Won't Turnover / POP - Bang - Burn

Hey Guys,

Last month I took the '74 on a fairly good trip from D.C. to New Jersey. She rode great the 250 miles there and the 100 around town. On trip back she started losing power under acceleration. By losing power I'm referring to a drastic loss in revs, almost like engine breaking. I pulled over and looked her over, eventually renting a U-Haul and heading home. Once home she was kicking back really hard and wouldn't turn over.

Over the last month...
- Installed new plugs
- cleaned the single Mikuni and installed new jets
- Checked for spark - I have spark on both plugs
- Reset the timing on the Boyer to the static 31 degrees
- Checked the wiring harness for burns or bad connections - All good.
- Charged the battery
- Checked compression. 90 PSI on both sides. I'm measuring cold.

Currently as she sits....
- Feels like there may be a loss in compression (Maybe an air leak).
- I can turn the crank with my hand if i push hard (wasn't able to do this before).
- If i kick her over it pop's and burnt gas fumes come out of the air cleaner
- There seems to be gas leaking out of the 2 into 1 carb manifold when i turn the crank

I'm guessing i have to pull the head and barrels off to check for a broken ring but before I do i wanted to get some other perspective.

Any thoughts or suggestions, as always will be appreciated,

Burnt inlet valve(s) ?

Checked the tappet clearances ??

Rocker clearances (Both Inlet and Exhaust) are correct. Obviously i will have to take the head off to examine the valves but everything in the Rocker's looks good. Valve train moving properly.

thanks for the suggestion though. I hadn't checked the clearances.

Issue with intake valve sealing as suggested would explain symptoms. Ride em and 'rench em.

THe kicking back is an ignition advance issue - too much advance at start. Boyers are known to do this with low batt voltage. Have you tried starting/running the bike since you reset the timing and charged the battery? Have you checked battery voltage while the engine is running to be sure the alternator is working and you are getting appropriate voltage?

Compression numbers are rather low but it can vary dramatically depending on the strength/consistency your "kick" and on whether the engine is cold or warmed up (should be warmed up for accurate results).

mike996 said:

THe kicking back is an ignition advance issue - too much advance at start. Boyers are known to do this with low batt voltage. Have you tried starting/running the bike since you reset the timing and charged the battery? Have you checked battery voltage while the engine is running to be sure the alternator is working and you are getting appropriate voltage?

Click to expand...

You are correct. When the battery was low (not that low actually) it was kicking back. Now, set at 31 degress, with a full battery, it's not kicking back but does have a loud "POP" when kicking over, followed by the smoke coming out of the air filter.

I can't get the bike to start or i would check voltage, timing etc. I had tried retarding the static timing a bit but it didn't seem to make a difference.

Oxidized fuse block piece of ka-ka ?

Bother .

Timings Slipped . (Ign . )

HOWEVER , if you STAND on the kick start , & push it though realy slowly , YOU can check for even Compression on Ea. Cylinder .
Or do it with a Comp Guage kicking it over . Sounds TOO ADVANCED , if its a bit of a jump in the foot crank as it spits / chuffs in the Intake .

See the blankety Battery is Fully Charged before you start playing around with it .

ARGGHHH . :shock: BOYER . indeed . The 81 Triumphs used to be incompatable with Japanese Batteries . The plates would get rattled .
Good News . Youre Battery may have falled to bits . :x

If the battery is up to full voltage it wont matter if you are making
juice or not. At least for a while. Full voltage though is critical.
Got another batt handy?
If you have a differential compression tester have a go. If not
put it on TDC, firing, put in a steady 80 pounds of air and
listen real careful for sound of hissing air in the intake .
Leaky valve will do that. If it leaks out the exhaust it wont
matter much.
It does sound too advanced.
If you had Amals on it Id tell you
to check the floats. But you dont.
Dump the fuel completely and go get fresh hitest.

Hi,
I think having fuel come out the 2-1 manifold is a good indication of carb troubles. Is gasket blown? u can hear air leak? if a burnt v/v you should know with a compression test. which ever cyl has burnt v/v should have little to no compression. as the v/v is burnt and no longer covers the seat properly cyl cannot build up pressure. so I don't think v/v's an issue because u have reported 90 psi per cyl. From my experience when v/v burns it literally melts and or breaks into pieces from the heat. I would check carb manifold gasket and that the carb float is seated correctly. u said u worked on carb {jets} I would re-check any work done here just to confirm nothing out of place, especially if u disturbed the float during your maintenance. Good luck let us know what u come up with.
Dox

About ten years ago I rebuilt my engine with only .035" clearance. It ran great for 1000+ mi. then one crisp October day your same symptoms happened to my bike. Turned out to have seized. When the engine cooled some clearance returned and I was able to get her started and home but the damage was done. New pistons of the same oversize as the rebuild with just a honeing of the damaged cylinders got me the extra .010" I needed for correct clearance. Been fine for 7000 or 8000mi. now.

If you have not redone both wire problems common with the Boyer, I would take a look there.