speed, you would expect worse performance when you rev the engine, not better as you observed.
A spark plug finds it much harder to fire under compression pressure, a good spark with the plugs out might break down when the engine is compressing the air around the spark tip.
There is a small chance that a low battery voltage will alow the Boyer to over advance the Ignition timing at tickover, it might be worth a quick battery voltage check- although I would expect that both cylinders were affected.

C) Mechanical.
Some things that might cause the engine to run on one cylinder.
I)Valve clearances too small and holding a valve open.
ii) A sticking or leaking or leaking valve seat.
iii) A leaking head gasket.
All of these faults should lower the compression in the cylinder, but you have said that you have checked the compression-what were the readings that you got.
If the compression is low due to a leak , then the compression pressure should rise as the revs increase allowing that cylinder to run, this is compatible with the observation that both cylinders fire at higher engine speeds.

Best regards
Peg.
 

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Discussion Starter #19
Thanks for the really thoughtful reply Peg.

To answer a few of your questions:
  • I did try brand new spark plugs. No difference.
  • The cold/dry compression was 125 lbs (for both the left and right cylinders) and the cold/wet compression was 130 lbs (again, for both cylinders). This almost exactly matches numbers I got four years ago, the last time I checked compression when the engine was running well as a baseline measure. It does take a few kicks to get to the max pressure.
  • The battery is about 5 years old now, but it still holds a charge quite well. With bike off, reading is 12.5V. I do top it off every few weeks with the battery tender.
The air leak on the inlet idea is intriguing. I'll replace the carb o-rings and try again. I don't see anything obviously wrong with where the manifold attaches to the cylinder head but I suppose an air leak could be very tiny. The throttle adjustment screws are pretty far out. Using the carb setup instructions at Bushmans, I had to loosen them quite a bit to get the slides to the proper initial height. How else can I assess if there's an air leak?

I also need to do a more thorough inspection of the valve train when I get some time.

Neal.
 

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Hi Neal,
This is a very strange fault. Usually when an engine is running on one cylinder consistently the cause is pretty easy to determine.
You have reasonable compression and every ancillary that could cause a problem has been swapped, all components allow the other cylinder to run and components that previously allowed the other side to run do not on the problem side.
I would expect an air leak would have to be fairly large to prevent the cylinder from firing.
To check the manifold start the engine at idle speed, spray wd40 around the joint, if it is sucked in, there is a leak.

12.5v is not that great, try checking the voltage with ignition off, then with ignition on, then with ignition and lights on for a minute. The voltage should hold up.
Boyer Brandsen EI units are sensitive to low voltage.
Regards
Peg.
 
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